P064 Predictive Value of Awake Supine Pulse Oxygen Saturation in Detecting Resting Hypercapnoea Prior to Polysomnography

INTRODUCTION: Arterial blood gases (ABG) are used during polysomnography (PSG) to assess gas exchange abnormalities in patients at risk of hypercapnoea. ABG sampling may cause discomfort and can be difficult to obtain. Normal, awake, supine finger pulse oxygen saturation (SpO2) immediately prior to PSG may reduce the need for ABG sampling. AIM: To determine if an awake supine evening SpO2 cutoff value can exclude awake hypercapnoea measured with ABG testing. Methods : Prospective observational study of all patients referred for diagnostic or treatment PSGs with ABG at Westmead Sleep Laboratory between January 2019 and December 2021. ABGs were requested to exclude hypercapnoea and were collected awake, supine and breathing room air prior to PSG with simultaneous SpO2 measurement. We defined resting hypercapnoea as PaCO2 > 45mmHg. We used 2x2 tables for SpO2 values of 93% to 98% to determine the negative predictive values (NPV) for SpO2 cutoffs and construct a receiver operating characteristic (ROC) curve. RESULTS: We analysed 146 patients (60±17 years; 68% male; Body-mass Index 32.7±12.0 kg/m2; mean±SD) and 36 patients (25%) had hypercapnoea. The NPVs for hypercapnoea on the ABG with cut off SpO2 values of 96%, 95%, 94% and 93% were 92%, 92%, 89% and 83%, respectively. The ROC area under curve was 0.75. CONCLUSION: Resting supine oxygen saturation of ≥95% was able to exclude hypercapnoea in 92% of subjects within this sleep population at high risk of hypercapnoea. Thus, an accurate, awake, supine SpO2 ≥95% will fail to diagnose hypercapnoea in ~8% of high risk patients.