1. Animal studies have shown that arterial baroreflexes are modulated by reflexes originating from the cardiopulmonary volume receptors, and that this modulation consists of a reduction of the inhibitory influence exerted by arterial baroreceptors on the heart and peripheral circulation. This has not been confirmed in man, however, in whom no reduction in the bradycardic response to carotid baroreceptor stimulation has been observed after the mild increase in central venous pressure (right atrial catheter) and cardiopulmonary receptor activity provided by passive leg raising.
2. In seven normotensive subjects carotid baroreceptors were gradedly stimulated by progressively increasing carotid transmural pressure through a neck chamber device, the resulting reflex lengthening in R–R interval being measured in the two–three cardiac cycles immediately after the baroreceptor stimulus. This manoeuvre was performed in control conditions and repeated during a head-out water immersion which increased central venous pressure (right atrial catheter) from 1.5 ± 0.2 to 12.0 ± 0.9 mmHg (mean ± SE), thereby providing a marked increase in the cardiopulmonary receptor stimulus.
3. In the control condition graded stimulation of the baroreceptors caused a progressive lengthening in R–R interval, the maximal effect being + 477.4 ± 57.2 ms. Immersion increased the R–R interval from 774.2 ± 3.2 to 961.6 ± 5.8 ms (P < 0.01) and reduced mean arterial pressure (cuff measurement) from 96.0 ± 1.0 to 82.3 ± 0.9 mmHg. The changes in R–R interval induced by carotid baroreceptor stimulation were virtually identical with those observed in the absence of immersion. However, owing to the lower baseline heart rate during immersion, this meant a lesser degree of reflex bradycardia to carotid baroreceptor stimulation (maximal reduction − 30.5 ± 3.0 beats/min and −19.3 ± 3.1 beats/min before and during immersion respectively).
4. Thus, although the baroreflex sensitivity as expressed by changes in R–R interval is altered, the arterial baroreceptor ability to modulate heart rate (and perhaps cardiac output) is reduced when cardiopulmonary receptor activity is markedly increased. This may have implications in a number of physiological and pathophysiological settings.
