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Use of Pulse Transit Time To Distinguish Respiratory Events From Tidal Breathing in Sleeping Children^*

^* From the School of Information Technology and Electrical Engineering (Drs. Foo, Wilson, and Bradley), University of Queensland, St. Lucia Campus, Brisbane; and Department of Respiratory and Sleep Medicine (Drs. Harris and Cooper and Mr. Williams), Mater Misericordiae Children’s Hospital, South Brisbane, QLD, Australia.

Correspondence to: Jong Yong A. Foo, PhD, School of Information Technology and Electrical Engineering, University of Queensland, St. Lucia Campus, Brisbane, Australia 4072; e-mail:jong{at}ieee.org

Study objectives: Currently, esophageal pressure monitoring is the "gold standard" measure for inspiratory efforts, but its invasive nature necessitates a better tolerated and noninvasive method to be used on children. Pulse transit time (PTT) has demonstrated its potential as a noninvasive surrogate marker for inspiratory efforts. The principle velocity determinant of PTT is the change in stiffness of the arterial wall and is inversely correlated to BP. Moreover, PTT has been shown to identify changes in inspiratory effort via the BP fluctuations induced by negative pleural pressure swings. In this study, the capability of PTT to classify respiratory events during sleep as either central or obstructive in nature was investigated.

Setting and participants: PTT measure was used in adjunct to routine overnight polysomnographic studies performed on 33 children (26 boys and 7 girls; mean ± SD age, 6.7 ± 3.9 years). The accuracy of PTT measurements was then evaluated against scored corresponding respiratory events in the polysomnography recordings.

Results: Three hundred thirty-four valid respiratory events occurred and were analyzed. One hundred twelve obstructive events (OEs) showed a decrease in mean PTT over a 10-sample window that had a probability of being correctly ranked below the baseline PTT during tidal breathing of 0.92 (p < 0.005); 222 central events (CEs) showed a decrease in the variance of PTT over a 10-sample window that had a probability of being ranked below the baseline PTT of 0.94 (p < 0.005). This indicates that, at a sensitivity of 0.90, OEs can be detected with a specificity of 0.82 and CEs can be detected with a specificity of 0.80.

Conclusions: PTT is able to categorize CEs and OEs accordingly in the absence of motion artifacts, including hypopneas. Hence, PTT shows promise to differentiate respiratory events accordingly and can be an important diagnostic tool in pediatric respiratory sleep studies.

Key Words: central sleep apnea • hypopnea • noninvasive technique • obstructive sleep apnea • pediatrics • pulse transit time • upper airway