The importance of the peripheral circulation in critical illnesses.
Explore the source record for details and available documents.
Baroreflex↗
Biomedical subjects
G W Mack
Publications and source records attributed to G W Mack.
Baroreceptor modulation of active cutaneous vasodilation during dynamic exercise in humans.
The hypothesis that baroreceptor unloading during dynamic limits cutaneous vasodilation by withdrawal of active vasodilator activity was tested in seven human subjects. Increases in forearm skin blood flow (laser-Doppler velocimetry) at skin sites with (control) and without alpha-adrenergic vasoconstrictor activity (vasodilator only) and in arterial blood pressure (noninvasive) were measured and used to calculate cutaneous vascular conductance (CVC). Subjects performed two similar dynamic exercise (119 +/- 8 W) protocols with and without baroreceptor unloading induced by application of -40 mmHg lower body negative pressure (LBNP). The LBNP condition was reversed (i.e., either removed or applied) after 15 min while exercise continued for an additional 15 min. During exercise without LBNP, the increase in body core temperature (esophageal temperature) required to elicit active cutaneous vasodilation averaged 0.25 +/- 0.08 and 0.31 +/- 0.10 degrees C (SE) at control and vasodilator-only skin sites, respectively, and increased to 0.44 +/- 0.10 and 0.50 +/- 0.10 degrees C (P < 0.05 compared with without LBNP) during exercise with LBNP. During exercise baroreceptor unloading delayed the onset of cutaneous vasodilation and limited peak CVC at vasodilator-only skin sites. These data support the hypothesis that during exercise baroreceptor unloading modulates active cutaneous vasodilation.
Adrenergic alpha-Antagonists↗
Effect of lymphatic outflow pressure on lymphatic albumin transport in humans.
The effects of posture on the lymphatic outflow pressure and lymphatic return of albumin were examined in 10 volunteers. Lymph flow was stimulated with a bolus infusion of isotonic saline (0.9%, 12.6 ml/kg body wt) under four separate conditions: upright rest (Up), upright rest with lower body positive pressure (LBPP), supine rest (Sup), and supine rest with lower body negative pressure (LBNP). The increase in plasma albumin content (Delta Alb) during the 2 h after bolus saline infusion was greater in Up than in LBPP: 82.9 +/- 18.5 vs. -28.4 mg/kg body wt. Delta Alb was greater in LBNP than in Sup: 92.6 vs. -22.5 +/- 18.9 mg/kg body wt (P < 0.05). The greater Delta Alb in Up and Sup with LBNP were associated with a lower estimated lymphatic outflow pressure on the basis of the difference in central venous pressure (Delta CVP). During LBPP, CVP was increased compared with Up: 3.8 +/- 1.4 vs. -1.2 +/- 1.2 mmHg. During LBNP, CVP was reduced compared with Sup: -3.0 +/- 2.2 vs. 1.7 +/- 1.0 mmHg. The translocation of protein into the vascular space after bolus saline infusion reflects lymph return of protein and is higher in Up than in Sup. Modulation of CVP with LBPP or LBNP in Up and Sup, respectively, reversed the impact of posture on lymphatic outflow pressure. Thus posture-dependent changes in lymphatic protein transport are modulated by changes in CVP through its mechanical impact on lymphatic outflow pressure.
Adult↗
Increased renal tubular sodium reabsorption during exercise-induced hypervolemia in humans.
We tested the hypothesis that renal tubular Na(+) reabsorption increased during the first 24 h of exercise-induced plasma volume expansion. Renal function was assessed 1 day after no-exercise control (C) or intermittent cycle ergometer exercise (Ex, 85% of peak O(2) uptake) for 2 h before and 3 h after saline loading (12.5 ml/kg over 30 min) in seven subjects. Ex reduced renal blood flow (p-aminohippurate clearance) compared with C (0.83 +/- 0.12 vs. 1.49 +/- 0.24 l/min, P < 0.05) but did not influence glomerular filtration rates (97 +/- 10 ml/min, inulin clearance). Fractional tubular reabsorption of Na(+) in the proximal tubules was higher in Ex than in C (P < 0.05). Saline loading decreased fractional tubular reabsorption of Na(+) from 99.1 +/- 0.1 to 98.7 +/- 0.1% (P < 0.05) in C but not in Ex (99.3 +/- 0.1 to 99.4 +/- 0.1%). Saline loading reduced plasma renin activity and plasma arginine vasopressin levels in C and Ex, although the magnitude of decrease was greater in C (P < 0.05). These results indicate that, during the acute phase of exercise-induced plasma volume expansion, increased tubular Na(+) reabsorption is directed primarily to the proximal tubules and is associated with a decrease in renal blood flow. In addition, saline infusion caused a smaller reduction in fluid-regulating hormones in Ex. The attenuated volume-regulatory response acts to preserve distal tubular Na(+) reabsorption during saline infusion 24 h after exercise.
Adult↗
Human cardiovascular and humoral responses to moderate muscle activation during dynamic exercise.
We examined the hypothesis that activation of the muscle metaboreflex during dynamic exercise would augment influences tending to cause a rise in arginine vasopressin, plasma renin activity, and catecholamines during dynamic exercise in humans. Ten healthy adults performed 30 min of supine cycle ergometer exercise at approximately 50% of peak oxygen consumption with or without moderate muscle metaboreflex activation by application of 35 mmHg lower body positive pressure (LBPP). Application of LBPP during the first 15 or last 15 min of exercise increased mean arterial blood pressure, plasma lactate concentration, and minute ventilation, indicating an activation of the muscle metaboreflex. These changes were rapidly reversed when LBPP was removed. During exercise at this intensity, LBPP augmented the release of arginine vasopressin and catecholamines but not of plasma renin activity. These results suggest that, although in humans hormonal responses are induced by moderate activation of the muscle metaboreflex during dynamic exercise, the thresholds for these responses may not be uniform among the various glands and hormones.
Adult↗
Intense exercise stimulates albumin synthesis in the upright posture.
We tested the hypothesis that an elevation in albumin synthetic rate contributes to increased plasma albumin content during exercise-induced hypervolemia. Albumin synthetic rate was measured in seven healthy subjects at 1-5 and 21-22 h after 72 min of intense (85% peak oxygen consumption rate) intermittent exercise and after 5 h recovery in either upright (Up) or supine (Sup) postures. Deuterated phenylalanine (d(5)-Phe) was administrated by a primed-constant infusion method, and fractional synthetic rate (FSR) and absolute synthetic rate (ASR) of albumin were calculated from the enrichment of d(5)-Phe in plasma albumin, determined by gas chromatography-mass spectrometry. FSR of albumin in Up increased significantly (P < 0.05) from 4.9 +/- 0.9%/day at control to 7.3 +/- 0.9%/day at 22 h of recovery. ASR of albumin increased from 87.9 +/- 17.0 to 141.1 +/- 16.6 mg albumin. kg body wt(-1). day(-1). In contrast, FSR and ASR of albumin were unchanged in Sup (3.9 +/- 0.4 to 4.0 +/- 1.4%/day and 74.2 +/- 8.9 to 85.3 +/- 23.9 mg albumin. kg body wt(-1). day(-1) at control and 22 h of recovery, respectively). Increased albumin synthesis after upright intense exercise contributes to the expansion of greater albumin content and its maintenance. We conclude that stimuli related to posture are critical in modulating the drive for albumin synthesis after intense exercise.
Adult↗
Reflex control of the cutaneous circulation during passive body core heating in humans.
The impact of body core heating on the interaction between the cutaneous and central circulation during blood pressure challenges was examined in eight adults. Subjects were exposed to -10 to -90 mmHg lower body negative pressure (LBNP) in thermoneutral conditions and -10 to -60 mmHg LBNP during heat stress. We measured forearm vascular conductance (FVC; ml. min(-1). 100 ml(-1). mmHg(-1)) by plethysmography; cutaneous vascular conductance (CVC) by laser-Doppler techniques; and central venous pressure, arterial blood pressure, and cardiac output by impedance cardiography. Heat stress increased FVC from 5.7 +/- 0.9 to 18.8 +/- 1.3 conductance units (CU) and CVC from 0.21 +/- 0.07 to 1.02 +/- 0.20 CU. The FVC-CVP relationship was linear over the entire range of LBNP and was shifted upward during heat stress with a slope increase from 0. 46 +/- 0.10 to 1.57 +/- 0.3 CU/mmHg CVP (P < 0.05). Resting CVP was lower during heat stress (6.3 +/- 0.6 vs. 7.7 +/- 0.6 mmHg; P < 0. 05) but fell to similar levels during LBNP as in normothermic conditions. Data analysis indicates an increased capacity, but not sensitivity, of peripheral baroreflex responses during heat stress. Laser-Doppler techniques detected thermoregulatory responses in the skin, but no significant change in CVC occurred during mild-to-moderate LBNP. Interestingly, very high levels of LBNP produced cutaneous vasodilation in some subjects.
Adult↗
Mechanism for the posture-specific plasma volume increase after a single intense exercise protocol.
To test the hypothesis that exercise-induced hypervolemia is a posture-dependent process, we measured plasma volume, plasma albumin content, and renal function in seven healthy subjects for 22 h after single upright (Up) or supine (Sup) intense (85% peak oxygen consumption rate) exercise. This posture was maintained for 5 h after exercise. Plasma volume decreased during exercise but returned to control levels by 5 h of recovery in both postures. By 22 h of recovery, plasma volume increased 2.4 +/- 0.8 ml/kg in Up but decreased 2.1 +/- 0.8 ml/kg in Sup. The plasma volume expansion in Up was accompanied by an increase in plasma albumin content (0.11 +/- 0.04 g/kg; P < 0.05). Plasma albumin content was unchanged in Sup. Urine volume and sodium clearance were lower in Up than Sup (P < 0.05) by 5 h of recovery. These data suggest that increased plasma albumin content contributes to the acute phase of exercise-induced hypervolemia. More importantly, the mechanism by which exercise influences the distribution of albumin between extra- and intravascular stores after exercise is altered by posture and is unknown. We speculate that factors associated with postural changes (e.g., central venous pressure) modify the increase in plasma albumin content and the plasma volume expansion after exercise.
Adult↗
Thermophysiological responses of human volunteers during controlled whole-body radio frequency exposure at 450 MHz.
Thermoregulatory responses of heat production and heat loss were measured in seven adult volunteers (four women and three men, aged 21-57 yr) during 45-min dorsal exposures of the whole body to 450 MHz continuous wave radio frequency (RF) fields. Two power densities (PD) (local peak PD = 18 and 24 mW/cm2; local peak specific absorption rate = 0.320 [W/kg]/[mW/cm2]) were tested in each of three ambient temperatures (Ta = 24, 28, and 31 degrees C) plus Ta controls (no RF). No changes in metabolic heat production occurred under any exposure conditions. Vigorous increases in sweating rate on back and chest, directly related to both Ta and PD, cooled the skin and ensured efficient regulation of the deep body (esophageal) temperature to within 0.1 degrees C of the normal level. Category judgments of thermal sensation, comfort, sweating, and thermal preference usually matched the measured changes in physiological responses. Some subtle effects related to gender were noted that confirm classic physiological data. Our results indicate that dorsal exposures of humans to a supraresonant frequency of 450 MHz at local peak specific absorption rates up to 7.68 W/kg are mildly thermogenic and are counteracted efficiently by normal thermophysiologic heat loss mechanisms, principally sweating.
Adult↗
Albumin infusion in humans does not model exercise induced hypervolaemia after 24 hours.
We rapidly infused 234 +/- 3 mL of 5% human serum albumin in eight men while measuring haematocrit, haemoglobin concentration, plasma volume (PV), albumin concentration, total protein concentration, osmolality, sodium concentration, renin activity, aldosterone concentration, and atrial natriuretic peptide concentration to test the hypotheses that plasma volume expansion and plasma albumin content expansion will not persist for 24 h. Plasma volume and albumin content were expanded for the first 6 h after infusion (44.3 +/- 1.9-47.2 +/- 2.0 mL kg-1 and 1.9 +/- 0.1-2.1 +/- 0.1 g kg-1 at pre-infusion and 1 h, respectively, P < 0.05), but by 24 h plasma volume and albumin content decreased significantly from 1 h post-infusion and were not different from pre-infusion (44.8 +/- 1.9 mL kg-1 and 1.9 +/- 0.1 g kg-1, respectively). Plasma aldosterone concentration showed a significant effect of time over the 24 h after infusion (P < 0.05), and showed a trend to decrease at 2 h after infusion (167.6 +/- 32.5(-1) 06.2 +/- 13.4 pg mL-1, P = 0.07). These data demonstrate that a 6.8% expansion of plasma volume and 10.5% expansion of plasma albumin content by infusion does not remain in the vascular space for 24 h and suggest a redistribution occurs between the intravascular space and interstitial fluid space.
Adult↗
Recovery after exercise in the heat--factors influencing fluid intake.
The restoration of body fluid balance following dehydration induced by exercise will occur through regulatory responses which stimulate ingestion of water and sodium ions. A number of different afferent signalling systems are necessary to generate appropriate thirst or sodium appetite. The primary sensory information of naturally occurring thirst is derived from receptors sensing cell volume and the volume of the extracellular fluid compartment. Sensory information from the oropharyngeal region is also an important determinant of thirst. The interaction of these various afferent signalling systems within the central nervous system determines the extent of fluid replacement following dehydration.
Afferent Pathways↗
Regulation of blood volume during training in post-menopausal women.
UNLABELLED: In younger people the increase in aerobic capacity following training is related, in part, to blood volume (BV) expansion and the consequent improvements in maximal cardiac output. This training-induced hypervolemia is associated with a decrease in cardiopulmonary baroreflex (CPBR) control of peripheral vascular tone. PURPOSE: To test the hypothesis that improvement in peak oxygen consumption (VO2peak) during training in older women is associated with specific central adaptations, such as BV expansion and a reduction in CPBR control of vascular tone. METHODS: Seventeen healthy older women were randomized into training (N = 9, 71 +/- 2 yr) and control (N = 8, 73 +/- 3 yr) groups. The training group exercised three to four times per wk for 30 min at 60% peak heart rate for 12 wk and then 40-50 min at 75% peak heart rate for 12 wk. The control group participated in yoga exercises over the same time period. We measured resting BV (Evans blue dye), VO2peak, and the forearm vascular resistance response to unloading low pressure mechanoreceptors during low levels of lower body negative pressure (through -20 mm Hg) before and after aerobic training. The slope of the increase in forearm vascular resistance (response) per unit decrease in central venous pressure (stimulus) was used to assess CPBR responsiveness. RESULTS: Aerobic training increased VO2peak 14.2% from 24.2 mL x kg(-1) x min(-1) to 27.7 mL x kg(-1) x min(-1) (P < 0.05), a smaller improvement than typically seen in younger subjects. Blood volume (59.9 +/- 1.9 and 60.9 +/- 1.9 mL x kg[-1]) and CPBR function (-3.98 +/- 0.92 and -3.46 +/- 0.94 units x mm(-1) Hg) were similar before and after training. CONCLUSIONS: These data indicate that the inability to induce adaptations in CPBR function may limit BV expansion during training in older women. In addition, the absence of these specific adaptations may contribute to the relatively poor improvements in VO2peak in older women during short (10-12 wk) periods of training.
Adaptation, Physiological↗
Albumin synthesis after intense intermittent exercise in human subjects.
We measured hepatic albumin synthesis in five volunteers (4 men and 1 woman) at 3 and 6 h after recovery from intense exercise. A primed-constant infusion of a stable isotopic tracer of phenylalanine was used to determine hepatic fractional synthetic rate (FSR) and absolute synthetic rate (ASR) of albumin from the enrichment of phenylalanine in albumin. The infusion of the stable isotope tracer began 2 h after upright exercise or upright rest. Albumin FSR and ASR were 6.39 +/- 0.48%/day and 120 +/- 9 mg.kg body wt-1.day-1, respectively, 3-6 h after recovery from exercise; the FSR and ASR on the time control study day were 5.94 +/- 0.47%/day and 104 +/- 9 mg.kg body wt-1.day-1, respectively. The 6 and 16% increases (P < 0.05) in FSR and ASR after exercise were associated with an elevated plasma albumin content at 5 and 6 h of recovery (P < 0.05), an increased total protein content throughout recovery (P < 0.05), and a negative free water clearance (P < 0.05) at 2, 3, and 6.5 h of recovery compared with baseline values; these variables were unchanged from their baselines on the time control study day. Increased albumin content and reduced free water clearance contribute to a retention of fluid within the circulation after intense exercise. The measured increase in albumin synthesis could not account for the entire increase in albumin content at 6 h of recovery from exercise. However, we estimate that if the increased activity was maintained for the next 18 h, it could account for the expected increase in albumin content at 24 h of recovery.
Adult↗
Effects of posture on cardiovascular responses to lower body positive pressure at rest and during dynamic exercise.
We tested the hypothesis that cardiovascular responses to lower body positive pressure (LBPP) would be dependent on the posture of the subject and also on the background condition (rest or exercise). We measured heart rate (HR), mean arterial blood pressure (MAP), and cardiac stroke volume in eight subjects at rest and during cycle ergometer exercise (76 +/- 3 W) with and without LBPP (25, 50, and 75 mmHg) in the supine and upright positions. At rest, the increase in MAP was proportional to the increase in LBPP and was greater in the supine (6 +/- 2, 15 +/- 3, and 26 +/- 3 mmHg) than in the upright (2 +/- 3, 9 +/- 3, and 17 +/- 3 mmHg) position. During dynamic exercise, the increases in MAP evoked by 25, 50, and 75 mmHg LBPP were greater in the supine (13 +/- 2, 28 +/- 3, and 40 +/- 3 mmHg) than in the upright (7 +/- 3, 12 +/- 3, and 25 +/- 3 mmHg) position. We conclude that the systemic pressure response to LBPP is clearly dependent on the body position, with the larger pressure responses being associated with the supine position both at rest and during dynamic leg exercise.
Adult↗
Assessment of cutaneous blood flow by using topographical perfusion mapping techniques.
The ability of laser Doppler scanning to reproduce the spatial pattern of cutaneous vascular conductance (CVC) in a 6.25-cm2 area of skin was evaluated at supine rest (28 degrees C), during thermal stress (cold and heat), and during baroreceptor unloading with -40-mmHg lower body negative pressure (LBNP). The spatial pattern of resting CVC was similar on 3 different days, varying by 6 +/- 3%. During cold stress, 89 +/- 2% of the skin area showed a decrease in skin blood flow (37 +/- 2%), whereas heat stress increased CVC in 94 +/- 5% of the skin area. During LBNP, the pattern of CVC response was not uniform, and frequency analysis indicated that 47 +/- 5% of the pixels showed a reduction in CVC (>1 SE), 28 +/- 2% of the skin area were unaffected, and the remaining 26 +/- 5% of the pixels showed some increase in CVC. These data indicate the ability of topographical perfusion mapping to provide quantitative and reproducible information about the spatial distribution of CVC. In addition, the site-to-site variability in reflex control of skin blood flow during LBNP is intriguing and requires more rigorous evaluation.
Baroreflex↗
Influence of hydrostatic pressure gradients on regulation of plasma volume after exercise.
The impact of posture on the immediate recovery of intravascular fluid and protein after intense exercise was determined in 14 volunteers. Forces which govern fluid and protein movement in muscle interstitial fluid pressure (PISF), interstitial colloid osmotic pressure (COPi), and plasma colloid osmotic pressure (COPp) were measured before and after exercise in the supine or upright position. During exercise, plasma volume (PV) decreased by 5.7 +/- 0.7 and 7. 0 +/- 0.5 ml/kg body weight in the supine and upright posture, respectively. During recovery, PV returned to its baseline value within 30 min regardless of posture. PV fell below this level by 60 and 120 min in the supine and upright posture, respectively (P < 0. 05). Maintenance of PV in the upright position was associated with a decrease in systolic blood pressure, an increase in COPp (from 25 +/- 1 to 27 +/- 1 mmHg; P < 0.05), and an increase in PISF (from 5 +/- 1 to 6 +/- 2 mmHg), whereas COPi was unchanged. Increased PISF indicates that the hydrostatic pressure gradient favors fluid movement into the vascular space. However, retention of the recaptured fluid in the plasma is promoted only in the upright posture because of increased COPp.
Adult↗
Influence of sodium replacement on fluid ingestion following exercise-induced dehydration.
This study investigated the hypothesis that addition of Na+ to a rehydration beverage would stimulate drinking and augment restoration of body water in individuals dehydrated during 90 min of continuous treadmill exercise in the heat. Following a 3.0 +/- 0.2% decrease in body weight (BW), 6 subjects sat in a thermoneutral environment for 30 min to allow body fluid compartments to stabilize. Over the next 3 hr, subjects rehydrated ad libitum using either flavored/artificially sweetened water (H2O-R) or a flavored, 6% sucrose drink containing either 25 (LNa(+)-R) or 50 (HNa(+)-R) mmol/L NaCl. Results demonstrated that rapid removal of the osmotic stimulus, during H2O-R, and the volume-dependent dipsogenic stimuli, during HNa(+)-R, are important factors in limiting fluid intake during rehydration, compared to LNa(+)-R. It was also found that the pattern of fluid replacement and restoration of fluid balance following dehydration is influenced by the dehydration protocol used to induce the loss in total body water and the sodium content of the rehydration beverage.
Adult↗
Mechanism of attenuated thirst in aging: role of central volume receptors.
To test the hypothesis that the inhibitory action of central blood volume expansion on thirst and renal fluid regulation is attenuated with aging, we monitored the drinking and renal responses of dehydrated older (70 +/- 2 yr, n = 6) and younger (24 +/- 1 yr, n = 6) subjects during 195 min of head-out water immersion (HOI), which shifts blood centrally and increases plasma volume (PV). Subjects dehydrated by exercising for 2 h at 36 degrees C in the evening and refraining from fluids overnight before HOI in 34 degrees C water or a seated control in water perfusion suit [time control (TC)] the next morning. Ad libitum water intake was allowed after 15 min of HOI. Dehydration decreased PV by 10.6 +/- 1 and 7.3 +/- 1.8% (P < 0.05) and increased plasma osmolality by 6 +/- 2 and 7 +/- 1 mosmol/kg H2O (P < 0.05) in older and younger subjects, respectively. Thirst ratings increased in both groups, but pre-HOI thirst perception on a line rating scale was lower in older (69 +/- 8 mm) than younger (94 +/- 6 mm, P < 0.05) subjects. Fifteen minutes of HOI restored PV by 7.8 +/- 1.0 and 5.7 +/- 1.0% in older and younger subjects, respectively, but suppressed thirst rating in younger subjects only (P < 0.05). Fluid intake was reduced in HOI compared with TC in younger (6.3 +/- 0.5 vs. 14.3 +/- 2.2 ml/kg, P < 0.05) but not in older (6.7 +/- 2.1 vs. 8.4 +/- 3.3 ml/kg) subjects. During HOI, older subjects had smaller suppression of plasma renin activity and aldosterone concentration but a greater increase in the plasma atrial natriuretic peptide concentration (P[ANP], P < 0.05). HOI increased fractional sodium excretion in both groups, but mean arterial pressure increased only in the older subjects (P < 0.05). We conclude that the inhibitory influence of central volume expansion on thirst and drinking behavior is diminished with aging. Furthermore, in contrast to younger people, HOI natriuresis is associated with exaggerated increases in P[ANP] and arterial blood pressure in older people, suggesting arterial baroreceptors may be involved in the fluid regulatory response to central blood volume expansion in older people.
Adult↗