Anesth Analg 2004;99:70-76
© 2004 International Anesthesia Research Society
doi: 10.1213/01.ANE.0000125113.03812.2A
AMBULATORY ANESTHESIA
Nocturnal Arterial Oxygen Desaturation and Episodic Airway Obstruction After Ambulatory Surgery
T. Andrew Bowdle, MD PhD
Departments of Anesthesiology and Pharmaceutics, University of Washington, Seattle, Washington
Address correspondence to T. Andrew Bowdle, MD, PhD, Department of Anesthesiology, Box 356540, Room AA117C, University of Washington, Seattle, WA 98195. Address e-mail to bowdle{at}u.washington.edu Reprints will not be available from the author.
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Abstract
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Some patients experience disordered breathing during sleep and arterial oxygen desaturation after major inpatient surgery. We performed this study to determine whether similar events occur after ambulatory surgery. Forty-five ambulatory surgery patients received an unrestricted anesthetic. Continuous unattended nocturnal recordings of breathing pattern and oxygen saturation were made in the patients’ homes before surgery and during the first and second postoperative nights. Nine patients had a respiratory disturbance index >10 and/or >1% of recording time with oxygen saturation <90% on at least one study night. These nine patients had a significantly older median age and a significantly larger median body mass index. Their median respiratory disturbance index and median percentage of time with oxygen saturation <90% were significantly higher on the first postoperative night than on the preoperative night.
IMPLICATIONS: We studied nocturnal breathing before and after ambulatory surgery in 45 patients without a history of the sleep apnea syndrome. Nine patients had abnormal breathing that frequently resulted in hypoxemia. Oxygen desaturation occurred in the presence and absence of obvious upper airway obstruction. Five of the nine abnormally breathing patients had abnormal breathing before and after surgery.
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Introduction
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There are significant alterations in the physiology of sleep after major surgery, especially a reduction in rapid eye movement and slow-wave sleep (1,2). The severity of the sleep disturbance appears to be related to the magnitude of the surgical procedure (1–3). In addition, some surgical patients experience episodes of disordered breathing during sleep. These episodes resemble obstructive sleep apnea and result in arterial oxygen desaturation (4–6). Ambulatory surgery is now the most common mode of surgical care. Breathing during sleep after ambulatory surgery has not been studied, possibly because of the inherent difficulty in performing sleep studies in patients’ homes. Surgical procedures performed on ambulatory patients are usually relatively minor and might not be expected to produce disruptions of sleep physiology as severe as those that occur after major surgery. However, if disordered breathing during sleep is caused or exacerbated by ambulatory surgery, then some patients may be at risk from the resulting hypoxemia. The purpose of this study was to examine nocturnal breathing before and after ambulatory surgery in a variety of ambulatory surgery patients to determine the nature and extent of disordered breathing.
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Methods
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For this IRB-approved study, ambulatory surgery patients were recruited during their visit to the preoperative evaluation clinic and gave written, informed consent. Eligible patients were at least 18-yr-old, with ASA status I–III. Otherwise eligible patients were excluded if they had a history of sleep apnea documented by polysomnography; features suggestive of sleep apnea, such as frequent snoring or daytime hypersomnolence; a history of substance abuse; or current use of opioid or sedative/hypnotic drugs. Exclusion of patients for definite or suspected sleep apnea was based on the ordinary clinical judgment of the preoperative evaluation clinic staff, and no special screening protocols were put in place for this study. All types of operations were included, except for operations on the upper airway or operations on the face that would prevent placement of the airflow sensor on the upper lip. All types of anesthesia were allowed. A control recording was made on one night during the week before surgery, followed by recordings on the first and second postoperative nights.
Breathing measurements were recorded with a multichannel recorder designed for the study of breathing during sleep (EdenTrace II; Nellcor Puritan Bennett). Air flow from the nose and mouth was detected by a thermistor applied to the upper lip (EdenTec Model 971 Adult Airflow Sensor; Nellcor Puritan Bennett). Oxygen saturation (SpO2) was measured by pulse oximetry, with an adhesive probe applied to a finger (Oxisensor II D-25; Nellcor Puritan Bennett). Heart rate and chest wall motion (thoracic impedance plethysmography) (7) were measured with a pair of electrocardiograph (ECG) electrodes applied in the anterior axillary line, high on the chest near the axilla. Discrepancy in heart rate between the pulse oximeter and the ECG (typically due to motion-induced errors in detection of the pulse oximetry signal and/or the ECG signal) resulted in an artifact marker on the record; these sections of the record were excluded from analysis. A small, flat microphone for recording airway sounds was attached to the neck overlying the larynx with an adhesive disk and tape. Sounds >90 dB were indicated on the record; during sleep, these sounds are primarily the result of snoring. The various sensors were connected to the recorder through a compact wiring harness attached around the chest with an elastic strap. The EdenTrace II recorder was connected and activated to record data continuously when the patient retired for the evening. Patients were able to detach the cable from the recorder to get out of bed if necessary. The recorder suspends data collection until the cable is reattached. After a night of recording, the data were downloaded into a personal computer (EdenTrace II Archiving Software, Version 4.0; Nellcor Puritan Bennett). Data were saved on diskette and also printed directly to a fanfold paper record (EdenTec Model 3710 Digital Printer; Nellcor Puritan Bennett). Patients were prepared for recording, and data were downloaded to a portable computer by a nurse who visited the patients in their homes before each recording session. The nurse left after connecting the patient to the recorder, and the recording sessions were otherwise unattended. The recorder was set to make no audible alarms.
Data were analyzed by using proprietary software (EAS Version 1.1; Nellcor Puritan Bennett) on a personal computer. This software reviews the recording and identifies abnormalities according to the thresholds entered by the investigator. Because the algorithms for detecting abnormalities are not perfect, the investigator overread each record in its entirety and edited the EAS results. Essentially, the software functioned as a convenient tool for reviewing the records and tabulating the results, but it was not relied on for identifying and categorizing abnormalities.
Thresholds for breathing abnormalities were chosen on the basis of common practice in clinical somnography and the report of the American Academy of Sleep Medicine Task Force (8). Apnea was defined as a pause in airflow for at least 10 s. Hypopnea was defined as a decrease in airflow of at least 50% less than average amplitude for at least 10 s with a decrease in SpO2 of at least 5%. The respiratory disturbance index (RDI) was defined as the sum of the number of apneas and hypopneas divided by the recording time (units of events per hour). The percentage of recording time with SpO2 <90%, the mean SpO2, and the percentage of recording time with snoring were tabulated.
Results from the control night, the first postoperative night, and the second postoperative night were compared by Friedman repeated-measures analysis of variance with post hoc comparison with the Tukey test. Comparisons between the group of patients with normal breathing and the group of subjects with abnormal breathing (see below) were performed with the Mann-Whitney ranked sum test. P < 0.05 was considered significant.
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Results
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Forty-five patients were enrolled (30 men and 15 women). The median age was 38 yr (range, 19–79 yr). Median body mass index (BMI; weight/height2) was 23.7 kg/m2 (range, 17.4–34.5 kg/m2). The distribution of ASA status was 25, 19, 1, and 0 for ASA status I, II, III, and IV, respectively. Twenty-one patients (47%) received general anesthesia, 16 patients (35%) received regional anesthesia, and 8 patients (18%) received local anesthesia with or without procedural sedation. The categories of surgical procedures (with numbers of each) were head and neck (n = 10), abdominal (n = 4), urologic (n = 6), orthopedic (n = 19), gynecologic (n = 4), and miscellaneous (n = 2) (see Appendix 1 for additional details).
Thirty-six patients had relatively normal breathing. The results for these patients are shown in Table 1. A subset of 9 patients had abnormal breathing, as indicated by an RDI >10 and/or >1% of recording time with SpO2 <90% during at least one study night. Data for these patients are shown in Tables 1 and 2 and Figure 1. Representative portions of the recordings from these patients are shown in Figure 2. The median age of the larger group with normal breathing was significantly younger than the median age of the subset with abnormal breathing (35.5 yr [range, 19–68 yr] versus 65 yr [range, 34–79 yr], respectively; P = 0.002). The median BMI of the main group was significantly smaller than the median BMI of the subset (23.4 kg/m2 [range, 17.4–32.6 kg/m2] versus 30.3 kg/m2 [range, 20.3–34.5 kg/m2], respectively; P = 0.004). For the subset of patients with abnormal breathing, the median RDI and median percentage of recording time with SpO2 <90% were significantly greater on the first postoperative night than on the preoperative study night (P = 0.042 and P = 0.010, respectively). Five of the nine patients (B, D, E, F, and G in Table 2) with abnormal breathing had abnormal studies on the preoperative study night. The five patients with abnormal breathing on the preoperative study night were similar in age and BMI to the four patients (A, C, H, and I in Table 2) with normal breathing studies on the preoperative study night.
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Table 2. Subset of Patients with Respiratory Disturbance Index (RDI) >10 and/or >1% of Recording Time with SpO2 <90%
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Figure 1. Respiratory disturbance index (RDI) and percentage of recording time with oxygen saturation (SpO2) <90% for the subset of patients with abnormal breathing (n = 9). The median (bar) and 90th percentile (error bar) are shown for each study session (preop = preoperative study; postop 1 = first postoperative night; postop 2 = second postoperative night). The RDI and percentage of recording time with oxygen saturation <90% were significantly greater on the first postoperative night compared with the preoperative night (*P < 0.05). Data for individual patients are shown in Table 2.
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Figure 2. Examples of nocturnal recordings. The top and bottom panels are from two different patients; the top panel (A) is from the patient designated as "H" in Table 2, and the bottom panel (B) is from the patient designated as "A" in Table 2. The time scale of the recording is 1 mm/s (the smallest vertical division is 1 mm). There are four main recording channels: heart rate, impedance, airflow, and oxygen saturation (SpO2). Snoring is indicated beneath the SpO2 channel by "boxes." The fine, rapid oscillations in the impedance tracing are caused by cardiac activity, whereas the larger, slower oscillations are due to breathing. Patient "H," represented in the top panel, was relatively hypoxemic, with oxygen saturation generally ranging from approximately 80% to the low 90% range. There were several episodic dips in oxygen saturation accompanied by reduced oscillation in the airflow sensor. A large amount of snoring was apparent by the boxes at the bottom of the record. Patient "A," represented in the bottom panel, had relatively normal oxygen saturation despite repeated episodes of apnea in which there was no chest wall movement and no air movement; the oscillations on the impedance tracing during the apneic episodes were cardiac in origin. The apneic episodes were accompanied by small desaturations. In contrast to Patient "H," no snoring was apparent.
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Discussion
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Nine patients were identified as having notably abnormal breathing by using the criteria of an RDI >10 and/or >1% of recording time with SpO2 <90% on at least one study night. The division of the 45 patients into 2 groups based on these criteria is arbitrary; however, in scrutinizing the data, it became apparent that the data were not continuously distributed. There were patients with relatively normal breathing and outliers whose breathing was clearly abnormal. The use of an RDI >10 is a relatively strict criterion for establishing the presence of significant episodic upper airway obstruction, because an RDI >5 is often cited as a threshold value for the diagnosis of sleep apnea (8). One percent of recording time with SpO2 <90% would correspond to approximately four minutes of hypoxemia during a six-hour sleep period. The statistically significant increase in the median RDI and the median percentage of recording time with SpO2 <90% between the preoperative study and the first preoperative night (Fig. 1) suggests that ambulatory surgery exacerbated breathing abnormalities during sleep.
For the subset of 9 patients, the median percentage of time with SpO2 <90% was 1.2% and 2.7% for the first and second postoperative nights, respectively. This magnitude of hypoxemia was similar to that found in several previous studies of breathing after major inpatient surgery (9–11). Among the nine patients with notably abnormal nocturnal breathing, there was evidence of multiple mechanisms. Not all patients with evidence of upper airway obstruction had severe hypoxemia, and not all of the patients with severe hypoxemia had evidence of upper airway obstruction. Of the six patients (A, D, E, G, H, and I in Table 2 ) with an RDI >10 (indicative of significant episodic airway obstruction), only 2 (G and H) had SpO2 <90% for more than 1% of recording time. Thus, significant episodic upper airway obstruction per se did not always produce severe oxygen desaturation. Three patients (B, C, and F in Table 2) had significant desaturation despite RDI values <5. The explanation for the presence or absence of severe oxygen desaturation in patients with upper airway obstruction is not evident from the data provided by this study. Pulmonary reserve is a possible factor. Patients with relatively poor pulmonary reserve (e.g., those with obesity or chronic lung disease) could be more vulnerable to hypoxemia induced by upper airway obstruction. Sleep has been previously reported to decrease the functional residual capacity of healthy volunteers, and this mechanism may be even more important in patients with abnormal pulmonary reserve (12). Another possibility is that more severe airway obstruction and hypoventilation may have resulted in lower SpO2. The RDI measures the frequency of apneic and hypopneic events but does not gauge their severity.
The median age of the subset of 9 patients with abnormal breathing was significantly older than the main group of 36 patients with normal breathing. The subset also had a significantly larger median BMI. These results are consistent with previous studies that found a relationship between aging (4,13) or obesity (14) and disordered breathing during sleep.
Recordings were made from the time the patients retired at night until they arose in the morning. Sleep stages were not determined. There were several reasons for taking this approach. First, determination of sleep stages requires full polysomnography with electroencephalogram, which would not have been possible outside of an inpatient or sleep laboratory setting; a key feature of this design was to study ambulatory surgery patients at home. Second, portable studies of sleep-disordered breathing have been shown to give results similar to laboratory polysomnography (15,16). Third, previous polysomnographic studies have shown that episodic nocturnal oxygen desaturation occurs primarily during sleep (4). However, the absence of sleep staging imposes certain limitations on the interpretation of the data. Recording time would exceed sleep time by the time required for onset of sleep, plus any interruptions of sleep. Therefore, the RDI, which is the ratio of apneas plus hypopneas to sleep time, will be underestimated in this study because of the overestimation of sleep time, and the severity of disordered breathing will be understated.
Whether the nocturnal oxygen desaturation identified in this study and previous studies of inpatient surgery results in morbidity and/or mortality is unknown. Previous anecdotal reports have attempted to link sleep-disordered breathing to postoperative delirium (17,18), stroke (19–21), myocardial infarction (22,23), arrhythmias (24), ST depression (25), and sudden death (26,27). A very large trial would be required to identify the relationship of nocturnal oxygen desaturation to infrequent events such as stroke and myocardial infarction. If a relationship between nocturnal oxygen desaturation and outcome were proven, then effort could be directed at identifying patients at risk and intervening to prevent hypoxemia. Interventions would probably include the administration of oxygen and/or SpO2 monitoring. Risk factors for postoperative nocturnal hypoxemia are poorly understood. The results of this study suggest that older age and larger BMI are risk factors. Gentil et al. (28) have suggested that a history of heavy snoring is a significant predictor of nocturnal hypoxemia after major inpatient surgery. Clearly, additional studies are needed to guide clinical management.
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Acknowledgments
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Supported in part by a grant from the Anesthesia Patient Safety Foundation and by a loan of equipment from Nellcor Puritan Bennett.
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Accepted for publication February 10, 2004.
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Abstract
Introduction
