1Department of Anaesthetics, Royal Prince Alfred Hospital, Missenden Road, Camperdown NSW 2050, Australia. 2Department of Cardiology and Sleep Disorders Centre, Royal Prince Alfred Hospital, Missenden Road, Camperdown NSW 2050, Australia
Accepted for publication: April 10, 2000
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Abstract |
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Br J Anaesth 2000; 85: 3548
Keywords: ventilation, apnoea; sleep; anaesthesia; surgery
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Introduction |
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To determine if this was an isolated finding, the records of a consecutive series of 148 unselected patients seen in consultation by one of the authors (J.L.) in the Sleep Disorders Consultative Service at the Royal Prince Alfred Medical Centre (Sydney, Australia) were reviewed.
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Patients and methods |
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Eleven of the records were unavailable at the time of the review. Twenty-nine patients were still waiting for their diagnostic sleep studies (DSS), and three had failed to attend.
Of the 105 patients who had already undergone DSS, 20 had an overall respiratory disturbance index (RDI the number of apnoeas and hypopnoeas per hour of sleep) of less than 10 per hour (considered normal), 10 had a non-OSA diagnosis (narcolepsy for example) and 10 had OSA but the REM/NREM differential for the RDI and minimum blood oxygen saturation (SpO2min) were not reported. One patient had no REM sleep on the night of the study. Sixty-four patients who had OSA and sufficient information to identify any sleep stage predominance therefore remained for inclusion in the study.
The variables recorded were the RDI and the minimum blood oxygen saturation using pulse oximetry (SpO2min) for both REM and non-rapid eye movement (NREM) sleep, as well as any demographic data available. A number of the study reports lacked satisfactory body position data, and this factor was not included in the analysis. Four patients had a previous diagnosis of asthma. Spirometric data were recorded for all but 11 patients. Three had an FEV1/FVC of less than 70%, two of whom had no known previous history of lung disease.
Diagnostic sleep study
The diagnostic sleep studies performed in each case were overnight polysomnographic studies including electro-encephalogram (C4/A1, O2/A1), electro-oculogram, submental and diaphragm electro-myogram, nasal airflow, chest and abdominal strain gauge, pulse oximeter, electro-cardiogram, leg movement sensors and position sensor. All diagnostic studies were carried out in a variety of sleep laboratories independent of the clinic and analysed manually, according to standard criteria, by experienced sleep study technicians, and further checked by one of several physicians specializing in sleep medicine. All episodes of oximetry artefact were manually rejected from analysis. No studies were scored by the authors.
Definitions
OSA was diagnosed if the overall RDI was greater than 10 per hour. While a small number of patients experienced some central events, there were no patients in this study with enough central apnoea to classify them as having either mixed or predominantly central sleep apnoea (CSA).
Statistics
All values are presented as mean (SD) unless otherwise stated. The Wilcoxon signed rank test was used for statistical analysis.
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Results |
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Discussion |
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There is some direct evidence to support this. Knill et al.11 studied six obese patients undergoing gastroplasty and found, despite a substantially higher number of apnoeas and hypopnoeas in NREM, that there were more severe desaturations during REM postoperatively. However, the mean BMI of that series was more than 50 kg m2 and our data concerning BMI could potentially explain a finding of REM predominance for desaturation. Rosenberg et al.8 reported a similar finding for a series of 10 patients, but a number of issues may have affected their observations. The method of sleep monitoring (using a modified F3-A2 electro-encephalogram channel) and scoring (using 6-s instead of 30-s epochs) does not appear to have been standard. Airflow was not measured, so that the oximetry data are open to question with respect to artefact. The data are tabulated in a way that makes it difficult to assess, but it appears that the greatest number of desaturations occurred during the night before the main REM rebound night, when there was less REM than preoperatively. It is also quite possible that their data were skewed by what appears to be a single patient with severe pre-existing sleep apnoea.
NREM/REM apnoea or desaturation differentials have been reported in only a few studies of OSA, often incidentally. Results similar to those in the current study have been reported.12 13 Despite this, REM predominance of OSA seems to be a widely held assumption in the anaesthesia literature. Central sleep apnoea, on the other hand, is reported to be most common in NREM stages one and two14 and actually reduced by REM sleep.15 Less common than OSA, it is nevertheless prevalent in certain conditions, especially men with left ventricular dysfunction.1618 Even when congestive heart failure is stable and optimally treated, the incidence of CSA has been reported to be as high as 75% in this group.19
Direct extrapolation of results in a diagnostic sleep laboratory to the postoperative condition is not possible. Sleep architecture may be affected by a variety of factors in each situation and any effect this might have on stage predominance of apnoea would be a matter of speculation. The effect of concurrent illness is also not known, although it could be assumed that surgical patients would have a similar incidence and degree of respiratory disease to that of our patients.
Despite these potential limitations, our findings suggest that the effects of postoperative sleep disturbance on sleep and breathing may be more complex than previously thought.
Apnoeas tend to be longer during REM,20 21 presumably as a consequence of reduced arousability.22 In patients having a very large number of apnoeas, any increase in the apnoea duration will actually limit the time available for other apnoeas to occur, perhaps increasing the degree of desaturation and reducing the RDI during REM. This is consistent with our data (Figs 1 and 2).
Sleep stage may be important for other reasons. REM apnoeas are associated with a greater increase in arterial pressure than those occurring in NREM, although the difference was fairly modest.23 While we were unable to control for body position in our analysis, others have demonstrated that sleep stage also determines the position dependency of obstructive apnoeas in some subjects.20 24 Interestingly, position dependence seems to be a largely NREM phenomenon. Patients tend to be nursed supine after many surgical procedures, so an increase in the NREM/REM ratio of apnoeas might be expected postoperatively. This is supported by the findings of Rosenberg-Adamsen et al.4 and would go against the assumption that REM rebound leads to greater risk. NREM supine apnoeic events may be an important factor in increasing apnoeic episodes in the early postoperative phase, before any period of REM rebound, consistent with the findings of Rosenberg et al.8
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Conclusion |
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Central apnoea and its sleep stage dependence has not been considered at all in the postoperative setting. Because CSA is common with patients suffering from cardiac dysfunction, a group already at increased perioperative risk, this disorder warrants further investigation. The reported NREM predominance of this form of sleep apnoea complicates the area of postoperative sleep disturbance considerably.
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Acknowledgements |
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References |
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