Baroreflex responses during dry resting and exercise apnoeas in air and pure oxygen

PURPOSE: We analysed the characteristics of arterial baroreflexes during the first phase of apnoea (phi1).METHODS: 12 divers performed rest and exercise (30W) apnoeas (air and oxygen). We measured beat-by-beat R-to-R interval (RRi) and mean arterial pressure (MAP). Mean RRi and MAP values defined the operating point (OP) before (PRE-ss) and in the second phase (phi2) of apnoea. Baroreflex sensitivity (BRS, ms·mmHg-1) was calculated with the sequence method.RESULTS: In PRE-ss, BRS was (median [IQR]): at rest, 20.3 [10.0-28.6] in air and 18.8 [13.8-25.2] in O2; at exercise 9.2[8.4-13.2] in air and 10.1[8.4-13.6] in O2. In phi1, during MAP decrease, BRS was lower than in PRE-ss at rest (6.6 [5.3-11.4] in air and 7.7 [4.9-14.3] in O2, p<0.05). At exercise, BRS in phi1 was 6.4 [3.9-13.1] in air and 6.7 [4.1-9.5] in O2. After attainment of minimum MAP (MAPmin), baroreflex resetting started. After attainment of minimum RRi, baroreflex sequences reappeared. In phi2, BRS at rest was 12.1 [9.6-16.2] in air, 12.9 [9.2-15.8] in O2. At exercise (no phi2 in air), it was 7.9 [5.4-10.7] in O2. In phi2, OP acts at higher MAP values.CONCLUSION: In apnoea phi1, there is a sudden correction of MAP fall via baroreflex. The lower BRS in the earliest phi1 suggests a possible parasympathetic mechanism underpinning this reduction. After MAPmin, baroreflex resets, displacing its OP at higher MAP level; thus, resetting may not be due to central command. After resetting, restoration of BRS suggests re-establishment of vagal drive.