To investigate the feasibility of pulse transit time (PTT) as a quantitative measure of inspiratory effort in patients with obstructive sleep apnea (OSA).Nineteen moderate to severe OSA patients were included to undergo overnight polysomnography simultaneously with esophageal pressure (P(es)) and PTT. The quantitative relationships between the size of P(es) variations (ΔP(es)) and PTT variations (ΔPTT) on a breath-by-breath basis in obstructive apneas were assessed.A total of 19,833 breaths from 6,087 obstructive apneas were analyzed. There were good correlations with r = 0.779 ± 0.095 (mean ± SD) between ΔP(es) and ΔPTT based on overnight sleep. The correlation coefficients for supine and lateral position were of the approximated magnitude (r = 0.783 ± 0.060 and 0.757 ± 0.106, respectively), whereas they were lower in rapid eye movement (REM) sleep (r = 0.564 ± 0.140) compared with non-rapid eye movement (NREM) sleep (r = 0.787 ± 0.071). In NREM sleep, the regression lines of ΔPTT against ΔP(es) were plotted with intercepts (5.1 ± 2.1 ms) and slopes (0.35 ± 0.08 ms·cm H(2)O(-1)).PTT showed good ability in detecting changes of inspiratory effort in overnight sleep and was proved to be a clinically useful method in quantifying increases in inspiratory effort in NREM sleep. Hence, PTT has prospects to become an alternative to P(es) in respiratory sleep studies.Copyright © 2011 S. Karger AG, Basel.