Predictors of Severity and Nocturnal Nadir of Postoperative Oxygenation Impairment after Shoulder Surgery in the Older Population - Abstract
Aim: Pain following shoulder surgery can be severe; interscalene brachial plexus blockade (ISB) and high dose opioid analgesia are commonly used management options, however both impair oxygenation by multiple mechanisms. We aimed to quantify the extent of, and explore the factors influencing, the largest decline/ nocturnal peak onset of postoperative oxygenation impairment.
Methods: 114 shoulder surgery patients over the age of 50 were included. Binary logistic regression analysis was used to identify factors predisposing to a significant fall in patient’s partial pressure of arterial oxygen: fraction inspired oxygen (PaO2:FiO2) ratios. Survival analysis was used to elucidate time to nocturnal PaO2
:FiO2nadir.
Results: 67 patients received ISB. Greatest postoperative reduction in PaO2:FiO2 ratio per patient was [mean (SD)] (179 [102]); in 22% of patients this occurred nocturnally. Only higher preoperative Sp02(OR 3.9) was associated with a significant fall in PaO2:FiO2 ratio in the multivariate logistic regression model; opioid usage, use of ISB and all other factors were not. ISB was however strongly associated with much earlier time to greatest nocturnal fall in PaO2:FiO2 ratio (median survival time 205 minutes vs. 635 minutes, hazard ratio 24.3, p<0.0001 by Kaplan-Meier survival curve). Higher preoperative SpO2 was also strongly correlated to fall
in PaO2 :FiO2ratio (Pearson’s R = 0.7, p<0.0001), but not absolute lowest PaO2:FiO2ratio per patient (Pearson’s R = 0.15, p = 0.12).
Conclusion: Whilst choice of analgesic technique does not influence the likelihood of profound impairment of oxygenation after shoulder surgery, use of ISB significantly shortens the time to onset of a nocturnal oxygenation nadir. Knowledge of this timing of peak respiratory impairment should be used to focus postoperative observation. As patients with normal preoperative room air SpO2 may still suffer a significant nadir in postoperative PaO2:FiO2 ratio, postoperative monitoring should be managed accordingly.