Osmosensitive mechanisms contribute to the water drinking-induced pressor response in humans

Hemodynamic Effects of the Osmopressor Response: A Systematic Review and Meta-Analysis

Background Rapidly consuming water may offer practical orthostatic hypotension therapy. However, its efficacy across disorders remains uncertain. This study aims to assess the impact of rapid 350- to 500-mL water intake on systolic and diastolic blood pressure (BP) and heart rate (HR) through a systematic review and meta-analysis. Methods and Results We systematically reviewed MEDLINE and Embase up to June 2023, including randomized controlled trials and prospective cohort studies. Using random-effects meta-analysis, we calculated pooled mean differences (MDs) for maximum hemodynamic effects of rapid 350- to 500-mL water bolus consumption. Participants with orthostatic hypotension experienced increased systolic BP (MD, 24.18 [95% CI, 15.48-32.88]) and diastolic BP (MD, 11.98 [95% CI, 8.87-15.09]) with decreased HR (MD, -3.46 [95% CI, -5.21 to -1.71]). Similar results were observed in multiple system atrophy and pure autonomic failure subgroup analysis. Healthy participants showed modest increases in systolic BP (MD, 2.33 [95% CI, 1.02-3.64]) and diastolic BP (MD, 2.73 [95% CI, 1.15-4.30]), but HR changes were not significant (MD, -2.06 [95% CI, -5.25 to 1.13]). Water had no significant hemodynamic effects in patients with seated or supine postural tachycardia syndrome, although standing effects were unassessed. Our data do not exclude water's potential standing effect in postural tachycardia syndrome. Conclusions In patients with orthostatic hypotension, rapid water intake elevated short-term systolic BP and diastolic BP, with mild HR reduction when seated or supine. Healthy participants exhibited similar but milder effects. However, patients with postural tachycardia syndrome did not experience these changes in seated or supine positions. Further research is needed to evaluate the promising impact of rapid water ingestion on patients with postural tachycardia syndrome in a standing position, which was not addressed in our study.

Oral and intravenous hydration in the treatment of orthostatic hypotension and postural tachycardia syndrome

Hydration with water and salt is the mainstay of treatment for autonomic nervous system disorders that impair orthostatic tolerance. The goal is to expand intravascular volume to compensate for the downward displacement of blood volume that occurs when standing and thereby sustain cerebral perfusion and restore quality of life. Despite strong consensus recommendations for salt supplementation as standard treatment of these disorders, published evidence of benefit is relatively weak, and no randomized clinical trials have occurred. This review summarizes the physiological rationale for hydration and evaluates the literature on oral and intravenous hydration in the treatment of neurogenic orthostatic hypotension, postural tachycardia syndrome, and recurrent vasovagal syncope. We conclude that oral salt replacement is indicated for treatment of neurogenic orthostatic hypotension because these patients have excessive renal sodium excretion, and for treatment of chronic orthostatic intolerance because these patients are often hypovolemic. As not all patients are able to tolerate sufficient oral hydration, there is also a role for intravenous volume-loading in severe cases of postural tachycardia syndrome. We offer guidance, based on review of the literature and the clinical judgment of a cardiologist and neurologist with experience treating autonomic disorders, regarding the option of ongoing intravenous hydration for treatment of severe, refractory cases of postural tachycardia syndrome.

Effect of temperature and/or sweetness of beverages on body composition in rats

Sweetened beverages are mainly consumed cold and various processes are activated in response to external temperature variations. However, the effect of internal temperature variations through the ingestion of cold beverages is far from clear. Two experiments were conducted to investigate the effect of beverage temperature on body composition. Sprague-Dawley rats (5-6-week-old males) had free access to food and beverage for 8 weeks. Energy intake, body weight and body composition were monitored. In Expt 1, two groups of rats (n 9) consumed water at room temperature (NW about 22°C) or cold (CW about 4°C). In Expt 2, rats were offered room-temperature (N) or cold (C) sweetened water (10 % sucrose CSu (n 7) and NSu (n 8); or 0·05 % acesulfame K CAk (n 6) and NAk (n 8)) for 12 h, followed by plain water. Our results show that in Expt 1, CW had higher lean body mass (P < 0·001) and lower body fat gain (P = 0·004) as compared with NW. In Expt 2, body weight (P = 0·013) and fat (P ≤ 0·001) gains were higher in the non-energetic sweetened groups, while lean body mass was not affected by the type of sweeteners or temperature. In conclusion, cold water ingestion improved lean body mass gain and decreased fat gain because of increased energy expenditure, while non-energetic sweetener (acesulfame K) increased body fat gain due to improved energy efficiency. Internal cold exposure failed to increase energy intake in contrast to that of external cold exposure.

Gastric cooling and menthol cause an increase in cardiac parasympathetic efferent activity in healthy adult human volunteers

NEW FINDINGS

What is the central question of this study? How do gastric stretch and gastric cooling stimuli affect cardiac autonomic control? What is the main finding and its importance? Gastric stretch causes an increase in cardiac sympathetic activity. Stretch combined with cold stimulation result in an elimination of the sympathetic response to stretch and an increase in cardiac parasympathetic activity, in turn resulting in a reduction in heart rate. Gastric cold stimulation causes a shift in sympathovagal balance towards parasympathetic dominance. The cold-induced bradycardia has the potential to decrease cardiac workload, which might be significant in individuals with cardiovascular pathologies.

ABSTRACT

Gastric distension increases blood pressure and heart rate in young, healthy humans, but little is known about the effect of gastric stretch combined with cooling. We used a randomized crossover study to assess the cardiovascular responses to drinking 300 ml of ispaghula husk solution at either 6 or 37°C in nine healthy humans (age 24.08 ± 9.36 years) to establish the effect of gastric stretch with and without cooling. The effect of consuming peppermint oil capsules to activate cold thermoreceptors was also investigated. The ECG, respiratory movements and continuous blood pressure were recorded during a 5 min baseline period, followed by a 115 min post-drink period, during which 5 min epochs of data were recorded. Cardiac autonomic activity was assessed using time and frequency domain analyses of respiratory sinus arrhythmia to quantify parasympathetic autonomic activity, and corrected QT (QTc) interval analysis to quantify sympathetic autonomic activity. Gastric stretch only caused a significant reduction in QTc interval lasting up to 15 min, with a concomitant but non-significant increase in heart rate, indicating an increased sympathetic cardiac tone. The additional effect of gastric cold stimulation was significantly to reduce heart rate for up to 15 min, elevate indicators of cardiac parasympathetic tone and eliminate the reduction in QTc interval seen with gastric stretch only. Stimulation of gastric cold thermoreceptors with menthol also caused a significant reduction in heart rate and concomitant increase in the root mean square of successive differences. These findings indicate that gastric cold stimulation causes a shift in the sympathovagal balance of cardiac control towards a more parasympathetic dominant pattern.

Evidence-based treatment of neurogenic orthostatic hypotension and related symptoms

Neurogenic orthostatic hypotension, postprandial hypotension and exercise-induced hypotension are common features of cardiovascular autonomic failure. Despite the serious impact on patient’s quality of life, evidence-based guidelines for non-pharmacological and pharmacological management are lacking at present. Here, we provide a systematic review of the literature on therapeutic options for neurogenic orthostatic hypotension and related symptoms with evidence-based recommendations according to the Grading of Recommendations Assessment, Development and Evaluation (GRADE). Patient’s education and non-pharmacological measures remain essential, with strong recommendation for use of abdominal binders. Based on quality of evidence and safety issues, midodrine and droxidopa reach a strong recommendation level for pharmacological treatment of neurogenic orthostatic hypotension. In selected cases, a range of alternative agents can be considered (fludrocortisone, pyridostigmine, yohimbine, atomoxetine, fluoxetine, ergot alkaloids, ephedrine, phenylpropanolamine, octreotide, indomethacin, ibuprofen, caffeine, methylphenidate and desmopressin), though recommendation strength is weak and quality of evidence is low (atomoxetine, octreotide) or very low (fludrocortisone, pyridostigmine, yohimbine, fluoxetine, ergot alkaloids, ephedrine, phenylpropanolamine, indomethacin, ibuprofen, caffeine, methylphenidate and desmopressin). In case of severe postprandial hypotension, acarbose and octreotide are recommended (strong recommendation, moderate level of evidence). Alternatively, voglibose or caffeine, for which a weak recommendation is available, may be useful.

Hepatic and renal mechanisms underlying the osmopressor response

Increased blood pressure (BP) is observed in patients with impaired baroreflexes after water drinking. The stimulus for this effect is low blood osmolality, and it has been termed the osmopressor response (OPR). The BP increase is associated with activation of the sympathetic nervous system and a requirement for transient receptor potential vanilloid 4 (TRPV4) channels. However, the mechanisms underlying the OPR are poorly understood. We tested the hypothesis that hypotonicity is sensed in the portal area to initiate the OPR. Sino-aortic denervated mice were used and BP was monitored for 30min after fluid infusion while mice were under anesthesia. Infusion of hypotonic fluid (0.45% saline), but not of isotonic 0.9% saline, directly into the portal vein, produced an immediate OPR (increase in BP with saline 0.45%: 15±13 vs. 0.9%: -7±2mmHg, p=0.003; AUC: 0.45%: 150±99, n=7 vs. 0.9%: -74±60mmHg·min, n=5, p=0.003). However, 0.45% saline was not able to trigger a similar response in TRPV4^-/- mice (ΔBP_TRPV4: -2±5mmHg, n=8, p=0.009). Hypotonic saline did not raise BP when infused at the same speed and volume into the jugular vein (jugular: -5±6mmHg, p=0.002, compared to portal). Denervation of the splanchnic nerve by celiac ganglionectomy (CGX) did not abolish the OPR (CGX: 15±11 vs. Sham: 16±6mmHg, p=0.34). Renal denervation diminished the OPR elicited by duodenal water infusion (denervation: 9±4 vs. sham: 31±15mmHg, p=0.016). Therefore, hypotonicity in the portal circulation, probably sensed by TRPV4 channels, triggers the OPR and intact renal nerves are needed for the full response.

The recommendations of a consensus panel for the screening, diagnosis, and treatment of neurogenic orthostatic hypotension and associated supine hypertension

Neurogenic orthostatic hypotension (nOH) is common in patients with neurodegenerative disorders such as Parkinson’s disease, multiple system atrophy, pure autonomic failure, dementia with Lewy bodies, and peripheral neuropathies including amyloid or diabetic neuropathy. Due to the frequency of nOH in the aging population, clinicians need to be well informed about its diagnosis and management. To date, studies of nOH have used different outcome measures and various methods of diagnosis, thereby preventing the generation of evidence-based guidelines to direct clinicians towards ‘best practices’ when treating patients with nOH and associated supine hypertension. To address these issues, the American Autonomic Society and the National Parkinson Foundation initiated a project to develop a statement of recommendations beginning with a consensus panel meeting in Boston on November 7, 2015, with continued communications and contributions to the recommendations through October of 2016. This paper summarizes the panel members’ discussions held during the initial meeting along with continued deliberations among the panel members and provides essential recommendations based upon best available evidence as well as expert opinion for the (1) screening, (2) diagnosis, (3) treatment of nOH, and (4) diagnosis and treatment of associated supine hypertension.

Sexual dimorphism in the osmopressor response following water ingestion

Compared with men, women exhibit a greater magnitude of increase in resting blood pressure after drinking a single water bolus of 500 ml. Accordingly, our study provides direct evidence of sexual dimorphism in the haemodynamic response to water intake.

There is conflicting evidence as to whether water drinking elicits a pressor response in healthy young adults. The inclusion of a variable number of women may have contributed to the discrepancies found in past research. Thus, we aimed at exploring whether the osmopressor response follows a sexually dimorphic pattern. In a randomized fashion, 31 healthy adults (16 men; 15 women, aged 18–40 years) ingested 50 and 500 ml of water before completing a resting protocol on two separate days. Arterial blood pressure, heart rate and spectral heart rate variability were measured in the seated position at pre- and post-25 min of water ingestion. Women responded to 500 ml of water with a greater proportion of change in diastolic and mean arterial pressure (MAP) (P<0.05). Conversely, the percent change in systolic blood pressure (SBP) and heart rate was not different between sexes after 500 ml of water. Overall, women demonstrated lower blood pressure, but higher resting heart rate compared with men (P<0.05). In contrast, heart rate variability was similar between sexes before and after ingesting either volume of water. There was a bradycardic effect of water and, irrespectively of sex; this was accompanied by increased high frequency power (HF) (P<0.05). We conclude that women display a greater magnitude of pressor response than men post-water ingestion. Accordingly, we provide direct evidence of sexual dimorphism in the haemodynamic response to water intake in young healthy adults.

The depressor response to intracerebroventricular hypotonic saline is sensitive to TRPV4 antagonist RN1734

Several reports have shown that the periventricular region of the brain, including the paraventricular nucleus (PVN), is critical to sensing and responding to changes in plasma osmolality. Further studies also implicate the transient receptor potential ion channel, type V4 (TRPV4) channel in this homeostatic behavior. In previous work we have shown that TRPV4 ion channels couple to calcium-activated potassium channels in the PVN to decrease action potential firing frequency in response to hypotonicity. In the present study we investigated whether, similarly, intracerebroventricular (ICV) application of hypotonic solutions modulated cardiovascular parameters, and if so whether this was sensitive to a TRPV4 channel inhibitor. We found that ICV injection of 270 mOsmol artificial cerebrospinal fluid (ACSF) decreased mean blood pressure, but not heart rate, compared to naïve mice or mice injected with 300 mOsmol ACSF. This effect was abolished by treatment with the TRPV4 inhibitor RN1734. These data suggest that periventricular targets within the brain are capable of generating depressor action in response to TRPV4 ion channel activation. Potentially, in the future, the TRPV4 channel, or the TRPV4–K_Ca coupling mechanism, may serve as a therapeutic target for treatment of cardiovascular disease.

Opportunities for intervention strategies for weight management: global actions on fluid intake patterns

Water is an essential nutrient for all physiological functions and particularly important for thermoregulation. About 60% of our body weight is made of water. Under standard conditions (18-20 °C and moderate activity), water balance is regulated within 0.2 % of body weight over a 24-hour period. Water requirement varies between individuals and according to environmental conditions. Concerning considerations related to obesity, the health impact of fluid intake is commonly overlooked. Fluid intake advices are missing in most of food pyramids offered to the public, and water requirements and hydration challenges remain often neglected. The purpose of this paper is to emphasize and discuss the role of water consumption in the context of other important public health measures for weight management. Attention will be focused on fluid intake patterns and hydration-related questions in the context of global interventions and/or physical activity programs settled in weight management protocols.

Effect of excessive water intake on body weight, body mass index, body fat, and appetite of overweight female participants

Objective:

Drinking plenty of water is commonly recommended in weight loss regimens; however, very limited scientific evidence is available to justify this. Here we studied the effect of drinking 1.5 L of water, over and above the usual intake on body weight, body mass index (BMI), body fat, and appetite score in overweight female participants.

Materials and Methods:

The study was done on 50 overweight female participants, who were instructed to drink 500 mL of water, three times a day 30 min before breakfast, lunch, and dinner. The 1.5 L water intake was over and above their daily water intake and was continued for 8 consecutive weeks. Body weight, BMI, body fat, and appetite score was measured pre and post-study; and the values were compared by paired t-test using Statistical Packages for Social Sciences (SPSS) version 14.0.1.

Results:

All the parameters tested were lower after 8 weeks: body weight (pre-study 65.86 ± 3.614 kg vs post-study 64.42 ± 3.704 kg; P < 0.01); BMI (pre-study 26.7002 ± 0.9423 vs post-study 26.1224 ± 1.0632; P < 0.01); sum of skinfold thickness (pre-study 79.626 ± 10.385 mm vs post-study 76.578 ± 10.477 mm; P < 0.01); and the appetite score (pre-study 36.880 ± 4.170 vs post-study 34.673 ± 3.845; P < 0.01).

Conclusions:

Our study establishes the role of drinking 1.5 L of excessive water in weight reduction, body fat reduction, and appetite suppression in overweight female participants.

Effect of 'water induced thermogenesis' on body weight, body mass index and body composition of overweight subjects

CONTEXT

Drinking lots of water is commonly suggested as a part of weight loss regimens. However, only few systematic studies have addressed this notion. In this study, the effect of drinking 1500 ml of water, over and above the daily water intake on body weight, body mass index (BMI) and body composition of overweight subjects was assessed.

AIM

To evaluate the role of drinking excessive water in weight reduction and body fat reduction of overweight subjects.

SETTINGS AND DESIGN

This study was conducted on 50 overweight girls for eight weeks, during which they were instructed to drink 500 ml of water, three times a day, half an hour before breakfast, lunch and dinner, which was over and above their daily water intake.

MATERIAL AND METHODS

Body weight was measured in kilograms (kgs). BMI was calculated as weight in kilograms, which was divided by the square of height in metres. Body composition score was calculated as a sum of skin fold thickness in millimetres at three different sites, which was measured by using skin fold calipers. Pre and post-study body weight, body mass index and body composition scores were measured.

STATISTICAL ANALYSIS

SPSS, version 14.0.1 and paired t-test were used to find out the statistical significance of the results.

RESULTS

The mean values of the pre-study and post-study body weight, body mass index and body composition scores were 65.86 kg and 64.42 kg, 26.7002 and 26.1224 and 79.626 mm and 76.578 mm respectively. All the three results were highly significant statistically.

CONCLUSIONS

The decrease in body weight, body mass index and body composition scores of overweight subjects at the end of study period establishes the role of water induced thermogenesis in weight reduction of overweight subjects.

Chronotropic and pressor effects of water ingestion at rest and in response to incremental dynamic exercise

Ingestion of water attenuates the chronotropic response to submaximal exercise. However, it is not known whether this effect is equally manifested during dynamic exercise below and above the ventilatory threshold (VT). We explored the effects of water ingestion on the heart rate response to an incremental cycle-ergometer protocol. In a randomized fashion, 19 healthy adults (10 men and nine women, age 20.9 ± 1.8 years) ingested 50 and 500 ml of water before completing a cycle-ergometer protocol on two separate days. The heart rate and oxygen uptake ( ) responses to water ingestion were analysed both at rest and during exercise performed below and above the VT. The effects of water intake on brachial blood pressure were measured only at rest. Resting mean arterial pressure increased and resting heart rate decreased, but only after 500 ml of water (P < 0.05). Compared with that seen after 50 ml of water, the 500 ml volume elicited an overall decrease in submaximal heart rate (P < 0.05). In contrast, drinking 500 ml of water did not affect submaximal . The participants' maximal heart rate, maximal and VT were similar between conditions. Our results therefore indicate that, owing to its effects on submaximal heart rate over a broad spectrum of intensities, the drinking of water should be recognized as a potential confounder in cardiovascular exercise studies. However, by showing no differences between conditions for submaximal , they also suggest that the magnitude of heart rate reduction after drinking 500 ml of water may be of minimal physiological significance for exercise cardiorespiratory capacity.

Lower body negative pressure-induced vagal reaction: role for the osmopressor response?

BACKGROUND

Water ingestion elicits an osmopressor response in patients with impaired baroreflexes. In young, healthy subjects, water elicits sympathetic vasoconstriction. This study investigated the effect of water on the lower body negative pressure (LBNP)-induced vasovagal reaction and also analyzed its effect on the change of regional cerebral blood flow during LBNP.

METHODS

Twelve young healthy subjects underwent LBNP (40 mm Hg) tolerance testing for 45 minutes or until presyncopal symptoms occurred. Subjects received either LBNP or no LBNP with or without prior water ingestion. The severity of vasovagal reaction was determined by participant self-report rating of orthostatic symptoms during the LBNP test. Changes of regional cerebral blood flow (rCBF) between LBNP and water ingestion with LBNP groups were assessed using statistical parametrical mapping analyses.

RESULTS

Water ingestion attenuated the severity of symptomatic scores during LBNP (P = 0.004). Water ingestion increased Total peripheral vascular resistance (P < 0.001) and attenuated the blood pressure drop (P < 0.001) at the cessation of study. LBNP decreased rCBF over the left superior prefrontal gyrus, limbic-parahippocampal gyrus, left sublobar-caudate body, and hypothalamus (P < 0.001). Water increased rCBF significantly over the right frontal lobe, including the inferior and medial prefrontal gyrus, subcallosal, and sublobar insula, during LBNP stimulation (P < 0.001).

CONCLUSIONS

Water ingestion strongly reduces symptomatic burden of the vasovagal reaction induced by LBNP stimulation. The cortical activation of limbic and prefrontal cortex likely indicates the involvement of osmopressor response in central autonomic cardiovascular physiology. The central cortical activation of osmopressor response might provide insight into the mechanisms by which water ingestion reduces the vasovagal reaction.

Non-pharmacological management of orthostatic hypotension in the older patient

Orthostatic hypotension (OH) occurs in up to 30% of community-dwelling older people. Its presence confers a greater risk of incident co-morbid disease and all-cause mortality. As per guidelines, first-line treatment should consist of non-pharmacological therapies. Effective lifestyle modification advice includes the avoidance of rapid postural changes and large meals. Physical counter-manoeuvres, when comprehensively described, effectively abate symptom progression. Patients should drink 1.5 to 2 litres of water daily, though reports suggest only half of older people comply with this regime. Moderate salt consumption is advised, though with caution as supine hypertension often co-exists. Compression hosiery benefits older people and, contrary to popular opinion, is well tolerated. Potential, future therapies include impedance threshold devices. Older patients with OH frequently have co-morbid disease such that a pharmacological approach is ill-advised. They respond well to non-pharmacological therapies and these should form the primary therapeutic approach.

Liver afferents contribute to water drinking-induced sympathetic activation in human subjects: a clinical trial

Water drinking acutely increases sympathetic activity in human subjects. In animals, the response appears to be mediated through transient receptor potential channel TRPV4 activation on osmosensitive hepatic spinal afferents, described as osmopressor response. We hypothesized that hepatic denervation attenuates water drinking-induced sympathetic activation. We studied 20 liver transplant recipients (44±2.6 years, 1.2±0.1 years post transplant) as model of hepatic denervation and 20 kidney transplant recipients (43±2.6 years, 0.8±0.1 years post transplant) as immunosuppressive drug matched control group. Before and after 500 ml water ingestion, we obtained venous blood samples for catecholamine analysis. We also monitored brachial and finger blood pressure, ECG, and thoracic bioimpedance. Plasma norepinephrine concentration had changed by 0.01±0.07 nmol/l in liver and by 0.21±0.07 nmol/l in kidney transplant recipients (p<0.05 between groups) after 30–40 minutes of water drinking. While blood pressure and systemic vascular resistance increased in both groups, the responses tended to be attenuated in liver transplant recipients. Our findings support the idea that osmosensitive hepatic afferents are involved in water drinking-induced sympathetic activation in human subjects.

Influence of climate on emergency department visits for syncope: role of air temperature variability

Background

Syncope is a clinical event characterized by a transient loss of consciousness, estimated to affect 6.2/1000 person-years, resulting in remarkable health care and social costs. Human pathophysiology suggests that heat may promote syncope during standing. We tested the hypothesis that the increase of air temperatures from January to July would be accompanied by an increased rate of syncope resulting in a higher frequency of Emergency Department (ED) visits. We also evaluated the role of maximal temperature variability in affecting ED visits for syncope.

Methodology/Principal Findings

We included 770 of 2775 consecutive subjects who were seen for syncope at four EDs between January and July 2004. This period was subdivided into three epochs of similar length: 23 January–31 March, 1 April–31 May and 1 June–31 July. Spectral techniques were used to analyze oscillatory components of day by day maximal temperature and syncope variability and assess their linear relationship.

There was no correlation between daily maximum temperatures and number of syncope. ED visits for syncope were lower in June and July when maximal temperature variability declined although the maximal temperatures themselves were higher. Frequency analysis of day by day maximal temperature variability showed a major non-random fluctuation characterized by a ∼23-day period and two minor oscillations with ∼3- and ∼7-day periods. This latter oscillation was correlated with a similar ∼7-day fluctuation in ED visits for syncope.

Conclusions/Significance

We conclude that ED visits for syncope were not predicted by daily maximal temperature but were associated with increased temperature variability. A ∼7-day rhythm characterized both maximal temperatures and ED visits for syncope variability suggesting that climate changes may have a significant effect on the mode of syncope occurrence.

Simple standing test predicts and water ingestion prevents vasovagal reaction in the high-risk blood donors

BACKGROUND

One of the serious complications of blood donation is vasovagal reaction (VVR) with syncope. This study was performed to determine if the measurement of hemodynamic responses to standing before blood collection (BC) was useful to identify the high-risk donors for VVR and also examined the effect of 300 mL of water ingestion in the prevention of VVR.

STUDY DESIGN AND METHODS

Blood pressure and heart rate (HR) during 5 minutes of standing were examined before and after BC in 93 donors. Because HR increase of 6 of 7 donors who developed syncopal VVR during standing after BC was 15 beats per minute (bpm) or greater, those with HR increase of 15 bpm or greater were determined as high-risk donors (n = 31). In another group (n = 117), 45 donors were identified as high risk based on the HR response before BC (15 bpm). The effect of 300 mL of water ingestion 15 minutes before BC on hemodynamic responses to standing and the rate of VVR after BC were analyzed.

RESULTS

Water ingestion given to the high-risk donors of the second group reduced HR increase with standing before BC (-6.6 ± 13.6 bpm, p < 0.02 vs. HR increase before water ingestion) and significantly suppressed VVR rate (2 of 45 donors with high risk, 4.4%,p < 0.04 vs. the first group; 6 of 31 high-risk donors, 19.4%).

CONCLUSION

HR response to standing before BC may detect the high-risk donors for VVR. For the high-risk donors, 300 mL of water ingestion may be a simple and effective way of prevention against syncopal VVR.

Acute fluid ingestion in the treatment of orthostatic intolerance - important implications for daily practice

BACKGROUND

Rapid water ingestion improves orthostatic intolerance (OI) in multiple system atrophy (MSA) and postural tachycardia syndrome (PoTS). We compared haemodynamic changes after water and clear soup intake, the latter being a common treatment strategy for OI in daily practice.

METHODS

Seven MSA and seven PoTS patients underwent head-up tilt (HUT) without fluid intake and 30 min after drinking 450 ml of water and clear soup, respectively. All patients suffered from moderate to severe OI because of neurogenic orthostatic hypotension (OH) and excessive orthostatic heart rate (HR) increase, respectively. Beat-to-beat cardiovascular indices were measured non-invasively.

RESULTS

In MSA, HUT had to be terminated prematurely in 2/7 patients after water, but in 6/7 after clear soup. At 3 min of HUT, there was an increase in blood pressure of 15.7(8.2)/8.3(2.3) mmHg after water, but a decrease of 11.6(18.9)/8.1(9.2) mmHg after clear soup (P < 0.05). In PoTS, HUT could always be completed for 10 min, but OI subjectively improved after both water and clear soup. The attenuation of excessive orthostatic HR increase did not differ significantly after water and clear soup drinking.

CONCLUSIONS

In MSA, clear soup cannot substitute water for eliciting a pressor effect, but even worsens OI after rapid ingestion. In PoTS, acute water and clear soup intake both result in improvement of OI. These findings cannot solely be explained by difference in osmolarity but may reflect some degree of superimposed postprandial hypotension in widespread autonomic failure in MSA compared to the mild and limited autonomic dysfunction in PoTS.

The osmopressor response to water drinking

Water drinking elicits profound pressor responses in patients with impaired baroreflex function and in sinoaortic-denervated mice. Healthy subjects show more subtle changes in heart rate and blood pressure with water drinking. The water-induced pressor response appears to be mediated through sympathetic nervous system activation at the spinal level. Indeed, water drinking raises resting energy expenditure in normal weight and obese subjects. The stimulus setting off the response is hypoosmolarity rather than water temperature or gastrointestinal stretch. Studies in mice suggest that this osmopressor response may involve transient receptor potential vanniloid 4 (Trpv4) receptors. However, the (nerve) cell population serving as peripheral osmosensors and the exact transduction mechanisms are still unknown. The osmopressor response can be exploited in the treatment of orthostatic and postprandial hypotension in patients with severe autonomic failure. Furthermore, the osmopressor response acutely improves orthostatic tolerance in healthy subjects and in patients with neurally mediated syncope. The phenomenon should be recognized as an important confounder in cardiovascular and metabolic studies.

Predonation hydration and applied muscle tension combine to reduce presyncopal reactions to blood donation

BACKGROUND

A randomized controlled trial was conducted to test the effects of hydration and applied muscle tensing on presyncopal reactions to blood donation. Both interventions are designed to prevent the decreases in blood pressure that can contribute to such reactions, but due to the distinct physiologic mechanisms underlying their pressor responses it was hypothesized that a combined intervention would yield the greatest benefit.

STUDY DESIGN AND METHODS

Before blood donation, first- and second-time blood donors (mean age, 20.2 years; SD, 4.9) were randomly assigned to 1) standard donation, 2) placebo (leg exercise before venipuncture), 3) predonation water, or 4) predonation water and leg exercise during donation.

RESULTS

Main effects of group were observed for phlebotomist classification of vasovagal reactions (chi(2)(3) = 8.38, p < 0.05) and donor reports of presyncopal reactions (chi(2)(3) = 13.16, p < 0.01). Follow-up analyses of phlebotomist classifications revealed fewer reactions in the predonation water and predonation water and leg exercise groups relative to placebo but not standard donation. Follow-up analyses of self-reported reactions revealed that women, but not men, had lower scores in both the predonation water and the predonation water and leg exercise groups relative to both placebo and standard donation.

CONCLUSIONS

Predonation hydration and a combination of hydration and leg exercise may help attenuate presyncopal reactions in relatively novice donors, although future studies with larger samples are required to confirm this effect.

Systemic postganglionic adrenergic studies do not distinguish Parkinson's disease from multiple system atrophy

BACKGROUND

Multiple system atrophy (MSA) affects the preganglionic adrenergic neuron and Parkinson's disease (PD) involves the postganglionic counterpart. Widespread postganglionic denervation should result in denervation supersensitivity and a failure of the axon to release norepinephrine (NE). We examined if pharmacological dissection of the adrenergic neuron can distinguish between MSA and PD.

METHOD

We measured blood pressure, heart rate, and plasma NE responses to direct (phenylephrine) and indirect (tyramine) acting adrenergic agonists in 15 patients with probable MSA, 16 patients with idiopathic PD, and 16 age- and gender-matched controls.

RESULTS

Baroreflex sensitivity was impaired in MSA and intact in PD. Pressor responses to phenylephrine (direct acting) were higher in MSA (p<0.01) and PD patients (p=0.04) than in controls. Blood pressure responses to tyramine (indirect acting) were increased in MSA only (p<0.01). Tyramine increased plasma catecholamine levels in all groups with no significant differences between groups.

CONCLUSION

There is denervation supersensitivity in PD patients that is, however, insufficient to shift the dose-response curve to the left. The excessive pressor responses to both tyramine and phenylephrine in MSA are due to baroreflex failure. We conclude that this diagnostic approach lacks sufficient sensitivity to differentiate PD and MSA.

Vasodilation in vasovagal syncope and the effect of water ingestion

Abnormal (increased, but also decreased) vasodilative responses have been observed in patients with vasovagal syncope (VVS). The objective was to assess reactive vasodilation in supine patients with VVS and its relation to severity of the syndrome. Reactive vasodilation was also assessed after a simple therapeutic intervention (water drinking). Thirty-four patients were studied, all with recurrent VVS and a recent positive head-up tilt test result. Seventeen matched healthy subjects served as controls. Venous occlusion plethysmography was used to assess forearm blood flow (FBF) and forearm vascular resistance resistance (1) at rest and (2) during reactive hyperemia. Clinical severity of the syndrome was related to the intensity and duration of the vasodilative reflex. The same plethysmographic measurements were repeated 60 minutes after drinking 500 ml of water. Before water drinking, no difference was observed between groups in baseline measurements. However, duration of hyperemia was longer in patients (p <0.05) and was related to the duration of the previous positive tilt test (r = -0.69, p <0.05) and total number of each patient's symptomatic vasovagal episodes (r = 0.49, p <0.05). After water ingestion, baseline FBF decreased in patients (p <0.05) and remained stable in controls. In patients, duration of hyperemia decreased to normal values. Hyperemic FBF remained similar between groups. In conclusion, increased reactive vasodilative reflexes were observed in patients with VVS. They seemed to be of significant pathophysiologic significance. Water drinking can normalize them for >or=60 minutes.

The effect of intraduodenal glucose on muscle sympathetic nerve activity in healthy young and older subjects

OBJECTIVE

The cardiovascular response to a meal is modulated by gastric distension and the interaction of nutrients, particularly carbohydrate, within the small intestine. We tested the hypothesis that the depressor effect of small intestinal glucose is greater in older than in young subjects, because the reflex increase in muscle sympathetic nerve activity (MSNA) is blunted by age.

METHODS

The effects of intraduodenal glucose infusion (IDGI) on blood pressure, heart rate and MSNA were evaluated in eight healthy young subjects (4 women; mean age +/- SEM: 28.8 +/- 3.4 years), eight healthy elderly (4 women; 75.3 +/- 1.6 years) and in two patients with symptomatic postprandial hypotension (PPH), one young (21 years), and one old (90 years).

RESULTS

In both young and elderly healthy subjects, IDGI decreased blood pressure (P < 0.05), but the fall in systolic blood pressure was greater in the older subjects (-17.0 +/- 4.1 vs. -6.5 +/- 1.6 mmHg, P < 0.03). MSNA increased similarly, after infusion in both young (9.0 +/- 3.4 bursts/min) and elderly (7.8 +/- 1.0 bursts/min) subjects. Baroreflex sensitivity for number of sympathetic bursts was attenuated in the elderly (P < 0.03). The increase in burst area in the young patient with PPH was attenuated (18 vs. 63% in the healthy young group).

INTERPRETATION

The fall in BP induced by IDGI was greater in healthy elderly compared to healthy young subjects. The reason for this is unclear, as they have similar increases in MSNA.

Water intake accelerates post-exercise cardiac vagal reactivation in humans

Post-exercise cardiac vagal reactivation is well-investigated; however, the effect of water intake during this period has not been well studied. Therefore, our aim was to assess the influence of water intake on the cardiac vagal reactivation after 30 min of a submaximal cycling exercise. Ten healthy subjects (eight men) aged 23-35 years were evaluated. A 3-day testing cycle duration, subjects were randomly chosen to drink either 500 ml (experimental visit) or 50 ml (control visit) of water immediately after the 30-min cycling exercise at a workload representing 80% of a previously measured anaerobic threshold. A cardiac vagal index (CVI) was obtained using the 4-s exercise test measured before and after (10 and 30 min) exercise at each testing day. Data analysis (2 x 3 ANOVA for repeated measures) showed higher cardiac vagal activity at the 30-min post-exercise period when 500 ml of water was ingested. CVI values for the 500 and 50 ml trials were 1.55 +/- 0.04 vs. 1.49 +/- 0.04, P = 0.003 (mean +/- SEM), respectively. Heart rate and blood pressure values were relatively the same. In conclusion, water intake of about 500 ml immediately after 30 min of cycling exercise accelerates post-exercise cardiac vagal reactivation. These results suggest that post-exercise hydration might be beneficial not only for thermoregulation, but also for vagal reactivation.

Water drinking induces thermogenesis through osmosensitive mechanisms

CONTEXT

Recently, we showed that drinking 500 ml water induces thermogenesis in normal-weight men and women.

OBJECTIVE

We now repeated these studies in a randomized, controlled, crossover trial in overweight or obese otherwise healthy subjects (eight men and eight women), comparing also the effects of 500 ml isoosmotic saline or 50 ml water.

RESULTS

Only 500 ml water increased energy expenditure by 24% over the course of 60 min after ingestion, whereas isoosmotic saline and 50 ml water had no effect. Heart rate and blood pressure did not change in these young, healthy subjects.

CONCLUSIONS

Our data exclude volume-related effects or gastric distension as the mediator of the thermogenic response to water drinking. Instead, we hypothesize the existence of a portal osmoreceptor, most likely an ion channel.

Effects of daily water drinking on orthostatic and postprandial hypotension in patients with multiple system atrophy

OBJECTIVE

It has been demonstrated that the increased blood pressure (BP) caused by a single dose of water alleviates orthostatic hypotension (OH) and postprandial hypotension (PPH) in patients with autonomic failure (AF). The aim of this study was to evaluate the practical effect of daily water drinking on OH and PPH in the morning when patients with AF are usually most affected.

METHODS

In five patients with multiple system atrophy (MSA) characterized by intractable OH and PPH, we measured seated, standing and postprandial BP in the morning without and with ingestion of 350 ml tap water at 07.30 hours for seven successive days. The changes from the basal BP level at 07.30 hours (DeltaBP) were assessed as an index of the effect of water drinking.

RESULTS

Water drinking elicited a rapid pressor response in all patients. The DeltaBP during sitting, standing and after a meal following water drinking (day 1 and day 7) was significantly higher than without water drinking (day 0). The effects of reducing OH and PPH on day 7 were equivalent to those on day 1. No adverse effects associated with daily water drinking were observed, except later diuresis, which occurred in one patient.

CONCLUSIONS

Daily water drinking demonstrated constant pressor effects in the morning with no severe adverse effects in MSA patients. This finding suggests that water drinking should be tried as a practical measure to prevent or reduce OH and PPH.

Endogenous substance P modulates human cardiovascular regulation at rest and during orthostatic load

Substance P (SP) is a peptide neurotransmitter identified in many central and peripheral neural pathways. Its precise role in human physiology has been difficult to elucidate. We used the selective neurokinin 1 (NK1) antagonist aprepitant as a pharmacological probe to determine the role of endogenous SP in human cardiovascular regulation. We performed a randomized, double-blind, placebo-controlled, crossover trial in healthy subjects. Blockade of endogenous NK1 receptors reduced resting muscle sympathetic activity 38% (P=0.002), reduced systemic vascular resistance by 25% (P=0.021), and increased cardiac index by 47% (P=0.006). This constellation of changes did not, however, alter either blood pressure or heart rate in the supine position. NK1 antagonism also raised orthostatic heart rate change by 38% (P=0.023), although during the incremental postural adjustment on the tilt table neither heart rate nor blood pressure was altered significantly. Despite a mildly attenuated vagal baroreflex with SP blockade, the depressor and pressor responses to nitroprusside and phenylephrine did not differ compared with placebo, suggesting other compensatory mechanisms. NK1 blockade manifests as a decrease in muscle sympathetic nerve activity and systemic vascular resistance. Our study suggests SP exerts a tonic enhancement of sympathetic outflow to some cardiovascular structures via its modulation of the NK1 receptor. Most likely, this ubiquitous neurotransmitter exerts effects at multiple sites that, in the aggregate, are relatively well compensated under many circumstances but may emerge with perturbations. This study is consistent with a role for SP afferents in supporting peripheral vascular resistance.

Sodium Paradoxically Reduces the Gastropressor Response in Patients With Orthostatic Hypotension

Orthostatic hypotension (OH) can cause syncope that is difficult to treat. We have found that 473 mL (16 oz) of water can increase systolic blood pressure (SBP) by >30 mm Hg in many OH patients (the gastropressor response). OH patients are routinely advised to increase their sodium intake to augment their blood volume. We tested the hypothesis that the ingestion of salt with water would increase the magnitude of the acute pressor response compared with water alone in patients with OH. Patients with OH (n=9; female=5; 65±3 years) underwent a randomized crossover trial of drinking water (H 2 O) and salt water (NaCl-H 2 O). Noninvasive heart rate and BP were measured with the patient seated for ≥60 minutes after ingestion. The area under the curve for SBP was greater with H 2 O than NaCl-H 2 O for the 30 minutes (714±388 mm Hg×min versus 364±369 mm Hg×min; P =0.002) and 60 minutes (1454±827 mm Hg×min versus 812±734 mm Hg×min; P =0.048) after ingestion. The increase in SBP with H 2 O was greater than with NaCl-H 2 O at 30 minutes (37±6 versus 18±5 mm Hg; P =0.006) but not at 60 minutes (17±6 versus 10±6 mm Hg; P =0.4). Norepinephrine increased after H 2 O ( P =0.018) but not after NaCl-H 2 O ( P =0.195). Both oral water and salt water increase BP in patients with OH. Instead of augmenting the gastropressor response, the additional salt paradoxically attenuates the pressor response to water. These data suggest a potentially important role for gastrointestinal osmolality in the activation of the sympathetic nervous system leading to cardiovascular reflexes responsible for the gastropressor response.