ABSTRACT
Obstructive sleep apnoea (OSA) is one of the most prevalent and detrimental of the many sleep disorders. In OSA, recurrent collapses of upper airways limit partially (hypopnea) or block completely (apnoea) the airflow during sleep. OSA is currently diagnosed based on medical examination, daytime symptoms, and the results of diagnostic sleep study. Diagnostic sleep study is used for severity assessment of OSA, being either polysomnography (PSG) or ambulatory polygraphic recording that both are data-rich physiological measurements. To a significant extent, the severity assessment of OSA is based on the value of the apnoea-hypopnea index (AHI). However, the AHI is defined as the average number of apnoeas and hypopneas per slept hour, thus providing an over-simplistic estimation of the severity of the disease i.e. it has major shortcomings.
The shortcomings of AHI include the poor association with the most common daytime symptoms of OSA: excessive daytime sleepiness (EDS) and reduced vigilance. An EDS assessment is based on subjective questionnaires such as the Epworth sleepiness scale (ESS) or objective measurements e.g. the Multiple sleep latency test (MSLT). In addition, the evaluation of vigilance can be conducted via the Psychomotor vigilance task (PVT). However, the AHI is able to explain only 10% of the variation in daytime sleep latencies and less than 2% of the variation in ESS scores. Moreover, the AHI lacks the capability to explain the prolonged reaction times in PVT, and the severity classification of OSA based on AHI displays little association with impaired vigilance.
Previous literature shows, however, that OSA-related hypoxemia contributes to the development of EDS and vigilance deterioration. Thus, the main hypotheses of this Ph.D. thesis were that a detailed parametric quantification of apnoeas, hypopneas, and related blood oxygen desaturations and frequency-domain features of pulse oximetry signals could be linked to objective EDS and poor PVT performance. The aim was to test the stated hypotheses by retrospectively investigating sub-populations from two large clinical datasets consisting of suspected OSA patients. The first dataset comprised patients with PSG and MSLT data (n = 2064) and the second consisted of patients with PSG and PVT data (n = 902).
The severity of intermittent hypoxemia was consistently found to have a significant (p < 0.05) association with more severe EDS and impaired vigilance. Relative 10% increases in the Desaturation Severity (DesSev) and in the time spent under 90% oxygen saturation (t90%) were significantly associated with higher odds of having severe (p < 0.001) or moderate EDS (p < 0.05). Similar increases in DesSev, t90%, and median desaturation depth were associated with prolonged mean and median reaction times and a higher number of lapses in PVT (p < 0.05).
Furthermore, the increase in power within the 15 - 35 mHz frequency band of blood oxygen saturation and heart rate signals indicated a significant (p < 0.05) risk of having severe EDS. In addition, an increase in photoplethysmogram (PPG)-based arterial pulsation frequency (APF) was significantly (p < 0.001) associated with a higher number of lapses and larger within-test variation in PVT in male OSA patients, and a higher number of lapses in female OSA patients (p < 0.05).
In conclusion, a more detailed quantification of desaturations is able to link the severity of OSA more strongly to EDS and impaired psychomotor vigilance than the conventionally used AHI. In addition, large fluctuations in heart rate combined with severe intermittent hypoxemia are a significant predictor of OSA-related EDS. Furthermore, a higher APF in PPG provides a marker for deteriorations in vigilance. These findings emphasize the potential of pulse oximetry-based methods for severity assessment and polysomnographic phenotyping of OSA.
The doctoral dissertation of MSc Samu Kainulainen, entitled Pulse oximetry-derived biomarkers for severity assessment of obstructive sleep apnoea – Associating parametric and frequency domain-features of SpO2 and PPG signals with daytime sleepiness and impaired vigilance will be examined at the Faculty of Science and Forestry on the 2nd of October in Kuopio. The opponent in the public examination will be Associate Professor Raquel Bailón Luesma, University of Zaragosa, Spain and the custos will be Docent Timo Leppänen, University of Eastern Finland. The public examination will be held in English.
