Using the ear photoplethysmographic waveform as an early indicator of central hypovolemia in healthy volunteers utilizing LBNP induced hypovolemia model

Objective. To study the photoplethysmographic (PPG) waveforms of different locations (ear and finger) during lower body negative pressure (LBNP) induced hypovolemia. Then, to determine whether the PPG waveform can be used to detect hypovolemia during the early stage of LBNP. Approach. 36 healthy volunteers were recruited for progressive LBNP induced hypovolemia, with an endpoint of −60 mmHg or development of hypoperfusion symptoms, whichever comes first. Subjects tolerating the entire protocol without symptoms were designated as high tolerance (HT), while symptomatic subjects were designated as low tolerance (LT). Subjects were monitored with an electrocardiogram, continuous noninvasive blood pressure monitor, and two pulse oximetry probes, one on the ear (Xhale) and one the finger (Nellcor). Stroke volume was measured non-invasively utilizing Non-Invasive Cardiac Output Monitor (NICOM, Cheetah Medical). The waveform morphology was analyzed using novel PPG waveforms indices, including phase hemodynamic index (PHI) and amplitude hemodyamaic index and were evaluated from the ear PPG and finger PPG at different LBNP stages. Main results. The PHI, particularly the phase relationship between the second harmonic and the fundamental component of the ear PPG denoted as ∇ φ 2 , during the early stage of LBNP (−15 mmHg) in the HT and LT groups is statistically significantly different (p value = 0.0033) with the area under curve 0.81 (CI: 0.616-0.926). The other indices are not significantly different. The 5 fold cross validation shows that ∇ φ 2 during the early stage of LBNP (−15 mmHg) as the single index could predict the tolerance of the subject with the sensitivity, specificity, accuracy and F1 as 0.771 ± 0.192, 0.71 ± 0.107, 0.7 ± 0.1 and 0.771 ± 0.192 respectively. Significance. The ear’s PPG PHI which compares the phases of the fundamental and second harmonic has the potential to be used as an early predictor of central hypovolemia.