We examined the usefulness of respiratory pulse transit time (PTT) variation as an intravascular volume index in young, healthy, spontaneous, paced breathing volunteers exposed to simulated central hypovolemia by lower body negative pressure (LBNP).

With paced breathing at 0.25 Hz, beat-to-beat finger blood pressure (BP), heart rate (HR), cardiac output (CO), stroke volume (SV), total peripheral resistance (TPR), and PTT were measured non-invasively in 18 healthy volunteers. Graded central hypovolemia was generated using LBNP from 0 to -20, -30, -40, and -50 mmHg. Respiratory PTT variation (PTTV) was calculated as the difference of maximal and minimal values divided by their respective means. Respiratory-frequency PTT variability (PTT_RF) using power spectral analysis was also estimated.

During LBNP, SV, CO and PTT_RF decreased, but PTT, PTTV and TPR increased significantly. PTTV did not correlate with SV changes (r = -0.08, P = 0.52), but PTT_RF (r = 0.58, P < 0.01) and PTT (r = 0.43, P < 0.01) did during progressive hypovolemia.

PTT_RF is more applicable to the changes in intravascular volume than PTT and PTTV, suggesting spectral analysis of PTT might be used as a dynamic preload index in patients with spontaneous and paced breathing condition, which needs further studies.