Incidence of hypoxemia in a post-anaesthesia care unit and relevant risk factors: a retrospective study of 14604 patients with general anaesthesia

Background This article is aim to investigate the incidence and risk factors for postoperative hypoxemia in a post-anaesthesia care unit (PACU). Methods The retrospective cohort assessed 14604 postoperative patients who were admitted to PACU between January 2015 and December 2015. A pulse oximeter was used to monitor and record pulse oxygen saturation (SpO2) every 5 minutes. Clinical data were collected for all these patients, and the incidence of and risk factors for postoperative hypoxemia were analysed. Results The total incidence of hypoxemia was 21.83% (SpO2 ≤ 95%) and 2.79% (SpO2 ≤ 90%). Multiple regression analysis indicated that the risk factors were age ≥50-year old, body mass index (BMI) ≥25kg/m2, American Society of Anaesthesiologists (ASA) II and III, limb surgery, and thoracic surgery. Conclusions Therefore, hypoxemia was common in postoperative patients in the PACU. Age, BMI, ASA classification, and surgical site are associated with postoperative hypoxemia. More attention should be paid to these patients to prevent hypoxemia in the PACU.

surgical wounds were more likely to be transferred to the intensive care unit(ICU)for further treatment, other patients were typically transferred to the PACU under the supervision of an anaesthesiologist. The PACU was close to the operating room; thus, no supplementary oxygen was given during patient transfer unless required by special circumstances. Upon patient arrival at the PACU, all patients were given supplementary oxygen (via a nasal tube, 3-4L/min). Aanaesthesiologist may give mechanical ventilation as warranted by the patient condition. A pulse oximeter (probe: Nellcor DS-100A, Shanghai, China; monitor: Philips Intelli Vue MP70, Boeblingen, Germany) would monitor and record SpO 2 of each patient after their arrival in the PACU. All monitor data were uploaded to the operating room information system in real time, the system will record every five minutes until the patient left the PACU.
Patients with key data missing were excluded (including SpO 2 and age). Finally, a total of 14,604 patients were enrolled in this study.

Statistical analysis
Continuous variables are expressed as the mean ± standard deviation, and the incidence is expressed as a percentage (%).  Figure 1 shows the enrollment, exclusions and patients available for analysis among 16316 patients admitted into the PACU from January 1, 2015 to December 31, 2015.

Results
Finally, 14,604 patients were analysed whose basic data are shown in Table1. Of all the patients, 40.6% were males, and 59.4% were females, with ages of 50.1 ± 14.6 years. Their BMI averaged 24.2± 3.7 kg/m 2 . Patients with a high ASA grade were usually transferred to the ICU for further treatment after surgery; in this study, a higher proportion of patients had ASA grades of I (45.7%) or II (50.8%). All patients were given general anaesthesia using inhaled anaesthetics, intravenous anaesthetics, opioids, neuromuscular blocker and muscle relaxant antagonist.
When hypoxemia was defined as SpO 2 ≤ 95%, the overall incidence of hypoxemia was 21.83% (3188 patients). When hypoxemia was defined as SpO 2 ≤ 90%, the overall incidence of hypoxemia was 2.79% (408 patients). Patients were divided into a hypoxemia group and a non-hypoxemia group for between-group comparison.
We performed multivariate regression analysis of the risk factors in hypoxemia and non-hypoxemia patients as SpO 2 ≤ 95% and SpO 2 ≤ 90% respectively. When hypoxemia was defined as SpO 2 ≤ 95%, multivariate regression analysis showed that age and BMI were closely related to the incidence of hypoxemia, especially in elderly patients over 50-year old (50-64 years old, OR = 1.

Discussion
Hypoxemia is usually defined as SpO 2 ≤ 90%, but in PACU, patients with SpO 2 ≤ 95% must be treated, so we defined hypoxemia with two criteria (SpO 2 ≤ 95% and SpO 2 ≤ 90%) and analysed respectively. The incidence of hypoxemia in PACU patients varies a great deal with the study population and sample size. 5 7 9 10 . In this study, we included 14604 adult patients admitted to the PACU over a period of one year and found that the overall incidence of hypoxemia was approximately 21.83% (SpO 2 ≤ 95%) and 2.79%( SpO 2 ≤ 90%). In our study, patients with a high ASA grade were usually in more serious condition and thus were transferred to the ICU for further treatment after surgery. PACU patients had fewer comorbidities, better preoperative condition, a higher cardiopulmonary reserve and all of them received muscle relaxant antagonist, but hypoxemia still occurs in some high-risk patients. We must pay more attention to potential hypoxemia in PACU patients to prevent it.
To accurately predict the risk of postoperative hypoxemia, many factors, such as patient condition, anaesthesia, and surgery factors before, during, and after operation must be taken into account, thus complicating the prediction of hypoxemia. At present, researchers are still debating the risk factors for hypoxemia.
This study showed that hypoxemia may be closely related to age, BMI, ASA classification, and surgical site, but may be unrelated to intraoperative position.
Previous studies have reported the relationship between the surgical site and postoperative hypoxemia. This study showed that thoracic surgery may be a probable risk factor which was consistent with literature reports. Xue 6 included 944 patients undergoing superficial plastic surgery, abdominal surgery, and thoracic surgery; the results showed that the incidence of postoperative hypoxemia was highest in patients undergoing thoracic surgery (52%; incidence of severe hypoxemia: 20%), followed by upper abdominal surgery (38%) and superficial surgery (approximately 7%; incidence of severe hypoxemia: 0.7%).The factors associated with hypoxemia may include direct compression of the lungs during operation, incomplete pulmonary re-expansion, postoperative pain-related weak chest wall and diaphragm movement, atelectasis, and increased pulmonary shunting. 18 The risk of hypoxemia varies a great deal with age. This study showed that age was a sensitive indicator for predicting the risk of hypoxemia. The incidence of hypoxemia was generally low in patients aged 18 to 34 but was much higher in elderly patients, especially in elderly patients over 50-year-old.
Elderly patients were susceptible to hypoxemia. 17 19 When patients were transferred from the operating room to the PACU without supplementary oxygen, the SpO 2 was lower in patients aged 60 or over upon approval at the PACU. 20 This reasons may be that elderly patients are more likely than younger patients to have residual postoperative muscle relaxation, 21 which affects the hypoxic ventilatory response and respiratory muscle strength, increasing the risk of airway obstruction and hypoxemia. Moreover, respiratory reserve decreases with age for elderly patients 22 ; low lung capacity, high residual volume, low ventilatory efficiency, low blood vessel elasticity, and low lung perfusion lead to an imbalance in the pulmonary ventilation/blood flow ratio, further increasing the risk of hypoxemia in cases with surgical and anaesthesia stress.

Conclusion
This study included a large data set and reflected the overall incidence of hypoxemia in PACU patients. The results showed that age, BMI, ASA classification, and surgical site were the main risk factors for hypoxemia in PACU patients, thus providing a valuable reference for predicting hypoxemia in clinical practice. In the future, we will conduct in-depth studies on individual factors or populations to further validate the predictive value of these factors. We believe that clinicians should carefully monitor at-risk patients before, during, and after operation and assess airway condition in a timely manner for supplementary oxygen or mechanical ventilation. Detailed knowledge on the mechanism of hypoxemia and relevant predictive factors will help clinicians make better clinical decisions to ensure patient safety and postoperative recovery.

Declaration section
Ethics approval and consent to participate This was a retrospective study approved by the Ethics Committee and Institutional   Figure 1 Study diagram, showing enrollment, exclusions and patients available for analysis.