Routine clinical parameters such as hypotension and tachycardia are suboptimal indicators of of blood volume loss during hemorrhage. A noninvasive monitor to help identify hemodynamic instability prior to its onset would allow rapid treatment of trauma patients to prevent oxygen debt, multisystem dysfunction and cardiovascular collapse. Near infrared spectroscopic (NIRS) measurement of muscle oxygenation (PO2) is noninvasive, continuous and can be applied quickly on a trauma patient. Lower body negative pressure (LBNP) has been used to create progressive central hypovolemia in healthy human volunteers (1). We hypothesized that noninvasively measured PO2 would provide an earlier indication of imminent hemodynamic instability during progressive hypovolemia compared to standard vital signs.
10 healthy human volunteers underwent LBNP in 5 min intervals of −15, −30, −45, and −60 mm Hg, then increasing by −10 mm Hg to presyncope. Mean arterial pressure, heart rate, cardiac output, stroke volume and pulse pressure were measured continuously and noninvasively throughout the study. Intramuscular (IM) PO2 was measured noninvasively with a NIRS sensor placed on the forearm. IM PO2 was determined from muscle oxygen saturation calculated with a novel application of Beer's Law (2). Average values for NIRS IM PO2 and each hemodynamic parameter were obtained and the percent change from baseline was calculated. Paired t-tests, corrected for multiple comparisons, were used to identify the lowest LBNP level where the tested parameter was statistically different from baseline.
NIRS IM PO2, stroke volume and pulse pressure decreased linearly with increasing negative pressure (R2 = 0.98) and were the earliest indicators of impending cardiovascular collapse (Table I). Change in blood pressure only reached statistical significance at −80 mm Hg.
NIRS IM PO2 has the sensitivity to detect a decrease in muscle perfusion. The NIRS PO2 sensor shows promise for detecting very early changes in IM PO2 during the progression toward hemorrhagic shock, obviating the need for more invasive monitors. This sensor can be easily used both in pre-clinical and in-hospital situations. This work was funded by the US Army Medical Research and Materiel Command.
1. Cooke WH, et al: J Appl Physiol
2. Soyemi OO et al. Am J Physiol. 2006; submitted.