Twenty-four-hour non-invasive monitoring of systemic haemodynamics and cerebral blood flow velocity in healthy humans

Acute short-term changes in blood pressure (BP) and cardiac output (CO) affect cerebral blood flow (CBF) in healthy subjects. As yet, however, we do not know how spontaneous fluctuations in BP and CO influence cerebral circulation throughout 24 h. We performed simultaneous monitoring of BP, systemic haemodynamic parameters and blood flow velocity in the middle cerebral artery (MCAV) in seven healthy subjects during a 24-h period. Finger BP was recorded continuously during 24 h by Portapres and bilateral MCAV was measured by transcranial Doppler (TCD) during the first 15 min of every hour. The subjects remained supine during TCD recordings and during the night, otherwise they were seated upright in bed. Stroke volume (SV), CO and total peripheral resistance (TPR) were determined by Modelflow analysis. The 15 min mean value of each parameter was assumed to represent the mean of the corresponding hour. There were no significant differences between right vs. left, nor between mean daytime vs. night time MCAV. Intrasubject comparison of the twenty-four 15-min MCAV recordings showed marked variations (P < 0.001). Within each single 15-min recording period, however, MCAV was stable whereas BP showed significant short-term variations (P < 0.01). A day-night difference in BP was only observed when daytime BP was evaluated from recordings in the seated position (P < 0.02), not in supine recordings. Throughout 24 h, MCAV was associated with SV and CO (P < 0.001), to a lesser extent with mean arterial pressure (MAP; P < 0.005), not with heart rate (HR) or TPR. These results indicate that in healthy subjects MCAV remains stable when measured under constant supine conditions but shows significant variations throughout 24 h because of activity. Moreover, changes in SV and CO, and to a lesser extent BP variations, affect MCAV throughout 24 h.     

Renal and hormonal responses to isotonic saline infusion after 3 days' head‐down tilt vs. supine and seated positions

Aim: The study aimed to determine whether prolonged exposure to simulated microgravity produces a level of thoracic volume receptor loading similar to that seen in the upright position or immediately after lying down. Methods: We used a cross‐over design to compare responses to a saline infusion in eight healthy subjects during a 4‐day, ?6° head‐down tilt (HDT) and in the acute seated and acute supine positions. Results: The first 24 h of HDT were associated with greater urinary excretion of water and sodium (UV, UNaV) than seated and acute supine [cumulative UV, 3035 ± 219, 2311 ± 156 (P < 0.05), and 2448 ± 182 mL (P < 0.05), respectively; cumulative UNaV, 256 ± 19, 180 ± 11 (P < 0.05), and 189 ± 15 mmol (P < 0.05), respectively]. Haemoglobin and haematocrit were increased after 24 h and plasma volume decreased after 48 h of HDT (P < 0.05). With prolongation of HDT, UV and UNaV returned near the baseline values, and plasma atrial natriuretic factor (ANF) and renin values returned to acute seated levels; in acute supine, ANF values were higher and renin lower than in the two other positions. After a 30‐min infusion of 20 mL kg^?1 isotonic saline on the fourth HDT day or during acute seated or acute supine, sodium excretion within 4 h was similar during HDT and acute seated (83 ± 6 and 84 ± 9 mmol, respectively) and greater during supine (104 ± 8 mmol, P < 0.05). The renin decrease was greater in HDT and seated than in supine. The plasma ANF increase was greater during HDT than during supine; during seated, plasma ANF was unchanged. Conclusion: These data suggest that, after 4 days of HDT, thoracic volume receptor loading returns to the same level as in the seated position, leading to blunted responses to volume expansion as compared with the acute supine position.     

Mechanisms of hypotensive effects of a posture change from seated to supine in humans

The hypothesis tested was that the hydrostatic stimulation of carotid baroreceptors is pivotal to decrease mean arterial pressure at heart level during a posture change from seated to supine. In eight males, the cardiovascular responses to a 15‐min posture change from seated to supine were compared with those of water immersion to the xiphoid process and to the neck, respectively. Left atrial diameter and cardiac output (rebreathing) increased similarly during the posture change and water immersion to the xiphoid process and further so during neck immersion. Mean arterial pressure decreased by 12 ± 2 mmHg during the posture change, by 5 ± 1 mmHg during xiphoid immersion, and was unchanged during neck immersion. Arterial pulse pressure increased by 12 ± 3 mmHg during the posture change (P < 0.05) and less during xiphoid and neck immersion by 7 ± 3 mmHg (P < 0.05). Total peripheral vascular resistance decreased similarly during the posture change and neck immersion and slightly less during xiphoid immersion (P < 0.05). In conclusion, the hydrostatic stimulation of carotid baroreceptors combined with some additional increase in arterial pulse pressure, which also stimulates aortic baroreceptors, accounts for more than half of the hypotensive response at heart level to a posture change from seated to supine.     

Objectively measured night‐to‐night sleep variations are associated with body composition in very elderly women

This cross‐sectional study examined the association between objectively measured sleep patterns and body composition in very elderly community‐dwelling women. Participants included 191 community‐dwelling adults aged ≥ 80 years (mean age: 83.4 ± 2.6 years; age range: 80–92 years). Sleep and physical activity were monitored via accelerometer (ActiGraph GT3X+) during at least five consecutive 24‐h periods. Night‐to‐night sleep pattern variability across all nights of recording was assessed using standard deviations (SDs). Body composition was assessed using dual‐energy X‐ray absorptiometry. Simple and multivariable linear regression analyses were performed. The mean number of nights with usable actigraphy data was 7.3 ± 1.3. On average, participants went to bed at 22:57 hours (SD: 1.11 h) and rose from bed at 6:27 hours (SD: 1.01 h). Night‐to‐night bedtime, sleep duration and sleep timing mid‐point variations correlated slightly with the percentage body fat and percentage lean mass (P < 0.05). Multiple linear regression analysis revealed significant associations of night‐to‐night bedtime variations and inconsistent sleep–wake patterns with all body composition indices after adjusting for potential confounding factors, including mean nightly sleep duration, self‐reported nap duration and daily physical activity. After further adjusting for night‐to‐night wake time, sleep timing mid‐point and sleep duration variations, greater bedtime variability remained associated significantly with all body composition indices except lean/fat mass ratio. Inconsistent sleep–wake patterns were associated independently with an increased fat mass and decreased lean mass among very elderly women. These findings suggest that in most elderly individuals, sleep patterns might be an important modifiable factor associated with obesity and sarcopenia development.     

Difference in human cardiovascular response between upright and supine recovery from upright cycle exercise

Cardiovascular responses were examined in seven healthy male subjects during 10 min of recovery in the upright or supine position following 5 min of upright cycle exercise at 80% peak oxygen uptake. An initial rapid decrease in heart rate (f _c) during the early phase of recovery followed by much slower decrease was observed for both the upright and supine positions. The average f _c at the 10th min of recovery was significantly lower (P < 0.05) in the supine position than in the upright position, while they were both significantly greater than the corresponding pre-exercise levels (each P < 0.05). Accordingly, the amplitude of the high frequency (HF) component of R-R interval variability (by spectrum analysis) in both positions was reduced with a decrease in mean R-R interval, the relationship being expressed by a regression line – mean R-R interval = 0.006 × HF amplitude + 0.570 (r = 0.905, n = 28, P < 0.001). These results would suggest that the slower reduction in f _c following the initial rapid reduction in both positions is partly attributable to a retardation in the restoration of the activity of the cardiac parasympathetic nervous system. Post-exercise upright stroke volume (SV, by impedance cardiography) decreased gradually to just below the pre-exercise level, whereas post-exercise supine SV increased markedly to a level similar to that at rest before exercise. The resultant cardiac output (Q˙ _c) and the total peripheral vascular resistance (TPR) in the upright and supine positions returned gradually to their respective pre-exercise levels in the corresponding positions. At the 10th min of recovery, both average SV and Q˙ _c were significantly greater (each P < 0.005) in the supine than in the upright position, while average TPR was significantly lower (P < 0.05) in the supine than in the upright position. In contrast, immediately after exercise, mean blood pressure dropped markedly in both the supine and upright positions, and their levels at the 10th min of recovery were similar. Therefore we concluded that arterial blood pressure is maintained relatively constant through various compensatory mechanisms associated with f _c, SV, Q˙ _c, and TPR during rest and recovery in different body positions. Accepted: 4 September 1999     

Achieving haemodynamic baseline values with Finapres in elderly subjects during supine rest.

BACKGROUND: Clear guidelines for the resting time necessary to achieve stable blood pressure (BP) levels are scant in gerontology research. Therefore, we aimed to determine the minimum period required for obtaining haemodynamic baseline values in elderly subjects during supine rest. In addition, we evaluated the effect of cardiovascular morbidity, such as diastolic heart failure, and the effect of complex comorbidity of geriatric patients, on haemodynamic changes during supine rest. METHODS: A total of 17 healthy subjects, 18 heart failure patients with normal systolic function and 24 geriatric patients, aged 70 years and more, participated. After an overnight fast, changes in systolic BP (SBP), diastolic BP (DBP), heart rate (HR), and stroke volume (SV) were determined by Finapres beat-to-beat non-invasive BP monitoring during a 20-min supine rest. The procedure was repeated in the healthy subjects and geriatric patients on a second day. RESULTS: Complete BP stabilization was reached in each group within 5 min of supine rest, as SBP remained essentially unchanged and DBP did not change significantly anymore after the fourth minute. In the heart failure patients, HR decreased and SV increased until the twelfth minute of rest. The SBP, DBP, HR, and SV changes during supine rest showed good reproducibility. CONCLUSIONS: A span of 5 min of supine rest ensured achievement of reliable and reproducible baseline BP values by Finapres in elderly subjects. However, we recommend at least 12 min of rest to obtain full haemodynamic stability in elderly patients with diminished cardiac compliance and diastolic function.     

The nocturnal secretion of cardiac natriuretic peptides during obstructive sleep apnoea and its response to therapy with nasal continuous positive airway pressure

The nocturnal secretion profile of the newly identified natriuretic peptide (NP), brain natriuretic peptide (BNP), was studied in 14 patients with obstructive sleep apnoea syndrome (OSAS) (apnoea hypopnoea index: 60.5±3.4, mean±SE) during two separate nights before and during nasal continuous positive airway pressure (NCPAP) therapy. Plasma levels of NPs (atrial natriuretic peptides; ANP and BNP) were measured at 2-h intervals during sleep. Simultaneously, blood pressure was measured by a non-invasive method (Finapres^®, Ohmeda, Englewood, CO, USA) and urine was collected for determing volume and catecholamine levels. Urinary and serum sodium concentration were determined before and after the study. Eight non-snoring subjects were also studied for the investigation of normal nocturnal profiles of BNP levels. To understand the discrete secretion profiles of the two NPs during sleep, blood was sampled from an additional seven patients every 5 min over a 30-min period around 00.00 and 04.00 hours before NCPAP. In patients with OSAS, plasma BNP levels increased from the beginning of sleep (22:00 h) to the morning (06:00 h) before NCPAP therapy (P< 0.01, anova ). Baseline BNP levels were not significantly correlated with patient's clinical and poly- somnographic parameters. However, in the latter half of the sleep period (02:00–06:00 h), increases in BNP levels during the night before NCPAP therapy were significantly correlated with blood pressure elevations (systolic: r=0.784 P< 0.01, diastolic: r=0.587 P< 0.01) and with apnoea duration (r=0.582 P< 0.01). In normal subjects BP and BNP levels were not changed significantly during sleep. Plasma BNP levels were well correlated with concomitant ANP levels (P< 0.001). NCPAP therapy reduced ANP and BNP levels during sleep and in the morning (P< 0.01). Plasma levels of BNP at 5 min intervals before NCPAP therapy revealed few variations. On the other hand, ANP levels fluctuated over the 30-min period. Changes in BNP levels during sleep in the patients with OSAS may be related to blood pressure variations, but may be too small to play a significant physiological role in regulating diuresis in OSAS. Further work is required to determine the precise role of dual natriuretic system in cardiovascular load and natriuresis in OSAS.     

Increasing sleep duration to lower beat‐to‐beat blood pressure: a pilot study

Strong evidence has accumulated over the last several years, showing that low sleep quantity and/or quality plays an important role in the elevation of blood pressure. We hypothesized that increasing sleep duration serves as an effective behavioral strategy to reduce blood pressure in prehypertension or type 1 hypertension. Twenty‐two participants with prehypertension or stage 1 hypertension, and habitual sleep durations of 7 h or less, participated in a 6‐week intervention study. Subjects were randomized to a sleep extension group (48 ± 12 years, N = 13) aiming to increase bedtime by 1 h daily over a 6‐week intervention period, or to a sleep maintenance group (47 ± 12 years, N = 9) aiming to maintain habitual bedtimes. Both groups received sleep hygiene instructions. Beat‐to‐beat blood pressure was monitored over 24 h, and 24‐h urine and a fasting blood sample were collected pre‐ and post‐intervention. Subjects in the sleep extension group increased their actigraphy‐assessed daily sleep duration by 35 ± 9 min, while subjects in the sleep maintenance condition increased slightly by 4 ± 9 min (P = 0.03 for group effect). Systolic and diastolic beat‐to‐beat blood pressure averaged across the 24‐h recording period significantly decreased from pre‐ to post‐intervention visit in the sleep extension group by 14 ± 3 and 8 ± 3 mmHg, respectively (P < 0.05). Though the reduction of 7 ± 5 and 3 ± 4 mmHg in the sleep maintenance group was not significant, it did not differ from the blood pressure reduction in the sleep extension group (P = 0.15 for interaction effect). These changes were not paralleled by pre‐ to post‐intervention changes in inflammatory or sympatho‐adrenal markers, nor by changes in caloric intake. While these preliminary findings have to be interpreted with caution due to the small sample size, they encourage future investigations to test whether behavioral interventions designed to increase sleep duration serve as an effective strategy in the treatment of hypertension.     

Effects of indomethacin on cerebrovascular response to hypercapnea and hypocapnea in breath-hold diving and obstructive sleep apnea

We tested whether breath hold divers (BHD) and obstructive sleep apnea (OSA) subjects had similar middle cerebral artery velocity (MCAV) responses to hypercapnea and hypocapnea. We analyzed changes in MCAV (cm/s) in response to hypocapnea and hyperoxic hypercapnea during placebo or after 90 min of oral indomethacin (100 mg) in BHD (N = 7) and OSA (N = 7). During control hypercapnea MCAV increased for 54.4% in BHD and 48.4% in OSA. Indomethacin blunted the MCAV increase in response to hypercapnea in BHD (P = 0.02), but not in OSA. Indomethacin attenuated the mean arterial pressure response in BHD, but not in OSA. The blunted MCAV responses to hypercapnea with indomethacin in BHD, but not in OSA patients suggests that (a) the normal contribution of local vasodilating mechanisms to the cerebrovascular responses to hypercapnea is absent in OSA patients and (b) exposure to chronic/repeated apneas is not causal per se in limiting the contribution of vasodilating mechanisms to the cerebrovascular responses to hypercapnea in OSA.     

Acute effects of exercise posture on executive function in transient ischemic attack patients

In patients with stroke or transient ischemic attacks (TIA), a decline in executive function may limit an individual's ability to process motor tasks and relearn motor skills. The purpose of this study was to assess the acute effect of exercise posture (seated vs. supine cycle ergometry) on executive function and prefrontal cortex perfusion in patients with TIA. Eleven TIA patients (65 ± 10 years) and 15 age‐matched, healthy controls (HC; 62 ± 7 years) completed two exercise tests to maximal capacity (one seated, one supine) and two 30‐min submaximal exercise tests (one seated, one supine). Executive function was assessed prior to and following (1.5 min post, 15 min post) the submaximal exercise tests using a Stroop task. Prefrontal cortex perfusion (total hemoglobin) was continuously recorded using near‐infrared spectroscopy. There was no Posture (seated, supine) × Group (TIA, HC) interaction for the Stroop task (p > .05). HC completed Stroop tasks significantly faster than TIA (51.9[SD = 10.3] vs. 64.2[8.5] s, respectively), while Stroop completion time significantly improved between baseline and 1.5 min post (61.3[10] vs. 58.1[9.4] s, respectively) and 1.5 min post and 15 min post (54.8[8.9] s). Posture and group had no significant influence on prefrontal cortex perfusion (p > .05). In summary, executive function improves to a similar extent in TIA and age‐matched, healthy controls following an acute bout of exercise, regardless of exercise posture. As acute improvements in executive function were maintained for 15 min, there could be an important window of opportunity for assigning executive tasks following exercise rehabilitation for patients with TIA.     

Redistribution of body fluids during postural manipulations

Inter-compartmental body-fluid distribution is contingent upon posture, exercise state and environmental temperature. This investigation aimed at quantifying the distribution of intra- and extravascular fluid volumes during postural manipulations. Fluid shifts were measured in eight males utilizing a simultaneous, radionuclide dilution technique, in which radioiodinated serum fibrinogen, radiochromated crythrocytes, radiobromine and tritiated water were used to measure plasma, red cell, extracellular and total body water volumes. Subjects were exposed to three postural changes [seated (control), supine and standing] for 30 min at an air temperature of 22.0 d?C, with each posture separated by 30 min seated rest. Total body water content remained stable throughout postural changes (P= 0.842). Relative to seated volumes, BV increased by 89 mL when supine, and decreased by 406 mL while standing (P= 0.003), with such shifts being primarily a result of plasma movement (P= 0.011). Red cell volume changes were not significant. Vascular fluid lost during standing was filtered into the interstitial compartment (P= 0.014), with the extracellular and intracellular volumes remaining unaffected (P= 0.271 and P= 0.800, respectively). These observations confirmed the influence of posture on inter-compartmental body-fluid distribution. The intravascular fluid loss when standing was caused by the filtration of plasma into the interstitium, while, during supine rest, intravascular volume increased, reflecting fluid flux from the interstitium to the circulation.     

Comparison of rhythmic masticatory muscle activity during non‐rapid eye movement sleep in guinea pigs and humans

Rhythmic masticatory muscle activity can be a normal variant of oromotor activity, which can be exaggerated in patients with sleep bruxism. However, few studies have tested the possibility in naturally sleeping animals to study the neurophysiological mechanisms of rhythmic masticatory muscle activity. This study aimed to investigate the similarity of cortical, cardiac and electromyographic manifestations of rhythmic masticatory muscle activity occurring during non‐rapid eye movement sleep between guinea pigs and human subjects. Polysomnographic recordings were made in 30 freely moving guinea pigs and in eight healthy human subjects. Burst cycle length, duration and activity of rhythmic masticatory muscle activity were compared with those for chewing. The time between R‐waves in the electrocardiogram (RR interval) and electroencephalogram power spectrum were calculated to assess time‐course changes in cardiac and cortical activities in relation to rhythmic masticatory muscle activity. In animals, in comparison with chewing, rhythmic masticatory muscle activity had a lower burst activity, longer burst duration and longer cycle length (P < 0.05), and greater variabilities were observed (P < 0.05). Rhythmic masticatory muscle activity occurring during non‐rapid eye movement sleep [median (interquartile range): 5.2 (2.6–8.9) times per h] was preceded by a transient decrease in RR intervals, and was accompanied by a transient decrease in delta elelctroencephalogram power. In humans, masseter bursts of rhythmic masticatory muscle activity were characterized by a lower activity, longer duration and longer cycle length than those of chewing (P < 0.05). Rhythmic masticatory muscle activity during non‐rapid eye movement sleep [1.4 (1.18–2.11) times per h] was preceded by a transient decrease in RR intervals and an increase in cortical activity. Rhythmic masticatory muscle activity in animals had common physiological components representing transient arousal‐related rhythmic jaw motor activation in comparison to human subjects.     

Respiratory muscle training increases cycling endurance without affecting cardiovascular responses to exercise

We tested whether the increased cycling endurance observed after respiratory muscle training (RMT) in healthy sedentary humans was associated with a training-induced increase in cardiac stroke volume (SV) during exercise, similar to the known effect of endurance training. Thirteen subjects underwent RMT by normocapnic hyperpnea, nine underwent aerobic endurance training (cycling and/or running) and fifteen served as non-training controls. Training comprised 40 sessions performed within 15 weeks, where each session lasted 30 min. RMT increased cycling endurance at 70% maximal aerobic power ( ) by 24% [mean (SD) 35.6 (11.9) min vs 44.2 (17.6) min, P<0.05], but SV at 60% was unchanged [94 (21) ml vs 93 (20) ml]. Aerobic endurance training increased both SV [89 (24) ml vs 104 (32) ml, P<0.01] and cycling endurance [37.4 (12.8) min vs 52.6 (16.9) min, P<0.01]. In the control group, no changes were observed in any of these variables. It is concluded that the increased cycling endurance that is observed after RMT is not due to cardiovascular adaptations, and that the results provide evidence for the role of the respiratory system as an exercise-limitingfactor. Electronic Publication     

Quantitative agreement between [15O]H2O PET and model free QUASAR MRI‐derived cerebral blood flow and arterial blood volume

The purpose of this study was to assess whether there was an agreement between quantitative cerebral blood flow (CBF) and arterial cerebral blood volume (CBVA) measurements by [¹⁵O]H₂O positron emission tomography (PET) and model‐free QUASAR MRI. Twelve healthy subjects were scanned within a week in separate MRI and PET imaging sessions, after which quantitative and qualitative agreement between both modalities was assessed for gray matter, white matter and whole brain region of interests (ROI). The correlation between CBF measurements obtained with both modalities was moderate to high (r²: 0.28–0.60, P < 0.05), although QUASAR significantly underestimated CBF by 30% (P < 0.001). CBV_A was moderately correlated (r²: 0.28–0.43, P < 0.05), with QUASAR yielding values that were only 27% of the [¹⁵O]H₂O‐derived values (P < 0.001). Group‐wise voxel statistics identified minor areas with significant contrast differences between [¹⁵O]H₂O PET and QUASAR MRI, indicating similar qualitative CBV_A and CBF information by both modalities. In conclusion, the results of this study demonstrate that QUASAR MRI and [¹⁵O]H₂O PET provide similar CBF and CBV_A information, but with systematic quantitative discrepancies. Copyright © 2016 John Wiley & Sons, Ltd.     

Comparison of a three‐quarter electrode band configuration with a full electrode band configuration for impedance cardiography

Impedance cardiography is a technique commonly used in psychophysiological studies. However, concerns about the utility of full circumferential band electrodes (FB) have been raised. The current study was designed to compare FB with a three‐quarter circumferential band configuration (PB). A total of 47 participants (66% female, mean [SD] age=20.4 [3.0] years) underwent 2 testing sessions, once using FB and once using PB. Session order was randomized and balanced. Each session consisted of 5 min of rest, math task, recovery, and cold pressor test. Average baseline and task pre‐ejection period (PEP), stroke volume (SV), cardiac output (CO), heart rate (HR), blood pressure (BP), and total peripheral resistance (TPR) was calculated from impedance cardiography and blood pressure monitoring. Participants were are asked to rate measures of comfort after each session. There were no significant difference between the mean levels of PEP, SV, CO, HR, and TPR for the PB versus the FB configurations. However, both systolic BP and diastolic BP were higher during the FB session. Intraclass correlations were high (r_icc=.63–.93) between PB and FB. Bland‐Altman analyses revealed a low level of bias (≤5%) between the configurations. Based on limits of agreement between ±30%, there was equivalence in PEP between the 2 configurations, and SV, CO, and TPR were close to reaching equivalence. Participants clearly indicated greater comfort with the PB configuration compared to the FB. The current study provides incremental evidence that suggests a three‐quarter PB configuration may be utilized for standard psychophysiological testing instead of the standard FB configuration. However, further studies are needed to validate the PB configuration against other techniques.     

Role of adenosine in exercise‐induced human skeletal muscle vasodilatation

The role of adenosine in exercise‐induced human skeletal muscle vasodilatation remains unknown. We therefore evaluated the effect of theophylline‐induced adenosine receptor blockade in six subjects and the vasodilator potency of adenosine infused in the femoral artery of seven subjects. During one‐legged, knee‐extensor exercise at ～48% of peak power output, intravenous (i.v.) theophylline decreased (P < 0.003) femoral artery blood flow (F_aBF) by ～20%, i.e. from 3.6 ± 0.5 to 2.9 ± 0.5 L min^?1, and leg vascular conductance (VC) from 33.4 ± 9.1 to 27.7 ± 8.5 mL min^?1 mmHg^?1, whereas heart rate (HR), mean arterial pressure (MAP), leg oxygen uptake and lactate release remained unaltered (P = n.s.). Bolus injections of adenosine (2.5 mg) at rest rapidly increased (P < 0.05) F_aBF from 0.3 ± 0.03 L min^?1 to a 15‐fold peak elevation (P < 0.05) at 4.1 ± 0.5 L min^?1. Continuous infusion of adenosine at rest and during one‐legged exercise at ～62% of peak power output increased (P < 0.05) F_aBF dose‐dependently to level off (P = ns) at 8.3 ± 1.0 and 8.2 ± 1.4 L min^?1, respectively. One‐legged exercise alone increased (P < 0.05) F_aBF to 4.7 ± 1.7 L min^?1. Leg oxygen uptake was unaltered (P = n.s.) with adenosine infusion during both rest and exercise. The present findings demonstrate that endogenous adenosine controls at least ～20% of the hyperaemic response to submaximal exercise in skeletal muscle of humans. The results also clearly show that arterial infusion of exogenous adenosine has the potential to evoke a vasodilator response that mimics the increase in blood flow observed in response to exercise.     

Effect of posture on high-intensity constant-load cycling performance in men and women

The time sustained during a graded cycle exercise is ~10% longer in an upright compared with a supine posture. However, during constant-load cycling this effect is unknown. Therefore, we tested the postural effect on the performance of high-intensity constant-load cycling. Twenty-two active subjects (11 men, 11 women) performed two graded tests (one upright, one supine), and of those 22, 10 subjects (5 men, 5 women) performed three high-intensity constant-load tests (one upright, two supine). To test the postural effect on performance at the same absolute intensity, during the upright and one of the supine constant-load tests subjects cycled at 80% of the peak power output achieved during the upright graded test. To test the postural effect on performance at the same relative intensities, during the second supine test subjects cycled at 80% of the peak power output achieved during the supine graded test. Exercise time on the graded and absolute intensity constant-load tests for all subjects was greater (P<0.05) in the upright compared with supine posture (17.9±3.5 vs. 16.1±3.1 min for graded; 13.2±8.7 vs. 5.2±1.9 min for constant-load). This postural effect at the same absolute intensity was larger in men (19.4±8.5 upright vs. 6.6±1.6 supine, P<0.001) than women (7.1±2 upright vs. 3.9±1.4 supine, P>0.05) and it was correlated (P<0.05) with both the difference in between positions during the first minute of exercise (r=0.67) and the height of the subjects (r=0.72). In conclusion, there is a very large postural effect on performance during constant-load cycling exercise and this effect is significantly larger in men than women.     

Intra- and extracranial artery blood velocity during a sudden blood pressure decrease in humans

The intra- and extracerebral Doppler artery blood velocity responses to a 10-mmHg abrupt blood pressure (BP) decrease in ten healthy men were studied. This decrease was obtained using two cuffs placed over both thighs. First, cuffs were inflated to pressures greater than the arterial BP for 5 min. Next, they were deflated to 60 mmHg in order to prevent venous return from the legs. We obtained a decrease in mean arterial BP of from 101 (10) to 90 (10) mmHg [mean (SD), P < 0.01] without modifications in the heart rate [HR, 88 (14) beats min⁻¹]. Middle cerebral artery mean blood velocity (MCAmv) decreased immediately from 50 (10) to 42 (12) cm s⁻¹ (P < 0.05). Simultaneously, temporal superficial artery mean blood velocity (TSAmv) decreased from 11 (3) to 7 (2) cm s⁻¹ (P < 0.05) and common carotid artery blood flow (CCAbf ) decreased from 305 (23) to 233 (33) ml min⁻¹ (P < 0.05). After 5 s, MCAmv and CCAbf returned to baseline values, whereas TSAmv [8 (2) cm s⁻¹], mean arterial BP [86 (10) mmHg] remained low and HR increased [92 (12) beats min⁻¹]. TSAmv, BP and HR returned to baseline values in 1 min. These data confirm that cerebral blood flow (CBF) is very rapidly regulated but that blood flow in extracranial territories is not and that it follows the arterial BP changes. Accepted: 8 April 1997     

The behaviour of the flow‐mediated brachial artery vasodilatation during orthostatic stress in normal man

Aims: Flow‐mediated brachial artery vasodilatation is an index of endothelial function. Published literature describes only supine data and no study has been performed during vertical displacement. This subject deserves investigation for two main reasons: humans spend the larger part of their life in the upright position; this position has significant effects on neural vascular regulation. Methods: In 21 healthy men (25 ± 2 years) the flow‐dependent brachial artery vasodilating response to distal circulatory arrest was assessed by Doppler ultrasound imaging, while supine and during 20° and 60° head‐up tilting (HUT). In 11 of these subjects the vasodilating response to nitroglycerine was also explored. Results: Absolute and percentage increments in brachial calibre during hyperaemia after deflation of the occluding cuff became increasingly greater at 20° (+0.44 mm) and 60° (+0.92 mm) HUT (P < 0.01), compared with the horizontal position (+0.27 mm), and the arterial dilatation for an increase in flow (0.98 ± 0.08 and 1.68 ± 0.06 mm mL^?1 min^?1 × 1000, respectively) was larger (P < 0.01) than occurred while supine (0.41 ± 0.05 mm mL^?1 min^?1 × 1000). Nitroglycerine‐mediated vasodilatation at 60° HUT was similar to that in the supine position. Conclusion: The orthostatic stimulus is associated with an increase of the flow‐mediated brachial artery vasodilatation, which is proportional to the degree of displacement. The mechanism of this effect does not consist of changes in nitric oxide sensitivity.