Postoperative complications in patients with obstructive sleep apnea: a retrospective matched cohort study
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Obstructive sleep apnea (OSA) is presumed to be a risk factor for postoperative morbidity and mortality, but the current evidence is incomplete. This retrospective matched cohort study tested the hypothesis that OSA is a risk factor for the development of postoperative complications.
Hospital ethics approval was obtained for the conduct of this study. The patients who were selected for the study were >18 yr of age, diagnosed preoperatively with OSA, and scheduled to undergo elective surgery. A cohort of surgical patients without OSA was used as a comparator group based on a one-to-one match. Matching criteria included gender, age difference <5 yr, type of surgery, and a <5 yr difference between two surgery dates. Summary data are presented and conditional logistic regression was used to identify risk factors for postoperative complications.
The 240 pairs of study subjects aged 57 ± 13 yr included 184 (77%) males and 56 (23%) females. The OSA patients had a higher mean body mass index relative to their non-OSA counterparts (35 ± 9 vs 28 ± 6 kg · m−2, respectively) and a higher frequency of co-morbidities, including hypertension (48% vs 36%, respectively) and obesity (61% vs 23%, respectively). Also, the incidence of postoperative complications in the OSA patients was significantly greater (44% OSA group vs 28% non-OSA group; P < 0.05). The most commonly observed between-group difference was oxygen desaturation < 90% (17% OSA group vs 8% non-OSA group). The OSA patients who did not use home continuous positive airway pressure (CPAP) devices prior to surgery but required the use of a CPAP device after surgery had the highest rate of complications. Conditional logistic regression was used to diagnose OSA and pre-existing stroke as significant risk factors for developing postoperative complications. The hazard ratio for OSA was 2.0 (1.25–3.19).
Patients with diagnosed OSA have an increased incidence of postoperative complications, the most frequent being oxygen desaturation.
Complications postopératoires chez des patients souffrant d’apnée obstructive du sommeil: une étude de cohorte appariée rétrospective
Il est présumé que l’apnée obstructive du sommeil (AOS) est un facteur de risque pour la morbidité et la mortalité postopératoires, mais les données probantes actuelles sont incomplètes. Cette étude de cohorte appariée rétrospective a testé l’hypothèse selon laquelle l’AOS est un facteur de risque pour l’apparition de complications postopératoires.
La réalisation de cette étude a été approuvée par le comité d’éthique de l’hôpital. Les patients sélectionnés pour participer à l’étude avaient > 18 ans, avaient reçu un diagnostic préopératoire d’AOS, et devaient subir une chirurgie non urgente. Une cohorte de patients chirurgicaux ne souffrant pas d’AOS a été utilisée comme groupe de comparaison, avec une correspondance injective. Les critères de correspondance incluaient le sexe, une différence d’âge < 5 ans, le type de chirurgie, et un délai < 5 ans entre les deux dates de chirurgie. Les données agrégées sont présentées et un modèle de régression logistique conditionnelle a été utilisé pour déterminer les facteurs de risque de complications postopératoires.
Les 240 paires de patients à l’étude, âgés de 57 ± 13 ans, étaient composées de 184 (77 %) hommes et de 56 (23 %) femmes. Les patients souffrant d’AOS avaient un indice de masse corporelle moyen plus élevé par rapport à leurs pendants sans AOS (35 ± 9 vs 28 ± 6 kg·m−2, respectivement) et une fréquence plus élevée de comorbidités, notamment l’hypertension (48 % vs 36 %, respectivement) et l’obésité (61 % vs 23 %, respectivement). De plus, l’incidence de complications postopératoires dans le groupe AOS était significativement plus élevée (44 % groupe AOS vs 28 % groupe non AOS; P < 0,05). La différence inter-groupe la plus communément observée était une désaturation en oxygène < 90 % (17 % groupe AOS vs 8 % groupe non AOS). Les patients AOS qui n’utilisaient pas d’appareil de ventilation en pression positive continue (PPC) à la maison avant la chirurgie mais qui ont nécessité l’utilisation d’un tel appareil après la chirurgie ont eu le taux de complications le plus élevé. La régression logistique conditionnelle a été utilisée pour diagnostiquer l’AOS et les accidents vasculaires cérébraux préexistants en tant que facteurs de risque significatifs pour l’apparition de complications postopératoires. Le taux de risque pour l’AOS était de 2,0 (1,25 – 3,19).
Les patients pour lesquels un diagnostic d’AOS a été posé sont plus à risque de souffrir de complications postopératoires, la plus fréquente étant la désaturation en oxygène.
Obstructive sleep apnea (OSA), the most prevalent breathing disturbance in sleep,1 affects 2–26% of the general population, depending on sex, age, and the criteria of diagnosis.2 A growing body of literature suggests that OSA has been implicated in the development of cardiorespiratory diseases.3, 4, 5, 6, 7, 8, 9, 10 OSA also has a significant impact on both acute and chronic cardiorespiratory function.11
Although no large epidemiologic studies have been undertaken to determine the prevalence of OSA in the general surgical population, reports of several studies suggest that the prevalence of OSA in the surgical population might be higher than in the general population, with variability among different surgical populations. In particular, as many as 29/41 (71%) of patients undergoing bariatric surgery were found to have OSA.12 In the general surgical population, 24% of surgical patients were identified by the Berlin questionnaire as being at a high risk of having OSA.13
Physiological studies show that the upper airway muscles are more sensitive to neuromuscular blocking drugs than either the diaphragm or the peripheral muscles.14 Patients with OSA are highly vulnerable to medications that suppress pharyngeal muscle activity.15, 16, 17 In addition to compromising upper airway muscle function, general anesthesia and narcotics can depress the patients’ ventilatory responses to obstruction and inhibit their normal arousal and awakening responses to hypoxia and hypercapnia.18 A high incidence of early and late postoperative nocturnal hypoxemic episodes in healthy patients undergoing surgery has been reported in a number of studies.19, 20, 21
However, the currently available literature on the occurrence of perioperative respiratory complications among surgical patients with OSA is too limited to make any major correlations. The objective of this study was to test the hypothesis that OSA is a risk factor for the development of postoperative complications.
We undertook a retrospective matched cohort study of a large cross section of surgical patients at the University Health Network, Toronto. Following research ethics board approval, the study subjects were selected from the hospital administrative database. The institutional research ethics board waived the requirement for individual informed consent.
Surgical patients in the OSA group were selected based on the following criteria: (1) age > 18 yr; (2) diagnosis with OSA when discharged from hospital (the discharge diagnosis of OSA was coded according to the international classification of disease (ICD-9) codes; (3) undergoing elective surgery under general, regional, or local anesthesia with monitored anesthesia care at the University Health Network during the period January 1, 1990 to December 31, 2005. Exclusion criteria included patients who were undergoing surgical procedures involving the upper airway, including tonsillectomy, septoplasty, uvuloplasty, uvulopalatoplasty, uvulopharyngoplasty, or uvulopalatopharyngoplasty, as these procedures were most likely to have been indicated to cure the primary disease process (OSA) and, thus, would be impossible to match. The non-OSA patients (non-OSA group) were based on a one-to-one match with the OSA patients and were selected from a cohort of surgical patients without an OSA diagnosis at the time of hospital discharge. The match criteria included gender, age difference < 5 yr, same type of surgery, and <5 yr between the two surgery dates.
The primary outcome variable was the incidence of the total postoperative complications. Data collection was implemented through chart review. The charts of the selected patients were reviewed by a research anesthesiologist (S.V.). The following data were collected: demographic data (including gender and age at the time of surgery), ASA physical status, pre-existing medical conditions, concurrent medications, type of surgery and anesthesia, postoperative complications and therapeutic interventions. The definitions of postoperative complications are shown in the Appendix.
In a previously published paper of a similar design,22 the incidence of postoperative complications was 39% among OSA patients and 18% among non-OSA patients. Assuming a similar incidence of postoperative complications in our study, with α = 0.05 and β = 0.9, we required 132 pairs of matching OSA and non-OSA patients.
The data collected were entered into a specifically designed Microsoft Access™ database (Microsoft Corporation, Redmond, WA, USA). SAS® 9.1.3 for Windows® (SAS Institute Inc., Cary, NC, USA) was used for data analysis. The demographic data and prevalence of the pre-existing co-morbidities were summarized. The incidence of postoperative complications and related treatments were compared between the OSA patients and the matched non-OSA patients. Testing of differences between the two groups was undertaken using the McNemar test for categorical data and Student’s paired t tests for numerical data.
Conditional logistic regression was used to adjust for potential confounding variables.23,24 The selection of potential risk factors was based mainly on the clinical relevance to the postoperative outcome. The correlation among the potential risk factors was also checked. When there was a correlation between two or more risk factors with a correlation coefficient >0.5, only one risk factor was retained. The chosen potential risk factors were used as independent variables, and the frequency of patients with one or more postoperative adverse events was used as the dependent variable for conditional logistic regression analysis. A backward automatic selection with P < 0.2 was used in conditional logistic regression. The appropriateness of the model was assessed based on fit statistics and testing for a global null hypothesis with β = 0.
Two hundred ninety-four patients with an OSA diagnosis underwent different types of surgery; however, 19 of these were excluded from our study due to having upper airway surgical procedures. To establish matched pairs, 275 OSA patient records were compared with the records of patients without diagnosed OSA. Two hundred forty patients with OSA were successfully matched with 240 patients without diagnosed OSA. The analysis is based on the data from the 240 matched pairs.
BMI (kg · m−2)
ASA physical status
Stroke or TIA
Type of surgery and anesthesia
Type of surgery
Type of anesthesia
General + regionala
Airway assessment and tracheal intubation
Total postoperative complications (case)a
Total respiratory complication (case)b
Mild desaturation (90% < SaO2 ≤ 95%)
Severe desaturation (SaO2 ≤ 90%)
Upper airway obstruction
Arrive intubated in PACU
Total cardiac complication (case)c
Total neurology complication (case)d
Stroke or TIA
Detailed information of patients suffering cardiac arrest
Type of anesthesia
Transphenoidal resection of pituitary
Hypertension asthma, hypothyroidism smoker
Occurred after induction of anesthesia, successfully resuscitated
Occurred after extubation, re-intubated and transferred to ICU. The patient was resuscitated and died 5 days later
Repair of colon parastomal hernia
General anesthesia with intubation
Hypertension, CAD, COPD, asthma, DM, acute MI
Acute MI induced cardiac arrest. The resuscitation was unsuccessful
Postoperative treatment and follow-up
Prolonged oxygen therapy
Pulmonary edema treatment
CPAP used postop
Total admission to ICUa
Stay in ICU (hr)b
Total hospital stay (days)b
Re-admission within 7 days
Re-admission within 8–30 days
ER visit within 30 days
Independent risk factors of postoperative complications
Risk factors for postoperative complicationsa
Hazard ratio (95% confidence interval)
ASA physical status
Postoperative complications and need for CPAP in OSA patients
Use of CPAP and postoperative complications in OSA patients
OSA patients not on home CPAP (n = 90)
OSA patients on home CPAP (n = 150)
No postoperative CPAP
No postoperative CPAP
Total postoperative complications (cases)
Total respiratory complications (cases)
Mild desaturation (SaO2 91–92%)
Severe desaturation (SaO2 90%)
Prolonged oxygen therapy
Total admissions to ICU
This retrospective cohort study demonstrates that patients with OSA have a higher incidence of postoperative complications compared with matched non-OSA surgical patients. Oxygen desaturation with SpO2 < 90% was the most common complication. The OSA patients who were not receiving home CPAP therapy before surgery and required CPAP after surgery had the highest incidence of postoperative complications. There was a need for a greater number of treatments and interventions in the OSA group, including prolonged oxygen therapy and additional monitoring. Multivariable logistic regression analysis demonstrated that the diagnosis of OSA and pre-existing stroke are risk factors for the occurrence of postoperative complications.
We recognize that this study has a number of important limitations. First, this was a retrospective analysis with all outcomes collected post hoc. The results must be considered as merely associations and causality cannot be implied. Second, patients with OSA were identified by using ICD-10 codes that are entered by non-physician coders after the patient has been discharged from hospital. Since the completion of this study, we have undertaken to quantitate the sensitivity and positive predictive value of ICD-10 codes to identify patients with OSA. During the year April 2008 to April 2009, there were 5542 elective non-cardiac surgical patients assessed prior to surgery. Of the 379 patients diagnosed with OSA (6.8%), 36 had an ICD-10 code which specified a preoperative diagnosis of OSA. Thus, the sensitivity of ICD-10 codes is 0.09 with a positive predictive value of 0.86. We are uncertain why only 10% of OSA patients were identified; perhaps our analysis preferentially identified a group of patients with severe OSA. Thus, our results may be applicable only to patients with moderate and severe OSA. Third, and related to the above point, there were no polysomnographic data available for the patients with OSA; thus, we could not quantitate the severity of OSA. Finally, our matching process was constructed so that age, gender, type of surgery, and surgery date were matched. However, this process did not match for individual patient characteristics, and our results show that there were more associated co-morbidities in the OSA patients, including higher ASA status, NYHA status, hypertension, diabetes, asthma, and gastroesophageal reflux disease (GERD). Thus, we cannot exclude the possibility that the associated co-morbidities may account for the higher complication rates rather than the diagnosis of OSA itself. Despite the above limitations, this study indicates that patients diagnosed with OSA had an increased incidence of postoperative adverse events, mainly oxygen desaturation with SpO2 < 90%. After adjusting for other confounding factors using conditional logistic regression, OSA remained a significant risk factor with an odds ratio of 2.00 (1.25–3.19).
Most previously published studies examining postoperative complications in OSA patients focused on patients who underwent upper airway surgeries.25, 26, 27, 28, 29, 30, 31 Only a few studies have evaluated postoperative complications in patients who underwent other types of surgery.22,32, 33, 34, 35, 36 Our study complements this growing body of literature by demonstrating that OSA patients undergoing different types of surgery have an increased rate of postoperative complications, mainly oxygen desaturation. In a retrospective study of 101 patients with OSA who were undergoing hip replacement or knee replacement, Gupta et al.22 demonstrated that patients with OSA have an increased incidence of total postoperative complications and major complications. The patients with OSA in that study also had a higher percentage of total and unplanned intensive care unit (ICU) transfers and a longer hospital stay.22 In another retrospective study of 37 patients with OSA undergoing cardiac surgery, Kaw et al.32 found that there was an increased incidence of postoperative encephalopathy and infection rates (mostly mediastinitis). Hwang et al.35 showed that surgical patients with the clinical features of OSA and oxygen desaturation index ≥5 on home nocturnal oximetry before surgery had a significantly higher rate of postoperative complications. In another study conducted by Chung et al.,34 the patients who had apnea-hypopnea index >5 on preoperative polysomnography had a higher incidence of postoperative complications. These studies all support that OSA in surgical patients is associated with an increased incidence of postoperative complications.
In this study, hypoxemia (oxygen desaturation) was the most common postoperative complication in patients with OSA. Three other studies have also demonstrated a similar result.22,34,35 However, a recent study of 31 OSA and 9 non-OSA morbidly obese patients did not find a difference in the number of hypoxemic episodes during the first 24 hr after surgery.36 This similarity of observed events may have been due to the high percentage of patients in both groups receiving oxygen therapy during the first 24 hr after surgery.36 Recently, we found that the apnea-hypopnea index (AHI) and the oxygen-desaturation index are greater among OSA patients on the third postoperative night compared with either the first postoperative night or preoperatively.37
In Gupta’s retrospective study on 101 matched pairs of patients undergoing hip or knee replacement, major complications (cardiac events and complications needing ICU transfer or urgent respiratory support) were significantly higher in patients with OSA.22 Our study did not show an increase in serious cardiopulmonary complications in OSA patients vs matched non-OSA patients. The cardiac arrests in two OSA patients, one during tracheal intubation and the other after extubation, were related to problems with airway management. This further supports the notion that airway management in OSA patients can be difficult. The cardiac arrest that occurred in one non-OSA patient was related to myocardial infarction.
Our results suggest that patients with OSA require more perioperative care than their non-OSA counterparts. Patients with OSA required prolonged oxygen therapy and additional monitoring and were admitted to ICU more frequently. Gupta’s study also demonstrated that more OSA patients required ICU care, both planned and unplanned.22 The unplanned ICU transfers occurred mainly in patients whose OSA was undiagnosed at the time of surgery. In our study, it is likely that patients were diagnosed before surgery, although we cannot exclude the possibility that OSA was diagnosed as a result of a perioperative event. The departmental policy that existed during the study period did not require OSA patients to be monitored more intensively or to be admitted to ICU. However, the anesthesiologist may have been aware of OSA when considering the need for ICU. This might account for the higher number of patients requiring ICU transfer in the OSA group and no increase in the unplanned ICU admission rate.
The recent ASA practice guideline for the perioperative management of patients with OSA recommends that the CPAP or nasal intermittent positive pressure ventilation (NIPPV) should be administered postoperatively to the OSA patients on home CPAP.38 As a matter of routine in most hospitals, a patient with OSA on home CPAP would be ordered to receive CPAP after surgery. However, our data suggests this guideline is not routinely observed, as only 63% of OSA patients on home CPAP received CPAP postoperatively. This observation suggests the need for education programs to ensure greater consistency of care for patients with OSA.
The patients with OSA who were not receiving home CPAP included two different patient populations, i.e., patients who did not require CPAP because of mild OSA and patients who were non-compliant with their CPAP devices. In our study, 27% of OSA patients not using home CPAP required CPAP postoperatively. These patients had the highest incidence of postoperative complications (Table 8). This finding is likely due to CPAP having been initiated in response to an adverse event, which may also have led to the chart abstracters coding for the disease. However, we must again reiterate that this is a retrospective study, and it is difficult to identify the exact reasons for the high incidence of postoperative complications in this group of patients.
In conclusion, these results suggest that one aspect of the preoperative assessment should focus on the evaluation and diagnosis of OSA. Patients who are non-compliant with their treatment of CPAP are at an increased risk of postoperative oxygen desaturation. These results await further prospective assessment.
We are grateful to Dr. Gordon Tait, PhD, Staff Scientist and Assistant Professor, Department of Anesthesia, University of Toronto, who generously shared the data regarding sensitivity and positive predictive values of ICD-10 coding on patients with obstructive sleep apnea.
Sources of financial support for the work (including institutional support): University Health Network Foundation, Physician Services Incorporated Foundation and University of Toronto, Toronto, Ontario, Canada.
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