Anesthesia for endoscopy--part 2 (Proceedings)

Laryngoscopy/tracheoscopy

Many patients undergoing this diagnostic procedure will have signs of obstructive upper airway disease.  They are dyspneic and easily stressed.  Thoracic radiographs should be added to the minimum data base if the images can be obtained without excessive stress to the patient.  Evaluation of laryngeal function is most frequently done under a light plane of general anesthesia.  A variety of injectable anesthetics have been used.  All sedative drugs and deeper planes of anesthesia tend to diminish arytenoid function with much individual variation.  The perfect technique of general anesthesia for laryngeal function evaluation has yet to be established, as the depth of anesthesia must be sufficient enough to open the jaw and protect the examiner and equipment, yet still maintain arytenoid cartilage movement for evaluation.  False positive exams can occur with most sedatives and anesthetic combinations.

Preoxygenation of the patient via oxygen mask or flow-by oxygen with the breathing circuit is very helpful.  Two to three liters per minute of oxygen should be given for 5 minutes immediately prior to drug administration.  This allows increased time for examination of the airway before the patient desaturates.  A variety of injectable anesthetic protocols have been evaluated.  It is helpful for an assistant to announce inspiration by the patient while evaluating arytenoid abduction.  Propofol may be administered at 6 mg/kg IV to effect or thiopental (if available) at 12 mg/kg to effect.  Administration of supplemental oxygen during the exam is useful, as is pulse oximetry to monitor oxygen saturation.  Some authors recommend doxapram administration (2-5 mg/kg IV) at the end of the exam to stimulate more vigorous respiratory movements and eliminate false positives.  The most important consideration is a light plane of anesthesia.   I think it is helpful to use the same anesthetic on most patients if medically appropriate, to minimize variations in assessment due to drug differences.

General anesthesia is used for tracheoscopy and bronchoscopy in animals in order to minimize laryngospasm and coughing and protect the endoscope.  Tracheoscopy/bronchoscopy is performed without an endotracheal tube in very small patients or via the endotracheal tube in patients with sufficient tracheal diameter (size 7 or 8 endotracheal tube).  Inhalant anesthesia can be used to maintain the patient during bronchoscopy is the patient is large enough for an endotracheal tube, using a special T-shaped adapter to accommodate the scope as well as administer oxygen and anesthetic gas.  There should be sufficient room inside the endotracheal tube for exhalation of gases without resistance.

Injectable anesthetics can be used to maintain anesthesia in patients with small tracheal diameter, while oxygen is administered through the scope or via a catheter placed beside the scope if there is sufficient room.  A variety of injectable protocols may be used, depending on the patient condition.  In general, an injectable protocol that has minimal cardiovascular effects and allows rapid recovery is preferable, as many patients undergoing bronchoscopy have significant respiratory impairment.  Short acting opioids can be used for premedication, such as fentanyl or butorphanol.  Butorphanol is a potent cough suppressant.  Acepromazine has little respiratory depression and is useful at low doses to calm patients with upper respiratory disease.  Propofol has little accumulative effect and can be administered in intermittent boluses or via CRI to maintain anesthesia.  The use of anticholinergics to “dry up” small airways is no longer recommended.  Oxygen supplementation post tracheoscopy or bronchoscopy is important to support patients through the recovery period until airway reflexes are normal.

Oxygen saturation should be monitored via pulse oximetry throughout the procedure, with the goal of maintaining saturation above 95%.  Mean arterial blood pressure should be greater than 60 mm Hg.  Administration of balanced, isotonic crystalloid fluids should be used with inhalant anesthesia and propofol CRI of moderate duration.

Thoracoscopy

When an instrument is placed in the thoracic cavity, the negative pressure of the thorax is compromised.  Deliberate collapse of the operable side lung is attempted in many instances to improve surgical conditions.  One lung ventilation may be achieved either through selective intubation of one bronchus or the use of a bronchial blocker to improve conditions for endosurgery.  Selective intubation can be done blindly or with the aid of an endoscope.  Thoracoscopy may also be performed with more conventional two lung ventilation techniques and the use of smaller tidal volumes to improve surgical conditions.  The use of bilateral ventilation techniques decreases general anesthesia time, as selective intubation is not done.  Although complete lung collapse does not occur, this tends to be the simplest way to manage the anesthesia for the patient.

One lung ventilation has minimal cardiopulmonary effects on healthy dogs with an intact chest. Nevertheless, opening the thoracic cavity will have adverse effects on gas exchange and may compromise the patient's oxygenation ability.  Significant decreases in arterial oxygen partial pressure (PaO2) and oxygen content can be expected.  Significant increases in shunt fraction and physiologic dead space can occur.  Arterial partial pressure of carbon dioxide may not be affected.

While laparoscopy requires insufflation of the abdominal cavity, thoracoscopy can be performed with or without carbon dioxide insufflation.  Thoracic insufflation decreases cardiac output at low insufflation pressures (3 mm Hg) and sustained insufflation should be used with caution. Monitoring patients undergoing general anesthesia for thoracoscopy should include capnometry, pulse oximetry, ECG and blood pressure monitoring.  Invasive blood pressure monitoring has the added advantage of arterial catheter placement for easier blood gas analysis.  Particular attention should be paid to the patient's ventilation and oxygenation status. 

Clinical candidates for thoracoscopy usually have pulmonary compromise, which can hamper their ability to withstand a sustained thoracoscopic procedure.  Conversion to thoracotomy may be necessary for patients who desaturate or deteriorate with a lengthy general anesthesia time.  One of the most common reasons for conversion to thoracotomy is insufficient lung collapse and visualization of the operative site.

Many of the complications of thoracoscopy are the same as conventional thoracotomy surgery.  Decreases in arterial oxygen tension and hypoventilation should be anticipated.  The use of 5 cm H2O of positive end expiratory pressure (PEEP) may help with desaturation, especially if one lung ventilation is used.  A mechanical ventilator will help sustain ventilation and free up personnel to monitor the patient.  Hemorrhage at the surgical site may occur and the patient should be monitored with serial PCV/total protein measurements.  Blood products should be administered if necessary.  Plasma or hetastarch can be helpful in maintaining a robust intravascular volume if cardiopulmonary compromise is anticipated.

Animals that experience thoracoscopy may have less pain than patients who experience lateral thoracotomy or median sternotomy, but have analgesic needs that should be addressed nevertheless.  Full mu agonist opioids are appropriate analgesic choices for most patients, despite the respiratory depression produced by the drug.  Oxygen therapy post procedure may be warranted in many clinical cases.

Laparoscopy

Laparoscopic noninvasive surgery has become common as more procedures are attempted in a non invasive fashion.   In order to perform this type of surgery, a pneumoperitoneum is established to allow room to place the trocar and cannula assemblies safely and improve visualization. Several gases have been used to insufflate the abdomen: carbon dioxide, nitrous oxide or room air.  Carbon dioxide is most frequently chosen as the insufflation gas for laparoscopy.  The use of medical air has increased potential for air embolism and increased potential to support combustion if electrocautery is used.  Carbon dioxide is able to diffuse across the peritoneal cavity and enter the blood stream, where it stimulates the sympathetic nervous system to release endogenous catecholamines.  Higher levels of arterial CO2 tend to increase heart rate, blood pressure and cardiac output.  Excessively high levels of CO2 will lead to narcosis, arrhythmia, acidemia, and myocardial depression.  Nitrous oxide does not alter the patient's acid-base status.

Regardless of the type of gas used, insufflation of gas increases intraabdominal pressure (IAP) in the patient, with the potential to cause decreased tidal volume and hypoventilation.  Functional residual capacity and lung compliance decrease during general anesthesia.  The increase in intraabdominal pressure from gas insufflation causes cranial displacement of the diaphragm.  All of these factors contribute to the need for increased ventilation support for the anesthetized patient undergoing laparoscopy.  Depression of ventilation increases with increasing intraabdominal pressure and intraabdominal pressure less than 20 mm Hg is recommended.

Increased IAP also leads to decreased venous return and a reduction in cardiac output.  Tissue blood flow may be compromised with increased IAP as elevated IAPs are associated with decreased hepatic blood flow and oliguria. Anesthetic conditions for the patient will be improved by using the least amount of IAP necessary to complete the procedure.

Changes in body position have the potential to adversely affect the anesthetized patient, especially when coupled with abdominal insufflation.  Inhalant anesthetics alter the baroreflex, leading to a depressed reflex control of circulation in response to changes in body posture.  Head down tilt of a dorsally recumbent patient (Trendelenburg position) allows better exposure of caudal organs in the operative field.  Reverse Trendelenburg position (head up and dorsally recumbent) is used when improved exposure of cranial organs is desired.  The head down tilt position has more effect on respiratory and cardiovascular mechanics; leading to decreases in minute ventilation and cardiac output, amongst other things.  Mean arterial pressure may increase.  The head up tilt position will also affect cardiovascular mechanics, leading to reflex vasoconstriction and increased heart rate and arterial blood pressure in dogs.

Excellent monitoring of the anesthetized patient undergoing laparoscopy is essential.  Increased intraabdominal pressure (IAP) results in hypoventilation, so the use of a mechanical ventilator is helpful to provide pulmonary support as normocapnia should be a monitoring goal.  If CO2 is the insufflation gas used, absorption of CO2 across the peritoneal membrane will lead to higher PaCO2, regardless of the respiratory status of the patient.  End-tidal CO2 monitoring and pulse oximetry will provide continuous monitoring of the respiratory system.  Invasive blood pressure monitoring is warranted in more critical patients undergoing laparoscopy, while non-invasive methods (Doppler or oscillometric cuff-based monitors) can be used in healthy patients undergoing elective laparoscopic procedures.  Arterial catheter placement will allow easier sampling for blood gas analysis if CO2 is the insufflation gas.  Abdominal insufflation must be monitored and the Rule of 15's is a good general guideline: No more than 15 mm Hg IAP, or 15 degrees of tilt.

While general anesthesia is most frequently used for laparoscopic procedures in small animals, it is important to consider the increased stress to the patient of abdominal insufflation and tilted body posture.  These effects are aggravated by general anesthesia.  A recent study compared the cardiopulmonary effects of laparoscopic-assisted jejunostomy feeding tube placement during sedation with epidural and local anesthesia versus general anesthesia.  Sedation and local anesthesia provided satisfactory conditions for the laparoscopic procedure and less cardiopulmonary depression.  Thus, sedation and epidural anesthesia may be considered for critical patients requiring a laparoscopic procedure.  Conversion to general anesthesia may be necessary if the duration of the procedure is extended and mechanical ventilation needed to offset excessive increases in PaCO2.

Complications of laparoscopy include hemorrhage, pneumothorax, or puncture of an organ with placement of the veress needle.  Splenic enlargement can occur depending on the induction agent.  Serial packed cell volume determination and total protein measurement can help assess the need for blood replacement products.  Packed red blood cells and plasma, or whole blood transfusion should be considered if the PCV falls below 20 and the total protein below 4.  Post operative analgesic needs can be met with parenterally administered opioids.  Lidocaine patch application at the port sites can be used to provide regional analgesia without systemic effects.

Suggested readings

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.