Degrees Head-up Tilt Research Articles

Cerebral Blood Flow Is Reduced in Severe Myalgic Encephalomyelitis/Chronic Fatigue Syndrome Patients During Mild Orthostatic Stress Testing: An Exploratory Study at 20 Degrees of Head-Up Tilt Testing.

Introduction: In a study of 429 adults with myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS), we demonstrated that 86% had symptoms of orthostatic intolerance in daily life. Using extracranial Doppler measurements of the internal carotid and vertebral arteries during a 30-min head-up tilt to 70 degrees, 90% had an abnormal reduction in cerebral blood flow (CBF). A standard head-up tilt test of this duration might not be tolerated by the most severely affected bed-ridden ME/CFS patients. This study examined whether a shorter 15-min test at a lower 20 degree tilt angle would be sufficient to provoke reductions in cerebral blood flow in severe ME/CFS patients. Methods and results: Nineteen severe ME/CFS patients with orthostatic intolerance complaints in daily life were studied: 18 females. The mean (SD) age was 35(14) years, body surface area (BSA) was 1.8(0.2) m2 and BMI was 24.0(5.4) kg/m2. The median disease duration was 14 (IQR 5–18) years. Heart rate increased, and stroke volume index and end-tidal CO2 decreased significantly during the test (p ranging from <0.001 to <0.0001). The cardiac index decreased by 26(7)%: p < 0.0001. CBF decreased from 617(72) to 452(63) mL/min, a 27(5)% decline. All 19 severely affected ME/CFS patients met the criteria for an abnormal CBF reduction. Conclusions: Using a less demanding 20 degree tilt test for 15 min in severe ME/CFS patients resulted in a mean CBF decline of 27%. This is comparable to the mean 26% decline previously noted in less severely affected patients studied during a 30-min 70 degree head-up tilt. These observations have implications for the evaluation and treatment of severely affected individuals with ME/CFS.

Repolarization variability independent of heart rate during sympathetic activation elicited by head-up tilt.

The fraction of repolarization variability independent of RR interval variability is of clinical interest. It has been linked to direct autonomic nervous system (ANS) regulation of the ventricles in healthy subjects and seems to reflect the instability of the ventricular repolarization process in heart disease. In this study, we sought to identify repolarization measures that best reflect the sympathetic influences on the ventricles independent of the RR interval. ECG was recorded in 46 young subjects during supine and then following 45 degrees head-up tilt. RR intervals and five repolarization features (QTend, QTpeak, RTend, RTpeak, and TpTe) were extracted from the ECG recordings. Repolarization variability was separated into RR-dependent and RR-independent variability using parametric spectral analysis. Results show that LF power of TpTe is independent of RR in both supine and tilt, while the LF power of QTend and RTend independent of RR and respiration increases following tilt. We conclude that TpTe is independent of RR and is highly affected by respiration. QTend and RTend LF power might reflect the sympathetic influences on the ventricles elicited by tilt. Graphical abstract.

A Technique of Forced Expiratory Noise Time Evaluation Provides Distinguishing Human Pulmonary Ventilation Dynamics During Long-Term Head-Down and Head-Up Tilt Bed Rest Tests Simulating Micro and Lunar Gravity.

Estimating the effect of microgravity/hypogravity on pulmonary ventilation function remains topical. Recently developed acoustic techniques based on the evaluation of the forced expiratory noise time (FETa) were hypothesized to be a promising tool for this aim. The aim of the protocol is to study the effect of two different modalities of bed rest space simulations (microgravity and lunar gravity) on FETa and spirometric indices. The FETa in the frequency band of 200–2000 Hz, recorded above human trachea, was evaluated. The 21st-day exposure to 6 degree head-down tilt (HDT) bed rest, simulating microgravity, and 9.6 degree head-up tilt (HUT) bed rest with head-zero tilt (HZT) rest intervals (HUT + HZT), simulating lunar gravity, in statistically identical subgroups of five and six healthy male volunteers, was studied. In the course of HDT bed rest, a significant elongation of FETa was found in relation to background measurements in “sitting” position (p = 0.016). The effect corresponded to a significant decrease of basic spirometric indices (p < 0.02). Moreover, FETa provided reliable discrimination of HDT and HUT + HZT bed rest tests (p = 0.018), while spirometric indices did not (p > 0.05). Based on previously found correlations (Korenbaum and Pochekutova, 2008; Malaeva et al., 2017), a FETa elongation in response to HDT bed rest was attributed to an increase of aerodynamic resistance of the respiratory tract. The technique seems promising to monitor human pulmonary ventilation dynamics in long-term space missions; however, new studies are welcome to verify it in real spaceflight.

Head-up tilt rapidly compromises hemodynamics in healthy anesthetized swine.

The aims were to explore the effect of head-up tilt (HUT) to 30 and 60 degrees on hemodynamics and tissue oxygenation in anesthetized healthy swine. The data serve as a reference for a study of resuscitation efficacy at HUT such as during transport. Nine healthy swine (49+/-4 kg) were anesthetized and multiple sensors including myocardial pressure-volume loops catheter, carotid flow probe, blood pressure catheters, near infrared spectroscopy (NIRS) tissue oximetry and mixed venous oximetry (SVO2) catheter were introduced and parameters continuously recorded. Experimental protocol consisted of baseline in supine position (15 min), 30 degrees HUT (15 min), recovery at supine position (15 min) and 60 degrees HUT (5 min). Vacuum mattress was used for body fixation during tilts. We found that 30 and 60 degrees inclination led to significant immediate reduction in hemodynamic and oximetry parameters. Mean arterial pressure (mm Hg) decreased from 98 at baseline to 53 and 39, respectively. Carotid blood flow dropped to 47 % and 22 % of baseline values, end diastolic volume to 49 % and 53 % and stroke volume to 47 % and 45 % of baseline. SVO2 and tissue oximetry decreased by 17 and 21 percentage points. The values are means. In conclusions, within minutes, both 30 and 60 degrees head-up tilting is poorly tolerated in anesthetized swine. Significant differences among individual animals exist.

Evaluation of the correlation between cardiac and sympathetic baroreflex sensitivity before orthostatic syncope.

The study investigates the two different aspects of the baroreflex control resulting in two baroreflex sensitivity (BRS) indexes: i) sympathetic BRS (sBRS); ii) cardiac BRS (cBRS). sBRS was assessed as the slope of the regression line of the conditional probability of detecting a burst on the integrated muscle nerve sympathetic activity (MSNA) given an assigned diastolic arterial pressure (DAP) on DAP. cBRS was estimated from spontaneous heart period (HP) and systolic arterial pressure (SAP) via a spectral approach in the low (0.04-0-15 Hz) and high (0.15-0.5 Hz) frequency bands respectively. Both sBRS and cBRS were assessed in eight healthy subjects undergoing three experimental sessions: supine resting position (REST), 80 degrees head-up tilt test (TILT) and before the occurrence of pre-syncope symptoms (TILT_PRE). Results showed a decrease of both sBRS and cBRS during TILT and a baroreflex impairment during TILT_PRE. sBRS and cBRS were linearly correlated during TILT but became uncorrelated during TILT_PRE. Findings suggest a failure of both "baroreflexes" and their disassociation during TILT_PRE.

Changes of Pre Ejection Period and Left Ventricular Ejection Time during Head up Tilt

Objective of the Study : Evaluate changes in Pre ejection period (PEP) and left ventricular ejection time (LVET) during head up tilt (HUT). Methods: Twenty healthy male subjects were involved in this study, with mean age 29.3±5 years, mean body mass index (BMI) 21.3±0.2Kg/m 2 . Measurement of PEP, LVET of Doppler wave form of the aortic flow were done at supine, 30 and 60 degree HUT. Measurement of HR and BP were done at these positions of tilting. Comparison of changes of these variables at different degrees of HUT was done by paired T-Test. Results: PEP values were significantly higher in 60 degree and 30 degree HUT than PEP values at supine position (p<0.0001). PEP values at 60 degree HUT were significantly higher than PEP values at 30 degree HUT (P=0.05). LVET values were significantly lower at 60 degree and 30 degree HUT than values at supine position (p<0.001), and LVET values were significantly lower at 60 degree HUT than values at 30 degree HUT (p<0.001). Conclusion: Key findings of PEP and LVET during HUT are progressive prolongation of PEP and shortening of LVET with increasing head up tilting.

Simulations of gravitational stress on normovolemic and hypovolemic men and women.

Earth-based simulations of physiologic responses to space mission activities are needed to develop prospective countermeasures. To determine whether upright lower body positive pressure (LBPP) provides a suitable space mission simulation, we investigated the cardiovascular responses of normovolemic and hypovolemic men and women to supine and orthostatic stress induced by head-up tilt (HUT) and upright LBPP, representing standing in lunar, Martian, and Earth gravities. Six men and six women were tested in normovolemic and hypovolemic (furosemide, intravenous, 0.5 mg x kg(-1)) conditions. Continuous electrocardiogram, blood pressure, segmental bioimpedance, and stroke volume (echocardiography) were recorded supine and at lunar, Martian, and Earth gravities (10 degrees, 20 degrees, and 80 degrees HUT vs. 20%, 40%, and 100% bodyweight upright LBPP), respectively. Cardiovascular responses were assessed from mean values, spectral powers, and spontaneous baroreflex parameters. Hypovolemia reduced plasma volume by approximately 10% and stroke volume by approximately 25% at supine, and increasing orthostatic stress resulted in further reductions. Upright LBPP induced more plasma volume losses at simulated lunar and Martian gravities compared with HUT, while both techniques induced comparable central hypovolemia at each stress. Cardiovascular responses to orthostatic stress were comparable between HUT and upright LBPP in both normovolemic and hypovolemic conditions; however, hypovolemic blood pressure was greater during standing at 100% bodyweight compared to 80 degree HUT due to a greater increase of total peripheral resistance. The comparable cardiovascular response to HUT and upright LBPP support the use of upright LBPP as a potential model to simulate activity in lunar and Martian gravities.

Cardiovascular models of simulated moon and mars gravities: head-up tilt vs. lower body unweighting.

In this study we compare two models [head-up tilt (HUT) vs. body unweighting using lower body positive pressure (LBPP)] to simulate Moon, Mars, and Earth gravities. A literature search did not reveal any comparisons of this type performed previously. We hypothesized that segmental fluid volume shifts (thorax, abdomen, upper and lower leg), cardiac output, and blood pressure (BP), heart rate (HR), and total peripheral resistance to standing would be similar in the LBPP and HUT models. There were 21 subjects who were studied while supine (simulation of spaceflight) and standing at 100% (Earth), 40% (Mars), and 20% (Moon) bodyweight produced by LBPP in Alter-G and while supine and tilted at 80 degrees, 20 degrees, and 10 degrees HUT (analogues of Earth, Mars, and Moon gravities, respectively). Compared to supine, fluid shifts from the chest to the abdomen, increases in HR, and decreases in stroke volume were greater at 100% bodyweight than at reduced weights in response to both LBPP and HUT. Differences between the two models were found for systolic BP, diastolic BP, mean arterial BP, stroke volume, total peripheral resistance, and thorax and abdomen impedances, while HR, cardiac output, and upper and lower leg impedances were similar. Bodyweight unloading via both LBPP and HUT resulted in cardiovascular changes similar to those anticipated in actual reduced gravity environments. The LBPP model/Alter-G has the advantage of providing an environment that allows dynamic activity at reduced bodyweight; however, the significant increase in blood pressures in the Alter-GC may favor the HUT model.

Recurrent orthostatic syncope due to left atrial and left ventricular collapse after a continuous-flow left ventricular assist device implantation

Left ventricular assist devices (LVADs) have become an established treatment for patients with advanced heart failure as a bridge to transplantation or for permanent support as an alternative to heart transplantation. Continuous-flow LVADs have been shown to improve outcomes, including survival, and reduce device failure compared with pulsatile devices. Although LVADs have been shown to be a good option for patients with end-stage heart failure, unanticipated complications may occur. We describe dynamic left atrial and left ventricular chamber collapse related to postural changes in a patient with a recent continuous-flow LVAD implantation.

Influence of Posture on the Regulation of Cerebral Perfusion

Posture has a major influence on cerebral blood flow (CBF). Unlike head-up tilt (HUT), less is known about how CBF is regulated during head-down tilt (HDT). We hypothesized that CBF would be elevated during HDT and decreased during HUT. In 21 healthy young adults, while controlling for end-tidal Pco2, we combined concurrent measurements of middle cerebral artery velocity and posterior cerebral artery velocity (MCAv and PCAv, respectively), blood pressure (BP), and heart rate (HR). Measures were made at rest and, in a randomized order, during -90 degrees HDT and +900 HUT. Dynamic cerebral autoregulation was quantified using transfer function analysis. In a subgroup, volumetric blood flow recordings were obtained in the common carotid artery (CCA; N=11), internal and external carotid arteries (ICA; N=8 and ECA; N=6), and vertebral artery (VA; N=4). End-tidal Pco2, CCA, ICA, VA, MCAv(mean) and PCAv(mean) remained unchanged during -90 degrees HDT and +90 degrees HUT compared to supine. During -90 degrees HDT, mean BP (+22 mmHg) and cerebral vascular resistance (CVR) in both the MCA and PCA were elevated relative to supine, whereas HR remained unchanged. During +900 HUT, when compared to supine, HR increased (+18 bpm), and mean arterial pressure (MAP) total power and low frequency (LF) power in the MCA and PCA increased. In both the very low frequency (VLF) and LF ranges, coherence during +90 degrees HUT increased (P < 0.05 vs. supine) in both the MCA and PCA. In contrast, coherence was reduced during -90 degrees HDT. Despite marked changes in perfusion pressure with HUT or HDT, our findings indicate that cerebral perfusion is well maintained during acute severe changes in posture.

3,4 Diaminopyridine - A Promising Novel Treatment Approach to Orthostatic Hypotension (P05.204)

Objective: To prove the hypothesis that 3,4-diaminopyridine (3,4-DAP) effectively improves blood pressure (BP) and symptoms in orthostatic hypotension (OH) with predominant effects in the upright position. Background Treatment options for OH are limited and often associated with aggravation of supine hypertension. We could recently demonstrate that acetylcholinesterase inhibition is efficacious in OH without worsening of supine hypertension, but with modest efficacy. We now hypothesized that the potassium channel blocker 3,4-DAP has similar but greater beneficial effects in patients with OH, as it should not only enhance adrenergic ganglionic neurotransmission but also neurotransmission at the adrenergic endorgan synapse, resulting in serially linked and therefore potentiated effects. Design/Methods: We studied 11 patients with neurogenic OH in a placebo-controlled patient-blinded protocol. Following a 20 minute period of supine rest, BP and heart rate (HR) were continuously monitored for 10 minutes in the supine position, followed by 5 minutes of 70 degree head-up tilt, 1 hour following administration of placebo and 1 hour after 20mg of 3,4-DAP. Cardiovascular parameters, orthostatic symptoms, QTc interval, and side effects were compared between treatment status in this pilot trial. Results: Orthostatic BP was significantly improved after administration of 3,4-DAP compared to placebo (systolic BP 111.9±6.2 vs. 88.7±6.3mmHg, diastolic BP 77.9±3.8 vs. 63.3±3.8, both p Conclusions: 3,4-DAP is safe and well tolerated, and dramatically improves OH and orthostatic symptoms. The improvement is much more pronounced than that seen with pyridostigmine, consistent with the hypothesis of potentiated, serially linked effects. A large double-blind, placebo-controlled trial is now planned to further confirm these findings. Supported by: Ronald F. Kinney Executive Dean for Research Career Development Award, NIH (BIRCWH 5K12HD065987-02 and NS 44233), and Mayo CTSA (RR00585). Disclosure: Dr. Singer has nothing to disclose. Dr. Gehrking has nothing to disclose. Dr. Gehrking has nothing to disclose. Dr. Sletten has nothing to disclose. Dr. Figueroa has nothing to disclose. Dr. Sandroni has nothing to disclose. Dr. Low has received personal compensation for activities with Chelsea, Pfizer, and WR Medical Inc. as a consultant.Dr. Low has received personal compensation in an editorial capacity for Autonomic Neuroscience.

Temporal Artery Doppler Spectrum Morphology Responses to Tilt and LBNP as an Early Indicator of Syncope

In a study to identify an early hemodynamic predictor of syncope, 12 men (25-40 yr) underwent 30 min of 80 degrees head-up tilt, followed by progressive lower body negative pressure (LBNP) until presyncope. Temporal (supplying extracranial tissues: TEMP), middle cerebral (MCA), and superficial femoral (FEM) arterial flow velocity (V) and vascular resistance indices (VR) were evaluated continuously using Doppler ultrasound. Ratios of the Doppler V(MEAN) (V(MCA)/V(FEM) or V(MCA)/ V(TEMP)) were used to assess flow redistribution between these areas. The progression of the testing protocol showed increases in vascular resistance in all territories. At presyncope, both MCA(VR) and FEM(VR) were reduced while there was a large increase in TEMP(VR). Vasoconstriction of the vascular bed supplied by the temporal artery occurred early during central hypovolemia resulting in the appearance of negative velocity deflections, which could be used for the early detection of impending syncope. Analysis of the velocity ratios showed little change until the onset of presyncope where there was an increase in V(MCA)/V(TEMP) which confirmed that vasoconstriction of the vascular bed supplied by the TEMP artery contributed to cardiac output redistribution in favor of the brain, and a reduction in V(MCA)/V(FEM) suggesting a redistribution of cardiac output toward the legs. In 67% of the tests, the appearance of the negative component of V(TEMP) was an early sign of increasing TEMP(VR) that occurred before visually detectable changes in VE(FEM) or V(MCA) and within 5 min before presyncope. Such easily identifiable in real time Doppler signs allowed experimenters to anticipate test termination.

Changes in cerebral hemodynamics in different body positions in healthy volunteers

Objective To investigate the changes in cerebral hemodynamics in different body positions in healthy volunteers.Methods Sixty right-handed healthy male volunteers,aged 22-26,height 167-178 cm,weighing 51-67 kg,were randomly divided into 4 groups ( n =15 each): 15 degrees head-down tilt group (group Ⅰ ),30 degrees head-down tilt group (group Ⅱ ),15 degrees head-up tilt group (group Ⅲ) and 30 degrees head-up tilt group (group Ⅳ ).Blood flow signals of right middle cerebral artery (MCA) and extracranial internal carotid artery (EICA) were detected by transcranial Doppler and systolic blood flow velocity (Vs),diastolic blood flow velocity (Vd ),mean blood flow velocity (Vm),pulsatility index (PI) and resistance index (RI) were recorded at supine position (baseline),immediately,10 and 30 min after body position change(T1-3 ).Lindegaard index was calculated.Results Compared with the baseline,in group [Ⅱ Lindegaard index was increased at T2,while Vs and Vm of MCA were decreased at T2,3,in group Ⅳ Vs of MCA and PI of EICA at T2,Vd and Vm of MCA at T2,3 were decreased ( P ＜ 0.05),while there was no significant difference in the variables mentioned above in the other two groups ( P ＞ 0.05 ).Conclusion In healthy adults,cerebral blood flow velocity decreases in 30 degrees headdown and head-up tilt positions,however there is no change in cerebral blood flow velocity in 15 degrees headdown and head-up tilt positions. Key words: Posture ; Hemodynamics ; Brain

Arterial wave reflection decreases gradually from supine to upright

Background. An increase in total peripheral resistance (TPR) usually increases arterial wave reflection. During passive head-up tilt (HUT), however, arterial wave reflection decreases with increasing TPR. This study addressed whether arterial wave reflection gradually decreases during HUT. Methods. In 10 healthy volunteers (22–39 years, nine males), we recorded finger arterial pressures in supine position (0°), and 30° and 70° degrees HUT and active standing (90°). Aortic pressure was constructed from the finger pressure signal and hemodynamics were calculated. Arterial wave reflection was quantified as the augmentation index (AIx) and the reflection magnitude (RM). Results. During HUT, heart rate increased (p < 0.001), stroke volume and cardiac output decreased (p < 0.001 and p < 0.01), diastolic blood pressure increased (p < 0.001), whereas systolic blood pressure did not change. TPR increased from 0.9 dyn s/cm5 at 0° to 1.2, 1.4 and 1.4 dyn s/cm5 at 30°, 70° and 90° (p < 0.001). AIx fell gradually from 25% at 0° to 16%, −1% and −10% at 30°, 70° and 90° (p < 0.001). The RM decreased from 0.572 at 0° to 0.456 at 90° (p < 0.001). Conclusion. From supine to upright, arterial wave reflection represented as AIx and RM gradually decreases in the presence of increasing TPR.

Gradient Compression Garments as a Countermeasure to Post-Spaceflight Orthostatic Intolerance

Post-spaceflight orthostatic intolerance affects approximately 30% of short-duration and 80% of long-duration crewmembers. While the current NASA antigravity suit is effective during Space Shuttle re-entry, it is not designed to be worn postflight and has several drawbacks. The purpose of this study was to evaluate the use of commercially available, thigh-high, gradient compression garments to prevent post-spaceflight orthostatic intolerance. Before spaceflight, five male Shuttle astronauts were fitted for compression garments. Postflight stand time, blood pressure, heart rate, stroke volume, cardiac output, and peripheral resistance during 10-min, 80 degrees head-up tilt test within 4 h of landing in these astronauts were retrospectively compared to a group of nine male astronauts not wearing the compression garments. On landing day, three of nine non-countermeasure astronauts developed presyncopal symptoms and could not complete the test, while no countermeasure subjects became presyncopal. Compared to the non-countermeasure subjects, the countermeasure subjects had higher systolic blood pressure (116 +/- 3 vs. 134 +/- 2 mmHg), stroke volume (42 +/- 5 vs. 57 +/- 6 ml), and cardiac output (3.1 +/- 0.3 vs. 4.6 +/- 0.4 L). Heart rate was not different between groups. In this small pilot study, the rate of presyncope in the non-countermeasure group was similar to that reported previously in subjects without a compression garment. In contrast, thigh-high graded compression garments mitigated the symptoms of orthostatic intolerance by improving stroke volume, cardiac output, and systolic blood pressure responses to standing.

Cardiovascular and Thermal Responses to Repeated Head-Up Tilts Following Exercise-Induced Heat Stress

We evaluated the acute cardiovascular and thermoregulatory responses to repeated 70 degrees head-up tilts (HUT) performed following exercise-induced hyperthermia. Eight male subjects underwent intermittent episodes of 70 degrees HUT in either a non-exercise/ non-heat stress state (NH) or an exercise-induced hyperthermic state (EIH). Subjects remained supine for 30 min in a thermoneutral environment (22 degrees C) and were subsequently exposed to three successive 6-min 70 degrees head-up tilts (HUT1, HUT2, HUT3), each separated by 10 min of supine resting. During EIH, subjects were rendered hyperthermic by exercise in the heat (core temperature of approximately 40.0 degrees C) and were then transferred to an ambient temperature of 22 degrees C. We evaluated the relative change in hemodynamic and thermal responses from the last minute in the supine position preceding the HUT to the final minute in HUT. While we measured a difference in the relative change in heart rate between conditions for all HUTs, no differences were observed in mean arterial pressure (MAP), total peripheral resistance, or cardiac output. A reduced change in baroreceptor sensitivity was measured in EIH for HUT1 only (-2 +/- 1 ms x mmHg(-1) following EIH compared to -13 +/- 3 ms x mmHg(-1) during NH). A significant transient reduction in cutaneous vascular conductance (CVC) occurred during HUT1 and HUT2 following EIH (-20 +/- 5% CVCmax and -9 +/- 3 %CVCmax, respectively), despite significant elevations in core temperature above resting levels (i.e., 1.4 degrees C and 0.9 degrees C for HUT1 and HUT2). We conclude that the maintenance of MAP following exercise in the heat is mitigated by reductions in skin perfusion despite significant elevations in core temperature.

Laparoscopic colecistectomy: anestesthetic implications in the elderly patient

BackgroundThe gallbladder stone is 20-40% of geriatric diseases. Inelderly patients with limited cardiac reserve the pneu-moperitoneum determines dangerous alterations of thecardiac output and other hemodynamic variables. Thefrequent co-morbidity in elderly people leads to a severeprognosis in these patients with surgical high mortalityand morbidity rates in open surgery (1) otherwise thelaparoscopic procedure is followed by lower mortalityand morbidity rates (0-0.6% vs 1.4%). (2,3,4). The gra-dual abdominal insufflation to 12 mmHg followed by alimited 10 degree head-up tilt is associated with cardio-vascular stability in elderly ASA III patients (5).The aim of our study was to analyze hemodynamicmodifications during surgical treatment by laparoscopiccolecistectomy and the anesthetic implications.Materials and methods121 patients aged over 65 years old were analyzed whounderwent laparoscopic cholecistectomy . The operationwas conducted with a low pressure of CO2 (8-10mmHg).We observed the hemodynamic trend duringthe anesthesia: blood pressure, heart rate, respiratoryrate, Tidal Volume and end Tidal C0

Identical pattern of cerebral hypoperfusion during different types of syncope

Syncope is caused by insufficient oxygen supply to the brain. There have been attempts to classify syncope on the basis of defects in the venous system, arterial system (that is impaired systemic vascular resistance) or a combination of the two (that is mixed). We examined the hypothesis that a comparable decrease in cerebral perfusion would be evident at pre-syncope irrespective of the category of dysfunction. Young healthy volunteers (N=37) participated. The protocol consisted of 15 min supine rest, followed by 60 degrees head-up tilt and lower body suction in increments of -10 mm Hg for 5 min each until pre-syncope. Beat-to-beat blood pressure (BP) (Finometer or intra-arterial), cardiac output (Finometer), middle cerebral artery blood velocity (MCAv), end-tidal CO(2) and cerebral oxygenation were monitored continuously. At pre-syncope, mixed dysfunction was common (21 out of 37 participants), followed by venular dysfunction (15 out of 37 participants). In the venular and mixed groups, comparable orthostatic tolerance and declines in BP (-37 vs -43% from baseline, respectively), end-tidal PCO(2), MCAv (-35 vs -38%) and cerebral oxygenation (-5 vs -7%) were evident despite distinct mechanisms purportedly being responsible for the hypotension. Although different determinants of hypotension do exist, cerebral hypoperfusion occurs to a similar extent.

Arterial baroreflex function in older adults with neurocardiogenic syncope

Neurocardiogenic syncope (formerly vasovagal) accounts for large numbers of falls in older adults and the mechanisms are poorly understood. This study examined the differences in baseline arterial baroreflex function in older adults with and without a neruocardiovascular response to orthostatic stress. Subjects were divided into two groups based on the presence (TT+ group) or absence (TT- group) of a neurocardiovascular response to upright tilting (70 degree head-up tilt for 10 minutes after 400 micrograms of sublingual nitroglycerin). A neurocardiovascular response was defined as presyncopal symptoms (lightheadedness) in association with at least a 30 mm Hg decrease in blood pressure. Before being divided into groups, baroreflex function was assessed using the spontaneous baroreflex method (baroreflex sensitivity, BRS). This method involves the analysis of "spontaneous" swings in blood pressure and heart rate that are mediated by the arterial baroreflexes. 42 older adults (mean age 70.3+/-0.7 yr) were recruited, of which 18 were in the TT+ and 24 were in the TT- group. At baseline, the TT+ group demonstrated increased arterial baroreflex sensitivity in response to negative blood pressure sequences only (BRSdown, 11.2+/-1.9 vs. 7.3+/-1.0 ms/mm Hg, P=0.011). During tilt, the TT+ group demonstrated a much larger decrease in overall arterial baroreflex sensitivity than the TT- group (-6.8+/-1.2 vs. -3.2+/-0.9 ms/mm Hg, P=0.012). There was a negative correlation between BRSdown and length of tilt table test (r=-0.329, P=0.041) in the TT+ subjects. Older adults with neurocardiogenic syncope have exaggerated arterial baroreflex sensitivity at baseline.

Compression Garments as Countermeasures to Orthostatic Intolerance

All astronauts experience some degree of orthostatic intolerance following spaceflight, ranging from tachycardia to orthostatic hypotension and syncope. The purpose of this study was to evaluate the ability of two compression garments, the National Aeronautics and Space Administration's inflatable antigravity suit (AGS) and the Russian Federal Space Agency's non-inflatable compression garment (Kentavr), to prevent hypovolemia-related orthostatic intolerance. To mimic the plasma volume loss experienced by astronauts during spaceflight 19 healthy subjects received an intravenous dose of a diuretic, furosemide (0.5 mg x kg(-1)), and then consumed a low-salt diet for 36 h. Thereafter, subjects participated in a 15-min 80 degrees head-up tilt test wearing either the AGS (N = 9) or Kentavr (N = 10). Compression garments were used in the fashion recommended by the respective agencies, delivering approximately 78 mmHg and approximately 30 mmHg of compression in the AGS and Kentavr, respectively. Incidence of presyncope and hemodynamic responses during upright tilt were compared to a separate group of hypovolemic control subjects (N = 16). Subjects wearing the AGS or Kentavr completed the full 15 min of upright tilt without incidence of orthostatic hypotension or presyncope. In contrast, only 9 control subjects (56%) were able to complete the tilt test. In addition, both types of compression garments maintained systolic blood pressure and significantly reduced tilt-induced tachycardia and reductions in stroke volume. Although both garments successfully countered hypovolemia-induced orthostatic intolerance, the Kentavr provided protection by using lower levels of compression. Determining the optimal compression level required for protection of intolerance may improve crewmember comfort and decrease restrictions on physical activities after spaceflight.