The objective of pre-operative evaluation is to provide information on the ability of the patient to respond to increased metabolic demands during and after an operation. A useful pre-operative test will help triage the patient according to operative risk, and will help direct treatment in order to improve post-operative outcome. This test should be cost effective, easily performed on all patients and simple to interpret. Pre-operative risk assessment should include evaluation of surgical risk, the functional capacity of the patient, and patient-specific risk.
Traditionally, pre-operative assessment of patient-specific risk has focused on detecting and quantifying ischaemic heart disease and cardiac failure. This is because the inability to increase cardiac output post-operatively is associated with increased mortality, and maintaining mean cardiovascular parameters in surgical patients is associated with improved outcome.
Ischaemic heart disease and cardiac failure has, until now, been detected using a combination of comprehensive medical history and the results of tests such as resting electrocardiogram, cardiac echocardiogram, angiogram, thalium scans and stress tests (dobutamine and exercise). These tests can provide useful information on underlying problems, but tend to be limited in their ability to quantify the impairment associated with it.
Due to a high rate of false negative and false positive results, there is some concern that current non-invasive screening for cardiac disease in non-cardiac surgery is not predictive of post-operative risk in low risk groups. Furthermore, it has been suggested that no test adequately mimics the physiological stress response to surgery. Major surgery is associated with an increased resting oxygen consumption of up to 44 per cent. For this increase in oxygen consumption to be maintained, the patient must be able to increase oxygen delivery, which is dependent on not only cardiac output, but also the adequacy of the lung, the pulmonary circulation and the peripheral circulation. If the oxygen demand cannot be met by the cardiovascular unit during incremental exercise (as seen when exercising anaerobically), exercise cannot be maintained indefinitely due to the build-up of fatiguing by-products.
The same concept can be applied to the hyper-metabolic post-operative period: if an imbalance between oxygen demand and oxygen delivery occurs as a result of any impairment of the cardiovascular system, cellular function will be impaired, resulting in an increased likelihood of post-operative mortality. Therefore, by testing a patient’s exercise tolerance in an objective and quantitative manner, their ability to respond to post-operative metabolic stress is also being tested.
Measurement of pre-operative exercise capacity has focused on patient reports and on ECG stress testing, during which a prediction of work rate is made. Other methods include the use of step tests and walking tests. These tests rely on estimations and are more often than not based on values measured during weight-bearing exercise. Subsequently, someone who is overweight will be working harder at the same work rate, since they must carry additional bodyweight. As a result of this, their exercise tolerance may be underestimated.
Furthermore, some commonly-used prediction equations use a sub-maximal heart rate measurement and age-predicted maximum heart rate in order to predict maximum exercise tolerance. This is based on the assumption that there exists a linear relationship between oxygen consumption and heart rate during incremental exercise. In patients with severe lung or heart disease, this relationship is not linear and often these patients will not be able to attain the age-predicted maximum heart rate, so exercise tolerance may be over-estimated.
Cardiopulmonary exercise testing (CPX) is now perceived as the gold standard for the assessment of cardiopulmonary function by organisations such as the American Heart Association, American Thoracic Society and the American College of Chest Physicians. CPX involves the measurement of expired gas during a graded exercise test, which is normally performed on a cycle ergometer. Variables measured include oxygen consumption, carbon dioxide production and ventilation. This data is then used for bivariate graphical analysis, which makes it possible to relate the rate of change of one variable with the rate of change of the other. In addition, a simultaneous and continuous 12-lead ECG is monitored and recorded, thereby allowing standard stress ECG interpretation. Exercise tolerance can also be objectively determined by measuring oxygen consumption directly.
CPX testing has a very low complication rate. The incidence of mortality is as low as two to five deaths per 100,000 exercise tests (American Thoracic Society / American College of Chest Physicians 2003) and this risk may be reduced even further if sub-maximal tests are used.
The anaerobic threshold (AT) is one physiological parameter measured during CPX testing. It is the point at which oxygen supply cannot keep up with demand from the exercising muscles. Anaerobic threshold is independent of the motivation of the patient and occurs well before maximum exercise tolerance. Since this measure does not require high levels of physical stress, it should result in the inclusion of patients who it may not have been able to test readily or accurately.
Values for normal populations are well described, as are guidelines for grading the severity of illness, prognosis and acceptance on to transplantation lists in several patient groups, including those with heart failure. The anaerobic threshold has also been found to be highly predictive of post-operative mortality.
One study demonstrated that when the anaerobic threshold was greater than 11 ml/min/kg, the chance of survival from major surgery was 99.2 per cent. However, if the anaerobic threshold was less than 11 ml/min/kg coexisting with myocardial ischaemia, survival was 58 per cent. From this we can conclude that in the absence of cardiac failure (an AT of >11ml/kg/min) as measured by CPX, ischaemia was not a potent cause of mortality in major surgery. Overall mortality in this study was 5.9 per cent.
In another study, patients were triaged to the ward, high-dependency unit or the intensive care unit according to anaerobic threshold, the presence or absence of ischaemia and by the severity of the stress response expected as a result of surgery. Fifty per cent of patients were sent to the ward. No cardiopulmonary deaths occurred in those patients deemed fit for surgery by CPX testing and triaged to receive care on the ward post-operatively. Overall mortality was 3.9 per cent. In addition, intensive care bed usage dropped from 600 bed days per 100 patients to just 60. The length of ICU stay was also reduced.
A CPX test is simple for the patient to perform, non-invasive and can be completed by virtually all patients. Furthermore, the cost of performing a CPX is lower than an exercise echocardiogram and dobutamine stress test comments on patient specific surgical risk. Therefore, CPX testing is a useful pre-operative test that could provide the clinical care team with information that could direct treatment in an objective manner.
