26 Operations with Cardiopulmonary Bypass
Synonym: extracorporeal circulation
26.1.1 Principle of the Cardiopulmonary Bypass Machine
The cardiopulmonary bypass machine assumes the pumping function of the heart and the central task of the lungs, gas exchange, during open-heart surgery.
The cardiopulmonary bypass machine consists mainly of a pump, an oxygenator, a heat exchanger, and a reservoir (Fig. 26.1). The principle is simple: the venous blood is drained from the heart through a system of tubes via a cannula in the right atrium or cannulas in the superior and inferior vena cava. The blood accumulates in a reservoir together with the blood suctioned from the surgical field. The blood is conducted to an oxygenator by a roller or centrifugal pump. A membrane oxygenator is usually used. The blood is oxygenated and carbon dioxide is removed by diffusion. A heat exchanger is also attached that can warm or cool the blood in order to affect the body temperature as desired.
To prevent embolisms, the blood is passed through a filter before being pumped back into the systemic circulation via the aortic cannula.
Because the blood is in continuous contact with foreign surfaces during bypass, anticoagulation with heparin is essential. At the end of the operation, the heparin is neutralized with protamine.
During cardiopulmonary bypass, the body temperature is usually cooled down to 25 to 28°C. This reduces the body’s demand for oxygen and increases the ischemia tolerance time of the myocardium.
After a gradual increase, the entire cardiac output is pumped through the cardiopulmonary bypass machine. The heart initially continues to beat as the coronaries are still perfused. The coronary arteries continue to receive blood until the aorta is clamped off proximal to the aortic cannula (Fig. 26.2). In addition, a cardioplegic solution is injected into the aortic root and the heart stops beating. Most intracardiac operations are performed on a bloodless and nonbeating heart. The cardioplegic solution, which contains potassium, magnesium, and buffer substances, is also used for cardioprotection. The cardioplegic solution is usually reapplied every 20 to 30 minutes during the operation to achieve optimal cardioprotection.
Transition to normal circulation
The heart does not begin to beat again until the myocardium is again perfused with blood and a normal membrane potential can be established after the aortic clamp has been opened. The heart either resumes activity spontaneously or the rhythm must be restored electrically. Later, the temperature of the blood and body is gradually warmed up again. The activity of the cardiopulmonary bypass machine is reduced in parallel.
Especially in neonates and small children, a modified ultrafiltration of the blood is also performed during this phase. This withdraws water from the circulatory system, increases hematocrit, and removes inflammatory mediators. Ultrafiltration has a favorable effect on postoperative course, which can otherwise often be complicated by a considerable capillary leak.
Circulatory arrest in deep hypothermia
Circulatory arrest is sometimes needed, especially in neonates or small infants. Some procedures that necessitate this are operations on the aortic arch distal to the aortic cannula, for example, in a Norwood procedure or the correction of a hypoplastic aortic arch or a total anomalous pulmonary venous connection. In these procedures, using the cardiopulmonary bypass machine would result in a continuous flow of blood to the operation field, making the correction impossible. In these cases, the cardiopulmonary bypass machine is either switched off entirely or only minimal blood flow via a cannula in the right carotid artery is maintained for the selective perfusion of the head. To protect the organism, the body is cooled down to a temperature of 14–20°C during circulatory arrest (Fig. 26.3, Fig. 26.4). In this way, neurological damage can be reduced surprisingly well. However, the risk of later neurological damage increases when the arrest lasts more than 45 to 60 minutes.
Side effects of extracorporeal circulation
Extracorporeal circulation triggers a number of systemic reactions in the organism that have a decisive effect on the postoperative course, including:
Increased risk of hemorrhage as a result of activating the coagulation and fibrinolytic system
Hemolysis as a result of the shear forces that act on the red blood cells by the roller pumps and suctioning
Activation of the complement system, activation of inflammatory mediators and neutrophils resulting in systemic inflammatory response syndrome (SIRS)
Capillary leak and generalized tendency for edema
Myocardial dysfunction, impaired myocardial contractility
Impaired renal function