Assessment of chemoreflex sensitivity in free breathing young subjects by correction for respiratory influence

Background: The assessment of autonomic function is an important tool for risk stratification in critically ill patients. Peripheral cardiac chemoreflex sensitivity has been considered a marker for increased risk of sudden cardiac death. In normals, the evaluation of peripheral cardiac chemoreflex sensitivity is performed under controlled breathing conditions during inhalation of hypoxic gas. Since this is poorly tolerated by patients, they are commonly studied under hyperoxic conditions, which are not physiological. Methods: We studied 20 healthy volunteers who underwent free and controlled breathing of a hypoxic gas mixture (10% O2 in N2) over 5 min. Values of peripheral cardiac chemoreflex sensitivity, corrected for respiratory influence, were compared with the results obtained experimentally under controlled breathing conditions in the same subjects. Results: We found a substantial difference between values obtained during free and controlled breathing (3.64±0.81 vs. 1.53±0.32 ms/mmHg, respectively; P<0.05). After application of a respiratory correction, described and validated in this article, no significant difference was seen for these values (0.89±0.91 vs. 1.53±0.32 ms/mmHg, P=0.46). Conclusions: This approach allows the evaluation of peripheral cardiac chemoreflex sensitivity in free breathing subjects. This correction could improve the assessment of cardiac chemoreflex sensitivity in patients with cardiorespiratory disorders, who find it difficult to control their breathing according to an experimental protocol.