Firing properties of single muscle vasoconstrictor neurons in the sympathoexcitation associated with congestive heart failure

Background-Congestive heart failure (CHF) in humans is associated with a marked sympathoexcitation, including an augmented muscle sympathetic nerve activity (MSNA) in intraneural multiunit recordings. In the present study, single-unit recording was used to evaluate whether the firing properties of individual muscle vasoconstrictor neurons can reveal underlying mechanisms for this increase in MSNA. Methods and Results-Eight patients with CHF (NYHA class II to IV; left ventricular ejection fraction, 29+/-5%, mean+/-SEM) were studied. In standard multiunit recordings, MSNA burst incidence (bursts/100 heartbeats) ranged from 65% to 100% (88+/-5%). Using selective tungsten microelectrodes, we made recordings from 16 single muscle vasoconstrictor axons. Mean unit firing probability (ie, the percentage of cardiac intervals in which a single axon fired) was 54.5+/-5.2% (range, 21 to 89%), and mean firing frequency was 0.98+/-0.22 Hz (0.14 to 3.86 Hz), both of which were higher than seen previously in healthy subjects (P<0.001). Although single neurons occasionally generated multiple spikes per sympathetic burst, such multiple firing was rare and was not different from that seen in healthy subjects. Conclusions-An increased firing frequency of individual vasoconstrictor neurons is one mechanism for the increased number of multiunit MSNA bursts at rest in CHF. The neurons discharge in more diastoles than in healthy subjects (ie, firing probability is increased), but the likelihood of discharging >1 impulse per sympathetic burst is not increased. Despite the intense multiunit activity at rest, the firing characteristics of individual vasoconstrictor axons indicate a remaining capacity for transient increases of MSNA in CHF.