Management of the Postoperative Cardiac Surgical Patient

Published on 22/03/2015 by admin

Filed under Critical Care Medicine

Last modified 22/03/2015

Print this page

rate 1 star rate 2 star rate 3 star rate 4 star rate 5 star
Your rating: none, Average: 0 (0 votes)

This article have been viewed 1249 times

194 Management of the Postoperative Cardiac Surgical Patient

The first days of care for the cardiac surgery patient present multiple challenges for the intensivist. The intensive care unit (ICU) stay for most of these patients lasts for only 24 to 48 hours, but during this period, life-threatening problems such as low cardiac output (CO), arrhythmias, and coagulopathy may become apparent. After 48 hours in the ICU, the problems encountered by postoperative cardiac surgery patients tend to become more like those experienced by other groups of critically ill patients.

image The Cardiac Surgery Patient in the Intensive Care Unit

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

image

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.

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.

image Separation from Cardiopulmonary Bypass and the End of Surgery

Successful management of the postoperative cardiac surgery patient begins by understanding what occurs in the OR. Problems encountered in the OR often persist after transfer to the ICU. An understanding of the technical and pathophysiologic aspects of CPB can help the intensivist better manage cardiac surgical patients in the ICU.

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.

image

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.)

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.

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.

image Monitoring the Postoperative Cardiac Surgery Patient

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.

image Clinical Manifestations of the Postbypass Period

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.

Preload

Preload refers to the stretch of the left ventricle at the end of diastole and is determined by the extent of ventricular filling during diastole. Adequate filling is required to ensure ejection in the subsequent systole. The most common cause of inadequate preload in postoperative patients is hypovolemia. Intravascular volume status should be continually monitored by assessing changes over time with respect to physical examination, chest tube output, and filling pressures (CVP, PAOP, or pulmonary artery diastolic pressure). Because none of the clinically measured filling pressures correlates perfectly with actual ventricular preload (i.e., end-diastolic volume), and correlation is particularly poor when the heart is diseased, it is often useful to obtain a “snapshot” of ventricular filling using echocardiography. By this means, it is possible to assess the relationship between measured filling pressures and actual preload in a specific patient. Preoperative catheterization data also can be helpful for determining this relationship. Hypovolemia should be treated with fluid replacement. Crystalloids are generally used. Surprisingly, there is no generally accepted hemoglobin concentration or hematocrit that should be used as a trigger for ordering transfusion of packed red blood cells. Red cell transfusion has been associated with early morbidity as well as long-term adverse sequelae.

In some cases, low preload is not caused by absolute hypovolemia but by relative or distributional hypovolemia. CPB and subsequent rewarming may lead to vasodilatation and a subsequent hypotension. Intravascular volume expansion may be required to maintain perfusion. An acceptable alternative is administration of a low dose of vasopressor such as phenylephrine or norepinephrine to maintain an adequate perfusion pressure. Recently, vasopressin in doses between 0.01 and 0.1 units/min has been demonstrated to be effective in this situation.33,34 Vasodilatation is usually a transient problem that resolves during the first several hours after separation from CPB. Continued vasodilatation after this period should prompt a search for another cause, particularly infection.

Pump Failure

Either or both ventricles may fail postoperatively. Decreased myocardial contractility may be caused by impaired preoperative function, inadequate revascularization at surgery, post-CPB reperfusion injury, or perioperative myocardial ischemia or MI. The incidence of infarction is approximately 5% in large series.35 Preoperative myocardial function and the adequacy of revascularization at surgery should be clear from the history. Determination of circulating levels of CK-MB or troponin postoperatively can provide evidence of perioperative ischemia or infarction.25,26 Often, diminished contractility after operation is caused by inadequate myocardial protection during surgery. Decreased myocardial contractility secondary to inadequate myocardial protection usually resolves within the first 24 hours postoperatively. ECG changes are nonspecific.

Persistent new myocardial dysfunction associated with ECG changes and echocardiographic evidence of new wall-motion abnormalities should raise suspicion that the problem is an occluded graft and MI. Measurements of CK-MB in serum are of limited usefulness because levels of this enzyme are commonly elevated after surgery due to manipulation of the heart and incision of the atria, structures that are rich in the enzyme. If CK-MB levels are very high, greater than 80 mg/dL, then perioperative MI is likely.36 Cardiac troponins are more specific for the diagnosis of perioperative infarction. A comparison of CK-MB, troponin-T, and troponin-I showed that a troponin-I level of greater than 5 µg/L was the most accurate indicator of MI, being superior to either troponin-T or CK-MB.37 Elevated serum concentrations of troponin-I are associated with a cardiac cause of death and with major postoperative complications.38 In addition, troponin-T concentrations measured after surgery are an independent predictor of in-hospital death after cardiac surgery.26 If ischemia or MI is diagnosed, the patient may be taken for angiography or re-exploration and revascularization.

Postoperative valvular insufficiency can occur not only in patients with preexisting valvular lesions but also as a result of injury during surgery. The mitral valve is most commonly affected. Ischemia of the papillary muscles due to inadequate myocardial protection or perioperative MI can lead to acute mitral regurgitation in the postoperative period. Diagnosis is often made by TEE in the OR, but inadequate CO and a new systolic murmur should prompt echocardiographic evaluation.

Buy Membership for Critical Care Medicine Category to continue reading. Learn more here