Recruitment pattern of sympathetic neurons during breath-holding at different lung volumes in apnea divers and controls.

We tested the hypothesis that breath-hold divers (BHD) attain higher level of sympathetic activation than controls due to the duration of breath-hold rather than a different recruitment strategy. In 6 control subjects and 8 BHD we measured muscle sympathetic neural activity (MSNA) prior to and during functional residual capacity (FRC) and total lung capacity (TLC) breath-holding. On a subset of subjects we applied a new technique for the detection of action potentials (APs) in multiunit MSNA. Compared with controls, BHD group had lower burst AP content (13±7 vs. 6±3AP/burst; P=0.05) and number of active clusters (5±1 vs. 3±1clusters/burst; P=0.05) at baseline. However, the overall sympathetic AP/unit-time was comparable between the groups (131±105 vs. 173±152AP/min; P=0.62) due to increased burst frequency in BHD group (20±4bursts/min) vs. controls (13±3bursts/min) (P=0.039). The achieved level in total MSNA during FRC breath-holds was higher in divers (2298±780 vs. 1484±575a.u./min; P=0.039). Total MSNA at the end of TLC breath-hold was comparable between the groups (157±50 (controls) vs. 214±41s (BHD); P=0.61). FRC and TLC breath-holds increased AP frequency, burst AP content and active clusters/bursts in both groups but the response magnitude was determined by the type of the breath-hold. The divers used fewer number of APs/burst and active clusters/burst. In both groups breath-holds resulted in similar increases in MSNA which were reached both by an increase in firing frequency and by recruitment of previously silent, larger (faster conducting) sympathetic neurons, and possibly by repeated firing within the same burst.