Anesthesia for endoscopy-part 1 (Proceedings)

Endoscopy is the process of looking inside the body by inserting a rigid or flexible tube into the body and examining an image of the interior of an organ or cavity.  An additional instrument may be inserted in order to biopsy tissue or retrieve foreign objects.  It is considered a minimally invasive diagnostic or medical procedure in animals, but most endoscopic procedures in dogs and cats will require general anesthesia.  Many of the potential complications of minimally invasive procedures are related to general anesthesia.  Endoscopic procedures are commonly used to examine the respiratory system (laryngoscopy/tracheoscopy/bronchoscopy), GI system (upper and lower GI endoscopy), thoracic cavity (thoracoscopy), abdomen (laparoscopy), urinary tract (cystoscopy) or joints (arthroscopy).    A thorough understanding of the physiologic changes produced by various endoscopic procedures is necessary to properly support an anesthetized patient.  Some endoscopic procedures require the use of an insufflation gas to facilitate visualization, with resulting physiologic changes to the patient.  Body position of the patient during the procedure may also have profound effects on the cardiovascular and respiratory systems.  A complete knowledge of all anesthetics and adjunctive drugs is necessary to support patient care.

Patients undergoing general anesthesia should have food withheld for 12 hours prior to the procedure.  The patient should have access to water up to an hour before the start of general anesthesia.  Pediatric patients and other patients at risk for hypoglycemia should have a shorter fasting period.  Baseline data include a CBC, chemistry panel with electrolytes and urinalysis.  Other ancillary tests that may be considered include thoracic and abdominal radiographs, ECG, echocardiogram, and blood gas analysis, depending on the body systems affected and the planned procedure.

Drug choices should be individualized. Premedication tranquilizers/sedatives such as acepromazine or benzodiazepines help calm patients prior to catheter placement and improve recovery conditions.  Opioids will add analgesia and sedation.  Minimally invasive procedures will still cause pain and discomfort, so analgesia must be considered.  The use of premeds will also reduce the amount of injectable and inhalant anesthetics needed by the patient, thus improving cardiovascular performance.  Anticholinergics (atropine, glycopyrrolate) should be used when patients require an increase in heart rate; they can counteract the increase in vagal tone produced by administered drugs (opioids) or procedure (cystoscopy or tracheoscopy).  Injectable anesthetics include propofol, thiopental, ketamine, or etomidate.  Isoflurane or sevoflurane are acceptable maintenance inhalant agents.  Table One includes a list of sample protocols for various endoscopic procedures. 

Monitoring of the anesthetized patient undergoing a minimally invasive procedure is just as important as any major procedure.  Cardiovascular and respiratory system monitors such as pulse oximetry, capnometry/capnography, invasive or noninvasive blood pressure, and ECG can be very useful in assessing and maintaining normal physiologic parameters in the anesthetized patient.  Mean arterial pressure should be greater than 60 mm Hg in dogs and cats.  End-tidal CO2 should be between 35 and 45 mm Hg and SpO2 >95 %. 

Crystalloid fluids should be administered to patients undergoing inhalant anesthesia for minimally invasive procedures, as inhalant anesthetics cause vasodilation and decreased venous return, regardless of the anticipated amount of blood loss.  Crystalloid fluids are generally administered at a rate of 10 mls/kg/hr unless the patient is hypoproteinemic, has cardiac disease, anuria, etc.  Patients that are dehydrated prior to the procedure should have their volume deficit corrected prior to general anesthesia. Some patients may benefit from the administration of colloids while undergoing the procedure. Hetastarch can be used in conjunction with crystalloid fluids.

Upper GI endoscopy

Anesthetic drugs may alter intestinal motility, sphincter function, and promote vomiting.  Upper GI endoscopy is impossible to perform without general anesthesia in dogs and cats.  If the patient has experienced prolonged vomiting, the animal should be carefully examined for dehydration and/or electrolyte disturbances.  Volume depletion and electrolyte imbalance should be corrected prior to general anesthesia.  The animal may be sedated with a mild tranquilizer like acepromazine if not dehydrated.  Full mu agonist opioids like morphine, oxymorphone, or hydromorphone may promote vomiting when administered IM.  Drugs that potentiate vomiting should be avoided in cases of esophageal or gastric foreign bodies.  Kappa agonist opioids such as butorphanol are less likely to promote vomiting.  The animal should be induced with an injectable anesthetic and intubated quickly in order to avoid aspiration.  Propofol, thiopental, ketamine or etomidate may be used for this purpose, depending on the rest of the animal's condition.  The patient may be maintained on inhalants after intubation.  An appropriately inflated endotracheal tube cuff should be maintained at all times to avoid inadvertent aspiration of fluid during the procedure. 

Balanced, isotonic crystalloid fluids (such as Norm-R or LRS) administered at 10 ml/kg/hr should be used for patients with normal oncotic pressure and plasma proteins.  Hypoproteinemic patients may benefit from colloid administration.  Plasma or Hetastarch can be used to assist in maintaining sufficient oncotic pressure.  Hetastarch can be used at a rate of 5 mls/kg/hr along with crystalloid fluid administration during the procedure.  Care must be taken to avoid fluid overload.

Insufflation of the stomach with air must be carefully monitored to avoid overinflation and attendant cardiovascular and respiratory compromise.  Pulse oximetry, blood pressure, and capnometry are very helpful to monitor anesthesia in these patients.  Frequently respiration must be supported with intermittent positive pressure ventilation if abdominal pressure is increased.  The size of the stomach should be continuously monitored during gastroscopy.

Care must be taken to avoid aspiration of gastric contents.  The endotracheal tube cuff should be properly inflated upon intubation and maintained throughout the procedure.  The cuff should not be deflated until the patient is extubated, ensuring that the patient can swallow and protect the airway.

The cardiac and pyloric sphincters can impede endoscopy. Comparison of premedication with atropine, glycopyrrolate, morphine, meperidine, acepromazine and saline prior to general anesthesia for gastroduodenoscopy in dogs resulted in more difficulty in entering the pyloric sphincter when a combination of morphine and atropine was used. This has led to the suggestion that all full mu opioid agonists be avoided when duodenoscopy is performed.  The use of atropine in dogs as a premedication does not facilitate duodenal intubation and may inhibit it.  Alpha-2 agonists such as medetomidine do not hinder passage of the endoscope through the pylorus in dogs, although vomiting may be an issue in some patients.

More recent work has evaluated the effects of various premedications on ease of duodenoscopy in the cat. (15).  Their results suggest that hydromorphone (a full mu opioid agonist), glycopyrrolate (anticholinergic), medetomidine (alpha-2 agonist) or butorphanol (agonist antagonist opioid) are all satisfactory for use as premeds prior to gastroduodenoscopy in the cat.

Experienced clinicians may not have any difficulty passing the endoscope into the duodenum, despite the anesthetic protocol used.  Butorphanol may be used without difficulty and has the additional benefit of not inducing as much vomiting as a full mu agonist when used as a premedication.  Its short duration is helpful in avoiding excessive post anesthetic sedation.

Colonoscopy

Colonoscopy is often performed in patients with signs of large bowel or rectal disease. In order to adequately visualize the colonic mucosa, the bowel is prepared for the procedure with food withdrawal, administration of a gastrointestinal lavage solution and a series of enemas. This preparation can cause dehydration in some patients.  Careful evaluation should be performed to ensure adequate hydration prior to general anesthesia.  Volume deficits should be corrected prior to general anesthesia with IV administration of crystalloid fluids.

Rhinoscopy

Rhinoscopy patients need to have very good analgesia in their anesthetic protocol, as the procedure requires a surgical plane of anesthesia.  A full mu agonist opioid such as hydromorphone, morphine or oxymorphone can be administered as part of the premed in addition to a tranquilizer like acepromazine or an alpha-2 sedative.  Short acting, potent opioids such as fentanyl can be bolused intravenously prior to biopsy to prevent excessively high vaporizer settings.  Regional anesthetic techniques like infraorbital blocks with lidocaine, mepivicaine, or bupivicaine will also improve patient comfort.  Post procedure bleeding can be minimized if the patient is well sedated after biopsies are taken, as excessive head shaking and activity can lead to continued bleeding and increased irritation of the area.

The endotracheal cuff should be properly inflated prior to rhinoscopy and the procedure halted anytime there is a concern about the cuff.  It can be helpful to extubate the patient with the cuff partially inflated to assist in clearing blood from the airway if it has not been packed prior to beginning the procedure.

Suggested reading

Weil AB. Anesthesia for endoscopy in small animals. In: Radlinsky MG. ed. Endoscopy. Veterinary Clinics of North America, Small Animal Practice. Philadelphia, PA: W.B. Saunders Company; 2009:39:839-848.