Peter Gauger

Cardiovascular parameters and neural sympathetic discharge variability before orthostatic syncope: role of sympathetic baroreflex control to the vessels

We tested the hypothesis that altered sympathetic baroreceptor control to the vessels (svBRS) and disrupted coupling between blood pressure (BP) fluctuations and muscle sympathetic activity (MSNA) discharge pattern in the low frequency band (LF, around 0.1 Hz) precede vasovagal syncope. Seven healthy males underwent ECG, BP, respiratory, and MSNA recordings at baseline (REST) and during a 15 min 80° head-up tilt, followed by a -10 mmHg step wise increase of lower body negative pressure up to presyncope. Spectral and coherence analyses of systolic arterial pressure (SAP) and MSNA variability provided the indexes of vascular sympathetic modulation, LFSAP, and of the linear coupling between MSNA and SAP in the low frequency band (around 0.1 Hz), K(2)MSNA-SAP(LF). svBRS was assessed as the slope of the regression line between MSNA and diastolic arterial pressure (DAP). Data were analyzed at REST, during asymptomatic and presyncope periods of tilt. svBRS declined during presyncope period compared to REST and asymptomatic tilt. The presyncope period was characterized by a decrease of RR interval, LFMSNA, LFSAP, and K(2)MSNA-SAP(LF) values compared to the asymptomatic one, whereas MSNA burst rate was unchanged. The reduction of svBRS producing an altered coupling between MSNA and SAP variability at 0.1 Hz, may provoke circulatory changes leading to presyncope.

Upright cardiac output measurements in the transition to weightlessness during parabolic flights.

OBJECTIVE Aims of this study were: 1) to determine cardiac output by inert gas rebreathing (CO(reb)) during transition into 0 Gz in the standing position; and 2) to compare impedance cardiography (ICG) and pulse contour method (PCM) with CO(reb) as a reference method. METHODS We measured baseline CO(reb) and heart rate (HR) on the ground, and CO(reb), CO(pcm), CO(icg), and HR in standing and supine positions in the transition to weightlessness in six subjects. We conducted repeated measures ANOVA, Bland and Altman analysis, and analysis of percentage error of each data set. RESULTS CO(reb) rose from 5.03 +/- 0.7 upright ground control to 11.45 +/- 3.6 L x min(-1) in 0 Gz. HR and stroke volume (SV) rose from 83 +/- 14 to 113 +/- 19 bpm and from 61 +/- 6 to 99 +/- 18 ml, respectively. Mean CO(reb), CO(pcm), and CO(icg) across all conditions were 10.45 +/- 3.04, 7.42 +/- 1.71, and 6.57 +/- 2.46 L x min(-1), respectively. Overall Bland and Altman analysis showed poor agreement for CO(pcm) and CO(icg) compared to CO(reb). DISCUSSION Large bias for both comparisons indicated that both PCM and ICG underestimate the true CO value. Paired CO values of individual subjects showed a better correlation between methods and a broad bias range, indicating a preponderant role for large between-subjects variability. Repeated CO(reb) determinations in 1 Cz (i.e., when the cardiovascular system is in a steady state) should be used for calibration of the PCM and of ICG data. PCM and ICG can then be used to track CO dynamics during rapid changes of acceleration profiles.

Influence of bed rest on plasma galanin and adrenomedullin at presyncope

The role of hormones in reduced orthostatic tolerance following long‐term immobilization remains uncertain. We have previously shown that plasma concentrations of adrenomedullin and galanin, two peptides with vasodepressor properties, rise significantly during orthostatic challenge. We tested the hypothesis that bedrest immobilization increases the rise in adrenomedullin and galanin during orthostatic challenge leading to presyncope.

Timing and source of the maximum of the transthoracic impedance cardiogram (dZ/dt) in relation to the H-I-J complex of the longitudinal ballistocardiogram under gravity and microgravity conditions

The transthoracic impedance cardiogram (ICG) and the acceleration ballistocardiogram (BCG) measured close to the center of mass of the human body are generated by changes of blood distribution. The transthoracic ICG is an integrated signal covering the whole thorax and spatial resolution is poor. Comparison between both signals can be used to elucidate the source of the ICG signal. We recorded the ECG, ICG, and BCG simultaneously in healthy subjects under resting and microgravity conditions during parabolic flights. The time interval between the R-peak of the ECG and the maximum of the ICG (R-dZ/dtmax) and the time interval between the R-peak of the ECG and the I-peak in the BCG (R-I) differed significantly (p<;0.0001). The I-peak in the BCG always occurred earlier during systole than dZ/dtmax. The delay of dZ/dtmax ranged between 23 and 28 ms at rest and was lowest under microgravity conditions (12±4 ms, p<;0.02). Our results suggest that both signals have different sources. Combination of modern imaging techniques with classical non invasive approaches to detect changes of blood distribution may provide new insights into the complex interaction between blood flow and mechanocardiographic signals like the BCG.

Pulse contour methods to estimate cardiovascular indices in micro- and hypergravity.

BACKGROUND The importance of noninvasive health monitoring in space increased as a result of the long-duration missions on the International Space Station (ISS). In order to monitor changes in cardiovascular indices such as cardiac output (CO) and total peripheral resistance (TPR), many methods have been developed using signal processing and mathematical modeling techniques. However, their performance in various gravitational conditions has not been known. METHODS The present study compared 10 methods to estimate CO and TPR by processing peripheral arterial blood pressure signals recorded from 8 subjects in multiple gravity levels (1 G, 0 G, and 1.8 G) during parabolic flights. For reference data sets, CO and TPR were simultaneously obtained by an inert gas rebreathing technique. Root normalized mean square errors and Bland-Altman plots were used to evaluate the estimation methods. RESULTS The corrected impedance method achieved the lowest estimation errors (20.0% CO error and 23.5% TPR error) over the three gravity levels. In microgravity, mean arterial pressure was also demonstrated to be an indicator of CO (24.5% error). DISCUSSION The corrected impedance method achieved low estimation errors for a wide range of the gravity levels. Gravity-dependent performance was observed in the mean arterial pressure method that achieved low errors in the short-term 0 G.

Changes in the linear relationship between cardiovascular parameters and neural sympathetic discharge variability before orthostatic syncope

To evaluate the cardiovascular parameters spontaneous fluctuations and neural sympathetic discharge variability relationship during the gravitational stimulus, seven healthy males underwent ECG, blood pressure, respiratory and post-ganglionic sympathetic discharge activity (MSNA) recordings. Data were recorded at rest and during a 15-minute 80°head-up tilt followed by a step wise lower body negative pressure increase up to the onset of presyncope. Spectral and coherence analyses of systolic arterial pressure (SAP) and MSNA variability provided the indexes of vessels sympathetic modulation in the low frequency (LF) band, LFSAP, and of the coupling between MSNA and SAP at LF, K2MSNA-SAP(LF), during asymptomatic (T1) and presyncope (T2) periods of tilt. During T2, RR interval, SAP, LFmsna, LFSAP and K2MSNA-SAP(LF) values decreased compared to T1. in the presence of unchanged MSNA values. This result suggests that a significant coupling between SAP and MSNA variability in the 0.1 Hz band is mandatory for a proper orthostatic tolerance.