Relationship between breathing and cardiovascular function at rest: sex-related differences.

AIM: to compare relationships at rest between breathing rate, levels of muscle sympathetic nerve activity, total peripheral resistance and cardiac output among young men and women.
METHODS: recordings were made of respiratory movements, sympathetic nerve activity (peroneal microneurography), intra-arterial blood pressure, electrocardiogram, cardiac output (open-circuit acetylene uptake technique) in 19 healthy men (age 27 (+/-) 2years, mean (+/-) SEM) and 17 healthy women (age 25 (+/-) 1years). Total peripheral resistance and stroke volume were calculated. Four minutes epochs of data were analysed.
RESULTS: breathing rates and sympathetic activity were similar in men and women but compared to men, women had significantly lower blood pressures, cardiac output and stroke volume. In men breathing rate correlated positively with sympathetic activity (r = 0.58, P < 0.05) but not in women (r = 0.12, P > 0.05). Furthermore, in men, respiratory rate correlated positively with total peripheral resistance (r = 0.65, P < 0.05) and inversely with cardiac output (r =-0.84, P < 0.05) and heart rate (r = -0.60, P < 0.05) but there were no such relationships in women (P > 0.05 for all).
CONCLUSIONS: the positive relationship between breathing and sympathetic activity in men, and the inverse coupling of breathing to cardiac output and heart rate suggest that influences of respiration may be important not only for dynamic but also for 'tonic' cardiovascular function. The lack of relationships among these variables in women shows that there are fundamental differences in basic blood pressure regulation between the sexes.