Our study has identified three measurements of PC previously unidentified, two of which were incorporated into a multivariate model, all of which may be significantly associated with a broad range of acute postoperative outcomes and suggests the possibility that other, yet undetermined, parameters of PC may also be useful indicators of risk. We also showed that both the ASA rank assignment and preoperative measurements of PC are significantly associated with a broad range of acute postoperative morbidities in the study patient population, and suggest that, compared with the ASA rank assignment, the multivariate model of PC may be a more sensitive predictor of risk, defining more accurately the patient population most likely to experience an acute postoperative morbid events.
It is difficult to draw a conclusion as to the predictive value of PC measures by comparing the results of the previous studies due to the diversity of: surgical procedures, postoperative endpoints, and the variety of measures evaluated. The use of PC as a preoperative cardiac risk assessment for elderly patients undergoing various abdominal operations identified an anaerobic threshold of <11 ml kg−1
as the critical measure of PC defining high risk for postoperative death resulting from cardiac dysfunction.12,13
This was the only study to identify the anaerobic threshold as the critical parameter of PC defining high risk. In a population of 82 morbidly obese patients undergoing bariatric surgery, peak oxygen uptake ≤15.8 ml kg−1
was identified as a critical measure of PC that defined high risk of overall postoperative morbidity.14
This study used a single surgical procedure, evaluated a variety of postoperative outcomes, and identified the peak oxygen uptake as the critical parameter of PC defining high risk.14
A peak oxygen uptake of <800 ml min−1
was proposed as a predictor of postoperative cardiopulmonary morbidity in 91 patients undergoing oesophagectomy.15
In contrast, an anaerobic threshold <11 ml kg−1
was a poor predictor of postoperative cardiopulmonary morbidity in 78 patients undergoing oesophagectomy.16
The peak oxygen uptake was significantly lower in patients developing cardiopulmonary complications, but CPET was thought to be of limited value in predicting postoperative cardiopulmonary morbidity. These studies15,16
evaluated the same surgical procedure and the same postoperative endpoint and identified the peak oxygen uptake as the measure defining high risk. However, they arrived at conflicting opinions about the value of PC as a preoperative risk assessment tool.
In our study, we evaluated 32 patients undergoing a mix of surgical procedures, and looked at a variety of postoperative outcomes. In our study, in contrast to the previous studies, peak oxygen uptake and the anaerobic threshold were not significantly associated with postoperative morbidity in our patient population. We identified three new and previously unreported measurements of PC that are potentially significant predictors of postoperative morbidity: HR3, ΔHR1, and PAT. ΔHR1 was identified based on a previous study17
which reported that a heart rate <99 beats min−1
after supine pedalling for 2 min (not measuring gas exchange) was a preoperative indicator of postoperative cardiac and pulmonary complications. These findings encouraged us to investigate such a measure in our data using a metabolic endpoint. PAT is a modification of the anaerobic threshold and was used in a multivariate model with ΔHR1 (PAT+ΔHR1). If our study had been limited to peak oxygen uptake and the anaerobic threshold, these additional measurements of PC would not have been identified as being significantly associated with postoperative morbidity in our study population. More importantly, we would have suspected that there was no relationship between measures of PC and postoperative outcomes in this study population. Taken together, all of these studies suggest that some measurements of PC are associated with postoperative morbidity. The specific details concerning which measures are associated with which postoperative endpoints, and under what conditions are not clear. Clarification of these details will require further studies of these potentially complex relationships.
CPET could have a significant value for patients undergoing a broad range of surgical procedures. Patients with cancer are frequently treated with cardiopulmonary toxic chemotherapy, radiation therapy, or both before surgery.18,19
Currently, there is no preoperative risk assessment tool that accurately assesses the physiological effect of preoperative chemotherapy or radiotherapy on postoperative outcomes. In the attempt to maximize the effects of cancer controlling surgical therapy, postoperative morbidities unrelated to the cancer prognosis may result in decreased patient survival.1,20
A patient's PC may be an effective means to assess the physiological effect of preoperative anti-cancer therapies and establish the recovery time needed after these therapies before surgical intervention to achieve the best therapeutic outcomes. The potential clinical values of data obtained from CPET are largely unexplored at this time.
The small number of patients in this pilot study and the number of measurements evaluated increase the possibility that some reported associations may be due to chance. This caveat notwithstanding, such limitations of statistical power should not be used as a pretence to ignore the potential implications of the newly reported measurements of PC. Given the inconsistencies in the literature regarding the predictive value of the reported measurements and this pilot study's identification of new potentially predictive measurements raises the possibility of others that predict postoperative outcomes. Systematic expression of CPET data may provide an opportunity to identify even more accurate and precise measures predictive of postoperative outcomes for specific types of surgeries, illnesses, or both. It is becoming increasingly clear that identifying which measurements of PC are associated with postoperative outcomes will require further studies to understand these critical and complex relationships.
In summary, assessing surgical risk requires consideration of both the anticipated therapeutic outcome related to the surgical prognosis and the probability of postoperative adverse outcomes unrelated to the surgical prognosis. This initial study was not designed to supply a definitive answer regarding the relationship between PC and postoperative outcome. The definitive answer will only result from more thorough, detailed studies of this potentially valuable, complex relationship. These studies must first identify measurements of potential risk, and then validate these as risk predictors. Continued investigations may ultimately result in the pre-emptive preoperative management of more precisely defined physiological risk status, thereby reducing postoperative complications.