Cimetidine for prophylaxis of aspiration pneumonitis: comparison of intramuscular and oral dosage schedules

Gastric tonometry: a new monitoring modality in the intensive care unit

In many critically ill patients, systemic measures of hemodynamic and O2 transport variables may not be sufficiently sensitive to portray the complex interaction between energy requirements and energy supply in all tissues. Gastric or intestinal tonometry has been proposed as a relative noninvasive index of the adequacy of aerobic metabolism in the gut mucosa, a tissue that is particularly vulnerable to alterations in perfusion and oxygenation. The gut mucosa lacks some of the microvascular control mechanisms that allow other tissues, such as the heart, skeletal muscle, and the brain, to increase tissue perfusion during times of stress, and, just like the canary, it will display metabolic changes indicative of dysoxia earlier than those more "vital" tissues. The tonometric measurement of mucosal PCO2 rests on the premise that equilibrium exists between mucosal and luminal PCO2. Increases in mucosal PCO2, or conversely, decreases in mucosal pH (pHi), are associated with the development of intestinal mucosa ischemia. The clinical utility of pHi to detect intestinal mucosal ischemia has been demonstrated in patients undergoing abdominal aortic surgery. Further, a low gastric mucosal pHi on admission to the ICU appears to be predictive of mortality and pHi-guided resuscitation may improve outcome in a subpopulation of patients admitted to the ICU with normal pHi, perhaps by preventing splanchnic ischemia and the development of a systemic oxygen deficit.

The effects of intravenous cimetidine and metoclopramide on gastric pH and volume in outpatients

STUDY OBJECTIVE

To evaluate and compare the preinduction effects of intravenously (IV) administered cimetidine alone and combined with metoclopramide on gastric contents and postoperative nausea and vomiting in outpatients undergoing elective surgery.

DESIGN

Patients were allocated randomly to 4 groups with 20 patients in each group.

SETTING

Ambulatory surgery at a university-affiliated city hospital.

PATIENTS

Eighty patients undergoing elective gynecologic or orthopedic procedures were studied.

INTERVENTIONS

Outpatients in Group 1 and inpatients in Group 2 served as controls. Outpatients in Group 3 received 300 mg of cimetidine, and outpatients in Group 4 received 300 mg of cimetidine and 10 mg of metoclopramide. All drugs were administered IV as an infusion over a 15-minute period, 30 to 45 minutes prior to induction of anesthesia.

MEASUREMENTS AND MAIN RESULTS

After induction of general anesthesia and endotracheal intubation, stomach contents were retrieved and volume and pH measured. Group 1 outpatients had a large residual gastric volume of 29.2 +/- 15.9 ml, with a very low pH of 2.32 +/- 1.23 and 15% frequency of postoperative vomiting. These patients are at high risk of developing significant pneumonitis in the event of the aspiration of gastric contents. The combination of cimetidine and metoclopramide in Group 4 provided the optimal, or safest, condition--i.e., high gastric pH [6.15 +/- 0.71 (p less than 0.005)] and low gastric volume [11.6 +/- 7.37 ml (p less than 0.001)], with no postoperative vomiting.

CONCLUSIONS

The combination of cimetidine and metoclopramide given to ambulatory patients during the preinductive phase may prevent severe pulmonary consequences should aspiration occur and is more effective in this regard than cimetidine alone.

Prophylaxis for pulmonary acid aspiration

Pulmonary acid aspiration in the perioperative period was first noted as a medical concern more than 40 years ago. Despite this awareness, identification of the patient at risk is by no means certain, and the true incidence of this condition is not known. Various therapeutic interventions have been proposed and tried. Physical therapies, such as pressure on the cricoid, have been successful, as has the administration of clear antacids prior to surgery. The histamine (H2)-receptor antagonists cimetidine and ranitidine have also been used successfully as prophylactic therapy when enough time is available prior to operation to make preoperative dosing practical. Metoclopramide, which can be given intravenously, may be useful in emergency surgery. When the degree of risk is high, concurrent use of physical and pharmaceutical interventions is recommended.

The effects of cimetidine and ranitidine with and without metoclopramide on gastric volume and pH in morbidly obese patients

The efficacy of preanaesthetic intravenous cimetidine versus ranitidine with and without metoclopramide for acid aspiration prophylaxis was assessed in 60 morbidly obese patients in a double-blind manner. Group 1 patients received cimetidine 300 mg + saline. Group 2 patients received cimetidine 300 mg + metoclopramide 10 mg. Group 3 patients received ranitidine 100 mg + saline. Group 4 patients received ranitidine 100 mg + metoclopramide 10 mg. Gastric fluid was aspirated for analysis of volume and pH following induction of anaesthesia. All four premedication regimens were equally effective in reducing the gastric volume and acidity and the inclusion of metoclopramide had no additive effect. Although statistically not significant, two patients in the cimetidine groups remained at risk (volume greater than 25 ml and pH less than 2.5) while no patients in the ranitidine groups remained so.

Premedication with cimetidine and metoclopramide. Effect on the risk factors of acid aspiration

A randomised, double-blind, placebo-controlled parallel study was conducted in adult females to evaluate the efficacy and safety of a combination of cimetidine 300 mg orally and metoclopramide 10 or 20 mg intravenously in reducing pre-operative residual gastric volume and raising gastric pH. The effect of preoperative metoclopramide on postoperative nausea and vomiting was also investigated. Oral cimetidine was given approximately 2-2.5 hours before, and intravenous metoclopramide either 15 or 30 minutes prior to induction of anaesthesia. The study showed that placebo-treated patients undergoing outpatient operations have an increased risk of acid aspiration because of high residual gastric volume and low pH and increased risk of serious pulmonary injury should acid aspiration occur. Metoclopramide 10 or 20 mg intravenously prior to induction of anaesthesia was effective in reducing the residual gastric volume significantly, but not in raising pH. The combination of cimetidine and metoclopramide, as well as cimetidine alone, reduced the risk factors of acid aspiration by raising gastric pH and reducing residual volume. No anti-emetic effect of metoclopramide was observed. Higher doses of metoclopramide (20 mg) produced significant side effects (flushing, dizziness, extrapyramidal side effects), but were only marginally more effective than 10 mg doses in reducing residual gastric volume.

Aspiration pneumonitis: predisposing conditions and prevention

Aspiration pneumonitis is a common complication of general anesthesia. The lower the pH of gastric fluid and/or the larger the volume, the greater is the potential danger. In spite of preoperative fasting, an alarmingly high percentage of patients who undergo surgery may be at risk. This review discusses conditions that predispose to aspiration pneumonitis and examines current strategies for its prevention.

Cimetidine in anesthesia: does it minimize the complications of acid aspiration?

The use of cimetidine, an H2-receptor antagonist, to minimize the complications of acid aspiration pneumonia is reviewed. Since Mendelson's early description of acid aspiration in 66 obstetrical patients, attempts have been made to decrease gastric acidity by administering perioperative antacids and/or antihistamine drugs such as atropine and glycopyrrolate. In this review of the literature, strict attention is given to study design and subject selection, methods of collecting gastric samples and reporting both pH and volume measurements, and medications taken on admission, given preoperatively, and as anesthetics. Clinical trials evaluating cimetidine alone and compared with other prophylactic regimens were reviewed, detailing the timing, route, and frequency of drug administration. Cimetidine appears to be effective in minimizing complications of acid aspiration since gastric acidity (pH less than 2.5) is a contributing factor in both the morbidity and mortality of this disease state. In general terms, cimetidine 300 mg po 1.5-2.0 h prior to intubation or 45-60 minutes iv before surgery will decrease gastric acidity below the critical level. This effect may be maintained for up to three hours. Perioperative cimetidine administration, to minimize the complications of acid aspiration, appears to be safe in terms of drug-drug interactions and effects on labor, the fetus, and the newborn child.

Preoperative cimetidine--effects on gastric fluid

A double blind study of 44 patients (24 placebo, 20 cimetidine) was undertaken to determine tha effect of cimetidine on the pH and volume of gastric fluid during anaesthesia. The pH was significantly higher on cimetidine-treated patients than placebo-treated patients at 0, 15, 30 and 45 minutes following intubation. At induction of anaesthesia, mean pH on cimetidine was 4.5 and on placebo mean H was 2.0. Approximately 70% of patients on cimetidine and 20% on placebo recorded a pH above 2.5 during anaesthesia. The reason for a pH recording of less than 2.5 in seven patients on cimetidine could not be ascertained. Mean volume of gastric aspirate was slightly lower in the cimetidine group at 0, 15, 30 and 45 minutes, but the differences were not significant.