Core Topics in General & Emergency Surgery: Companion to Specialist Surgical Practice (74 page)
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Authors: Simon Paterson-Brown MBBS MPhil MS FRCS
In 1963 the American Society of Anesthesiologists (ASA) adopted a five-point classification system for assessing the physical status of a patient prior to elective surgery. A sixth category was added later (
Box 15.4
).
Box 15.4
ASA classification
A normal healthy patient
A patient with mild systemic disease
A patient with severe systemic disease
A patient with severe systemic disease that is a constant threat to life
A moribund patient who is not expected to survive without the operation
A declared brain-dead patient whose organs are being removed for donor purposes
Note: If the surgery is an emergency, the ASA grade is followed by ‘E’ (for emergency), for example ‘3E’. Category 5 is always an emergency so should not be written without ‘E’.
The ASA grade is essentially a combination of the subjective opinion of the anaesthetist taken in conjunction with a more objective assessment of the patient's general fitness for surgery, and is used routinely in most centres in the UK. There are a number of studies assessing the utility and accuracy of the ASA grade in determining surgical risk and, as anticipated given the nature of this scoring system, the literature is conflicting.
One study of 113 anaesthetists in the UK demonstrated such marked variation in the inter-individual assessment of 10 hypothetical patients that the authors concluded that the ASA grade should not be used on its own to predict surgical risk.
29
A further study of 97 anaesthetists demonstrated that the agreement for the assessment of each hypothetical patient varied from 31% to 85%. The overall correlation was only
fair
, and the inter-observer inconsistency was similar to that in a study from 20 years previously.
30
However, the largest study to date is more encouraging.
31
Of 16 227 patients undergoing elective surgery over a 5-year period, 215 died within 4 weeks of operation. There was a significant correlation between perioperative mortality and the ASA grade. The mortality was lowest (0.4%) when the ASA grade was less than or equal to 2 and increased up to 7.3% in ASA grade 4 patients. The authors concluded that perioperative mortality can be predicted using the ASA grade.
The ASA classification remains a quick, simple, widely used and reasonably accurate assessment of surgical risk in both the elective and emergency settings.
The
surgical mortality probability model
(SMPM) was derived from a retrospective data analysis of almost 300 000 patients using the American College of Surgeons National Surgical Quality Improvement Program Database.
32
The primary outcome was 30-day mortality for patients undergoing non-cardiac surgery. The model identified three risk factors – ASA status; emergency or elective surgery; and surgery risk class – as the main determinants of outcome. Points are allocated in accordance with these three factors and predicted 30-day mortality is calculated from the total score (
Table 15.2
). Patients with a total risk score less than 5 had a predicted mortality of less than 0.5%, whereas a risk score greater than 6 predicted a mortality of more than 10%. This new model has yet to be fully externally and prospectively validated but may prove to be a useful risk tool for the future.
Table 15.2
The surgical mortality probability model (SMPM)
| Risk factor | Points |
| ASA | |
| I | 0 |
| II | 2 |
| III | 4 |
| IV | 5 |
| V | 6 |
| Procedure severity | |
| Low | 0 |
| Intermediate | 1 |
| High | 2 |
| Urgency | |
| Elective | 0 |
| Emergency | 1 |
| Total points | Mortality risk |
| 0–4 | < 0.5% |
| 5–6 | 1.5–4% |
| 7–9 | > 10% |
The SMPM was developed to predict 30-day mortality for patients undergoing non-cardiac surgery. The model utilises three risk factors (ASA status, severity of procedure and urgency of the procedure). A score is awarded for each variable and the mortality risk is calculated form the total score.
The surgical mortality probability model (SMPM) is a newly devised risk assessment tool to predict 30-day surgical mortality. It has the advantage of using simple data readily available at the bedside. Whether the SMPM becomes widely established as a risk model will depend on the results of future validation studies.
The risk models described thus far have been designed to predict general mortality and morbidity for a patient population. Some risk models have been developed with the aim of predicting
specific
complications, such as risk of cardiac or pulmonary complications in the postoperative period. Of these, the
Revised Cardiac Risk Index
(RCRI) is the most commonly used. This model was published in 1999 with the aim to develop an index of risk for cardiac complications in major elective non-cardiac surgery.
33
Six independent predictors of complications were identified and included high-risk type of surgery, history of ischaemic heart disease, history of heart failure, history of stroke, diabetes requiring insulin, and elevated baseline serum creatinine. The risk of myocardial infarction and cardiac death could then be predicted according to the number of risk factors that were present (
Table 15.3
,
Fig. 15.1
). A recent systematic review was performed to evaluate the current accuracy of the RCRI to predict cardiac complications and death after non-cardiac surgery.
34
The authors concluded that the RCRI discriminated moderately well between patients at low versus high risk for cardiac events, but it did not perform well at predicting cardiac events after vascular surgery or at predicting death within 30 days.
Table 15.3
The Revised Cardiac Risk Index: six risk factors to predict mortality and cardiovascular complications following surgery
| Risk factor | Number of risk factors | Risk of death/ myocardial infarction |
| Major (high risk) surgery | 0 | 0.4% |
| History of ischaemic heart disease | 1 | 1% |
| History of heart failure | 2 | 2.4% |
| History of cerebrovascular disease | 3 | 5.4% |
| Diabetes requiring insulin treatment | ||
| Serum creatinine concentration > 177 μmol/L |
Figure 15.1
Risk of major cardiac complications predicted by the Revised Cardiac Risk Index according to type of surgical procedure performed. The greater the number of risk factors present, the greater the risk of complications, irrespective of the type of surgery undertaken.
Reproduced from Lee TH, Marcantonio ER, Mangione CM et al. Derivation and prospective validation of a simple index for prediction of cardiac risk of major noncardiac surgery. Circulation 1999; 100(10):1043–9. With permission from Wolters Kluwer Health.
There are many other published risk prediction models that have not been presented here for reasons of brevity. Despite showing initial promise, many of these models have been disappointing when validated against external patient populations. The inability of these models to reproduce initial predictive accuracy has resulted in many failing to gain widespread acceptance. It is fair to say that at present there is no single model that can accurately predict surgical risk for all patient populations.
Assessment of exercise capacity provides useful information about the functional status of a patient and their response to physiological stress. This information can then be used to inform about how the patient might respond to surgical stress and may therefore be used to predict perioperative risk. Patients with higher exercise tolerance usually have lower risk. Evaluation of exercise capacity may be subjective or objective, where formal exercise testing is performed.
Subjective assessment of exercise tolerance can usually be easily undertaken by asking some simple questions to assess the functional capacity of the patient. Determination of how many stairs a patient can climb before stopping due to limitation by symptoms, or how far they can walk on the flat without stopping, are commonly employed questions. There is evidence to suggest that these simple assessments of exercise capacity correlate with surgical risk. In a study of 600 patients undergoing major non-cardiac surgery serious postoperative complications, especially cardiac complications, were twice as common for those patients who were unable to climb two flights of stairs preoperatively.
35
Inability to climb two flights of stairs was associated with a positive predictive value of 82% for the development of cardiopulmonary complications in patients undergoing major thoracic and abdominal surgery and stair-climbing ability was inversely related to duration of hospital stay.
36
Exercise capacity may be more objectively measured.
Metabolic equivalent of tasks
(METs) is a measure of energy expenditure related to physical activity. One MET may be considered as the resting metabolic rate (RMR) and is defined as energy consumption at a rate of 3.5 mL O
2
per kg per minute. Physical activities may be measured as a ratio compared to the RMR. For example, ironing clothes is equivalent to 1.8 METs and climbing two flights of stairs is equivalent to 4 METs. Some further examples are given in
Table 15.4
. This process may then be used to identify patients with reduced exercise capacity and who may benefit from a more objective assessment of their functional status. A full list of physical activities and the MET equivalents can be found at the web address listed in Ref.
37
.
Table 15.4
Examples of common activities and their metabolic equivalents (METs)
| Activity | MET value |
| Watching television | 1 |
| Showering | 2 |
| Playing the piano | 2.3 |
| Washing the dishes | 2.5 |
| Playing snooker | 2.5 |
| Walking the dog | 3 |
| Slow ballroom dancing | 3 |
| Lawn bowls | 3 |
| Moderate housework | 3.5 |
| Climbing two flights of stairs | 4 |
| Golf (using an electric cart) | 3.5 |
| Golf (carrying clubs) | 4.3 |
| Mowing the lawn | 5.5 |
| Moderate swimming | 5.8 |
| Jogging | 7 |
| Running (10 minute/mile pace) | 9.8 |
Cardiopulmonary exercise testing
(CPEX) has been introduced in an attempt to provide a measurable, objective assessment of cardiorespiratory function for the assessment of surgical risk. In 1993 Older et al. performed CPEX testing among a group of elderly patients undergoing major surgery. An anaerobic threshold (AT) of less than 11 O
2
mL/min/kg was associated with a mortality rate of 18% compared with a mortality rate of less than 1% for those patients with an AT greater than 11 O
2
mL/min/kg.
38
Subsequent studies have similarly shown that an AT of less than 11 O
2
mL/min/kg was associated with increased hospital mortality following major elective abdominal and vascular surgery.
39
,
40
CPEX testing was also predictive of longer-term outcome. In a study of 102 patients undergoing elective abdominal aortic aneurysm repair, CPEX testing was not only predictive of 30-day mortality, but was also predictive of longer-term survival at 30 months.
40
Other studies have identified different values for the optimal discriminatory level for the anaerobic threshold. CPEX testing was undertaken in patients with a low functional capacity (less than 7 METs) who subsequently underwent major surgery. A lower AT value was associated with increased likelihood of postoperative complications and the optimal AT threshold for the study group was identified at 10.1 O
2
mL/min/kg.
41
An AT cut-off of 11 mL/kg/min was also a poor predictor of postoperative cardiopulmonary morbidity for patients undergoing oesophagectomy for cancer.
42
However, this study did demonstrate an association between lower exercise capacity and risk of complications, suggesting that this study was underpowered and/or an alternative AT threshold may be more suitable for this group of patients. The optimal cut-off value for the anaerobic threshold is generally accepted at 11 O
2
mL/min/kg.
43
It is interesting that this value closely relates to 4 METs (14 O
2
mL/min/kg) and, in turn, ability to climb two flights of stairs. This would suggest that patients who are able to climb two flights of stairs would have an anaerobic threshold greater than11 O
2
mL/min/kg. Stair climbing therefore has the potential to be used as a screening tool for the identification of patients who would benefit from further assessment by CPEX testing. However, it remains unclear whether this is the optimal AT value or indeed if alternative thresholds should be adopted for different patient groups or for different surgical procedures.
Other limitations of CPEX testing relate to the process of conducting the test itself. Patients are required to exercise, usually on a cycle ergometer, and full assessment may be limited by physical ability rather than limitations due to cardiorespiratory function – for example, patients with arthritis or amputees. Another potential limitation of CPEX testing relates to availability and cost. The equipment and expertise to perform the test are not widely available in the UK at present. A survey conducted in England during 2008 found that only 30 (17%) hospitals had a CPEX service, with an additional 12 (7%) in the process of setting one up.
43
Despite these limitations, CPEX testing is becoming an increasingly adopted tool for preoperative assessment of higher risk patients undergoing major surgery.
Cardiopulmonary exercise testing (CPEX) is the ‘gold standard’ measure of cardiorespiratory function. An anaerobic threshold (AT) less than 11 O
2
mL/min/kg has been associated with increased risk of postoperative complications and mortality, although the exact threshold AT value may need to be modified for different patient groups or different surgical procedures. CPEX testing requires specialist equipment and expertise to perform, and it is not widely available in the UK at present, but it is likely to be increasingly used for assessment of perioperative risk in selected high-risk patient populations.
The incremental shuttle walk test (ISWT) requires the patient to walk between two markers placed 10 metres apart within a set time period. This time period becomes progressively shorter, requiring more effort from the patient to make the distance within the shorter time. The test stops when the patient cannot reach the end of the 10-metre course within the given time. The ISWT has been shown to correlate with measured oxygen consumption in patients with cardiac and chronic lung disease.
44
A small study investigated the role of ISWT to predict 30-day mortality following oesophagogastrectomy.
45
No patients with a walk distance greater than 350 metres died in the postoperative period. Patients who managed to walk a distance less than 350 metres had a 50% 30-day mortality. Distance achieved on the shuttle walk test was compared with CPEX measurements in a study of 50 patients undergoing abdominal surgery. All patients who walked in excess of 360 metres had an anaerobic threshold (AT) greater than 11 O
2
mL/min/kg.
46
It was also noted that some patients who walked less than 360 metres may also have had satisfactory CPEX results, suggesting that the ISWT was good at identifying patients with a good AT, but could not accurately identify those who had a poor anaerobic threshold (i.e. a good positive predictive value, but poor negative predictive value). The study was not, however, sufficiently powered to investigate surgical outcomes. These data suggest that the ISWT may be used as a screening tool to identify patients who may then benefit from more formal exercise testing with CPEX (i.e. those who walked less than 350–360 metres).
The 6-minute walk test is another standardised assessment tool for estimation of exercise capacity. The test involves measuring the distance that a patient can cover during a 6-minute period. The patient is instructed to walk as fast as they can to cover the maximum possible distance. The AT determined by CPEX testing was compared with maximum distance achieved during the 6-minute walk test in a study of 110 patients awaiting major general surgery. Patients who completed in excess of 563 metres during the 6-minute test had an AT greater than 11 O
2
mL/min/kg and those who managed less than 427 metres had an AT less than 11 O
2
mL/min/kg.
47
The authors recommended that those patients who completed 563 metres did not require formal exercise testing, whereas those who could not manage more than 427 metres should undergo CPEX assessment. Those patients who walked between 427 and 563 metres belong to a group of ‘clinical uncertainty’, and other clinical risk factors and magnitude of surgery should be incorporated into the decision-making process.
The incremental shuttle walk test (ISWT) and the 6-minute walk test are simple tools to objectively assess exercise capacity. They are indirect tests of oxygen consumption and have been shown to correlate with formal exercise testing values (CPEX). The main value of these tests is to identify higher-risk patient populations who may benefit from formal exercise testing.