Anesthesia for sick patients (Proceedings)
Objectives of the presentation:
To identify some of the life-threatening changes that may develop during anesthesia of patients with serious abdominal disease.
To use cases to illustrate anesthetic management for prevention and treatment of these complications.To discuss different anesthetic protocols with specific reference to their cardiovascular effects and impact on these high risk patients.
Summary of important points:
Prevalence of complications during anesthesia is increased in patients with major systemic disease.
Decreased anesthetic requirement, hypotension, and hypoventilation occur regularly in these patients.
Opioids and benzodiazepines are useful agents for these high-risk patients.
Thiopental and propofol cause greatest cardiovascular depression, and ketamine and etomidate the least.
Isoflurane and sevoflurane are frequently used for maintenance of anesthesia.
Fentanyl and lidocaine infusions in dogs are useful adjuncts to maintenance of anesthesia.
Patients that have physiologic or pathologic abnormalities are more likely to develop complications during or after anesthesia and surgery. A system for classification of risk associated with anesthesia has been found to closely correlate with occurrence of complications. The risk classification is 1 for completely healthy patients through 3 with compensated major systemic disease, 4 is major disease that could result in death in the near future, and 5 includes patients that are dying or severely ill that require emergency surgery in order to have a chance for survival. Surveys of both veterinary and human patients show that patients that have risk classification of 3 or above have significantly more problems with anesthesia. When the patient also has underlying cardiac disease, the risk for complications increases. Anesthesia risk evaluation in humans with cardiac disease undergoing non-cardiac surgery has revealed a significantly increased risk over healthy patients, with the risk of complications (morbidity) for patients with cardiac disease increasing from 2.9% for a minor procedure to 19% if the surgery is complex and to 49% in patients with an uncontrolled medical problem.
Dogs and cats with abdominal disease that present as high anesthetic risks include gastric dilatation-volvulus, splenectomy, intestinal volvulus, intestinal perforation, traumatic abdominal crush or perforation, protein losing enteropathy, pancreatitis, biliary calculi or mucocoele, exploratory for neoplasia, and urinary tract rupture. These patients should be managed before anesthesia to bring physiologic parameters to as close as normal as possible. These patients are prime candidates for hypotension and decreased peripheral perfusion during anesthesia if they are hypovolemic and hypoproteinemic. Blood volume and colloid osmotic pressure should be normalized before anesthesia and some animals that are anemic or have been bleeding may need a blood transfusion.
It is imperative to practice "Defensive Anesthesia" for these high-risk patients. Defensive anesthesia involves evaluating the problems before anesthesia, anticipating the complications, and making contingency plans.
Two intravenous catheters should be inserted in these animals before anesthesia in anticipation of the need for multiple drug administrations. Arterial blood pressure, heart rate, gum color, and capillary refill time should be measured perioperatively to monitor the patients' cardiovascular progress. Measurement of central venous pressure when a jugular catheter is in place (tip must be within the thorax) will provide additional guide to the patient's need for fluid. Dosage of emergency drugs should be calculated before anesthesia. This will shorten time to treatment of an abnormality and removes risk of inadvertent inaccurate drug administration. A prepared bag of dopamine or dobutamine should be present in the surgery room.
Animals that are obviously depressed, or are septicemic or endotoxemic, will have a reduced requirement for anesthetic agents and may need very little drug for endotracheal intubation and maintenance of anesthesia.
Animals with a large intra-abdominal mass are at risk for hypotension from aortocaval compression when they are turned into dorsal position. Manipulation of ischemic tissue, some neoplasms, or mesenteric traction may release catecholamines or other vasoactive products that cause abrupt hypotension. Reperfusion injury may develop in some organs that have been ischemic and blood supply restored during surgery.
Balanced electrolyte solutions, such as lactated Ringer's solution or Normosol-R, are used for basic fluid therapy during anesthesia. Patients that are hypoglycemic or have liver disease should receive 5% dextrose at 3ml/kg/h and the rate adjusted based on measurements of blood glucose. Cardiovascular support with any or all of the following may be necessary in these patients: dobutamine infusion at 5 micrograms/kg/min, dopamine at 7 micrograms/kg/min, ephedrine at 0.1 mg/kg, hetastarch, 10-20 ml/kg, and hypertonic saline, 2-4 ml/kg, IV.
Adequacy of ventilation and oxygenation should be monitored during anesthesia and in recovery. Controlled ventilation may be necessary. Regurgitation may occur in some of these animals and the pharynx must be cleaned before recovery from anesthesia. Passage of a stomach tube may be necessary to empty the stomach and lavage with water is indicated in some cases. The eyes must be protected from contact with gastric fluid; imperative in short-nosed brachycephalic breeds.
Abdominal distension resulting from gastric distension in experimental dogs decreases cardiac output as much as 90% from normal and the decrease in coronary blood flow causes myocardial ischemia. Measurement of increased serum concentrations of cardiac troponin I and cardiac troponin T in clinical patients with gastric dilatation volvulus (GDV) is supportive evidence of myocardial ischemia. The increased serum concentrations of these markers of myocardial cell injury were correlated with the severity of ECG abnormalities and outcome. Ventricular premature depolarizations may first manifest within 36 hours after the onset of the disease and are further evidence of myocardial damage. A recent retrospective study identified cardiac dysrhythmias in 51% of 166 dogs with GDV, and the majority of patients developed these during anesthesia.2
Mean intra-abdominal pressures (IAP) of 24.7 cm H2O, range 16 to 37 cm H2O, were measured in 8 dogs with GDV before gastric decompression using an indwelling transurethral catheter.3 Although arterial blood pressure may be unaffected by this value, cardiac output is significantly decreased at IAP of 13 cm H2O (10 mm Hg). Pressures as high as measured in GDV also significantly decrease blood flow to the intestinal tract, leading to intestinal ischemia and translocation of bacteria, decreased hepatic blood flow and decreased renal blood flows, leading to anuria. The mean IAP was 10.5 cm H2O after decompression and this is still higher than IAP of 4.5 cm H2O in normal dogs. This elevation could result from abdominal muscle splinting from pain as well as residual lavage fluid and intestinal gas.
Hypovolemia and dehydration are frequently present in dogs with GDV on admission to the hospital. Hypotension occurring at any time during hospitalization has been correlated with increased risk for death in dogs with GDV, as are the presence of sepsis, DIC, and peritonitis. Re-expansion of plasma volume before anesthesia is advisable to decrease the severity of cardiovascular depression on induction of anesthesia. Treatment with hypertonic 7.5% saline solution (HSS) or HSS-dextran at 4-5 ml/kg in addition to lactated Ringer's solution (LRS) will restore arterial blood pressure and peripheral perfusion more rapidly than treatment with LRS alone. 7.5% HSS has been reported to have several other beneficial effects in other species, including modulation of systemic inflammation and increasing urine output and intestinal motility post-operatively. HSS can be given rapidly over 10-15 min if considered necessary. Hetastarch, 10-20 ml/kg given IV over 30 min can also be used to expand blood volume and improve cardiac output. Not all dogs with GDV are in shock and preoperative treatment should be modified accordingly.
Increased IAP is partly alleviated by bulging of the diaphragm into the thorax. Ventilation is compromised and hypoxemia is more likely to develop during administration of anesthetic agents. Gastric decompression before induction of anesthesia and pre-oxygenation (administration of oxygen for a few minutes before anesthesia and throughout induction of anesthesia) serve to limit the impact of increased abdominal pressure and anesthetic-induced respiratory depression on PaCO2 and PaO2.