The prognostic strength of gas analysis measurement during maximal exercise testing

The search to assess the risk of dying prematurely has a long history in medicine, and numerous anthropometric, physiological and biochemical variables have been related to death. Exercise testing1–3 has also been used to assess prognosis, and in the past cardiologists placed great emphasis on ECG abnormalities such as arrhythmias4,5 and ST changes6,7 during exercise. For several decades now it has been well established that exercise capacity is a very strong prognostic indicator in the population at large8–11 as well as in specific groups with cardiovascular12–14 or other chronic15–17 diseases. Most studies used peak oxygen uptake (pVO2) as the gold standard of exercise tolerance, but significant relations with other expressions of functional capacity, such as exercise duration, 6-minute walking distance or aerobic thresholds with total, cardiovascular or cardiac death have also been reported.15–18 The diagnostic and prognostic value of different exercise variables in various patient populations has recently been described,19,20 illustrating the great importance of maximal exercise testing. In this issue, Laukkanen and co-workers report on the prognostic value of peak oxygen pulse in a cohort of more than 2200 men living in the Kuopio region of Finland.21 They used relative peak oxygen pulse (peO2p = (peak VO2/peak heart rate (HR)/body weight) × 100) and related it to sudden cardiac death, fatal coronary heart disease, fatal cardiovascular disease and all-cause mortality, after correction of several models with cardiovascular risk factors or treatment. Per 1 standard deviation (SD) increase in peO2p and after correction of all significant variables, the hazard ratios showed a decrease of 20–25% in all fatal outcomes. When comparing the lowest quartile of peO2p with the other quartiles, an important lower mortality was found; when comparing the lowest with the highest quartiles, decreases in fatal outcomes after correction for all significant covariates ranged from 39% to 46%. The authors therefore suggested adding this important variable, an expression of peak stroke volume, in the reports of cardiopulmonary exercise, as it adds important prognostic information beyond several conventional risk factors. These data clearly show again the great importance of gas exchange variables obtained during maximal exercise testing: pVO2,8–14 peak circulatory power,18 oxygen uptake efficiency slope,22,23 minute ventilation/carbon dioxide production relationship (VE/VCO2) slope,24 oscillatory ventilation,25 end-tidal carbon dioxide26 and now peak oxygen pulse.21 The question is now which of these variables is the strongest to predict prognosis? In different pathologies which parameter is prognostically the strongest? Or should we evaluate all these parameters during graded maximal exercise testing? And related to the paper by Laukkanen et al., will the measurement of haemodynamic parameters at maximal exercise provide us with more prognostic information? Laukkanen et al. used oxygen pulse as a surrogate of stroke volume, but related to the Fick equation (VO2 = HR × SV × A-VO2 difference), oxygen pulse is also related to A-VO2 difference. Therefore, this paper not only strengthens the importance of gas exchange measurements at maximal exercise for prognostic information but also raises new research questions and challenges. The author declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article. The author received no financial support for the research, authorship, and/or publication of this article.