Reproducibility of the alterations in circulation and cerebral oxygenation from supine rest to head-up tilt.

Cardiovascular stability, as affected by several diseases, may be assessed by head-up tilt testing. Follow-up studies are essential in both evaluating interventions and assessing progression. However, data on the reproducibility of the changes in circulatory status and cerebral oxygenation provoked by head-up tilt testing are fundamental to follow-up studies. The aim of this study was, therefore, to assess the reproducibility of the alterations in stroke volume (SV), mean arterial pressure (MAP), as well as oxygenated ([O2Hb]) and deoxygenated haemoglobin ([HHb]) concentration in cerebral tissue from supine rest (SUP) to head-up tilt (HUT). SV was calculated by Modelflow, a pulse contour method, from the finger arterial pressure wave measured by Portapres, the portable version of Finapres. [O2Hb] and [HHb] were measured using near-infrared spectroscopy (NIRS). Ten healthy individuals visited the laboratory on two different days. On both days, they underwent 10 min SUP followed by 10 min 70 degrees HUT twice. SV decreased, which was (in part) compensated for by an increased heart rate, while MAP increased slightly during HUT compared with SUP. Although [HHb] increased during HUT, no presyncope symptoms were experienced. The circulatory variables (SV, HR and MAP) as well as [HHb] showed an acceptably small systematic and random error as well as reproducibility error compared with the observed difference between HUT and SUP and were similar between and within visits. Therefore, it is concluded that MAP measured by Portapres and SV calculated by Modelflow as well as HHb measured by NIRS seem to be reproducible and may therefore be used in follow-up studies.     

Effects of triglycyl-lysine-vasopressin on cardiovascular responses to orthostatic stress

The influence of triglycyl-lysine-vasopressin (TGLVP) on cardiovascular responses to orthostatic stress was studied. Arterial pressures, heart rate (HR) and stroke volume (SV) were measured in eight healthy males subjected to 20 min 70 degrees head-up tilt. On different days they received either 0.01 mg/kg b.w. of TGLVP or a corresponding volume of 0.9% saline i.v. after 15 min supine rest. After the drug injection, in supine subjects, HR had decreased from 58 to 50 beats min-1, total peripheral resistance (TPR) was elevated by 29%, systolic (SAP) and diastolic pressure (DAP) had increased by 7 and 8 mmHg, respectively. During tilt, values for HR and SAP were similar with and without TGLVP whereas DAP and MAP were elevated 8 and 7 mmHg, respectively, by the drug. 4-8 min into the tilt, TGLVP caused an 8% sustained curtailment of SV. Both with and without the drug TPR increased by about 30% in response to head-up tilt. Thus, the marked peripheral arteriolar constriction after vasopressin in the supine position was not affected by head-up tilt. Tilting also abolished the drug-induced elevation in SAP, most likely explained by the reduction in SV. Although TPR was markedly increased by TGLVP during head-up tilt, reflected in the behaviour of DAP, the response of SV speaks against any beneficial effect of this drug on orthostatic tolerance in healthy subjects.     

Thermal stress modulates arterial pressure variability and arterial baroreflex response of heart rate during head-up tilt in humans

To examine the effects of thermal stress on the blood pressure variability and the arterial baroreceptor-cardiac reflex during orthostatic stress, 11 male volunteers underwent whole body thermal stress using a cool or hot water-perfused suit during 5 min of 70° head-up tilt (HUT). The spontaneous variability in arterial pressure was quantified by power spectrum analysis. The sensitivity of the arterial baroreceptor-cardiac reflex was calculated from the spontaneous changes in beat-to-beat arterial pressure and heart rate (f _c). During supine rest the variability of arterial pressure decreased during cooling, while it remained unchanged during heating. The variability increased with HUT; it was greater (P<0.05) with heating than with cooling. In the supine condition, the arterial baroreflex sensitivity of f _c increased during cooling, while it did not change during heating. The sensitivities decreased (P<0.05) with HUT during both thermal stresses; the decreased rate of sensitivity from the pre-tilt value was greater during heating [mean 63 (SEM 4)%] and smaller during cooling [mean 11 (SEM 24)%] than during normothermia [mean 47 (SEM 4)%] (both, P<0.05). There were significant negative correlations between the sensitivities and the amplitude of the arterial pressure variability during normothermia and heating (P<0.0001). The results suggest that the spontaneous baroreflex response of f _c is a modulatory factor for the changes of arterial pressure variability brought about by thermal stress during orthostatic stress. Electronic Publication     

Carotid-cardiac baroreflex function does not influence blood pressure regulation during head-up tilt in humans

The influence of the carotid-cardiac baroreflex on blood pressure regulation was evaluated during supine rest and 40 degrees head-up tilt (HUT) in 9 healthy young subjects with and without full cardiac vagal blockade. The carotid baroreflex responsiveness, or maximal gain (G(MAX)), was assessed from the beat-to-beat changes in heart rate (HR) and mean arterial pressure (MAP) by the variable neck pressure and suction technique ranging in pressure from +40 to -80 Torr, with and without glycopyrrolate (12.0 +/- 1.0 microg/kg body weight; mean +/- SE). In the supine position, glycopyrrolate increased the HR to 91 +/- 3 bpm, from 54 +/- 3; MAP to 89 +/- 2 mmHg, from 76 +/- 2; and cardiac output to 6.8 +/- 0.3 l.min(-1), from 4.9 +/- 0.3 (P < 0.05). The G(MAX) of the carotid baroreflex control of HR was reduced to -0.06 +/- 0.01 bpm.mmHg(-1), from -0.30 +/- 0.02 (P < 0.05) with no significant effect on the G(MAX) of the carotid baroreflex control of MAP. During HUT the carotid baroreflex control of MAP was unchanged, though the G(MAX) of the carotid baroreflex control of HR was increased (P < 0.05). During HUT, central blood volume, assessed by electrical thoracic admittance, and total vascular conductance were decreased with and without glycopyrrolate. Furthermore, glycopyrrolate reduced G(MAX) of the carotid baroreflex control of HR during HUT (P < 0.05) with no significant effect on G(MAX) of the carotid baroreflex control of MAP. These data suggest that during supine rest and HUT-induced decreases in central blood volume, the carotid baroreflex control of HR is mediated primarily via parasympathetic activity. Furthermore, the maintenance of arterial blood pressure during postural stress is primarily mediated by arterial and cardiopulmonary reflex regulation of sympathetic activity and its effects on the systemic vasculature.     

Reduced orthostatic tolerance following 4 h head-down tilt

Summary The cardiovascular responses to a 10-min 1.22 rad (70°) head-up tilt orthostatic tolerance test (OST) was observed in eight healthy men following each of a 5-min supine baseline (control), 4 h of 0.1 rad (6°) head-down tilt (HDT), or 4 h 0.52 rad (30°) headup tilt (HUT). An important clinical observation was presyncopal symptoms in six of eight subjects following 4 h HDT, but in no subjects following 4 h HUT. Immediately prior to the OST, there were no differences in heart rate, stroke volume, cardiac output, mean arterial pressure and total peripheral resistance for HDT and HUT. However, stroke volume and cardiac output were greater for the control group. Mean arterial pressure for the control group was less than HDT but not HUT. Over the full 10-min period of OST, the mean arterial pressure was not different between groups. Heart rate increased to the same level for all three treatments. Stroke volume decreased across the full time period for control and HDT, but only at 3 and 9 min for HUT. There was a higher total peripheral resistance in the HDT group than control or HUT. The pre-ejection period to left ventricular ejection time ratio was less in HDT than for control or HUT groups. These data indicate a rapid adaptation of the cardiovascular system to 4 h HDT that appears to be inappropriate on reapplication of a head to foot gravity vector. We speculate that the cause of the impaired orthostatic tolerance is decreased tone in venous capacitance vessels so that venous return is inadequate.     

Prone sleeping impairs circulatory control during sleep in healthy term infants: implications for SIDS

STUDY OBJECTIVES: To determine the effects of sleeping position on development of circulatory control in infants over the first 6 months of postnatal age (PNA). DESIGN: Effects of sleeping position, sleep state and PNA on beat-beat heart rate (HR) and mean arterial pressure (MAP) responses to a head-up tilt (HUT) were assessed during sleep in infants at 2-4 wks, 2-3 mo and 5-6 mo PNA. MEASUREMENTS: Daytime polysomnography was performed on 20 full-term infants (12 F/8 M) and MAP was recorded continuously and noninvasively (Finometer). HUTs of 15 degrees were performed during active sleep (AS) and quiet sleep (QS) in both the prone and supine sleeping positions. MAP and HR data were expressed as the percentage change from baseline, and responses were divided into initial, middle and late phases. RESULTS: In the supine position HUT usually resulted in an initial increase (P < 0.05) in HR and MAP, followed by decreases (P < 0.05) in HR and MAP in the middle phase; subsequently HR and MAP returned to baseline in the late phase. By contrast, in the prone position the initial HUT-induced rises in HR and MAP were usually absent, and at 2-3 mo MAP actually decreased (P < 0.05); subsequently HR but not MAP returned to baseline. At 2-3 mo, MAP was lower (P < 0.05) in prone than supine sleeping throughout the HUT. CONCLUSIONS: Prone sleeping alters MAP responses to a HUT during QS at 2-3 mo PNA. Decreased autonomic responsiveness may contribute to the increased risk for SIDS of infants sleeping in the prone position.     

Stroke volume and systolic time intervals: Beat-to-beat comparison between echocardiography and ambulatory impedance cardiography in supine and tilted positions

The aim of the study was to compare stroke volume (SV), ejection time (ET) and pre-ejection period (PEP) measurements obtained using a central haemodynamics ambulatory monitoring device based on impedance cardiography (ICG), in supine and tilted positions (60°), with pulsed Doppler echocardiography as a noninvasive reference method. The Holter-type ICG device was used for off-line, beat-to-beat, automatic determination of SV, ET and PEP. ICG data were compared with those obtained simultaneously using pulsed Doppler echocardiography in the ascending aorta from a suprasternal projection, 1 min before and 10 min after tilting. The tests were performed in 13 young, healthy subjects (six men and seven women, aged 23–33 years). Linear regression between the measured values obtained for all subjects was described by the following formulas: SV_icg=13.9+0.813*SV_echo (r=0.857 SEE=9.03, n=496), ET_icg=16.8+0.987*ET_echo (r=0.841, SEE=21.3, n=496), PEP_icg=22.8+0/890*PEP_echo (r=0.727, SEE=14.6, n=496). The data showed that ambulatory impedance cardiography gives useful absolute values of SV and systolic time intervals measured in supine and tilted positions.     

直立倾斜引起的心率和血压的耦合性变化分析

从动态和稳态两个视角,研究直立倾斜(HUT)引起体位改变前后以及不同速度改变体位过程中RR间期(RRI)与收缩压(SBP)间耦合性的变化。所用数据来自PhsioNet发布的体位变化所引起的生理响应数据库(PRCP),含有10位健康受试者(5男5女)在HUT过程中记录的连续心电和动脉血压信号。慢速体位变化(ST)和快速体位(RT)变化分别为在50和2 s之间从水平仰卧升至75°倾斜。提取逐拍RRI和SBP数据后,运用交叉时频分析和信息分解方法,结合时域和短时分形指数(α₁),进行RRI和SBP时间序列的联合分析。信息分解分析结果表明,所有的显著差异集中在压力反射导致心率变化的后向反馈回路(SBP→RRI),ST后心率的可预测性较平卧时显著增高(0.416±0.067 vs 0.626±0.127),压力反射支路的SBP-RRI耦合性升高。而在RRI→SBP方向,HUT对其几乎没有影响。ST和RT之前,所有的同类指标相比均无显著差异。ST和RT之后的稳态,虽然RRI无显著差异,但较之ST之后,RT之后RRI的变异系数显著升高(0.054±0.014 vs 0.074±0.027),α₁显著降低(1.45±0.25 vs 1.28±0.27)。同时,交叉时频分析结果揭示了ST和RT过程中自主神经不同的动态反应行为。研究证明了信息分解方法的有效性,可明确区分心率与血压相互作用时的前向反馈和后向反馈的主导因果方向,而且可反映HUT前后信号可预测性的变化。     

Effect of hydration on some orthostatic and haematological responses to head-up tilt

Summary Experiments were undertaken to determine the effects of hydration status on a) orthostatic responses, and on b), relative changes in intravascular volume and protein content, during 70 head-up tilt (HUT). Six men underwent 45 min of HUT, preceded by 45 min supine, first dehydrated, and again 105 min later after rehydration with water. Heart rate was consistently lower following rehydration (p<0.01), while supine diastolic pressure was higher (p<0.02). Systolic pressure fell during dehydrated HUT (p<0.01), but not during rehydrated HUT. Postural haemoconcentration, which was reduced after rehydration (p<0.001), was accompanied by a decrease in intravascular albumin content (p<0.05). Two subjects experienced severe presyncopal symptoms during dehydrated HUT, but not during rehydrated HUT. Thus, it appears that rehydration after fluid restriction improves orthostatic tolerance. Furthermore, extravascular hydration status may be more important than intravascular hydration status in determining orthostatic tolerance.     

Norepinephrine and epinephrine responses during orthostatic intolerance in healthy elderly men

In young individuals, orthostatic intolerance is associated with marked increases in plasma epinephrine (EPI) concentrations and attenuated rises in plasma norepinephrine (NE) concentrations. This study investigated the cardiovascular, EPI and NE responses of healthy elderly males during orthostatic stress. Twelve men (68 +/- 1 yr) with a recent history of orthostatic hypotension and who exhibited orthostatic intolerance (HYPO) during 90 degrees head-up tilt (HUT) were compared with 12 men (69 +/- 1 yr) without a history of orthostatic hypotension and who remained normotensive (NORMO) throughout 90 degrees HUT. Beat-by-beat recordings of heart rate (HR), mean (MAP), systolic (SBP), diastolic (DBP), and pulse (PP) pressures were made throughout 90 degrees HUT. Blood samples obtained during supine rest and 90 degrees HUT were analyzed for changes in EPI and NE concentrations, hematocrit, hemoglobin and plasma volume. Compared to supine rest, orthostatic intolerance was characterized by significant reductions (p < 0.0001) in MAP, SBP, DBP, and PP. The HR, MAP, SBP, DBP, and PP at the termination of 90 degrees HUT was significantly lower (p < 0.0001) for HYPO than NORMO. The 90 degrees HUT position resulted in significant increases (p < 0.01) in NE for both HYPO and NORMO, with the rise in NE significantly lower (p < 0.05) in HYPO. There were no differences between groups regarding EPI concentrations at the termination of 90 degrees HUT. These results suggest that the magnitude of arterial pressure (AP) reduction does not influence the EPI response during orthostasis in healthy elderly men. However, marked reductions in AP, leading to orthostatic intolerance, are associated with inadequate increases in NE in these individuals.     

Assessment of autonomic nervous system with analysis of heart rate variability in children with spastic cerebral palsy

The purpose of this study was to investigate the function of the autonomic nervous system in children with spastic cerebral palsy (CP) through an analysis of heart rate variability (HRV) occurring with orthostatic stress. Twelve children with spastic CP and twelve normal children participated in this study. The echocardiogram (ECG) signals were recorded for 3 minutes in both the supine and 70 degrees C head-up tilt positions, and then the HRV signals underwent power spectrum analysis at each position. Two components were measured; a low- frequency (LF) component (0.05 - 0.15 Hz) primarily reflecting sympathetic activity during orthostatic stress and a high-frequency (HF) component (0.15 - 0.4 Hz) reflecting parasympathetic activity. In the supine position, there was no significant difference between any of the HRV components of the two groups. In the head-up tilt position, absolute and normalized LF were significantly increased and absolute HF was significantly decreased in the normal children (p < 0.05), but not in the children with spastic CP. The results of this study suggest that cardiac autonomic functions, such as vagal withdrawal and sympathetic activation which occur during head-up tilt position, are not sufficient to overcome the orthostatic stress arising in spastic CP children.     

A comparative assessment of two techniques for investigating initial cardiovascular reflexes under acute orthostatic stress

The physiological differences between active and passive changes in posture have been previously established. This study determined the extent of the differences in the initial cardiovascular responses to the passive head-up tilt (HUT) and the active squat-stand test (SST). Eleven females and 13 males underwent one +75° HUT and one SST. Beat-to-beat diastolic blood pressure (DBP), systolic blood pressure (SBP), mean arterial pressure (MAP) and heart rate (HR) were determined non-invasively. Data were recorded 10 s prior to (control) and 30 s after tilt or stand (event). Blood pressure and HR responses were analysed by calculating the deviation from control at 10 s (T10), 20 s (T20) and 30 s (T30) after the onset of each test. The DBP response (reflecting changes in systemic vascular resistance) at T10 was –10 (2) mmHg [mean (SEM)] for the HUT and –25 (2) mmHg for the SST (P<0.01). DBP returned to control levels by T30 for the HUT, but remained depressed for the SST. MAP responses directly reflected these changes in DBP. HR significantly increased from control values (P<0.001) for the HUT [+14 (1) bpm] and the SST [+16 (1) bpm], and remained elevated for the entire 30-s period for both tests. This study demonstrates that although the magnitude of the initial blood pressure decrease is greater for the active SST compared with the passive HUT, the reflex compensatory response is no different, making the SST a greater challenge for the cardiovascular reflexes.     

Relationship between blood pressure and heart-rate variability during graded head-up tilt

To investigate the relationship between change in blood pressure and autonomic nerve activity, two types of head-up tilt experiments were performed. One was a 30 degrees, 45 degrees, 60 degrees, and 90 degrees graded tilt-up, in which tilt angles were changed at 6-min intervals and 5 min were spent at each angle. The other was a 10-min lasting head-up tilt at 60 degrees. Electrocardiogram (ECG) and blood pressure (Finapres) of 18 healthy non-smoking subjects (9 men, 9 women) were recorded during the experiments. Heart-rate variability was examined by general spectral analysis (GSA). The high-frequency/total-area ratio (HF/TO) showed a decrease as the tilt angle increased. Compared with the values at the 0 degrees position, these changes were statistically significant (P < 0.05). The low-frequency/HF ratio (LF/HF) showed a significant (P < 0.05) difference between 0 degrees and 90 degrees, and between 30 degrees and 90 degrees. Some of the subjects could not maintain their blood pressure during either of the head-up tilt experiments, and they showed only a slight change in HF/TO and LF/HF. This result confirmed that immediate responses to head-up tilt reflect autonomic nerve activity. Hence, changes in the frequency components were found to be an index of autonomic nerve activity, and they explained the individual differences observed in the ability to control blood pressure during a transition to upright posture.     

Decreased tolerance to orthostatic stress in diabetics

Cardiovascular responses to head up tilt (HUT) were investigated in chronic diabetics (having disease of 8 to 10 years duration) and in control subjects. The parameters recorded were heart rate (HR), blood pressure (BP), forearm blood flow (FABF) and forearm vascular resistance (FAVR). Recordings were made first in the supine position, next after HUT, and thirdly in the recovery period after return to the supine position. Normal subjects responded to HUT by a marked increase in HR (P less than 0.001), decrease in FABP (P less than 0.001) and increase in FAVR (P less than 0.001). The diastolic blood pressure (DBP) and mean blood pressure (MBP) showed appreciable increase (P less than 0.001, P less than 0.01 respectively) without significant fall in systolic blood pressure (SBP). The maximum alteration in cardiovascular responses was observed immediately after HUT (within 15 sec). The cardiovascular responses to HUT in diabetics were found to be significantly impaired compared to control subjects. The impairment of cardiovascular responses in diabetics in indicative of autonomic neuropathy which can be detected by these tests before the development of clinical signs of the neuropathy.     

Reliability of orthostatic responses in healthy men aged between 65 and 75 years

The purpose of this study was to investigate the short-, medium- and long-term reproducibility of cardiovascular responses during 90 degrees head-up tilt (HUT) in healthy older men. Twenty-eight healthy male subjects aged 69 (95% confidence intervals, 68-70) years participated in the study. Eight subjects underwent duplicate 90 degrees HUT tests on consecutive days, while 20 subjects underwent four 90 degrees HUT tests performed at baseline, and after 1 week, 1 month and 1 year. Following a 20-min supine resting period, each subject was rapidly tilted to the upright vertical position (90 degrees HUT) and remained in that position for 15 min. Beat-by-beat recordings of mean (MAP), systolic (SBP) and diastolic (DBP) pressures were made via Finapres, while heart rate (HR) was monitored continuously from an electrocardiogram. No significant test-retest differences (P > 0.05) were observed for the changes in HR, MAP, SBP or DBP during 90 degrees HUT. These measurements demonstrated high reproducibility (intraclass correlation coefficient, r = 0.91-0.99, P < 0.05). The supine resting and tilted HR, MAP, SBP and DBP over the 1-week, 1-month and 1-year period were not significantly different (P > 0.05) from baseline, and demonstrated high reproducibility (intraclass correlation coefficient, r = 0.82-0.98, P < 0.05). The results of this study demonstrate that in healthy older men, cardiovascular responses during orthostasis are highly reproducible, and this reproducibility is maintained over a 12-month period. These findings demonstrate that the 90 degrees HUT test offers a reproducible method of monitoring longitudinal orthostatic responses in healthy older men.     

Role of Baroreflex Sensitivity in Predicting Tilt Training Response in Patients with Neurally Mediated Syncope

PurposeAn association between baroreflex sensitivity (BRS) and the response to tilt training has not been reported in patients with neurally mediated syncope (NMS). This study sought to investigate the role of BRS in predicting the response to tilt training in patients with NMS.ResultsAfter tilt training, 52 patients (91.2%) achieved three consecutive negative responses to the HUT. In the supine position before upright posture during the first session of tilt training for responders and non-responders, the mean BRS was 18.17±10.09 ms/mm Hg and 7.99±5.84 ms/mm Hg (p=0.008), respectively, and the frequency of BRS ≥8.945 ms/mm Hg was 45 (86.5%) and 1 (20.0%; p=0.004), respectively. Age, male gender, frequency of syncopal events before HUT, type of NMS, phase of positive HUT, total number of tilt training sessions, and mean time of tilt training did not differ between the study groups. In the multivariate analysis, BRS <8.945 ms/mm Hg in the supine position (odds ratio 23.10; 95% CI 1.20-443.59; p=0.037) was significantly and independently associated with non-response to tilt training.ConclusionThe BRS value in the supine position could be a predictor for determining the response to tilt training in patients with NMS who are being considered for inpatient tilt training.     

Differential effect of head-up tilt on cardiovagal and sympathetic baroreflex sensitivity in humans

The purpose of this study was to determine the effect of baroreceptor unloading on the sensitivity of the cardiovagal and sympathetic arms of the baroreflex during upright posture. Beat-by-beat R-R interval, arterial blood pressure and cardiac output (Doppler ultrasound), as well as muscle sympathetic nerve activity (MSNA) were recorded during periods in supine (Supine) and 60 deg head-up tilt (HUT) positions (n = 8 volunteers). Cardiovagal baroreflex sensitivity (BRS) was measured by the spontaneous sequence analysis method using systolic blood pressure and R-R interval, while sympathetic BRS was determined using the slope of the linear relationship between decreasing segments of diastolic blood pressure (DBP) and corresponding increases in MSNA. On changing to HUT, mean R-R interval and cardiac output decreased, while mean measures of MSNA, DBP and total peripheral resistance increased (P < 0.05). Cardiovagal BRS decreased from Supine to 60 deg HUT (19 +/- 2 ms mmHg(-1) versus 7.6 +/- 1.2 ms mmHg(-1); P < 0.01). In contrast, sympathetic BRS increased from -6.1 +/- 1.4 a.u. mmHg(-1) in Supine to -14 +/- 2 a.u. mmHg(-1) in HUT (P < 0.01). Thus, HUT produced differential effects on cardiac versus sympathetic BRS. The data suggest that dynamic baroreflex-mediated cardiovascular control is dominated by sympathetic control during baroreceptor unloading.     

Determination of baroreflex gain using auto-regressive moving-average analysis during spontaneous breathing.

The heart rate component of the arterial baroreflex gain (BRG) was determined with auto-regressive moving-average (ARMA) analysis during each of spontaneous (SB) and random breathing (RB) protocols. Ten healthy subjects completed each breathing pattern on two different days in each of two different body positions, supine (SUP) and head-up tilt (HUT). The R-R interval, systolic arterial pressure (SAP) and instantaneous lung volume were recorded continuously. BRG was estimated from the ARMA impulse response relationship of R-R interval to SAP and from the spontaneous sequence method. The results indicated that both the ARMA and spontaneous sequence methods were reproducible (r = 0.76 and r = 0.85, respectively). As expected, BRG was significantly less in the HUT compared to SUP position for both ARMA (mean +/- SEM; 3.5 +/- 0.3 versus 11.2 +/- 1.4 ms mmHg-1; P < 0.01) and spontaneous sequence analysis (10.3 +/- 0.8 versus 31.5 +/- 2.3 ms mmHg-1; P < 0.001). However, no significant difference was found between BRG during RB and SB protocols for either ARMA (7.9 +/- 1.4 versus 6.7 +/- 0.8 ms mmHg-1; P = 0.27) or spontaneous sequence methods (21.8 +/- 2.7 versus 20.0 +/- 2.1 ms mmHg-1; P = 0.24). BRG was correlated during RB and SB protocols (r = 0.80; P < 0.0001). ARMA and spontaneous BRG estimates were correlated (r = 0.79; P < 0.0001), with spontaneous sequence values being consistently larger (P < 0.0001). In conclusion, we have shown that ARMA-derived BRG values are reproducible and that they can be determined during SB conditions, making the ARMA method appropriate for use in a wider range of patients.     

Sympathetic responses to Valsalva's manoeuvre following bed rest.

The purpose of this study was to examine whether 14 days of head-down tilt bed rest (HDBR) alters autonomic regulation during Valsalva's manoeuvre (VM) and if this would predict blood pressure control during a 60 degrees head-up tilt (HUT) test. To examine autonomic control of blood pressure, we measured the changes in systolic (delta SBP) and diastolic (delta DBP) blood pressure between baseline and the early straining (Phase IIE) period of VM (20 sec straining to 40 mmHg; N = 7) in conjunction with changes in muscle sympathetic nerve activity (MSNA; microneurography) burst frequency (B/min) and total activity (% delta) from baseline over the 20-sec straining period. MSNA data were successfully recorded from 6 of the 7 individuals. The averaged responses from three repeated VMs performed in the supine position were compared between the pre- and post-HDBR tests. Compared with the pre-HDBR test, a greater reduction in SBP, DBP, and MAP was observed during Phase IIE following HDBR, p < 0.05. The increase in MSNA burst frequency during straining was augmented in the post- compared with the pre-HDBR test, p < 0.0001, as was the Phase IV blood pressure overshoot, p < 0.05. Although all subjects completed the 20-min pre-HDBR tilt test without evidence of hypotension or orthostatic intolerance, the post-HDBR test was stopped early in 5 of the 7 subjects due to systolic hypotension. The responses during the VM suggest that acute autonomic adjustments to rapid blood pressure changes are preserved after bed rest. Furthermore, MSNA and blood pressure responses during VM did not predict blood pressure control during orthostasis following HDBR.     

Initial circulatory responses to changes in posture: influence of the angle and speed of tilt

To assess if changes in the angle or speed of tilt could account for the differences between the initial (first 30 s) circulatory responses induced by active and passive changes in posture, as found in previous studies, we investigated the initial heart rate and blood pressure responses induced by stand up from supine and various head-up tilt manoeuvres in 12 healthy, male subjects. Comparison was made between 70 degrees head-up tilt in 3 s, 90 degrees head-up tilt also in 3 s and 70 degrees head-up tilt in 1.5 s, using an automatic pneumatic-driven tilt table with foot support. It was found that the initial heart rate and blood pressure responses induced by the three tilt manoeuvres were almost identical in time course and amplitude, but significantly different from those induced by stand up. The results of this study prove that regardless of the angle and speed of tilt, the initial circulatory responses induced by passive changes in posture are essentially different from the responses induced by active changes in posture.