Many possible complications need to be considered when formulating an anesthetic regimen for the animal with GDV. The abdomen is distended and restricts movement of the diaphragm and thorax. As a result, functional residual capacity and tidal volume are decreased, causing hypoventilation and hypoxemia (Jacobson et al. 1995). It also results in an increase of CO2leading to respiratory acidosis, further compounding the metabolic acidosis already present (Rasmussen 2003). The PaCO2 is monitored with blood gas analysis or approximated with capnographic ETCO2 readings (Haskins 1999). Manual or mechanical ventilation is necessary to improve the ventilatory status of the patient (Rasmussen 2003).
The bloated and torsed stomach compresses the caudal vena cava and the portal vein, which decreases the amount of venous blood returned to the heart. This in turn can negatively affect cardiac output and arterial blood pressure (Rasmussen 2003). Tissue perfusion is ultimately compromised, including perfusion to the already compromised organs (Hedlund and Fossum 2007). The anesthetist must monitor arterial blood pressure to ensure adequate tissue perfusion during surgery. Blood pressure can also decline during surgical manipulation of the spleen and torsed stomach. Aggressive fluid administration combined with inotropic agents such as dobutamine or dopamine (2–10 μm/kg/ min IV) are used to support blood pressure.
Gastric ischemia can lead to the breakdown of the gastric mucosa, leading to tissue necrosis of the stomach. Bacteria and endotoxins can enter the circulation through the compromised tissue precipitating septic shock to an already cardiovascular-compromised patient (Monnet 2003). If gastric perforation occurs, peritonitis may exacerbate the problem.
Electrolyte imbalances, hypoxemia, and inadequate myocardial perfusion can lead to cardiac arrythmias in the patient with GDV (Evans and Wilson 2007). The most common arrythmias observed are ventricular tachycardia and ventricular premature complexes (VPCs) (Rasmussen 2003). Multifocal VPCs and sustained ventricular tachycardia should be addressed and often respond to lidocaine (2.0 mg/kg IV bolus followed by 25–75 μg/kg/min IV CRI) (Fig. 30.1). Procainamide (2–10 mg/kg IV given slowly, followed by 20–50 μg/kg/min IV CRI) is used if the arrythmias are unresponsive to lidocaine. Arrythmias may persist for many hours postoperatively and must be monitored and corrected if they result in decreased cardiac output and poor tissue perfusion (Hedlund and Fossum 2007).
Electrolyte imbalances, hypoxemia, and inadequate myocardial perfusion can lead to cardiac arrythmias in the patient with GDV (Evans and Wilson 2007). The most common arrythmias observed are ventricular tachycardia and ventricular premature complexes (VPCs) (Rasmussen 2003). Multifocal VPCs and sustained ventricular tachycardia should be addressed and often respond to lidocaine (2.0 mg/kg IV bolus followed by 25–75 μg/kg/min IV CRI) (Fig. 30.1). Procainamide (2–10 mg/kg IV given slowly, followed by 20–50 μg/kg/min IV CRI) is used if the arrythmias are unresponsive to lidocaine. Arrythmias may persist for many hours postoperatively and must be monitored and corrected if they result in decreased cardiac output and poor tissue perfusion (Hedlund and Fossum 2007).
Regurgitation and aspiration of stomach contents are complications of the treatments for GDV dogs. When passing the orogastric tube pre- or intraoperatively and when the surgeon manipulates the stomach during surgery, fluid and food material can drain around the orogastric tube into the pharynx and into the trachea. The contents can be accidentally aspirated or even leak past the inflated cuff of the endotracheal tube. Aspiration leads to infection, edema, and inflammation of the airways. If this happens during anesthesia, the anesthetist must check the cuff to be sure it is properly inflated. The oropharyngeal area is flushed with water or saline and suctioned until all fluids are clear and no stomach contents remain. This can be done during surgery as well as recovery to assure that all fluids are removed prior to extubation (Rasmussen 2003).
Rapid induction and intubation of the patient for anesthesia can be facilitated by different drug protocols. Ketamine (5 mg/kg) combined with diazepam (0.25 mg/kg) can be titrated intravenously to effect. Diazepam (0.1–0.2 mg/kg IV) followed by etomidate (1.0–2.0 mg/kg IV) can also be used. Another induction option is lidocaine followed by thiopental. Lidocaine (2.0 mg/ kg IV) is given first, and then thiopental (9 mg/ kg IV) is titrated to effect. Either isoflurane or sevoflurane is used as the inhalant to maintain a surgical plane of anesthesia.
Intraoperative and postoperative analgesia is provided by hydromorphone or oxymorphone (0.05–0.1 mg/kg IV for either drug), administered as needed (usually every 4–6 hours). If arrhythmias persist following extubation, the ECG should be monitored and a lidocaine or procainamide CRI is continued. Postoperative monitoring and correction of electrolyte, fluid, and acid-base imbalances are also recommended (Aronson et al. 2000; Hedlund and Fossum 2007). Oxygen therapy via nasal cannula or face mask is used if the patient becomes hypoxic during recovery (Jacobson et al. 1995).
Foreign Bodies and Obstructions
Dogs and cats indiscriminately eat objects such as bones, rocks, sticks, strings, clothing, carpet, fish hooks, needles, and coins. These objects can become lodged in the pharynx or trachea; under the tongue; or in the esophagus, stomach, or small intestine, causing partial or complete obstructions. Systemic toxicities can also develop from ingested objects containing lead or zinc. In addition, intraluminal intestinal obstructions can also be the result of an intussusception or neo-plasia (Macintire et al. 2005).
Airway obstructions can cause severe respiratory distress and anxiety. The increased respiratory effort and decreased heat exchange through respiration can lead to hyperthermia, and inadequate ventilation can lead to hypercapnia (Perkowski 2000). Inflammation of tissues surrounding the object can cause further complications. The administration of acepromazine (0.005–0.05 mg/kg IV, IM) can decrease anxiety and sedate the patient in order to provide oxygen via face mask or with flowby oxygen (Ford and Mazzaferro 2006). Be aware that acepromazine causes vasodilation, which can affect blood pressure, especially in patients under anesthesia. It is also not reversible.
Establishing a patent airway is top priority when presented with a patient having an upper airway obstruction. This is achieved by endotracheal intubation or by emergency tracheostomy, if required, in order to bypass the obstruction (Ludwig 2000; Syring and Drobatz 2000). Thiopental (12–15 mg/kg IV) or propofol (4–6 mg/kg IV) is given slowly in small, incremental doses to allow intubation and visualization of the foreign body.
Once the airway is secured and oxygen is delivered, the clinician can locate and remove the object and further evaluate any tissue damage via tracheoscopy. It is important to monitor the patient’s oxygen saturation and ventilatory status throughout this procedure and provide manual ventilation if the oxygen saturation level drops below acceptable reference ranges (Ford and Mazzaferro 2006).
Recovery complications include possible respiratory distress due to occlusion of the airway secondary to tissue swelling (Hedlund 2007b). The anesthetist should monitor the patient for signs of respiratory depression and, if painful during recovery, butorphanol (0.2–0.4 mg/kg IM) or buprenorphine (5–15 μ g/kg IM) is administered.
Animals with esophageal foreign bodies can show signs of retching, gagging, ptyalism, and food regurgitation (Aronson et al. 2000). Thoracic radiographs and contrast esophagrams are used to locate the foreign body and identify thoracic complications (i.e., pneumothorax or pneumomediastinum) secondary to an esophageal perforation. An esophagoscopy is performed to identify the foreign body and attempt to remove it. If the patient requires initial sedation, the anesthetist can administer acepromazine (0.02–0.05 mg/kg IV, IM). For rapid induction, thiopental (12–15 mg/kg IV) or propofol (6 mg/ kg IV) is titrated to effect to allow for intubation followed by inhalant anesthesia (isoflurane or sevoflurane) (Kyles 2003).
The clinician should attempt to pull out the object with grasping forceps or, if necessary, advance the object into the stomach for dissolution by gastric acid or retrieval via gastrotomy (Macintire et al. 2005). If bradycardia develops during the endoscopy, glycopyrrolate (0.01 mg/ kg IV) or atropine (0.02 mg/kg IV) can be given.
If endoscopic extraction of the object is successful, the patient will have discomfort or pain from the manipulation of the esophagus. Butorphanol (0.2 mg/kg IV, IM) is administered before or after extubation as needed for pain control. A potential complication of endoscopy is regurgitation with possible aspiration. The clinician can suction the esophagus with the endoscope during the procedure, but the oropharynx must be examined during recovery and suctioned if any fluids remain.
During the procedure, the esophagus is evaluated for any perforations that may have been caused by the foreign body or by attempts to retrieve it. Changes in respiration rate and effort, oxygenation, and ventilatory status can indicate a possible tension pneumothorax caused by air escaping through an esophageal tear (Kyles 2003). If a large perforation is present or if the object is firmly lodged and cannot be retrieved during endoscopy, a thoracotomy is necessary to access and repair the esophagus (Macintire et al. 2005).
Gastric and intestinal foreign bodies can obstruct the outflow of ingesta or cause perforations of the wall, allowing contents to leak into the abdominal cavity. The dissolution of the object in the stomach can lead to systemic illness if toxic chemicals are absorbed (Rasmussen 2003). Septic peritonitis and shock result from gastrointestinal leakages through intestinal perforations into the abdomen (Aiello 1998).
Vomiting is the most frequent clinical sign associated with gastric and intestinal obstructions, but it varies from acute to chronically intermittent, depending on the location of the foreign body in the intestinal tract (Brown, D.C. 2003; Macintire et al. 2005; Rasmussen 2003). Other clinical signs include dehydration, lethargy, diarrhea, abdominal distention, and pain (Aiello 1998). Anemia as a result of bleeding or toxicity may be present along with abnormal electrolyte and acid-base values (Macintire et al. 2005; Rasmussen 2003).
Abdominal radiographs help locate the obstruction or suspected foreign bodies, and should be repeated just prior to anesthesia to evaluate whether the foreign body has moved (Hedlund and Fossum 2007). Radiographs can reveal enlarged loops of bowel with trapped gas or fluids just proximal to the obstruction (Macintire et al. 2005). The small intestine may appear plicated if a linear foreign body is present (Brown, D.C. 2003).
The location and duration of the obstruction or foreign body will influence the anesthetic plan. Animals that are presented to the clinic soon after ingesting foreign material may be otherwise healthy. Electrolyte and acid-base disturbances may be profound with obstructions of the small intestine and require medical therapy. Appropriate intravenous fluid therapy is initiated prior to anesthesia in debilitated animals (Macintire et al. 2005). Patients exhibiting signs of toxicity, such as from zinc or lead, may require medical management in addition to the removal of the object (Ford and Mazzaferro 2006).
If the object is in the stomach or just past the pylorus, the veterinarian can use an endoscope to examine the foreign body and possibly retrieve it with grasping forceps or other endoscopic tools. Surgical removal of a foreign body is required if endoscopic retrieval is not possible or if the obstruction is in the small intestine (Macintire et al. 2005). The veterinarian performs an exploratory laparotomy to gain access to the stomach and small intestine to remove the object and evaluate the organs for signs of perforation, ischemia, and necrosis (Hedlund and Fossum 2007).
The anesthetic plan must address any fluid, electrolyte, or acid-base disturbances throughout the perioperative period. When ischemic necrosis is likely or intestinal contents have leaked into the abdominal cavity, surgery should not be delayed (Macintire et al. 2005). The anesthetist can collect intraoperative blood samples to be analyzed for changes in arterial blood gas values. The choice of fluids to be administered (crystalloids, colloids, blood products) will be based on serum chemistry and complete blood count values prior to anesthesia.
For sedation, the anesthetist can administer acepromazine (0.02–0.05 mg/kg IV, IM) only if the patient is stable. The compromised or hypovolemic patient may not require sedation, but if necessary, diazepam or midazolam (0.2–0.4 mg/ kg IM, IV) can be given instead of acepromazine. Cats may also receive ketamine (5 mg/kg IM) for preoperative restraint if indicated. Analgesics include hydromorphone (0.1–0.2 mg/kg IM, IV) or oxymorphone (0.05–0.1 mg/kg IM, IV) that can be included with premedications or can be administered intravenously after anesthetic induction. Additional doses of intraoperative opioids are administered as needed.
For rapid induction, thiopental (12–15 mg/kg IV), propofol (6 mg/kg IV), or ketamine (5 mg/kg IV) combined with diazepam (0.25 mg/kg IV) are titrated to effect to allow for intubation, followed by inhalant anesthesia (isoflurane or sevoflurane). If bradycardia develops during the exploratory celiotomy, glycopyrrolate (0.01 mg/kg IV) or atropine (0.02 mg/kg IV) can be used for treatment.
Once the patient is anesthetized, the oral cavity and tongue are examined for linear foreign bodies (i.e., string) that may be anchored underneath the tongue (Macintire et al. 2005). If the animal requires surgery, direct arterial blood pressure should be monitored because manipulation of abdominal viscera can cause a profound hypotensive crisis. Colloids (hetastarch or plasma) or inotropic drugs can be administered to elevate blood pressure and improve tissue perfusion (Muir et al. 2007). If the surgeon elects for open abdominal drainage, the anesthetist needs to provide appropriate amounts of compensatory IV fluids and needs to keep the patient warm with circulating hot water pads or forced-air blankets (Rasmussen 2003).
Prior to extubation, any regurgitated fluid in the oropharynx needs to be suctioned out to avoid aspiration pneumonia (Rasmussen 2003). If endoscopic extraction of the object is easily performed, the patient may only require an analgesic such as butorphanol (0.2–0.4 mg/kg IM, IV) or buprenorphine (5–15 μg/kg IM, IV, also buccal in cats) for postoperative pain.
Animals undergoing exploratory celiotomies require postoperative analgesics. The surgeon can infiltrate the subcutaneous and muscle layers around the incision with bupivicaine (0.5–1.0 mg/ kg for cats; 1.0–2.0 mg/kg for dogs) just prior to surgical closure for local pain management (Carroll 2008). Postoperative hydromorphone (0.1–0.2 mg/kg IV, IM) or oxymorphone (0.05–0.1 mg/kg IV, IM) are administered as needed after extubation.
Intervertebral Disk Disease (IVDD)
Intervertebral cartilaginous disks are pads that lie between two adjacent vertebrae, which absorb shock and assist in vertebral movement. A disk consists of an outer layer of fibrocartilaginous material (the annulus fibrosus) surrounding a center of gelatinous material (the nucleus pulposus) (Toombs and Waters 2003). Degenerative changes in the disk can result in its herniation dorsally into the vertebral canal, compressing and injuring the spinal cord. The type of change and the resulting extrusions or protrusions of the disk are classified as either a Hansen type I or Hansen type II degeneration (Kapatkin and Vite 2000; Toombs and Waters 2003).
A common emergency presentation of a thoracolumbar disk rupture is a dachshund or other chondrodystrophoid breed having progressive signs of back pain, then loss of proprioception and motor function, and superficial and potentially deep pain. A complete neurological examination allows the veterinarian to localize the lesion, but survey radiographs followed by a myelogram, a computed tomography (CT) scan, or magnetic resonance imaging (MRI) of the area is needed to provide a definitive diagnosis (Toombs and Waters 2003).
Spinal imaging and decompressive surgery of the lesion require the patient to be anesthetized. These dogs are usually metabolically stable (Kapatkin and Vite 2000), and owners describe them as healthy just prior to the acute onset of the back pain or paraparesis. For preoperative analgesia and sedation, diazepam or midazolam (0.2–0.4 mg/kg IM, IV) in combination with hydromorphone (0.1 mg/kg IM, IV) or oxymorphone (0.05–0.1 mg/kg IM, IV) can help reduce anxiety and pain. For rapid induction, use thiopental (12–15 mg/kg IV) or propofol (6 mg/kg IV) titrated to effect to allow for intubation followed by inhalant (isoflurane or sevoflurane) anesthesia. Intravenous fluids are initiated following intubation to maintain intravascular volume and blood pressure (10 mL/kg/hr for the normovolemic patient) (Pascoe 2006).
Survey radiographs and myelogram require that the patient be repositioned frequently to obtain several views. The anesthetist must be attentive to accidental extubation of the patient, dislodgement of the intravenous catheter and fluid lines, and detachment of the various monitoring instruments. The clinician injects an iohexol contrast agent through a spinal needle into the subarachnoid space following the completion of survey radiographs. The flow of the contrast agent in the spinal canal is evaluated with the aid of fluoroscopy to determine the exact location of the lesion (Seim 2007). During the subarachnoid injection of iohexol, severe bradycardia can develop due to vagal stimulation (Jacobson et al. 1995). If the patient’s heart rate is low or drops rapidly during the injection, the anesthetist can administer glycopyrrolate (0.01 mg/kg IV) or atropine (0.02 mg/kg IV) to prevent or correct bradycardia. Iohexol may flow intracranially as well, creating the risk of seizures at recovery (Harvey et al. 2007).
If CT or MRI is available for spinal imaging, the contrast media used to highlight the lesion (i.e., Ultravist® or Magnevist®, Berlex, Montville, NJ) is injected intravenously as needed and can cause temporary hypotension or an increased heart rate, rarely requiring intervention.
Figure 30.2. Gelatin sponge soaked with preservative-free morphine placed at the laminectomy site.
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