Cardiopulmonary exercise testing in patients with moderate-severe obesity: a clinical evaluation tool for OSA?

Abstract

Purpose: Obstructive sleep apnea (OSA) is a widespread comorbidity of obesity. Nasal continuous positive airway pressure (CPAP) has been demonstrated very effective in treating patients with OSA. The aims of this study were to investigate whether or not cardiopulmonary exercise testing (CPET) can characterize patients with OSA and to evaluate the effect of nasal CPAP therapy.

Methods: An observational study was conducted on patients with moderate to severe obesity and suspected OSA. All patients underwent cardiorespiratory sleep study, spirometry, and functional evaluation with ECG-monitored, incremental, maximal CPET.

Results: Of the 147 patients, 94 presented with an apnea-hypopnea index (AHI) ≥ 15 events/h and were thus considered to have OSA (52 receiving nasal CPAP treatment; 42 untreated) while 53 formed a control group (AHI < 15 events/h). Patients with untreated OSA showed significantly lower oxygen uptake (VO₂), heart rate, minute ventilation (VE), and end tidal carbon dioxide (PETCO₂) at peak exercise compared to controls. Patients receiving nasal CPAP showed higher VE and VO₂ at peak exercise compared to untreated patients. A difference in PETCO₂ between the maximum value reached during test and peak exercise (ΔPETCO₂ max-peak) of 1.71 mmHg was identified as a predictor of OSA.

Conclusion: Patients with moderate to severe obesity and untreated OSA presented a distinctive CPET-pattern characterized by lower aerobic and exercise capacity, higher PETCO₂ at peak exercise associated with a lower ventilatory response. Nasal CPAP treatment was shown to positively affect these cardiorespiratory adaptations during exercise. ΔPETCO₂ max-peak may be used to suggest OSA in patients with obesity.

Keywords: Cardiorespiratory fitness; Cardiorespiratory sleep study; Continuous positive airway pressure; End-tidal carbon dioxide; Obstructive sleep apnea.

Fig. 1

Study flow-chart. AHI, apnea–hypopnea index; Ob, patients affected by obesity; Ob-OSA, patients affected by obesity and OSA; Ob-CPAP, patients affected by obesity and OSA treated with CPAP

Fig. 2

Relationship between PETCO₂ peak and AHI. Figure 2 shows the positive correlation between end tidal carbon dioxide at peak exercise (PETCO₂ peak) and apnea–hypopnea index (AHI)

Fig. 3

Response of PETCO₂ during incremental exercise. Figure 3 shows the response of end tidal carbon dioxide (PETCO₂) during incremental cardiopulmonary exercise testing in Ob (blue), Ob-CPAP (red), and Ob-OSA (green). Although PETCO₂ max is similar among the three groups, it diverges at peak exercise (graphic above). In the graphic, it is possible notice that these differences in PETCO₂ peak occur after RCP is achieved. REST, at rest; AT, anaerobic threshold; RCP, respiratory compensation point; PEAK, at peak exercise; MAX, maximum value reached during exercise testing

Fig. 4

ROC curve analysis of ΔPETCO₂ max-peak. Figure 4 shows receiver operating characteristic curve analysis of the difference in end tidal carbon dioxide pressure between the maximum value reached during exercise and peak exercise (ΔPETCO₂ max-peak), as predictor of OSA. The red circle indicates the value of 1.71 mmHg, having a sensitivity of 81% and a specificity of 67%. AUC, area under the curve; CI 95%, 95% confidence interval