The purpose of this talk today is to discuss a variety of issues for general anesthesia of patients undergoing non-elective procedures. This will include anesthetic tips for urinary tract obstruction, cesarean section, gastric dilation volvulus, and foreign body removal.
The purpose of this talk today is to discuss a variety of issues for general anesthesia of patients undergoing non-elective procedures. This will include anesthetic tips for urinary tract obstruction, cesarean section, gastric dilation volvulus, and foreign body removal. Patients requiring surgical intervention for these procedures can present with a wide spectrum of clinical signs. Some will be in similar preoperative condition to a patient undergoing an elective surgery, and others will be so ill that general anesthesia may be life threatening.
Basic laboratory data should include CBC, chemistry with electrolytes, and lactate. Other ancillary diagnostics may include thoracic and abdominal radiographs, blood gas analysis, ultrasound, or echocardiography.
Assessment is of paramount importance: Many of these animals will be moderately to severely hypovolemic. Volume deficits must be made up as quickly as possible prior to surgery. However, sometimes a surgical intervention must be performed before the animal's deficits are completely corrected. Robust fluid administration is often necessary during general anesthesia. “Regular” fluid rates for patients undergoing general anesthesia with inhalants are generally recommended at 5-10 mls/kg/hr of crystalloid fluid. Patients undergoing emergency exploratory laparotomy may require higher rates than this, and/or the inclusion of colloid therapy. One caveat to the above mentioned information: if the patient has ongoing hemorrhage into the abdomen, then zealous fluid rates will only increase the blood loss into the abdomen. Care must be taken to deliver enough fluid volume to help deliver oxygen to vital tissues, without increasing the blood pressure unduly and exaggerating blood loss.
Basic monitoring parameters should include blood pressure, capnography, pulse oximetry, and ECG if possible. Direct blood pressure monitoring via arterial catheterization can be very helpful both for monitoring and blood gas analysis. Care should be taken to insure that excessive amounts of time are not used to place the catheter in unstable patients. I generally recommend that all patients who undergo non-elective surgical procedures have at least two intravenous catheters, as these patients may have multiple fluids, analgesic agents, blood products, or antibiotics that need to be administered. Central lines may be very helpful in the more critical patient for central venous pressure monitoring, as well as the ease of blood sampling for serial PCV/TP/electrolyte/ lactate monitoring as well as multiple ports available for fluid and drug administration.
Opioid therapy is the mainstay of analgesia for these patients, as opioids have the advantage of providing supraspinal analgesia with minimal effects on cardiac contractility. Choices include µ-agonists such as morphine, fentanyl, hydromorphone, or oxymorphone. Kappa agonists like butorphanol or partial µ-agonists like buprenorphine are also options. Amount of analgesia needed, duration of analgesia, and route and ease of administration should be considered on an individual basis. Analgesic therapy should be selected to meet pre-op, intra-op and post-op needs. Combinations of opioids can be used if a single agent does not meet all needs. Benzodiazepines such as diazepam or midazolam can be very helpful to enhance sedation in debilitated patients, help lower the amount of induction drug needed, and augment muscle relaxation. Patients that are in good shape prior to surgery may benefit from a low dose of acepromazine to improve sedation prior to general anesthesia and minimize emergence delirium.
Induction of general anesthesia can be done via injectable or inhalant means. Injectable anesthetic choices include propofol, dissociative anesthetics (ketamine or Telazol®), or etomidate. Propofol causes vasodilation and reductions in cardiac output and blood pressure, resulting in a compensatory increase in heart rate when administered. Ketamine stimulates the sympathetic nervous system, with resultant endogenous catecholamine release. This results in increased heart rate, blood pressure, and cardiac output. It should be noted that if a patient is catecholamine depleted, then ketamine is a negative inotrope as well. Etomidate can be considered the most cardiac friendly of the injectables---it maintains the patient's cardiac output “status quo” so to speak, and is not arrhythmogenic. Regardless of which injectable is selected, it should be administered “to effect” to facilitate endotracheal intubation and keep the dose as low as possible.
Maintenance of these patients with inhalant agents has the advantage of oxygen administration as well as the capability of ventilation support if necessary. Either isoflurane or sevoflurane are appropriate choices for maintenance. Both agents will support vital organ blood flow and cardiac function in a similar manner. Sevoflurane will recover patients more quickly, while isoflurane is more cost effective to use.
Comments on general anesthesia for specific procedures
Management of anesthesia for cesarean section in the small animal is a somewhat controversial, highly opinion-based science. The goal of Cesarean section is to produce lively, thriving puppies or kittens which are minimally affected by anesthetic and perianesthetic drugs, and to prevent anesthesia-related problems in the bitch or queen. There are two different types of patients presented to the veterinarian for cesarean section: the stable patient presented for elective surgery and the animal that has experienced dystocia for a variable time frame and must have surgery on an emergent basis. The emergency patient may be exhausted, hypovolemic, hypotensive, hypo or hyperthermic and painful. The anesthetic management of such a patient must take these factors into consideration. Anesthetic management may include sedative and analgesic drugs, general anesthesia, regional anesthesia or any combination of the above.
The patient may be at risk of increased gastric pressure and more prone to aspiration. Thus it is best if intubation is accomplished quickly so that the endotracheal tube cuff is inflated (properly) in a timely fashion and the patient's airway protected. If the patient has been laboring for a prolonged period, you can expect that they will be hypovolemic, exhausted, and have acidosis or have other metabolic derangements. Hypovolemia should be corrected prior to general anesthesia so that blood pressure can be appropriately maintained in the dam. If time or viability of the fetuses becomes an issue, than a fluid challenge dose of 10 mls/kg/hr balanced isotonic crystalloid fluid or crystalloids combined with colloid therapy may be administered quickly prior to induction.
The use of opioids as preanesthetic drugs in cesarean section candidates have the advantage of providing analgesia to the dam and reversibility in the neonate if their effects compromise vigor at the time of birth. An anticholinergic may be given if there is a concern about falling heart rate. Atropine will cross the placental barrier while glycopyrrolate will not. A catheter should be placed in the dam after premedication. Fluids may be bolused to the patient if hypovolemic. Preoxygenation of the dam via face mask (2-3 l flow) for five minutes is beneficial if an injectable induction method is chosen. Injectable agents commonly available include ketamine, propofol or etomidate. Other practitioners may elect to induce anesthesia via mask, although there is some concern for additional aspiration risk due to the increased pressure against the diaphragm of the pregnant patient.
Maintenance with either sevoflurane or isoflurane is appropriate. Sevoflurane has the advantage of being less soluble with the result of quicker recovery of both the neonate and the mother. Regardless, the lowest possible level of inhalant should be utilized until the neonates are out of the uterus. Fluid therapy is needed at rates of 10 ml/kg/hr crystalloid fluids or higher. Regional anesthestic techniques can be utilized to advantage in these patients, including line blocks and epidurals. Regardless of the techniques used, time is of the essence for a good outcome.
Foreign body removal
Most patients presented for foreign body removal will benefit from a fairly straightforward anesthetic protocol. Many have been vomiting for some time, and need volume management prior to anesthesia and surgery. I will frequently use an opioid IV just prior to intubation to minimize vomiting at the time of induction, as these patients have significant risk of aspiration.
I often use a protocol similar to those for elective ovariohysterectomy in the pyometra patient that is still relatively “healthy”. For those that are systemically ill, I choose a protocol that minimizes reductions in cardiac output. These patients often require significant volumes of fluid to manage hypotension.
Gastric dilatation volvulus
Most of the dogs presented for surgery for gastric dilation and/or volvulus have significant pain and distress. One should expect to treat problems associated with metabolic derangement (acidosis or alkalosis), cardiac arrhythmias, shock, dying tissue, decreased venous return, respiratory impairment, and peritonitis. The distended stomach should be decompressed as quickly as possible. Volume depletion and electrolyte abnormalities should be addressed and corrected as soon as possible. Anesthetic drug choices should minimize impact on cardiac output and avoid arrhythmias. Analgesic therapy is needed. Crystalloid, colloids, hypertonic saline or various combinations of fluid therapy will be needed to restore circulating plasma volume.
Urinary tract obstruction
Evaluation of blood work for azotemia and electrolyte disturbances are very important prior to choosing anesthetic protocols. These patients are often hypovolemic, need fluid support for general anesthesia and cannot pass urine. Judicious fluid use is necessary to ensure circulating plasma volume support. Hyperkalemia should be corrected prior to general anesthesia if at all possible.
Sample Protocol for the Critical Abdominal Patient
Hydromorphone 0.05-0.1 mg/kg IM, SC, or IV
100% oxygen via face mask
Propofol 6 mg/kg IV to effect or
Diazepam 0.2 mg/kg IV with Etomidate 1-2 mg/kg IV to effect or
Mask induction with sevoflurane or isoflurane
Sevoflurane or Isoflurane
Fentanyl CRI 2-15 µg/kg/hr
Hydromorphone 0.05 mg/kg
LRS or Norm-R
Lumb & Jones' Veterinary Anesthesia and Analgesia, 4th edition. Blackwell Publishing, 2007