The effect of head up tilting on bioreactance cardiac output and stroke volume readings using suprasternal transcutaneous Doppler as a control in healthy young adults.

Abstract

To compare the performance of a bioreactance cardiac output (CO) monitor (NICOM) and transcutaneous Doppler (USCOM) during head up tilting (HUT). Healthy young adult subjects, age 22±1years, 7 male and 7 female, were tilted over 3-5s from supine to 70° HUT, 30° HUT and back to supine. Positions were held for 3min. Simultaneous readings of NICOM and USCOM were performed 30s into each new position. Mean blood pressure (MBP), heart rate (HR), CO and stroke volume (SV), and thoracic fluid content (TFC) were recorded. Bland-Altman, percentage changes and analysis of variance for repeated measures were used for statistical analysis. Pre-tilt NICOM CO and SV readings (6.1±1.0L/min and 113±25ml) were higher than those from USCOM (4.1±0.6L/min and 77±9ml) (P<0.001). Bland-Altman limits of agreement for CO were wide with a percentage error of 38%. HUT increased MBP and HR (P<0.001). CO and SV readings decreased with HUT. However, the percentage changes in USCOM and NICOM readings did not concur (P<0.001). Whereas USCOM provided gravitational effect proportional changes in SV readings of 23±15% (30° half tilt) and 44±11% (70° near full tilt), NICOM changes did not being 28±10 and 33±11%. TFC decreased linearly with HUT. The NICOM does not provide linear changes in SV as predicted by physiology when patients are tilted. Furthermore there is a lack of agreement with USCOM measurements at baseline and during tilting.