Journal of Strength & Conditioning Research:
The Influence of Rehydration Mode After Exercise Dehydration on Cardiovascular Function
McDermott, Brendon P.1; Casa, Douglas J.2; Lee, Elaine C.2; Yamamoto, Linda M.2; Beasley, Kathleen N.2; Emmanuel, Holly2; Pescatello, Linda S.2; Kraemer, William J.2; Anderson, Jeffrey M.2; Armstrong, Lawrence E.2; Maresh, Carl M.2
1Department of Health, Human Performance, and Recreation, University of Arkansas, Fayetteville, Arkansas; and
2Human Performance Laboratory, Department of Kinesiology, University of Connecticut, Storrs, Connecticut
Address correspondence to Dr. Brendon P. McDermott, email@example.com.
Research was completed at the Human Performance Laboratory, University of Connecticut.
Partial funding received from the National Athletic Trainers' Association Research and Education Foundation.
Abstract: McDermott, BP, Casa, DJ, Lee, EC, Yamamoto, LM, Beasley, KN, Emmanuel, H, Pescatello, LS, Kraemer, WJ, Anderson, JM, Armstrong, LE, and Maresh, CM. The influence of rehydration mode after exercise dehydration on cardiovascular function. J Strength Cond Res 27(8): 2086–2095, 2013—Our purpose was to compare the common modes of rehydration (REHY) on cardiovascular and fluid regulation recovery after exercise dehydration (EXDE). Twelve nonheat-acclimatized trained subjects (age: 23 ± 4 years, weight: 81.3 ± 3.7 kg, height: 180 ± 6 cm, V[Combining Dot Above]O2max: 56.9 ± 4.4 ml·min−1·kg−1 , and body fat: 7.8 ± 3.0%) completed 20-hour fluid restriction and 2-hour EXDE to −4% body mass, and then were rehydrated to −2% body mass in a randomized, crossover design. The REHY methods included no fluid (NF), ad libitum, oral (OR), intravenous (IV), and a combination of IV and OR (IV + OR) of 1/2-normal saline (0.45% NaCl). The REHY occurred for 30 minutes, and the subjects were observed during rest for 30 minutes. Seated, standing, and mean arterial pressure (MAP) and blood pressure (BP) were measured every 15 minutes throughout REHY. Heart rate (HR), plasma arginine vasopressin concentration [AVP], and thirst perception were measured throughout REHY. The EXDE resulted in a body mass loss of 4.32 ± 0.22%. The REHY returned the subjects to −2.13 ± 0.47% body mass for controlled trials. Seated systolic BP was greater for IV + OR compared with that for OR (p = 0.015). Seated systolic BP and MAP during REHY showed that IV + OR was greater than OR, independent of time (p ≤ 0.011). Upon standing, IV + OR demonstrated a greater BP than both NF (p = 0.012) and OR (p = 0.031) did. The HR was reduced by IV and IV + OR to a greater extent than NF at REHY30 and REHY60 (p < 0.05). The IV + OR [AVP] demonstrated a strong trend for decreasing over time (p = 0.054) and was significantly less than NF at REHY60 (p = 0.003). Practical application seeking to restore cardiovascular function after EXDE, the combined use of IV + OR rather than a single REHY method seems to be most expedient.
Hypohydration resulting from inadequate fluid replacement and exercise dehydration (EXDE) in the heat is detrimental to cellular (25) and cardiovascular function (2,5,8,10,12,16,19,31,36), heat dissipation (2,5,16,36), and subsequent exercise performance (7,23,24). The influence of hypohydration on recovery from exercise, however, is not entirely clear. Impaired cardiovascular function after EXDE may impede recovery because of a limited capacity to supply appropriate nutrition to muscles, circulate hormones, and remove waste products (6,35). Further, the interplay between fluid regulation and cardiovascular function is well supported (2,3,5,8,10,15). Impaired or delayed fluid restoration after EXDE can increase recovery time and affect future performance (7,23,24). An ideal fluid replacement method to facilitate recovery from EXDE has yet to be identified.
When fluid replacement is inadequate and marked EXDE occurs, cardiovascular strain is apparent with poor heart rate (HR) and blood pressure (BP) recovery (8,10,12,19,21,31,37). This delay in cardiovascular control, resulting from impaired receptor sensitivity, decreased blood volume, or both, manifests itself as decreased BP and increased HRs during exercise recovery (5,8,31).
Previous research comparing modes of rehydration (REHY) suggests that intravenous (IV) offers limited, if any, benefit and that oral (OR) may actually be a superior means of REHY (7,22–24,26,34). Oral REHY offers perceptual and oropharyngeal reflex benefits bypassed with IV fluid replenishment (13,26,34). However, IV may facilitate recovery because of rapid plasma volume and osmolality restoration (3,7,8,22,27). Sports medicine professionals lack irrefutable evidence for the most efficient method of REHY for fluid regulation or cardiovascular restoration after EXDE.
This study was completed to compare methods of REHY after EXDE regarding outcomes related to cardiovascular function and fluid regulation. Practically, an ideal modality to treat EXDE would expedite cardiovascular recovery, maximize the benefits of the previous workout, restore homeostasis, and perhaps increase future performance. We used the most common used and recommended REHY methods (OR, ad libitum, and IV), added a combination of IV and OR (IV + OR) REHY, and included a control condition. The purpose of this study was to compare fluid replacement methods on BP, HR, fluid regulatory hormones, perceptual thirst, and stomach feeling ratings during REHY and recovery. Strength coaches and athletic trainers working with athletes are constantly seeking advantageous routes to maximize performance and facilitate recovery. If a single method of REHY facilitates recovery superior to other methods, strength coaches and athletic trainers would be supported in their recommendations or current practices. We hypothesized that stimulating oropharyngeal and baroreceptor mechanisms would lead to more efficient fluid regulation and restoration of cardiovascular function via IV + OR compared with singular REHY methods.
Experimental Approach to the Problem
The study followed a randomized, control comparison, crossover design. All the subjects completed 5 trials that consisted of baseline, EXDE, and REHY (Figure 1). The only difference between trials was the mode of REHY to isolate effects of the REHY method itself. We compared all methods within the participants on the independent variables listed below related to cardiovascular, fluid regulation, and perceptual recovery from EXDE. To establish scientific control, 1 trial included no fluid (NF), but mirrored data collection for all other trials. The REHY trials consisted of (a) fluid REHY via ad libitum consumption, where the subjects were instructed to drink “as little or as much as they desired according to thirst,” (b) metered OR REHY where 2% body mass was quantified and separated into equal boluses provided at REHY0 and every 5 minutes throughout REHY, (c) an IV REHY in which 2% of the body mass was provided evenly over 30 minutes, or (d) IV + OR REHY in which 1% of the body mass was provided via IV and the other 1% was OR, metered as described for IV and OR trials, respectively. The IV fluid provided in the IV and IV + OR trials was 1/2-normal saline (0.45% NaCl, 150 mOsm·L−1, 77 mEq·L−1 Na+ and Cl−), and the OR fluid for ad libitum, OR, and IV + OR trials was the identical fluid with a noncaloric lemon flavoring (Supervalu Inc., Eden Prarie, MN, USA) added for palatability. Each trial was separated by a minimum of 5 days to avoid potential heat acclimation. Subject start time for baseline and trial days was identical between trials (within 1 hour) to ensure circadian consistency in outcome variables.
Twelve well-trained nonheat-acclimatized men (age: 18–39 years) from the University community were recruited for participation. The subjects were participating in a moderate fitness program involving a minimum of 8 hours of moderately intense exercise training per week. All testing occurred during winter months when outdoor high temperatures were <50° F consistently. Screening information was obtained to ensure that the subjects did not have (a) chronic health problems; (b) a history of heat illness within the past 3 years; (c) a history of cardiovascular, metabolic, or respiratory disease; (d) a regiment of consuming any muscle-building supplements within the past year; and (e) a history of smoking within the past 2 years. The subjects were instructed to continue regular participation in their training regime to maintain cardiovascular fitness throughout the study. Physical characteristics of our subjects included the following: age: 23 ± 4 years, weight: 81.3 ± 3.7 kg, height: 180 ± 6 cm, V[Combining Dot Above]O2max: 56.9 ± 4.4 ml·min−1·kg−1 , and body fat: 7.8 ± 3.0% . All the subjects attended a study briefing and had ample time to ask questions before voluntary participation. This study was approved by the University of Connecticut Institutional Review Board, and all the subjects signed a human subjects’ informed consent form before participating. The subjects were financially compensated for their participation.
The subjects initially attended baseline testing and familiarization visits to gain baseline characteristics and ensure they have an understanding of the experimental protocol. Baseline skinfold calculated body fat % was collected by the same trained clinician (3-site Jackson-Pollock method). Maximal oxygen uptake (V[Combining Dot Above]O2max) testing was completed via an incremental run to exhaustion on a treadmill while using metabolic measuring apparatus (ParvoMedics, Sandy, UT, USA). The subjects began with 4 minutes of sustained output at a comfortable pace with 0% grade. Every 2 minutes following, the grade was incrementally increased by 2% until they reached volitional exhaustion, and rating of perceived exertion ≥ 18. After a 20-minute rest, the V[Combining Dot Above]O2max was verified by starting the subjects at the previous exhaustive velocity and incline grade % and they again ran until volitional exhaustion. The V[Combining Dot Above]O2max testing occurred in the heat (35° C, 40% RH) to match experimental conditions. Baseline euhydrated body mass was measured on 3 consecutive days by having the subjects report to the laboratory after a 12-hour fast and consuming extra fluid the night before and morning of baseline body mass measures. Euhydration was verified by a urine specific gravity ≤ 1.020. The subjects also practiced on the treadmill and cycle ergometer to verify relative exercise intensity for experimental trials as a percentage of the V[Combining Dot Above]O2max and to familiarize them with the testing protocol.
Experimental trials began with a baseline day blood draw. The subjects reported to the laboratory after a 12-hour overnight fast (no food or fluid, except water) and 24-hour without exercising, drinking alcohol, or consuming stimulants (including caffeine). To ensure euhydration, the subjects were asked to drink an extra 36 fluid ounces of water the night before each baseline visit and 36 fluid ounces of water the morning of each visit before they arrived at the laboratory. The subjects voided their bladder and researchers recorded body mass. The subjects then completed baseline thirst perception followed by a euhydrated sit-to-stand test (Figure 1). The sit-to-stand test began with seated BP recorded in triplicate (separated by 1 minute). The subjects were then instructed to stand up not using their arms as fast as they could. The BP inflation began when the subject reached standing posture. Standing BP was recorded at baseline, 5 minutes after EXDE and every 15 minutes during REHY, immediately after the third seated reading was taken. Comparisons included systolic, diastolic, and calculated mean arterial pressure (MAP) in both seated and standing positions, and a change variable was calculated to identify the change in pressures from the seated to standing positions (i.e., seated systolic BP − standing systolic BP). For the rest of the baseline day, the participants refrained from exercise. Beginning at 1 PM, they did not consume fluids and were asked to eat low-moisture foods and refrain from ingesting stimulants (including caffeine).
The subjects reported to the laboratory the subsequent day for the evaluation of the EXDE and REHY components of trials (Figure 1). Upon arrival, the subjects were weighed and provided a urine sample to verify their hydration state (with a goal of between −1 and −2% body mass) and ate a standard meal (bagel, banana, with 3 g of cream cheese; 490 kcal, 11 g fat, 86 g CHO, 12 g PRO, and 356 mg Na+) provided by researchers. The meal was maintained consistent between trials by weighing the amount voluntarily consumed for trial 1 and weighing subsequent meals to ensure that the same amount of all foods was provided for every trial. After breakfast, the subjects had a 20-G Teflon cannula inserted at the antecubital vein of both arms, kept patent with normal saline and heparin (9:1).
The subjects then entered the environmental chamber (Model 2000, Minus Eleven, Inc., Malden, MA, USA) for the remainder of the testing protocol. After a 15-minute seated equilibration, blood was drawn. For EXDE, the subjects alternated (30 minutes each) between walking on a treadmill at 5.6–6.8 km·h−1 with 5–10% grade (40–60% V[Combining Dot Above]O2max), and cycling on an ergometer at a preset power output set to elicit a relative output of 40–60% V[Combining Dot Above]O2max for a total of 120 minutes of EXDE (30 minutes of walking, 30 minutes of cycling, etc.). Body mass was monitored every 30 minutes, whereas the HR was measured every 15 minutes during EXDE. The subjects were provided a small snack at 90 minutes of EXDE (230 or 240 kcal with equal amounts of macronutrients, Na+, K+; Powerbar, Glendale, CA, USA). Each subject was provided with the same weight and bar flavor in each trial after eating volitionally (up to the provided nutrition information) during the first trial. The subjects walked or cycled for 120 minutes with the goal of eliciting a total −4% body mass loss.
After EXDE, the participants were helped to a seated position and were rehydrated (except in the NF trial) to −2% of their baseline body mass for 30 minutes. From 30- to 60-minutes post-REHY, the subject was observed for specific responses described below.
Diet records for food intake were completed by the subjects including the 3 days preceding the EXDE experimental protocol. The subjects were asked to match their diet for these days leading into each trial, and copies were provided for each trial. Diet records were entered into nutrition analysis software (Nutritionist Pro Version 4.2.0, Axxya Systems, Stafford, TX, USA) to analyze total caloric, carbohydrate, protein, and fat intake.
All BP measures were taken with an automated stress test BP monitor (Tango+, SunTech Medical, Morrisville, NC, USA) applied to the subjects' left arm. The device uses an automatic inflation-deflation protocol with 3-lead electrocardiogram and microphone technology to doubly verify heart beat detection while deflating. Monitor setup and maintenance throughout the study occurred according to manufacturer guidelines. The BP during EXDE was taken every 15 minutes. Seated systolic and diastolic pressures were compared with an average of the 5 baseline day measures (average hydrated resting BP) to quantify differences throughout REHY. The BP variables were all included as dependent on trial REHY method and are valid indicators of cardiovascular physiology responses (5,8,10,19,32).
The HR was continuously measured via a cardiotachometer (Polar, Lake Success, NY, USA) and recorded every 15 minutes throughout the EXDE and every minute during REHY. The HR has been consistently used as a reflection of cardiovascular stress and recovery from exercise and dehydration (2,5,8,9,16,39). Blood samples were drawn and immediately transferred to tubes containing ethylenediaminetetraacetic acid. Separate tubes were used for serum analysis. Plasma or serum was separated, after centrifugation. Serum samples were immediately analyzed for osmolality via freezing point depression (Advanced Instruments, Inc., Model 3250, Norwood, MA, USA). Plasma samples were transferred to storage containers and stored at −80° C for later analysis. After thawing the samples, they were extracted and analyzed in duplicate for plasma arginine vasopressin concentration [AVP] using a commercially available enzyme-linked immunosorbent assay kit (Assay Designs, Ann Arbor, MI, USA). Interassay coefficients of variation were 28%, so %Δ plasma [AVP] was used to identify differences during REHY. Plasma [AVP] values were not corrected for %Δ plasma volume. These methods are consistent with other methods assessing fluid regulatory hormone concentrations (13–15,22,23,38). Plasma volume shifts were calculated via microhematocrit and hemoglobin (Hb 201+ Analyzer, Hemocue, Lake Forest, CA, USA) using the equation presented by Dill and Costill (11). Plasma volume is considered a reliable measure of blood and extracellular volume and, in our case, it is indicative of acute changes related to restoration levels (11,12,30).
Thirst perception was measured via a 9-point visible scale with verbal anchors ranging from 1 (not thirsty at all) to 9 (very, very thirsty). Thirst sensation was recorded at baseline, before, and every 30 minutes during EXDE, and every 5 minutes during REHY. Thirst has been evaluated previously and is a practical measure of subject approval or preference (18,20,26,34). A stomach feeling questionnaire composed of 6 Likert-scale (10 cm) lines ranging from not at all to extremely was administered on a laptop computer and included stomach contents sloshing, nausea, stomach bloat, hunger, stomach upset, and stomach fullness. The place where the subjects clicked on the line was automatically measured and recorded by the computer.
A 5 × 5 (trial × time) repeated measures analysis of variance was used to identify trial, time, or interaction differences for dependent variables. Greenhouse-Geisser corrections were applied when the assumptions of sphericity were compromised. A t-test post hoc comparison with appropriate Bonferroni correction was used to detect the differences within or between trials when trial × time interactions were present. The 0.05 level of significance was selected for identified differences. A prior power analysis for 2 variables (HR and systolic BP) revealed that, for sufficient (0.80) power, we required analysis of. Data are presented as mean ± SD. Statistical analyses were completed using SPSS software (International Business Machines Corporation, SPSS 18, Chicago, IL, USA).
Environmental conditions for all trials (mean ± SD; air temperature, 35.5 ± 1.5° C; relative humidity, 33 ± 8%; barometric pressure, 748 ± 6 mm Hg) were similar (p > 0.05). Nutritional analysis for 3 days before trials revealed no significant differences between trials in kilocalories, fat, carbohydrate, protein, Na+, or K+ intake (p > 0.05). Relative exercise intensities for EXDE treadmill walking and ergometer cycling were not significantly different between trials (p > 0.05).
Body mass measures are presented in Table 1 and Figure 2. Fluid restriction and EXDE successfully induced a hypohydrated state of −4.32 ± 0.22% body mass and REHY returned the subjects to −2.13 ± 0.47% body mass in the OR, IV, and IV + OR trials. Noteworthy, during ad libitum REHY, the subjects voluntarily replaced similar amounts of fluid overall (1.8 ± 0.5 L) compared with the OR trial (1.8 ± 0.4 L). The subjects did, however, replace significantly more fluid (p < 0.05) during the initial 10 minutes of REHY during ad libitum (1.06 ± 0.34 L) vs. OR (0.6 ± 0.14 L).
Systolic BP during EXDE showed significant decreases at EXDE90 and EXDE120 compared with EXDE30 (p ≤ 0.009). Seated systolic BP during REHY showed that, independent of time, IV + OR was greater than OR (p = 0.004, η2 = 0.341). Compared with baseline systolic seated BP, IV changed significantly less than ad libitum (p = 0.028) and OR (p = 0.041) did at REHY15. At REHY45 and REHY60, both IV and IV + OR changed significantly less than OR (p ≤ 0.045) did. Oral REHY was the only trial with significant systolic BP decreases over time, with REHY60 being decreased more compared with baseline than REHY30 and REHY0 (p ≤ 0.007). There was a main effect difference (p = 0.011, η2 = 0.299) with IV + OR seated MAP being significantly greater than OR values. Standing systolic BP values during REHY responses are presented in Figure 3. The change in systolic BP with the sit-to-stand test was significantly decreased over time with all trials (p ≤ 0.036, η2 = 0.441) with no between trial differences (p > 0.05).
The HR was significantly increased (p < 0.001, η2 = 0.993) independent of trial, from 75 ± 13 b·min−1 at rest to 158 ± 18 b·min−1 during EXDE. During REHY, the HR was decreased during IV + OR by a greater amount than HR was during NF, ad libitum, and OR at REHY15 (p ≤ 0.048). The IV HR was significantly less than ad libitum (p = 0.009) at REHY15. At REHY30, both IV and IV + OR significantly decreased the HR compared with NF, OR, and ad libitum (p ≤ 0.044). The IV + OR and IV had decreased the HR compared with NF at REHY60 (p ≤ 0.027) and the IV HR was significantly less than for ad libitum (p = 0.022; Figure 4).
We used a %Δ variable compared with baseline plasma [AVP] values. Percent change in plasma [AVP] during REHY is shown in Figure 5. Serum osmolality responses during trials are shown in Table 2.
Plasma Volume Change
Plasma volume changes are shown in Figure 6 for pre-EXDE through REHY60. Plasma volume demonstrated a graded response in restoration after a marked decrease during fluid restriction and EXDE. The IV most rapidly restored plasma volume, whereas NF failed to return the volume to normal levels.
Thirst sensation was not significantly different at post-EXDE between trials (p = 0.602, η2 = 0.856). Thirst sensation ratings during REHY are shown in Figure 7. Thirst sensation for NF was not decreased over time (8 ± 2; p = 0.852). Ad libitum thirst at REHY15, REHY30, and REHY45 was significantly decreased when compared with post-EXDE (p ≤ 0.048), but REHY60 was not (p = 0.604). For IV, REHY45 was significantly less than it was post-EXDE (p = 0.043). For IV + OR and OR, all REHY time points were significantly reduced compared with post-EXDE (p ≤ 0.007) and REHY15 (p ≤ 0.039) values.
Ad libitum demonstrated significantly greater ratings of stomach content sloshing vs. IV and NF at REHY15 (p ≤ 0.042). Oral stomach content sloshing was rated significantly greater than NF at REHY30 and REHY45 (p ≤ 0.038), and greater than IV at REHY60 (p = 0.038). There were no trial differences in hunger (p = 0.115) from baseline through post-EXDE, but there was a time difference with pre-EXDE greater than baseline (p = 0.026) values. Stomach fullness ratings at REHY15 demonstrated that ad libitum was significantly greater than NF (p = 0.017), IV (p = 0.016), and IV + OR (p = 0.038). The IV was significantly less with stomach fullness than were ad libitum (p = 0.020) and OR (p = 0.024) at REHY30. At REHY60, OR exhibited significantly greater stomach fullness than did NF (p = 0.050) and IV (p = 0.013). No other significant differences over time or between trials existed for other stomach feeling questions (p > 0.05).
The main contribution of this study was the initial comparison of a variety of modes of REHY, with a focus on cardiovascular variables during REHY and recovery after EXDE. This was also the first study to include ad libitum and a combination of OR and IV administered fluids. We successfully controlled dietary intake, environmental conditions, work output during EXDE, the extent of hypohydration and REHY, and time between trials to isolate the effects of REHY mode on cardiovascular variables and fluid regulation after EXDE. Our findings suggest that combining IV and OR fluids offers some minor benefits (limited BP decreases, faster HR restoration, quicker plasma [AVP] restoration) over providing a single method of REHY. Interestingly, we found that the subjects during ad libitum replaced fluids adequately up to −2% body mass loss, demonstrating that thirst may not be an optimal guide for REHY after EXDE.
We expected that, as a result of exercise, cardiovascular drift and a decreased plasma volume, EXDE would cause a decrease in the BP because of reduced blood volume (8,21,32). One of the means by which we assessed recovery was to compare values to baseline measures, to quantify a return to normal individual capacity. We found that the IV + OR condition was superior to OR REHY alone in restoring systolic BP. This restoration was maintained better than both NF and OR, when the subjects assumed a standing posture. This suggests that IV + OR decreases the orthostatic intolerance associated with hypohydration. That is, IV and IV + OR may lead to a more efficient restoration of the BP as a result of bypassing the transit and absorption time of OR fluid replacement. Davis and Fortney and Schroeder et al. (10,37) previously reported that fluid ingestion improves cardiovascular recovery, but the present findings indicated that the BP was maintained when half the fluid was administered via IV (during the IV + OR trial).
A dehydrated patient in a medical tent or sports medicine treatment facility should be initially placed supine with legs elevated until symptoms subside (1,4,33). Once dehydration is confirmed and treated, the patient should be gradually transitioned to a seated and then a standing posture (33). Provided the symptoms do not return after standing for a brief period of time, patient discharge normally follows. If the patients are hypotensive or feel dizzy upon standing, they should be returned to a supine position and REHY should continue. The absence of orthostatic hypotension is an important sign of recovery, readiness for discharge, or discontinuation of treatment, for a previously hypohydrated athlete (1,4,8,33). The BP results in Figure 3 suggest that treatment time required for effective recovery after EXDE can be reduced if IV + OR is used. This method offers the simultaneous stimulation of oropharyngeal reflexes and cardiovascular baroreceptors. This stimulation causes a cascade of events leading to a more efficient restoration of BP and improved maintenance of MAP. We chose to control the amount of fluid replaced during REHY rather than replacing as much as possible in a short period of time. Obviously, an athletic clinical situation requiring rapid REHY would be ideal when including both IV and OR REHY. Future research should attempt to maximize the REHY capacity and compare similar variables.
The use of IV + OR provided a superior means of facilitating HR restoration by REHY15. Interestingly, there were no significant differences in the HR between OR, ad libitum, and NF throughout REHY. Previous research suggests that OR resulted in similar benefits when compared with IV (7,23). Explanations for differences in our results compared with that in previous research may be the fact that we only rehydrated the subjects to a level of −2% body mass. This is the previously identified threshold for detriments (e.g., exercise performance, symptoms, and increased body temperature) because of hypohydration (2,16,36). In terms of HR recovery, the IV + OR trials likely were not affected because of profound increases in plasma volume, actually above baseline (9). However, Figure 4 shows that all REHY trials were significantly better than NF administration. This is similar to the findings of previous research suggesting that rapid REHY benefits are comparable between mode when cardiovascular variables are considered (7,23).
We used serum osmolality as an indication of hydration status, while it is also influential in AVP regulation. Previous research demonstrates that plasma osmolality is involved in mechanistic regulation of plasma [AVP], and thirst concurrently decreases with plasma [AVP] with REHY after EXDE (3,14,15,27,28,30,38). We found differences between the IV and IV + OR trial, supporting previous research. Our results demonstrated an AVP decrease in the absence of marked osmolality alterations, suggesting nonosmotic influence. Our data demonstrate that when osmotic and nonosmotic mechanisms are stimulated (via IV + OR REHY), plasma [AVP] decreases more than when these mechanisms are stimulated independently. It is not clear at this point whether this serves as a benefit, although a more efficient return to homeostasis is demonstrated (3,15,38).
We purposely facilitated a reduction in the plasma volume via fluid restriction and EXDE. Consistent with previous studies, we found an average plasma volume reduction of 8 ± 3% from baseline at post-EXDE (7,22,23). During REHY, we found an expected graded plasma volume restoration with IV providing the most benefit, followed by IV + OR, and then the OR trials (Figure 6), consistent with that in previous studies (7,22,23).
Thirst was significantly decreased compared with post-EXDE in all the trials with the exception of NF. The superior method in immediately decreasing thirst was ad libitum, when the subjects were instructed to consume as much fluid as they wanted, according to thirst. As expected, this resulted in rapid oral fluid replacement (>1 L within 10 minutes). Interestingly, by REHY30, IV + OR, which included metered oral fluid intake of half the amount, showed no significant thirst differences compared with ad libitum. A partial, or gradual, stimulation of the oropharyngeal reflex resulted in a similar decrease in thirst. The oropharyngeal reflex was likely responsible for decreased thirst in the IV + OR trial, and this is supported with differences when oral fluid was added to IV (13,30).
Voluntary partial fluid replacement occurred in this study, consistent with the findings of previous studies (17,18,20). That is, the subjects consumed fluids to return body mass losses up to −2.3% in the ad libitum trial. This was accompanied by a thirst rating of 3 ± 2 (3 = a little thirsty). These data show that the thirst mechanism is significantly decreased when REHY returns to about 2% body mass loss. Previous research has demonstrated increases in thirst at a similar threshold during EXDE (6,17,18,20,35). Rapid gastric pressure increases and oropharyngeal reflex stimulation cause a concomitant decrease in thirst (13,30).
Results from our ad libitum trial are in contrast to suggestions made by some that individuals should consume fluids according to thirst (29). If an athlete or soldier were to follow a moderate workout during which they dehydrated to −4% body mass, and they rehydrated according to thirst, they would only partially rehydrate, perhaps delaying recovery. This could mount over time and be increased during subsequent workouts, potentially leading to impaired cardiovascular function and heat dissipation (2,5,8–10,16,24,36).
When the diagnosis of EXDE is made, and rapid REHY is indicated, there are minor benefits of including both OR and IV fluids compared with a single method of REHY. When IV fluids are used, adding simultaneous OR fluids, if tolerated, seems to expedite cardiovascular and fluid regulation recovery. These data apply to treatment of patients at both large-scale endurance events (marathons, ironman triathlons) and sporting events that include a halftime (American football, soccer) or limited time between matches (tennis, basketball). The IV usage is common in these settings and an optimal REHY strategy including a combination of IV and OR fluids may optimize recovery and subsequent athletic performance.
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hypohydration; subsequent exercise; fluid regulation; heat stress
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