Modulation of muscle sympathetic activity during spontaneous and artificial ventilation and apnoea in humans

Respiratory modulation of muscle sympathetic activity was compared in relaxed subjects breathing spontaneously and in anaesthetized and non-anaesthetized subjects ventilated artificially with intermittent positive pressure. Muscle sympathetic activity was recorded directly from the peroneal nerve using the microneurographic technique. Arterial pressure was monitored continuously either by finger-pulse photoplethysmography (Finapres) or intra-arterially. Respiratory modulation of sympathetic activity, heart rate and arterial pressure was measured by averaging consecutive breaths to the ECG R-wave closest to the onset of inspiration. In relaxed subjects (n = 15) breathing quietly the averaged sympathetic activity was greatest during late expiration and the first half of inspiration and minimal after the peak of inspiration, after correcting for delays within the baroreflex loop. Systolic and diastolic pressures fell during inspiration. In anaesthetized or awake subjects ventilated artificially at normal tidal volumes the pattern of respiratory modulation of sympathetic activity was preserved but the changes in arterial pressure were reversed and respiratory sinus arrhythmia abolished. Ventilation with positive end-expiratory pressure (20 cmH₂O) increased the overall level of sympathetic activity and enhanced the breath-to-breath modulation. We conclude that, although baroreceptors provide potent modulation of muscle sympathetic activity in humans, the inspiratory inhibition of sympathetic activity does not depend on an increase in arterial pressure and hence an increase in baroreceptor input.