Enemas buffering

The Austrian Adverse Drug Reactions Advisory Committee received three reports of severe electroljde disturbances associated with an oral bowel-cleansing solution containing sodium phosphate solution (Fleet Phospho-Soda Buffered Saline Laxative Mixture), used as a bowel preparation for colonoscopy (5). Prescribers are advised to be aware of complications of the use of phosphate enemas, particularly in infants, elderly or debilitated patients, patients with congestive heart failure, and patients with impaired renal function. 

Early use of barium as a contrast medium involved the swallowing of a meal containing barium sulfate or, alternatively, the infusion of the salt rectally via a saline-buffered barium enema. This produced a dense mass in the intestine which really gave only an outline of the gross anatomy, useful only for diseases involving major structural changes. 

Rectal absorption depends on solubility and lipophilicity of the active substance (see further Sect. 16.1.5), and is influenced therefore by volume, pH, and buffering capacity of the enema (see Sects. 11.3 and 16.2.4). 

The pH of enemas is important for stabihty and absorption of the active substance. Regarding irritation, a pH-value between 4 and 10 is acceptable for an enema up to 20 mL [54], but preferably the pH should approach the physiological value (pH 7-8). In particular if the active substance is a weak acid or a weak base the pH may be adjusted to shift the equihbrium to the unionised form. Absorption of an active substance appears to be better from a buffered solution than from an tmbuffered solution with equal pH, so adjusting is best done by a buffer [55]. A phosphate buffer is frequently used for this purpose. 

The rectum is the lowest part of the large intestine. It is 15-20 cm long with a diameter of about 5 cm. In the rectum 1-5 mL of viscous fluid with a pH between 6.4 and 7.4 and a small buffer capacity is present. The temperature (under physiological conditions) is 36.2—37.6 °C. The rectum is a flat tube, because of the pressure of the bowels. After administration of a fluid into the rectum the Uquid will spread due to this pressure. Larger volumes (enemas) are spread into the colon as well. A defecation reflex will occur when volumes exceeding 100 mL are applied, which limits the volume of enemas. 

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