Many cases reported complications such as respiratory distress, tracheal displacement, pleural effusion, pulmonary edema, pneumothorax and possibly death after the shoulder arthroscopy surgery [4]. Postoperative hypoxemia, as a life-threatening postoperative complication, might be attributed to prolonged mechanical ventilation and PACU length of stay, which shouldn't be ignored after shoulder arthroscopic surgery. To the different purposes and methods of the research, the incidence of hypoxemia after shoulder arthroscopic surgery was 12.6% [5]and 16% [6], while it was 14% in our study. Through univariate analysis, the risk factors were age, BMI, smoking history, chronic pulmonary disease history, abnormal chest computed tomography, operating time and irrigation fluid volume. Furthermore, through logistic regression, the independent risk factors of hypoxemia in the PACU after shoulder arthroscopic surgery were as follows: age, BMI, operating time, irrigation fluid volume.
Among the preoperative factors, age and BMI were independent risk factors for hypoxemia. Khirfan G et al confirmed that patients with hypoxemia had older age, greater body mass index, higher prevalence of cardiovascular risk factors, worse functional capacity and pulmonary function [7]. Lumachi et al also indicated that in obese patients, the severity of hypoxemia was mainly related to age. Age was associated with postoperative hypoxemia, which might be due to the increasing of cardiopulmonary complications with older age [8]. For obese patients, respiratory resistance was increasing, lung compliance was worse, and cardiovascular system and respiratory complications were often accompanied. At the same time, proinflammatory factor and vascular active substances were releasing more, oxidative stress might lead to alveolar damage directly. Compared with normal patients, obese patients were more likely to suffer postoperative hypoxemia [9]. Smoking, chronic pulmonary disease, and abnormal lung computed tomography were not independent risk factors for hypoxemia in our study, but they were risk factors through univariate analysis. Decreased lung compliance, decreased vital capacity and maximal ventilation and alveolar damage, might cause restrictive or obstructive ventilation disorders, which could also influence ventilation/perfusion imbalance, ultimately impairing the gas exchange function, leading to the occurrence of hypoxemia [10]. In conclusion, we could reduce the incidence of postoperative hypoxemia by lowering preoperative BMI and optimizing the pulmonary condition of patients with chronic pulmonary diseases.
Among the intraoperative factors, operative time and irrigation fluid volume were independent factors for hypoxemia. Irrigation fluid accumulated in the neck, flowed into the para-tracheal space and the para-carotid space, obstructed lymphatic return, and formed edema in these areas, resulting in airway compression and even obstruction [11]. Edema of chest wall tissue might reduce the compliance of the lung and affect the function of respiratory muscles. Irrigation fluid accumulating in the chest and neck might also spread into the thoracic cavity or the lung interstitium, leading to limited oxygen diffusion function, even life-threatening pulmonary edema [12].While there was no established upper limit regarding the amount of irrigation fluid to be used during shoulder arthroscopic surgery, studies of symptomatic patients reported a range of volumes from 20 to 36 L, and as such, volumes lower than 20 L might be considered safe until more precise estimates were determined [4]. Total operative time was directly proportional to the volume of irrigation fluid used and subsequent weight gain by the patient, presumably from absorption of the irrigation fluid. Although there was no clear safe upper limit for operative times, some authors suggested limiting surgical time to between 90 and 120 minute to reduce this risk [13].
Continuous high-pressure perfusion was used for intraarticular irrigation by a gravity fluid control system for visual clarity in the surgery. We conducted a relative analysis on factors related to postoperative edema, although these factors were not risk factors for postoperative hypoxemia. There was a direct correlation between systolic blood pressure (SBP), subacromial space pressure (SASP), and the clarity of the visual field. Maintaining a pressure difference (SBP-SASP) of 49 mm Hg on average could less bleeding and permit good visual clarity. Furthermore, hypotensive anesthesia permitted lower irrigation pressure and reduced the risk of fluid extravasation into the subcutaneous tissues of the shoulder significantly [14]. Since the exact pressure of the irrigation fluid could not be recorded in our study, it was not statistically analyzed as a variable. It was an alternative way of intravenous administration of TXA to reduce the bleeding and improve visual clarity in arthroscopic shoulder surgery, in the meantime, administration of TXA could also reduce edema caused by fluid extravasation [15] to reduce the incidence of hypoxemia. In patients undergoing arthroscopic shoulder surgery, general anesthesia combined with ISB was associated with a lower heart rate, lower pain scores, lower intraoperative systolic blood pressure, shorter extubation time and lower incidence of adverse events compared with general anesthesia alone [16]. But for patients with BMI ≥ 30 kg/m2 undergoing arthroscopic shoulder surgery, high volume ISB was found to be responsible for diaphragmatic paralysis, dyspnea, occurrence of hypoxic episodes. In our anesthesiology department, lower volume and concentration ISB was used, and the injection site was as far away as possible from the location of the phrenic nerve, which might be helpful to reduce diaphragmatic paralysis.
There were some limitations to this study. First, routine blood gas analysis after surgery was not available for every patient, the SpO2 was the only indicator for evaluating hypoxemia. There might be errors and inaccuracies. Second, not every patient got routine blood test after the surgery, the effect of hemoglobin reduction to hypoxemia was not analyzed in this study. At last, this was a single-center retrospective study. It was impossible to collect and analyze all the relevant factors, multi-center, large sample, randomized controlled clinical trials are needed for further confirmation.