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 ma rked sympathoexcitation, including an augmented muscle sympathetic nerve ac tivity (MSNA) in intraneural multiunit recordings. In the present study, si ngle-unit recording was used to evaluate whether the firing properties of i ndividual muscle vasoconstrictor neurons can reveal underlying mechanisms f or this increase in MSNA. Methods and Results-Eight patients with CHF (NYHA class II to IV; left vent ricular ejection fraction, 29+/-5%, mean+/-SEM) were studied. In standard m ultiunit recordings, MSNA burst incidence (bursts/100 heartbeats) ranged fr om 65% to 100% (88+/-5%). Using selective tungsten microelectrodes, we made recordings from 16 single muscle vasoconstrictor axons. Mean unit firing p robability (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 previou sly in healthy subjects (P<0.001). Although single neurons occasionally gen erated 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 neu rons is one mechanism for the increased number of multiunit MSNA bursts at rest in CHF. The neurons discharge in more diastoles than in healthy subjec ts (ie, firing probability is increased), but the likelihood of discharging >1 impulse per sympathetic burst is not increased. Despite the intense mul tiunit activity at rest, the firing characteristics of individual vasoconst rictor axons indicate a remaining capacity for transient increases of MSNA in CHF.