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Oklahoma City, OK
Dr. Keddissi is Assistant Professor of Medicine, and Dr. Kinasewitz is Professor and Chief, Pulmonary/Critical Care Medicine, OU Health Sciences Center.
Correspondence to: Gary T. Kinasewitz, MD, FCCP, Professor and Chief, Pulmonary/Critical Care Medicine, OU Health Sciences Center, 920 Stanton L. Young Blvd, WP 1310, Oklahoma City, OK 73104
More Americans die of lung cancer than of any other malignancy. In 2004, there will be 173,770 cases and 160,440 deaths due to the disease.1 The majority of these cases will be locally advanced or metastatic when they are diagnosed, precluding any surgical resection. When technically feasible, surgery offers the only possibility for cure.
Because of the presence of a common risk factor (ie, smoking), lung cancer tends to develop in patients with concomitant cardiopulmonary limitations. Lung resection poses a significant stress to the cardiopulmonary system, so the importance of identifying patients who will tolerate surgery from among those who are anatomically resectable becomes evident. The search for the ideal preoperative test that would identify these patients with certainty has been ongoing for decades. Routine pulmonary function tests including spirometry and measurement of the diffusion capacity of the lung as well as quantitative radionuclide perfusion lung scanning have all been utilized, with variable degrees of accuracy.
Exercise testing simulates the operative stress faced by the cardiac and pulmonary systems. Therefore, it represents an attractive way of identifying a patient at high risk for postoperative complications. It is intuitive that the higher the exercise tolerance, the more likely an individual patient will be able to successfully tolerate the stress of lung resection.
Different techniques can be utilized and multiple parameters can be measured during a cardiopulmonary exercise test. They range from the distance covered during a 6-min walk on level ground to the maximal oxygen uptake (
O2max) during incremental exercise in a laboratory.
Van Nostrand et al2 showed that in patients who are unable to walk on level ground or climb one flight of stairs, the mortality after lung resection is 50%. Holden et al3 found that patients who died following pulmonary resection had a significantly shorter 6-min walk distance and a lower stair climbing ability compared to patients who survived. Additional studies456 have confirmed that symptom-limited stair climbing can be used to predict post-lung resection complications. This has been verified by a prospective trial7 in patients undergoing thoracic and upper abdominal surgeries.
Measuring oxygen uptake during maximal or submaximal exercise as a predictor of postoperative complications has been studied for years. Colman8 failed to show a predictive value for O2max, but he included "technical" complications, such as blood loss and the development of pneumothorax. Such complications are not expected to be predicted by a preoperative exercise testing. Eugene et al9 found that O2max < 1 L/min predicted a poor outcome. Smith et al10 showed that correcting the oxygen uptake for the body weight improved its ability to predict postoperative outcome: all patients with a O2max < 15 mL/kg/min and only 10% of patients with a O2max > 20 mL/kg/min had cardiopulmonary complications.
Bechard and Wetstein11 extended these observations and found that patients with O2max < 10 mL/kg/min are at an extremely high (71%) risk for complications with a mortality of 29%, while those able to achieve a level > 20 mL/kg/min are at minimal risk. Patients with a O2max between these values are at moderate risk.
The study by Win and colleagues in the current issue of CHEST (see page 1159) revisits the concept of using the O2max as a percentage of the predicted, rather than an absolute value. It is theoretically possible, by using an absolute value (in liters per minute or milliliters per kilogram per minute), to discriminate against the elderly, female patients and people with a short stature. By failing to reach a certain cut-off value, these groups could be denied a potential curative surgery. Thirteen of 99 patients in the study by Win et al had predicted O2max < 15 mL/kg/min. When prior studies12 looked at this potential bias, different conclusions were reached. Bolliger et al12 found that O2 max as a percentage of predicted was better than the absolute O2max as an indicator for postoperative complications. In contrast, Smith and colleagues10 found that the absolute value expressed in milliliters per kilogram per minute was better, even though the oxygen uptake expressed as a percentage of predicted was also able to predict postoperative complications.
Win et al studied patients with lung cancer undergoing potentially curative resection. Two different outcomes were assessed: (1) a complicated postoperative course, which included heart failure, renal failure, pulmonary embolism, septicemia, pneumonia, respiratory failure, myocardial infarction, and postoperative death; and (2) poor outcome, defined as only the latter three complications. They found that O2max as a percentage of predicted was a good indicator of the postoperative course, but that the absolute value of O2max was not. A complicated course, poor outcome, and mortality were all more common in patients with a low O2max as a percentage of predicted. Two of three patients with a O2max < 50% of predicted died but, due to the small number of patients with a low O2max who actually had surgery, a definite threshold recommendation cannot be made.
The finding of a good correlation between the O2max (as a percentage of predicted) and outcome is expected. The lack of association between the absolute value of O2max and outcome is not expected since the two methods of expressing oxygen uptake should be collinearly related. Most,91011121314 but not all,815 previous studies have found that the outcome is related to both the absolute level and percentage of predicted O2max achieved.
Over the years, pulmonary exercise testing has become an integral part of the preoperative evaluation of patients selected for lung resection. It is particularly important in patients with poor pulmonary function testing. Measuring oxygen uptake has evolved as a useful objective part of that evaluation. Whether this should be expressed as an absolute value or as a percentage of predicted may still be debated. What is not in doubt is the fact that the more the patient can do, the better the outcome is likely to be.
References
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