Inspiratory CO2 increases orthostatic tolerance during repeated tilt.

INTRODUCTION: The partial pressure of end tidal CO2 (PetCO2) is known to decrease with head-up tilt. Decreases in arterial CO2 reduce cerebral blood flow (CBF) and may increase the incidence of presyncope. We measured cerebral and central cardiovascular responses to repeated tilt where: 1) PetCO2 was allowed to change with tilt (eucapnic): and 2) PetCO2 was clamped at supine levels (isocapnic). METHODS: In eight healthy subjects breath-by-breath measurements were made of ventilation (VE) and PetCO2 along with beat-by-beat measurements of blood pressure (BP), heart rate (HR) and middle cerebral artery mean flow velocities (MFV). Following 30-min in the supine position, a series of six 10-min 90 degrees head-up tilts were performed, with 30-s of supine between each. Presyncopal subjects were returned immediately to the supine position. RESULTS: Statistical comparisons were made between the supine, and the first and last minute of the first tilt. BP, HR responses were not different between the eu- and isocapnic conditions; however, by the end of the first tilt VE was significantly higher than supine. MFV and BP at brain level decreased and HR increased from supine to tilt. MFV was higher in the isocapnic compared with the eucapnic condition but decreased from the beginning to the end of the first tilt in both conditions (i.e., tilt #1: eucap. 49.4 to 46.7; isocap. 65.0 to 59.6 cm s(-1); p < 0.05) while the BP remained constant. Five subjects were presyncopal in the study. With isocapnic tilt, presyncopal time was not reduced but was extended in four of the five subjects (2.2, 5.5, 6.3 and 31 min) yet at presyncope the values for MFV, BP and HR were the same in both conditions. CONCLUSIONS: Inspiratory CO2 contributed to increased MFV at the beginning of tilt and increased orthostatic tolerance.     

累加角度头高斜位下心率、血压、心率变异性与脑血流变化的特征

目的探讨累加角度头高斜位暴露下 ,正常人体血压、心率、脑血流及心率变异性 (HRV)变化特征 ,为建立飞行员晕厥检查的方法学提供生理依据。方法采用自行研制的多功能双向式医用倾斜床 ,结合先进的十二导联同步心电图描记方法和经颅多普勒脑血流成像技术进行累加角度 ( + 4 5°→ 60°→75°)头高斜位 (HUT)暴露下的心率、血压、HRV和脑血流速率的测量与分析。结果 1 )随着倾斜角度加大 ,暴露时程延长 ,HR逐渐增快 ,SBP逐渐下降 ,DBP逐渐升高 ,PP逐渐缩小 ,与暴露前比较 ,HR、PP和DBP相差非常显著 (P <0 .0 5 ) ;脑血流速率 (包括 :Vs、Vm、Vd)逐渐下降 ,无论与暴露前比 ,或倾斜角度之间比 ,差异十分显著 (P <0 .0 1 ) ;mRR和sdRR逐渐减小。 2 )生理指标与技术指标直线相关分析发现 :在 1min时 ,SBP、DBP、MAP、Vd、sdRR与倾斜角度呈明显正相关 (P <0 .0 1 ) ;在 2 0min时 ,HR和sdRR与倾斜角度呈明显正相关 (P <0 .0 1 ) ,与SBP、PP、Vs、mRR明显负相关 (P <0 .0 1 ) ;暴露时程与HR呈明显正相关 (P <0 .0 1 ) ,与SBP、Vm、Vd、mRR呈明显负相关 (P <0 .0 1 )。 3)生理指标间相关分析发现 :HR与SBP、Vs、Vm、Vd 呈明显负相关 (P <0 .0 1 ) ;SBP与Vd 和mRR呈明显正相关 (P <0 .0 1 ) ;Vs、Vm、Vd 与mRR和sdR     

Cerebral hemodynamic changes after carotid angioplasty and stenting

BACKGROUND AND PURPOSE: Carotid angioplasty stent placement (CAS) is an accepted alternative to endarterectomy. However, little is known about its effect on cerebral hemodynamics. We prospectively studied the early and delayed hemodynamic changes after CAS. MATERIALS AND METHODS: Sixty patients underwent CAS for >or=70% internal carotid artery stenosis. Transcranial Doppler (TCD) was done before, 6 hours, and 30 days after CAS. Cerebrovascular reserve to apnea (CVR) was measured. RESULTS: On the ipsilateral side, basal middle cerebral artery (MCA) middle flow velocity (MFV) and pulsatility index (PI) increased from 49.7 cm/s and 0.85, respectively, to 62.5 cm/s and 1.09 immediately after CAS (P < .05); anterior cerebral artery (ACA) MFV and PI increased from 50.7 cm/s and 0.90, to 58.9 cm/s and 1.08 (P < .05); and posterior cerebral artery (PCA) MFV decreased from 47.5 to 36.5 cm/s, with no change in PI (P < .05). On the contralateral side, nonsignificant changes were seen on MCA, immediately after CAS; ACA and PCA MFV decreased from 63.7 and 45.3 cm/s to 50.3 and 38.6 cm/s, respectively (P < .05); ACA and PCA PI increased from 0.96 and 1.00, respectively, to 1.04 and 1.04 (P < .05). At 30 days, ipsilateral MCA MFV and PI were 52.8 cm/s and 1.12, respectively (P < .05), and contralateral values were 49.6 cm/s (P < .05) and 1.02 (nonsignificant), respectively. Basal ipsilateral and contralateral CVR improved from 26.0% to 37.0% (P < .05), and from 30.1% to 33.5% (nonsignificant), respectively, at 30 days. CONCLUSIONS: CAS produces an early significant increase of MFV and PI in the ipsilateral anterior circulation. This effect is maintained, though minor, 30 days later. In addition, CAS results in an improvement of CVR at 30 days.     

Regional cerebral perfusion in cardiovascular reflex syncope

Little is known about the regional cerebral perfusion in subjects with presyncope or syncope, and the impact that autonomic nervous dysfunction has on it. Seven subjects with cardiovascular vasodepressor reflex syncope were studied. A baseline test was performed with the patients standing in the 70° upright position, while the passive head-up tilt table test with and without isoprenaline infusion was employed for provocation. Regional cerebral perfusion was assessed by means of single-photon emission tomography with technetium-99m labelled V-oxo-1,2-N,N ₁-ethylenedylbis-l-cysteine diethylester (baseline, and during blood pressure decline in the provocation test) and the autonomic nervous function by means of spectral analysis of heart rate variability (baseline, and before blood pressure decline in the provocation test). Every subject showed an abrupt decline in blood pressure in the provocation test (five with presyncope and two with syncope). The systolic and diastolic blood pressures decreased significantly (P<0.001) between the baseline and the provovation study time points (radiopharmaceutical injection and lowest systolic blood pressure). Mean cerebral perfusion as average count densities decreased upon provocation as compared with baseline (190±63 vs 307±90 couts/voxel, respectively,P=0.0013). Hypoperfusion was most pronounced in the frontal lobe. These results suggest that cerebral perfusion decreases markedly during presyncope or syncope with systemic blood pressure decline in subject with cardiovascular vasodepressor syncope. Furthermore, the autonomic nervous function remains unchanged before the systemic blood pressure decline.     

经颅多普勒在前循环动脉瘤破裂致脑血管痉挛中的应用价值

目的探讨经颅多普勒(TCD)在前循环动脉瘤破裂致症状性脑血管痉挛中的应用价值。方法回顾性分析我院神经外科2010年4月~2020年4月收治的前循环动脉瘤性蛛网膜下腔出血,诊断为症状性脑血管痉挛的患者62例,所有患者在发病后连续每天行TCD检查,连续观察大脑中动脉血流速度,诊断为症状性脑血管痉挛后行脑血管造影检查(DSA),并行选择性脑动脉内盐酸法舒地尔灌注治疗,治疗后继续每日TCD监测;以DSA结果为标准,对比分析TCD诊断脑血管痉挛的准确性,分析TCD与DSA诊断痉挛程度的一致性。结果62例症状性脑血管痉挛患者,男性26例,女性36例,平均年龄(57.7±7.5)岁,其中前交通动脉瘤9例,大脑中动脉瘤21例,后交通动脉瘤32例;临床诊断症状性脑血管痉挛后,TCD诊断脑血管痉挛57例,行DSA检查,61例证实血管痉挛;TCD检出率93.4%(57/61)。选择性脑血管灌注法舒地尔前后,TCD测量M1平均血流速度,轻度痉挛组分别是(130.8±6.4)cm/s、(83.8±14.6)cm/s,中度痉挛组分别是(171.3±12.8)cm/s、(82.3±8.5)cm/s,灌注治疗效果明显,TCD血流速度有明显差异。术后TCD继续监测,4例患者在TCD血流速度再次升高后复查DSA,证实并治疗。结论针对前循环动脉瘤破裂所致症状性脑血管痉挛,TCD筛查检出率高;选择性脑血管内灌注治疗前后TCD血流速度有明显差异,可预测和指导下一步的动脉内治疗,并可作为治疗后的复查手段。TCD检查是症状性脑血管痉挛安全、有效、便捷的筛查手段。     

Cerebrovascular responses during lower body negative pressure-induced presyncope

BACKGROUND: Reduced orthostatic tolerance is commonly observed after spaceflight, occasionally causing presyncopal symptoms which may be due to low cerebral blood flow (CBF). It has been suggested that CBF decreases in early stages of exposure to orthostatic stress. The purpose of this study was to investigate cerebrovascular responses during presyncope induced by lower body negative pressure (LBNP). HYPOTHESIS: Although CBF decreases during LBNP exposure, blood pressure (BP) or heart rate (HR) contributes more to induce presyncopal conditions. METHODS: Eight healthy male volunteers were exposed to LBNP in steps of 10 mm Hg every 3 min until presyncopal symptoms were detected. Electrocardiogram (ECG) was monitored continuously and arterial BP was measured by arterial tonometry. CBF velocity at the middle cerebral artery was measured by transcranial Doppler sonography (TCD). Cerebral tissue oxygenation was detected using near-infrared spectroscopy (NIRS). We focused our investigation on the data obtained during the final 2 min before the presyncopal endpoint. RESULTS: BP gradually decreased from 2 min to 10 s before the endpoint, and fell more rapidly during the final 10 s. HR did not change significantly during presyncope. CBF velocity did not change significantly, while cerebral tissue oxygenation decreased prior to the presyncopal endpoint in concert with BP. Our results suggest that CBF is maintained in the middle cerebral artery during presyncope, while BP decreases rapidly. CONCLUSIONS: Cerebrovascular hemodynamics are relatively well maintained while arterial hypotension occurs just prior to syncope.     

Transcranial Doppler ultrasound in the evaluation of collateral blood flow in patients with internal carotid artery occlusion: correlation with cerebral angiography.

PURPOSETo determine the accuracy of transcranial Doppler (TCD) ultrasound for evaluation of collateral supply through the circle of Willis in patients with internal carotid artery (ICA) occlusion.METHODSThe evaluation of the collateral pathways through the circle of Willis with TCD ultrasound and with cerebral angiography was compared in 40 patients (30 men, 10 women; mean age, 55 +/- 9 years) in a total of 44 ICA occlusions of which 43 had a suitable ipsilateral temporal bone window for TCD examination.RESULTSBy TCD, a patent anterior communicating artery is indicated by a reversal blood flow in the A1-segment of the anterior cerebral artery or by a prompt fall of blood velocity in the middle cerebral artery after compression of the nonoccluded contralateral carotid artery. In 42 of 43 instances of ICA occlusion, TCD and angiography agreed in the evaluation of a present or absent anterior communicating artery collateral supply. TCD''s sensitivity was 95%, its specificity 100%. A collateral supply through the basilar artery was assumed with TCD when there was: (a) a basilar artery blood velocity of more than 70 cm/s; (b) a marked increase of basilar artery blood velocity after compression of the nonoccluded carotid artery; (c) an evident side-to-side asymmetry of the blood velocity of the posterior cerebral arteries with high blood velocity ipsilateral to the ICA occlusion. For evaluating the collateralization via the basilar artery, TCD and angiography agreed in 37 of 40 ICA occlusions. TCD''s sensitivity was 87%, its specificity 95%.CONCLUSIONSTCD is a reliable tool for the evaluation of the collateral supply in patients with ICA occlusions.     

Brain SPECT used to evaluate vasospasm after subarachnoid hemorrhage: correlation with angiography and transcranial Doppler

PURPOSE: The primary objective of this study was to correlate Tc-99m HMPAO and ethyl cysteine dimer perfusion brain SPECT imaging with angiography and transcranial Doppler (TCD) to identify vasospasm after subarachnoid hemorrhage. METHODS: A retrospective analysis of consecutive patients who had cerebral blood flow SPECT imaging for subarachnoid hemorrhage and aneurysm clipping was made. Flow velocity measurements were correlated using TCD and cerebrovascular angiography. RESULTS: Of the 129 patients were included in this study, 84 were female and 45 were male, with a mean age of 51.9 years and a median age of 51 years (range, 9 to 84 years). Eighty-nine patients had brain SPECT evidence of hypoperfusion. Concordance was found between SPECT and TCD with vasospasm in 57 of 89 (64%) patients and nonconcordance was evident in 32 patients (36%). Eleven patients who had concordance between SPECT and TCD had nonconcordant results of angiography for vasospasm. CONCLUSIONS: These findings suggest that all three methods are complementary to each other in the evaluation of patients with vasospasm after subarachnoid hemorrhage. Concordance of 64% between SPECT and TCD is acceptable and explicable by the differences in technique and measurement of cerebral blood flow compared with vascular narrowing, respectively.     

Heart rate and blood pressure changes with endurance training: the HERITAGE Family Study

PURPOSE: The purpose of this study was to determine the magnitude of change in resting and exercise heart rate (HR) and blood pressure (BP), by race, sex, and age, after a 20-wk endurance training program in 507 healthy and previously sedentary subjects from the HERITAGE Family Study. METHODS: After baseline measurements, subjects exercised on cycle ergometers 3 d x wk(-1) for a total of 60 exercise sessions starting at 55% of VO2max for 30 min x session(-1) and building to 75% of VO2max for 50 min x session(-1) for the last 6 wk. HR and BP at rest and during exercise (50 W, 60% of VO2max maximal exercise) were each determined in duplicate on two different days both before and after training (resting values at 24-h and 72-h posttraining). RESULTS: After the period of training, there was a small decrease in resting HR (-2.7 to -4.6 beats x min(-1) across groups at 72-h posttraining), and small changes (i.e., < 3 mm Hg) in resting systolic (SBP), diastolic (DBP), and calculated mean BP (MBP), which varied by race, sex, and age. During exercise at the same absolute work rate (50 W), HR, SBP, DBP, and MBP were all significantly reduced, with greater reductions in HR in women compared with men, and greater reductions in BP in blacks and older subjects compared with whites and younger subjects, respectively. At the same relative work rate (60% VO2max), HR, DBP, and MBP were reduced, but SBP remained unchanged. Blacks had a greater reduction in DBP, but whites had a greater reduction in HR. Finally, at maximal exercise, there was a small decrease in HR, with men and whites decreasing more than women and blacks; an 8 mm Hg increase in SBP, with men increasing more than women; a 4 mm Hg decrease in DBP, with blacks decreasing more than whites; and no change in MBP. CONCLUSION: In conclusion, the reductions in resting HR and BP with training were generally small, but the reductions during exercise were substantial and clinically important, with the older and the black populations experiencing greater reductions.     

糖尿病患者的经颅多普勒变化

目的:应用经颅多普勒检查糖尿病患者颅内血管状况,以了解糖尿病与血液动力学变化的关系。方法:采用德国EME公司2020型经颅多普勒检查仪。对55例糖尿病患者及60例健康对照组进行了经颅多普勒检查,观察大脑中动脉、大脑前动脉、大脑后动脉,椎、基底动脉的血流速度,搏动指数及血流频谱进行检测。结果:55例糖尿病患者中血流速度异常占81.82%,其中血管痉挛8例,血管硬化30例,血管狭窄7例,正常10例。两组搏动指数值比较,糖尿病组搏动指数值显著高于健康对照组。结论:经颅多普勒超声检查可以较明确反映糖尿病脑血管病变的部位、范围和性质。     

24h头低位卧床对心血管功能及立位应激反应的影响

目的 观察２４ｈ头低位卧床对心血管系统功能和头高位倾斜心血管反应变化的影响。方法健康男性学员６名,年龄２２～２３岁,进行了２４ｈ头低位卧床实验,卧床期间用阻法测量了心脏泵血功能指标,并观测了卧床前后头高位倾斜时的心血管反应和卧床期间尿量的变化。结果 在卧床期间受试者心率较卧床前（立位）显著减慢,第６、１２、１８ｈ的心率较卧床开始时（０ｈ）显著降低。ＳＶ和ＳＩ在卧床０、６、１２和２４ｈ较卧床前显著增     

Diastolic blood pressure influences cerebrovascular reactivity measured by means of123I-iodoamphetamine brain single photon emission computed tomography in medically treated patients with occlusive carotid or middle cerebral artery disease

OBJECTIVE: Impaired cerebrovascular reactivity (CVR) to vasodilating agents is a predictor of the onset and prognosis of ischemic stroke. It is realized that the CVR improves or worsens when measured periodically during the clinical course in medically treated patients with occlusive cerebrovascular disease. In these patients, we investigated the possible relationship between the interval change in CVR and that in systemic blood pressure (BP). METHODS: Forty-two patients (14 females and 28 males, mean age +/- SD: 65.3 +/- 8.8 years) with severe stenosis or occlusion of the common carotid, internal carotid, or middle cerebral arteries repeatedly underwent single photon emission computed tomography (SPECT) studies using 123I-iodoamphetamine to measure cerebral blood flow (CBF) distribution and CVR at a more-than-6-month interval (mean +/- SD: 18.5 +/- 8.8 months). The CVR was separately estimated in cerebral hemispheres ipsilateral and contralateral to the most severe vascular lesion as the % increase in CBF after acetazolamide loading to CBF at rest. Systemic BP was measured four times at enrollment and the follow-up SPECT studies during resting and acetazolamide loading. Average BP at each SPECT study was an average of BP measurements during resting and acetazolamide loading. Interval changes in CVR were correlated with those in average systolic BP, average diastolic BP, and average mean arterial BP. RESULTS: The interval changes in CVR were significantly correlated with those in average diastolic BP in the ipsilateral hemisphere (y = 0.71x + 1.43, r2 = 0.11, p < 0.05) and in the contralateral hemisphere (y = 0.88x - 0.46, r2 = 0.16, p < 0.05) but not with those in average systolic BP or average mean arterial BP. CONCLUSIONS: In medically treated patients with steno-occlusive carotid artery or middle cerebral artery lesions, the interval change in CVR to acetazolamide by means of 123I-IMP SPECT was influenced by the diastolic BP at the SPECT studies. Monitoring diastolic BP is important to evaluate interval change in CVR.     

Can relaxation lower metaboreflex-mediated blood pressure elevations?

PURPOSE: Relaxation can lower resting blood pressure, and this investigation sought to determine whether relaxation could reduce mean blood pressure (MBP) elevations produced by postexercise circulatory occlusion (PECO). METHODS: Sixteen volunteers trained with relaxation and were able to decrease MBP at rest by at least 5 mm Hg within 2 min. Subjects performed four tests assigned randomly: i). rest with cuff occlusion, ii). rest and cuff occlusion with relaxation, iii). hand-grip exercise followed by PECO rest with cuff occlusion, and iv) hand-grip exercise followed by PECO with relaxation. Data for HR and MBP were collected using a Finapres; ratings of relaxation and discomfort from cuff occlusion were obtained using a 1- to 10-unit scale. Stroke volume (SV) and HR were collected from six subjects to calculate cardiac output and total peripheral conductance (TPC). Dependent variables were compared using an ANOVA. RESULTS: HR (mean +/-SD) was lower during both relaxation conditions as compared with control (-7 +/- 4 bpm vs -2 +/- 3 bpm; P< 0.05). The MBP was reduced during relaxation alone (-6 +/- 3.7 mm Hg; < 0.05) but not during PECO with relaxation. The rating of relaxation was higher during relaxation (6.8 +/- 1.3 units) versus control (3.5 +/- 1.2 units), but ratings were not different between relaxation conditions. Ratings of discomfort were higher during PECO ( P< 0.05). Relaxation did not significantly alter CO or SV (N= 6). During relaxation alone, TPC was increased (0.046 +/- 0.001 vs 0.049 +/- 0.002 L.min.mm Hg; P< 0.05). However, TPC was significantly increased during PECO with relaxation. CONCLUSIONS: These findings suggest that although relaxation can affect cardiovascular regulation and lower HR and MBP at rest, this central signal cannot lower reflex increases in blood pressure originating from a peripheral metabolic stimulus.     

Bispectral analysis as a tool to investigate dynamics of cardiorespiratory physiology

INTRODUCTION: Recent studies show that cardiorespiratory variables are nonlinearly coupled. In the present study, we tested the feasibility of using bispectral analysis to estimate changes in nonlinear phase coupling. METHODS: We stimulated chemo- and baro-reflexes by switching inspired gas between air and air with 5% CO2 added during supine and 70 degrees head-up tilt. Bispectra were estimated using Fourier transform of the triple cumulant and were integrated between 0.04 and 0.3 Hz. Variables analyzed included cardiac RR intervals, systolic BP (SBP), tidal volume (VT), end tidal pressure of CO2 (PETCO2) and mean cerebral blood flow velocity (CBFM). Data were collected in 30 healthy adults who were then grouped according to whether they become presyncopal (PS) during tilt or remained normal with no signs or symptoms of presyncope (NPS). RESULTS: Six subjects (20%) developed presyncope. During tilt with room air breathing, phase coupling in RR, SBP, and between RR-SBP and RR-VT was lower in PS than in NPS, while phase coupling between CBFM-SBP was higher. In contrast, during tilt with 5% inhaled CO2, phase coupling between CBFM-PETCO2 was lower for PS than for NPS. DISCUSSION: Perturbations to baro- and chemo-reflexes led to changes in nonlinear phase coupling that were altered in presyncope. Although physiological interpretation of changes in phase coupling are less than clear at this time, our results show that such nonlinear indexes may provide a helpful perspective in understanding the complex phenomenon of orthostatically mediated syncope.     

直立倾斜试验与晕厥评价

晕厥是一种突发性，短暂性，一过性的意识丧生，由于一时性，广泛性脑缺血，缺氧引起，并在短时间内自然恢复。60％－70％的晕厥患者找不到明确原因，为不明原因晕厥，近年来开展的直立倾斜试验及异丙基肾上腺素激发的倾斜试验对诊断原因不明晕厥是一种有价值的方法。本文总结了倾斜试验诊断不明原因晕厥的价值，试验方法，临床应用情况，注意事项及研究进展。     

高血压病126例经颅多普勒脑血流测定临床分析

目的　探讨经颅多普勒脑血流测定对高血压病诊断和分期的价值。方法　对 1 2 6例高血压病患者 ,采用经颅多普勒仪探测大脑中动脉、大脑前动脉及大脑后动脉 ,记录平均血流速度 ,测定结果进行统计分析。结果　高血压病 期组 ,病程较短 ,年龄较轻者 ,各颅内血管的 Vm值与对照组比较无显著差异。 期组 ,各颅内血管的 Vm值明显增快。 期组 ,病程较长 ,年龄较大者 ,各颅内血管的 Vm值明显减慢。结论　TCD测值改变提供了高血压病诊断和分期的重要辅助指标。     

Assessment of left ventricular volume by an ambulatory radionuclide monitoring system during head-up tilt in patients with unexplained syncope: Relation to autonomic activity assessed by heart rate variability

BACKGROUND: Decreased left ventricular volume during head-up tilt plays an important role in triggering syncope in patients with neurally mediated syncope. However, precise changes in left ventricular volume during head-up tilt have not been well investigated. This study was conducted to test the hypothesis that the decline in left ventricular volume during tilt could trigger ventricular mechanoreceptor activation. METHODS AND RESULTS: To investigate the mechanisms of tilt-induced syncope, we measured the temporal changes in left ventricular volume, ejection fraction, cardiac output, and heart rate variability indices during head-up tilt in 25 patients with syncope of undetermined etiology. Eleven patients had a cardioinhibitory response (CI group), 7 patients showed a vasodepressor response (VD group), and 7 patients demonstrated a negative response (NG group). Before syncope, ejection fraction increased most in the CI group, the left ventricular end-diastolic volume declined most in the VD group (VD group, -11.0% +/- 3.3%; CI group, -2.8% +/- 4.8%; NG group, -3.4% +/- 2.2%; P <.005), and the high-frequency spectra increased most in the CI group (CI group, 25.0% +/- 21.0%; VD group, -4.1% +/- 11.7%; NG group, -5.3% +/- 12.7%; P <.01). The vasodepressor response was dependent on left ventricular volume, whereas the cardioinhibitory response was related to the vagal activity reflected by high-frequency spectra. CONCLUSIONS: The precise evaluation of left ventricular volume by an ambulatory radionuclide monitoring system combined with a heart rate variability analysis is considered useful for clarifying the pathophysiology of neurally mediated syncope.     

Hemodynamic responses to upright tilt at sea level and high altitude

Hemodynamic responses to upright tilt were studied in eight young men at sea level (SL); after 1 h at 4,300 m simulated altitude (SA); and at 18 h, 66 h and 114 h during residence at 4,300 m (HA). Heart rate (HR), stroke volume (SV), cardiac output (CO), calf blood flow (CBF), blood pressure (BP) and total peripheral resistance (TPR) were obtained during supine rest and after 13 min of 60 degrees head-up tilt using an impedance monitor and electrosphygmomanometer. SL to HA changes in blood volume (BV) were calculated from hematocrit and hemoglobin values. Plasma norepinephrine (Nor) was measured at SL and after 18 h and 66 h of HA. Supine HR, TPR and BP were increased while SV, CO and CBF were reduced SL to HA (p less than 0.05). HR and BP in the upright position were increased SL to HA (p less than 0.05). The responses to tilt (delta supine to upright) were unaltered SL vs SA. With prolonged exposure, SV, CO, TPR and CBF responses to tilt were reduced (p less than 0.05). The reduced responses to tilt at HA were associated with a 10% decline in BV (p less than 0.01) and a 40% increase in Nor (p less than 0.05). It was concluded that the reduction in SV during tilt at SL and SA was compensated for by increases in HR and TPR in order to maintain BP. After 18 h HA, BP in the upright position was maintained only by an increase in HR.     

Cerebral blood flow velocity by transcranial Doppler during a vertical-rotating table simulation of the push-pull effect

BACKGROUND: The push-pull effect (PPE) has been suspected of causing many aircraft accidents. The perfusion and then withdrawal of cerebral blood during the PPE may change the state of the cerebral blood vessel. HYPOTHESIS: During head-down tilt (HDT) cerebral vasoconstriction occurs in response to the elevated perfusion pressure to maintain cerebral blood flow, and during subsequent head-up tilt (HUT) the increased resistance of the cerebral blood vessel recovers slowly. METHODS: Ten healthy male non-pilots were exposed to the following protocol using a rotating-table to simulate the push-pull maneuver: HUT (+1 Gz) for 1 min followed by transition to HDT (-1 Gz) 10 s followed by transition to HUT (+1 Gz) 1 min. Cerebral blood flow velocity and pulsatility indices in the left middle cerebral artery were continually measured with a transcranial Doppler (TCD) instrument. RESULTS: Mean blood flow velocity (Vm) increased significantly by 10%, during the first 5 s of HDT, recovered to baseline during HDT 5 10 s, and remained unchanged during subsequent HUT. Systolic blood flow velocity (Vs) increased by 9% during HDT 5-10 s and 11% during HUT 0-5 s. Diastolic blood flow velocity (Vd) decreased by -9% during HDT 5-10 s, and -22% during HUT 0-5 s. Vs-Vd increased by 26% during HDT 5 10 s, and 41%, during HUT 0-5 s. Pulsatile indices (PI) and resistance index (RI) increased by 26%) and 15% during HDT 5-10 s, and by 40% and 27% during HUT 0-5 s, respectively. Vs, Vs-Vd, PI, and RI remained at the higher level, and Vd remained at the lower level to HDT 15-20 s. CONCLUSIONS: The results indicate that cerebral vasoconstriction occurred to prevent brain over-perfusion during HDT. During HUT, the elevated resistance of the cerebral vessel remained at the higher level for about 20 s, and may have worsened the cerebral perfusion from exposure to +Gz. This may be one of the mechanisms of PPE.     

Cerebral blood flow in hypertensive patients with cerebrovascular disease: technique for measurement and effect of captopril

Nineteen patients with unilateral cerebrovascular disease underwent cerebral blood flow (CBF) measurements; ten had been receiving conventional therapy and then were studied after treatment with captopril without or with a diuretic and nine on conventional therapy were studied twice as a control group. CBF (ml min-1) was measured after an intravenous injection of 99Tcm-labelled patient's red cells with a computer linked gamma camera over the vertex and a probe over the aorta. With deconvolution analysis regional CBF is given by regional cerebral volume divided by regional mean transit time. Results in the captopril group showed on average a 10% fall in mean blood pressure and a 10% rise in blood flow to the affected hemisphere. In the control group there was on average a 4% fall in pressure, together with an 11% fall in CBF to the affected hemisphere. Captopril appears to maintain autoregulation in cerebrovascular disease.