Objectives: To investigate the feasibility of pulse transit time (PTT) as a quantitative measure of inspiratory effort in patients with obstructive sleep apnea (OSA). Methods: Nineteen moderate to severe OSA patients were included to undergo overnight polysomnography simultaneously with esophageal pressure (P-es) and PIT. The quantitative relationships between the size of P-es variations (Delta P-es) and PTT variations (Delta PTT) on a breath-by-breath basis in obstructive apneas were assessed. Results: 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 Delta P-es and Delta PTT 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 Delta PTT against Delta P-es were plotted with intercepts (5.1 +/- 2.1 ms) and slopes (0.35 +/- 0.08 ms.cm H2O-1). Conclusions: 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 (C) 2011 S. Karger AG, Basel