3/14/2020

The Cardiopulmonary
Bypass Circuit
Lec.BY.Noor Talib

NOOR Talib
AL ISRAA UNIVERSITY
 The Cardiopulmonary Bypass Circuit
(heart -lung machine)

Introduction :

It is one of the truly revolutionary machines of medical equipments has been

the invention and development of heart-lung machine.
During an open heart surgery such a bypass surgery the heart lung machine takes
over the functions of heart and lung and allows the surgeon to carefully stop the
heart while the rest of the patients body continues to receive oxygen-rich blood
The surgeon can perform the work on: heart without interference from bleeding or heart
pumping motion.
once the procedure is over, the surgeon starts the heart and disconnects the heart lung
machine.

The principle of the heart-lung machine

(also known as pump-oxygenator or cardiopulmonary bypass)
•
•
Deoxygenated blood withdrawn into the reservoir.
• From there, the blood is pumped through an artificial lung.
• This component is designed to expose the blood to oxygen.
•
As the blood passes through the artificial lung (also known as an oxygenator)
• the blood comes into intimate contact with the fine surfaces of the devoce itself
Oxygen gas is delivered to the interface between the blood and the device
Med. Inst. Eng. Dept. Fourth Stage Medical Inst. Lec.10

• permitting the blood cells to absorb oxygen molecules directly.

• The blood becomes oxygenated blood.
• Finally the heart-lung machine actively pumps the oxygen
the oxygenated blood back into the patient by the tube connected to the atria

Typically, blood is drained cannulas in the superior and inferior vena cavae
(SVC, IVC) or IVC and right atrium (RA) to the heart–lung machine where it is
pumped through the artificial lung back into the body via an arterial cannula placed
in the ascending aorta.

Modern Machines
It can more Tasks during an open heart surgery for the safe completion of the surgery
Firstly : blood which escapes the circulation and spills into the operating field around the
heart can be suctioned and returned to the pump
. Returning shed blood into the machine
Med. Inst. Eng. Dept. Fourth Stage Medical Inst. Lec.10

Secondly: the patient's body temperature can be controlled by selectively

cooling or heating the blood as it moves through the heart-lung machine.
Thus the
surgeon can use low body temperatures as a tool to preserve the function of the heart
and other vital organs during the period of artificial circulation.

And the bypass pump has connectors into which medications and anesthetic
drugs can be given

To initiate heart-lung bypass, the surgeon must first impair the body’s own
clotting system. Otherwise, the patient’s blood would immediately clot upon
exposure to the plastic tubing and artificial surfaces inside the heart-lung machine
itself. Thinning of the blood (or anticoagulation) is done by first administering a
powerful anticoagulant called heparin.

Once clotting is impaired, a large drainage tube is placed in the upper chamber of the heart
(called the right atrium). This tube drains the deoxygenated blood from the patient into
the heart-lung machine. Then a smaller tube is placed into the arterial system so can be
returned to the patient’s body where it is needed.
The most common site for this tube is in the aorta.
So the modern Heart-lung machine can perform the following tasks:
1- Oxygenation of blood
2- Controlling the patient body temperature.
3- Adding medication and anesthetic drugs to the blood through the machine
during the surgery.

In spite of these advantages, there are some substantial disadvantages in this

machine, some of these are:
1- Formation of blood clots which may lead to a stroke, heart attack or kidney
 failure.
2- This machine can also trigger inflammatory process that can damage many
body systems and organs.
3- In rare cases, memory loss may occur.

THE HEART LUNG MACHINE IS COMPOSED OF 3 MAIN PARTS

1- The Pumps: these pumps are required to circulate the blood between the body
and the machine.
2- Oxygenator: this part is required to oxygenate the blood.
3- Heat Exchanger: this part is necessary to regulate the temperature of the blood
and thus the temperature of the body.
 Design Aspects of Heart-Lung Machine
To serve the natural function of heart and lung, the machine should be able to
arterialize up to 5 liters per minute of venous blood from 65% oxygen saturation to
more than 95%, the carbon dioxide must be adequate. It should cause the least
trauma and damage to the blood particles.

Pumps design,
The pump must be designed to have flow rate up to 5 liters per
minute with a pressure head up to 200mm Hg. The roller occludes the tube and
displaces the fluid. The pump should be double roller that has a wide flow range
which is varied by varying the speed of rotation.
The pump rotation speed should be variable from 0-180rprn. The pump has
two heads and each head is driven by 1/8 hp single phase AC motor. Speed variation
can be accomplished by Voltage/frequency speed variation principle.
For viewing only

☺Complications associated with roller pumps include: malocclusion, miss calibration,

or miscalculation including setting the wrong tube size into the pump controller;
fracture of the pump tubing; loss of power; spallation; and pumping of large
amounts of air. If the outflow becomes occluded, pressure in the line will
progressively rise until the tubing in the pump ruptures or connectors and tubing
separate. This can be avoided by use of a pressure-regulated shunt between the
outflow and inflow lines of the roller pump or use of servoregulation of the pump to
arterial line pressure so that it turns off when excessive pressures are detected. If
inflow becomes limited, the roller pumps will develop high negative pressures
.producing cavitation, and microbubbles
Centrifugal pumps consist of a nest of smooth plastic cones or a vaned
impeller located inside a plastic housing. When rotated rapidly (2,000 to 3,000 rpm),
these pumps generate a pressure differential that causes the movement of fluid.
Unlike roller pumps, they are totally nonocclusive and afterload dependent (i.e., an
increase in downstream resistance or pressure decreases forward flow delivered to
the patient if no adjustment is made in the rpm). This has both favorable and
unfavorable consequences.☺

Flow is not determined by rotational rate alone, and

therefore a flowmeter must be incorporated in the outflow line to quantitate pump
flow. Furthermore, when the pump is connected to the patient's arterial system but is
not rotating, blood will flow backward through the pump and out of the patient
unless the CPB systemic line is clamped. This can cause exsanguination of the
patient or aspiration of air into the arterial line. Thus, whenever the centrifugal
pump is not running, the arterial line must be clamped, or a check valve is used to
prevent this problem.

if the arterial line becomes occluded, these pumps will not generate excessive pressure
the maximum is only about 700 to 900 mm Hg) and will not rupture the systemic flow
flow line. Likewise, they will not generate as much negative pressure.
and hence as much cavitation and microembolus production as a roller pump
because the maximum is only about –500 mm Hg if the inflow becomes occluded

A reputed advantage of centrifugal pumps over roller pumps is less risk of passing
massive air emboli into the arterial line. This is because they will become deprimed
and stop pumping if more than approximately 50 mL of air is introduced into the
circuit. However, they will pass smaller but still potentially lethal quantities of
smaller bubbles.
 Design of the Oxygenator
The oxygenator must be able to oxygenate up to 5 liters per minute from 65%
oxygen saturation to 95% before the blood enter the physiological system.
To provide such an environment, the disk oxygenator is used as shown in the
next figure. It consists :
• cylindrical glass vessel of 15cm internal diameter
• 38cm long in which 80-100 stainless steel disks of 0.6mm thickness and 14cm
diameter
• are mounted axially with 3mm spacers between the discs.
• The shaft on which the discs are mounted is hollow and supported by 3 ill bearings
at the end.
oxygen is fed through the axis of the shaft and it enters the oxygenator through the
distributing apertures on the circumstance of the shaft.

To oxygenate 5 liters per minute of

blood would require a rotating speed of about 150r.p.m. The oxygenation capacity
can be varied, by varying rotating speed or by changing the number of discs.
The blood level is a photo-electric level sensor. The sensing element is a
photo conducting cell which conducts depending on the light radiation it receives.
The light source is fixed on the top of the oxygenator shown in the next figure, and
the light reaching the photodiode depends on the blood level in the oxygenator. If
we know the signal corresponding to the blood level 0.7R, we can suitably adjust
the pump flow rate. Manual control is replaced by feedback control of the power
supply of the motor using the signal from the photo sensor.
 Heat Exchanger:
The heat exchanger is single pass, shell and tube heat exchanger with blood
flowing on the shell side and water flowing on the tube side. The priming volume of
heat exchanger is about 250 ml and blood flows as thin film in the space between
shell and tube. The temperature of blood stream and water stream are monitored.

The circuit of heart-lung bypass circuit is shown below. One of the pumps is
connected to the suction line and the other on the arterial line. Oxygen flow is
regulated in the range of 0-15 liters per minute with a rotameter and needle valve
flow controller. The arterial and venous pressures gauges are provided. The blood
from the heat exchanger is passed through a bubble trap before it is returned to the
patient. The role of physical factors such as oxygen saturation of incoming blood,
the blood distributing system in the oxygenator, oxygen carrying capacity of the
blood under given conditions, the blood flow rate, temperature of the oxygenation,
partial pressures of the oxygen etc.. All tell upon the performance of the oxygenat