Cardiopulmonary bypass

Cardiopulmonary bypass

Cardiopulmonary bypass is used in the majority of intra-cardiac surgical procedures.

The purpose of cardiopulmonary bypass is

· adequate tissue perfusion

· oxygenation

· dry, quiet operative field

The cardiopulmonary bypass consists of the pump, an oxygenator and plastic circuitry. Cardiopulmonary bypass is accomplished by large bore catheters diverting blood flow from the right atrium, to the pump and oxygenator (oxygenation, filtration, dilution, temperature alteration) and back to the patient via a single catheter inserted into the femoral artery or ascending aorta.

The principles of cardiopulmonary bypass

Haemodilution

The CPB pump is primed with a combination of colloids and crystalloids to reduce the patients hematocrit by 25%. By reducing the hematocrit

· the colloidal pressure and the oxygen carrying capacity of the blood is reduced

· the viscosity of the blood is reduced

· haemolysis of the red blood cells is reduced

· systemic vascular resistance is lowered

· post-operative diuresis is promoted

Anticoagulation

Total haemostasis is essential for cardiac surgery. The goal of anticoagulant therapy is to stop thrombogenesis (ACT 400 seconds). The patient is heparinized fully before the insertion of the cannula. Heparin is reversed on completion of CPB with protamine sulphate (ratio=1mg/100 units)

Myocardial protection

Myocardial protection is achieved by maintaining a balance between oxygen supply and demand. Myocardial oxygen demand is affected by heart rate, preload, afterload and contractility. Oxygen supply is affected by oxygen saturation, haemoglobin, oxygen delivery and coronary blood flow. Myocardial protection is accomplished by intermittent cross clamping, hypothermia and cardioplegia.

The effects of cardio pulmonary bypass

Cardiovascular system

· myocardial infarction

· low cardiac output after surgery

· increased afterload

· hypertension

Pulmonary system

· respiratory insuficiency

· atelectasis

Neurological system

· cerebrovascular accidents

· cerebral haemorrhage

Gastrointestinal system

· gastrointestinal bleeding

· intestinal ischaemia or infarction

· acute pancreatitis

Renal system

· acute renal failure

Fluid and electrolyte balance

· interstitial oedema

· intavascular hypovolaemia

· hypokalaemia

· hyperkalaemia

· hyponatraemia

· hypocalcaemia

· hypomagnesaemia

Endocrine system

· water and sodium retention

· hypothyroidism

· hyperglycaemia

Immune system

· infection

· post perfusion syndrome

Hematologic factors

· bleeding

Preoperative assessment

History

· cardiac disease

· past medical/surgical history

· medication therapy

· exertional tolerance.

· arterial blood gas

· pulmonary function test

· smoking history

Physical examination

· cardio-respiratory signs

· peripheral Vascular Disease

· diabetes

· gastric ulcers

· renal function

Investigations

· haematological profile

· biochemistry profile

· coagulation studies

· HIV, Hep B&C

· Chest x-ray, PFT

· ECG

· Echocardiograph

· Cardiac catheter studies to delineate coronary artery anatomy and left ventricular function.