CARDIAC CYCLE I (I, II)

By
Prof. Dr. Samina Malik
 Learning outcomes Ia
By the end of this interactive session,
students are expected to:

• Define cardiac cycle

• Relate phases of cardiac cycle with

pressure changes (in left and right
ventricles).
 Learning outcomes Ib
By the end of this interactive session,
students are expected to:
• Relate JVP (atrial pressure changes) with
cardiac cycle.
• Relate ECG with cardiac cycle
• Relate volume changes with cardiac cycle.
• Relate valvular changes with heart sounds
in cardiac cycle.
 Learning outcomes II
By the end of this interactive session,
students are expected to:
• Explain the functioning of heart working as
a pump
• Define preload & afterload
 Learning outcomes IIIa & IIIb
(Another PDF file)
By the end of this interactive session,
students are expected to:

• Define cardiac output and cardiac index

• Summarize factors affecting venous return

• Summarize factors affecting cardiac output.

 DEFINITION OF
CARDIAC CYCLE:

• Period between start of one beat to start of

next.
• It consists of one complete heart beat.
• It consists of one systole & one diastole.
INITIATION OF CARDIAC CYCLE:
• Initiated by Cardiac Impulse, which
originates from SA node.
 EVENTS THAT OCCUR IN THE
CARDIAC CHAMBERS
DURING CARDIAC CYCLE
• Pressure Changes.
• Volume Changes.
• Production of Heart Sounds (separate
lecture)
• Closure & Opening of Cardiac Valves.
• Electric Changes /ECG recording
(separate lectures)
Summary of phases of cardiac cycle
VENTRICULAR SYSTOLE 0.31 sec
(Peak of R wave of QRS
complex to the end of T wave)
ISO-VOLUMETRIC CONTRACTION 0.06 sec
MAXIMUM EJECTION (2/3) 0.11 sec
REDUCED EJECTION (1/3) 0.14 sec
VENTRICULAR DIASTOLE 0.52 sec
(End of T wave to the peak
of R wave of QRS complex)
PROTODIASTOLE 0.04 sec
ISO-VOLUMETRIC RELAXATION 0.06 sec

RAPID INFLOW 0.11 sec

SLOW INFLOW / DIASTASIS 0.2 sec
ATRIAL SYSTOLE (after P wave) 0.11 sec

8 Phases of CARDIAC CYCLE 0.8 sec

 PRESSURE CHANGES:
Heart has 4 chambers:

A) Pressure changes in left ventricle

during cardiac cycle.
B) Pressure changes in right ventricle
during cardiac cycle.
C) Pressure changes in atria.
Pressure changes in Left Ventricle
during cardiac cycle:

‘Phase 1’ of cardiac cycle / Iso-volumetric

contraction of ventricle:

At the start of ventricular systole  L.V is full of

blood (received from left atrium during previous
diastole).

Pressure in L.V at this stage = 1-3 mm Hg.

Now L.V begins to contract  I.V.P (Intra-
ventricular pressure) begins to rise  closure of
mitral valve / Left AV Valve  1st phase starts:
ISOMETRIC CONTRACTION PHASE OR
ISOVOLUMETRIC CONTRACTION PHASE.
 Pressure changes in Left Ventricle
during cardiac cycle:
• With closure of mitral valve  ventricle is a closed
chamber  no change in blood volume  ISO-
VOLUMETRIC, as both valves are closed  ISO-
METRIC CONTRACTION (no change in length of
muscle but rapid increase in I.V.P).

• When IVP rises just above 80 mm Hg  opening of

Aortic or Semi-lunar valve.

• Duration of I.V.C of Ventricle = 0.06 sec.

• With opening of Aortic valve, 2nd phase starts:

MAXIMAL EJECTION PHASE.
Pressure changes in Left Ventricle
during cardiac cycle:

‘Phase 2’ of cardiac cycle / Maximal Ejection Phase

(M.E.P) / Rapid Ejection Phase (R.E.P):

Ventricle muscle is contracting powerfully with opening of

Aortic valve.

Blood is ejected from ventricle (2/3 of stroke volume) 

Aorta (at maximum rate). 70% EMPTYING occurs in first
1/3 of ejection phase.

In this phase: I.V.P  maximum = 120 mm Hg.

Duration of M.E.P = 0.11 sec.

 Pressure changes in Left Ventricle
during cardiac cycle:
• ‘Phase 3’ of cardiac cycle / Reduced Ejection
Phase (R.E.P):

• Blood ejection (remaining 1/3 of stroke volume,

30% EMPTYING occurs in last 2/3 of ejection
phase) from L.V  Aorta, continues but at a
reduced rate.

• I.V.P  falls from maximum.

• This phase ends when I.V.P becomes equal to OR

slightly less than AORTIC PRESSURE.

• Duration of R.E.P = 0.14 sec.

 Duration of ventricular systole
(3 phases):
Isovolumetric contraction = 0.06 sec
Maximum Ejection Phase = 0.11 sec
Reduced Ejection Phase = 0.14 sec
Ventricular Systole = 0.31 sec
 Pressure changes in Left Ventricle
during cardiac cycle:
• ‘Phase 4’ of cardiac cycle / Protodiastole:
• A short phase = 0.04 sec.
• At the junction of systole & diastole, but included in
diastole.
• At this stage, I.V.P = Aortic Pressure or
I.V.P is slightly less than Aortic pressure, BUT
SMALL AMOUNT OF BLOOD CONTINUES TO
OOZE, because of momentum.

• In protodiastole: THIS MOMENTUM IS OVERCOME

due to further fall in I.V.P & there is some retrograde
flow of Aortic blood in 1st part of Aorta  closure of
Aortic valve  end of Protodiastole.
Pressure changes in Left Ventricle
during cardiac cycle:

• ‘Phase 5’ of cardiac cycle /

Isovolumetric Relaxation Phase
(I.V.R):
• Starts with closure of Aortic valve.
• Why it is called ISOVOLUMETRIC
RELAXATION?
 ANSWER
• Ventricle  relaxation without change in
volume, because: BOTH VALVES ARE
CLOSED  no change in blood volume 
Rapid fall in I.V.P because of relaxation.
When
(left ventricular pressure) < (left atrial pressure)
Opening of left AV valve / mitral valve  end
of I.V.R phase.
• Duration of I.V.R = 0.06 sec.
Pressure changes in Left Ventricle
during cardiac cycle:
• ‘Phase 6’ of cardiac cycle / Rapid Inflow
Phase (R.I.P) / Rapid filling phase (R.F.P):

• Starts with opening of mitral valve.

• Blood from Left Atrium  rapidly flows into Left

Ventricle.

• 2/3 of ventricular filling occurs in this phase

(during first 1/3 of ejection phase)

• Duration of R.I.P = 0.11 sec.

 Pressure changes in Left Ventricle
during cardiac cycle:
• ‘Phase 7’ of cardiac cycle / Slow Inflow Phase / Diastasis:

It appears that: No blood is flowing from Lt. Atrium  Lt.

Ventricle because:
• During last phase (R.I.P), most of blood  Lt. Vent.

• Mitral valve is open  Lt. atrium & Lt. Ventricle = common

chamber  whatever blood that returns in small amount from
pulmonary veins  Lt. Atrium  Lt. Ventricle (through open
valve)  so it appears that no blood is flowing.

• Only slight (1/3) filling of Lt. Ventricle in this phase.

• Duration of diastasis / Slow Inflow Phase = 0.2 sec.

• THE LONGEST PHASE OF CARDIAC CYCLE.
Pressure changes in Left Ventricle
during cardiac cycle:
• ‘Phase 8’ of cardiac cycle / Atrial Systole:

• Last phase of cardiac cycle

• Lt. Atrium contracts  pushes the blood from its cavity  Lt.
Ventricle  20% ventricular filling by atrial contraction.

• Atria contract towards the end of ventricular diastole.

• With atrial contraction, ventricular filling is complete.

• Duration: 0.11 sec.

• Mechanism: AV nodal delay. It allows atria to contract before

ventricles begin to contract, at the end of ventricular filling.
 Duration of ventricular diastole
(5 phases):
• Protodiastole = 0.04 sec
• Isovolumetric Relaxation = 0.06 sec
• Rapid Inflow Phase = 0.11 sec
• Slow Inflow Phase = 0.20 sec
• Atrial Systole = 0.11 sec
• Ventricular Diastole = 0.52 sec
 Duration of Cardiac Cycle
(8 phases):
= Duration of systole + diastole =
• (0.31) + (0.52) = [0.8 sec]
 Pressure Changes in
Right Ventricle:
• Same phases as for Lt. ventricle.

• Same duration as for Lt. ventricle.

• Only change in pressure levels & in names of

valves.

• Aortic valve is replaced by pulmonary valve.

• Mitral valve is replaced by Tricuspid valve.

 Pressure Changes in
Right Ventricle:
At the beginning of Rt. Vent. Systole:
• Pressure = 0-1 mm Hg.

• During I.V.C  Pressure increases on the right

side  just exceeds 8 mmHg  opening of
pulmonary valve (above 80 mmHg, there was
opening of Aortic valve).

• Maximum increase in pressure in Rt. Vent

systole = 25 mmHg (it was120 mm Hg in Lt.
vent.)
 Pressure Changes in
Right Ventricle:
• Right ventricle & Pulmonary artery is a low
pressure system.

Pulmonary artery pressure variation:

• 8 – 25 mm Hg

Aortic pressure variation:

• 80 – 120 mm Hg
 Duration of cardiac cycle &
heart rate:
• Duration of cardiac cycle = 0.8 sec at heart
rate = 70 beats / min.
• When heart rate increases  duration of
cardiac cycle decreases.
• Diastole is more affected as compared to
systole with rapid heart rate.
• At heart rate = 180 / min, cardiac cycle
duration = 0.33 sec: (systole = 0.18 sec,
diastole = 0.15 sec).
 QUESTION
• At a very rapid heart rate: Cardiac output
decreases, Why???
 ANSWER
• Because diastole becomes too short 
ventricular filling decreases  decrease
stroke volume & decrease in cardiac output,
in spite of increase in heart rate.
 Pressure changes in Atria during
the Cardiac Cycle:
• Atrial systole duration = 0.11 sec
• Atrial diastole duration = 0.7 sec
• Atrial systole + Atrial diastole = 0.8 sec = cardiac cycle.

• Atrial diastole > Atrial systole, because basic function of

atria is to receive blood from large veins & it can receive
blood only when it is relaxed.

3 waves can be recorded from atria which represent

atrial pressure changes:
• a-wave, c-wave & v-wave (Seen as Jugular Venous
Pulse, not a true pulse, but a reflection of pressure
changes in right atrium.
 Pressure changes in Atria during
the Cardiac Cycle:
 a-wave: Due to increase in atrial pressure
during atrial systole.
 Pressure changes in Atria during
the Cardiac Cycle:
 c-wave: Recorded at beginning of
contraction of ventricle. During
isovolumetric contraction, ventricular
pressure increases  Cusps of AV valves
are pushed into atrial cavity  pressure
rises in atria  ascent of c-wave.
 Pressure changes in Atria during
the Cardiac Cycle:
 The top of c-wave coincides with opening of
semi-lunar valves (Aortic & Pulmonary).

 With opening of semi-lunar valves, 2nd phase

starts, which is maximum ejection phase.

 It is later on followed by iso-volumetric relaxation

of ventricle  muscle length increases  now
AV valve is pulled to ventricular cavity  atrial
cavity increases  pressure falls in the atria 
descent of c-wave.
 Pressure changes in Atria during
the Cardiac Cycle:
• v-wave: Due to gradual increase in atrial
pressure, resulting from venous filling of
blood (from the venae cavae) into the
atria, with closed AV valves  ascent of v-
wave.

• Top of v-wave coincides with opening of

AV valves  rapid inflow phase 
decrease pressure in atria  descent of v-
wave.
 Right atrial pressure =
Central Venous Pressure.
• During most of cardiac cycle, this pressure
remains almost zero.
• During wave a, c & v  pressure rises.
Otherwise  remains almost zero.
• 4-6 mm Hg  Rt. Atrium (during a, c, v)
• 7-8 mm Hg  Lt. Atrium (during a, c, v)
 JUGULAR VENOUS PULSE:
(a, c, v waves)
• Normally arteriolar pulse ends in arterioles
& in veins  no pulsation.

• But we can record pulsation in jugular

vein, which is not a true pulse.

• It is just backward transmission of

pressure changes in Rt. Atrium (a, c. v
waves) transmitted in neck veins.
 Significance of J.V.P:
• ac interval coincides with PR interval of
ECG.

• ac interval increases in delayed AV

conduction.
 Significance of J.V.P:
• a waves are absent in: ATRIAL FIBRILLATION.

• (a wave) > (c wave) in COMPLETE AV BLOCK.

• ‘Giant a waves’ in TRICUSPID & PULMONARY

STENOSIS.

• Pulsating Neck Veins in CCF (Congestive

Cardiac Failure).
 Volume changes in Ventricles
during Cardiac Cycle:
Beginning of ventricular systole:

Ejection Phases: (Maximum Ejection Phase &

Reduced Ejection Phase)

Iso-volumic Relaxation Phase:

Rapid Inflow Phase:

Diastasis / Slow Inflow Phase:

Atrial Systole:
Beginning of ventricular systole:
• Ventricle is full of blood (collected during
previous diastole, EDV = 120 ml) at the onset of
ventricular systole.

• This much volume is there at the start of

ventricular systole.

• With this volume  onset of Isovolumic /

Isometric Contraction, with no change in blood
volume.
 Ejection Phases: (Maximum Ejection Phase &
Reduced Ejection Phase)

Maximum Ejection Phase:

• 2/3 of Stroke Volume (total = 70 ml) is ejected
out.
Reduced Ejection Phase:
• Remaining 1/3 is ejected out.

After ejection phases, the volume of blood left

behind is ESV = 50 ml.
 Diastolic phases
Iso-Volumic Relaxation Phase:
• No change in blood volume occurs.
Rapid Inflow Phase:
• 2/3 of ventricular filling.
Diastasis / Slow inflow phase:
• Only slight filling occurs.
Atrial Systole:
• Remaining 1/3 filling (30%). Now filling of
ventricles is complete & EDV of 120 ml is left.
Sequence of systole in chambers
of heart:

2
1 LA
RA
3
4 LV
RV 6 AORTA PRESSURE

5
PULMONARY
ARTERY
PRESSURE
 Sequence of systole in chambers
of heart:
• Right atrial systole begins earlier as compared
to left atrial systole.

• Left ventricular systole begins earlier as

compared to right ventricular systole.

• But blood ejection from right ventricle 

pulmonary artery, starts earlier as compared to
ejection from left ventricle  aorta, BECAUSE
pressure in pulmonary artery < Aortic pressure.
Closure & Opening of Heart Valves:
AV VALVES:
• Are closed at the beginning of Isovolumic contraction
Phase.

• Are open at the beginning of Rapid Inflow phase.

• AV valve closure is slow & soft & does not require

backward flow of blood.

• Cusps of AV valves are soft & thin because they are not
subjected to increase in pressure & rapid blood flow.
 Closure & Opening of
Heart Valves:
SEMILUNAR VALVES:
• Are closed at the beginning of Isovolumic
relaxation phase.

• Cusps of these valves are thick & heavier (as

they are subjected to increased pressure & rapid
blood flow).

• Their closure is rapid & requires backward flow

of blood (incisura in case of Aortic valve).
 Closure & Opening of
both AV & Semilunar Heart Valves:
• Forward pressure gradient  opening.
• Backward pressure gradient  closure.

• AV valves prevent, leakage of blood from

ventricle  atria, during ventricular systole
(when pressure rises in ventricle).

• Semilunar valves prevent leakage of blood from

large arteries  ventricles, during ventricular
diastole (when pressure falls in ventricle)
 Preload & Afterload
PRELOAD AFTERLOAD
• Degree of tension • Load against which
on muscle when it muscle exerts its
begins to contract contractile force

• End-diastolic • Systemic arterial

pressure pressure
• Venous return • Resistance in
circulation
• Significance ? • Significance ?