Is obstructive sleep apnoea a rapid eye movement-predominant phenomenon?

J. A. Loadsman1 and I. Wilcox2

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


    Abstract
 Top
 Abstract
 Introduction
 Patients and methods
 Results
 Discussion
 Conclusion
 References
 
Obstructive sleep apnoea (OSA) is thought to be worse during rapid eye movement (REM) sleep. REM rebound in the late postoperative period can follow the REM suppression shown to occur after some types of surgery. This is thought to worsen nocturnal episodic hypoxaemia, leading to greater cardio-respiratory risk. We set out to determine if OSA was a REM-predominant phenomenon. We reviewed the sleep clinic records of 64 consecutive patients with a diagnosis of OSA on full overnight polysomnography and sufficient data to determine the presence of a sleep stage predominance. OSA was diagnosed if the number of apnoeas/hypopnoeas per hour of sleep, the respiratory disturbance index (RDI), was greater than 10. The variables recorded for the purposes of this study were the RDI and the minimum blood oxygen saturation using pulse oximetry (SpO2min) for both REM and non-rapid eye movement (NREM) sleep. All values are presented as mean (SD). The Wilcoxon signed rank test was used for statistical analysis. The means for NREM and REM RDI were, respectively, 36 (26) and 38 (27) per hour (P=0.96). In 32 of the 64 patients (50%) the RDI in NREM was greater than in REM. Thirty-one (48%) had a larger number during REM. One patient had identical RDIs for both REM and NREM. Sixty-two patients had satisfactory pulse oximetry recordings for both NREM and REM, and the mean SpO2min values were, respectively, 84 (7) and 82 (13)% (P=0.15). Twenty-nine patients (47%) had a lower SpO2min in REM (seven by more than 10% and two by more than 40%), while 24 (39%) were lower in NREM (two by more than 10%). Nine patients (14%) had identical values in REM and NREM. In contrast to suggestions that OSA is a REM-predominant phenomenon, this study suggests that respiratory disturbance is not greatly affected by sleep stage, in most patients. While a small number clearly desaturate much more during REM, the majority do not. Thus, postoperative REM rebound may worsen OSA in some patients, but in many it may do otherwise. The implications of postoperative sleep disturbance are therefore likely to be more complex than previously suggested.

Br J Anaesth 2000; 85: 354–8

Keywords: ventilation, apnoea; sleep; anaesthesia; surgery


    Introduction
 Top
 Abstract
 Introduction
 Patients and methods
 Results
 Discussion
 Conclusion
 References
 
Anaesthetic articles suggest that obstructive sleep apnoea (OSA) is a rapid eye movement (REM)-predominant phenomenon, and that REM rebound in the late postoperative period, which follows REM suppression after some types of surgery, can worsen nocturnal episodic hypoxaemia.15 The results of the first preoperative polysomnogram (PSG) carried out as part of another investigation currently in progress suggested that this might not always be the case. This particular patient had previously undiagnosed, mild obstructive sleep apnoea, which was non-rapid eye movement (NREM) stage two predominant.

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.


    Patients and methods
 Top
 Abstract
 Introduction
 Patients and methods
 Results
 Discussion
 Conclusion
 References
 
Patient selection
The medical records of 148 consecutive new patients seen by one physician (over a period of ~2 yr) in a sleep disorders clinic were reviewed retrospectively. The patients referred to the clinic are assigned randomly, according to time available on a first come first served basis, to one of several physicians by the secretarial staff when the booking is made.

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.


    Results
 Top
 Abstract
 Introduction
 Patients and methods
 Results
 Discussion
 Conclusion
 References
 
Respiratory disturbance index
The mean values for NREM and REM RDI were, respectively, 36 (26) and 38 (27) per hour (P=0.96). In 32 of the 64 studies (50%), the RDI in NREM was greater than in REM. Thirty-one (48%) had a larger number during REM. One patient had identical values for both REM and NREM. The REM versus NREM value for each patient is plotted in Fig. 1.



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Fig 1 The respiratory disturbance index (events per hour) in REM plotted against the respiratory disturbance index in NREM for each individual patient in the study. Those above the diagonal represent patients with a higher RDI during REM and vice versa.

 
Minimum saturation
Two patients did not have the SpO2min recorded for one or both of REM or NREM, leaving 62 for analysis. The mean values for NREM and REM SpO2min, respectively, were 84 (7) and 82 (13)% (P=0.15). Twenty-nine patients (47%) had a lower SpO2min in REM (seven by more than 10% and two by more than 40%), while 24 (39%) were lower in NREM (two by more than 10%). Nine patients (14%) had identical values in REM and NREM. The REM versus NREM value for each patient is plotted in Fig. 2. Those patients with the lowest overnight saturation had a tendency for this to occur during REM, while the milder apnoeics had similar nadirs for both REM and NREM.



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Fig 2 The minimum blood oxygen saturation (per cent) by pulse oximetry in REM plotted against the minimum saturation in NREM for each individual patient in the study. Those above the diagonal represent patients with a higher saturation nadir during REM and vice versa.

 
Effect of age and body habitus
To determine if age or body habitus had any significant influence on sleep stage predominance, the REM values for both RDI and SpO2min were subtracted from the corresponding NREM values, and the results plotted against both the age of the individuals and their body mass index (BMI). The plots are shown in Figs 3 and 4. Age had no discernible effect on sleep stage predominance in OSA with either RDI (slope=0.27, r2=0.019) or SpO2min (slope=0.015, r2=0.000). With increasing BMI there would appear to be a slight trend towards REM predominance, more so for SpO2min (slope=0.47, r2=0.093) than RDI (slope=–1.12, r2=0.083). At best, our data suggest that there is an increased likelihood of REM predominance in terms of minimum saturation only if the BMI is greater than 35 kg m–2.




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Fig 3 (A) The difference between the NREM and REM respiratory disturbance indices (events per hour) for each patient plotted against his/her age in years. (B) The difference between the NREM and REM saturation nadirs (per cent) for each patient plotted against his/her age in years.

 



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Fig 4 (A) The difference between the NREM and REM respiratory disturbance indices (events per hour) for each patient plotted against his/her body mass index (kg m–2). (B) The difference between the NREM and REM saturation nadirs (per cent) for each patient plotted against his/her body mass index (kg m–2).

 

    Discussion
 Top
 Abstract
 Introduction
 Patients and methods
 Results
 Discussion
 Conclusion
 References
 
Major surgery is followed by a period of significant alteration in sleep architecture.2 610 Most notably, REM, slow wave sleep (SWS – NREM sleep stages three and four) and total sleep time are variably reduced in the nights immediately following surgery, and there is a tendency for these to rebound for one or two nights thereafter.2 8 9 Perhaps based on an assumption that sleep apnoea is worse during REM, some consider that REM rebound is likely to be associated with increased risk of late postoperative nocturnal hypoxaemia and consequently ischaemia-related complications.

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 m–2 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


    Conclusion
 Top
 Abstract
 Introduction
 Patients and methods
 Results
 Discussion
 Conclusion
 References
 
We have limited data on the role of sleep in the recovery of patients from surgery and anaesthesia. Most of the views expressed have been largely speculative. Although a REM rebound-related increase in apnoea is a possible cause of postoperative morbidity, our data from the sleep laboratory suggest that this assumption should be considered critically, and that more direct evidence from the postoperative ward is required. A small number of patients, mainly those with severe OSA, do appear to have a tendency to REM predominance of their apnoea. REM rebound might be more important to this group. Preoperative identification of such patients remains a major problem. The effects of sleep stage on factors other than upper airway patency also need to be determined.

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.


    Acknowledgements
 
John Loadsman is the recipient of grants from both the Australian and New Zealand College of Anaesthetists and the Australian Society of Anaesthetists. We are also grateful for the financial support provided to the University of Sydney Department of Anaesthetics by Roche Products, Australia, Novartis Pharmaceuticals, Australia, and Hewlett-Packard, Australia.


    References
 Top
 Abstract
 Introduction
 Patients and methods
 Results
 Discussion
 Conclusion
 References
 
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