Increases in Heart Sound Intensity following Respiratory Events are Positively Correlated with Drops in Oxygen Saturation during the Events

Kabir, Muammar 1, 2 ; Gavrilovic, Bojan 2 ; Saha, Shumit 1, 2 ; Zhu, Kaiyin 2 ; Yadollahi, Azadeh 1, 2

1. Institute of Biomaterials and Biomedical Engineering, University of Toronto; 2. Toronto Rehabilitation Institute, University Health Network

Background: In patients with obstructive sleep apnea (OSA), intermittent hypoxia caused by closure of the pharyngeal airway (apnea or hypopnea) leads to increases in the sympathetic nervous activity. These increases are suggested to lead to hypertension and long-term cardiovascular disorders in OSA patients. According to the previous studies, changes in blood pressure are associated with changes in heart sound intensity (HSI). Therefore, the purpose of this study is to investigate the changes in HSI from pre- to post- termination of apneas/hypopneas in association with the drops in oxygen saturation (SaO2) during apneas/hypopneas.        

Methods: Subjects suspected for OSA underwent overnight polysomnography (PSG). We developed a small wearable device (The Patch) to record tracheal respiratory and heart sounds over the neck simultaneously with PSG. An automated algorithm was developed to localize and separate heart sounds from respiratory sounds recording. For each subject, 20 obstructive apnea/hypopneas during Non-REM sleep stage-2 were considered. Subsequently, 10-second segments before the start of each event (pre-event), before the termination of the event or onset of arousal (pre-termination) and after the termination of the event/arousal (post-termination) were extracted. For each segment the average HSI was calculated. One-way ANOVA was used to compare changes in HSI in the pre-event, pre-termination and post-termination segments. For post-hoc analysis, Tukey’s multiple comparisons test was used. Pearson’s correlation was used to investigate the correlation between drop in SaO2 and change in HSI from pre- to post-termination segments.       

Results: Data from 39 subjects (20 females), age of 50.2±14.6 years, BMI of 28.3±5.8 kg/m2 and apnea/hypopnea index of 15.0±18.5 events/hour were investigated. There was a significant increase in HSI from pre-termination to post-termination (6.9±2.4dB vs. 9.8±2.5dB, p<0.0001) and from pre-event to post-termination (8.2±2.3dB vs. 9.8±2.5dB, p<0.001). In addition, there was a significant positive correlation between drop in SaO2 and the change in HSI from pre-termination to post-termination, with (r=0.94, p=0.002) and without (r=0.92, p=0.004) arousal. 

Conclusion: Our results indicate that increases in HSI following termination of respiratory events are directly associated with the magnitude of hypoxia.  After validation in a larger patient population, heart sound analysis can be used to investigate the effects of apneas and hypopneas on cardiac abnormalities during sleep and long-term cardiovascular disorders in OSA patients.