Management of the High-Risk Surgical Patient
A high-risk surgical procedure can be considered as one in which there is an accepted postoperative mortality rate of more than 1%. This includes cardiothoracic surgery, vascular surgery and major intra-abdominal cancer surgery, either as elective or emergency procedures. There are a number of factors that can put a patient at risk from such procedures. These can be divided into two broad categories: first, the technical hazards of the surgical procedure itself, e.g. the construction of a gastrointestinal tract anastomosis and the potential for it to break down; and second, the presence of significant co-morbidities in the patient before surgery, usually of a cardiorespiratory nature, which are severe enough to cause impaired preoperative functional status. In most patients, poor outcome from major surgery arises from a combination of these factors, in which the patient with impaired physiological reserve is unable to cope with the physiological demands of the surgery, leading to multi-organ dysfunction syndrome (MODS), multi-organ failure (MOF), and death in the worst cases.
Cardiothoracic anaesthesia and emergency anaesthesia are considered elsewhere (Chs 33, 34 and 37), and this chapter will concentrate on the patient undergoing scheduled major non-cardiac surgery. However, the principles of treatment, particularly fluid management, generally apply also to the patient undergoing an emergency procedure.
Major surgery generates a systemic inflammatory response which is driven by the release of pro-inflammatory cytokines such as tumour necrosis factor (TNF) and interleukin-6 (IL-6). The magnitude of the inflammatory response, as judged by the levels of pro-inflammatory cytokines in the circulation, is associated directly with postoperative outcome, with higher concentrations of circulating IL-6 associated with an increased incidence of postoperative complications.
Pro-inflammatory responses are particularly marked in surgery involving the gastrointestinal tract, major vascular surgery and cardiac surgery. Other factors which increase the inflammatory response include the need for major blood transfusion, emergency surgery and the presence of decreased tissue perfusion, particularly in the gastrointestinal tract.
The effect of the inflammatory responses is a postoperative increase in oxygen requirements of up to 50% above basal levels. This substantial increase in oxygen demand is met normally by increases in cardiac output and tissue oxygen extraction. Most patients can meet the increased oxygen demand by increasing cardiac output and usually recover well after surgery. However, there is a group who may not have the physiological reserve to increase cardiac output to the required level and these patients are at higher risk of complications after surgery.
In addition to the systemic inflammatory response, major surgery generates a neuroendocrine stress response. Although this stress response may be attenuated to a degree, it is difficult to modify an established systemic inflammatory response and treatment strategies for the high-risk patient have relied on identifying patients early and optimizing various aspects of patient care in order to reduce risk and improve outcome.
Large-scale audits of surgical deaths in the UK have found that patients at risk are usually elderly, and 60–70% have established cardiorespiratory disease. When cardiac output monitoring is used in patients undergoing major surgery, it has been found that patients are more likely to die if they are unable to increase their cardiac output spontaneously in response to the physiological demands of the procedure. Poor outcome after major surgery is also associated with other related physiological factors which are all markers of impaired tissue perfusion, either globally or more specifically.
Because these abnormalities may manifest themselves only during surgery or in the postoperative period, the challenge for the anaesthetist is to identify high-risk patients before surgery, wherever possible.
The American Society of Anesthesiologists’ scoring system (ASA score) was developed as a guide to patient risk, and depends on a subjective assessment by an anaesthetist of the impact of co-morbidities on an individual patient’s risk. The system has only 5 categories, and the subjective nature of the assessment means that there is a large degree of variability between anaesthetists when classifying patients. In practice, it has some limited use as a rough guide, but does not have specific risk-prediction value for individual patients.
In the 1980s, Shoemaker noted that a decreased oxygen delivery (DO2) was associated with poor outcome, and described a list of clinical risk factors associated with decreased survival, known as the Shoemaker criteria.
The Physiological and Operative Severity Score for the enUmeration of Mortality and Morbidity (POSSUM score) was developed specifically to provide predicted risk scores for complications and death after surgery.
The POSSUM system assigns different scores to degrees of abnormalities demonstrated by the variables. The total scores for the physiological and operative components are entered into an equation which gives predicted percentage values for the risks of mortality and morbidity (a specified range of complications). The original POSSUM system was devised 20 years ago in a general surgical population and the values of predicted mortality and morbidity reflect the current standards of care at that time. Subsequent research has produced more procedure-specific and location-specific versions of the POSSUM score but the overall structure of the system remains unchanged. The requirement for intraoperative data severely limits the real-time use of POSSUM as a tool for predicting the risk of an individual patient prior to surgery, but it has been shown to be a useful tool for retrospective assessment of the risk of comparative patient groups for audit and research purposes.
The Revised Cardiac Risk Index (RCRI) is a more system-specific score, designed to predict the risk of a patient developing a cardiac-related complication following non-cardiac surgery. Six variables are identified as independent predictors.
The risk of cardiac events ranges from 0.4% without any factors to 11% if three or more factors are present. Although simple to use, the RCRI predicts only specific cardiac morbidity. In addition, the score was derived during the 1990s, and subsequent developments in treatment for secondary prevention of ischaemic heart disease are likely to have led to a reduction in the incidence of postoperative events, and to an over-prediction of risk.
In patients presenting for elective surgery, investigations such as full blood count, or serum urea and electrolyte concentrations, will not, in general, be of any significant value in predicting risk, although they may be useful as components of more comprehensive scoring systems such as POSSUM (see above), and they may highlight specific abnormalities, such as severe anaemia, which should be corrected before surgery.
Biomarkers are biochemical substances that can be assayed from plasma samples and abnormal levels of biomarkers may be associated with certain disease states. B-type natriuretic peptide (BNP) is a hormone secreted by cardiac cells in response to stretching of the myocardium. BNP results in increased sodium excretion and decreased systemic vascular resistance, with the net result of decreasing blood volume. BNP levels are raised in patients with heart failure and correlate with the degree of severity of the disease. In the patient undergoing major surgery, the presence of a raised concentration of BNP is associated with higher risks of adverse cardiac events and mortality after surgery, and normal levels are powerful negative predictors of complications.
Resting echocardiography in the elderly patient is useful in diagnosing and categorizing aortic stenosis, which has been highlighted as a significant problem in these patients. Normal left ventricular function in terms of ejection fraction or resting echocardiogram does not necessarily mean that the patient’s risk of developing postoperative cardiac complications is low, as a significant proportion of patients with heart failure have a preserved ejection fraction. These patients may have diastolic dysfunction or heart failure and will also have an increased risk of adverse cardiac events after surgery.
If the demand on the myocardium is increased by an infusion of a positive inotropic agent such as dobutamine, the onset of wall-motion abnormalities indicates that a patient is at increased risk of postoperative cardiac-related complications. Conversely, a normal stress response is highly predictive of a very low risk of cardiac complications. However, dobutamine stress echocardiography is not readily available, interpretation is very user-dependent, and it is useful only in predicting specific cardiac risk.
The physiological response to major surgery is a dynamic situation and assessment of the patient’s preoperative functional capacity has been recognized as a useful test with some predictive value. A patient who has limitation of cardiorespiratory reserve on exercise may be less able to elevate cardiac output in response to postoperative demands, and therefore may be considered at greater risk of complications.
Traditionally it has been taught that patients are at higher risk of complications after surgery if they are unable to perform exercise to a level that equates to 4 metabolic equivalents (METs), where MET is the energy expenditure at rest of a 40-year-old, 70 kg male. The degree of exercise equating to 4 METs would be climbing two flights of stairs. Questionnaires have been devised which allow practitioners to estimate a patient’s level of fitness, but these inevitably rely on the patient giving an accurate and honest appraisal of their levels of activity, and are therefore subject to bias.
Objective testing of exercise capacity can provide a reasonable estimate of risk. This can vary from simple stair-climbing to more formal tests such as the shuttle walk test, in which the subject walks between two cones placed 10 m apart until unable to keep pace with a timed beep. The results of the test are expressed in metres walked, and the further the subject walks, the better the outcome after surgery is likely to be.
Although simple walking or climbing tests are useful in giving an overall impression of a patient’s reserve, they have significant limitations; for example, some elderly patients have significant mobility problems of the lower limb due to osteoarthritis and may find walking difficult. The main value of these simple tests lies in their negative predictive value, which means that fit patients can be identified who are very unlikely to have complications after surgery. However, these tests give little useful information about the underlying causes of impairment of functional capacity in patients who do not perform well in the test. This is an important limitation, because some patients perform poorly due to an underlying disease state, most commonly cardiac impairment, whilst others perform poorly simply through being out of condition due to lack of physical activity. Recognition of the cause of decreased functional capacity may allow effective preoperative interventions to improve the patient’s performance and reduce the risk of surgery. To evaluate patients in this way, more sophisticated testing is required.
During CPET, a patient undergoes exercise of increasing intensity which requires an increase in metabolic activity in the exercising muscles, which in turn demands increases in ventilation and cardiac output. Limitations in either respiratory or cardiac function (or both) result in decreased oxygen uptake by the exercising muscles.
CPET is performed with the patient pedalling on a static bicycle with a flywheel to which increasing resistance is applied. During the test, oxygen uptake and carbon dioxide production are measured using a metabolic monitoring cart. A 12-lead ECG is recorded simultaneously. Cardiorespiratory performance during exercise is usually defined by the measurement of oxygen uptake by the tissues (oxygen consumption, VO2), either as the maximum oxygen uptake measurable (VO2max), or as the oxygen uptake at the onset of lactate production, commonly known as the anaerobic threshold (AT). Other useful parameters obtained include measures of ventilatory efficiency and the ability to diagnose myocardial dysfunction from heart failure or ischaemic heart disease.
AT occurs usually at 50–60% of VO2max, and is independent of patient motivation. If the AT is the prime objective of the CPET, the test can be stopped after it has been reached, and this may be advantageous in the frail or elderly surgical patient. This variant of CPET is known as a submaximal test, as the intention is not to test the patient to the maximum effort.
AT is identified as the point at which there is onset of lactate production through the activation of anaerobic pathways. The lactate produced is buffered by bicarbonate to produce water and carbon dioxide. The net effect is an increase in the slope of the graph of carbon dioxide production relative to oxygen uptake (Fig. 23.1