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February 1998
A Comparison of the Volume and pH of Gastric Contents of Obese and Lean
Surgical Patients
Harter RL, Kelly WB, Kramer MG, Perez CE, Dzwonczyk RR;Anesth Analg 1998;86:147-52
[ see abstract below ]
Pulmonary aspiration of gastric contents during anesthesia occurs rarely but is feared because of the significant morbidity which can result. The mortality rate for pulmonary aspiration has been reported as 1 in 71,000 general anesthetics [1993]. The conventionally-held threshold at which an
individual is considered to be at risk for pulmonary injury after aspiration is when gastric pH is less than 2.5 with gastric volume greater than 25 ml. Drugs which increase pH and decrease volume can be given, but it is important to identify those patient populations which are at increased risk and therefore will benefit from this premedication.
Obese patients have considered to be such an at-risk patient group since a paper by Vaughan et al. in 1975 (Anesthesiology 43:686). However, more recent reports had suggested that obesity may not be associated with increase risk of pulmonary aspiration or gastroesophageal reflux, and therefore these authors have reevaluated the gastric contents of obese vs lean patients.
The authors collected data from three groups of patients. Group L were those patients defined as lean, based on a body mass index < 30 (BMI = kg / meter2). Group O/n consisted of 75 patients who were obese (BMI > 30) who had received no preoperative antacid or gastric prokinetic drug, at the
discretion of the attending anesthesiologist. Group O/p consisted of 24 obese patients who had received antacid and/or gastric prokinetic premedication.
Excluded patients were those with known esophageal or gastric pathology, including hiatal hernia or peptic ulcer disease requiring treatment, current treatment with any antacid or gastric motility enhancing agent, any condition precluding placement of a gastric sump tube, history of diabetes mellitus and suspected or known difficult endotracheal intubation. For all patients, an18-Fr sump tube was placed after induction of general anesthesia and intubation, and gastric contents were aspirated using a syringe. Gastric volume was recorded immediately using graduated markings from the syringe; the pH was measured later.
Median age was 36-41 in all groups and median NPO duration was 10 hours. The incidence of smoking and ASA physical status 3 were also not different. Group L patients had median BMI = 24.7 (weight 68.2 kg), and had median gastric volume 20 ml and pH 1.69. The patients in Group O/n had BMI = 33.7 (100.9 kg) with median gastric volume 18 ml and gastric pH 1.65. Patients in Group O/p had BMI = 39.3 (102.3 kg) who had gastric volume 26 ml and gastric pH 5.64. The premedications given to Group O/p were at the discretion of the anesthesiologist and included 30 ml sodium citrate, metoclopramide, ranitidine and/or ondansetron.
These authors found that there was no difference in median pH between the two unmedicated groups, L and O/n, and the median pH was higher in Group O/p as would be expected from the administration of antacid. The median volume of gastric contents was not significantly different among the
three groups. The authors then looked at the combination criteria, defining HVLP (high volume, low PH) gastric contents that with combined volume > 25 ml and pH < 2.5. Group L patients were significantly more likely exhibit HVLP gastric contents than were those in Group O/n or O/p:
42.0% vs 26.7% or 4.2%, respectively. [Group O/p had received antacids.] The authors further analyzed the incidence of HLVP in relation to BMI. They subdivided patients in the O/n group into mildly obese (30 < BMI < 35), moderately obese (35 < BMI < 40) and severely obese (BMI > 40), and compared those results to Group L. The incidence of unmedicated patients reaching HVLP criteria ranged from 42%- 29.4%- 28.1%- 18.2% from the lean through severely obese groups.
Several questions remain unanswered. The surgical subpopulations which are at risk for aspiration have not been well identified. Furthermore, it is not certain whether the popular threshold criteria do predict risk of aspiration injury in surgical populations. If premedicating all patients is considered, the risks and cost-benefit of that approach must also be researched; antacids, for example, may cause nausea.
Why do these results differ from the report from the 1975 report that forms the basis of our current practice? Patients in the earlier study had all been premedicated with intramuscular Innovar" (fentanyl plus droperidol) and diphenhydramine; in the present study, patients received up
to 2 mg midazolam immediately before leaving the preoperative holding area. A significant increase in PaCO2 was also noted preoperatively among the obese patients in Vaughan et al study, and this may have contributed to an increased H+ concentration in gastric secretions.
It is also important to reemphasize that the current study excluded patients with history of gastroesophageal pathology and with anticipated difficult intubation. Nonetheless, in this population of fasted, minimally medicated, nondiabetic obese surgical patients, the incidence of HVLP
gastric contents was significantly lower than that of lean patients. This intriguing result warrants further research and corroboration.
Return to the Current Literature Review Front Page, or read the abstract:
ABSTRACT
Obese surgical patients are typically considered to be more likely than lean patients to possess high-volume and low pH (HVLP) gastric contents after a standard preoperative fast, based on a study of a population predominately consisting of patients receiving intramuscular preoperative sedation. We revisited this issue in a study population of 256 fasted surgical patients, of which 232 received no preoperative antacid or gastric prokinetic drug. Immediately after endotracheal intubation, an 18-French sump tube was placed, and gastric contents were defined as HVLP if they exhibited a combination of a volume > 25mL and a pH < 2.5. Obesity was defined as a body mass index > 30.
Among nonmedicated obese patients, the proportion with HVLP gastric contents was 20 of 75 (26.6%). The proportion of lean patients with HVLP gastric contents was 66 of 157 (42.0%). The difference between the HVLP proportions for these two groups was found to be significant (P < 0.05) using X2 analysis.
Obesity seems to be associated with a significantly decreased risk of HVLP gastric contents among surgical patients with no history of gastroesophageal pathology after a normal interval of preoperative fasting.
Implications: Previous studies have shown that obese surgical patients have a greater volume of acidic stomach contents than lean patients, despite a routine preoperative fast. We have reexamined this issue and found that among otherwise healthy, fasted, obese surgical patients, there is a lower incidence of combined high-volume, low-pH stomach contents compared with lean patients.
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