Sleep Investigations
4.5 Nocturnal Pulse Oximetry .1 Use
Nocturnal pulse oximetry can be a useful case-finding tool to detect sleep- disordered breathing and is usually performed in the patient’s home (Zamarrón et al. 2003). A pulse oximeter is typically the size of a wristwatch and provides continuous infor- mation on two channels—oxygen saturation and pulse—overnight. Its small size allows the device to be posted to patients.
4.5.2 Technique
The oximeter resembles a wristwatch with an oximetry probe attached to the finger.
The oximeter records oxygen saturation (SaO2) and pulse rate every 1–6 s. The record- ing is started by the patient pressing a button, and it can be turned off in the morning so that two or three nights’ worth of data can be collected. Typically the oximeter collects about 6–18 h of data. Information from the device is downloaded and provides both numerical data and a trace of pulse rate and oxygen saturations. The oxygen desatura- tion index (ODI) and the pattern of the trace are both used in the identification of disease.
4.5.3 Patterns of Disease
A normal pulse oximetry trace is flat with oxygen saturations between 94% and 98% during the recording. There is a small reduction (1–3%) in oxygen saturation at the onset of sleep compared to wake. There is an increase in heart rate variability during REM sleep.
Obstructive sleep apnoea (OSA) is characterized by repetitive dips in oxygen saturation for part or all of the recording (Fig. 4.4). Oxygen desaturation occurs because of obstruction of the patient’s upper airway, apnoea and hypoxia.
Desaturation is usually accompanied by a rise in heart rate, but occasionally there can be an associated bradycardia. The fall in oxygen saturation is followed by a return of oxygen saturations back to baseline. If there is a persistent failure of oxy- gen saturations, to return to baseline, then hypoventilation should be suspected.
Evidence of hypoventilation is often associated with daytime respiratory failure and may indicate a more severe form of obstructive sleep apnoea called obesity hypoven- tilation syndrome (Pickwickian syndrome).
Severity of obstructive sleep apnoea can be approximated using the ODI. The 3%
oxygen desaturation index (3% ODI) has been shown to most closely resemble the
apnoea hypopnea index in obese patients (Ling et al. 2012). A 3%ODI of 5–14/h accords with mild OSA, 15–29/h with moderate OSA and ≥30/h with severe OSA.
Clusters of desaturations that occur during only part of the recording suggest sleep apnoea that is related to body position, head position or sleep stage. Sleep apnoea is typically worse in the supine position and REM sleep and can be isolated to these times.
Other disease can have distinct oximetry trace patterns. The pulse oximetry in Cheyne-Stokes respiration shows even and regularly spaced desaturations with the resaturations often above the baseline. However, it can be difficult to distinguish central from obstructive apnoeas despite the different pattern (Sériès et al. 2005).
Neuromuscular disease (motor neurone disease, diaphragm weakness, muscular dystrophy) is characterized by periods of prolonged desaturation for 30–90 min due
Fig. 4.4 Patterns of Nocturnal Pulse Oximetry. (Images courtesy of Dr. Nicholas Hart) The top trace shows oxygen saturations of haemoglobin (SaO2), the bottom trace shows pulse rate. (a) shows a patient with obstructive sleep apnoea with repetitive oxygen dips that return to baseline accompanied by an increase in heart rate. (b) shows a patient with nocturnal hypoventilation from respiratory muscle weakness, with normal baseline oxygen saturations of haemoglobin but with prolonged periods of desaturation lasting approximately 30 min. These periods of desaturation probably correspond to REM sleep when hypoventilation is most severe. (c) Is a patient with obe- sity hypoventilation syndrome with both repetitive desaturations of obstructive sleep apnoea and evidence of nocturnal hypoventilation-where there is a failure of oxygen saturations to recover to baseline
a
b
c
to REM hypoventilation. This occurs because during REM sleep, the accessory muscles that support respiration are lost due to REM sleep muscle atonia. Chronic lung disease (chronic obstructive lung disease, asthma) is characterized by low baseline saturations, usually less than 90%.
Periodic limb movements in sleep also have a characteristic trace on pulse oximetry. There is large pulse rate variability with a normal oxygen saturation trace (Krishnaswamy et al. 2010). Heart rate variability is probably due to autonomic arousal.
4.5.4 Advantages
The great advantage of pulse oximetry is its ease of use, small size and low cost. Its ease of use means the patient can put it on at home, whilst other home-based sleep studies require a technician to set up the leads. Its small size means that pulse oxim- etry can be posted to patients. It can also be used outside the sleep lab such as in a general or psychiatric hospital bed. It is a cost-effective way of screening patients with symptoms of obstructive sleep apnoea by avoiding a more detailed and expensive sleep study. Pulse oximetry has been shown to reduce the number of inpatient PSGs for patients referred to a sleep centre with symptoms of OSA (Chiner et al. 1999).
4.5.5 Disadvantages
The main limitation of pulse oximetry is that it provides only two channels of data (SaO2 of haemoglobin and pulse rate). Sleep and wake states cannot be differenti- ated nor does it provide information regarding sleep stage or position. A sleep diary completed during the oximetry study can help to identify the patient’s sleep times.
Pulse oximetry is not helpful in the assessment of parasomnias or narcolepsy unless comorbid sleep-disordered breathing is suspected. Oximetry can miss sleep- disordered breathing in patients with normal healthy lungs who arouse when their airway is partly obstructed but do not desaturate (upper airway obstructive syndrome).
4.5.6 Reliability
Nocturnal pulse oximetry lacks sensitivity and specificity in the detection of obstructive sleep apnoea when compared with (the gold standard) nocturnal PSG. The sensitivity and specificity of nocturnal oximetry varies in studies due to differences in methods, limiting the ability to pool results. Older studies used oximetry devices that had less frequent sampling times, which may have affected the sensitivity of the device to detect obstructive sleep apnoea. However, Ling et al.
analysed 11,448 patients undergoing polysomnography. They established that 3%
ODI performed best, and this index most closely approximated the apnoea hypopnea index (Ling et al. 2012). Pulse oximetry has a higher sensitivity to detect severe
obstructive sleep apnoea compared with mild or moderate OSA. False-positive results for OSA may occur in patients with chronic lung disease, as they are more prone to oxygen desaturation. False-negative results may occur in patients who are of normal body mass index and have healthy lungs. Oximetry should be used with caution in these two groups.
We recommend two consecutive nights of data collection to improve assessment of the severity of obstructive sleep apnoea (Wallberg et al. 2010). Current devices have sufficient memory for 2–3 nights’ recording. If pulse oximetry is not diagnos- tic and there is high clinical suspicion for OSA, then one should proceed to noctur- nal PSG.