There are four types of laxatives.Osmotics (saline)
Osmotic laxatives (hyperosmolar) are salts or saline products, lactulose, and
glycerin. The saline products are composed of sodium or magnesium, and a
small amount is systemically absorbed. They pull water into the colon and
increase water in the feces to increase bulk, which stimulates peristalsis. Saline
cathartics cause a semiformed-to-watery stool depending on dose. However,
they are contraindicated for patients who have congestive heart failure.
Osmotic laxatives contain three types of electrolyte salts: sodium salts (sodium
phosphate or phospho-soda, sodium biphosphate), magnesium salts (magnesium
hydroxide (milk of magnesia), magnesium citrate, magnesium sulfate (Epsom
salts), and potassium salts (potassium bitartrate, potassium phosphate). Serum
electrolytes should be monitored to avoid electrolyte imbalance. Good renal
function is needed to excrete any excess salts.
High doses of salt laxatives are used for bowel preparation for diagnostic and
surgical procedures. Also used for bowel preparation is polyethylene glycol
(PEG) with electrolytes, commonly referred to as Colyte or GoLYTELY. PEG
is administered in amounts of 3 to 4 liters over 3 hours. Patients may be advised
to keep GoLYTELY refrigerated to make it more palatable. PEG is an isotonic,
nonabsorbable osmotic substance that contains sodium salts. Therefore, patients
with renal impairment or cardiac disorder can use potassium chloride.
Lactulose, another saline laxative, is not absorbed and draws water into the
intestines and promotes water and electrolyte retention. Lactulose decreases the
serum ammonia level and is useful in liver diseases such as cirrhosis. Glycerin acts
like lactulose by increasing water in the feces in the large intestine. The bulk that
results from the increased water in the feces stimulates peristalsis and defecation.
Patients who have diabetes mellitus should avoid lactulose because it contains
glucose and fructose.
Adequate renal function is needed to excrete excess magnesium. Patients who
have renal insufficiency should avoid magnesium salts. Hypermagnesemia can
result from continuous use of magnesium salts, causing symptoms such as
drowsiness, weakness, paralysis, complete heart block, hypotension, flushing,
and respiratory depression. Side effects from excess use are flatulence, diarrhea,
abdominal cramps, nausea, and vomiting.Stimulants (contact or irritants)
Stimulant laxatives increase peristalsis by irritating sensory nerve endings in the
intestinal mucosa. Stimulant laxatives include those containing phenolphthalein(^338) CHAPTER 18 Gastrointestinal System