Mechanical bowel preparation (MBP) is commonly used for colonoscopy and elective colorectal surgery. Its goal is to rid the colon of solid fecal material. It is believed that the “clean” bowel has a lower bacterial load and has a lower risk of contaminating the wound and peritoneal cavity during surgery. The prepped bowel is much easier to manipulate during surgery and enables the surgeon to perform intraoperative colonoscopy. Disruption of the anastomosis and the sequela of an anastomotic leak may be attenuated by the MBP as well. Other surgeries in which the MBP is commonly used include abdominal and pelvic operations involving the aorta, kidneys, bladder, and reproductive structures. Absolute contraindications to the mechanical bowel prep include complete bowel obstruction and free perforation.

For colonoscopy and elective colorectal surgery, MBP is the standard of care worldwide. Historically, agents such as castor oil, senna, bisacodyl, and magnesium citrate have all been used in conjunction with a low-residue diet. Whole-gut lavage with large volumes of isotonic solutions via a nasogastric tube has also been used. However, this technique has been associated with side effects including electrolyte abnormalities, abdominal distention, nausea, and vomiting. Whole-gut lavage with mannitol has also been used, but this practice has been shown to be associated with catastrophic intraoperative explosions when gases produced from fermentation of mannitol by Escherichia coli were ignited by electrocautery. Currently, the two most widely used agents for MBP are polyethylene glycol and sodium phosphate.

Polyethylene glycol (PEG) was developed in the 1980s as an oral agent for MBP. It is an iso-osmotic, nonabsorbable sodium sulfate-based solution that cleans the bowel by washout of ingested material without significant fluid and electrolyte shifts. Patients are required to drink at least 2 to 4 L of solution along with additional fluids. Abdominal cramping, nausea, and vomiting are common side effects, and patients are typically prescribed prophylactic antiemetics.

Sodium phosphate (NaP) was developed in response to patient dissatisfaction with PEG and has generally been found to be more tolerable and associated with a higher rate of compliance. NaP is a hyperosmotic electrolyte solution that provides high-quality bowel cleansing while avoiding
the need to ingest large volumes of solution. It can be taken in two 45-mL doses or in a series of 40 tablets. Abdominal cramping, nausea, and vomiting are common side effects.

Because NaP is a hyperosmolar solution, a watery diarrhea is induced with its administration. A negative fluid balance can ensue, which can lead to hypovolemia that may become especially evident at the induction of anesthesia if adequate fluid replacement has not taken place. PEG, on the other hand, leaves the fluid balance virtually unaffected. Water and electrolytes are neither absorbed nor excreted from the bowel. Therefore, patients do not usually exhibit hemodynamic instability during the induction of anesthesia. For these reasons NaP may have a narrower therapeutic index than PEG, especially in patients who may be vulnerable to shifts in intravascular volume. NaP has been linked more frequently than PEG to serious electrolyte and metabolic derangements. Most commonly reported are transient decreases in pH, magnesium, calcium, and potassium. Hypernatremia and hyperphosphatemia can also occur, because these solutions contain 48 g of monobasic sodium phosphate and 18 g of dibasic sodium phosphate per 100 mL of solution. Although most patients are not clinically affected by these electrolyte abnormalities, NaP should be avoided in patients with renal disease, congestive heart failure, ascites, and in those who cannot adequately hydrate themselves. Furthermore, severe hyperphosphatemia and associated electrolyte and metabolic derangements following administration of NaP have been reported, as well as at least one case report of fatal hypocalcemic, hyperphosphatemic, metabolic acidosis following sequential NaP enema administration.