MEASUREMENT OF VOLUME AND pH OF GASTRIC CONTENTS DURING GENERAL ANAESTHESIA: A COMPARISON BETWEEN PREGNANT AND NONPREGNANT PATIENTS

Abstract

Gastric content values of volume and pH are considered surrogate measures of the risk for pneumonitis should aspiration occur. The accepted criterion for defining the risk of pneumonitis is a combination of pH less than 2.5 and a volume greater than 25ml of stomach contents. These arbitrarily set critical values, originally extrapolated from animal work by Mendelson’s 1946 rabbit experiment, were validated by Roberts and Shirley (1974). The clinical significance of these criteria has not been well investigated, and has since been challenged. The gastric contents are considered to be more in obstetric patients and the risk of pulmonary aspiration increased during induction of anaesthesia than in nonobstetric patients. Because of this, many cases of Caesarean sections are done under regional anaesthesia. But there are patients that are contraindicated to the use of regional anaesthesia, and the next alternative is general anaesthesia. Therefore, identifying accurately and treating the surgical patients that are at risk of pulmonary aspiration will help in reducing the incidence and severity of aspiration. This is possible when the gastric volume and acidity (the determinants of the severity of aspiration) in the stomach are aspirated and measured.
In this study, gastric contents, aspirated from 30 pregnant patients undergoing elective Caesarean section under general anaesthesia, were compared to that of 30 non-pregnant patients going for elective myomectomies (control group). This was done to identify the group of patients that would be more at risk, since the gastric contents are variable at the time of induction of anaesthesia. After overnight fasting, the patients were induced and intubated inside the theatre. After the establishment of adequate anaesthesia, the gastric contents were aspirated using an 18FG Salem Sump tube with anti-reflux valve. The gastric volumes were measured using graduated markings from 50ml syringes. The gastric pH was measured using B Braun pH meter (B-Braun Scientific & Instrument Company, England).
The results revealed majority of the patients from both groups had high levels of gastric volumes and low pH, despite the overnight fasting. About 20 pregnant patients had gastric contents greater than defined criteria values, and 22 patients from the non-pregnant group.
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This study clearly shows the necessity of taking preventive measures against pulmonary aspiration in patients scheduled for general anaesthesia, even if they have been fasted overnight, this is not a guarantee for an empty stomach.