Phase relationship between normal human respiration and baroreflex responsiveness.

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Abstract
Influences of respiration phase and breathing frequency on human sinus node responses to arterial baroreceptor stimulation were studied. Carotid baroreceptors were stimulated with brief (0.6 s), moderate (30 mm Hg) neck suction during early, mid and late inspiration, or expiration at usual breathing rates, or, during early inspiration and expiration at breathing rates of 3, 6, 12 and 24 breaths/min. Baroreceptor stimuli applied during early and mid inspiration and late expiration provoked only minor sinus node inhibition; stimuli begun during late inspiration and early expiration provoked maximum sinus node inhibition. At breathing rates of 3, 6, 12 breaths/min, expiratory baroreflex responses were significantly greater than inspiratory responses; at 24 breaths/min inspiratory and expiratory baroreceptor stimuli produced comparable degrees of sinus node inhibition. Human baroreflex responsiveness oscillates continuously during normal, quiet respiration. This is an important central biological rhythm in normal man. The phase shift of baroreflex reponsiveness on respiration suggests that this interaction cannot be ascribed simply to gating synchronous with central inspiratory neuron activity. Regularization of heart rate during rapid breathing is associated with loss of the differential inspiratory-expiratory baroreflex responsiveness, which is present at usual breathing rates.