History of Cardiac Surgery Linked to the History of Intensive Care

The development of modern cardiac surgery has been intimately related to the development of the ICU. This relationship has worked in both directions. Until the 1950s, cardiac surgery was limited to control of traumatic injuries and the closed repair of valves. Development of the extracorporeal pump oxygenator in 1953 by Gibbon ushered in the era of open-heart surgery.¹ Heart valve replacement then became possible. Subsequently, in the 1960s, coronary artery bypass grafting (CABG) for ischemic heart disease was developed and rapidly popularized.²

Several studies have demonstrated that risk-adjusted mortality rates after CABG vary significantly among surgeons and hospitals and that mortality is related both to the number of surgeries performed by each surgeon and the total volume of procedures performed at the hospital.^3–5 For high-risk surgical patients, survival is also related to the characteristics of the ICU care.⁶

The Changing Epidemiology of Cardiac Surgery

Over the last decade, the population of patients treated with cardiac surgery has changed dramatically. Advances in cardiology including reperfusion therapy, angioplasty, stenting, and drug-eluting stents, have obviated the need for surgical approaches to treatment except for particularly complex problems or after failure of other less invasive modalities. In the year 2000, 561,000 patients in the United States underwent percutaneous transluminal coronary angioplasty (PTCA), an increase of 262% relative to 1987. In the same year, 314,000 patients underwent CABG. Multiyear trends, represented in Figure 194-1, show a leveling off and subsequent decrease in the overall number of patients undergoing CABG.⁷ The recently developed sirolimus-coated coronary stent has been associated with even better results.⁸ Studies comparing the use of stents versus CABG for left main disease have found no significant difference in rates of death or of the composite endpoint of death, Q-wave infarction, or stroke between patients receiving stents and those undergoing CABG. However, stenting, even with drug-eluting stents, was associated with higher rates of target-vessel revascularization than was CABG.⁹

Figure 194-1 Trends in cardiovascular operations and procedures in the United States, 1979-2000. PTCA, percutaneous transluminal coronary angioplasty.

(From American Heart Association. Heart disease and stroke statistics—2003 update. Dallas, TX: AHA; 2003.)

Even as younger patients are being treated with interventional techniques, the elderly are increasingly referred for operation. Although these operations are successful even in most octogenarians, they are associated with increased hospital mortality and longer ICU and hospital stays. It is clear, however, that good results in terms of long-term survival and quality of life are achievable.

Alternative Techniques for Cardiac Surgery

The increasing age of patients undergoing cardiac surgery and the relatively high incidence of adverse effects related to cardiopulmonary bypass (CPB) in these patients have led to the development of less invasive cardiac surgical techniques. These techniques are intended to decrease postoperative morbidity, reduce hospital length of stay, reduce costs, and hasten recovery of lifestyle (Table 194-1). Three major techniques have been proposed.

TABLE 194-1 Comparison of Minimally Invasive Cardiac Surgery Techniques

Minimally invasive direct coronary artery bypass (MIDCAB) differs from conventional CABG mainly in the type of incision used for access. In place of the conventional median sternotomy, access is obtained via a left or right thoracotomy, a parasternal incision, or a partial sternotomy. The proposed benefit of such an approach is the reduction in morbidity related to median sternotomy. This proposed advantage has not been demonstrated. MIDCAB grafting is a challenging technique and should be performed only in selected patients with favorable coronary anatomy. Both bare metal and drug-eluting stenting have been shown to be inferior to MIDCAB for proximal left anterior descending (LAD) coronary artery lesions, owing to higher reintervention rates with similar results in mortality and morbidity.^4,10

Off-pump coronary artery bypass (OPCAB) is performed on a beating heart without benefit of CPB. The proposed benefit of this procedure is reduction of morbidity related to hypothermia and CPB. The procedure is undertaken using partial to full heparinization. Extubation may be achieved earlier in these patients because they do not require rewarming and are less coagulopathic. A subset of patients cannot tolerate the extent of retraction of the heart required for the surgery and need to be urgently placed on CPB. These patients may suffer ischemic myocardial injury and require support with inotropes or intraaortic balloon pumping (IABP) during the postoperative period. A retrospective study of 1398 patients showed that use of the OPCAB technique for multivessel myocardial revascularization in high-risk patients significantly reduced the incidence of perioperative myocardial infarction (MI) and other major complications, length of stay in the ICU, and mortality.¹¹ In a single-center non-randomized registry, the incidence of major cardiac events were similar in OPCAB versus sirolimus-eluting stents in diabetic patients with multivessel disease.¹²

A third method of minimally invasive cardiac surgery is the port-access technique. This operation entails obtaining access for CPB with the use of endovascular catheters. This allows surgery to be performed using CPB via either a left or right thoracotomy. The technique is particularly useful for mitral valve replacement through a right thoracotomy and for redo CABG (avoiding the complications associated with repeat sternotomy). The port-access technique has been shown to be safe and is associated with shorter lengths of stay, reduced transfusion requirements, fewer infections, decreased incidence of renal failure, and less atrial fibrillation when compared with conventional techniques.¹³ In outcome data using propensity score analysis for mitral valve repair, minimally invasive repair had similar results to open repair. There was an increase in cross-clamp and bypass times, but early outcome was similar.¹⁴ Widespread adoption of this technique has been limited by the technical complexity of placing the required catheters, which requires both extra time and a specially trained and skilled operative team.

The techniques of minimally invasive cardiac surgery are still evolving. The intensivist caring for cardiac surgical patients must continue to keep abreast of these new methods.

Organization of the Postoperative Cardiac Surgery Unit

Optimal results from cardiac surgery require a skilled, dedicated, and multidisciplinary ICU team. Patients undergoing cardiac surgery are usually admitted to the hospital on the day of surgery. They arrive in the ICU directly from the operating room (OR). The typical patient is transferred to a step-down unit on the morning after surgery. This unit allows continued monitoring with telemetry for an additional 24 to 48 hours. Patients remaining in the ICU beyond 48 hours tend to become similar to a standard ICU population, as they develop secondary complications such as sepsis, pneumonia, and acute respiratory distress syndrome (ARDS).

Guidelines developed by the American Heart Association and the American College of Cardiology outline the requirements for cardiac surgical ICUs.¹⁵ These include the development of protocol-driven care, a minimum number of cardiac surgical ICU beds that is half the number of surgeries performed per week, and one-to-one nursing care during the first night in the unit. ICU coverage by a dedicated intensivist has been shown to improve outcomes in other types of major surgery and should be recommended after cardiac surgery as well.⁶

Cardiopulmonary Bypass

The goal of CPB is to separate the heart and lungs from the systemic circulation so that the heart can be arrested while the surgical repair is constructed. Blood is drained from the right side of the heart, either by gravity or with vacuum assistance, via a cannula in the right atrium directly or via a cannula in the femoral vein that is advanced into the right atrium. The blood is collected in a reservoir and then pumped through an oxygenator that contains a membrane where the blood is oxygenated and carbon dioxide is removed (Figure 194-2). The perfusionist controls both the fraction of inspired oxygen and the rate of oxygen flow through the circuit, thereby controlling the patient’s arterial oxygen and carbon dioxide levels, respectively. The treated blood then passes through an air filter and is returned to the patient via an arterial cannula placed in either the ascending aorta or the femoral artery. The perfusionist controls the amount of flow provided to the patient (i.e., CO). Mild to moderate systemic hypothermia (28°C-34°C) is used during bypass to minimize oxygen consumption by both the body and the brain. After adequate CPB is established, an aortic cross-clamp is applied to the ascending aorta, between the aortic cannula and the heart. The interval when the cross-clamp is applied is referred to as “ischemic” time, because no blood is circulated through the heart during this period. The heart is arrested by infusion of a high-concentration potassium solution into the native coronary arteries (antegrade cardioplegia) via a cannula placed between the aortic cross-clamp and the heart. Cardioplegia may also be given “backwards,” through the venous system of the myocardium (retrograde cardioplegia) via a catheter placed in the coronary sinus. Potassium is used as the arresting agent because it stops the heart from beating and minimizes myocardial oxygen consumption.

Figure 194-2 Cardiopulmonary bypass circuit.

(Adapted with permission from Gravlee GP, Davis RF, Kurusz M, Utley JR, editors. Cardiopulmonary bypass: principles and practice. 2nd ed. Baltimore: Lippincott Williams & Wilkins; 2000, p. 70.)

Myocardial Protection

Several measures are taken to protect the heart during ischemic time, because irreversible myocardial damage may otherwise occur. Electromechanical arrest is the most important protective measure, because the beating action of the heart accounts for about 85% of the heart’s total oxygen consumption. The heart is usually cooled to about 10°C with a cold cardioplegia solution (4°C) supplemented with topical ice slush. Additionally, the left ventricle is “vented” to prevent distention, which could lead to subendocardial ischemia. Finally, various additives are included in the cardioplegia solution to minimize myocardial edema, maintain normal intramyocardial pH, and provide substrates for anaerobic metabolism. The adequacy of intraoperative myocardial protection is critical for determining the subsequent course and final outcome of the patient

Separation from Cardiopulmonary Bypass

Weaning from CPB is the process whereby cardiopulmonary function is transferred from the bypass system back to the patient’s own heart and lungs. Successful separation from CPB requires that the metabolic, cardiac, and respiratory parameters are as close to normal as possible. Separation from CPB implies that the native circulation will be required to support the body’s metabolic demands. The surgical team manipulates the heart rate and rhythm, preload, afterload, and myocardial contractility to achieve this goal.

In most cases, normal sinus rhythm is restored after discontinuation of cardioplegia and rewarming of the heart. Occasionally, discontinuation of cardioplegia and rewarming leads to the onset of ventricular fibrillation; in such cases, electrical defibrillation is required. Other dysrhythmias commonly encountered are atrioventricular disassociation and atrial fibrillation. An attempt should be made to convert these to sinus rhythm by pharmacologic means. Bradyarrhythmias are treated by pacing, using temporary epicardial wires placed by the surgeon after completion of the repair. A heart rate of 70 to 90 beats/min usually is optimal. Pharmacologic support of the circulation may be needed to provide appropriate afterload or systemic vascular resistance (SVR) during separation from CPB. Most patients are vasodilated to some extent, possibly as a result of a systemic inflammatory response to CPB or the effects of rewarming, or both. As a consequence, infusion of a vasoconstrictor is often required. Care must be taken to strike a proper balance so that increased SVR maintains adequate arterial blood pressure without excessively increasing left ventricular afterload and compromising CO.

Most often, myocardial function is adequate, and infusion of an inotrope is not necessary. However, inotropic support often is needed for patients with a poor preoperative ventricular function or inadequate myocardial protection or revascularization during CPB. The optimal inotrope in this situation is a matter of considerable debate, and data are lacking to support a strong recommendation for a specific agent. Epinephrine, norepinephrine, dopamine, dobutamine, amrinone, and milrinone have all been used successfully. Intraoperative monitoring using transesophageal echocardiography (TEE) is particularly useful for titration of inotropic therapy.

Once all preparations for separation have been made, the perfusionist begins to wean the patient from bypass. This is done by slowly decreasing the amount of blood drained from the right atrium while simultaneously reducing flow into the aorta. Once the patient is off bypass (i.e., no blood is being drained from the right atrium into the CPB circuit), the perfusionist, at the direction of the anesthesiologist or surgeon, may continue to infuse through the aortic cannula. This maneuver allows optimization of ventricular filling or preload. Care must be taken, however, not to overdistend the heart; again, during this period, TEE is extremely useful.

Reversal of Anticoagulation

After weaning from CPB, protamine is given to neutralize any residual heparin. Dosing can be based on the patient’s weight, the total amount of heparin given, or an assay of residual heparin activity. Institutional preference governs the technique employed, and all have been proven effective. Several adverse responses to protamine administration are possible, including histamine-induced systemic hypotension, immunoglobulin E–mediated allergic reactions, and complement-mediated catastrophic pulmonary hypertension.

Transport and Admission to the Intensive Care Unit

After chest closure, confirmation of hemodynamic stability, and adequate medical and surgical hemostasis, the patient may be transferred to the ICU. Transport of a critically ill patient is a potentially dangerous process and requires extreme vigilance. Transport between the operating room and the ICU should be done with the same degree of monitoring as would be available at either end. This usually includes continuous monitoring of arterial blood pressure, pulmonary artery pressure and/or central venous pressure (CVP), electrocardiogram (ECG), and pulse oximetry. The transport bed should be equipped with a full oxygen tank, Ambu bag and mask, intubation equipment, resuscitation drugs, and a defibrillator. Care must be taken to ensure that infusions of vasoactive drugs are not interrupted.

On arrival in the ICU, the ICU team assumes care of the patient. A detailed sign-out from the operative team ensures continuity of care. The sign-out should include a detailed history including an assessment of preoperative cardiac functional status, a list of preoperative medications, and a detailed description of the surgery. Key facts are the type of repair performed, target vessels (if the patient has undergone CABG), duration of CPB and cross-clamping, difficulties encountered in separation from CPB, presence of abnormal bleeding, and postoperative assessment of cardiac function. All treatments administered in the OR should be detailed—in particular, fluids, blood products, and vasoactive drugs.

Once care has been handed over to the ICU team, a thorough examination of the patient should immediately follow. This examination should include verification of endotracheal tube placement, type and position of arterial or central venous lines, chest tube position and patency, and the presence and location of any epicardial pacing wires.

Hemodynamic Monitoring

All patients admitted to the ICU after cardiac surgery will have their blood pressure continuously monitored using an intraarterial line. This is usually placed in either a radial or femoral artery. Accuracy of the measurements depends on strict attention to calibration, leveling, and removal of air from the tubing. After CPB, femoral arterial pressure may more accurately reflect central aortic pressures,¹⁶ but this problem has usually resolved by the time the patient arrives in the ICU. If the radial artery is cannulated, the hand should be examined for signs of ischemia.¹⁷ Vascular complications of femoral arterial lines are extremely rare, but femoral catheters may be associated with an increased incidence of infection.¹⁸

Central venous access is required in all patients for drug administration and hemodynamic monitoring. In the low-risk patient, a CVP catheter may be all that is needed, particularly if echocardiography is available as a backup. Pulmonary artery catheters have the advantage of allowing measurement of pulmonary artery occlusion pressure (PAOP), thermodilution, and CO, as well as sampling of the mixed venous blood saturation (SvO₂). Use of the pulmonary artery catheter remains controversial. Improved outcome due to use of a pulmonary artery catheter for monitoring of cardiac surgical patients has not been demonstrated.¹⁹ Some studies showed an increased risk of death or adverse outcome when treatment was guided by the use of a pulmonary artery catheter.^20,21 However, many of these studies have been criticized on methodological grounds, and use of the catheter in cardiac surgery remains widespread.²² Current guidelines recommend use of the pulmonary artery catheter in high-risk patients undergoing surgery in an appropriate practice setting.²³ Such a setting is one in which the physician and nursing staff are familiar with the catheter and trained to properly interpret the information obtained. If echocardiography is readily available, it is possible to manage even high-risk patients using a CVP catheter.

Electrocardiography

On admission to the ICU, the patient is connected to a continuous ECG monitor, and a formal 12-lead ECG is obtained. The cardiogram is examined for rate, rhythm, QRS complex morphology, and signs of myocardial ischemia. For patients who are being paced postoperatively, the type of pacing and the degree of capture should be assessed.

Continuous ECG monitoring allows detection of arrhythmias. If an arrhythmia is detected, a 12-lead ECG should be obtained, and serum electrolyte concentrations should be measured. Treatment of arrhythmias should be carried out using established protocols.²⁴ If a malignant arrhythmia occurs, myocardial ischemia should be considered as a possible precipitating cause.

Monitoring of trends in ST-segment elevation or depression allows early detection of postoperative myocardial ischemia. Although transient ST-segment changes are relatively common and of unclear significance, persistent changes should be investigated by obtaining a 12-lead ECG and measuring circulating levels of creatine kinase myocardial band (CK-MB), troponin-T, or troponin-I.^25,26 If ischemia is strongly suspected, then echocardiography followed by coronary angiography should be considered. Findings from these studies may indicate the need for further coronary revascularization.

Chest Radiography

The postoperative chest radiograph should be systematically evaluated. Proper placement of the endotracheal tube and any central lines inserted should be confirmed. If a pulmonary artery catheter is place, the location of its tip should be noted and adjusted as needed. The lung fields should be examined for the presence of pneumothorax or collapse. Additional air may be noted as subcutaneous emphysema or as pneumopericardium, although these findings are of little clinical significance. Further examination of the lung fields commonly shows small areas of atelectasis and pleural effusion. The cardiac silhouette is often enlarged after surgery as a result of myocardial edema and accumulation of fluid in the open pericardial sac. Increasing size of the cardiac silhouette or pleural effusions on serial chest radiographs may be evidence of ongoing mediastinal bleeding.

Echocardiography in the Intensive Care Unit

Echocardiography is an excellent tool for evaluating chamber size and function and the adequacy of valve repair or replacement. Indications include postoperative assessment of left ventricular function, assessment of unexplained sudden hemodynamic deterioration, evaluation to rule out pericardial tamponade, and workup of new cardiac ischemia. Limitations to transthoracic echocardiography (TTE) include inadequate windows early after operation due to air and edema in the soft tissues and wound dressings.

TEE is being used as a tool to facilitate decision making in the management of critically ill patients, including cardiac surgical patients. In the cardiac surgical ICU, this modality may have a particularly high yield when it is used to establish the cause of postoperative hypotension.²⁷ In one large series, a new diagnosis was established or an important pathology was excluded in 45% of TEE examinations performed in the ICU. Pericardial tamponade was diagnosed in 34 cases (11%) and excluded in 36 cases (12%). Other diagnoses included severe left ventricular failure and presence of large pleural effusions. The results of TEE had an impact on therapy in 220 cases (73%) by leading to a change of pharmacologic treatment and/or fluid administration, reoperation, or a decision that reoperation was unnecessary.²⁸

The Normal Course

Patients are typically admitted to the ICU intubated and ventilated. Sedation with a short-acting agent, typically propofol, is continued until the patient is ready for extubation. Once hemodynamic stability is ascertained and chest tube drainage is judged to be under control, the patient is allowed to awaken. There is no need for prolonged weaning from mechanical ventilation. A short trial of spontaneous ventilation is sufficient to determine whether respiration will be adequate without mechanical support. The rapid shallow breathing index (RSBI) has been shown to be a sensitive way to assess the likelihood of successful extubation.²⁹ The RSBI is calculated by dividing the respiratory rate (in breaths per minute) by the tidal volume (in liters). A value of lower than 105 predicts successful extubation. Chest tubes are commonly removed on the first postoperative day. The pulmonary artery catheter, if present, is discontinued, and the patient may be transferred to a step-down unit.

Fast-tracking of cardiac surgical patients refers to a comprehensive program designed to reduce both length of stay and hospital costs.^30,31 As a part of this program, multiple anesthetic techniques designed to allow earlier postoperative extubation have been proposed, studied, and shown to be safe. These techniques may allow extubation in the OR.³² The key to proper use of this technique is patient selection. Although the criteria are expanding, patients with unstable angina or a high degree of congestive heart failure are generally not appropriate candidates for fast-tracking. In a retrospective review comparing 4020 patients undergoing cardiac surgery with a conventional anesthetic versus 3969 patients with a fast-track anesthetic, the fast-track group had shorter extubation times, shorter ICU or PACU stays, and a lower incidence of low cardiac output syndrome.

Low Cardiac Output

Low CO is the most common problem encountered in the postoperative cardiac surgical patient. A hallmark of low CO is low blood pressure. However, a patient may have a low CO with tissue hypoperfusion and still maintain what appears to be an adequate blood pressure. In the postoperative state, the physician must continuously examine and monitor the patient for signs of hypoperfusion. Physical signs of inadequate tissue perfusion include altered mental status; cool, pale, or even cyanotic extremities; diaphoresis; and low urine output. Global measures of hypoperfusion include increasing base deficit, elevated blood lactate concentration, and decreased SvO₂. Although the clinician must consider CO in terms of adequacy of perfusion, blood pressure per se is still important. Both the brain and kidneys depend on adequate blood pressure to maintain tissue perfusion. Additionally, coronary artery blood flow is dependent on a diastolic blood pressure, a key determinant of coronary artery perfusion pressure.

When assessing a patient with hypotension or signs of hypoperfusion, it is useful to consider the problem in relation to the components of CO; namely, preload, contractility, afterload, and rate and rhythm.