PREOPERATIVE ASSESSMENT OF ELDERLY SURGICAL PATIENTS 

Paul Older        MB, FRCA, FANZCA, FFICANZCA, FJFICM  
Adrian Hall,      MB, FANZCA, FJFICM 

The assessment of cardiac risk for non-cardiac surgery

ACC/AHA Guidelines

alternatives to cpet (cpx) as a screening test

diagnosis of myocardial failure and cardiac failure by cpet (cpx)

incidence and significance of myocardial ischaemia and cardiac failure

influence of cardiopulmonary exercise  on selection of surgical patients

current patient triage system

references

Introduction 

In the late 1970's and early 1980's there was an explosion in the use of high technology in industry. Medicine, traditionally conservative, did not rapidly embrace this new ‘industrial revolution’. Certainly in most hospitals at that time, the Intensive Care Units (ICU) relied more on clinical expertise than invasive measurements and high technology. The mid 1980’s saw a large increase in the use of invasive monitoring systems in the treatment of critically ill patients. Much of the workload of ICU’s at that time comprised patients with post surgical complications. However, the Intensive Care Specialists were not involved until late in the postoperative period when the patient had already deteriorated on the ward. Patients were only admitted to ICU when they were acutely ill with multisystem failure and a high APACHE score ⁽¹⁾. The delays in referral and definitive treatment resulted in a high mortality ⁽²⁾. Furthermore the duration of stay was excessively prolonged due to high morbidity. The latter was a major drain on Intensive Care resources and remains so today.

Clowes and Del Guercio identified the fact that perioperative mortality was related to poor ventricular function in 1960 ⁽³⁾ but this work was clearly not known to a wide audience. Shoemaker elaborated on this concept in 1972 by characterizing the differing physiologic variables in surviving and non-surviving surgical patients (4). Goldman et al published their seminal article on clinical evaluation of cardiac risk in non-cardiac surgical patients in 1977 ⁽⁵⁾. Despite this information, throughout the seventies, the concept of risk identification and subsequent triage of patients to differing levels of perioperative management was not practiced. It was not until Professor Louis Del Guercio and Joseph Cohn used pulmonary artery catheters to identify poor ventricular function preoperatively in 1980 ⁽³⁾ that any concept of preoperative identification of risk based on physiological measurement was developed.

In 1988 our group published the results of preoperative assessment of physiological variables by pulmonary artery catheter ⁽⁷⁾. This study was the genesis of our integrated approach to perioperative risk assessment and management. Before our study the documented mortality rate at our hospital was 19%; this information was obtained from a retrospective analysis of major abdominal surgery in patients over 65 years from 1980 to 1983.  Post-surgical management at that time followed the model described above. 

Over a period of three years, starting in 1984, we studied 100 elderly patients scheduled for major surgery. Using similar methods to Del Guercio we used pulmonary artery catheters preoperatively, as a screening test, to identify patients at increased surgical risk. At that time the pulmonary artery catheter had been in clinical use in Australia for only six years and was not widely utilized by Intensive Care Physicians. 

This study highlighted three major issues. Firstly, mortality compared to the three preceding years had been improved from 19% to 6%. The only change was that the patients were managed perioperatively by Intensive Care Specialists in an Intensive Care Unit (ICU). Secondly, 11% of the patients had severe ventricular dysfunction with a resting cardiac index of less than 2.2 ml/min/M². Thirdly, as no pre- or postoperative problem was demonstrated in many of the patients who had been admitted to ICU, an expensive resource was being wasted.

In 1987 The Confidential Enquiry into Perioperative Deaths (CEPOD) ⁽⁸⁾ was published and demonstrated in a series 500,000 patients that postoperative deaths occurred predominantly in patients over 70 years of age having major surgery with preexisting cardiac or pulmonary disease. Three main points need emphasis, elderly patients; major surgery; preexisting disease. This is the same group of patients identified by Goldman et al. in 1977 ⁽⁵⁾. We saw the need for a better method of evaluation of surgical risk. We hypothesized that exercise gas exchange could be employed to achieve this.

In 1988 we began to use Cardiopulmonary Exercise (CPX) testing to look for preexisting cardiac and pulmonary disease preoperatively on a routine basis. The major consideration was that it was to be used as a tool for the routine evaluation of all elderly patients scheduled for major surgery, i.e. a screening test. We suspected that cardiopulmonary disease exists in an occult state in the elderly. If the test was restricted to those with clinically detectable heart disease then ‘occult’ disease would be missed and we would miss the very group we were trying to identify.

The increase in oxygen uptake under exercise conditions, as a concept, was not new; it had been understood since the late 1700’s. Antoine Laurent Lavoisier, the man who named ‘oxygen’, demonstrated the increase in air consumption of a subject lifting a 15 pound weight an accumulated height of 600 feet in 15 minutes. In May 1794 he went to the guillotine for his work; “the revolution has no need of scientists”. The direct measurement of oxygen uptake as a non-invasive clinical procedure had to wait for the development of the Metabolic Cart which became available in the early 1980’s.  

Under conditions of exercise, oxygen uptake is a direct function of cardiac output. Using a Metabolic Cart it is easy to evaluate cardiac and pulmonary performance, during exercise, by measurement of oxygen consumption and other parameters by respiratory gas exchange measurements. This is the basis of CPX testing. As it is non-invasive, cheap to perform and may be performed on out-patients, it offered us the ideal method of screening patients for cardiopulmonary risk assessment. Such testing need not be to maximum exercise capacity but may be stopped shortly after the anaerobic threshold (AT) is reached. This point is easy to detect during the test and is non volitional. 

The assessment of cardiac risk for non-cardiac surgery 

Age is not a predictor of individual risk. In terms of group statistics AT decreases with advancing age (Figure 1), but one standard deviation embraces the population from the age of 55 years to 85 years. 

Professor Wasserman has pointed out that we age at different rates and therefore the use of age as a discriminator will result in fit elderly patients being denied life saving surgery whilst allowing a younger patient with occult cardiovascular disease to proceed to surgery, only to succumb to it ⁽⁹⁾. 
Clinical assessment of risk will detect a large group of patients with overt cardiopulmonary disorders but can miss those with more occult disease. CPX testing will detect the group that such clinical examination will miss.
For many years attention has focused preoperatively on detection of myocardial ischemia. Some published studies have made assumptions of myocardial ischemia on the basis of a history of risk factors ^(10,11) without any proof of the diagnosis. This approach is flawed from two perspectives. Firstly some patients who are asymptomatic and have no ‘risk factors’, may have myocardial ischemia. Secondly not all patients with risk factors have coronary artery disease. Other studies have not distinguished the type of surgery for which the patient is scheduled ⁽¹¹⁾. As will be discussed later, this is an important issue.

Little attention has been paid to issues of cardiac failure ⁽¹²⁾ and postoperative stress response, despite the original work of Goldman, Clowes, Del Guercio and Shoemaker.

We hypothesize that the major determinant of perioperative mortality is the inability of the heart to increase output to match the increase in oxygen demand mandated by major surgery. Myocardial ischemia may be a cause of cardiac failure or may be caused by an increase in myocardial oxygen demand consequent on an increase in cardiac output. In two published studies involving over 700 elderly patients we showed that mortality is a function of cardiac failure ^(13,14). Certainly the association of myocardial ischemia with cardiac failure increases the risk but patients exhibiting myocardial ischemia in the absence of cardiac failure appear not to be at risk. RETURN TO START

The ACC/AHA Guidelines ⁽¹⁵⁾ 

These guidelines were published in 1996 and are a Consensus document published by a Taskforce. The patient population is classified into three risk groups on the basis of cardiac risk. These are low, intermediate and high risk. The low risk group includes those under 60 years of age with no history of cardiopulmonary disease whilst the high risk group includes patients with acute coronary syndromes, decompensated cardiac failure and supraventricular arrhythmias. There could be little argument about these groupings. The problem lies in the intermediate group where clinical indicators are unreliable, age is a poor discriminator and occult cardiac failure and/or myocardial ischemia do occur. In our own studies some 25% of patients over 60 years of age have an AT below 11 ml/min/kg and 25% have silent myocardial ischemia ^(13,14,25). Whilst some patients fall into both groups either pathology may exist independent of the other. It is this group where CPX testing will reveal such problems. In our studies myocardial ischemia was only detected in 35% of the patients at risk ⁽¹³⁾. 

The guidelines also introduce the important concept of ‘surgery specific risk’. This concept is based on the fact that minor surgery (e.g. peripheral surgery) is not associated with the large increase in oxygen demand of major intracavity surgery. We prefer to define ‘surgery specific risk’ in terms of postoperative increase in oxygen consumption (VO₂). We classify low risk surgery where the postoperative VO₂ is likely to be less than 120 ml/M²; intermediate where the VO2 is likely to be between 120-150 ml/M² and high risk where it is likely to be in excess of 150 ml/M². We have demonstrated the average VO₂ following major intra-abdominal surgery is 170 ml/M². ⁽⁷⁾ 

The guidelines endorse the use of maximal exercise capacity estimated as METS. One MET or metabolic equivalent is the VO₂ of a resting 40 year old 70 kg male and is approximately 3.5 ml/min/kg. Whilst we agree in general terms with these guidelines we feel that estimation of functional capacity in terms of METS leads to unacceptable inaccuracy. The guidelines suggest that patients unable to reach 4 METS are at increased risk and our work suggests that patients unable to meet a 3 MET demand at AT, are at an increased risk. Most estimates of METS are based on treadmill exercise studies where METS are literally estimated not measured. The “Clinical exercise stress testing - Safety and performance guidelines” as published in the Medical Journal of Australia in 1996 ⁽¹⁶⁾ suggest that METS may be estimated from nomograms. This statement we would challenge as such estimations are woefully inaccurate. Kleber has acknowledged that CPX testing is the ‘gold standard’ for evaluation of cardiac failure ⁽¹⁷⁾ and the guidelines state that CPX testing has proved to be reliable and important in evaluation of patients with heart failure. Clearly if one is able to accurately measure VO₂ then one has the ‘gold standard’ for evaluation of cardiac failure. In fact the ACC/AHA guidelines for exercise testing state “One of the strongest and most consistent prognostic markers identified in exercise testing is maximum exercise capacity, which is at least partly influenced by the extent of resting left ventricular function and the amount of further left ventricular dysfunction induced by exercise”.   RETURN TO START

The alternatives to CPX testing as a screening test

We have recently published a review article in which we evaluate various preoperative screening tests ⁽¹⁸⁾.  Exercise ECG testing is by far the most common test prescribed preoperatively. It can evaluate myocardial ischemia and estimates METS. One author suggests that a patient who achieves an estimated 7 METS or a heart rate >130 is low risk ⁽¹⁹⁾. This equates to a VO₂ of 25 ml/min/kg. In our series of elderly patients the average peak VO₂ for 900 patients is 15.2 ml/min/kg (Table 1). 

These errors occur because of estimation of METS rather than actual measurement. 
A much more sensitive test than this is required. 
Ejection fraction determinations, by any means, do not correlate with aerobic capacity, with or without myocardial ischemia ^(20,21). A low ejection fraction with an enlarged heart may well be associated with a normal stroke volume.

Transthoracic echocardiography is non invasive and easy to perform but the Perioperative Ischemia Research Group failed to support the use of this technique in assessment of cardiac risk preoperatively ⁽²²⁾. The same group found that dipyridamole-thallium scintigraphy was not a valid screening test for detection of postoperative cardiac events – even in vascular surgery patients ⁽²³⁾.

Dobutamine stress echocardiography has good sensitivity for detection of myocardial ischemia but does not allow for accurate assessment of functional capacity. The test is very expensive and interpretation is operator dependent ⁽¹⁹⁾.

CPX testing has the advantage of being low cost, noninvasive and applicable to most patients regardless of age It is quick to perform, and able to detect both myocardial ischemia and to evaluate cardiac function and respiratory function objectively. The patient is not able to ‘cheat’ in that there is no volitional component to the test. Finally the test is extremely repeatable and minimally open to operator interpretation.  RETURN TO START 
Diagnosis of Myocardial Ischemia and Cardiac Failure by CPX testing

CPX testing involves the computerized analysis of gas exchange data. The computer is usually configured by the operator to perform many different analyses. 
Evaluation of cardiac function is usually performed by determination of the anaerobic threshold (AT), the peak VO₂, the heart rate/VO₂ relationship and the work rate/VO₂ relationship. Peak VO₂ is not the same as Vmax and many people fail to distinguish between the two. Vmax is defined as the point, where despite increases in work rate, there is no increase in VO₂. Peak VO₂ is the highest VO2 reached during a specific test and may or may not be Vmax. It is extremely rare for any of our patients to reach Vmax and thus most exercise testing performed on the elderly would be best described as symptom limited.
We define cardiac failure in terms of the oxygen consumption at AT, using the classes suggested by Weber and Janicki ⁽²⁴⁾ (Table 2).

Respiratory function is evaluated by such relationships as the Ve/VO₂ and analysis of flow volume loops during exercise as well as at rest.
In our laboratory the patient is always monitored via a computerised ECG monitor (Mortara ELI-100XR, Mortara Instruments, Milwaukee W1). This machine gives an interference free 12 lead display as well as tracking and storing ST depression and slope. This data is printed out at the end of each test. The X-axis of all printouts is time in minutes, thus we are able to see at which point in the test ST depression occurred and to establish the extent of that depression in millimeters. The criterion used for diagnosis of myocardial ischemia was more than 1 mm ST depression 60 millisecs after the J-point. This was determined by the Mortara ECG machine and was not therefore subject to observer bias. In a previous study ⁽²⁵⁾ a cardiologist, blinded to both the CPX test and the Mortara report, had reported all ECG’s thus validating the computer generated reporting. RETURN TO START   

Incidence and Significance of Myocardial Ischemia and Cardiac Failure 

The incidence of ischemia, using the above criteria, has been remarkably constant at about 25% throughout our studies. (52 out of 214 – 24.3%; 44 out of 187 – 23.5%) ^(25,13). In our latest study 51 out of 186 (27%) patients tested showed myocardial ischemia. Symptomatic angina is very uncommon; almost all ischemia being diagnosed on the ECG criteria. Some patients developed a supra ventricular tachycardia which gave rise to symptoms thus limiting the test. 

The average oxygen consumption during exercise of 850 elderly surgical patients, at peak was 15.2 ml/min/kg with an AT of 12.4 ml/min/kg. The oxygen extraction ratio (OER) of elderly patients during exercise averages over 75%. Even following major abdominal surgery oxygen consumption rarely exceeds 7 ml/min/kg (250 ml/M²), however OER is normally only 30%. For these reasons a direct comparison of exercise VO₂ and post surgical VO₂ is impossible but, for any specified oxygen consumption, post surgery the cardiac output would need to be a minimum of 2.5 times higher than for the same oxygen consumption during exercise. To place this in perspective in a study we published in 1989 in which a group of nine patients were exercised and monitored both by pulmonary artery catheter and metabolic gas exchange. This showed that average cardiac output increased from 4.6 l/min (SD +/-0.3) to 9.3 l/min (SD +/-0.6) after 4 minutes work at 50 watts. The oxygen consumption index rose from 114 ml/min/M² (SD +/-5) to 515 ml/min/M² (SD +/-30) ⁽²⁶⁾.  

In 1993 we pointed out that just the presence of myocardial ischemia did not correlate with postoperative mortality ⁽¹³⁾ (Table 3). This study showed that if ischemia was associated with an AT of less than 11 ml/min/kg, then mortality was 42%. If the AT was greater than 11 ml/min/kg then mortality was only 4%. In 1999 we showed that of nine cardiovascular deaths in 548 elderly surgical patients only three had myocardial ischemia whilst seven of this nine had cardiac failure defined as an AT less than 11 ml/min/kg. No patient with an AT greater than 11 ml/min/kg died even if they had associated myocardial ischemia ⁽¹⁴⁾.

Myocardial ischemia will tend to limit ventricular function. If ischemia occurs early in exercise it is deemed the cause of the cardiac failure. If however ventricular function is good and the ischemia occurs late in exercise then it is viewed differently, then the exercise is deemed to have caused the ischemia. In reality this is the same issue; myocardial ischemia will finally limit ventricular function. One could argue that the limiting factor in all exercise is cardiac failure – whether that is at 20 watts on a bicycle for an elderly patient or the inability to run a marathon in less than two hours in an athlete. The difference is relative. Myocardial ischemia is merely one cause of cardiac failure at whatever level of workload it occurs. Clearly if the surgery causes a rise in myocardial oxygen demand to the equivalent point in exercise where ischemia became apparent, that patient is more likely to develop myocardial ischemia than the patient in whom ischemia occurred at a much higher work rate.

From the viewpoint of clinical relevance, myocardial ischemia during CPX testing may be divided into two main groups. The first group exhibits ischemia generally within two minutes of the onset of exercise. The second group does not show ischemia until levels of exercise approaching or exceeding the AT ⁽²⁵⁾. In those patients in whom the ischemia occurred early, the average AT was 10.4 ml/min/kg compared to those patients with late ischemia of 13.9 ml/min/kg. Our latest, study which examined this issue in detail, showed a similar pattern (Table 4).  

Early ischemia is therefore associated with cardiac failure, Classes C and D, as defined by Weber and Janicki ⁽²⁴⁾. 

From examination of Table 3 it will be clear that the association of myocardial ischemia and an AT of less than 11 ml/min/kg has a high morbidity. The presence of myocardial ischemia, as a sole variable, did not influence mortality. This data was from a paper published in 1993 ⁽¹³⁾, before any patient selection bias was apparent. The implication of this is discussed later. 

Table 5 shows the significance of early myocardial ischemia compared to late in the incidence of postoperative events. These 51 patients, with ischemia, represented 27% of our series of 186. The cardiovascular mortality/morbidity comprised of four patients; three needing treatment of significant tachyarrhythmia. One of these three suffered an AMI and survived. The fourth patient suffered an AMI and died. 

If myocardial ischemia is the precipitating factor for morbidity or mortality then one would expect this would occur predominantly in the group with ischemia, regardless of whether or not the patient had cardiac failure. 

If cardiac failure is the precipitating factor then one would expect the morbidity and mortality to occur predominantly in the group with cardiac failure, regardless of whether or not the patient had ischemia.

Reference to Tables 3 and 5 shows that the mortality and morbidity occurs in the group with cardiac failure i.e. AT less than 11 ml/min/kg.

 In our 1999 study of over 700 patients ⁽¹⁴⁾ only one patient died of a myocardial infarction. Of the 9 cardiac related deaths in this study, only two patients had myocardial ischemia preoperatively as shown by CPX testing. Eight patients died from cardiac complications and of these six had an AT of less than 11 ml/min/kg. Certainly early ischemia and a low AT is the worst combination but even then the main cause of death is cardiac failure; rather than an acute myocardial event.

It is our contention, supported by analysis of over 2000 patients since 1990, that cardiac failure is the precipitating factor for postoperative morbidity and mortality. Myocardial ischemia occurring late in exercise with otherwise good ventricular function does not appear to be associated with either morbidity or mortality. The explanation for this is that following surgery, the myocardial oxygen demand in many patients may not reach levels associated with myocardial ischemia, even allowing for the increased myocardial work consequent on the postoperative rise in VO₂. RETURN TO START 

The influence of CPX on selection of patients for surgery 

The first paper that we published relating to CPX and surgical mortality was in 1993 ⁽¹³⁾. At that time there was no selection process either by us or the surgeons as the concept of using CPX was new; consequently the patients went to surgery with knowledge of their CPX test result but without understanding the risk involved. Consequently many patients with an AT well below 11 ml/min/kg as well as ischemia underwent major surgery. The mortality of patients with a low AT and ischemia was very high (Table 3). At that time we were unable to accurately assess the temporal relationship between ischemia and AT  as we were able following the introduction of the Mortara ECG machine in 1993.

By the time our second paper was published in 1999 ⁽¹⁵⁾ the surgeons were beginning to be influenced by mortality figures and the CPX test result. In other words a selection bias was evolving. Initially this was an occult process but currently it has gained considerable momentum to the point where the CPX test is often performed before the definitive surgical procedure is scheduled. Surgeons are now influenced by the presence of an AT in single figures associated with early myocardial ischemia.  These patients are now referred to cardiologists with a view to angiography and possible myocardial revascularisation.  

In our latest study (Table 5)  

it is apparent that only 7 patients out of 186 were operated on with a combination of an AT of less than 11 ml/min/kg and early myocardial ischemia. In 1993 (Table 4) there were 19 out of 187 operated on with ischemia and an AT of less than 11 ml/min/kg. 

These changes are almost certainly secondary to the process of selection by the surgeons as we make no effort to influence the decision for surgery. In other words patients who are tested and found to have early ischemia and a low AT only proceeded to operation if there is no alternative.  

A selection process would only be vindicated if mortality figures actually improve by its use. Current mortality from cardiovascular causes has consistently improved at our hospital since 1988 (Table 6). 

RETURN TO START   

Current patient triage system 

To be of value a comprehensive evaluation should assess the patient risk and include recommendations for patient management based on these findings.

Up to the completion of this, our latest study, we have used a triage system for patients following CPX as shown in Figure 2. below

The basis of this is that the high risk patients i.e. those with an AT of less than 11 ml/min/kg (with or without ischemia) always were admitted to ICU and invasively monitored via a pulmonary artery catheter. Patients defined as having ‘surgery specific risk’ e.g. abdominal aortic aneurysms were also admitted to ICU. Those with an AT better than 11 ml/min/kg but with concomitant ischemia were admitted to a post surgical high dependency area (HDU) for non invasive monitoring.

Our latest study shows that the latter group do not develop post surgical cardiovascular complications. Consequently we have now adopted a new triage system shown in Figure 3, in which only patients with a low AT are admitted to ICU or HDU. Those patients with a low AT and myocardial ischemia are routinely referred to the cardiologists for assessment via angiography where indicated. Patients with an AT greater than 11 ml/min/kg are sent to the ward if they do not comply with the definition of ‘surgery specific risk’.

  RETURN TO START  

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