Haemodynamic and ADH responses to central blood volume shifts in cardiac-denervated humans

Vagal blockade suppresses the phase I heart rate response but not the phase I cardiac output response at exercise onset in humans

Purpose

We tested the vagal withdrawal concept for heart rate (HR) and cardiac output (CO) kinetics upon moderate exercise onset, by analysing the effects of vagal blockade on cardiovascular kinetics in humans. We hypothesized that, under atropine, the φ₁ amplitude (A₁) for HR would reduce to nil, whereas the A₁ for CO would still be positive, due to the sudden increase in stroke volume (SV) at exercise onset.

Methods

On nine young non-smoking men, during 0–80 W exercise transients of 5-min duration on the cycle ergometer, preceded by 5-min rest, we continuously recorded HR, CO, SV and oxygen uptake (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \dot{V} $$\end{document}V˙O₂) upright and supine, in control condition and after full vagal blockade with atropine. Kinetics were analysed with the double exponential model, wherein we computed the amplitudes (A) and time constants (τ) of phase 1 (φ₁) and phase 2 (φ₂).

Results

In atropine versus control, A₁ for HR was strongly reduced and fell to 0 bpm in seven out of nine subjects for HR was practically suppressed by atropine in them. The A₁ for CO was lower in atropine, but not reduced to nil. Thus, SV only determined A₁ for CO in atropine. A₂ did not differ between control and atropine. No effect on τ₁ and τ₂ was found. These patterns were independent of posture.

Conclusion

The results are fully compatible with the tested hypothesis. They provide the first direct demonstration that vagal blockade, while suppressing HR φ₁, did not affect φ₁ of CO.

Career perspective: Victor A. Convertino

This review focuses on a career of unique opportunities to participate in various areas of research related to extreme physiology and medicine. My experience as a volunteer subject in exercise experiments conducted at NASA included the study of acute and chronic physiological responses and adaptations to exercise in environments of hypoxia, heat stress, and simulated microgravity (bed rest), and eventually to my doctoral work on mechanisms underlying expansion of plasma and blood volume with acute and repeated exercise and heat exposure. My career has taken me to research positions at NASA, the Stanford University School of Medicine, the University of Arizona, and the U.S. Air Force Research Laboratory before assuming my present location at the U.S. Army Institute of Surgical Research. As a result of these multiple experiences across a period of 45 years, I have had opportunities to translate basic research to astronauts, high-performance aircraft pilots, and critically ill patients who are challenged by conditions of extreme physiology and medicine.

Plasma clearance of polyfructosan and extracellular body fluid distribution in idiopathic dilated cardiomyopathy and after heart transplantation

The total extracellular fluid volume and distribution in plasma and interstitial spaces, and the microvascular permeability properties were studied in 16 nonedematous patients with congestive heart failure (CHF) due to idiopathic dilated cardiomyopathy and 17 such patients who underwent heart transplantation (HT) by analyzing the 3-hour plasma disappearance curve of polyfructosan. Eighteen healthy subjects served as controls. Polyfructosan (3.5 kD) is an extracellular marker and inulin analog transported almost solely by diffusion. The initial capillary membrane plasma clearance (i.e., the permeability-surface area product), the interstitial plasma clearance determined at 10 minutes (clearance[10), and the extracellular volume were determined from the polyfructosan curves. I-131-albumin was used as a plasma volume reference. Permeability-surface area product was elevated in both patient groups (6.6 +/- 1.9 ml/ kg/min in the CHF group and 6.7 +/- 2.0 ml/kg/min in the HT group vs 5.1 +/- 1.3 ml/kg/min in controls, p <0.01 for both), whereas clearance(10) was normalized in the HT group (4.5 +/- 0.9 ml/kg/min in the HT group, 4.4 +/- 0.7 ml/kg/min in controls vs 5.0 +/- 0.9 ml/kg/ min in the CHF group, p <0.1 and p <0.05, respectively). The normalization of interstitial plasma clearance of polyfructosan was associated with time since HT (r = 0.49, p <0.05). Plasma volumes were similar in all 3 groups (41 +/- 8 ml/kg in controls, 44 +/- 13 in the CHF group and 39 +/- 8 in the HT group). In contrast, total extracellular volume was elevated in both patients groups (177 +/- 27 ml/kg in the CHF group and 173 +/-27 in the HT group vs 152 +/- 12 in controls, p <0.01). The results strongly suggest a microvascular permeability defect in both patient groups that perhaps plays a role in the extravascular distribution of the excess extracellular fluid volume.

Pathophysiologic basis for vasodepressor syncope

The current knowledge regarding the pathophysiologic basis of the vasodepressor response was reviewed. The balance of evidence indicates that the mechanoreceptor hypothesis seems unlikely to be the sole afferent alteration that leads to the vasodepressor response. Alternative afferent mechanisms should include neurohumoral mediated sympathoinhibition triggered by opioid mechanisms as well as impaired endothelial and NO responses to orthostatic stress in susceptible individuals. It is possible that impaired cardiovagal and sympathetic outflow control of arterial baroreceptors is enhanced by the aforementioned mechanisms. The role of central sympathoinhibition and vagal excitation triggered directly from pathways within the temporal lobe or triggered by alterations in regional cerebral blood flow should be considered as potential alternative mechanisms. Efferent autonomic outflow during vasodepressor syncope include sympathetic neural outflow withdrawal in addition to activation of parasympathetic outflow to the heart and abdominal viscera. Further human research is needed to understand the underlying mechanisms that result in the described neural and vascular responses.

Vasopressin Levels in Major Head and Neck Surgery

In an effort to further understand the perioperative intravascular volume status of major head and neck surgery patients, serum antidiuretic hormone (ADH) and osmolality levels were assessed at four perioperative junctures. Thirty-five major head and neck surgical patients were randomly selected for examination with placement of a central venous pressure monitor. Serum osmolality and serum vasopressin levels were obtained at four junctures perioperatively. ADH levels were lower both after patients were anesthetized and five hours into the procedure than at either baseline or 24 hours after the end of the procedure. ADH levels after patients were anesthetized did not differ from those at five hours into the procedure, nor did ADH levels at baseline differ from those 24 hours after the end of the procedure. In addition, osmolality levels did not change over time. Additional analyses examining relationships between preoperative, intraoperative, and postoperative characteristics and ADH levels after patients were anesthetized and five hours into the procedure, as well as changes from baseline at these times and the baseline levels themselves, detected no significant relationships. This study provides information about the perioperative intravascular volume status of major head and neck surgery patients which may be important to intraoperative care, especially to decisions regarding invasive intraoperative fluid monitoring. Specifically, the data provide additional evidence against the need for the routine placement of central venous catheters to guide fluid administration during major head and neck surgery.

Training effects on the hydromineral endocrine responses of cardiac transplant patients

Cardiac transplant patients manifest several haemodynamic changes as well as altered peripheral responses to exercise which may disrupt body fluid regulation. This study examined the effect on an endurance training programme on the exercise-induced hydromineral endocrine responses of heart transplant patients. Seven patients underwent a square-wave exercise test before and after a 6-week training programme. The tests were performed at the same absolute intensity but, during training, the workload was increased to maintain the same relative exercise intensity. Pretraining results were compared to those obtained from age-matched controls. Training improved physical capacity, producing a significant increase in maximal tolerated power and workload between the first and last training session (P < 0.05, P < 0.001, respectively). Haematocrit and osmolality were increased in both groups at the end of exercise (P < 0.01) but changes observed post-training did not differ from pretraining values. Apart from atrial natriuretic peptide (ANP), which showed significantly higher concentrations at rest and during exercise (P < 0.01), the changes in hydromineral hormones of the patients resembled those of the controls. Basal plasma renin activity (PRA) was slightly raised prior to training (P < 0.07) compared to the controls and post-training. For both PRA and aldosterone, a significant training effect was revealed when both the exercise-stimulated increase and postexercise decline were considered (P < 0.05), possibly reflecting lower noradrenaline concentrations. Training had no effect on either basal or exercise stimulated ANP levels, which is compatible with the theory that ANP regulation is largely under mechanical rather than sympathetic nervous system control.(ABSTRACT TRUNCATED AT 250 WORDS)

Acute and delayed hormonal changes in mitral stenosis treated by balloon valvulotomy

The role of left atrial and aortic pressures on the secretion of the main hormones controlling blood volume is still subject to debate in humans. Because of increased mean left atrial pressure and decreased mean aortic pressure produced by balloon inflation in patients with mitral stenosis treated with balloon valvulotomy, the hormonal changes occurring acutely (group II of patients) were measured. The same studies (group I patients) were also performed 48 hours after this treatment, a period at which left atrial pressure permanently diminished. Inflation of the balloon resulted in a decrease in plasma renin activity and increases in plasma atrial natriuretic factor (ANF) and plasma arginine vasopressin (AVP). Forty-eight hours after balloon valvulotomy, which had produced a decrease in left atrial pressure, plasma ANF was lower (58.9 +/- 7.9 vs 95.3 +/- 11.9 pg/ml; p < 0.001), and plasma renin activity (2,575 +/- 533 vs 960 +/- 113 pg/ml/hour; p < 0.01), plasma angiotensin II (25.0 +/- 4.1 vs 9.3 +/- 1.3 pg/ml; p < 0.001) and plasma aldosterone (181.7 +/- 36.7 vs 139.9 +/- 19.8 pg/ml; p < 0.05) were higher than their respective control levels 24 hours before treatment of the stenosis. In contrast, plasma AVP (3.7 +/- 0.25 vs 4.4 +/- 0.31 pg/ml; p = 0.001) diminished moderately along with plasma osmolality (282.4 +/- 0.1 vs 286.2 +/- 0.6 mOsm/kg; p < 0.001). Urinary sodium excretion was also examined before and after balloon valvulotomy.(ABSTRACT TRUNCATED AT 250 WORDS)

Gender differences in endocrine responses to posture and 7 days of -6 degrees head-down bed rest

Endocrine regulation of fluids and electrolytes during 7 days of -6 degrees head-down bed rest (HDBR) was compared in male (n = 8) and, for the first time, female (n = 8) volunteers. The subjects' responses to quiet standing for 2 h before and after HDBR were also tested. In both sexes, diuresis and natriuresis were evident during the first 2-3 days of HDBR, resulting in a marked increase in the urinary Na(+)-to-K+ ratio and significant Na+ retention on re-ambulation. After the 1st day of HDBR, plasma renin activity (PRA) was increased relative to aldosterone (Aldo), plasma volume was decreased, and the renal response to Aldo appeared to be appropriate. Circulating levels of arginine vasopressin, cortisol, and ACTH were unchanged during HDBR. Plasma testosterone decreased slightly on day 2 of HDBR in males. The ratio of early morning ACTH to cortisol was lower in females than in males because ACTH was lower in females. Urinary cortisol increased and remained elevated throughout the HDBR in males only. There were no gender differences in the responses to 7 days of HDBR, except those in the pituitary-adrenal system; those differences appeared unrelated to the postural change. The provocative cardiovascular test of quiet standing before and after HDBR revealed both sex differences and effects of HDBR. There were significant sex differences in cardiovascular responses to standing before and after HDBR. Females had greater PRA and Aldo responses to standing before HDBR and larger Aldo responses to standing after HDBR than males.(ABSTRACT TRUNCATED AT 250 WORDS)