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

Abstract

Purpose: We analysed the characteristics of arterial baroreflexes during the first phase of apnoea (φ1).

Methods: 12 divers performed rest and exercise (30 W) 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 (φ2) 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 φ1, 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 φ1 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 φ2, BRS at rest was 12.1 [9.6-16.2] in air, 12.9 [9.2-15.8] in O₂. At exercise (no φ2 in air), it was 7.9 [5.4-10.7] in O₂. In φ2, OP acts at higher MAP values.

Conclusion: In apnoea φ1, there is a sudden correction of MAP fall via baroreflex. The lower BRS in the earliest φ1 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.

Keywords: Baroreflex resetting; Baroreflex sensitivity; Breath holding; Closed loop; Sequence method.

Fig. 1

Operating points during the steady states, in all resting and exercise apnoeas. Data are presented as means and standard error of the mean. Steady state during resting (black symbols) and exercise (white symbols) apnoeas. Steady state before starting apnoeas (dots) and during φ2 (squares). RRi: R-to-R interval; MAP: mean arterial pressure; *: MAP significantly different (p < 0.05) than corresponding value at pre-apnoea steady state; ^#: RRi significantly different (p < 0.05) than corresponding value at rest; ^$: MAP significantly different (p < 0.05) than corresponding value at exercise

Fig. 2

Closed-loop contour plot of the relationship between R-to-R interval (RRi) and mean arterial pressure (MAP) during φ1 in resting apnoea. Data obtained from a representative subject, showing the presence of a baroreflex responses before the attainment of minimum of mean arterial pressure (MAPmin, black diamond). The resetting phase starts from the diamond point: during this phase, RRi and MAP vary in opposite directions toward a new operating point in φ2 (white circle). White square: operating point at steady state before apnoea. Black tringle: beginning of apnoeas

Fig. 3

Tuckey representation of baroreflex sensitivity (BRS) computed during the φ1_up of resting apnoeas. For apnoeas characterised by multiple BRS sequences during φ1_up, the first, the second, and the third BRS are reported separately, besides the single BRS computed in apnoeas with only one BRS sequence during late φ1. *p < 0.05