Ventilatory expired gas techniques during cardiopulmonary exercise testing (CPET) have become more widely applied because they significantly increase the precision and yield of information from the exercise test [1]. The identification of peak as an important independent predictor of severity of heart disease and also prognosis has been recently updated by the evidence that ventilation plays a role in improving predictive accuracy of CPET. Responses such as VT, slope, at peak exercise, oscillatory ventilation, oxygen uptake kinetics, rate of recovery of and oxygen uptake efficiency slope have been used with greater frequency to classify functional limitations and to stratify risk in patients with heart disease. Many of these are expressions of ventilatory efficiency and reflect the various underlying pathophysiological factors leading to inefficient breathing associated with heart failure or pulmonary disease. There has been a particular focus on the clinical significance of the slope in patients with heart failure. This response is usually expressed as the slope of the best-fit linear regression line relating VE and . Among patients with heart failure, the slope has been demonstrated to predict mortality, hospitalisation and other outcomes at least as well as, and independently from, peak [2–4]. A slope value ≥34 has been reported to indicate an abnormal response [3]. In fact, a heightened slope reflects the combination of factors that underlie ventilatory inefficiency in heart failure, is independent of patient effort and powerfully predicts health outcomes. In conditions where heart efficiency is normal, the utility of ventilatory efficiency as a limiting factor of exercise capacity has not been demonstrated. Patients with pulmonary disease have more marked alterations in ventilation that become more evident during exercise. However, information is lacking about the role of parameters from CPET, which may improve prognostic accuracy of resting spirometry in patients with chronic obstructive pulmonary disease (COPD) and/or other pulmonary disease. In the study by Torchio and colleagues [5], published in this issue of the European Journal of Cardiothoracic Surgery, the authors aimed to investigate the role of slope as preoperative mortality and morbidity predictor in COPD patients submitted to lung resection for non-small-cell lung cancer (NSCLC) according to current standards. They performed a retrospective analysis in 145 consecutive mild-to-severe COPD patients with lung cancer (128 males and 17 females), with a mean age of 64 years (range 41–82 years) that were referred for preoperatory evaluation. All these patients were deemed operable only after a CPET demonstrated a preserved cardiopulmonary function. They found that slope was the only independent predictor of mortality (odds ratio: 1.24, z = 2.77, p ≪ 0.007), while peak was instead the best predictor for the occurrence of severe cardiopulmonary postoperative complications (odds ratio: 0.05, z = –2.39, p ≪ 0.02). These results suggest a differential interpretation of parameters deriving from CPET analysis. Peak is the most widely used parameter indicating the level of functional capacity reached by a subject during a symptom-limited exercise test, because it summarises the contribution of the heart, lungs, oxygen transport system and skeletal muscles to external work. Thus, we may expect that peak retains the information that is derived from the lungs, together with the information from other systems involved in work production, while slope is the expression of lung efficiency which may or may be not compensated for by the other systems. For instance, in heart failure, the combination of peak and slope confers a more accurate interpretation not only of the severity of heart failure, but also regarding the prognosis. In one study, heart-failure patients with a peak between 10 and 18 ml kg−1 min−1 had a worse prognosis when the slope was >35 [3]. However, the importance of was lost when peak was below 10 ml kg−1 min−1, suggesting that functional capacity maintains its leading role of predictor of prognosis when it is very depressed or when it is high. Ventilatory efficiency seems to be more important in stratifying the prognosis of patients with a wide range of peak between 10 and 18 ml kg−1 min−1. Taking into account the important limitations of the study, such as its retrospective nature, which may permit inherent problems of recording and definition of variables and outcomes, the limited number of events that preclude a reliable statistical analysis, particularly for mortality (five cases only), the absence of a multicollinearity test to prevent correlated variables (such as peak and slope, usually highly and inversely correlated) to be entered simultaneously in the regression analysis, Torchio and colleagues [5] should be commended for their effort to look beyond the peak . CPET provides a wealth of information that can assist during preoperative risk stratification not only in identifying those patients at increased risk, but also in identifying those systems responsible for a reduced aerobic capacity. Once those systems are identified, corrections can be implemented (i.e., coronary revascularisation, medical treatment, physical rehabilitation, etc.) to improve the fitness of the patients and minimise their surgical risk. We should not forget that ventilatory efficiency is only one of the several possible limitations of exercise tolerance. Many other factors along the so-called ‘aerobic cascade’ may play a role, and the combination of different parameters is probably the best way to improve the diagnostic accuracy of CPET.
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Brunelli et al. (2010) studied this question.
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