Haemodynamic responses to varying postural stress

September 11, 2017 | Autor: Al Rony | Categoria: Human Body, Impedance cardiography, Cardiac Output Monitoring, Ejection Fraction
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Hemodynamic Responses to Varying Postural Stress R. Paz, B.S., S.AI-Dhaheri M.S., H. H. Sun, Ph.D., L. Hrebien, Ph.D. Biomedical Engineering and Science Institute Drexel University, Philadelphia, PA 19104

Abstract. The purpose of this study is to present data demonstrating early cardiovascular responses t o simulated gravity levels between 1G and -1G. A noninvasive, cardiac impedance cardiography system (The IQ System) was used to evaluate cardiac output (CO), stroke volume ( S V ) , ventricular ejection time (VET), ejection fraction ( E F ) , and pre-ejection period (PEP). Seven subjects were positioned at selected angles of +go', +45', O', -45', b -90" for five minutes at each angle. These postural changes simulate different gravitational forces on the human body ranging from +ZG to -1G. This study's findings demonstrate an overall increasing trend in CO, SV, VET and EF, and a slight decrease in PEP with postural changes from +90' t o -90'. Findings may aid in understanding cardiovascular responses to low and negative gravity environments. Introduction Postural stress has been widely used as a tool for studying gravitational effects on human hemodynamics. Postural exposures in the head-up tilt (HUT) and headdown tilt (HDT) positions bring about changes in circulation. There is a lack of information on these changes primarily due to the associated methodological problems of direct measurement. The purpose of this study is to present data 'demonstrating early cardiovascular responses to simulated gravity levels between +1G and -lG. The parameters being evaluated are cardiac output, stroke volume, ventricular ejection time, ejection fraction, and pre-ejection period. All of which are measured by a newly designed non-invasive impedance cardiograph, the IQ Impedance Cardiograph. This study's goals are to better the understanding of cardiovascular responses to low and negative gravity environments. The results may serve as comparators of the processes responsible for circulatory homeostasis both on earth at +1G and in OG. These results may also serve as a comparison for positive pressure breathing and lower body positive pressure inflation studies [l]. Finally, this paper suggests a new application for the impedance cardiograph method as an excellent noninvasive technique for demonstrating hemodynamic trends [21. The IQ Impedance Cardiograph may prove to be a possible solution to the methodological problems associated with the measurement of hemodynamic parameters in healthy subjects.

Technique In order to measure the hemodynamic changes, a non-invasive, cardiac impedance technique was used. In this study, the IQ Impedance Cardiograph, developed at Drexel University and approved by the F.D.A. in 1993, was employed to measure the selected parameters [3,4]. It is touch screen driven and virtually operator independent. The IQ ICG uses sophisticated digital signal processing of the time-frequency transform to reduce the noise and to identify the parameters for the measurement of the cardiac functions [5,6]. This signal processing technique is known as the spectrogram method. Fourteen hemodynamic parameters including cardiac output, stroke volume, ventricular ejection time, pre-ejection period and ejection fraction were updated automatically by the IQ Impedance Cardiograph and recorded to disk for later analysis. The criteria for updating the parameters is based on ten consecutively clean impedance waveforms [5,61. The IQ ICG also displayed the subject's EKG continuously d u r i n g the experiment, allowing monitoring of cardiac rhythms. Methods

Subjects Seven healthy subjects from a student community took part in this experiment. The subjects' weights and ages ranged from 125-190 lbs. and 22-28 yrs. old, respectively. The mean weight was 167.5 lbs. and mean age was 25 yrs. old, respectively. The subject's had similar levels of activity. Before taking part in the experiment the subjects were fully informed of the protocol and precautions. Any concerns were encouraged to be expressed before performing the experiment in order to remove any anxieties which may effectively bias the results. The subjects were also given dietary and clothing instructions for the day of the experiment. Electrode Placement The electrode placement protocol used was the spot electrode system for Impedance Cardiography. The sites were prepared by removing body oils with alcohol wipes. Lead-LokTM gel, spot electrodes were used. A total of 11 electrodes were placed, three of which were EKG leads, and eight of which were the ICG leads. The first electrode pair, labeled as Pair 2 in Figure 1, was located on the side of the neck's base above the clavicle, at the intersection of the shoulder and the side of the neck. The second electrode pair, Pair 1, was placed 5 cm above the first pair.

0-7803-1930-3/94/$3.00 0 1994 IEEE

The third pair, shown as Pair 3, was placed on the sides of the thorax, level to the xyphoid process. The fourth electrode pair was placed 5 cm below Pair 3. Electrode symmetry was inspected and ensured for reproducibility. Impedance was measured between electrode pairs 2 and 3 where an input current source was connected between electrode pairs 1 and 4 [6].

Results This study's findings, shown in Figure 3., demonstrate an overall trend for four of the evaluated parameters to increase in the prescribed range. This was expected due to the resulting increase in pre-load. Stroke volume demonstrated a virtually linear increase from the +90' position to the -90' position, more than doubling its baseline value. Its corresponding percentage Cardiac output, increase was 119.8% (P
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