Clinical assessment or home oximetry for obstructive sleep apnea
ACP J Club. 1993 July-Aug;119:23. doi:10.7326/ACPJC-1993-119-1-023
To determine whether suspected obstructive sleep apnea in patients seen in a sleep disorders clinic can be identified by clinical assessment and home oximetry measurements.
A blinded comparison of unsupervised home oximetry with clinical assessment and assessment by polysomnography.
A sleep disorders clinic in Australia.
98 consecutive patients (mean age 50 y, 77 men) referred because of suspected obstructive sleep apnea. All patients were habitual snorers. Exclusion criteria were significant chronic lung disease, grossly abnormal oximetry results, or residence not near the clinic.
Description of tests and diagnostic standard
A clinical examination collected patient age and reports of apnea during sleep, of obesity, and of hypertension. Within 1 week of clinical assessment, pulse oximeters (Model Biox 3700, Ohemeda) were used for 1 night to monitor continuous oxygen saturation levels and to record the lowest value for each 12-second interval. Computer analysis included the number of times per hour oxygen saturation levels fell ≥ 2%, ≥ 3%, and ≥ 4% below baseline levels (assuming a threshold of > 15 episodes/h as abnormal) and the percentage of time spent with oxygen saturation below 90% (CT90). All data were then interpreted by clinicians who assessed whether the probability of obstructive sleep apnea was > 15%. Laboratory polysomnography was then done by technical staff blinded to other results. Obstructive sleep disorder was defined as ≥ 15 apnea (cessation of oronasal airflow for > 10 seconds) and hypoapnea (reduction of oronasal airflow to ≥ 10 seconds) events per hour from polysomnography results.
Main outcome measures
Sensitivity and specificity of home oximetry at various cut points.
43 of 98 patients had obstructive sleep apnea and 31 were subsequently treated with continuous positive airway pressure. Clinical assessment had a sensitivity of 79% and a specificity of 50%. For oximetry, sensitivity and specificity for clinician scanning of the data were 72% and 88%, respectively; for 1% CT90, 93% and 51%; for desaturations of 2%, 65% and 74%; for desaturations of 3%, 51% and 90%; and for desaturations of 4%, 40% and 98%.
Home oximetry with a cut point of ≥ 1% of time spent at oxygen saturation below 90% represented a good screening test for obstructive sleep apnea. > 15 episodes of desaturation for > 4% below baseline levels were specific for the disease. Less conclusive results probably require further investigation.
Source of funding: National Health and Medical Research Council of Australia.
For article reprint: Dr. L.G. Olson, Sleep Disorders Centre, Royal Newcastle Hospital, P.O. Box 664J, Newcastle, 2300, Australia. FAX 61-49-266-520.
The study by Gyulay and colleagues is thorough, elegant, and timely. It indicates an effective way to triage patients with the potential for a serious breathing disorder (obstructive sleep apnea), thus protecting scarce medical resources (such as clinicians' time and shrinking financial support) and limiting the number of patients unnecessarily sent to the sleep laboratory for full polysomnography.
The authors used a reasonable operational definition of obstructive sleep apnea. A replication of this study would be useful because the number of participants was small; however, the authors' analysis of the data was thorough and impressive. The authors showed that, depending on the parameter used, home oximetry can be a more sensitive test and therefore a better screening test than an excellent clinical assessment that includes age, reports of sleep apnea, obesity, and hypertension. Using home oximetry, patients with < 1% of sleep time with an arterial oxygen saturation below 90% are sufficiently unlikely to have obstructive sleep apnea so that polysomnography is not warranted. Patients with > 15 episodes per hour of 4%, or even 3%, desaturation are almost certain to have the disease, so further testing may not be needed.
If the results of this study are applied in usual clinical settings, some patients screened using home oximetry, particularly those living far from a sleep laboratory, may be treated for obstructive sleep apnea without first having polysomnography, and some may be reassured that the disease is probably not present.
Robert Lebow, MD
University of Massachussetts Worcester, Massachusetts, USA