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From Wikipedia, the free encyclopedia

(Redirected from Human heart)

This article is about the internal organ. For other uses, see Heart (disambiguation).
"Cardiac" redirects here. For the computer programming tool, see CARDIAC. For the comics
character, see Cardiac (character).

Heart

The human heart

 Details

System Circulatory

Artery Aorta,[a] pulmonary trunk and right and

left pulmonary arteries,[b] right coronary artery, left
main coronary artery[c]

Vein Superior vena cava, inferior vena cava,[d] right and

left pulmonary veins,[e] great cardiac vein, middle
cardiac vein, small cardiac vein, anterior cardiac
veins[f]

Nerve Accelerans nerve, vagus nerve

Identifiers

Latin cor

Greek καρδία (kardía)

MeSH D006321

Anatomical terminology

[edit on Wikidata]

The heart is a muscular organ found in humans and other animals. This organ
pumps blood through the blood vessels.[1] Heart and blood vessels together make
the circulatory system.[2] The pumped blood carries oxygen and nutrients to the tissue,
while carrying metabolic waste such as carbon dioxide to the lungs.[3] In humans, the
heart is approximately the size of a closed fist and is located between the lungs, in the
middle compartment of the chest, called the mediastinum.[4]

In humans, the heart is divided into four chambers: upper left and right atria and lower
left and right ventricles.[5][6] Commonly, the right atrium and ventricle are referred together
as the right heart and their left counterparts as the left heart.[7] In a healthy heart, blood
flows one way through the heart due to heart valves, which prevent backflow.[4] The
heart is enclosed in a protective sac, the pericardium, which also contains a small
amount of fluid. The wall of the heart is made up of three
layers: epicardium, myocardium, and endocardium.[8]

The heart pumps blood with a rhythm determined by a group of pacemaker cells in
the sinoatrial node. These generate an electric current that causes the heart to contract,
traveling through the atrioventricular node and along the conduction system of the heart.
In humans, deoxygenated blood enters the heart through the right atrium from
the superior and inferior venae cavae and passes to the right ventricle. From here, it is
pumped into pulmonary circulation to the lungs, where it receives oxygen and gives off
carbon dioxide. Oxygenated blood then returns to the left atrium, passes through the left
ventricle and is pumped out through the aorta into systemic circulation, traveling
through arteries, arterioles, and capillaries—where nutrients and other substances are
exchanged between blood vessels and cells, losing oxygen and gaining carbon dioxide
—before being returned to the heart through venules and veins.[9] The adult heart beats
at a resting rate close to 72 beats per minute.[10] Exercise temporarily increases the rate,
but lowers it in the long term, and is good for heart health.[11]

Cardiovascular diseases are the most common cause of death globally as of 2008,
accounting for 30% of all human deaths.[12][13] Of these more than three-quarters are a
result of coronary artery disease and stroke.[12] Risk factors include: smoking,
being overweight, little exercise, high cholesterol, high blood pressure, and poorly
controlled diabetes, among others.[14] Cardiovascular diseases do not frequently have
symptoms but may cause chest pain or shortness of breath. Diagnosis of heart disease
is often done by the taking of a medical history, listening to the heart-sounds with
a stethoscope, as well as with ECG, and echocardiogram which uses ultrasound.
[4]
Specialists who focus on diseases of the heart are called cardiologists, although many
specialties of medicine may be involved in treatment.[13]

A teenager's heartbeat

Duration: 32 seconds.0:32

Sounds of a healthy 16-year-old child's heart beating normally, as heard with a stethoscope.

Problems playing this file? See media help.

Structure
 Human heart during an autopsy

Computer-generated animation of a beating human

heartCardiology video
See also: Anatomy of the human heart

Location and shape

Duration: 16 seconds.0:16Subtitles available.CCReal-time MRI of the human heart

The human heart is in the middle of the thorax, with its apex
pointing to the left. [15]

The human heart is situated in the mediastinum, at the level of thoracic vertebrae T5–
T8. A double-membraned sac called the pericardium surrounds the heart and attaches
to the mediastinum.[16] The back surface of the heart lies near the vertebral column, and
the front surface, known as the sternocostal surface, sits behind the sternum and rib
cartilages.[8] The upper part of the heart is the attachment point for several large blood
vessels—the venae cavae, aorta and pulmonary trunk. The upper part of the heart is
located at the level of the third costal cartilage.[8] The lower tip of the heart, the apex, lies
to the left of the sternum (8 to 9 cm from the midsternal line) between the junction of the
fourth and fifth ribs near their articulation with the costal cartilages.[8]

The largest part of the heart is usually slightly offset to the left side of the chest
(levocardia). In a rare congenital disorder (dextrocardia) the heart is offset to the right
side and is felt to be on the left because the left heart is stronger and larger, since it
pumps to all body parts. Because the heart is between the lungs, the left lung is smaller
than the right lung and has a cardiac notch in its border to accommodate the heart.
[8]
The heart is cone-shaped, with its base positioned upwards and tapering down to the
apex.[8] An adult heart has a mass of 250–350 grams (9–12 oz).[17] The heart is often
described as the size of a fist: 12 cm (5 in) in length, 8 cm (3.5 in) wide, and 6 cm
(2.5 in) in thickness,[8] although this description is disputed, as the heart is likely to be
slightly larger.[18] Well-trained athletes can have much larger hearts due to the effects of
exercise on the heart muscle, similar to the response of skeletal muscle.[8]

Chambers

Heart being dissected showing right and left ventricles,

from above

The heart has four chambers, two upper atria, the receiving chambers, and two
lower ventricles, the discharging chambers. The atria open into the ventricles via
the atrioventricular valves, present in the atrioventricular septum. This distinction is
visible also on the surface of the heart as the coronary sulcus.[19] There is an ear-shaped
structure in the upper right atrium called the right atrial appendage, or auricle, and
another in the upper left atrium, the left atrial appendage.[20] The right atrium and the
right ventricle together are sometimes referred to as the right heart. Similarly, the left
atrium and the left ventricle together are sometimes referred to as the left heart.[7] The
ventricles are separated from each other by the interventricular septum, visible on the
surface of the heart as the anterior longitudinal sulcus and the posterior interventricular
sulcus.[19]

The fibrous cardiac skeleton gives structure to the heart. It forms the atrioventricular
septum, which separates the atria from the ventricles, and the fibrous rings, which serve
as bases for the four heart valves.[21] The cardiac skeleton also provides an important
boundary in the heart's electrical conduction system since collagen cannot
conduct electricity. The interatrial septum separates the atria, and the interventricular
septum separates the ventricles.[8] The interventricular septum is much thicker than the
interatrial septum since the ventricles need to generate greater pressure when they
contract.[8]

Valves
Main article: Heart valves

With the atria and major vessels removed, all four valves are clearly visible. [8]

The heart, showing valves, arteries and veins. The white arrows show the normal direction of blood
flow.

Frontal section showing papillary muscles attached to

the tricuspid valve on the right and to the mitral valve on the left via chordae tendineae.[8]
The heart has four valves, which separate its chambers. One valve lies between each
atrium and ventricle, and one valve rests at the exit of each ventricle.[8]
The valves between the atria and ventricles are called the atrioventricular valves.
Between the right atrium and the right ventricle is the tricuspid valve. The tricuspid valve
has three cusps,[22] which connect to chordae tendinae and three papillary
muscles named the anterior, posterior, and septal muscles, after their relative positions.
[22]
The mitral valve lies between the left atrium and left ventricle. It is also known as the
bicuspid valve due to its having two cusps, an anterior and a posterior cusp. These
cusps are also attached via chordae tendinae to two papillary muscles projecting from
the ventricular wall.[23]

The papillary muscles extend from the walls of the heart to valves by cartilaginous
connections called chordae tendinae. These muscles prevent the valves from falling too
far back when they close.[24] During the relaxation phase of the cardiac cycle, the
papillary muscles are also relaxed and the tension on the chordae tendineae is slight.
As the heart chambers contract, so do the papillary muscles. This creates tension on
the chordae tendineae, helping to hold the cusps of the atrioventricular valves in place
and preventing them from being blown back into the atria.[8][g][22]

Two additional semilunar valves sit at the exit of each of the ventricles. The pulmonary
valve is located at the base of the pulmonary artery. This has three cusps which are not
attached to any papillary muscles. When the ventricle relaxes blood flows back into the
ventricle from the artery and this flow of blood fills the pocket-like valve, pressing
against the cusps which close to seal the valve. The semilunar aortic valve is at the
base of the aorta and also is not attached to papillary muscles. This too has three cusps
which close with the pressure of the blood flowing back from the aorta.[8]

Right heart
The right heart consists of two chambers, the right atrium and the right ventricle,
separated by a valve, the tricuspid valve.[8]

The right atrium receives blood almost continuously from the body's two major veins,
the superior and inferior venae cavae. A small amount of blood from the coronary
circulation also drains into the right atrium via the coronary sinus, which is immediately
above and to the middle of the opening of the inferior vena cava.[8] In the wall of the right
atrium is an oval-shaped depression known as the fossa ovalis, which is a remnant of
an opening in the fetal heart known as the foramen ovale.[8] Most of the internal surface
of the right atrium is smooth, the depression of the fossa ovalis is medial, and the
anterior surface has prominent ridges of pectinate muscles, which are also present in
the right atrial appendage.[8]

The right atrium is connected to the right ventricle by the tricuspid valve.[8] The walls of
the right ventricle are lined with trabeculae carneae, ridges of cardiac muscle covered
by endocardium. In addition to these muscular ridges, a band of cardiac muscle, also
covered by endocardium, known as the moderator band reinforces the thin walls of the
right ventricle and plays a crucial role in cardiac conduction. It arises from the lower part
of the interventricular septum and crosses the interior space of the right ventricle to
connect with the inferior papillary muscle.[8] The right ventricle tapers into the pulmonary
trunk, into which it ejects blood when contracting. The pulmonary trunk branches into
the left and right pulmonary arteries that carry the blood to each lung. The pulmonary
valve lies between the right heart and the pulmonary trunk.[8]

Left heart
The left heart has two chambers: the left atrium and the left ventricle, separated by
the mitral valve.[8]

The left atrium receives oxygenated blood back from the lungs via one of the
four pulmonary veins. The left atrium has an outpouching called the left atrial
appendage. Like the right atrium, the left atrium is lined by pectinate muscles.[25] The left
atrium is connected to the left ventricle by the mitral valve.[8]

The left ventricle is much thicker as compared with the right, due to the greater force
needed to pump blood to the entire body. Like the right ventricle, the left also
has trabeculae carneae, but there is no moderator band. The left ventricle pumps blood
to the body through the aortic valve and into the aorta. Two small openings above the
aortic valve carry blood to the heart muscle; the left coronary artery is above the left
cusp of the valve, and the right coronary artery is above the right cusp.[8]

Wall
Further information: Cardiac muscle

Layers of the heart wall, including visceral and parietal

pericardium

The heart wall is made up of three layers: the inner endocardium,

middle myocardium and outer epicardium. These are surrounded by a double-
membraned sac called the pericardium.

The innermost layer of the heart is called the endocardium. It is made up of a lining
of simple squamous epithelium and covers heart chambers and valves. It is continuous
with the endothelium of the veins and arteries of the heart, and is joined to the
myocardium with a thin layer of connective tissue.[8] The endocardium, by
secreting endothelins, may also play a role in regulating the contraction of the
myocardium.[8]
 The swirling pattern of myocardium helps the heart pump
effectively

The middle layer of the heart wall is the myocardium, which is the cardiac muscle—a
layer of involuntary striated muscle tissue surrounded by a framework of collagen. The
cardiac muscle pattern is elegant and complex, as the muscle cells swirl and spiral
around the chambers of the heart, with the outer muscles forming a figure 8 pattern
around the atria and around the bases of the great vessels and the inner muscles,
forming a figure 8 around the two ventricles and proceeding toward the apex. This
complex swirling pattern allows the heart to pump blood more effectively.[8]

There are two types of cells in cardiac muscle: muscle cells which have the ability to
contract easily, and pacemaker cells of the conducting system. The muscle cells make
up the bulk (99%) of cells in the atria and ventricles. These contractile cells are
connected by intercalated discs which allow a rapid response to impulses of action
potential from the pacemaker cells. The intercalated discs allow the cells to act as
a syncytium and enable the contractions that pump blood through the heart and into
the major arteries.[8] The pacemaker cells make up 1% of cells and form the conduction
system of the heart. They are generally much smaller than the contractile cells and have
few myofibrils which gives them limited contractibility. Their function is similar in many
respects to neurons.[8] Cardiac muscle tissue has autorhythmicity, the unique ability to
initiate a cardiac action potential at a fixed rate—spreading the impulse rapidly from cell
to cell to trigger the contraction of the entire heart.[8]

There are specific proteins expressed in cardiac muscle cells.[26][27] These are mostly
associated with muscle contraction, and bind with actin, myosin, tropomyosin,
and troponin. They include MYH6, ACTC1, TNNI3, CDH2 and PKP2. Other proteins
expressed are MYH7 and LDB3 that are also expressed in skeletal muscle.[28]

Pericardium
Main article: Pericardium

The pericardium is the sac that surrounds the heart. The tough outer surface of the
pericardium is called the fibrous membrane. This is lined by a double inner membrane
called the serous membrane that produces pericardial fluid to lubricate the surface of
the heart.[29] The part of the serous membrane attached to the fibrous membrane is
called the parietal pericardium, while the part of the serous membrane attached to the
heart is known as the visceral pericardium. The pericardium is present in order to
lubricate its movement against other structures within the chest, to keep the heart's
position stabilised within the chest, and to protect the heart from infection. [30]

Coronary circulation

Arterial supply to the heart (red), with other areas labelled

(blue).
Main article: Coronary circulation

Heart tissue, like all cells in the body, needs to be supplied with oxygen, nutrients and a
way of removing metabolic wastes. This is achieved by the coronary circulation, which
includes arteries, veins, and lymphatic vessels. Blood flow through the coronary vessels
occurs in peaks and troughs relating to the heart muscle's relaxation or contraction.[8]

Heart tissue receives blood from two arteries which arise just above the aortic valve.
These are the left main coronary artery and the right coronary artery. The left main
coronary artery splits shortly after leaving the aorta into two vessels, the left anterior
descending and the left circumflex artery. The left anterior descending artery supplies
heart tissue and the front, outer side, and septum of the left ventricle. It does this by
branching into smaller arteries—diagonal and septal branches. The left circumflex
supplies the back and underneath of the left ventricle. The right coronary artery supplies
the right atrium, right ventricle, and lower posterior sections of the left ventricle. The
right coronary artery also supplies blood to the atrioventricular node (in about 90% of
people) and the sinoatrial node (in about 60% of people). The right coronary artery runs
in a groove at the back of the heart and the left anterior descending artery runs in a
groove at the front. There is significant variation between people in the anatomy of the
arteries that supply the heart.[31] The arteries divide at their furthest reaches into smaller
branches that join at the edges of each arterial distribution.[8]

The coronary sinus is a large vein that drains into the right atrium, and receives most of
the venous drainage of the heart. It receives blood from the great cardiac vein (receiving
the left atrium and both ventricles), the posterior cardiac vein (draining the back of the
left ventricle), the middle cardiac vein (draining the bottom of the left and right
ventricles), and small cardiac veins.[32] The anterior cardiac veins drain the front of the
right ventricle and drain directly into the right atrium.[8]

Small lymphatic networks called plexuses exist beneath each of the three layers of the
heart. These networks collect into a main left and a main right trunk, which travel up the
groove between the ventricles that exists on the heart's surface, receiving smaller
vessels as they travel up. These vessels then travel into the atrioventricular groove, and
receive a third vessel which drains the section of the left ventricle sitting on the
diaphragm. The left vessel joins with this third vessel, and travels along the pulmonary
artery and left atrium, ending in the inferior tracheobronchial node. The right vessel
travels along the right atrium and the part of the right ventricle sitting on the diaphragm.
It usually then travels in front of the ascending aorta and then ends in a brachiocephalic
node.[33]

Nerve supply

Autonomic innervation of the heart

The heart receives nerve signals from the vagus nerve and from nerves arising from
the sympathetic trunk. These nerves act to influence, but not control, the heart
rate. Sympathetic nerves also influence the force of heart contraction.[34] Signals that
travel along these nerves arise from two paired cardiovascular centres in the medulla
oblongata. The vagus nerve of the parasympathetic nervous system acts to decrease
the heart rate, and nerves from the sympathetic trunk act to increase the heart rate.
[8]
These nerves form a network of nerves that lies over the heart called the cardiac
plexus.[8][33]

The vagus nerve is a long, wandering nerve that emerges from the brainstem and
provides parasympathetic stimulation to a large number of organs in the thorax and
abdomen, including the heart.[35] The nerves from the sympathetic trunk emerge through
the T1–T4 thoracic ganglia and travel to both the sinoatrial and atrioventricular nodes,
as well as to the atria and ventricles. The ventricles are more richly innervated by
sympathetic fibers than parasympathetic fibers. Sympathetic stimulation causes the
release of the neurotransmitter norepinephrine (also known as noradrenaline) at
the neuromuscular junction of the cardiac nerves[citation needed]. This shortens the
repolarisation period, thus speeding the rate of depolarisation and contraction, which
results in an increased heart rate. It opens chemical or ligand-gated sodium and calcium
ion channels, allowing an influx of positively charged ions.[8] Norepinephrine binds to
the beta–1 receptor.[8]

Development
Main articles: Heart development and Human embryogenesis

Development of the
human heart during the first eight weeks (top) and the formation of the heart chambers (bottom).
In this figure, the blue and red colors represent blood inflow and outflow (not venous and arterial
blood). Initially, all venous blood flows from the tail/atria to the ventricles/head, a very different
pattern from that of an adult.[8]

The heart is the first functional organ to develop and starts to beat and pump blood at
about three weeks into embryogenesis. This early start is crucial for subsequent
embryonic and prenatal development.

The heart derives from splanchnopleuric mesenchyme in the neural plate which forms
the cardiogenic region. Two endocardial tubes form here that fuse to form a primitive
heart tube known as the tubular heart.[36] Between the third and fourth week, the heart
tube lengthens, and begins to fold to form an S-shape within the pericardium. This
places the chambers and major vessels into the correct alignment for the developed
heart. Further development will include the formation of the septa and the valves and
the remodeling of the heart chambers. By the end of the fifth week, the septa are
complete, and by the ninth week, the heart valves are complete.[8]

Before the fifth week, there is an opening in the fetal heart known as the foramen ovale.
The foramen ovale allowed blood in the fetal heart to pass directly from the right atrium
to the left atrium, allowing some blood to bypass the lungs. Within seconds after birth, a
flap of tissue known as the septum primum that previously acted as a valve closes the
foramen ovale and establishes the typical cardiac circulation pattern. A depression in
the surface of the right atrium remains where the foramen ovale was, called the fossa
ovalis.[8]

The embryonic heart begins beating at around 22 days after conception (5 weeks after
the last normal menstrual period, LMP). It starts to beat at a rate near to the mother's
which is about 75–80 beats per minute (bpm). The embryonic heart rate then
accelerates and reaches a peak rate of 165–185 bpm early in the early 7th week (early
9th week after the LMP).[37][38] After 9 weeks (start of the fetal stage) it starts to
decelerate, slowing to around 145 (±25) bpm at birth. There is no difference in female
and male heart rates before birth.[39]

Physiology
Main article: Cardiac physiology

Blood flow

Blood flow through the valvesBlood flow through the

heartVideo explanation of blood flow through the heart

The heart functions as a pump in the circulatory system to provide a continuous flow of
blood throughout the body. This circulation consists of the systemic circulation to and
from the body and the pulmonary circulation to and from the lungs. Blood in the
pulmonary circulation exchanges carbon dioxide for oxygen in the lungs through the
process of respiration. The systemic circulation then transports oxygen to the body and
returns carbon dioxide and relatively deoxygenated blood to the heart for transfer to the
lungs.[8]

The right heart collects deoxygenated blood from two large veins,
the superior and inferior venae cavae. Blood collects in the right and left atrium
continuously.[8] The superior vena cava drains blood from above the diaphragm and
empties into the upper back part of the right atrium. The inferior vena cava drains the
blood from below the diaphragm and empties into the back part of the atrium below the
opening for the superior vena cava. Immediately above and to the middle of the opening
of the inferior vena cava is the opening of the thin-walled coronary sinus.[8] Additionally,
the coronary sinus returns deoxygenated blood from the myocardium to the right atrium.
The blood collects in the right atrium. When the right atrium contracts, the blood is
pumped through the tricuspid valve into the right ventricle. As the right ventricle
contracts, the tricuspid valve closes and the blood is pumped into the pulmonary trunk
through the pulmonary valve. The pulmonary trunk divides into pulmonary arteries and
progressively smaller arteries throughout the lungs, until it reaches capillaries. As these
pass by alveoli carbon dioxide is exchanged for oxygen. This happens through the
passive process of diffusion.

In the left heart, oxygenated blood is returned to the left atrium via the pulmonary veins.
It is then pumped into the left ventricle through the mitral valve and into the aorta
through the aortic valve for systemic circulation. The aorta is a large artery that
branches into many smaller arteries, arterioles, and ultimately capillaries. In the
capillaries, oxygen and nutrients from blood are supplied to body cells for metabolism,
and exchanged for carbon dioxide and waste products.[8] Capillary blood, now
deoxygenated, travels into venules and veins that ultimately collect in the superior and
inferior vena cavae, and into the right heart.

Cardiac cycle
Main articles: Cardiac cycle, Systole, and Diastole

The cardiac cycle as correlated to the ECG

The cardiac cycle is the sequence of events in which the heart contracts and relaxes
with every heartbeat.[10] The period of time during which the ventricles contract, forcing
blood out into the aorta and main pulmonary artery, is known as systole, while the
period during which the ventricles relax and refill with blood is known as diastole. The
atria and ventricles work in concert, so in systole when the ventricles are contracting,
the atria are relaxed and collecting blood. When the ventricles are relaxed in diastole,
the atria contract to pump blood to the ventricles. This coordination ensures blood is
pumped efficiently to the body.[8]

At the beginning of the cardiac cycle, the ventricles are relaxing. As they do so, they are
filled by blood passing through the open mitral and tricuspid valves. After the ventricles
have completed most of their filling, the atria contract, forcing further blood into the
ventricles and priming the pump. Next, the ventricles start to contract. As the pressure
rises within the cavities of the ventricles, the mitral and tricuspid valves are forced shut.
As the pressure within the ventricles rises further, exceeding the pressure with the aorta
and pulmonary arteries, the aortic and pulmonary valves open. Blood is ejected from the
heart, causing the pressure within the ventricles to fall. Simultaneously, the atria refill as
blood flows into the right atrium through the superior and inferior vena cavae, and into
the left atrium through the pulmonary veins. Finally, when the pressure within the
ventricles falls below the pressure within the aorta and pulmonary arteries, the aortic
and pulmonary valves close. The ventricles start to relax, the mitral and tricuspid valves
open, and the cycle begins again.[10]

Cardiac output
Main article: Cardiac output

The x-axis reflects time with a recording of the heart

sounds. The y-axis represents pressure.[8]

Cardiac output (CO) is a measurement of the amount of blood pumped by each

ventricle (stroke volume) in one minute. This is calculated by multiplying the stroke
volume (SV) by the beats per minute of the heart rate (HR). So that: CO = SV x HR.
[8]
The cardiac output is normalized to body size through body surface area and is called
the cardiac index.

The average cardiac output, using an average stroke volume of about 70mL, is 5.25
L/min, with a normal range of 4.0–8.0 L/min.[8] The stroke volume is normally measured
using an echocardiogram and can be influenced by the size of the heart, physical and
mental condition of the individual, sex, contractility, duration of
contraction, preload and afterload.[8]

Preload refers to the filling pressure of the atria at the end of diastole, when the
ventricles are at their fullest. A main factor is how long it takes the ventricles to fill: if the
ventricles contract more frequently, then there is less time to fill and the preload will be
less.[8] Preload can also be affected by a person's blood volume. The force of each
contraction of the heart muscle is proportional to the preload, described as the Frank-
Starling mechanism. This states that the force of contraction is directly proportional to
the initial length of muscle fiber, meaning a ventricle will contract more forcefully, the
more it is stretched.[8][40]

Afterload, or how much pressure the heart must generate to eject blood at systole, is
influenced by vascular resistance. It can be influenced by narrowing of the heart valves
(stenosis) or contraction or relaxation of the peripheral blood vessels.[8]

The strength of heart muscle contractions controls the stroke volume. This can be
influenced positively or negatively by agents termed inotropes.[41] These agents can be a
result of changes within the body, or be given as drugs as part of treatment for a
medical disorder, or as a form of life support, particularly in intensive care units.
Inotropes that increase the force of contraction are "positive" inotropes, and
include sympathetic agents such as adrenaline, noradrenaline and dopamine.
[42]
"Negative" inotropes decrease the force of contraction and include calcium channel
blockers.[41]

Electrical conduction
Main article: Electrical conduction system of the heart

Transmission of a cardiac action potential through the

heart's conduction system
The normal rhythmical heart beat, called sinus rhythm, is established by the heart's own
pacemaker, the sinoatrial node (also known as the sinus node or the SA node). Here an
electrical signal is created that travels through the heart, causing the heart muscle to
contract. The sinoatrial node is found in the upper part of the right atrium near to the
junction with the superior vena cava.[43] The electrical signal generated by the sinoatrial
node travels through the right atrium in a radial way that is not completely understood. It
travels to the left atrium via Bachmann's bundle, such that the muscles of the left and
right atria contract together.[44][45][46] The signal then travels to the atrioventricular node.
This is found at the bottom of the right atrium in the atrioventricular septum, the
boundary between the right atrium and the left ventricle. The septum is part of
the cardiac skeleton, tissue within the heart that the electrical signal cannot pass
through, which forces the signal to pass through the atrioventricular node only. [8] The
signal then travels along the bundle of His to left and right bundle branches through to
the ventricles of the heart. In the ventricles the signal is carried by specialized tissue
called the Purkinje fibers which then transmit the electric charge to the heart muscle.[47]

Conduction system of the heart

Heart rate
Main article: Heart rate

A racing heartbeat

Duration: 1 minute and 28 seconds.1:28

Heart sounds of a 16 year old girl immediately after running, with a heart rate of 186 BPM.

Problems playing this file? See media help.

The prepotential is due to a slow influx of sodium ions

until the threshold is reached followed by a rapid depolarisation and repolarisation. The
prepotential accounts for the membrane reaching threshold and initiates the spontaneous
depolarisation and contraction of the cell; there is no resting potential.[8]
The normal resting heart rate is called the sinus rhythm, created and sustained by
the sinoatrial node, a group of pacemaking cells found in the wall of the right atrium.
Cells in the sinoatrial node do this by creating an action potential. The cardiac action
potential is created by the movement of specific electrolytes into and out of the
pacemaker cells. The action potential then spreads to nearby cells.[48]

When the sinoatrial cells are resting, they have a negative charge on their membranes.
A rapid influx of sodium ions causes the membrane's charge to become positive; this is
called depolarisation and occurs spontaneously.[8] Once the cell has a sufficiently high
charge, the sodium channels close and calcium ions then begin to enter the cell, shortly
after which potassium begins to leave it. All the ions travel through ion channels in the
membrane of the sinoatrial cells. The potassium and calcium start to move out of and
into the cell only once it has a sufficiently high charge, and so are called voltage-gated.
Shortly after this, the calcium channels close and potassium channels open, allowing
potassium to leave the cell. This causes the cell to have a negative resting charge and
is called repolarisation. When the membrane potential reaches approximately −60 mV,
the potassium channels close and the process may begin again.[8]

The ions move from areas where they are concentrated to where they are not. For this
reason sodium moves into the cell from outside, and potassium moves from within the
cell to outside the cell. Calcium also plays a critical role. Their influx through slow
channels means that the sinoatrial cells have a prolonged "plateau" phase when they
have a positive charge. A part of this is called the absolute refractory period. Calcium
ions also combine with the regulatory protein troponin C in the troponin complex to
enable contraction of the cardiac muscle, and separate from the protein to allow
relaxation.[49]

The adult resting heart rate ranges from 60 to 100 bpm. The resting heart rate of
a newborn can be 129 beats per minute (bpm) and this gradually decreases until
maturity.[50] An athlete's heart rate can be lower than 60 bpm. During exercise the rate
can be 150 bpm with maximum rates reaching from 200 to 220 bpm.[8]

Influences
The normal sinus rhythm of the heart, giving the resting heart rate, is influenced by a
number of factors. The cardiovascular centres in the brainstem control the sympathetic
and parasympathetic influences to the heart through the vagus nerve and sympathetic
trunk.[51] These cardiovascular centres receive input from a series of receptors
including baroreceptors, sensing the stretching of blood vessels and chemoreceptors,
sensing the amount of oxygen and carbon dioxide in the blood and its pH. Through a
series of reflexes these help regulate and sustain blood flow.[8]

Baroreceptors are stretch receptors located in the aortic sinus, carotid bodies, the
venae cavae, and other locations, including pulmonary vessels and the right side of the
heart itself. Baroreceptors fire at a rate determined by how much they are stretched,
[52]
which is influenced by blood pressure, level of physical activity, and the relative
distribution of blood. With increased pressure and stretch, the rate of baroreceptor firing
increases, and the cardiac centers decrease sympathetic stimulation and increase
parasympathetic stimulation. As pressure and stretch decrease, the rate of baroreceptor
firing decreases, and the cardiac centers increase sympathetic stimulation and
decrease parasympathetic stimulation.[8] There is a similar reflex, called the atrial reflex
or Bainbridge reflex, associated with varying rates of blood flow to the atria. Increased
venous return stretches the walls of the atria where specialized baroreceptors are
located. However, as the atrial baroreceptors increase their rate of firing and as they
stretch due to the increased blood pressure, the cardiac center responds by increasing
sympathetic stimulation and inhibiting parasympathetic stimulation to increase heart
rate. The opposite is also true.[8] Chemoreceptors present in the carotid body or adjacent
to the aorta in an aortic body respond to the blood's oxygen, carbon dioxide levels. Low
oxygen or high carbon dioxide will stimulate firing of the receptors.[53]

Exercise and fitness levels, age, body temperature, basal metabolic rate, and even a
person's emotional state can all affect the heart rate. High levels of the
hormones epinephrine, norepinephrine, and thyroid hormones can increase the heart
rate. The levels of electrolytes including calcium, potassium, and sodium can also
influence the speed and regularity of the heart rate; low blood oxygen, low blood
pressure and dehydration may increase it.[8]

Clinical significance
Diseases

The stethoscope is used for auscultation of the heart, and is one of the most iconic symbols
for medicine. A number of diseases can be detected primarily by listening for heart murmurs.
 Atherosclerosis is a condition affecting the circulatory system. If the coronary arteries are
affected, angina pectoris may result or at worse a heart attack.

Cardiovascular diseases, which include diseases of the heart, are the leading cause of
death worldwide.[54] The majority of cardiovascular disease is noncommunicable and
related to lifestyle and other factors, becoming more prevalent with ageing. [54] Heart
disease is a major cause of death, accounting for an average of 30% of all deaths in
2008, globally.[12] This rate varies from a lower 28% to a high 40% in high-income
countries.[13] Doctors that specialise in the heart are called cardiologists. Many other
medical professionals are involved in treating diseases of the heart,
including doctors, cardiothoracic surgeons, intensivists, and allied health
practitioners including physiotherapists and dieticians.[55]

Ischemic heart disease

Main article: Coronary artery disease

Coronary artery disease, also known as ischemic heart disease, is caused

by atherosclerosis—a build-up of fatty material along the inner walls of the arteries.
These fatty deposits known as atherosclerotic plaques narrow the coronary arteries,
and if severe may reduce blood flow to the heart.[56] If a narrowing (or stenosis) is
relatively minor then the patient may not experience any symptoms. Severe narrowings
may cause chest pain (angina) or breathlessness during exercise or even at rest. The
thin covering of an atherosclerotic plaque can rupture, exposing the fatty centre to the
circulating blood. In this case a clot or thrombus can form, blocking the artery, and
restricting blood flow to an area of heart muscle causing a myocardial infarction (a heart
attack) or unstable angina.[57] In the worst case this may cause cardiac arrest, a sudden
and utter loss of output from the heart.[58] Obesity, high blood pressure,
uncontrolled diabetes, smoking and high cholesterol can all increase the risk of
developing atherosclerosis and coronary artery disease.[54][56]

Heart failure
Main article: Heart failure

Heart failure is defined as a condition in which the heart is unable to pump enough
blood to meet the demands of the body.[59] Patients with heart failure may experience
breathlessness especially when lying flat, as well as ankle swelling, known as peripheral
oedema. Heart failure is the result of many diseases affecting the heart, but is most
commonly associated with ischemic heart disease, valvular heart disease, or high blood
pressure. Less common causes include various cardiomyopathies. Heart failure is
frequently associated with weakness of the heart muscle in the ventricles (systolic heart
failure), but can also be seen in patients with heart muscle that is strong but stiff
(diastolic heart failure). The condition may affect the left ventricle (causing
predominantly breathlessness), the right ventricle (causing predominantly swelling of
the legs and an elevated jugular venous pressure), or both ventricles. Patients with
heart failure are at higher risk of developing dangerous heart rhythm disturbances
or arrhythmias.[59]
Cardiomyopathies
Main article: Cardiomyopathy

Cardiomyopathies are diseases affecting the muscle of the heart. Some cause
abnormal thickening of the heart muscle (hypertrophic cardiomyopathy), some cause
the heart to abnormally expand and weaken (dilated cardiomyopathy), some cause the
heart muscle to become stiff and unable to fully relax between contractions (restrictive
cardiomyopathy) and some make the heart prone to abnormal heart rhythms
(arrhythmogenic cardiomyopathy). These conditions are often genetic and can
be inherited, but some such as dilated cardiomyopathy may be caused by damage from
toxins such as alcohol. Some cardiomyopathies such as hypertrophic cardiomopathy
are linked to a higher risk of sudden cardiac death, particularly in athletes.[8] Many
cardiomyopathies can lead to heart failure in the later stages of the disease.[59]

Valvular heart disease

Main article: Valvular heart disease

Mitral Valve Prolapse murmur

Duration: 12 seconds.0:12

Heart sounds of a 16 year old girl diagnosed with mitral valve prolapse and mitral regurgitation.
Auscultating her heart, a systolic murmur and click is heard. Recorded with the stethoscope over the
mitral valve.

Problems playing this file? See media help.

Healthy heart valves allow blood to flow easily in one direction, and prevent it from
flowing in the other direction. A diseased heart valve may have a narrow opening
(stenosis), that restricts the flow of blood in the forward direction. A valve may otherwise
be leaky, allowing blood to leak in the reverse direction (regurgitation). Valvular heart
disease may cause breathlessness, blackouts, or chest pain, but may be asymptomatic
and only detected on a routine examination by hearing abnormal heart sounds or
a heart murmur. In the developed world, valvular heart disease is most commonly
caused by degeneration secondary to old age, but may also be caused by infection of
the heart valves (endocarditis). In some parts of the world rheumatic heart disease is a
major cause of valvular heart disease, typically leading to mitral or aortic stenosis and
caused by the body's immune system reacting to a streptococcal throat infection.[60][61]

Cardiac arrhythmias
Main article: Arrhythmia

While in the healthy heart, waves of electrical impulses originate in the sinus
node before spreading to the rest of the atria, the atrioventricular node, and finally the
ventricles (referred to as a normal sinus rhythm), this normal rhythm can be disrupted.
Abnormal heart rhythms or arrhythmias may be asymptomatic or may cause
palpitations, blackouts, or breathlessness. Some types of arrhythmia such as atrial
fibrillation increase the long term risk of stroke.[62]

Some arrhythmias cause the heart to beat abnormally slowly, referred to as

a bradycardia or bradyarrhythmia. This may be caused by an abnormally slow sinus
node or damage within the cardiac conduction system (heart block).[63] In other
arrhythmias the heart may beat abnormally rapidly, referred to as a tachycardia or
tachyarrhythmia. These arrhythmias can take many forms and can originate from
different structures within the heart—some arise from the atria (e.g. atrial flutter), some
from the atrioventricular node (e.g. AV nodal re-entrant tachycardia) whilst others arise
from the ventricles (e.g. ventricular tachycardia). Some tachyarrhythmias are caused by
scarring within the heart (e.g. some forms of ventricular tachycardia), others by an
irritable focus (e.g. focal atrial tachycardia), while others are caused by additional
abnormal conduction tissue that has been present since birth (e.g. Wolff-Parkinson-
White syndrome). The most dangerous form of heart racing is ventricular fibrillation, in
which the ventricles quiver rather than contract, and which if untreated is rapidly fatal. [64]

Pericardial disease
The sac which surrounds the heart, called the pericardium, can become inflamed in a
condition known as pericarditis. This condition typically causes chest pain that may
spread to the back, and is often caused by a viral infection (glandular
fever, cytomegalovirus, or coxsackievirus). Fluid can build up within the pericardial sac,
referred to as a pericardial effusion. Pericardial effusions often occur secondary to
pericarditis, kidney failure, or tumours, and frequently do not cause any symptoms.
However, large effusions or effusions which accumulate rapidly can compress the heart
in a condition known as cardiac tamponade, causing breathlessness and potentially
fatal low blood pressure. Fluid can be removed from the pericardial space for diagnosis
or to relieve tamponade using a syringe in a procedure called pericardiocentesis.[65]

Congenital heart disease

Main article: Congenital heart defect

Some people are born with hearts that are abnormal and these abnormalities are known
as congenital heart defects. They may range from the relatively minor (e.g. patent
foramen ovale, arguably a variant of normal) to serious life-threatening abnormalities
(e.g. hypoplastic left heart syndrome). Common abnormalities include those that affect
the heart muscle that separates the two side of the heart (a "hole in the heart",
e.g. ventricular septal defect). Other defects include those affecting the heart valves
(e.g. congenital aortic stenosis), or the main blood vessels that lead from the heart
(e.g. coarctation of the aorta). More complex syndromes are seen that affect more than
one part of the heart (e.g. Tetralogy of Fallot).

Some congenital heart defects allow blood that is low in oxygen that would normally be
returned to the lungs to instead be pumped back to the rest of the body. These are
known as cyanotic congenital heart defects and are often more serious. Major
congenital heart defects are often picked up in childhood, shortly after birth, or even
before a child is born (e.g. transposition of the great arteries), causing breathlessness
and a lower rate of growth. More minor forms of congenital heart disease may remain
undetected for many years and only reveal themselves in adult life (e.g., atrial septal
defect).[66][67]

Channelopathies
Main article: Channelopathy

Channelopathies can be categorized based on the organ system they affect. In the
cardiovascular system, the electrical impulse required for each heart beat is provided by
the electrochemical gradient of each heart cell. Because the beating of the heart
depends on the proper movement of ions across the surface membrane, cardiac ion
channelopathies form a major group of heart diseases.[68][69] Cardiac ion channelopathies
may explain some of the cases of sudden death syndrome and sudden arrhythmic
death syndrome.[70] Long QT syndrome is the most common form of cardiac
channelopathy.

 Long QT syndrome (LQTS) – Mostly hereditary. On EKG can be observed as longer

corrected QT interval (QTc). Characterized by fainting, sudden, life-threatening heart rhythm
disturbances – Torsades de pointes type ventricular tachycardia, ventricular fibrillation and
risk of sudden cardiac death.[71]
 Short QT syndrome.
 Catecholaminergic polymorphic ventricular tachycardia (CPVT).[72]
 Progressive cardiac conduction defect (PCCD).[73]
 Early repolarisation syndrome (BER) – common in younger and active people, especially
men, because it is affected by higher testosterone levels, which cause increased potassium
currents, which further causes an elevation of the J-point on the EKG. In very rare cases, it
can lead to ventricular fibrillation and death.[74]
 Brugada syndrome – a genetic disorder characterized by an abnormal EKG and is one of
the most common causes of sudden cardiac death in young men.[75]
Diagnosis
Heart disease is diagnosed by the taking of a medical history, a cardiac examination,
and further investigations, including blood tests, echocardiograms, electrocardiograms,
and imaging. Other invasive procedures such as cardiac catheterisation can also play a
role.[76]

Examination
Main articles: Cardiac examination and Heart sounds

The cardiac examination includes inspection, feeling the chest with the hands
(palpation) and listening with a stethoscope (auscultation).[77][78] It involves assessment
of signs that may be visible on a person's hands (such as splinter haemorrhages), joints
and other areas. A person's pulse is taken, usually at the radial artery near the wrist, in
order to assess for the rhythm and strength of the pulse. The blood pressure is taken,
using either a manual or automatic sphygmomanometer or using a more invasive
measurement from within the artery. Any elevation of the jugular venous pulse is noted.
A person's chest is felt for any transmitted vibrations from the heart, and then listened to
with a stethoscope.

Heart sounds

3D echocardiogram showing the mitral valve (right),

tricuspid and mitral valves (top left) and aortic valve (top right).
The closure of the heart valves causes the heart sounds.

Normal heart sounds

Duration: 20 seconds.0:20

Normal heart sounds as heard with a stethoscope

Problems playing this file? See media help.

Typically, healthy hearts have only two audible heart sounds, called S1 and S2. The first
heart sound S1, is the sound created by the closing of the atrioventricular valves during
ventricular contraction and is normally described as "lub". The second heart sound, S2,
is the sound of the semilunar valves closing during ventricular diastole and is described
as "dub".[8] Each sound consists of two components, reflecting the slight difference in
time as the two valves close.[79] S2 may split into two distinct sounds, either as a result of
inspiration or different valvular or cardiac problems.[79] Additional heart sounds may also
be present and these give rise to gallop rhythms. A third heart sound, S3 usually
indicates an increase in ventricular blood volume. A fourth heart sound S4 is referred to
as an atrial gallop and is produced by the sound of blood being forced into a stiff
ventricle. The combined presence of S3 and S4 give a quadruple gallop.[8] Heart
murmurs are abnormal heart sounds which can be either related to disease or benign,
and there are several kinds.[80] There are normally two heart sounds, and abnormal heart
sounds can either be extra sounds, or "murmurs" related to the flow of blood between
the sounds. Murmurs are graded by volume, from 1 (the quietest), to 6 (the loudest),
and evaluated by their relationship to the heart sounds, position in the cardiac cycle,
and additional features such as their radiation to other sites, changes with a person's
position, the frequency of the sound as determined by the side of the stethoscope by
which they are heard, and site at which they are heard loudest.[80] Murmurs may be
caused by damaged heart valves or congenital heart disease such as ventricular septal
defects, or may be heard in normal hearts. A different type of sound, a pericardial
friction rub can be heard in cases of pericarditis where the inflamed membranes can rub
together.

Blood tests
Blood tests play an important role in the diagnosis and treatment of many
cardiovascular conditions.

Troponin is a sensitive biomarker for a heart with insufficient blood supply. It is released
4–6 hours after injury and usually peaks at about 12–24 hours.[42] Two tests of troponin
are often taken—one at the time of initial presentation and another within 3–6 hours,
[81]
with either a high level or a significant rise being diagnostic. A test for brain natriuretic
peptide (BNP) can be used to evaluate for the presence of heart failure, and rises when
there is increased demand on the left ventricle. These tests are
considered biomarkers because they are highly specific for cardiac disease.[82] Testing
for the MB form of creatine kinase provides information about the heart's blood supply,
but is used less frequently because it is less specific and sensitive.[83]

Other blood tests are often taken to help understand a person's general health and risk
factors that may contribute to heart disease. These often include a full blood
count investigating for anaemia, and basic metabolic panel that may reveal any
disturbances in electrolytes. A coagulation screen is often required to ensure that the
right level of anticoagulation is given. Fasting lipids and fasting blood glucose (or
an HbA1c level) are often ordered to evaluate a person's cholesterol and diabetes
status, respectively.[84]

Electrocardiogram
Main article: Electrocardiography

Cardiac cycle shown against ECG

Using surface electrodes on the body, it is possible to record the electrical activity of the
heart. This tracing of the electrical signal is the electrocardiogram (ECG) or (EKG). An
ECG is a bedside test and involves the placement of ten leads on the body. This
produces a "12 lead" ECG (three extra leads are calculated mathematically, and one
lead is electrically ground, or earthed).[85]

There are five prominent features on the ECG: the P wave (atrial depolarisation),
the QRS complex (ventricular depolarisation)[h] and the T wave (ventricular
repolarisation).[8] As the heart cells contract, they create a current that travels through
the heart. A downward deflection on the ECG implies cells are becoming more positive
in charge ("depolarising") in the direction of that lead, whereas an upward inflection
implies cells are becoming more negative ("repolarising") in the direction of the lead.
This depends on the position of the lead, so if a wave of depolarising moved from left to
right, a lead on the left would show a negative deflection, and a lead on the right would
show a positive deflection. The ECG is a useful tool in detecting rhythm
disturbances and in detecting insufficient blood supply to the heart.[85] Sometimes
abnormalities are suspected, but not immediately visible on the ECG. Testing when
exercising can be used to provoke an abnormality or an ECG can be worn for a longer
period such as a 24-hour Holter monitor if a suspected rhythm abnormality is not
present at the time of assessment.[85]

Imaging
Main article: Cardiac imaging

Several imaging methods can be used to assess the anatomy and function of the heart,
including ultrasound (echocardiography), angiography, CT, MRI, and PET, scans. An
echocardiogram is an ultrasound of the heart used to measure the heart's function,
assess for valve disease, and look for any abnormalities. Echocardiography can be
conducted by a probe on the chest (transthoracic), or by a probe in
the esophagus (transesophageal). A typical echocardiography report will include
information about the width of the valves noting any stenosis, whether there is any
backflow of blood (regurgitation) and information about the blood volumes at the end of
systole and diastole, including an ejection fraction, which describes how much blood is
ejected from the left and right ventricles after systole. Ejection fraction can then be
obtained by dividing the volume ejected by the heart (stroke volume) by the volume of
the filled heart (end-diastolic volume).[86] Echocardiograms can also be conducted under
circumstances when the body is more stressed, in order to examine for signs of lack of
blood supply. This cardiac stress test involves either direct exercise, or where this is not
possible, injection of a drug such as dobutamine.[78]

CT scans, chest X-rays and other forms of imaging can help evaluate the heart's size,
evaluate for signs of pulmonary oedema, and indicate whether there is fluid around the
heart. They are also useful for evaluating the aorta, the major blood vessel which leaves
the heart.[78]

Treatment
Diseases affecting the heart can be treated by a variety of methods including lifestyle
modification, drug treatment, and surgery.

Ischemic heart disease

Main articles: Coronary artery disease, Coronary artery bypass surgery, and Coronary stent

Narrowings of the coronary arteries (ischemic heart disease) are treated to relieve
symptoms of chest pain caused by a partially narrowed artery (angina pectoris), to
minimise heart muscle damage when an artery is completely occluded (myocardial
infarction), or to prevent a myocardial infarction from occurring. Medications to improve
angina symptoms include nitroglycerin, beta blockers, and calcium channel blockers,
while preventative treatments include antiplatelets such as aspirin and statins, lifestyle
measures such as stopping smoking and weight loss, and treatment of risk factors such
as high blood pressure and diabetes.[87]

In addition to using medications, narrowed heart arteries can be treated by expanding

the narrowings or redirecting the flow of blood to bypass an obstruction. This may be
performed using a percutaneous coronary intervention, during which narrowings can be
expanded by passing small balloon-tipped wires into the coronary arteries, inflating the
balloon to expand the narrowing, and sometimes leaving behind a metal scaffold known
as a stent to keep the artery open.[88]

If the narrowings in coronary arteries are unsuitable for treatment with a percutaneous
coronary intervention, open surgery may be required. A coronary artery bypass
graft can be performed, whereby a blood vessel from another part of the body
(the saphenous vein, radial artery, or internal mammary artery) is used to redirect blood
from a point before the narrowing (typically the aorta) to a point beyond the obstruction.
[88][89]

Valvular heart disease

Main article: Artificial heart valve

Diseased heart valves that have become abnormally narrow or abnormally leaky may
require surgery. This is traditionally performed as an open surgical procedure to replace
the damaged heart valve with a tissue or metallic prosthetic valve. In some
circumstances, the tricuspid or mitral valves can be repaired surgically, avoiding the
need for a valve replacement. Heart valves can also be treated percutaneously, using
techniques that share many similarities with percutaneous coronary
intervention. Transcatheter aortic valve replacement is increasingly used for patients
consider very high risk for open valve replacement.[60]

Cardiac arrhythmias
Main articles: Heart arrhythmia, Radiofrequency ablation, and Artificial cardiac pacemaker

Abnormal heart rhythms (arrhythmias) can be treated using antiarrhythmic drugs. These
may work by manipulating the flow of electrolytes across the cell membrane (such
as calcium channel blockers, sodium channel blockers, amiodarone, or digoxin), or
modify the autonomic nervous system's effect on the heart (beta blockers and atropine).
In some arrhythmias such as atrial fibrillation which increase the risk of stroke, this risk
can be reduced using anticoagulants such as warfarin or novel oral anticoagulants.[62]

If medications fail to control an arrhythmia, another treatment option may be catheter

ablation. In these procedures, wires are passed from a vein or artery in the leg to the
heart to find the abnormal area of tissue that is causing the arrhythmia. The abnormal
tissue can be intentionally damaged, or ablated, by heating or freezing to prevent further
heart rhythm disturbances. Whilst the majority of arrhythmias can be treated using
minimally invasive catheter techniques, some arrhythmias (particularly atrial fibrillation)
can also be treated using open or thoracoscopic surgery, either at the time of other
cardiac surgery or as a standalone procedure. A cardioversion, whereby an electric
shock is used to stun the heart out of an abnormal rhythm, may also be used.
Cardiac devices in the form of pacemakers or implantable defibrillators may also be
required to treat arrhythmias. Pacemakers, comprising a small battery powered
generator implanted under the skin and one or more leads that extend to the heart, are
most commonly used to treat abnormally slow heart rhythms.[63] Implantable defibrillators
are used to treat serious life-threatening rapid heart rhythms. These devices monitor the
heart, and if dangerous heart racing is detected can automatically deliver a shock to
restore the heart to a normal rhythm. Implantable defibrillators are most commonly used
in patients with heart failure, cardiomyopathies, or inherited arrhythmia syndromes.