Abstract
PurposeWe analysed the characteristics of arterial baroreflexes during the first phase of apnoea (φ1).Methods12 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.ResultsIn 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.ConclusionIn 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.
Highlights
The time course of cardiovascular parameters during dry apnoeas has been investigated at rest and exercise, in air and after breathing pure oxygen (Perini et al 2008; Tocco et al 2012; Costalat et al 2013, 2015; Sivieri et al 2015; Fagoni et al 2015, 2017; Taboni et al 2018b, 2019)
When the apnoeas are performed at lung volumes close to the individual total lung capacity, there might be a reduction of venous return during φ1, due to the high intrathoracic pressure exerted at elevated lung volumes, which entails an increase in central venous pressure and a decrease in cardiac output and in mean arterial pressure (MAP) (Potkin et al 2007; Breskovic et al 2011a; Batinic et al 2011; Stembridge et al 2017)
The R-to-R interval (RRi) values at exercise were significantly lower than at rest, either in air or in oxygen; MAP values were higher during exercise apnoeas in oxygen (p < 0.05) and in air the trend was similar but did not reach the statistical significance
Summary
The time course of cardiovascular parameters during dry apnoeas has been investigated at rest and exercise, in air and after breathing pure oxygen (Perini et al 2008; Tocco et al 2012; Costalat et al 2013, 2015; Sivieri et al 2015; Fagoni et al 2015, 2017; Taboni et al 2018b, 2019). When the apnoeas are performed at lung volumes close to the individual total lung capacity, there might be a reduction of venous return during φ1, due to the high intrathoracic pressure exerted at elevated lung volumes, which entails an increase in central venous pressure and a decrease in cardiac output and in MAP (Potkin et al 2007; Breskovic et al 2011a; Batinic et al 2011; Stembridge et al 2017) These cardiovascular changes would be similar to those that intervene in the course of a Valsalva manoeuvre (Korner et al 1976; Palmero et al 1981). An analysis of the baroreflex dynamics during apnoea could shed light on the characteristics of the baroreflex response to MAP perturbations at the beginning of breath-holding
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