Читать книгу Successful Training in Gastrointestinal Endoscopy - Группа авторов - Страница 117

One of the most important steps in a successful colonoscopy is adequate cleaning of the colon. Without this, polyp detection or pathology identification may be hindered and adequate visualization to enable safe scope advancement may not be possible.

There are a number of different methods that have been used to prepare the colon. These fall under two main groups: osmotic and nonosmotic agents. Ingestion of highly osmotic agents, such as sodium phosphate, magnesium citrate, or mannitol, function by creating a large osmotic gradient between the bowel lumen and interstitial tissue, resulting in a large influx of fluid from the bowel lining into the lumen. This fluid is then passed, cleansing the colon in the process. These agents have been used successfully; however, they do have significant drawbacks that have led to limitations or even the discontinuation of their use in some instances. As a group, the osmotic agents result in large intravascular and intracellular fluid shifts. In healthy individuals, symptoms of dehydration are not uncommon, however, in patients with significant heart or renal disease, these fluid shifts can lead to significant heart failure or worsening of renal function. Additionally, magnesium citrate and sodium phosphate result in some absorption of their elements and can lead to dangerous elevations of magnesium or phosphate in patients. This is especially true in the elderly or patients with renal insufficiency. Mannitol, on the other hand, is a nondigested carbohydrate and does not get absorbed, which would limit the problem of electrolyte disturbances; however, colonic bacteria can metabolize this carbohydrate, leading to the production of methane and hydrogen gases within the colon. Not only do these gases lead to distention and greater patient discomfort but they are also extremely flammable and can lead to combustion with the use of electrocautery during polyp removal.

Lavage of the colon using nonosmotic agents has become the most common method of preparation. These agents use various preparations of polyethylene glycol (PEG) in electrolyte solutions. Like mannitol, PEG is a nondigestible and nonabsorbable molecule; however, it cannot be fermented by colonic bacteria. PEG preparations are formulated so that concentrations of electrolytes and PEG are isoosmotic to patients' interstitial fluids. This results in no significant fluid shifts in either direction, leaving the patient's fluid status largely unaffected by the preparation with no dangerous electrolyte imbalances. This makes PEG a safer alternative to cleanse the colon, especially in patients with cardiac or renal disease. The strengths of these PEG preparations, however, are also their weaknesses. Since there is no fluid recruitment from the patient's interstitial fluid as seen with the osmotic agents, the entire volume of cleansing fluid must be ingested. This requires drinking large volumes of solution to achieve the preparation goals. These solutions are generally not very palatable, also limiting the tolerability of these. For many years, PEG‐based preparations consisted of 4 liters of solution taken the night before a procedure; however, preparations are now commonly split into two 1‐liter doses, one taken the night before and the second the morning of the procedure. Split‐dose preparation has been found to be superior in cleansing the colon and better tolerated and is currently the recommended method. Other variations have tried to reduce the volume of PEG solution needing to be ingested by concentrating the solution into a smaller volume followed by drinking a predetermined amount of free water or placing PEG in capsules, so the patient does not have to taste the medication. This pill preparation, however, requires ingesting >30 capsules over a short period of time and still requires ingesting large amounts of water or clear soda to provide the fluid volume needed to clean the colon and ensure an isoosmotic concentration in the intestines. Despite these drawbacks, PEG preparations still remain the safest, most effective, and most commonly used preparations.