Backrest angle and cardiac output measurement in critically ill patients.

Abstract

Cardiac output is an extremely important measurement in the care of critically ill patients, but the accuracy of measurement is unknown when patients are in positions other than flat and supine. The purpose of this study was to compare the effects of varying degrees of backrest elevation on continuous cardiac output measurements in critically ill patients at head-of-bed angle of 0 degrees, 30 degrees, and 45 degrees, and at time points of 0 minutes, 5 minutes, and 10 minutes after each position change. A within participants design using a convenience sample (N = 26). Data were collected in a 24-bed adult Medical/Surgical/Trauma Intensive Care Unit. A continuous cardiac output catheter was used for all continuous cardiac measurements and continuous cardiac output values were indexed to continuous cardiac index values. Four repeated measures analyses of variance (ANOVA) were run, one for each dependent variable (continuous cardiac index, stroke volume, heart rate, and mean arterial pressure). There were two within participant factors with three levels each (time and head-of-bed angle). The results indicated no overall significant differences in continuous cardiac index values at the various head-of-bed angle and time points (p =.715). In addition, no significant differences were found for stroke volume (p =.614), heart rate (p =.289) or mean arterial pressure (p =.246). No differences in the continuous cardiac index values across the nine different measurement conditions were found. An examination of the determinants of cardiac output (stroke volume and heart rate) indicated that the lack of change in continuous cardiac index was not a result of a compensatory change in either stroke volume or heart rate. These data indicate that in daily clinical practice with critical medical surgical patients it may be unnecessary to reposition patients solely for the purpose of obtaining continuous cardiac index measurements. The measurements appear to be reproducible at head-of-bed angle up to 45 degrees.