Respiratory modulation of blood flow in normal and sympathectomized skin in humans

Sympathetic vasoconstrictor neurons innervating hairless skin of the cat show a respiratory rhythm of activity discharging in inspiration. The following questions arise: (1) Is it possible to detect respiratory variations in cutaneous blood flow in humans? (2) Are these variations actively mediated by rhythmic activity in vasoconstrictor neurons (active rhythms), or do they depend on blood flow changes induced passively due to respiratory blood pressure waves (passive rhythms)? Three patients who had been sympathectomized unilaterally and four healthy controls were studied. Cutaneous blood flow was measured bilaterally using a laser-Doppler flowmeter during physiological breathing (14/min, tidal volume 500-600 ml. minute volume 81/min) and during slower respiratory rate with a higher tidal and smaller minute volume (5/min, 11, 51/min). The temporal pattern of skin blood flow was analyzed with respect to respiration by constructing peri-event-time histograms after summation and averaging of 10-15 respiratory cycles. During physiological breathing no or minimal variation of cutaneous blood flow could be detected. During slower respiratory rate with higher tidal and smaller minute volume a potentiation of variations appeared. In controls the inspiratory phase was followed by a considerable decrease in cutaneous blood flow with a latency of 4.6 s. Identical rhythms were also present on the unoperated side of the patients. In contrast, on the sympathectomized side a respiratory rhythm appeared that was lower in amplitude and phase shifted by about half a cycle. We conclude: (1) Respiration related cutaneous blood flow variations can be detected, in particular if slower respiratory rates, higher tidal and smaller minute volumes are present. (2) Passive oscillations can be differentiated from active rhythms due to sympathetic vasoconstrictor activity by their temporal pattern. (3) The observations suggest that the neurons responsible for the active rhythm discharge during inspiration.