The PFT Lab I am associated with performs cardiopulmonary exercise tests for pre-op cardiothoracic surgery patients with lung cancer. Surgeons have to make a decision to operate or not based at least in part on the amount of function they think will remain after a lung resection and this depends on which and how many lobes are involved. This calculation is done by taking a percentage based on the amount of lung tissue that will be lost which is weighted using ventilation-perfusion scan results off the baseline DLCO and vital capacity. When the result is inconclusive or there are other medical factors that can affect a patient’s prognosis the results from a CPET can help with the surgeon’s decision.
The criteria that has been most often cited as an indicator of acceptable surgical risk is a peak VO2 greater than 15 ml/kg/min. About ten years ago we encountered a patient with a significantly low body weight. His max VO2 was about 16 ml/kg/min which appeared to make him a good candidate for surgery but his actual maximum oxygen consumption in LPM was 45% of predicted. This discrepancy was due to the fact that his BMI was about 14. We re-calculated what his peak VO2 would have been if his body weight had been normal and that was less than 10 ml/kg/min. For this reason our recommendation at that time was that the patient was a poor candidate for surgery.Up until that time we had used a set of predicted peak VO2 equation that was based solely on height, age and sex. Our analysis of this patient’s results seemed inadequate so with a bit of research we found and decided to change to a set of predicted equations that incorporated body weight.
At that time we were aware of and were using the Ve/VCO2 at anaerobic threshold as part of the exercise interpretation process but we have since come to rely far more on the Ve-VCO2 slope. Because determining anaerobic threshold using ventilatory parameters is a “soft” measurement some investigators have recommended using the lowest observed Ve/VCO2 instead. From my point of view however, the Ve-VCO2 slope is measured from a series of data points rather than a single one and for this reason it is probably a more reliable measurement than Ve/VCO2 at AT.
I will mention in passing that the lowest Ve/VCO2 whether or not it occurs at AT is a function of the Ve-VCO2 slope and its intercept. Ve-VCO2 slope and Ve/VCO2 at AT have been investigated and reported many times but little has been reported about the Ve-VCO2 intercept and what, if anything, the intercept means is still unclear.
We recently had a similar low body weight patient for a pre-op CPET and I think our ability to analyze this patient’s results has improved. The patient had a diagnosis of lung cancer and COPD and had a BMI of 18.2. PFT data showed a FVC that was 60%VO2, FEV1 36% and DLCO 65% of predicted. CPET data showed a peak VO2 in LPM that was 52% of predicted, the peak VO2 in ml/kg/min was 16.2, peak Ve was 94% of predicted (FEV1x40), Ve/VCo2 at AT was 44, VO2 at AT was 45% of predicted in LPM and 14.0 ml/kg/min, Ve-VCO2 slope was 30.7 (intercept 9.9), the max heart rate was 101% of predicted and the SpO2 at peak exercise was 97%.
The maximum heart rate and peak minute ventilation shows the patient gave an excellent test effort. Overall the results indicate the patient had a primarily pulmonary mechanical limitation to exercise. Despite the low DLCO the lack of desaturation and normal Ve-VCO2 slope indicates that pulmonary vascular disease was not a primary limiting factor.
The literature concerning the effect of low body weight on exercise capacity is quite sparse. Interestingly, although there is an ERS statement on cardiopulmonary exercise testing it contains no recommendations about reference equations and does not discuss the effects of body weight on exercise results.
Patients with a low body weight are expected to have a lowered resting and peak oxygen consumption and this is due primarily to a loss of lean body mass (muscle), but because of their body weight this causes quite different expectations in max VO2 when expressed in LPM and when expressed in ml/kg/min.
A peak VO2 less than 15 ml/kg/min is not the only indicator of post-op complications and mortality and in this case in particular may not necessarily be the best one. An alternate criteria that we think is more widely pertinent than peak VO2 is a Ve-VCO2 slope greater than 34. A review of the literature shows that other possible negative pre-op criteria can include a peak VO2 (in LPM) less than 60% of predicted, a DLCO less than 60% of predicted and a VO2 at anaerobic threshold less than 11 ml/kg/min. Depending on what factors are emphasized there are different implications for this patient’s post-surgery prognosis.
Since the peak VO2 in ml/kg/min is likely overestimated in this patient I think its prognostic value is low and should be ignored. More pertinent is the fact that the peak VO2 was 52% of predicted and the reference equation used was specific for underweight individuals. On the other hand, the Ve-VCO2 slope and the percent predicted DLCO were above their cutoffs and these would tend to indicate a favorable prognosis. In addition the VO2 at AT was normal and there was no desaturation. Our final recommendation was that despite questions about the peak VO2 the normal Ve-VCO2 slope indicated the patient was likely a reasonable candidate for surgery.
When we originally encountered the problem of assessing the peak VO2 in ml/kg/min in a low body weight patient we interpreted the results by noting that the peak VO2 would have been markedly reduced if the patient’s body weight was normal which in retrospect was not the proper way to analyze the results. This time we had a better notion about what the patient’s predicted VO2 should be and as importantly we were able to use other criteria to help make an informed decision about the patient’s prognosis.
References:
Brutsche MH, Spiliopoulos A, Bolliger CT, Licker M, Frey JG, Tschopp JM. Exercise capacity and extent of resection as predictors of surgical risk in lung cancer. Eur Respir J 2000; 15: 828-832.
Corra U, Mezzani A, Bosimini E, Scapellator F, Imparato A, Giannuzzi P. Ventilatory response to exercise improves risk stratification in patients with chronic heart failure and intermediate functional capacity. Amer Heart J 2002; 143: 418-426.
Francis DP, Shamim W, Davies LC, Piepoli MF, Ponikowski P, Anker SD, Coats AJS. Cardiopulmonary exercise testing for prognosis in chronic heart failure: continuous and independent prognostic value for Ve/VCO2 slope and peak VO2. Eur Heart J 2000; 21: 154-161.
Horwich TB, Leifer ES, Brawner CA, Fitzgerald MB, Forarow GC. The relationship between body mass index and cardiopulmonary exercise testing in chronic systolic heart failure. Am Heart J 2009; 158(4-Suppl): S31-S36.
Morice RC, Peters EJ, Ryan MB, Putnam JB, Ali MK, Roth JA. Exercise testing in the evaluation of patients at high risk for complications from lung resection. Chest 1992; 101:356-361.
Palange P et al. ERS Task Force: Recommendations on the use of exercise testing in clinical practice. Eur Respir J 2007; 29: 185-209.
Salvadori A, Fanari P, Mazza P, Agosti R, Longhini E. Work capacity and cardiopulmonary adaptation of the obese subject during exercise testing. Chest 1992; 101:674-679.
Smith TB, Stonell C, Pukayastha S, Paraskevas P. Cardiopulmonary exercise testing as a risk assessment method in non cardio-pulmonary surgery: a systematic review. Anaesthesia 2009; 64: 883-893.
Tardie GB, Dorsey DA. Pre-opeartive cardiopulmonary exercise testing for a severely malnourished lung cancer resection surgery candidate. JEPonline 2005; 8:34-43.
Wang J, Olak J, Ferguson MK. Diffusing capacity predicts operative mortality but not long-term survival after resection for lung cancer. J Thorac Cardiovasc Surg 1999; 117: 581-587.
Wasserman K, Hansen JE, Sue DY, Stringer WW, Whipp BJ. Principles of Exercise Testing and Interpretation Including Pathophysiology and Clinical Applications Fourth Edition. Copyright 2005 by Lippincott Williams & Wilkins.
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