![]() In healthy controls, on OPO, the oxygen saturation is maintained above 90% throughout the night. Most clinicians also look at the oxygen saturation waveform patterns, which provide a good view of the oxygenation status of the patient for the entire night's recording. Its low cost, easy availability, and being able to perform it on an outpatient basis make this an attractive alternative to polysomnographic studies for the screening of obstructive sleep apnea (OSA) in patients with a high pretest suspicion. OPO has been used extensively in the field of sleep medicine and classified as a type 4 monitoring device. It provides the clinician a clue of the presence of coexistence of pulmonary hypertension, heart failure, and hypoventilation symptoms. An elevated T-90 during overnight sleep study represents the hypoxic burden during the study. A decreased value points to an underlying cardiopulmonary problem.Īnother parameter which is clinically helpful and is widely reported in most studies is time spent by the patient, under 90% oxygen saturation (T-90) during the duration of the study. In general healthy controls, the normal overnight mean oxygen saturation is 96%. Generally, continuous oximetry is done during the night time hence, it is called OPO.ĭuring a routine OPO, one can get an idea about the mean overnight saturation (mean SaO 2) and lowest SaO 2 during the entire night recording. Over the past few years, the advancement in technology has resulted in the possibility of continuous pulse oximetry recording which gives a better idea of a patient's oxygenation and respiratory status. The microprocessor gives a new reading every 0.5–1 s that averages out the readings over the last 3–6 s. These values are accepted or rejected using specific formulas. A large number of measurements of relative light absorption are made multiple times every second, and these are fed into a microprocessor that compares the ratio of absorption of the two spectra against a set of stored reference values in the form of a calibration curve obtained empirically by measuring the relative red: infra-red modulation ratios in healthy volunteers whose saturations were altered from 100% to approximately 70%. This photodiode detects the amount of red and near-infrared light absorbed by the vascular tissues from the light transmitted through the finger held in between the arms of the probe. A photoreceptor is present opposite to the LEDs with patient tissues with a higher vascular density such as the finger, toes, nose, forehead, or earlobe held in between. Based on this principle of differential absorption of light, pulse oximeters emit two wavelengths of light, red at 660 nm and near infrared at 940 nm from two light-emitting diodes (LEDs) located in one arm of the probe. Other wavelengths of the light such as the yellow, green, blue, and far infrared are absorbed by nonvascular tissues. On the contrary, deoxyhemoglobin absorbs the red component of light more and scatters the near infrared, making it look less red. The oxyhemoglobin absorbs near-infrared light and dissipates the red component of light which makes it appear bright red. Pulse oximetry is a spectrophotometry technology which determines the arterial oxygen saturation by the detection of pulsatile blood flow and is based on differing absorption spectra of oxyhemoglobin and deoxyhemoglobin which includes the red and near-infrared wavelengths of light.
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