Validation of a novel index of hemorrhage using a lower body negative pressure shock model

BACKGROUND: Vital signs are late indicators of blood loss in trauma patients. Indexed Heart to Arm Time (iHAT ) is a non-invasive index based on a modified pulse transit time (mPTT) indexed to the time between R waves on the electrocardiogram (RR interval). We aimed to investigate how early iHAT is able to detect central hypovolemia during the progression from mild to severe simulated hemorrhage induced by applying lower body negative pressure (LBNP). METHODS: Thirty healthy volunteers were enrolled. Central hypovolemia was induced by application of increasing LBNP from 0 to -80 mmHg. At every step, non-invasive blood pressure, heart rate, cardiac echo Doppler measurements and iHAT were recorded. RESULTS: Aortic flow Velocity Time Integral (VTI) reduction from 21.8±3.7 (baseline) to 11.2±3 cm (-70 mmHg) (P<0.001) was progressive with LBNP increase and represented a significant change in stroke volume and preload and induced an increase in heart rate from 69±2 to 107±4 bpm. iHAT increased from 34.2±4.65% (baseline) to 53.9±14.34% (-80 mmHg), P<0.001. The increase in iHAT became significant after -30 mmHg level was reached, corresponding to 500-1000 mL blood loss. CONCLUSIONS: iHAT measures both the reduction in preload and the parabolic heart rate increase due to the linear decrease in stroke volume. iHAT was able to detect a progressive central volume loss in a model of hemorrhage in healthy volunteers undergoing LBNP. A rising trend in iHAT can be a useful marker for progressive volume loss during moderate to severe bleeding.