Introduction Pulse Transit Time (PTT) represents a non-invasive indirect marker of sleep fragmentation in OSAS. Little is known regarding PTT indices in persons presenting with sleepiness where sleep studies exhibit “flow limited” breaths in the absence of clinically significant OSA (a so called Inspiratory “Flow Limitation” (IFL) cohort) and whether PTT indices differ when compared to OSAS subjects and with a “control” group exhibiting no evidence of OSAS or IFL (“Non-Flow Limited” or NFL cohort).

Methodology 20 subjects meeting criteria for the IFL cohort (mean AHI = 3.84/hr; RDI = 17.71/hr) were aged (± 2 yrs) and gender matched with 20 OSAS subjects (mean AHI = 48.93 hr) and 20 control “NFL” subjects (no sleep disordered breathing; mean AHI = 1.01/hr; RDI = 2.63/hr) underwent respiratory limited polysomnography, including pulse oximetry and ECG monitoring. PTT was defined as interval between the electrocardiographic R wave and point corresponding to 50% height of the ascending plethysmographic (pulse) waveform; PTT arousal (deceleration) defined by decline in PTT signal of ≥15 ms, lasting 5 seconds; PTT Deceleration index (PTT Di) defined by number of PTT arousals per hour.

Results Table 1 outlines key demographics in the cohorts. Of the NFL cohort, 14 presented with snoring in absence of sleepiness. 72% and 84% were deemed “responders” to CPAP within the IFL and OSAS cohorts respectively. The PTT DI in the IFL cohort (33.67 ± (23.34)/hr) was significantly higher than that measured in the control NFL cohort (23.89 ± (18.88)/hr) but significantly lower than that measured in the OSAS cohort (55.21 ± (29.30)/hr; 3-way ANOVA; F = 8.76; p < 0.001). PTT Di was positively correlated with AHI within the whole study population (CC = 0.46; p < 0.001). Within the IFL cohort, PTT Di was positively correlated with age (CC = 0.501; p = 0.024) but not with gender and BMI.

Conclusion The PTT Deceleration Index increased proportionately with SDB, with significantly higher markers of arousal in sleepy subjects exhibiting nocturnal IFL in comparison to control subjects, but not as high as those with clinically significant OSA. These findings support the relevance of IFL as a potentially significant pathogenic entity in the development of daytime sleepiness. The utility of PTT Deceleration Index as a therapeutic target for CPAP Titration in OSAS requires further evaluation.