European Heart Journal Advance Access originally published online on May 25, 2006
European Heart Journal 2006 27(13):1633-1634; doi:10.1093/eurheartj/ehl055
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Obstructive sleep apnoea–hypopnoea syndrome reversibly depresses cardiac response to exercise: reply
Servicio de Neumología
Hospital Universitario Son Dureta
Palma de Mallorca
Spain
Servicio de Neumología
Hospital Universitario La Paz
Madrid
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Servicio de Cardiología
Complejo Hospitalario de Jaén
Jaén
Spain
We appreciate the comments by Patrice et al. regarding our recent article. As we pointed out, the CO2 rebreathing manoeuvre is an indirect and reliable method to measure Qt, but it may overestimate its values. However, although the agreement interval of Qt during incremental exercise between non-invasive techniques and direct Fick method is ±2–10 L/min,1 the inaccuracy of the CO2 rebreathing method seems acceptable in previous studies,2 and it is not higher than that in other indirect procedures.
In our study, we asked the subjects to rebreathe CO2 for 
20 s. The CO2 concentration was 15% in the last determinations, which is poorly tolerated and it may lead to an increase in the stress level. Thus, the intensity of exercise might be >60% of the theoretical maximum work intensity (W), and it could be comparable with a peak assessment. In fact, V'O2 at 60% of W (Figure 2) was actually close to the theoretical V'O2 peak (Table 2). Under such conditions, a Qt of 28 L/min is lower than that obtained from the well-trained subjects (36 L/min),3 and it is only slightly higher than that of sedentary subjects when the direct Fick method is used.4
We agree with Patrice et al. that there is a clear relationship between Qt and V'O2 during exercise; however, their recalculations should be interpreted cautiously. Predicting Qt from a cardiopulmonary exercise test, i.e. the percentage of predicted V'O2 max values, could lead to inaccuracies.5 In addition, it has been demonstrated that Qt is a non-linear function of V'O2 during incremental exercise,6 even so, and assuming a mean oxygen extraction of 0.12 L/min, V'O2 recalculation (41 mL/min/kg) would be concordant with typical values for V'O2max recorded in male subjects (40–50 mL/kg/min).5 However, these values are quite different from the mean V'O2 peak of our control subjects (Table 2). We must take into account that the test could have ended prematurely, because the rebreathing of CO2 is uncomfortable to perform during heavier stages of exercise and therefore V'O2 peak may be lower than that could be achieved without applying this technique. We also must remember that we used the slope of the relationship between Qt and stroke volume with V'O2 and W (instead of absolute values) in order to reduce all these potential limitations.
Finally, it is difficult to accept (in the context of a randomized crossover double-blind-controlled trial) that Qt was overestimated in nine patients only during the CPAP period. Furthermore, on successive studies, we have tried to define who would have a better response to CPAP, and preliminary data suggest that they would be subjects with variable grades of left ventricular diastolic dysfunction.
We thank Patrice et al. for their observations, and we cannot rule out absolutely an overestimation of Qt with a non-invasive method, but we believe that our data are robust enough to maintain the main conclusions of the study.
References
- Bougault V, Lonsdorfer-Wolf E, Charloux A, Richard R, Geny B, Oswald-Mammosser M. (2005) Does thoracic bioimpedance accurately determine cardiac output in COPD patients during maximal or intermittent exercise? Chest 127:1122–1131.
[Abstract/Free Full Text] - Auchincloss JH, Gilbert R, Morales R, Peppi D. (1991) The effect of progressive exercise on the equilibrium rebreathing cardiac output method. Med Sci Sports Exerc 23:1111–1115.[Medline]
- Kremser CB and Rajfer SI. (1986) The normal cardiovascular response to exercise. In Leff AR (Ed.). Cardiopulmonary Exercise Testing (Grune and Stratton, Orlando) pp. p107–121.
- Stringer WW, Hansen JE, Wasserman K. (1997) Cardiac output estimated noninvasively from oxygen uptake during exercise. J Appl Physiol 82:908–912.
[Abstract/Free Full Text] - Cooper CB and Storer TW. (2001) Exercise Testing and Interpretation. A Practical Approach (Cambridge University Press, Cambridge).
- Stringer WW, Whipp BJ, Wasserman K, Porszasz J, Christenson P, French WJ. (2005) Non-linear cardiac output dynamics during ramp-incremental cycle ergometry. Eur J Appl Physiol 93:634–639.[Medline]
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