Overnight Pulse Oximetry and Its Use In The Diagnosis of OSA

Introduction
Sleep apnea, with an estimated incidence between three and seven percent in the general population, is a common disorder.¹ Currently, either an at-home sleep test or overnight polysomnographic test is necessary to diagnose sleep apnea. Both of these evaluations are at times cumbersome. Consequently, patients may be unwilling to undergo these procedures. A less intrusive screening test like overnight pulse oximetry would likely increase adherence to study protocols.

Overnight pulse oximetry (OPO) is a straightforward testing modality that carries the potential to screen for patients with moderate to severe obstructive sleep apnea (OSA). This technology provides adequate information regarding a patient’s oxygenation and respiratory patterns, aiding in the assessment of a patient’s cardiopulmonary dysfunction. As such, it is important that respiratory therapists acquaint themselves with this technology and its uses.

What is pulse oximetry?
Pulse oximetry has been used to measure oxygen status for more than 40 years. It has even been called the “fifth vital sign”. Using simple spectrophotometry, this technology allows us to detect differences in the absorption spectra of oxyhemoglobin and deoxyhemoglobin as blood passes through the body in a pulsatile fashion.² It is well documented that oxyhemoglobin absorbs near-infrared light and dissipates red light. (Fig. 1)

Figure 1 – absorbance patterns of oxyhemoglobin and deoxyhemoglobin

Most commercially available pulse oximeters are placed on bodily areas that are rich in blood vessels such as the fingertip or the earlobe. Most pulse oximeters emit light at two distinct wavelengths. The first, at 660 nm, appears as red light and the second, at 940 nm, is infrared light. The lights are emitted from the diodes on one end of the device and pass through the vascular tissue before being detected by a component on the other side. This component is capable of measuring the amount of light that is transmitted through the tissue.

This technology has advanced over the years to the point that continuous monitoring is possible. Traditional pulse oximetry is useful in its ability to capture a snapshot of a patient’s oxygen saturation at a moment in time. The advantage of continuous monitoring is that practitioners are able to evaluate changes in oxygen saturation in real-time. Since continuous monitoring is generally done overnight, this technology is commonly referred to as overnight pulse oximetry (OPO). In general, healthy patients have an overnight oxygen saturation of 96%.³ Any decrease below this value can indicate to a health care provider that the patient may possibly have an underlying derangement in his or her cardiopulmonary status.

OPO has been extensively studied in sleep studies and the American Academy of Sleep Medicine has classified it as a type 4 monitoring device.⁴ Continuous pulse oximetry measures three parameters; these include the pulse rate, the oxygen saturation, and lastly, the time spent with an oxygen saturation below 90% during the duration of the study (T-90).^{5, 6} Analyzing the oxygen saturation waveform after OPO for specific patterns can reveal information regarding the patient’s overnight oxygenation status.

How is overnight pulse oximetry useful in OSA?
From a practical point of view, waveforms can be classified into one of three distinct patterns. The first pattern is seen in healthy individuals without underlying cardiopulmonary issues. In these patients, the oxygen saturation remains above 90% for the duration of the study. (Fig. 2)

Figure 2 – normal overnight pulse oximetry tracing in a healthy patient

A second pattern is characterized by multiple clustered episodes of oxygen desaturation, sometimes referred to as a “sawtooth” or “icicles on rooftop” pattern. (Fig. 3) This pattern is seen in patients with moderate to severe OSA. These episodes show a strong correlation to REM sleep-related desaturations.

Figure 3 – overnight pulse oximetry tracing showing “sawtooth” or “icicles on rooftop” pattern in a patient with OSA

In the third pattern, the patient’s oxygen saturation remains below 90% for the majority of the test’s duration resulting in a high T-90. (Fig. 4) This pattern suggests an underlying severe ventilation-perfusion mismatch as is seen in patients with conditions such as advanced chronic obstructive pulmonary disease or interstitial lung disease.

Figure 4 – overnight pulse oximetry tracing showing desaturation pattern

Desaturation is important to monitor given the association between hypoxemia and poor patient outcomes. We can use OPO to measure the number of desaturation events per hour. A desaturation event is defined as a drop in the oxygen saturation by greater than four percent from the baseline for longer than 10 seconds.⁷ The number of desaturation events per hour is referred to as the oxygen desaturation index (ODI).

Many authors have used various cut-offs for this parameter to diagnose OSA with varying sensitivity. For example, an ODI greater than 10 is highly sensitive (93.3%) when diagnosing suspected moderate to severe OSA.⁷ The utility of the ODI in diagnosing OSA lies in the fact that a higher ODI reflects a higher pre-test probability that the patient has OSA.⁸

In cases of suspected moderate to severe OSA, OPO can be a powerful screening tool.^9-12 Practitioners should be aware, however, that OPO as a screening tool is inadequate in patients with a low suspicion of OSA.

OPO can also assess a patient’s response to OSA treatment. Among patients who have been treated using CPAP or orodental interventions, subsequent OPO post-intervention may show improvement in the T-90 and thus improvement in the patient’s oxygenation status.

However, it should be clear that overnight OPO cannot be used to distinguish between OSA and central sleep apnea. Additionally, it cannot be used as a qualifying test for initiation of CPAP therapy. It bears mentioning that patients with suspected OSA on OPO will still need a formal home sleep study or polysomnography to confirm the diagnosis of OSA.

Conclusion
In conclusion, OPO is a far less cumbersome means of screening patients for suspected moderate to severe sleep apnea. It also allows practitioners to identify abnormalities in oxygenation among patients with underlying structural lung disease.

Industry Perspective
“At VirtuOx we have been using Overnight Pulse Oximetry (OPO) for over 15 years to help qualify patients for nocturnal oxygen and as a sleep apnea screener/sleep apnea therapy validation tool using Oxygen Desaturation Index (ODI),” said Kyle Miko, Chief Marketing Officer at VirtuOx.

“Over the past 15 years, we have processed over 6 million OPO tests and have realized not all OPO devices were built with the best technology. Typical devices in the market record in signal averaging a method of only recording SPO2/HR every 4-seconds which can miss important desaturations, devices automatically erase the data when batteries are removed and there is no Good Study Indicator (GSI) to allow patients and providers if there is sufficient data to produce a report. That is why we manufacture High-Resolution Pulse Oximetry devices (HRPO) that record SPO2/HR in 1-second and software that allow clinicians to cost-effectively implement programs like this.”

 

Authors
Justin Panthappattu, MD, Sara Z. Khan, MD, Dilbagh Singh, MD, Sameer Verma, MD, Arunabh Talwar, MD, FCCP, Northwell Health Department of Pulmonary, Critical Care and Sleep Medicine, New Hyde Park, NY

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