Relative Change In Plasma Volume Research Articles

Impact of sodium citrate ingestion during recovery after strenuous exercise in the heat on heart rate variability: A randomized, crossover study

Changes in hydration status influence plasma volume (PV) which is associated with post‐exercise parasympathetic reactivation. The present study hypothesized that, after dehydrating cycling exercise in the heat (DE), stimulation of PV expansion with sodium citrate (CIT) supplementation would promote heart rate variability (HRV) recovery in endurance‐trained men. Twelve participants lost 4% of body mass during DE. During subsequent 16‐h recovery, participants consumed water ad libitum (CIT =5.5‐L, PLC =5.1‐L) and ate prescribed food supplemented with CIT or placebo in a randomized, double‐blind, crossover manner. Relative changes in PV were assessed across DE and 16‐h recovery. HRV was analyzed before and 16 h after DE in three conditions for altogether four 5‐min periods: supine in a thermoneutral environment, supine in the heat (32°C, 46% relative humidity; 2 periods), and standing in the heat. A larger expansion of PV across 16‐h recovery occurred in CIT compared to placebo trial (p < 0.0001). However, no between‐trial differences appeared in HRV parameters (lnRMSSD, lnSDNN, lnLF/HF) in any 5‐min period analyzed before or 16 h after DE (in all cases p > 0.05). Increases in HR (p < 0.001) and lnLF/HF (p = 0.005) and decreases in lnRMSSD (p < 0.001) and lnSDNN (p < 0.001) occurred following DE in both trials. Larger PV expansion induced by CIT supplementation after DE does not improve recovery of HRV at rest and has no influence on HRV responsiveness in endurance‐trained men.

Systolic blood pressure on admission as a marker of fluid re-distribution in acute heart failure

BackgroundThe relation between systolic blood pressure (sBP) on admission and the extent of fluid re-distribution in patients with acute heart failure (AHF) remains unclear. This study aimed to investigate this relation. MethodsWe enrolled consecutive patients who were admitted for AHF in our cardiology department and divided them into three groups according to the tertiles of sBP on admission as follows: low, intermediate, and high sBP groups. Weight changes and estimated relative plasma volume changes (ΔePV) on admission and 24h later were determined in each patient. ΔePV were calculated with the Strauss formula using hemoglobin and hematocrit levels. Univariate and multiple regression analyses were performed to investigate the relation between sBP and ΔePV. ResultsThe ΔePV of low, intermediate, and high sBP groups were 104.3%, 108.2%, and 121.3%, respectively. High sBP group showed a significantly larger ΔePV than the other two groups (p<0.001 and 0.004, respectively). The body weight of patients in the high sBP group slightly but significantly decreased within 24h (−0.64±0.92kg, p=0.002). The initial sBP had a significant correlation with ΔePV (R2, 0.295; p<0.001). Multiple regression analysis showed significant association between initial sBP with ΔePV. ConclusionsThese results indicate that large amounts of extravasated fluid existed on admission in patients with a high initial sBP. The sBP on admission could be a simple and useful indicator for the extent of fluid re-distribution in AHF.

The effects of core and skin temperatures and dehydration on heart rate response during interval exercise in warm and cool conditions

Heart rate (HR) gradually increases during exercise at a constant work load, which is known as cardiovascular drift (CVD). The CVD is exacerbated when exercise is performed under condition with high ambient temperatures (Ta). It is suggested that increased esophageal temperature (Tes) is associated with the level of CVD. In the present study, we examined the effect of skin temperatures (Tsk) and the dehydration on CVD during high intensity interval exercise.METHODSEight healthy young men underwent interval exercise on a cycling ergometer for 72 min (8 sets of 4 min at 80% VO2max – 5 min at 20% VO2max) in cool (Ta of 20□, 40% relative humidity) and warm (Ta of 30□, 40% relative humidity) conditions. Tes, Tsk, heart rate (HR), sweat rate (SR) and skin blood flow (SkBF) at chest were measured continuously. Blood samples were taken before and at the end of exercise, and relative change in plasma volume (%ΔPV) after exercise were determined from hematocrit and hemoglobin concentration of the blood samples.RESULTSHR increased during exercise at 80% VO2max while decreased during exercise at 20% VO2max in both trials. HR in warm condition was higher than cool condition (p &lt; 0.05) throughout trial. More importantly, HR at the end of each set was gradually increased during exercise and the increase was significantly higher in warm than in cool condition. Tes increased during exercise at 80% VO2max while decreased during exercise at 20% VO2max in both trials, it reached the peak value at the end of the 4th set of exercise at 80% VO2max (37.93 ± 0.04□ (means ± SE) in cool and 37.84 ± 0.04□ in warm), then kept these values until the end of trial. There was no significant difference in Tes between trials throughout trial. Tsk was higher (p &lt; 0.05) in warm than cool condition throughout trial. SR at chest was higher (p &lt; 0.05) in warm than cool condition from 9 min to the end of exercise, while SkBF at chest was not significantly different between conditions throughout trial. The %ΔPV was not different among conditions (−13.9 ± 1.0 % in cool, −12.7 ± 1.5 % in warm).CONCLUSIONSExercise in a warm condition increased HR during interval exercise compared to a cool condition despite the level of esophageal temperature and dehydration were same between conditions. The increased HR would be associated with an increased skin temperatures.Support or Funding InformationThis work was supported by a Grant‐in‐Aid for Scientific Research (B) and a Grant‐in‐Aid for Challenging Exploratory Research (K. Okazaki) from the Ministry of Education, Culture, Sports, Science and Technology of Japan.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Effects of skin surface cooling before exercise on lactate accumulation in cool environment.

We assessed whether plasma lactate accumulation increased and the lactate threshold (LT) declined when the skin temperature was lowered by whole body skin surface cooling before exercise in cool, but not temperate, conditions, and whether the lowered LT was associated with sympathetic activation or lowered plasma volume (PV) by cold-induced diuresis. Ten healthy subjects performed a graded maximal cycling exercise after pre-conditioning under three different conditions for 60min. Ambient temperature (using an artificial climatic chamber) and water temperature in a water-perfusion suit controlled at 25 and 34°C in temperate-neutral (Temp-Neut); 25 and 10°C in temperate-cool (Temp-Cool); and at 10 and 10°C in cool-cool (Cool-Cool) conditions, respectively. Esophageal (Tes) and skin temperatures were measured; plasma lactate ([Lac]p) and noradrenaline concentrations ([Norad]p), and relative change in PV (%ΔPV) were determined before and after pre-conditioning and during exercise, and LT was determined. After pre-conditioning, Tes was not different among trials, whereas the mean skin temperature was lower in Cool-Cool and Temp-Cool than in Temp-Neut (P < 0.001). During exercise, [Lac]p and [Norad]p were higher (P = 0.009 and P < 0.001, respectively) and LT was lower (P = 0.013) in Cool-Cool than in the other trials. The %ΔPV was not different among trials. LT was correlated with [Norad]p during exercise (R = 0.50, P = 0.005). Whole body skin surface cooling before exercise increases lactate accumulation and decreases LT with sympathetic activation when exercise is performed in a cool, but not in a temperate, environment.

PlanHab: Hypoxia counteracts the erythropoietin suppression, but seems to exaggerate the plasma volume reduction induced by 3weeks of bed rest.

The study examined the distinct and synergistic effects of hypoxia and bed rest on the erythropoietin (EPO) concentration and relative changes in plasma volume (PV). Eleven healthy male lowlanders underwent three 21‐day confinement periods, in a counterbalanced order: (1) normoxic bed rest (NBR; PIO 2: 133.1 ± 0.3 mmHg); (2) hypoxic bed rest (HBR; PIO 2: 90.0 ± 0.4 mmHg, ambient simulated altitude of ~4000 m); and (3) hypoxic ambulation (HAMB; PIO 2: 90.0 ± 0.4 mmHg). Blood samples were collected before, during (days 2, 5, 14, and 21) and 2 days after each confinement to determine EPO concentration. Qualitative differences in PV changes were also estimated by changes in hematocrit and hemoglobin concentration along with concomitant changes in plasma renin concentration. NBR caused an initial reduction in EPO by ~39% (P = 0.04). By contrast, HBR enhanced EPO (P = 0.001), but the increase was less than that induced by HAMB (P < 0.01). All three confinements caused a significant reduction in PV (P < 0.05), with a substantially greater drop in HBR than in the other conditions (P < 0.001). Thus, present results suggest that hypoxia prevents the EPO suppression, whereas it seems to exaggerate the PV reduction induced by bed rest.

Concordance of absolute and relative plasma volume changes and stability of Fcells in routine hemodialysis.

Central hematocrit (H) measurements are currently used to track the degree of ultrafiltration-induced hemoconcentration with the aim to detect and prevent excessive intravascular fluid depletion during hemodialysis (HD). Failure to maintain hemodynamic stability is commonly attributed to the misinterpretation of H caused by an unaccountable increase in Fcells , the ratio of whole-body hematocrit to H. It was the aim to examine Fcells under everyday conditions in a group of stable HD patients. Absolute plasma volume (Vp ) and H were concomitantly measured during routine HD in the extracorporeal system in hourly intervals by noninvasive and continuous technology (CritLine-Instrument-III) and indocyanine green dye dilution to derive relative plasma volumes from Vp and H (RPVp , RPVH ), respectively, and to calculate Fcells . Thirteen patients were studied during two midweek treatments (n = 26). Both absolute Vp (P < 0.05) and relative plasma volumes RPVH (P < 0.001) decreased during HD. Vp at any time point was positively correlated to RPVH (r = 0.52). Moreover, relative plasma volumes RPVH and RPVp determined by independent techniques were identical and showed negligible bias (-0.2%) but considerable limits of agreement (-15.6% to +15.3%). Fcells was stable and in the range of 0.9 ± 0.05 throughout HD and not different from the value assumed at the beginning of HD. Although Fcells remains constant in patients on routine dialysis and relative plasma volumes (RPVH and RPVp ) determined by independent techniques are therefore comparable, the variability of experimental conditions during dialysis and the limited accuracy of absolute volume measurements using available technology continues to complicate the ultrafiltration control problem.

Difference between true functional haemoglobin and pre-dialysis haemoglobin is associated with plasma volume variation: a multicentre study.

Due to the well-known intradialytic haemoglobin (Hb) change, it has been suggested that true functional Hb differs from pre-dialysis Hb. However, mechanisms of this change are not understood properly. We aimed at studying the role of plasma volume variation. This is a multicentre study with two consecutive phases. First, true functional Hb was predicted by Krisper's formula that estimates time-averaged haemoglobin concentration (TAC-Hb) and by the average (Avg-Hb) of Pre-Hb and Post-Hb (n = 346). Erythropoiesis-stimulating agent (ESA) doses were estimated according to the FDA instructions. Second, we investigated the correlations between Hb changes (ΔHb), body weight loss (ΔBW) and relative plasma volume changes (%ΔPV) in subjects with a weight gain higher than 1 kg (n = 208). First: Avg-Hb and TAC-Hb were similar to each other and higher than midweek Pre-Hb (p < 0. 002). Odds of having an Hb change >1 g/dL were 0.415. Odds were similar in both women and men [odds ratio (OR) 0.923; 95 % confidence interval (CI) 0.591-1.441]. If ESA doses were estimated by TAC-Hb or Avg-Hb instead of Pre-Hb, they should be lower in 40 % of the 346 patients. Second: Mean %ΔPV was -12.1 % (95 % CI -10.72/-13.48), and mean ΔHb was 1.1 g/dL (95 % CI 0.97/1.25). Correlation between ΔHb and %ΔPV was -0.92 (r (2) 0.84) whereas that of ΔHb/ΔBW was just -0.45 (r (2) 0.21; p < 0.000). Prevalence of ΔHb >1 g/dL was independent of ΔBW from 1.4 to 6.3 kg. By contrast, prevalence of ΔHb >1 g/dL in %ΔPV quartiles was clearly linear: 0, 52, 90 and 100 %, respectively. Plasma volume variations play a major role in intradialytic Hb change. True functional Hb should be estimated in all patients with a weight gain higher than 1 kg.

Acute effect of oral water intake during exercise on post-exercise hypotension

Post-exercise hypotension (PEH) is a sustained reduction in mean arterial blood pressure (MAP) after prolonged exercise. As water drinking is known to elicit a large acute pressor response, we aimed to explore the effect of drinking water during exercise on PEH. Ten normotensive male volunteers performed the control protocol: 30 min supine rest, 60 min cycling exercise in moderate intensity, and 60 min supine rest recovery. In the water drinking protocol, the same procedure was followed but with water intake during exercise to compensate for exercise-induced body weight lost. Heart rate, MAP, cardiac output and blood flow in the brachial artery were measured pre- and post-exercise. The total vascular conductance (TVC) and the vascular conductance (VC) in the brachial artery were calculated pre- and post-exercise, and the relative change in plasma volume (ΔPV) was also measured. Body weight loss during exercise was 0.65 ± 0.24 kg in the control. ΔPV was not different during recovery in either protocol. MAP in the control was significantly reduced during the latter half of the recovery compared with baseline. In contrast, MAP in the water drinking showed no reduction during recovery, and was significantly higher than in the control. TVC and VC in the brachial artery were lower in the water drinking, in which vasoconstriction was relatively exaggerated. Prevention of dehydration after exercise by oral water intake, or oral water intake per se has a role in maintaining post-exercise MAP and it may be related to reduction in TVC.

Sweat loss during heat stress contributes to subsequent reductions in lower‐body negative pressure tolerance

The contribution of sweating to heat stress-induced reductions in haemorrhagic tolerance is not known. This study tested the hypothesis that fluid loss due to sweating contributes to reductions in simulated haemorrhagic tolerance in conditions of heat stress. Eight subjects (35 ± 8 years old; 77 ± 5 kg) underwent a normothermic time control and two heat stress trials (randomized). The two heat stress trials were as follows: (i) with slow intravenous infusion of lactated Ringer solution sufficient to offset sweat loss (IV trial); or (ii) without intravenous infusion (dehydration; DEH trial). Haemorrhage was simulated via progressive lower-body negative pressure (LBNP) to presyncope after core body (intestinal) temperature was raised by ~1.5 °C using a water-perfused suit or a normothermic time control period. The LBNP tolerance was quantified via a cumulative stress index. Middle cerebral artery blood velocity (transcranial Doppler) and mean blood pressure (Finometer®) were measured continuously. Relative changes in plasma volume were calculated from haematocrit and haemoglobin. Increases in core body temperature and sweat loss (~1.6% body mass deficit) were similar (P > 0.05) between heat stress trials. Slow intravenous infusion (1.2 ± 0.3 litres) prevented heat-induced reductions in plasma volume (IV trial, -0.6 ± 6.1%; and DEH trial, -6.6 ± 5.1%; P = 0.01). Intravenous infusion improved LBNP tolerance (632 ± 64 mmHg min) by ~20% when compared with the DEH trial (407 ± 117 mmHg min; P = 0.01), yet tolerance remained 44% lower in the IV trial relative to the time control normothermic trial (1138 ± 183 mmHg min; P < 0.01). These data indicate that although sweat-induced dehydration impairs simulated haemorrhagic tolerance, this impairment is secondary to the negative impact of heat stress itself.

Using dialysis machine technology to reduce intradialytic hypotension

Intradialytic hypotension remains the most frequent complication associated with routine outpatient hemodialysis. Although increasing dialysis frequency and also lengthening dialysis session duration can reduce the risk of intradialytic hypotension, in practice, these options are limited to a small minority of dialysis patients. To help reduce intradialytic hypotension, a number of technological developments have been incorporated into the hemodialysis machine, based around relative blood volume monitoring, an indirect assessment of plasma volume. Further developments based on so called "fuzzy" logic feedback systems designed to adjust either or both the ultrafiltration rate and dialyzate sodium concentration according to relative changes in plasma volume. In addition, cooling and dissipation of the heat generated during dialysis also reduces the risk of intradialytic hypotension, and this can be regulated by cooling of the dialyzate using thermal control systems. In addition, convective therapies, such as online hemodialfiltration, have also been reported to reduce the frequency of intradialytic hypotension; whether this effect is simply due to increased cooling remains to be determined. Although all these developments have been reported to reduce the frequency of serious intradialytic hypotensive episodes, they have not been able to totally abolish hypotension, as they can not alone compensate for excessive weight gains and consequent excessive ultrafiltration requirements. Thus, in addition to the advances in hemodialysis machine technology designed to reduce intradialytic hypotension, attention also needs to be focused on reducing interdialytic weight gains, so reducing ultrafiltration requirement.

High-sweat Na+ in cystic fibrosis and healthy individuals does not diminish thirst during exercise in the heat.

Sweat Na(+) concentration ([Na(+)]) varies greatly among individuals and is particularly high in cystic fibrosis (CF). The purpose of this study was to determine whether excess sweat [Na(+)] differentially impacts thirst drive and other physiological responses during progressive dehydration via exercise in the heat. Healthy subjects with high-sweat [Na(+)] (SS) (91.0 ± 17.3 mmol/l), Controls with average sweat [Na(+)] (43.7 ± 9.9 mmol/l), and physically active CF patients with very high sweat [Na(+)] (132.6 ± 6.4 mmol/l) cycled in the heat without drinking until 3% dehydration. Serum osmolality increased less (P < 0.05) in CF (6.1 ± 4.3 mosmol/kgH(2)O) and SS (8.4 ± 3.0 mosmol/kgH(2)O) compared with Control (14.8 ± 3.5 mosmol/kgH(2)O). Relative change in plasma volume was greater (P < 0.05) in CF (-19.3 ± 4.5%) and SS (-18.8 ± 3.1%) compared with Control (-14.3 ± 2.3%). Thirst during exercise and changes in plasma levels of vasopressin, angiotensin II, and aldosterone relative to percent dehydration were not different among groups. However, ad libitum fluid replacement was 40% less, and serum NaCl concentration was lower for CF compared with SS and Control during recovery. Despite large variability in sweat electrolyte loss, thirst appears to be appropriately maintained during exercise in the heat as a linear function of dehydration, with relative contributions from hyperosmotic and hypovolemic stimuli dependent upon the magnitude of salt lost in sweat. CF exhibit lower ad libitum fluid restoration following dehydration, which may reflect physiological cues directed at preservation of salt balance over volume restoration.

Age related decreases in thermoregulation and cardiovascular function in horses

Older horses have an increased risk of hyperthermia due to impaired cardiovascular function. While many studies have investigated thermoregulation in horses during exercise, none have investigated the effects of ageing. To test the hypothesis that there is a difference in thermoregulation during exercise and plasma volume (PV) in young and old horses. Study 1: 6 young (Y, 7.7 ± 0.5 years) and 5 old (O, 26.0 ± 0.8 years) unfit Standardbred mares (507 ± 11 kg, mean ± s.e.) ran on a treadmill (6% grade, velocity calculated to generate a work rate of 1625 watts) until core temperature reached 40 °C. Core (CT), skin (ST), rectal temperature (RT) and heart rate (HR) were measured every min until 10 min post exertion. Packed cell volume (HCT), lactate (LA) and plasma protein (TP) were measured in blood samples collected before, at 40 °C and every 5 min until 10 min post exercise. Sweat loss was estimated using bodyweight. Study 2: Plasma volume was measured in 26 young (8.2 ± 0.7 years) and 8 old (26.6 ± 0.7 years) Standardbred mares (515 ± 12 kg) using Evans Blue dye. Pre-exercise blood (rBV) and red cell (rRCV) volumes were calculated using PV and HCT. Data analysis utilised repeated measures ANOVA and t tests and data are expressed as mean ± s.e. Old horses reached 40 °C faster (998 ± 113 vs. 1925 ± 259 s; P < 0.05) with a greater HR at 40 °C (184 ± 6 vs. 140 ± 5 beats/min; P < 0.05) and greater sweat losses (P < 0.05). Heart rate did not differ (P > 0.05) post exercise. Age did not alter (P > 0.05) CT, ST, RT, LA, HCT or TP. Plasma volume was greater in Y vs. O horses (P < 0.05, 28.5 ± 1.4 vs. 24.1 ± 1.6 l) as was rBV (41.3 ± 2.0 vs. 35.3 ± 2.3 l) and rRCV (13.3 ± 0.6 vs. 11.1 ± 0.8 l). Ageing compromises the ability to handle the combined demand of exercise and thermoregulation in part due to decreased absolute pre-exercise PV.

Exercise-induced plasma volume expansion and post-exercise parasympathetic reactivation

The purpose of this study was to investigate the effect of exercise-induced plasma volume expansion on post-exercise parasympathetic reactivation. Before (D(0)) and 2 days after (D(+2)) a supramaximal exercise session, 11 men (21.4 +/- 2.6 years and BMI = 23.0 +/- 1.4) performed 6-min of submaximal running where heart rate (HR) recovery (HRR) and HR variability (HRV) indices were calculated during the first 10 min of recovery. Relative plasma volume changes (PV) were calculated using changes in hematocrit and hemoglobin measured over consecutive mornings from D(0) to D(+2). Parasympathetic reactivation was evaluated through HRR and vagal-related indexes calculated during a stationary period of recovery. Compared with D(0), PV (+4.8%, P < 0.01) and all vagal-related HRV indices were significantly higher at D(+2) (all P < 0.05). HRR was not different between trials. Changes in HRV indices, but not HRR, were related to PV (all P < 0.01). HRR and HRV indices characterize distinct independent aspects of cardiac parasympathetic function, with HRV indices being more sensitive to changes in plasma volume than HRR.

The Effects of Creatine Loading on Thermoregulation and Intermittent Sprint Exercise Performance in a Hot Humid Environment

The purpose of this study was to determine the effects creatine (Cr) loading may have on thermoregulatory responses during intermittent sprint exercise in a hot/humid environment. Ten physically active, heat-acclimatized men performed 2 familiarization sessions of an exercise test consisting of a 30-minute low-intensity warm-up followed by 6 x 10 second maximal sprints on a cycle ergometer in the heat (35 degrees C, 60% relative humidity). Subjects then participated in 2 different weeks of supplementation. The first week, subjects ingested 5 g of a placebo (P, maltodextrin) in 4 flavored drinks (20 g total) per day for 6 days and were retested on day 7. The second week was similar to the first except a similar dose (4 x 5 g/day) of creatine monohydrate (Cr) replaced maltodextrin in the flavored drinks. Six days of Cr supplementation produced a significant increase in body weight (+1.30 +/- 0.63 kg), whereas the P did not (+0.11 +/- 0.52 kg). Compared to preexercise measures, the exercise test in the heat produced a significant increase in core temperature, a loss of body water determined by body weight change during exercise, and a relative change in plasma volume (%PVC); however, these were not significantly different between P and Cr. Sprint performance was enhanced by Cr loading. Peak power and mean power were significantly higher during the intermittent sprint exercise test following 6 days of Cr supplementation. It appears that ingestion of Cr for 6 days does not produce any different thermoregulatory responses to intermittent sprint exercise and may augment sprint exercise performance in the heat.

Effects of Acute Ingestion of Sodium Citrate on Metabolism and 5-km Running Performance: A Field Study

The purpose of the study was to (a) assess the effects of sodium citrate ingestion on metabolism and performance capacity in a 5-km competitive outdoor stadium run in trained male runners, and (b) elucidate the potential relationship between citrate-induced changes in plasma volume, body mass, and performance. Ten subjects (age 22.1 +/- 2.5 yrs, body mass 74.1 +/- 6.1 kg, height 180.1 +/- 5.7 cm, (.)VO(2)max 60.8 +/- 5.5 ml x kg(-1) x min(-1)) participated in the study. There was no effect of treatment on 5-km running time: 1100.0 +/- 79.1 and 1082.7 +/- 62.0 s in citrate (CIT) and in placebo (PLC) trials, respectively, p = 0.09. Blood pH increased from 7.34 +/- 0.07 to 7.49 +/- 0.07 (p = 0.002) as a result of administering sodium citrate in the amount of 0.5 g x kg(-1) body mass in 1.5 litres of solution but remained stable while the equal volume of placebo drink was consumed: 7.40 +/- 0.04 and 7.44 +/- 0.09. The relative change in plasma volume after administering the drink was -1.99 +/- 3.49% in the PLC and 9.75 +/- 6.51% in the CIT trial (p = 0.001). Body mass did not differ before drinking; however, before the start the subjects were heavier in the CIT trial (74.2 +/- 6.1 kg) vs. the PLC trial (73.4 +/- 6.2 kg, p = 0.048). The shifts in plasma volume and body mass were not related to changes in performance. The results suggest that ingestion of sodium citrate induces an increase in water retention, plasma volume, and blood pH before exercise but does not improve performance in a 5-km competitive run in field conditions in trained male runners.

Effect of hypervolemia on heart rate during 4 days of prolonged exercises.

The aim was to evaluate the cardiodynamic adjustment during 4 days of prolonged exercises and to check if the plasma volume (PV) expansion which is observed generally during such events plays a role in this adaptation. Thirteen subjects exercised 5 hours per day on a cycle ergometer alternately with a treadmill for 4 days (D1 to D4) (6 x 50 min sessions per day). The individual cycle ergometer load and the treadmill speed were unchanged during each exercise session and throughout all the sessions, and corresponded to a moderate exercise intensity: 58 - 63 % of peak oxygen uptake (VO (2)peak). Heart rate (HR) was recorded every 15 s during each exercise session and VO (2) was measured from the expired air at the beginning and the end of each exercise session. Relative PV changes were measured from haematocrit and haemoglobin changes in the morning before the exercise bouts. No significant changes of VO (2) were observed between the first and the last exercise session i. e. for cycling: 2.1 +/- 0.2 l/min and for running: 2.4 +/- 0.3 l/min. Between the first and the last day, HR decreased from 143 to 129 bpm for cycle (p < 0.0001) and from 147 to 137 bpm (p = 0.01) for treadmill. As compared to D1, PV increased gradually from D2 (+ 1.8 % +/- 4.7 %) to D4 (+ 8.5 % +/- 4.7 %). The individual PV increases were significantly correlated with cycling HR decreases from D1 to D4 (r (2) = 0.40, p = 0.02). In conclusion, the 4 days' prolonged exercise induced a HR decrease during submaximal exercise without VO (2) drift. Here we suggested that this HR decline could be in part linked to the transient PV expansion.

THERMAL DEHYDRATION: CENTRAL RENIN-ANGIOTENSIN SYSTEMAND PLASMA FLUID SHIFTS AND OSMOLARITY

We measured relative changes in plasma volume (PV) and osmolarity duringheat stress at 40 C in untreated and in intracerebroventricular (icv) losartan(LOS)-administered rats. The rats were in different hydration states and various phases of heat acclimation (at 34 C), differing in response to LOS.Icv LOS accelerated the rate of water loss, which led to a marked increase inplasma osmolarity, and augmented plasma fluid efflux during heat stress in all experimental groups. The data obtained fit with the hypothesis that icv LOS mediates plasma composition and volume via a linkage to the adreno sympathetic vasomotor thermoregulatory reflex and modulation of vasopress in release. The results suggest that during thermal dehydration central autonomic control may be involved in plasma fluid shifts. The temporally varying responsiveness of this pathway with acclimation may suggest that this control mechanism is subjected to adaptive modulations. The relative importance of this pathway, however, remains to be elucid...

Endurance training, overtraining and baroreflex sensitivity in female athletes.

We examined heavy training-induced changes in baroreflex sensitivity, plasma volume and resting heart rate and blood pressure variability in female endurance athletes. Nine athletes (experimental training group, ETG) increased intense training (70-90% VO2max) volume by 130% and low-intensity training (< 70% Vo2max) volume by 100% during 6-9 weeks, whereas the corresponding increases in six control athletes (CG) were 5% and 10% respectively. Maximal oxygen uptake (VO2max) in the ETG and CG did not change, but in five ETG athletes VO2max decreased from 53.0 +/- 2.2 (mean +/- SEM) (CI 46.8-59.2) ml kg-1 min-1 to 50.2 +/- 2.3 (43.8-56.6) ml kg-1 min-1 (P < 0.01), indicating overtraining. Baroreflex sensitivity (BRS) measured using the phenylephrine technique and blood pressure variability (BPV) did not change, but the low-frequency power of the R-R interval variability increased in the ETG (P < 0.05). The relative change in plasma volume was 7% in the ETG and 3% in the CG. The changes in BRS did not correlate with the changes in plasma volume, heart rate variability and BPV. We conclude that heavy endurance training and overtraining did not change baroreflex sensitivity or BPV but significantly increased the low-frequency power of the R-R interval variability during supine rest in female athletes as a marker of increased cardiac sympathetic modulation.

Underestimation of plasma volume changes in humans by hematocrit/hemoglobin method.

During water immersion in humans, the use of changes in hematocrit (Hct) and hemoglobin concentration (Hb) underestimates the relative changes in plasma volume (PV) as measured directly with Evans blue (EB). It is not known whether the same is the case during posture changes. Therefore, changes in PV were determined with an EB dilution technique in 10 males before, during, and after an acute posture change from seated to 6 degrees head-down tilt (HDT). The EB method was improved to take into account changes in transcapillary escape rate of albumin-bound EB. Furthermore, blood was sampled from a central venous catheter. Hct and Hb were simultaneously measured. During HDT, PV determined with EB increased by 9.3 +/- 2.0% but increased only 4.5 +/- 0.9% when calculated with the Hct/Hb method (P < 0.05 vs. EB measurements). Thus use of the Hct/Hb method in humans leads to underestimation of the change in PV by as much as 50% during an acute change in posture. Therefore, a direct tracer-dilution method must be used for accurate estimations of changes in PV during changes in posture or other antiorthostatic maneuvers.

Cardiovascular instability and baroreflex activity in a patient with tetanus

In a patient with tetanus we tested the hypothesis that the hyperadrenergic cardiovascular instability might be due to impairment of the baroreceptor reflex by the tetanus toxin. Baroreflex sensitivity assessed with the phenylephrine method was found to be normal. Changes in arterial pressure correlated inversely with relative changes in plasma volume but not with plasma catecholamine levels. There were both extreme hypo- and hyper-adrenergic episodes. We conclude that sympathetic overactivity in tetanus temporarily overrules a functionally intact baroreflex leading to severe blood pressure instability with episodes of hypertension.