Vascular sympathetic nerve function in congestive heart failure

Recovery kinetics of oxygen uptake and heart rate in patients with coronary artery disease and heart failure

BACKGROUND

Patients with congestive heart failure exhibit a prolonged period of recovery to baseline levels of oxygen consumption, but the decline of heart rate during recovery from exercise has been shown to be similar to that in healthy subjects, and the results of studies on the response of ventilation in recovery have been mixed. Patients with coronary artery disease have a reduced exercise capacity, but it is unknown whether the patterns of the decline in oxygen uptake (VO2), ventilation, or heart rate are similar to those in patients with heart failure.

METHODS

We performed a cardiopulmonary exercise test with a ramping protocol in 18 healthy subjects, 18 patients with coronary artery disease, 19 patients with class A or B congestive heart failure, and 19 patients with class C congestive heart failure, according to the Weber classification. Peak oxygen uptake and the kinetics of oxygen uptake, ventilation, and heart rate were calculated and expressed as the slope of a single exponential relation between VO2 levels and time during the first 3 min of recovery as y(VO2) = y0Ae(-x/t).

RESULTS

A difference in time of recovery of VO2 was found only between healthy subjects and patients with more severe heart failure (class C) (p < 0.05); no significant differences were observed among any of the groups in ventilation or heart rate recovery responses.

CONCLUSION

VO2 recovery time is prolonged only in the presence of more severe heart failure. The presence and degree of heart disease has no effect on ventilation or heart rate recovery time.

Modulation of noradrenaline release through presynaptic alpha 2-adrenoceptors in congestive heart failure

Stimulation of presynaptic alpha 2-adrenoceptors inhibits the release of noradrenaline from sympathetic nerve endings; however, the extent to which it operates in patients with congestive heart failure is still unknown. To investigate the degree of negative feedback to the release of noradrenaline via presynaptic alpha 2-adrenoceptors at sympathetic nerve endings, we measured plasma noradrenaline levels before and after the injection of phentolamine (i.e., plasma noradrenaline concentration at rest, plasma noradrenaline concentration after phentolamine injection [NAph], and the phentolamine-induced increase in plasma noradrenaline [delta NAph]). Plasma noradrenaline concentration at rest, NAph, and delta NAph increased in a stepwise manner from New York Heart Association class I to class III. A positive correlation was found between the plasma noradrenaline at rest and delta Naph (n = 123, r = 0.697, p < 0.001). These results suggest that the enhanced release of plasma noradrenaline is substantially buffered by the mechanism of noradrenaline release-inhibitory presynaptic alpha 2-adrenoceptors in patients with congestive heart failure, and this buffer serves to protect organs such as the heart from excess sympathetic stimulation.

Prolonged kinetics of recovery of oxygen consumption after maximal graded exercise in patients with chronic heart failure. Analysis with gas exchange measurements and NMR spectroscopy

BACKGROUND

Patients with chronic heart failure (CHF) often complain of prolonged dyspnea after exercise. The determinants of oxygen consumption after exercise in these patients are unknown. We hypothesized that the kinetics of oxygen consumption recovery after graded exercise was prolonged in parallel with the recovery of muscle energy stores, was not affected by the exercise level, and could be used to assess the circulatory response to exercise.

METHODS AND RESULTS

Seventy-two patients with CHF in Weber's class A (n = 28), B (n = 21), and C/D (n = 23) and 13 healthy subjects performed maximal upright bicycle exercise with breath-by-breath respiratory gas analysis. Kinetics of recovery of ventilation (VE), oxygen consumption (VO2), and CO2 production (VCO2) after exercise were characterized by T1/2, the time to reach 50% of the peak value. T1/2 VO2 (seconds) increased with the severity of CHF (97 +/- 17 for CHF A [P < .05 versus CHF B, P < .05 versus CHF C/D], 119 +/- 22 for CHF B [P < .05 versus control subjects, P < .05 versus CHF A, and P < .05 versus CHF C/D], 155 +/- 55 for CHF C/D [P < .05 versus control subjects, P < .05 versus CHF A, and P < .05 versus CHF B] compared with 77 +/- 17 for control subjects). T1/2 VCO2 and T1/2 VE also increased similarly with the worsening of CHF. T1/2 VO2 was correlated negatively with peak VO2 (r = .65) and was reproducible (r = .96). To study the relation between T1/2 VO2 and the duration of exercise, 10 healthy subjects and 22 patients underwent a second graded test at 75% and/or 50% of peak workload. T1/2 VO2 was minimally shortened, at only 50% of peak workload (P = .02). Finally, 19 patients underwent 31P nuclear magnetic resonance spectroscopy of the anterior compartment of the leg during exercise; the half-time of recovery of the ratio of inorganic phosphate to creatine phosphate (T1/2 Pi/PCr), reflecting the level of involvement of oxidative metabolism in the restoration of energetic metabolites after exercise, was linearly correlated with the half-time of VO2 recovery (r = .70, P < .01).

CONCLUSIONS

Postexercise T1/2 VO2 increases when CHF worsens, perhaps in part a result of slower kinetics of recovery of muscle energy stores. The time course of oxygen consumption recovery may represent a simple new criterion for measuring the impairment of the circulatory response to exercise in CHF, even submaximal exercise.

Carotid sinus baroreceptor reflex in dogs with experimental heart failure

We have previously demonstrated a decrease in baroreceptor discharge sensitivity in dogs with experimental heart failure. In the present study, we determined the sensitivity of the carotid sinus baroreceptor reflex in dogs with pacing-induced heart failure. The carotid sinus baroreceptor reflex sensitivity was determined by pressurizing one carotid sinus with all other baroreceptor and cardiopulmonary receptor inputs removed. The data were analyzed by plotting carotid sinus pressure-mean arterial pressure curves and carotid sinus pressure-renal sympathetic nerve activity curves in the two groups of dogs. The peak arterial pressure during carotid hypotension was significantly depressed in dogs with heart failure compared with normal dogs (107.1 +/- 5.7 versus 139.8 +/- 7.0 mm Hg, p less than 0.001). Mean arterial pressure range, renal sympathetic nerve activity range, and peak slope were significantly decreased in the heart-failure group. To determine if this depression was completely due to depression of baroreceptor discharge per se, or to alterations in central or end-organ responsiveness, similar experiments were performed by stimulating the carotid sinus nerve and evaluating frequency, voltage, and duration of stimulation on the resultant mean arterial pressure and renal sympathetic nerve activity. As was the case with carotid sinus pressurization, electrical stimulation caused a significantly smaller change in mean arterial pressure in heart-failure dogs compared with the normal dogs. However, there was no significant difference between normal and heart-failure dogs for the renal sympathetic nerve activity-electrical stimulation curves. These data strongly suggest that the depressed carotid sinus baroreceptor reflex in heart failure is not solely the result of depressed baroreceptor responsiveness but may be related to poor end-organ responses and normal central control of renal sympathetic outflow.