Response of Ventricular Repolarization to Simulated Microgravity Measured by Periodic Repolarization Dynamics Using Phase-Rectified Signal Averaging

Head-Down Bed Rest (HDBR) microgravity simulation induces cardiovascular deconditioning, including effects on ventricular repolarization. The index of Periodic Repolarization Dynamics (PRD) was developed to quantify low-frequency oscillations of cardiac repolarization. In this study, PRD was quantified by Phase Rectified Signal Averaging (PRDPRSA) and Continuous Wavelet Transform (PRDCWT) methods. PRD was analyzed in ECGs from 22 volunteers at rest and during orthostatic Tilt-Table Test (TTT) performed before and after -6° 60-days HDBR. Significant correlation was found between PRD measured by PRSA and CWT (Pearson''s ¿ = 0.93, p < 10-54 and Kendall''s t = 0.79 p < 10-38). A highly significant increase was found when PRDPRSA values were measured at POST-HDBR with respect to PRE-HDBR in the tilt phase: 1.40 [1.10] deg and 0.97 [0.90] deg (median [IQR]), p = 0.008, respectively. PRDPRSA also increased significantly in the tilt phase with respect to baseline, both at POST-HDBR (0.90 [0.57] deg, p = 0.003) and at PRE-HDBR (0.75 [0.45] deg, p = 0.011). PRD, either measured with PRSA or with CWT, is able to measure changes in ventricular repolarization induced by microgravity simulation as well as following sympathetic provocation

Relationship Between Blood Pressure and Heart Rate Circadian Rhythms in Normotensive and Hypertensive Subjects

This paper focuses on the relationship between blood pressure (BP) and heart rate (HR) during 24 hours in 423 normotensive (NO) and 205 hypertensive (HE) subjects. Although considerable knowledge has been gained about BP and HR signals, their relationship over 24 hours has never been completely described. By using a Holter Blood Pressure Monitor, it was possible to record BP and HR for 24 hours. Systolic, Diastolic and Mean BP in both NO and HE subjects showed four different time intervals presenting well-defined trends The results demonstrated that changes in HR present closely parallel changes in BP with a marked reduction of both signals during nocturnal rest. On the contrary, in the period between 15:30 and 19:30, HR and BP showed an inverse relationship with decreasing heart rate and increasing blood pressure

Computational models of cardiovascular response to orthostatic stress

Thesis (Ph. D.)--Harvard-MIT Division of Health Sciences and Technology, 2004.Includes bibliographical references (p. 163-185).The cardiovascular response to changes in posture has been the focus of numerous investigations in the past. Yet despite considerable, targeted experimental effort, the mechanisms underlying orthostatic intolerance (OI) following spaceflight remain elusive. The number of hypotheses still under consideration and the lack of a single unifying theory of the pathophysiology of spaceflight-induced OI testify to the difficulty of the problem. In this investigation, we developed and validated a comprehensives lumped-parameter model of the cardiovascular system and its short-term homeostatic control mechanisms with the particular aim of simulating the short-term, transient hemodynamic response to gravitational stress. Our effort to combine model building with model analysis led us to conduct extensive sensitivity analyses and investigate inverse modeling methods to estimate physiological parameters from transient hemodynamic data. Based on current hypotheses, we simulated the system-level hemodynamic effects of changes in parameters that have been implicated in the orthostatic intolerance phenomenon. Our simulations indicate that changes in total blood volume have the biggest detrimental impact on blood pressure homeostasis in the head-up posture. If the baseline volume status is borderline hypovolemic, changes in other parameters can significantly impact the cardiovascular system's ability to maintain mean arterial pressure constant. In particular, any deleterious changes in the venous tone feedback impairs blood pressure homeostasis significantly. This result has important implications as it suggests that al-adrenergic agonists might help alleviate the orthostatic syndrome seen post-spaceflight.by Thomas Heldt.Ph.D

Microgravity Exposure Alters Sympathetic Modulation of Ventricular Repolarization Quantified from the ECG via Periodic Repolarization Dynamics

Prolonged micro gravity exposure induces cardiovascular deconditioning, including orthostatic intolerance due to dysregulation of autonomic modulation of the cardiovascular system. Recent studies suggest that sympathetic modulation of ventricular repolarization can be assessed by measuring the index of Periodic Repolarization Dynamics (PRD), which quantifies low frequency oscillations in ECG T-waves. In this study PRD was analyzed in ECGs from 22 volunteers at rest and during orthostatic Tilt-Table Test ( TTT ) performed before and after 60-day Head-Down (-6 degrees) Bed Rest (HDBR) experiments to simulate microgravity effects. A very notable increase was found in resting PRD values measured at POST-HDBR with respect to PRE-HDBR: 2.70[3.21] deg(2) vs 2.05[1.18] deg(2) (median[IQR]). When PRD was evaluated during TTT, an even more remarkable increase was found in POST-HDBR with respect to PRE-HDBR: 4.25[6.46] deg(2) vs 2.39[3.32] deg(2). A jump-based countermeasure was only able to counteract microgravity-induced effects in response to TTT but not at baseline. In conclusion, prolonged exposure to simulated microgravity induces changes in ventricular repolarization that are measurable by PRD and are more manifested when assessed following sympathetic provocation. A jump-based countermeasure is only partially effective in counteracting such effects

Life Sciences Program Tasks and Bibliography

This document includes information on all peer reviewed projects funded by the Office of Life and Microgravity Sciences and Applications, Life Sciences Division during fiscal year 1995. Additionally, this inaugural edition of the Task Book includes information for FY 1994 programs. This document will be published annually and made available to scientists in the space life sciences field both as a hard copy and as an interactive Internet web pag

Pacing with restoration of respiratory sinus arrhythmia improved cardiac contractility and the left ventricular output: a translational study

Introduction: Respiratory sinus arrhythmia (RSA) is a prognostic value for patients with heart failure and is defined as a beat-to-beat variation of the timing between the heart beats. Patients with heart failure or patients with permanent cardiac pacing might benefit from restoration of RSA. The aim of this translational, proof-of-principle study was to evaluate the effect of pacing with or without restored RSAon parameters of LV cardiac contractility and the cardiac output