BYPASS

PRINCIPLES OF CARDIOPULMONARY BYPASS
cardiopulmonary bypass (CPB) and cardioplegia provides a still bloodless heart by diverting blood into a
heart-lung machine (extracorpeal circuit) that performs the functions of respiration, circulation and
temperature regulation while the heart and lungs are not functioning CPB is required when
manipulation of the heart significantly compromises systemic blood pressure and when intracardiac
surgery is performed the cardiopulmonary circuit (see Figure 1)
venous blood drains by gravity from the right atrium (RA) or vena cavae into a reservoir, passes
through an oxygenator/heat exchanger attachedto a heating/cooling machine, and is returned to the
arterial system through a filter using either a roller or centrifugal pump
the membrane oxygenator is ventilated with 100% O 2, run through a blender to control the pO 2
between 100-200 mmHg and pCO 2 between 35-40 mmHg
a priming solution (mostly crystalloid) is circulated through the lines to remove air
the arterial cannula is usually placed in the ascending aorta or arch
occasionally, the arterial cannula is placed in the femoral or axillary artery (e.g. aortic dissection
surgery)
additional suction lines can be used for intracardiac venting and scavenging of blood from the
operative field, and this blood passes througha microporous filter (to remove particulate matter) before
returning to the cardiotomy reservoir.

Initiating bypass
3-4 mg/kg heparin is administered systemically, and the activated clotting time (ACT) is monitored (a
value over 400 is required before bypass is started)
Blood pressure is usually maintained between 55-65 mmHg using vasodilators or vasopressors
(cerebral blood flow is usually maintained by autoregulation until the pressure falls below 40 mmHg)
The lungs are not ventilated during bypass
Pump flows are usually around 2.24 L/min/m2 and is non-pulsatile (pulsatile flow used if significant
renal disease)
The patient may be warmed or cooled depending on the procedure and the surgeon's preference

Terminating bypass
The patient is warmed to normothermia
Air is removed from the LV and aorta with a needle or venting cannula
The lungs are ventilated and cardiac pacing is initiated as necessary
The heart is filled by restricting venous return as bypass flow is reduced and turned off
Alpha agents (e.g. dopamine 1-5 ug/kg/min) and calcium chloride (1 g) are often used to improve
contractility and systemic blood pressure and facilitate weaning from CPB
Inotropic support is considered for poor cardiac performance
When the patient is stable, protamine is administered to reverse the heparin effect (1.0 to 1.5 times
the original heparin dose), and the cannulas are removed
Blood remaining in the oxygenator and lines at the end of bypass (as well as shed mediastinal blood)
can be transfused back to the patient at the conclusion of the operation

Adverse effects of CPB
CPB activates numerous cascades (coagulation, complement, fibrinolytic, and kallikrein systems)
proinflammatory cytokines are released that can cause a systemic inflammatory response and
contribute to myocardial reperfusion damage, lung injury, and generalized capillary leak
CPB can also cause a coagulopathy (dilution of clotting factors and platelets, platelet dysfunction) and
renal and splanchnic hypoperfusion

Circulatory arrest
necessary for some operations on the ascending aorta and all aortic arch operations to allow the
surgeon to operate without the constraints of vascular clamps and a blood obscured field
The patient is cooled systemically to 18-20C at which the EEG is flat, and the head is packed with ice
The arterial line is clamped, the CPB machine is turned off, and blood is drained from the circulation
(the venous line is intermittently clamped to prevent excessive drainage from the patient)
after completing the distal anastomosis of the prosthetic graft to the distal ascending/ transverse
aorta, the arterial inflow cannula is positioned through the graft, the open end of the prosthetic graft is
clamped, and full bypass is resumed before the proximal anastomosis is performed
The "safe" upper limit for circulatory arrest is 45-60 minutes at 18C
administering blood retrograde into the brain through a cannula in the superior vena cava (SVC) may
extend this safe upper limit by providing additional cooling and possibly some oxygen and nutrition to
the brain
retrograde perfusion also maintains cerebral hypothermia and flushes air and debris out of the
cerebral vessels

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PRINCIPLES OF CARDIOPULMONARY BYPASS

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