Intramuscular tramadol increases gastric pH during anesthesia

Tramadol, [(1RS, 2RS)-2-dimethylamino) methyl-1-(3-methoxyphenyl)-cyclohexanol hydrochloride], is an analgesic in clinical use. It has been reported that tramadol inhibits muscarinic type 3 receptor function, which primarily mediates smooth muscle contraction and glandular secretion. We investigated the effects of tramadol on the pH of gastric juices during anesthesia to determine whether tramadol inhibits secretion from the gastric glands.

ASA physical status I or II adult patients (n = 30) presenting for major elective orthopedic surgery of the upper extremities or mastectomy were enrolled. Patients were randomly assigned to receive treatment with tramadol (n = 10), famotidine (n = 10), or saline (n = 10). General anesthesia was then induced using propofol, vecuronium bromide, and fentanyl. After inducing anesthesia, the gastric pH was measured using pH test paper and, then, 100 mg tramadol, 20 mg famotidine, or saline were injected into the deltoid muscle. Three hours after starting the operation, gastric juice was again aspirated and its gastric pH measured.

There were no differences in the pH before anesthesia between the three groups. By contrast, gastric pH was increased in the tramadol group by the same amount as it was in the famotidine group three hours after administering the drugs. Gastric pH of the saline, famotidine, and tramadol groups was 2.6 +/- 2.5, 6.3 +/- 2.0, and 6.4 +/- 0.8, respectively.

These results suggest that tramadol inhibits the secretion of gastric acid.