Introduction: A key feature of gastric acid aspiration that result in chemical pneumonitis is aspiration of a high volume >50mL  at a pH of <2.5 . Prevention of acid aspiration following cardiac surgery is variable even within the same unit. All patients for routine cardiac surgery are starved for a minimum of 6 hours, and may be prescribed anxiolytics and H2-receptor antagonists. Following surgery, patients remain intubated and ventilated for variable periods of time. We therefore decided to assess the risk of gastric acid aspiration at the time of extubation in our cardiac surgical unit.
Method: 45 non-randomized patients undergoing routine cardiac surgery in our unit were studied. Premedication, anaesthetic management and postoperative care were unaltered by the study and were as prescribed by the individual anaesthetist. Current policies regarding extubation criteria were followed. Just prior to extubation the gastric contents were aspirated via an oro-gastric tube, the volume noted and its pH measured. Data is mean and range and analysed using Student's t-test assuming equal variance.
Results: Data was collected from 45 patients. 19 did not receive any premedication, and 26 were given temazepam (10-30mg) and ranitidine (150-300 mg) preoperatively. The duration of starvation prior to surgery, the length of operation and the time from ICU admission to extubation were not different between groups.
Just prior to extubation the pH of the residual gastric contents in unpremedicated patients was significantly lower (pH 4.9, range 1-8) than in premedicated patients (pH 7.8, range 6-10). The volume of the residual gastric contents was higher in unpremedicated patients (23 mL, range 0-150mL) with a wider distribution, than in premedicated patients (11 mL, range 0-80 mL), but this was not statistically significant. There was no correlation between time to extubation and either gastric residual volume or pH.
Conclusions: Before cardiac surgery routine use of anxiolytics together with H2-receptor antagonists prevents the residual gastric contents having a clinically significant acidity. In addition premedication also appears to reduce the wide variability in residual volumes seen in unpremedicated patients and in the majority of patients (25 out of 26 patients) premedication reduces this volume to less than the perceived ‘at risk’ volume (<50mL) identified by Raidoo et al .
1 Raidoo DM, Rocke DA, Brock-Utne JG, et al. Critical volume for pulmonary acid aspiration: reappraisal in a primate model. Br J Anaesth
2 Mendelson CL. Aspiration of stomach contents into lungs during obstetric anesthesia. Am J Obstet Gynec