Anesth Analg 2001;93:917-921
© 2001 International Anesthesia Research Society
AMBULATORY ANESTHESIA
Ambulatory Surgery: Room Air Versus Nasal Cannula Oxygen During Transport After General Anesthesia
Donald D. Mathes, MD*,
Mark R. Conaway, PhD
, and
William T. Ross, MD*
*Department of Anesthesiology and Department of Health Evaluation Sciences, Division of Epidemiology & Biostatistics, University of Virginia, Charlottesville, Virginia
Address correspondence and reprint requests to Donald D. Mathes, MD, 885 Charter Oaks Dr., Charlottesville, VA 22901. Address e-mail to jmathes{at}usa.net
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Abstract
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We compared outpatients transported to the postanesthesia care unit (PACU) while breathing room air to 2–4 L/min nasal cannula oxygen (O2) to test the hypothesis that routine supplemental O2 during transport is not required after general anesthesia in an ambulatory surgery center. We also examined whether the arbitrary arrival PACU O2 saturations of >92% may be used to predict an infrequent incidence of subsequent significant desaturations (<90%) in the PACU. One-hundred-ninety patients were randomized to receive either room air or 2–4 L/min nasal cannula for transport to PACU after receiving general anesthesia. O2 saturations were recorded before surgery, just before leaving the operating room, and upon arrival in the PACU. The lowest O2 saturation occurring in the PACU was also recorded. The mean arrival PACU O2 saturation was 95.0 in the Room Air group, compared with 97.2 for the Nasal Cannula (NC) group, a statistically significant difference (P < 0.001). In the Room Air group, 20% had arrival O2 saturations 
92%, and half of these (10%) had O2 saturations <90%. In the NC group, 6% had O2 saturations 92%, of which one third (2%) were <90% on arrival in the PACU. All of these initial desaturations were easily corrected with face-tent O2 administration, deep breathing, or both. Subgroup analysis revealed that patients whose ages were 60 yr or older or those weighing 100 kg or more had lower arrival room air saturations than their younger or slimmer counterparts. In the Room Air group, only three (3.9%) of the patients that arrived in PACU with O2 saturations >92% had subsequent desaturations <90%, compared with seven (7.9%) in the NC group. We conclude that most adult patients undergoing ambulatory surgery can be transported safely to the PACU breathing room air after general anesthesia. However, patients whose age was 
60 yr or weight was 100 kg, or for whom transient O2 desaturation on transport may be harmful, should be transported while breathing nasal O2 via nasal cannula.
IMPLICATIONS: Most adult patients undergoing ambulatory surgery can be transported safely to the PACU breathing room air after general anesthesia. However, patients whose age was 60 yr or weight 100 kg, or for whom transient O2 desaturation on transport may be harmful, should be transported while breathing oxygen via nasal cannula.
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Introduction
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Several studies have found a significant incidence of oxygen (O2) desaturation on arrival in the postanesthesia care unit (PACU) when transporting patients from the operating room while breathing room air (RA) after general anesthesia (1–3). These studies were performed in hospital operating rooms where the distance from the operating rooms to the PACU may be considerable and many patients may have undergone major abdominal or thoracic surgery. These studies recommend the routine use of supplemental O2 for transport to the PACU. However, for ambulatory surgery, no study cited in MEDLINE has investigated the incidence of desaturations during transport to the PACU while the patient breathes RA after general anesthesia.
We hypothesized that patients in an ambulatory surgery center that are transported to the PACU breathing RA have a low incidence of arrival desaturations in the PACU because of its proximity to the operating room and the nature of surgery typically performed in an ambulatory center. Hence, we examined whether patients may be transported to the PACU safely while breathing RA after general anesthesia in ambulatory surgery. We also investigated whether arbitrary O2 saturations of >92% upon arrival in the PACU can be used as a predictor of an infrequent incidence of subsequent desaturations in the PACU. This value is based on the observation of Gift et al. (4), who found an infrequent incidence of subsequent desaturations for patients with arrival desaturations >92% when they were transported to the PACU breathing 4 L/min O2 via nasal cannula (NC).
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Methods
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A randomized, prospective study was performed on 190 patients undergoing general anesthesia in an ambulatory surgery center. The study was approved by the University of Virginia Health Science Center Human Investigation Committee. Patients were randomly assigned before the induction of general anesthesia to be transported to PACU on a stretcher while breathing either RA or 2–4 L/min NC O2. The altitude at the University of Virginia is approximately 600 feet above sea level. The distance from the operating rooms to the PACU ranged from 37 to 53 m. Average transport time from discontinuing 100% O2 in the operating room until arrival in the PACU was 3 to 5 min. O2 delivery systems used during transport to the PACU came from E cylinders with regulators and either a dial flow controller or a flowmeter. The NC used consisted of two prongs. The dial flow controller or flowmeter was set at a 4 L/min flow rate. Approximately 25% of the patients receiving NC O2 had O2 delivered from flowmeters set at 4 L/min, but the O2 tank under the stretcher sat at an angle approximately 30 degrees to the floor. These tanks, when placed perpendicular to the floor, revealed the flowmeter to be set at 2–2.5 L/min. Hence, the O2 delivery rate ranged from 2 to 4 L/min and was not always 4 L/min in patients receiving O2.
O2 saturations were recorded before surgery, just before leaving the operating room, and initially upon arrival in the PACU. Additionally, the lowest O2 saturation occurring in the PACU was recorded. Preoperative and operating room pulse oximetry determinations used Nellcor DS100 A sensors measured on a Hewlett-Packard Component Monitoring System/Anesthesia (Palo Alto, CA), and PACU pulse oximetry determinations used Nellcor DS100 A sensors (Nellcor, Pleasanton, CA) with Hewlett-Packard Omnicare Model 1204 monitors. Patients were excluded from the study for surgeries involving the airway, age <18 yr, preoperative O2 saturations 92%, pulse oximetry determinations that did not register O2 saturations at any of the measurement points, and transportation to the PACU with either laryngeal mask airway or endotracheal tube in the airway. Patients whose NC became displaced on transport to the PACU were also excluded.
How the general anesthetic was performed was determined by the anesthesiologist conducting the anesthetic. The induction of general anesthesia was predominately performed with IV propofol. General anesthesia was maintained predominately with a combination of isoflurane or sevoflurane volatile anesthesia, nitrous oxide, O2, and 1–2 µg/kg of IV fentanyl. Where nondepolarizing neuromuscular blocking drugs were used, the effect was antagonized before extubation. During emergence and before transfer to the stretcher, 100% O2 was administered with the patient breathing spontaneously. The duration of 100% O2 exposure before transfer to the PACU was not controlled. The position of the patient on transfer to PACU was not recorded or controlled. The discontinuation of O2 in the NC group or the addition of face-tent O2 in the PACU was at the clinical discretion of the recovery room nurse. In the NC group, O2 was discontinued typically between 5 and 20 min after arrival in the PACU.
Comparisons of O2 saturation outcomes between treatment groups were made with the two-sample Wilcoxon test. This test was also used to compare treatment groups within subgroups defined by asthma, tobacco use, age, and weight. Proportional odds regression models were used to compare treatment groups, adjusting for patient characteristics such as asthma, tobacco use, age, and weight; 
2 tests were used to compare treatment groups with respect to dichotomous outcomes, such as whether or not O2 levels on arrival to the PACU were 92%. Logistic regression models were used to compare treatment groups with respect to dichotomous outcomes, adjusting for characteristics such as asthma, tobacco use, age, and weight.
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Results
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Patient demographics revealed an even distribution of type of surgery, method of airway maintenance, ASA classification, weight, and age (Table 1). The RA group had a 12% incidence of asthma, compared with 6% in the NC group (P = 0.15), and there was a 41% incidence of tobacco use in the RA group, compared with 29% in the NC group (P = 0.08). In addition to the 190 patients in the study, 7 patients were eliminated because the NC became displaced on transport. Five patients were excluded from the study because of being transferred to PACU with a laryngeal mask airway or endotracheal tube in the airway. One patient was excluded because of developing laryngospasm on emergence.
Preoperative O2 saturations and O2 saturations upon leaving the operating room were similar (Table 2). Arrival O2 saturations in PACU were statistically higher in the NC group. The arrival O2 mean saturation was 95.0 in the RA group and 97.2 in the NC group (Table 2). Regression models were used to compare treatment groups, adjusting for the presence of asthma, tobacco use, surgical operating service, age, weight, and ASA classification. With regression models, patients receiving 2–4 L/min NC O2 had an average arrival O2 saturation in the PACU of 1.8% units more (95% confidence interval, 0.8–2.8; P < 0.001) than patients receiving no O2 support. Subgroup analysis revealed lower arrival PACU O2 saturations with RA for weight 100 kg (2.7% units difference), age 60 yr (1.9% units difference), and asthma (1.3% units difference). However, RA PACU arrival O2 saturations were 0.2% units higher for tobacco use (Table 3). In the RA group, 20% had O2 saturations 92% and 10% < 90% on arrival in the PACU, and in the NC group, 6% had O2 saturations 92% and 2% < 90% on arrival in the PACU (2 test, P = 0.0016) (Fig. 1). Three patients transported on RA had arrival O2 saturations <85%. Each of these patients had a single different demographic factor of age 60 yr old, weight 100 kg, or asthma.
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Figure 1. Initial arrival O2 saturation in the postanesthesia care unit (PACU) for patients transported breathing room air versus 2–4 L nasal cannula (NC) O2.
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All arrival desaturations <92% were easily corrected by instructing the patient to breathe deeply, sitting the patient up, placing face-tent O2 supplementation, or a combination of these. Eighteen patients in the RA group received face-tent O2 supplementation while in the PACU, compared with eight face tents used in the NC group. Four of the eight face tents placed in the NC group were placed for O2 desaturations after discontinuing NC O2 without using the initial NC used for transport. Placement of NC O2 was not attempted for desaturations in the RA group. The acquired cost analysis for the NC was US$0.52 per unit and US$1.50 per face-tent unit. The average acquired cost of all O2 delivery systems used for the RA group was US$0.29 per patient versus US$0.65 per patient for the NC group.
While in the PACU, the RA group’s mean lowest O2 saturation was 94.0, and the NC group’s mean lowest O2 saturation was 95.4. In the NC group, many of the lowest O2 saturations occurred after discontinuing NC O2. Of the 77 patients in the RA group that arrived in the PACU with O2 saturations >92%, only 3 (3.9%) had subsequent desaturations <90% in the PACU, compared with 7 of 88 patients (7.9%) in the NC group who had desaturations <90% when they were taken off O2 (P = 0.28, 2 analysis).
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Discussion
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Patients transported to the PACU with 2–4 L/min NC O2 after general anesthesia in an outpatient surgery setting have statistically significant higher arrival O2 saturations compared with using no O2 in transport. It remains unclear if there is any true benefit in transferring patients while they are breathing supplemental O2. All arrival PACU desaturations were easily corrected with deep breathing, sitting the patient up, supplemental face-tent O2, or a combination of these. We did not examine the incidence of myocardial ischemia, nausea and vomiting, and wound infection, in which supplemental O2 may decrease the incidence of these adverse events (5–8).
Subgroup analysis revealed that patients 60 years old and who weighed 100 kg had lower arrival PACU O2 saturations with RA than other patients. This subgroup of patients gained the most benefit in being transported while breathing NC O2. Our findings confirmed the previous study of Hudes et al. (9), which found obesity and older age to be the highest risk factors for arrival PACU desaturations. The most likely reason for patients age 60 years or older and patients weighing 100 kg or more having lower arrival PACU saturations is that these groups both have underlying abnormalities in gas exchange as a result of low ventilation to perfusion regions. In obese supine patients, this results from severe reductions in functional reserve capacity. Older patients, on the other hand, have low ventilation to perfusion regions as a result of increasing closing capacity. Any added atelectasis as a result of general anesthesia compounds the gas exchange abnormality.
We found that asthmatics had a smaller decrease in arrival O2 saturations with RA and that tobacco use did not affect arrival PACU O2 saturations. It is possible that increased carboxyhemoglobin concentrations in some of these smokers may have resulted in an overestimate of O2 saturation because of similar absorption characteristics of oxyhemoglobin to carboxyhemoglobin (10). However, no definite conclusion can be made about these particular subgroups in our study because of the small number of patients in each subgroup (Table 3).
The study revealed a small cost savings in the average acquired cost of the O2 delivery system needed to treat O2 desaturations in the RA group. Further cost saving could be accrued if NC O2 was used instead of face-tent O2 for desaturations. Hudes et al. (9) did find that NC was as effective as face-tent 40% O2 in treating desaturations in the PACU. Also, cost savings could have been accrued for the NC group by treating desaturations after weaning off O2 with the patient’s initial NC instead of using face-tent O2.
Patients with arrival O2 saturations >92% while breathing RA had a low incidence of subsequent desaturations (<90%) in the PACU. Arrival O2 saturations >92% with RA may be a predictor of which patients can be moved quickly through the PACU. Furthermore, transporting patients with RA to the PACU after ambulatory surgery may allow the PACU staff to decide immediately which patients are at low risk of further desaturations and may need less intensive respiratory care. It remains unclear whether arrival O2 saturations >92% in the PACU while the patient is breathing 2–4 L/min NC is as reliable a predictor for a low incidence of subsequent desaturations <90% while in the PACU. Our study found a correlation (but not a statistically significant difference) between arrival O2 saturations >92% breathing RA having a decreased incidence of subsequent desaturations <90% compared with the 2–4 L/min NC group. We hypothesize that the most likely explanation for the trend for higher desaturations in the NC group is that these patients would have likely presented on arrival in the PACU with O2 saturations <92% if they had been transported breathing RA. No conclusion can be made whether arrival O2 saturations >92% breathing RA is a better marker for risk stratifying patients than 2–4 L/min NC. Because of the low incidence of desaturations in both groups in this study, a larger number of patients would be required to determine whether there is a statistically significant difference between these groups. Further studies are warranted to see whether PACU arrival saturations for patients transported breathing RA or NC can be used to risk stratify patients to those who can be moved quickly to less intensive nursing care and subsequently be discharged from the PACU earlier.
This study was limited by several factors. Patients were randomized before the start of general anesthesia, and this could have introduced bias into the anesthetic management. The amount of narcotics or how the general anesthetic was performed was not controlled. In addition, there was a lack of a control period of delivering 100% O2 to the patients before transfer to the PACU. Such issues as inexperienced residents giving excessive narcotics or having longer transport times could have contributed to O2 desaturations. We chose not to control many of the stated limitations because we wanted to determine whether our clinical practice of transporting patients to PACU while they breathed RA is safe with our current methods of general anesthesia.
Since our ambulatory surgery center opened in 1985, no adverse respiratory events related to our usual practice of transporting patients breathing RA have occurred. On the basis of the results of this study, we will continue this practice. However, on the basis of our observation of lower O2 saturations in patients breathing RA compared with NC O2, we recommend the use of 2–4 L/min NC O2 for the transport of patients with age 60 years or weight 100 kg. In addition, on the basis of previous studies, we recommend the use of supplemental O2 for patients for whom transient desaturations could be harmful, such as those having risk factors for myocardial ischemia or wound infection (5,8).
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Acknowledgments
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Supported in part by the University of Virginia, Department of Anesthesiology.
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Accepted for publication May 18, 2001.
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Abstract
Introduction
