70: Cardiopulmonary Bypass

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CHAPTER 70 Cardiopulmonary Bypass

Nathaen Weitzel, MD

1 What are the main functions of a cardiopulmonary bypass circuit?

A cardiopulmonary bypass (CPB) circuit functions as the temporary equivalent of the native cardiopulmonary system. The CPB circuit allows for perfusion of the patient’s vital organs while oxygenating the blood and removing carbon dioxide (CO₂). Isolation of the cardiopulmonary system allows for surgical exposure of the heart and great vessels along with cardiac electrical silence and a bloodless field.

2 What are the basic components of the cardiopulmonary bypass circuit?

A CPB circuit has a venous line, which siphons central venous blood from the patient into a reservoir. This blood is then oxygenated and has CO₂ removed before being returned to the patient’s arterial circulation. Pressure to perfuse the arterial circulation is supplied by either a roller head or a centrifugal pump, usually resulting in nonpulsatile arterial flow, although some roller pumps can deliver pulsatile flow. The machine also has roller head pumps for cardioplegia administration, a ventricular vent to drain the heart during surgery, and a pump sucker to remove blood from the surgical field. In addition, the circuit contains filters for air and blood microemboli because both can cause devastating central nervous system injury if delivered to the arterial circulation. A heat exchanger is present to produce hypothermia on bypass and warm the patient before separating from CPB. The venous reservoir must never be allowed to empty while on CPB because a life-threatening air embolism could result.

3 Define the levels of hypothermia. What are adverse effects of hypothermia?

Mild: 32–35° C

Moderate: 26–31° C

Deep: 20–25° C

Profound: 14–19° C—This level of hypothermia is achieved if total circulatory arrest is planned.

Typical cardiopulmonary bypass temperatures range from 28° C to 33° C. Adverse effects of hypothermia include platelet dysfunction, reduction in serum ionized calcium concentration caused by enhanced citrate activity, impaired coagulation, arrhythmias, increased risk of infection, decreased oxygen unloading, potentiation of neuromuscular blockade, and impaired cardiac contractility.

4 Why is hypothermia used on cardiopulmonary bypass?

Systemic oxygen demand decreases 9% for every degree of temperature drop. Therefore hypothermia allows for lower CPB pump flows while providing adequate oxygen supply to vital organs. The main concern of CPB is the prevention of myocardial injury and central nervous system injury, along with renal and hepatic protection.

5 Discuss the common cannulation sites for bypass

Venous blood is typically obtained through cannulation of the right atrium using a two-stage cannula that drains both the superior and inferior vena cava. Alternatively for open heart procedures bicaval cannulation is used with direct, separate cannulation of the superior and inferior vena cavae. Arterial blood is returned to the ascending aorta proximal to the innominate artery. The femoral artery and vein can be used as alternative cannulation sites. Drawbacks to femoral bypass include ischemia of the leg distal to the arterial cannula, inadequate venous drainage, possible inadequate systemic perfusion secondary to a small inflow cannula, and difficulty in cannula placement because of atherosclerotic plaques. Axillary artery cannulation can be used for repeat sternotomies and is often carried out before the sternotomy itself to allow for arterial fluid or blood administration from the CPB machine during the sternal dissection if necessary.

6 What are the basic anesthetic techniques used in cardiopulmonary bypass cases?

Historically many cardiac cases were managed using high-dose opioid anesthetic techniques for fear of cardiac depression with use of inhalational gases or induction agents such as propofol. During the past 20 years improvement in surgical technique and CPB circuit design, along with added experience using inhalation agents, has resulted in a moderate reduction in opioid dosing. The main benefit of lower opioid dosing is a decreased time to extubation on arrival in the intensive care unit—so-called fast tracking. Important details in determining anesthetic choice include degree of systolic dysfunction caused by coronary disease, degree of valvular involvement, and overall exercise tolerance. These critically ill cardiac patients require delicate anesthetic management. Patients undergoing CPB have been identified as a high-risk group for intraoperative awareness, likely a result of opioid-based techniques. Amnestic agents such as midazolam are routinely used along with inhalational agents, which can be administered by the perfusionist during CPB. Neuromuscular blocking agents prevent patient movement and shivering, decreasing systemic oxygen demand during bypass and diaphragmatic contraction during the surgical procedure.

7 List the two basic types of oxygenators

1. Bubble oxygenators work by bubbling oxygen (O₂) through the patient’s blood and then defoaming the blood to minimize air microemboli.

2. Membrane oxygenators use a semipermeable membrane that allows diffusion of O₂ and CO₂. Membrane oxygenators have a lower risk of gas microemboli, cause less damage to blood elements, and are the main type used in clinical practice.

8 What is meant by pump prime? What is the usual hemodynamic response to initiating bypass?

Priming solutions of crystalloid, colloid, or blood are used to fill the CPB circuit. When bypass is initiated, the circuit must contain fluid to perfuse the arterial circulation until the patient’s blood can circulate through the pump. Priming volumes historically were 1.5 to 2 L, but newer CPB circuits can achieve priming volumes as low as 800 ml for an open circuit and even less with a so-called closed or mini-bypass circuit. Reductions in priming volume have resulted in decreased inflammatory response and a reduction in transfusion requirements. The acute hemodilution from the patient’s circulating blood volume mixing with the prime volume can cause an acute reduction in mean arterial pressure and the hemoglobin concentration.

9 Why is systemic anticoagulation necessary?

Contact activation of the coagulation system occurs when nonheparinized blood contacts the synthetic surfaces of the CPB circuit, resulting in widespread thrombosis, oxygenator failure, and death. In a dire emergency a minimum standard dose of 300 units/kg of heparin must be given through a central line before the initiation of bypass. Following separation from the CBP machine, protamine is used to bind heparin and reverse the anticoagulant effect.

10 How is the adequacy of anticoagulation measured before and during bypass?

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