Respiratory influences on muscle sympathetic nerve activity and vascular conductance in the steady state.

In patients with hypertension, volitional slowing of the respiratory rate has been purported to reduce arterial pressure via withdrawal of sympathetic tone. We examined the effects of paced breathing at 7, 14, and 21 breaths/min, with reciprocal changes in tidal volume, on muscle sympathetic nerve activity, forearm blood flow, forearm vascular conductance, and blood pressure in 21 men and women, 8 of whom had modest elevations in systemic arterial pressure. These alterations in breathing frequency and volume did not affect steady-state levels of sympathetic activity, blood flow, vascular conductance, or blood pressure (all P > 0.05), even though they had the expected effect on sympathetic activity within breaths (i.e., increased modulation during low-frequency/high-tidal volume breathing) (P < 0.001). These findings were consistent across subjects with widely varied baseline levels of sympathetic activity (4-fold), mean arterial pressure (78-110 mmHg), and vascular conductance (15-fold), and those who became hypocapnic during paced breathing vs. those who maintained normocapnia. These findings challenge the notion that slow, deep breathing lowers arterial pressure by suppressing steady-state sympathetic outflow.