Intensive care after cardiac surgery

Chapter 22 Intensive care after cardiac surgery



Raymond F. Raper


Coronary artery bypass surgery is one of the most frequently undertaken surgical procedures. Because of the prevalence of cardiac disease, cardiac surgery has significant health and economic implications. Intensive care may account for up to 40% of the total hospital costs for these patients and much of the short-term morbidity and mortality is based on perioperative events.


The overall mortality of cardiac surgery is low (approximately 3%). However, this ranges from less than 1% for elective coronary artery bypass grafting to in excess of 30% for more complicated surgery in patients with significant myocardial dysfunction and associated disorders. Usually, intensive care post-cardiac surgery involves a short period of recovery before discharge to the ward but, for a small percentage of patients, at least potentially remediable complications may require the complete intensive care armamentarium, with a highly significant impact on the intensive care unit (ICU) and hospital budgets and resources.



ORGANISATION


The convergence of large numbers of patients with very similar problems in the cardiac ICU provides an ideal environment for the standardisation of care based on protocols and clinical pathways. One of the traps of postoperative cardiac care is that the sameness of the patients tends to obscure their particularity. Individual patient assessment must carefully address the multisystemic manifestations of cardiovascular and degenerative diseases.


Cardiac surgery involves a continuum of care from presentation to postdischarge management and rehabilitation. The intensive care specialist must be involved in this continuum, not functioning in isolation from surgeons, anaesthetists, cardiologists and family practitioners.


The postoperative stage is largely set by the preoperative and operative phases of management. A component of postoperative management is active participation both in principle and in particular in patient selection and preparation as well as in the conduct of anaesthesia and surgery. Relevant aspects include suitability and preparation for surgery, advanced care planning and more technical issues such as temperature management, invasive monitoring, haemodynamic management and transport. Movement from the operating theatre to the ICU and later to the ward will only appear seamless if the management systems have been well coordinated in advance.



CARDIOVASCULAR MANAGEMENT


The first step in the intensive care management of the newly arrived patient who has undergone cardiac surgery involves a simple transfer of ventilation, monitoring and drug administration from transport to ICU systems. This should be structured to minimise disruption.


• Confirm integrity and position of endotracheal and gastric tubes and intravascular catheters.

• Re-establish mechanical ventilation of both lungs.

• Undertake chest radiography; to assess lung expansion, pleural integrity and catheter placement.

• Perform early 12-lead electrocardiography to exclude or identify acute ischaemia.

Management is conveniently dictated by standardised protocols, which should cover investigations, fluid and electrolyte management, vasoactive and other drug administration and mechanical ventilation. Standardisation is probably more important than the particulars of the protocol, which might vary considerably among institutions. Optimal cardiovascular management requires a sound knowledge of normal and abnormal cardiovascular physiology. It also requires an understanding of the haemodynamic changes usually seen in the postoperative patient (Figure 22.1).1


image

Figure 22.1 Haemodynamic changes following cardiac surgery. Systemic vascular resistance index (SVRI).



MONITORING


Electrocardiography and continuous invasive blood pressure and central venous pressure monitoring are standard. Flotation pulmonary artery (PA) catheterisation has become somewhat controversial. It is now reasonably well established that PA catheterisation is safe but may not alter outcomes.2


• Surgery can be safely undertaken at least in low-risk patients without PA catheterisation.3

• Collateral evidence supports PA catheter use in more complicated cases.

• It is essential that the known limitations and complications of PA catheters are considered in usage and interpretation.4

Cardiac output measurement by pulse contour analysis and a variety of ultrasound techniques is becoming more commonplace. A precise role for these techniques is not established, especially in the postsurgical patient, where Doppler-enabling windows may be difficult to obtain. Pulse contour analysis is invalidated by intra-aortic counterpulsation. Nevertheless, these techniques and continuous mixed venous saturation monitoring are attractive and very useful in individual patients.


Transoesophageal echocardiography is now almost routine in the operating theatre, at least for more complicated patients. The technique is less suitable for monitoring in the ICU but is helpful in the diagnosis and management of cardiovascular instability in postoperative patients.



FLUID AND ELECTROLYTE MANAGEMENT


Despite generous intraoperative fluid administration, effective hypovolaemia is common in the early postoperative period, especially as warming with associated vasodilatation occurs.



RESUSCITATION FLUID



• Use of isotonic fluid is essential.

• No benefit for any particular resuscitation fluid has been established. The suggestion that albumin may not be safe has been essentially dispelled.5

• Larger volumes of crystalloid than colloid solutions are required.

Polyuria is frequently observed in the early postoperative period, possibly related to hypothermia, haemodilution and the after-effects of non-pulsatile cardiopulmonary bypass on stretch and baroreceptors. This usually settles within the first 6 hours but often necessitates considerable volume replacement in the meantime.



POTASSIUM AND MAGNESIUM HOMEOSTASIS


Hypomagnesaemia and hypokalaemia are frequent in the early postoperative stage. These are exacerbated by polyuria. Late hyperkalaemia is also quite common, especially in patients with renal impairment or prior angiotensin-converting enzyme (ACE) inhibitor administration. Treatment for hyperkalaemia is rarely required in the absence of significant renal impairment. Especially in patients with atrial or ventricular ectopy or tachydysrhythmias, potassium and magnesium levels must be maintained in the high normal range.


Calcium homeostasis is generally not threatened by cardiopulmonary bypass. Massive transfusion, however, frequently causes hypocalcaemia. Ionised calcium levels are now easily monitored by ward-based blood gas machines.



HYPOTENSION


Hypotension is both a consequence and a cause of myocardial dysfunction. The major causes include:


• hypovolaemia

• vasodilation

• pericardial tamponade

• heart failure

Other causes of low cardiac output state (Table 22.1) should be considered and excluded. Whatever the cause, hypotension frequently causes myocardial ischaemia and consequent heart failure, especially if it occurs in the first 12–24 hours postoperatively. Angiography during this time almost always shows a significant reduction in the calibre of native coronary arteries, indicating increased coronary resistance and, presumably, altered coronary vasodilator reserve. Treatment of hypotension is urgent if a spiral of ischaemia and heart failure is to be averted.


Table 22.1 Low cardiac output















Preload

• Hypovolaemia, including haemorrhage

• Tamponade, pericardial constriction

• Left ventricular diastolic dysfunction

• Hypertrophy

• Ischaemia

• Oedema

• Cardiomyopathy

• Pulmonary hypertension

• Right ventricular failure
Afterload

• Excessive vasoconstriction

• Aortic stenosis

• Functional left ventricular obstruction

• Obstructive cardiomyopathy

• Systolic anterior motion of the mitral valve
Myocardial function

• Mechanical (ventricular septal defect, valve pathology)

• Cardiomyopathy

• Ischaemia, postischaemic stunning

• Metabolic, electrolyte abnormalities, pharmacolgical depression


MANAGEMENT


Successful management of hypotension depends on:


• rapid diagnostic assessment

• diagnosis-specific intervention

Context is very helpful. Early hypotension in patients with well-preserved ventricular function and no obvious ischaemia usually responds well to fluid administration. Patients with poor ventricular function and increasing inotrope requirement are likely to have a more sinister pathophysiology.


Vasoconstrictors may be useful in breaking the cycle of hypotension–ischaemia–hypotension but must be used cautiously in patients with impaired ventricular function, and especially in patients with major vascular or aortic pathology, for whom a hypertensive overshoot may be catastrophic.



HYPERTENSION


Complications associated with hypertension include:


• bleeding

• heart failure

• vascular (especially aortic) injury

• myocardial ischaemia

Significant postoperative hypertension is more common in patients having a history of hypertension and with cessation of β-blockade, but is reasonably common in the early postoperative period. Both absolute pressure and dp/dt are important factors in vascular injury. Vascular resistance declines over the first few hours (see Figure 22.1), so that therapy during this period is best undertaken with agents with a short duration of action. Nitroglycerine is theoretically more appropriate than nitroprusside because of the possibility of coronary steal with the latter agent.6 In practice, this is almost never apparent and nitroprusside appears to be more effective. Simple measures such as the provision of adequate analgesia and sedation should also be considered.


The target blood pressure varies with the indication. Excessive reduction of blood pressure risks reducing myocardial oxygen supply more than demand. Under most circumstances, a mean arterial pressure between 90 and 100 mmHg (12.0 and 13.5 kPa) seems optimal.7 Much lower target pressures may be applicable for the management of heart failure or in the presence of a vulnerable aorta, such as occurs with aortic trauma, dissection or aneurysm. Reduction of dp/dt with beta-blockade is more important than absolute blood pressure control in the management of aortic dissection.



LOW CARDIAC OUTPUT


The aetiology of a low cardiac output following heart surgery is diverse (see Table 22.1). The most common causes include:


• intravascular volume depletion

• systolic heart failure

• pericardial tamponade

Transient, reversible myocardial depression may follow an episode of acute ischaemia (preoperative or intraoperative) and is especially problematic where intraoperative myocardial protection has been difficult or suboptimal. Recognition of a low-output state in the absence of invasive monitoring may be difficult. Many of the usual signs of low output are also consequences of anaesthesia and surgery. Tachycardia may be obscured by drugs, hypothermia and heart disease, and even lactic acidosis may be an unreliable marker in this patient group.1


In the early postoperative period, a relatively low cardiac output may not warrant intervention, providing tissue oxygen delivery is adequate. Since beta-blockade is beneficial in the postoperative cardiac patient, beta-agonists should not be used unquestioningly. Nevertheless, optimisation of cardiac function does confer some benefit8 and intervention is clearly required when tissue oxygen delivery is inadequate.


Low cardiac output is associated with increased gastrointestinal, renal and neurological complications.



MANAGEMENT OPTIONS



1 Establish the diagnosis, including aetiology. Distinguishing tamponade from heart failure may require cardiac imaging with transthoracic or transoesophageal echocardiography. This should be the first recourse when uncertainty remains following conventional haemodynamic assessment. Tamponade may not be detectable using transthoracic echocardiography in the early phase after surgery.9 The principal role of echocardiography is thus the investigation of possible alternate diagnoses.

2 Correct all easily reversible factors, including hypovolaemia, tamponade, acute myocardial ischaemia, electrolyte abnormalities and dysrhythmias.

3 Vasodilators are helpful in the presence of hypertension and ventricular dilatation. Postsurgical vasodilation after the first 4–8 hours often obviates any ongoing role for these agents.

4 Use of inotropic agents. Because of the associated vasodilatation, ino-constricting agents are frequently required, alone or in combination. Useful agents include:
• dobutamine, an essentially pure β1-agonist with inotropic, lusotropic and some chronotropic effects

• noradrenaline (norepinephrine), metaraminol, vasopressin and high-dose dopamine which, being predominantly vasoconstrictors, are hazardous in the presence of hypovolaemia and should probably not be used alone in the presence of a low cardiac output

• adrenaline (epinephrine), the metabolic effects of which (especially lactic acidosis) make usage in cardiac surgical patients problematic.10 Adrenaline is, nevertheless, a potent ino-constrictor and recent studies suggest that it is as effective and as safe as noradrenaline, in the general patient population at least

• milrinone, which has a long duration of action and potent vasodilating properties. These make milrinone relatively difficult to introduce and to wean. It is, however, a very effective inotropic agent, especially in patients with beta-receptor downregulation

• levosimendan, a novel calcium-sensitising agent, has a promising role both perioeratively and as rescue therapy for postsurgery heart failure11

5 Intra-aortic balloon counterpulsation (IABC; see below).

6 Ventricular assist devices (VAD) are expensive and technically far more demanding than IABC. They are effective in resting the heart while supporting organ and tissue oxygen delivery. Indications for implantation have not been well established but include intractable heart failure, and failure to wean from cardiopulmonary bypass in spite of full cardiovascular support. A combination of haemodynamic and cardiac support-level criteria seems most useful and early utilisation offers improved outcomes.12,13 Usage is relatively infrequent so that management expertise is difficult to acquire. Some centres thus prefer to re-establish or continue cardiopulmonary bypass as short-term, biventricular support. Most published VAD data relate to non-surgical patients in whom the role of heart assist devices is increasingly established as both ‘bridge to transplant’ and as ‘destination therapy’.
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Jul 7, 2016 | Posted by in CRITICAL CARE | Comments Off on Intensive care after cardiac surgery

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