Comparison of cardiac output monitoring methods for detecting central hypovolemia due to lower body negative pressure.

Reduction in mean arterial pressure (MAP) is a late indictor of progressive circulatory pathology. Non-invasive monitoring methods that are superior indicators of circulatory compromise would be clinically valuable. With IRB approval, 21 healthy volunteers were subjected to progressive lower body negative pressure (LBNP) until the onset of presyncopal symptoms. We evaluated the usefulness of four investigational methods of arterial blood pressure waveform analysis during progressive hypovolemia: mean arterial pressure (MAP); the ModelFlow cardiac output algorithm (MF); the long time interval method (LTI); and the product of pulse pressure and heart rate (PP*HR). Electrical bioimpedance measurement of cardiac output (EBI) provided a reference. When results were analyzed, we found significant differences between the methods. MF, LTI, and EBI all corresponded with LBNP severity, while MAP and PP*HR did not. In terms of discriminating between (a) decompression to -45 mmHg; versus (b) recovery five minutes after LBNP cessation, there was a significant difference between MF and LTI: the receiver operating characteristic area-under-the-curve (ROC AUC) for MF was 0.57 and for LTI was 0.76. In terms of discriminating between (a) the 11 subjects who tolerated the protocol (i.e., tolerated higher levels of LBNP); versus (b) the 10 non-tolerant subjects, there was also a significant difference between MF and LTI: the ROC AUC for MF was 0.40 and for LTI was 0.66. There were no significant differences between MF nor EBI, however. In conclusion, LTI is notable as the only method which (a) correlated with decompression: (b) distinguished between decompression to -45 mmHg versus recovery; and (c) distinguished between those subjects who adequately compensated for central hypovolemia (tolerant) and those who did not have such robust physiologic compensation (non-tolerant).