Abstract
Background: 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.
Objective: 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.
Method: 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.
Results: 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).
Conclusion: 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].
Cardiac output (CO) 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. When patients are moved from supine to more upright positions, pressure on the carotid baroreceptor decreases causing a reflexive increase in the total vascular pressure. These two mechanisms work together to maintain a constant mean arterial pressure (MAP) throughout the position changes (Ogoh, Fadel, Monteiro, Wasmund, & Raven, 2002). The efficacy of these compensatory mechanisms depends on both the heart and the vascular system. The purpose of this research was to study the effect of these types of positional changes in critically ill patients on the measurement of CO.
Cardiac output is defined as the amount of blood in liters ejected from the left ventricle per minute. Cardiac output is a product of heart rate (HR) and stroke volume (SV) (the amount of blood ejected from the heart with each contraction). Stroke volume is influenced by preload (diastolic volume), afterload (the resistance the ventricle must pump against), and contractility (the ability of the heart muscle to contract and distend). While the normal range of CO is 4-8 liters/minute, it is variable over a 24 period, peaking during activity and troughing during sleep. The range of difference from peak to trough can be 0.26 to 2 liters per minute (Smolensky, 1976;Woods & Osguthorpe, 1993). Cardiac output is a primary determinant of end organ perfusion and oxygen delivery to the tissues. In the presence of critical illness, morbidity and mortality are directly related to oxygen delivery. The higher the degree of oxygen deprivation at the tissue level, the higher the likelihood of severe morbidity or mortality in critically ill patients (Riedinger & Shellock, 1984;Whalen & Kelleher, 1998).