Cold stress provokes sympathoinhibitory presyncope in healthy subjects and hemodialysis patients with low cardiac output

Effects of multielectrode renal denervation on elevated sympathetic nerve activity and insulin resistance in metabolic syndrome

OBJECTIVE

This study aimed to investigate the effects of renal denervation (RDN) on sympathetic nerve activity and insulin resistance in patients with metabolic syndrome.

METHODS

Seventeen patients fulfilled at least four of five criteria for metabolic syndrome and under stable use of at least two antihypertensive drugs were randomized in 3 : 1 ratio to RDN (n = 13, 12 men, age: 58 ± 7 years) and control groups (n = 4, three men, age: 60 ± 5 years) and followed up for 3 months. Muscle sympathetic nerve activity (MSNA) at rest and during standard 75 g oral glucose tolerance test (OGTT) was assessed.

RESULTS

In the RDN group, office and average 24-h blood pressures reduced by 16 ± 21/10 ± 11 mmHg (P = 0.01/0.007) and 14 ± 16/5 ± 8 mmHg (P = 0.008/0.03) respectively; waist circumference reduced by 3.1 ± 3.6 cm (P = 0.008); and resting MSNA reduced from 55 ± 9 bursts per minute to 46 ± 8 bursts per minute (P = 0.0008) at month 3 post-RDN. During OGTT, although blunted MSNA responses were noted at baseline throughout the 120-min test, improved MSNA responses with burst frequency increased to 52 ± 8 bursts per minute (P < 0.001 vs. the resting MSNA, n = 13) at 30 min and to 54 ± 8 bursts per minute (P = 0.004 vs. the resting MSNA, n = 10) at 120 min and were observed at month 3 post-RDN. No such improvements were observed in the controls. No significant change was observed in the HOMA-IR in both groups at month 3.

CONCLUSION

In this pilot study of patients with metabolic syndrome and associated hypertension, RDN reduced elevated sympathetic nerve activity and restored the normal neural response to oral glucose loading.

Cardiovascular and autonomic responses to physiological stressors before and after six hours of water immersion

The physiological responses to water immersion (WI) are known; however, the responses to stress following WI are poorly characterized. Ten healthy men were exposed to three physiological stressors before and after a 6-h resting WI (32-33°C): 1) a 2-min cold pressor test, 2) a static handgrip test to fatigue at 40% of maximum strength followed by postexercise muscle ischemia in the exercising forearm, and 3) a 15-min 70° head-up-tilt (HUT) test. Heart rate (HR), systolic and diastolic blood pressure (SBP and DBP), cardiac output (Q), limb blood flow (BF), stroke volume (SV), systemic and calf or forearm vascular resistance (SVR and CVR or FVR), baroreflex sensitivity (BRS), and HR variability (HRV) frequency-domain variables [low-frequency (LF), high-frequency (HF), and normalized (n)] were measured. Cold pressor test showed lower HR, SBP, SV, Q, calf BF, LFnHRV, and LF/HFHRV and higher CVR and HFnHRV after than before WI (P < 0.05). Handgrip test showed no effect of WI on maximum strength and endurance and lower HR, SBP, SV, Q, and calf BF and higher SVR and CVR after than before WI (P < 0.05). During postexercise muscle ischemia, HFnHRV increased from baseline after WI only, and LFnHRV was lower after than before WI (P < 0.05). HUT test showed lower SBP, DBP, SV, forearm BF, and BRS and higher HR, FVR, LF/HFHRV, and LFnHRV after than before WI (P < 0.05). The changes suggest differential activation/depression during cold pressor and handgrip (reduced sympathetic/elevated parasympathetic) and HUT (elevated sympathetic/reduced parasympathetic) following 6 h of WI.

Differential effects of acute and sustained cyclosporine and tacrolimus on sympathetic nerve activity

BACKGROUND

We studied the effect of acute and sustained cyclosporine and tacrolimus on muscle sympathetic nerve activity (MSNA) in groups of healthy male volunteers.

METHODS AND RESULTS

Acute cyclosporine in normal dose (2.5 mg/kg) increased MSNA from 11 +/- 6 to 19 +/- 8 bursts/min (P < 0.05). Acute cyclosporine in high dose (10 mg/kg) increased MSNA from 13 +/- 6 to 25 +/- 4 bursts/min (P < 0.05) and increased heart rate and mean arterial pressure (heart rate from 64 +/- 8 to 74 +/- 6 b.p.m., MAP from 92 +/- 10 to 105 +/- 8 mmHg; both P < 0.05). Sustained cyclosporine (2.5 mg/kg b.i.d. for 2 weeks) suppressed MSNA from 14 +/- 6 to 8 +/- 7 bursts/min (P < 0.05). Blood pressure increased from 89 +/- 6 to 98 +/- 6 mmHg (P < 0.05). Body weight increased and plasma renin activity was suppressed. Acute tacrolimus in regular dose (0.05 mg/kg) and high dose (0.20 mg/kg) had no effect on MSNA and blood pressure. Sustained tacrolimus (0.05 mg/kg b.i.d. for 2 weeks) had no effect on blood pressure, body weight and plasma renin activity, but decreased MSNA from 14 +/- 6 to 8 +/- 5 bursts/min (P < 0.05).

CONCLUSION

Sympathetic overactivity plays a role in the acute hypertensive action of cyclosporine. Cyclosporine given during 2 weeks increases blood pressure and suppresses MSNA, possibly by volume retention. Tacrolimus, in the presently applied dosages, does not cause hypertension or sympathetic overactivity. However, sustained tacrolimus also suppresses sympathetic activity, the reason of which is unclear.

A model of orthostatic hypotension in the conscious monkey using lower body negative pressure

INTRODUCTION

Methods most commonly used for detecting susceptibility to orthostatic hypotension in humans include head-up tilt and the application of lower body negative pressure (LBNP). The objective of this study was to evaluate the use of LBNP for detecting drug-induced changes in susceptibility to orthostatic hypotension in conscious monkeys (Macaca fascicularis).

METHODS

Orthostatic responses were produced using an airtight chamber, which sealed around the stomach (umbilical area) and enclosed the lower body, to which were applied successive decrements of 10 mmHg chamber pressure every 5 min until the orthostatic response was observed. Cardiovascular measurements, involving arterial pressures, heart rate, and left ventricular pressures were recorded. The hypotensive agents prazosin and minoxidil were administered to evaluate the ability of the procedure to detect drug-induced changes in the susceptibility to orthostatic hypotension.

RESULTS

A rapid decrease in systolic arterial pressure of >20 mmHg occurring within a 30 s time period was determined to be the best indicator of an orthostatic response. The application of LBNP produced an orthostatic response in all monkeys and on all occasions (100% response). The onset, rate and magnitude of the decrease in systolic blood pressure were also consistent for each monkey. Prazosin (>or=0.16 mg/kg, iv) produced an increase in the susceptibility to the orthostatic response, whereas minoxidil (10 mg/kg, po) had no effect. These results are consistent with previous findings in humans, where similar decreases in arterial pressures occur following the administration of prazosin and minoxidil, whereas increased susceptibility to orthostatic hypotension only occurs with prazosin.

DISCUSSION

The results of this study demonstrate that the application of the LBNP is a reliable method for producing an orthostatic hypotensive response in conscious monkeys. In addition, the use of positive (prazosin) and negative (minoxidil) controls demonstrated that the use of LBNP is a valid method for evaluating the effect of drug treatment on susceptibility to orthostatic hypotension.

Sympathetic nervous system contributes to the age-related impairment of flow-mediated dilation of the superficial femoral artery

The physiological aging process is associated with endothelial dysfunction, as assessed by flow-mediated dilation (FMD). Aging is also characterized by increased sympathetic tone. Therefore, the aim of the present study is to assess whether acute changes in sympathetic activity alter FMD in the leg. For this purpose, the FMD of the superficial femoral artery was determined in 10 healthy young (22 +/- 1 yr) and 8 healthy older (69 +/- 1 yr) men in three different conditions: 1) at baseline, 2) during reduction of sympathetic activity, and 3) during sympathetic stimulation. Reduction of sympathetic activity was achieved by performing a maximal cycling exercise, leading to postexercise attenuation of the sympathetic responsiveness in the exercised limb. A cold pressor test was used to increase sympathetic activity. Nitroglycerin (NTG) was used to assess endothelium-independent vasodilation in all three conditions. Our results showed that, in older men, the FMD and NTG responses were significantly lower compared with young men (P = 0.001 and P = 0.02, respectively). In older men, sympathetic activity significantly affected the FMD response [repeated-measures (RM) ANOVA: P = 0.01], with a negative correlation between the level of sympathetic activity and FMD (R = -0.41, P = 0.049). This was not the case for NTG responses (ANOVA; P = 0.48). FMD and NTG responses in young men did not differ among the three conditions (RM-ANOVA: P = 0.32 and P = 0.31, respectively). In conclusion, in older men, FMD of the femoral artery is impaired. Local attenuation of the sympathetic responsiveness partly restores the FMD in these subjects. In contrast, in young subjects, acute modulation of the sympathetic nervous system activity does not alter flow-mediated vasodilation in the leg.

Cold pressor test protocol to evaluate cardiac autonomic function

The primary objective of this study was to develop a cold pressor test (CPT) protocol to evaluate cardiac autonomic function. Secondary objectives were to assess CPT protocol reliability and to examine gender differences in response to orthostatic stress and the CPT. Healthy, normotensive men and women (n = 12 per group) completed 2 trials on different days in the left lateral decubitus and standing postures and during a 6 min CPT (hand immersion while seated). Measurements included R-R interval, blood pressure, ventilatory responses, spontaneous baroreflex sensitivity, and heart rate variability indices. During the CPT, blood pressure and the sympathetic nervous system (SNS) indicator increased significantly and low-frequency power, high-frequency power (normalized for tidal volume), and the parasympathetic nervous system (PNS) indicator decreased significantly. Standing caused significant increases in the SNS indicator and decreases in the R-R interval in both genders. The PNS indicator was higher in women than in men in the left lateral decubitus posture. The 6 min hand-immersion CPT provoked cardiac sympathetic activation and parasympathetic withdrawal; however, it is best suited to studies with a repeated measures design, as analysis of reliability suggests that responses are highly variable between individuals. Performing the CPT in the left lateral decubitus position may prevent vasovagal responses.Key words: cold pressor test, sympathetic nervous system, parasympathetic nervous system, gender, posture.

Noninvasive investigation of blood pressure changes using the pulse wave transit time: a novel approach in the monitoring of hemodialysis patients

Severe blood pressure changes are well known in hemodialysis. Detection and prediction of these are important for the well-being of the patient and for optimizing treatment. New noninvasive methods for this purpose are required. The pulse wave transit time technique is an indirect estimation of blood pressure, and our intention is to investigate whether this technique is applicable for hemodialysis treatment. A measurement setup utilizing lower body negative pressure and isometric contraction was used to simulate dialysis-related blood pressure changes in normal test subjects. Systolic blood pressure levels were compared to different pulse wave transit times, including and excluding the cardiac preejection period. Based on the results of these investigations, a pulse wave transit time technique adapted for dialysis treatment was developed and tried out on patients. To determine systolic blood pressure in the normal group, the total pulse wave transit time was found most suitable (including the cardiac preejection period). Correlation coefficients were r = 0.80 +/- 0.06 (mean +/- SD) overall and r = 0.81 +/- 0.16 and r = 0.09 +/- 0.62 for the hypotension and hypertension phases, respectively. When applying the adapted technique in dialysis patients, large blood pressure variations could easily be detected when present. Pulse wave transit time is correlated to systolic blood pressure within the acceptable range for a trend-indicating system. The method's applicability for dialysis treatment requires further studies. The results indicate that large sudden pressure drops, like those seen in sudden hypovolemia, can be detected.

Effect of gastric distension on cardiovascular parameters: gastrovascular reflex is attenuated in the elderly

Stretching the stomach wall in young healthy subjects causes an increase in muscle sympathetic nerve activity and in blood pressure, the gastrovascular reflex. We compared healthy elderly subjects with healthy young subjects to find out whether the gastrovascular reflex attenuates in normal ageing and we studied whether there was a difference in autonomic function or gastric compliance that could explain this possible attenuation. Muscle sympathetic nerve activity, finger blood pressure and heart rate were continuously recorded during stepwise isobaric gastric distension using a barostat in eight healthy young (6 men and 2 women, 27 +/- 3.2 years, mean +/-s.e.m.) and eight healthy elderly subjects (7 men and 1 woman, 76 +/- 1.5 years). Changes in cardiac output and total peripheral arterial resistance were calculated from the blood pressure signal. The baseline mean arterial pressure and muscle sympathetic nerve activity were higher in the elderly group (both P < 0.05) and muscle sympathetic nerve activity increase during the cold pressor test was lower in the elderly group (P = 0.005). During stepwise gastric distension, the elderly subjects showed an attenuated increase in muscle sympathetic nerve activity compared to the young subjects (P < 0.01). The older group tended to show a higher increase in mean arterial pressure (P = 0.08), heart rate (P = 0.06) and total peripheral arterial resistance (P = 0.09) The cardiac output rose slightly in both groups without significant difference between groups. The fundic compliance did not differ between groups. We conclude that stepwise gastric distension caused an increase in muscle sympathetic nerve activity in both groups, but the increase in the elderly was attenuated.

Subtle involvement of the sympathetic nervous system in amyotrophic lateral sclerosis

The literature on the involvement of the autonomic nervous system (ANS) in amyotrophic lateral sclerosis (ALS) is conflicting. We therefore investigated several aspects of autonomic function, namely muscle sympathetic nerve activity (MSNA), blood pressure, cardiac function (electrocardiogram; ECG), and respiration in 16 patients with sporadic ALS and in 12 age-matched healthy volunteers, both at rest and during sympathoexcitatory stimulation. We measured MSNA by provoking venous pooling during short-lasting lower body negative pressure (LBNP) and during the cold pressor test (CPT). To assess the vagal (baroreflex) control of heart rate (HR), we measured spontaneous baroreflex sensitivity (BRS). To assess the involvement of the ANS beyond the cardiovascular system, we measured the sympathetic skin response (SSR). The stand-up test showed that none of the subjects had orthostatic intolerance. In comparison with the control group, the ALS patients had an increased HR and a decreased BRS at rest, and a reduced MSNA response to LBNP. The CPT response was normal and the total MSNA at rest did not differ significantly from that of controls. The latencies of the palmar and plantar SSR were prolonged, and in 3 ALS patients there was no plantar SSR. The results indicate that the sympathetic nervous system shows subtle abnormalities in ALS, predominantly sympathetic overactivity. They also point to the involvement of the preganglionic sympathetic column as the cause of the higher sympathetic activity and the absence of SSR. The higher sympathetic activity is postulated to be due to changes in modulation of the sympathetic system, whereas the absence of the SSR is probably caused by disruption of the reflex pathway.

Sympathetic activation markedly reduces endothelium-dependent, flow-mediated vasodilation

OBJECTIVES

We sought to evaluate whether increased sympathetic outflow may interfere with flow-mediated dilation (FMD).

BACKGROUND

Endothelial function, assessed as FMD, is frequently used as an intermediate end point in intervention studies. Many disease states with increased sympathetic tone are also characterized by endothelial dysfunction.

METHODS

Sixteen healthy volunteers underwent FMD studies with and without concomitant sympathetic stimulation. Intra-arterial nitroglycerin (NTG) infusion was used to assess endothelium-independent vasodilation. Pathophysiologically relevant sympathetic stimulation was achieved by baroreceptor unloading, using a lower body negative pressure box. In a subset of eight volunteers, this protocol was repeated during loco-regional alpha-adrenergic blockade by intra-arterial infusion of phentolamine (PE). Reactive hyperemic flow was assessed with strain-gauge phlethysmography.

RESULTS

Overall, FMD responses (8.3 +/- 3.4%) were significantly attenuated by concomitant sympathetic stimulation (3.6 +/- 3.4%, p < 0.01). Loco-regional alpha-adrenergic blockade had no effect on baseline FMD responses (10.7 +/- 4.7%), whereas the attenuation by sympathetic stimulation was abolished completely during PE co-infusion (11.5 +/- 3.3%). During intra-arterial NTG infusions, arterial diameters relative to baseline were not significantly different between the four possible stages.

CONCLUSIONS

Sympathetic stimulation, at a clinically relevant range, significantly impairs the FMD response by an alpha-adrenergic mechanism.

Role of short-term regulatory mechanisms on pressure response to hemodialysis-induced hypovolemia

BACKGROUND

A large inter-subject variability exists in the arterial pressure response to hemodialysis-induced blood volume (BV) withdrawal. We investigated the hypothesis that this variability is due to the inter-subject differences in the short-term reflex capacity to compensate for hypovolemia.

METHODS

Mean arterial pressure (MAP), heart rate (HR) and the percentage reduction in BV (%R-BV) were recorded in 32 subjects during their regular hemodialysis sessions. On the basis of absolute MAP changes between the beginning and the end of the session with respect to %R-BV at the end of the session, three distinct pressure responses were identified: (1) hypotension-prone response; (2) unstable response with delayed hypotension; and (3) hypotension-resistant response. For each kind of response, one patient was selected and a computer model of the cardiovascular system including the main short-term reflex compensatory mechanisms was used to analyze data collected over five consecutive sessions. The %R-BV and HR were used as model inputs, while simulated arterial pressure was fitted to the measured MAP by the tuning model parameters representing the efficiency in the control of venous capacity, microvascular resistance and heart inotropism.

RESULTS

The model-based analysis related the hypotension-prone response to a lack of efficacy in capacity and resistance regulation. In the unstable response with delayed hypotension, the control of venous capacity was not effective and resistance control alone kept the pressure stable only for a limited %R-BV reduction (<5%). In the hypotension-resistant response, an efficient compensation of capacitance vessels was evidenced, and the slightly increasing arterial pressure was referred to a prevalence of cardiopulmonary pathway in the compensatory process.

CONCLUSIONS

The model ascribes differences in pressure response to differences in the effectiveness of reflex compensatory mechanisms.

Hemodynamic patterns and spectral analysis of heart rate variability during dialysis hypotension

Intradialytic hypotension, a major source of morbidity during hemodialysis and ultrafiltration, is often accompanied by paradoxical bradycardia. Relatively little is known about the sequential changes in autonomic nervous system activity up to and during the hypotensive episode. Continuous, beat-to-beat measurements of BP and heart rate were made during hemodialysis in patients prone (n = 8) and not prone (n = 11) to develop intradialytic hypotension. Off-line spectral analysis of heart rate variability (HRV) was performed to assess changes in autonomic nervous system activity during dialysis sessions both with and without hypotension. The low frequency (LF) component of HRV is thought to correlate with sympathetic nervous system activity, the high frequency (HF) component with that of the parasympathetic nervous system. In the sessions not complicated by symptomatic hypotension (n = 26), mean arterial BP (MAP) hardly fell, whereas heart rate increased from 77 +/- 2 to 89 +/- 5 bpm (P < 0.05). The LF component of HRV increased from 45.2 +/- 5.0 normalized units (nu) to 59.9 +/- 4.9 nu (P < 0.05), whereas the HF component fell from 54.8 +/- 5.0 to 40.2 +/- 4.4 nu (P < 0.05). These changes agree with compensatory baroreflex-mediated activation of the sympathetic nervous system (and suppressed parasympathetic activity) during ultrafiltration-induced intravascular volume depletion. In the sessions complicated by severe symptomatic hypotension (n = 22), the changes in heart rate and the results of spectral analysis of HRV were similar to those reported above up to the moment of sudden symptomatic (nausea, vomiting, dizziness, cramps) hypotension, whereas MAP had already fallen gradually from 94 +/- 3 to 85 +/- 3 mmHg (P < 0.05). The sudden further reduction in MAP (to 55 +/- 2 mmHg, P < 0.02) was invariably accompanied by bradycardia (heart rate directly before hypotension 90 +/- 2 bpm, during hypotension 69 +/- 3 bpm, P < 0.002). The LF component of HRV fell from 62.8 +/- 4.6 nu directly before to 40.0 +/- 3.7 nu (P < 0.05) during hypotension, whereas the HF component increased from 37.9 +/- 4.7 to 60.3 +/- 3.7 nu (P < 0.05). These findings agree with activation of the cardiodepressor reflex, involving decreased sympathetic and increased parasympathetic nervous system activity, respectively. These findings indicate that activation of the sympatho-inhibitory cardiodepressor reflex (Bezold-Jarisch reflex), which is a physiologic response to a critical reduction in intravascular volume and cardiac filling, is the cause of sudden intradialytic hypotension.

Reduction of sympathetic hyperactivity by enalapril in patients with chronic renal failure

BACKGROUND

Inhibition of angiotensin-converting enzyme (ACE) reduces the risk of cardiovascular problems in patients with chronic renal failure. This effect may be due in part to a decrease in sympathetic nervous activity, but no direct evidence of such an action is available.

METHODS

We studied muscle sympathetic-nerve activity in 14 patients with hypertension, chronic renal failure, and increased plasma renin activity before, during, and after administration of the ACE inhibitor enalapril. Ten other patients with similar clinical characteristics were studied before and during treatment with the calcium-channel blocker amlodipine. Normal subjects matched for age and weight were included in both studies.

RESULTS

At base line, mean (+/-SD) muscle sympathetic-nerve activity was higher in the group of patients who received enalapril than in the control subjects (35+/-17 vs. 19+/-9 bursts per minute, P=0.004). The baroreflex curve, which reflects changes in muscle sympathetic-nerve activity caused by manipulations of blood pressure with sodium nitroprusside and phenylephrine, was shifted to the right in the patients, but baroreflex sensitivity was similar to that in the control subjects (-2.1+/-1.9 and -2.7+/-1.3 bursts per minute per mm Hg, respectively; P=0.36). A single dose of the sympatholytic drug clonidine caused a greater fall in blood pressure in the patients than in the control subjects. Treatment with enalapril normalized blood pressure and muscle sympathetic-nerve activity (at 23+/-10 bursts per minute) in the patients and shifted the baroreflex curve to the left, reflecting normal blood-pressure levels, without significantly changing sensitivity (-2.3+/-1.8 bursts per minute per mm Hg, P=0.96). In the patients who received amlodipine, treatment also lowered blood pressure but increased muscle sympathetic-nerve activity, from 41+/-19 to 56+/-14 bursts per minute (P=0.02).

CONCLUSIONS

Increased sympathetic activity contributes to hypertension in patients with chronic renal disease. ACE inhibition controls hypertension and decreases sympathetic hyperactivity.

Stomach distension increases efferent muscle sympathetic nerve activity and blood pressure in healthy humans

Although the enteric nervous system is usually described as a separate and independent entity, animal studies show that gastric distension causes a reflex increase in arterial pressure and a sympathetically mediated increase in heart rate and peripheral vascular resistance. To assess the influence of gastric distension on sympathetic outflow and blood pressure, we recorded muscle sympathetic nerve activity (MSNA) from the peroneal nerve by microneurography in eight healthy volunteers. The stomach was distended by means of a barostat, using a single staircase protocol by which pressure was increased by 2 mmHg every 3 min. Gastric sensory function was assessed at each distension step by using a visual analog scale (VAS) for sensations of fullness, nausea and pain. For comparison, we also performed a cold pressor test. The MSNA increased on barostat-induced gastric distension with an almost concomitant elevation of blood pressure. The increase in both was proportional to the intragastric pressure and both decreased towards initial values after the end of distension. Heart rate increased inconsistently and only at higher distension pressures that were associated with high VAS scores. The opposite was found for the cold pressor test. The results of this study confirm the existence of a functional relationship between gastrointestinal distension and cardiovascular function. Decrease in this gastrovascular response may play a role in postprandial hypotension in the elderly, since the MSNA responses to simulated microgravity decrease with age.