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?

Activated clotting time (ACT) is measured about 3 to 4 minutes after heparin administration and every 30 minutes on CPB. An ACT of 400 seconds or longer is considered acceptable. Heparin levels are measured frequently, but only the ACT is a measure of anticoagulant activity. This is particularly important in patients with heparin resistance (seen with preoperative heparin infusions) and antithrombin III deficiency.

11 What must be ascertained before placing the patient on cardiopulmonary bypass?

Adequate arterial inflow of oxygenated blood with acceptable line pressures

Sufficient venous return to the bypass pump

ACT of at least 400 seconds

Appropriate placement of retrograde cardioplegia cannula if being used.

Arterial line monitor of mean blood pressure

Core temperature monitoring

Adequate depth of anesthesia

12 Why is a left ventricular vent used?

Left ventricular distention during bypass can be caused by aortic regurgitation or blood flow through the bronchial and thebesian veins. The resultant increase in myocardial wall tension can lead to myocardial ischemia by precluding adequate subendocardial cardioplegia distribution and elevating myocardial oxygen demands. A left ventricular vent, placed through the right superior pulmonary vein, decompresses the left side of the heart and returns this blood to the CPB pump.

13 What are the characteristics of cardioplegia?

Cardioplegia is a hyperkalemic solution containing various metabolic energy substrates. Perfused through the coronary vasculature, cardioplegia induces diastolic electromechanical dissociation. Myocardial oxygen and energy requirements are reduced to those of cellular maintenance. Cardioplegia is perfused either anterograde via the aortic root coronary ostia or retrograde through the right atrial coronary sinus.

14 Discuss myocardial protection during cardiopulmonary bypass. What elements should be in place to optimize myocardial protection?

Cellular integrity must be maintained during CPB to ensure cardiac performance after CPB. A critical factor to prevent cellular damage is intraoperative myocardial protection. Preservation of the balance between myocardial oxygen consumption and delivery is essential, and the following are key elements in achieving this:

Adequate cardioplegia

Hypothermia, specifically myocardial hypothermia with myocardial temperature goals below 12 to 15° C

Topical cooling of the heart with icy saline slush

Left ventricular venting to prevent distention

Insulating pad on the posterior cardiac surface to prevent warming from mediastinal blood flow

Minimizing bronchial vessel collateral flow (which rewarms the arrested heart)

Inadequate preservation manifests after CPB as impaired cardiac output, ischemic electrocardiogram (ECG) changes, and wall motion abnormalities on transesophageal echocardiogram (TEE), dysrhythmias, and the requirement for inotropic support.

15 What is the function of an aortic cross-clamp?

Clamping across the proximal aorta isolates the heart and coronary circulation. The arterial bypass inflow enters the aorta distal to the clamp. Cardioplegia is infused between the clamp and aortic valve, thus entering the coronary circulation. This isolation of the heart from the systemic circulation allows for prolonged cardioplegia activity, diastolic arrest of the heart, and profound myocardial cooling.

16 Review the physiologic responses to cardiopulmonary bypass

Stress hormones, including catecholamines, cortisol, angiotensin, and vasopressin, increase in part because of decreased metabolism of these substances.

Exposure of blood to the CPB circuit results in complement activation, initiation of the coagulation cascade, and platelet activation. A systemic inflammatory response is also initiated.

Platelet dysfunction associated with CPB may contribute to post-CPB bleeding.

Hemodilution associated with onset of CPB decreases the serum concentrations of most drugs, but decreased hepatic and renal perfusion during CPB will eventually increase the serum concentration of drugs administered by continuous infusion.

17 What are the pH-stat and α-stat methods of blood gas measurement?

There are differing opinions as to whether blood gases should be corrected for the temperature during CPB because the solubility of gases decreases with hypothermia. All blood gases are analyzed at 37° C. In pH-stat measurements, the obtained value is corrected on a nomogram, and the reported values refer to the partial pressure at the hypothermic temperatures. CO₂ is then added to the system to correct the acid-base status. More commonly blood gases are reported uncorrected for temperature, a method referred to as α-stat blood gas management. Studies comparing the two methods show conflicting results, with the main concern being the alteration in cerebral vascular tone caused by the CO₂ management. In adults α-stat management leads to improved outcomes and is commonly used. Neonatal data indicates a trend toward neurologic improvement using pH-stat management, which is therefore used in this population.

19 How is the heparin effect reversed? What are potential complications?

Protamine, a positively charged protein molecule, binds the negatively charged heparin, and this complex is removed from the circulation by the reticuloendothelial system. Although there are different regimens to determine how much protamine should be administered, the simplest and most common method is to dose protamine based on the heparin administered (roughly 1 mg protamine per 100 units heparin). The efficacy of heparin reversal should be assessed by ACT but can also be determined using thromboelastography. Protamine has been associated with systemic hypotension because of histamine release or true anaphylaxis, along with catastrophic pulmonary hypertension due to anaphylactoid thromboxane release. Risk factors include preexisting pulmonary hypertension, patients with diabetes treated with NPH insulin preparations, bolus protamine administration, and central administration of protamine.

20 Why is cardiac pacing frequently useful after bypass?

Between the ischemic insult of bypass, the residual effect of cardioplegia, and effects from hypothermia, cardiac conduction may be impaired, and myocardial wall motion is suboptimal. Sequential cardiac pacing at a rate of 80 to 100 beats/min can significantly improve cardiac output.

21 What are some therapies for the patient with impaired cardiac performance or difficulty weaning from cardiopulmonary bypass?

From a surgical standpoint the adequacy of the surgical procedure (whether it be coronary artery bypass, valve replacement, or otherwise) should be reconsidered. TEE can evaluate regional wall motion abnormalities and valvular competency. Filling pressures should be assessed both by TEE and invasive monitors. Hemodynamic variables (cardiac index, mixed venous oxygen concentration, pulmonary artery pressure, pulmonary artery occlusion pressure, systemic vascular resistance) must be interpreted.

Typical scenarios encountered during the weaning process include reduced vascular resistance (so-called vasoplegia), which often requires vasopressor support. Contractility problems with the heart itself necessitate inotropic agents or possibly an aortic balloon pump to assist in the weaning from bypass. Right heart dysfunction and/or pulmonary hypertension may also complicate weaning from bypass. Agents such as nitric oxide or vasodilator therapy targeted toward the pulmonary system can be useful in this situation. TEE is invaluable in guiding these decisions along with interpretation of the pulmonary artery catheter. Inotropic and vasodilator therapies are discussed in Chapter 15.

22 Review the central nervous system complications of cardiopulmonary bypass

There is about a 1% to 3% incidence of new neurologic events, defined as stroke (including vision loss), transient ischemic attack, or coma. There is also about a 3% incidence of deterioration in intellectual function, memory defects, or seizures, although close neurocognitive testing reveals a much higher incidence (20% to 60%) of cognitive dysfunction at 1- and 6-month intervals. Much of the cognitive dysfunction will resolve over a number of months. Cerebral microemboli, in particular platelet microemboli, are believed to be a contributing factor.

KEY POINTS: Cardiopulmonary Bypass

1. There are multiple approaches to the anesthetic technique during CPB. The overall cardiac reserve based on the exercise tolerance and severity of disease (valvular or ischemic) should guide the choice of anesthetic, using cardiac depressant agents cautiously.

2. Patients must be completely anticoagulated before initiating CPB or face the risk of massive intravascular clot formation.

3. The CPB reservoir should never be allowed to empty during CPB since massive air embolism is a consequence.

4. Factors involved in myocardial preservation include cardioplegia, myocardial hypothermia, and ventricular venting. Consequences of inadequate myocardial preservation include decreased cardiac output, ischemia, dysrhythmias, and failure to wean from CPB.

5. Always consider inadequate surgical technique (e.g., kinking of graft, valve failure) for the patient failing to wean from CPB.

6. Neurologic complications, in particular neurocognitive deficits, are distressingly common after CPB.

23 What might be done to decrease the incidence of such complications?

Reversible factors may be identified before surgery. For instance, patients with significant carotid stenosis may require correction of this problem before CPB (perhaps at the same surgery). Significant aortic atherosclerosis is an independent risk factor for stroke. Therefore avoiding aortic cross-clamp is important; an off-pump surgical strategy may benefit these patients.

Perioperative aspirin therapy and minimally invasive off-pump coronary artery surgery may also have some benefit.

Meticulous attention intraoperatively to decreasing cerebral oxygen consumption through hypothermia, along with maintenance of perfusion pressure and mixed venous oxygen levels, may optimize cerebral supply and demand.

Transesophageal echocardiography may identify patent foramen ovale, significant atheromatous disease at the site of aortic cannulation, left atrial thrombi, and intracardiac air and change subsequent management. Consider aortic ultrasound to help assess disease in the ascending aorta.

Tight glucose control is likely beneficial.

In the future some pharmacologic interventions may decrease neurologic complications.

SUGGESTED READINGS

1. Gravlee G. Cardiopulmonary Bypass, Principles and Practice, ed 3. Philadelphia: Lippincott Williams & Wilkins, 2008.

2. Hogue C., Palin C., Arrowsmith J. Cardiopulmonary bypass management and neurologic outcomes: an evidence-based appraisal of current practices. Anesth Analg. 2006;103:21-37.

3. Mora-Mangano C.T., Chow J.L., Kanevsky M. Cardiopulmonary bypass and the anesthesiologist. In: Kaplan J.A., editor. Cardiac anesthesia. ed 5. Philadelphia: Saunders Elsevier; 2006:893-935.

4. Murphy G.J., Angelini G.D. Side effects of cardiopulmonary bypass: what is the reality? J Card Surg. 2004;19:481-488.

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