Special considerations in anesthesia (Proceedings)

Anesthesia is concerning for every patient, this is especially so when the patient has pre-existing conditions. The value of a thorough physical examination and detailed history cannot be underestimated as part of a balanced anesthetic plan because the goal of the plan is to maximize its' effects while minimizing patient complications.

The Anesthesiologist Society of America (ASA) created a physical status scale that is often used in veterinary medicine to rate patients for potential anesthetic risk. The scale is as follows:

      ASA status I is a healthy patient

      ASA status II is a patient with mild systemic disease with no functional limitations

      ASA status III is a patient with severe systemic disease with functional limitations

      ASA status IV is a patient with severe systemic disease that is a constant threat to life

      ASA status V is a moribund patient that is not expected to survive 24 hours with or without surgery

      E denotes an emergency

This section will cover some conditions in which special consideration needs to be given regarding anesthesia.

Pediatric

Puppies and kittens are generally considered neonates for the first 6 weeks of life and pediatrics for the first 12 weeks of life. Compared to adults, neonatal and pediatric patients have immature cardiovascular systems and ventilation muscles, decreased alveolar surface in the lungs, underdeveloped organs, and less body fat. These cause a decreased ability to respond to physiological challenges, exaggerated or prolonged effects of drugs, and a tendency to develop hypothermia, hypoglycemia, and fluid overload.

Successful anesthetic management depends on several actions:

     • Careful dosing and administration of drugs.

     • Consider using drugs that cause less respiratory depression such as partial opioid agonists (i.e. Buprenorphine instead of Hydromorphone)

     • Close monitoring of the cardiovascular status, respiratory rate, and body temperature.

     • Ventilate more often for their oxygen consumption is approximately twice that of the adult patient. Also, due to their immature intercostal muscles and diaphragm, pediatrics and neonates must work harder to breathe. This causes a greater potential for respiratory fatigue.

     • Include anticholinergics in the anesthetic plan because these patients have less functional contractile tissue and rely on the heart rate for cardiac output.

     • It may be prudent to administer a Dextrose CRI depending on the duration of the anesthetic procedure. Do not withhold food if still suckling and fast for no more than four hours if eating solid food. Periodic checks of the patient's blood glucose may be indicated during a lengthy procedure, and post-operatively.

     • Hypotension and proper pain management is still a concern as with all anesthetic patients.

     • Administer intravenous fluids to maintain cardiovascular support, and use a fluid pump to prevent fluid overload.

     • Closely monitor in the post-operative period since they cannot thermoregulate and can become hyperthermic/ hypothermic quickly.

Geriatric

Older patients are less resilient because organ function diminishes with age. It may be difficult for these patients to compensate for physiologic disturbances that can occur from general anesthesia and surgery. They also may have impaired vision and hearing causing them to startle easily. Calming them with compassion along with appropriate sedation may be necessary since stress can increase complications and inhibit recovery.

Several actions are indicated to care for these patients properly:

     • Thorough preanesthetic bloodwork and physical exam

     • Decreased drug doses to minimize cardiovascular effects and organ elimination

     • Monitor closely for hypothermia as most geriatric patients have less body fat, and cannot adequately compensate for heat loss. Also hypothermia's negative effects such as bradycardia, hypotension, fluid overload, and arrhythmias can take a greater toll on these patients.

     • Supportive care in the post operative period includes proper fluid administration, lots of bedding, and comforting nursing care.

Heart disease

In general patients with heart disease are prone to arrhythmias, susceptible to fluid overload, and unable to compensate for dramatic heart rate increases, decreases, or the depressant effects of anesthetic agents.

Numerous complications can arise from anesthetizing a patient with heart disease that can result in a domino effect due to their inability to compensate. For example, cardiac output is a function of stroke volume and heart rate. Contractility affects the stroke volume, and anesthetic agents can depress contractility. Decreased contractility can cause decreased cardiac output, resulting in hypotension that can be difficult to treat because these patients often cannot tolerate fluids, and develop pulmonary edema.

The International Small Animal Cardiac Health Council guidelines divide cardiac patients into three classes:

Class I - Patients with confirmed cardiac disease but no signs of heart failure. These patients would be considered ASA category II if clinically normal without cardiac medications and ASA category III if cardiac medications are required to achieve clinical normalcy. These patients should tolerate a properly managed anesthetic/sedative event without difficulty.

Class II - Patients with mild to moderate signs of heart failure at rest or with mild exertion. These patients would be considered ASA category IV. These patients should have additional diagnostics and therapeutic adjustments to stabilize prior to anesthesia or sedation.

Class III - Patients in fulminate heart failure requiring aggressive measures to stabilize prior to any procedures. These patients would be considered ASA category IV to V.

There are many ways to approach every cardiac affliction in small animals. Though they are not listed here, there are some common actions that benefit cardiac patients when undergoing anesthesia:

     • Always perform a thorough physical exam and detailed history for knowledge of the heart disease and its severity is needed.

     • Preoxygenate (administer flow-by oxygen for 5 minutes) before induction

     • Minimize stress

     • Minimize fluid administration

     • Avoid or minimize doses of the following:

     • Acepromazine causes vasodilation and has no antagonist.

     • Alpha-2 agonists such as (Dex)medetomidine cause bradycardia, and can increase the potential for arrhythmias.

     • Anticholinergics can cause tachycardia that overworks the heart, and increase oxygen consumption.

     • Pure agonist opioids can cause bradycardia and cardiovascular depression.

     • Ketamine and Tiletamine increase sympathetic tone. Adequate preanesthetic medication and administering two parts Diazepam to each part Ketamine can minimize the amount of Ketamine used.

     • Thiopental has an arrhythmic potential

     • Propofol is a potent cardiac and respiratory depressant. Adequate preanesthetic medication and administering 0.2-0.4 mg/kg Diazepam just before induction can minimize the amount of Propofol used.

     • Mask induction is stressful and should be avoided.

     • Etomidate has the least impact of the induction agents on contractility and heart rate, but can cause muscle tremors and retching. This is minimized by effective preanesthetic medications and by delivering 0.2 to 0.4 mg/kg Diazepam IV just before induction.

     • Vasopressors or sympathomimetics may be needed to manage hypotension.

     • Monitor the temperature closely and provide supplemental heat because hypothermia increases cardiac stress and may cause bradycardia. Also, shivering can greatly increase oxygen consumption.

Diabetes

The goal in anesthetizing diabetic patients is to get them back on their routine at home as soon as possible. Stress and an erratic feeding schedule can cause the patient to become unregulated, and, if prolonged, can cause diabetic ketoacidosis (DKA). The patient should receive their normal feedings and insulin up until the morning of anesthesia at which time they should receive a half dose of insulin and food is withheld. Schedule the surgery as early as possible so the patient can return to normal feedings sooner.

Short-acting induction agents such as Propofol, Thiopental, Ketamine/Diazepam, or Etomidate are safe to use with diabetic patients. Alpha-2 agonists such as (Dex)medetomidine should be avoided because they can cause hypoinsulinemia and hyperglycemia.

The patient's blood glucose should be monitored prior to induction of anesthesia, hourly while under anesthesia and through recovery. The BG should remain between 150-250 mg/dl. If hypoglycemia occurs then a 2.5-5% dextrose solution should be administered at a rate of 10-15ml/kg/hr, and continue to monitor.

Ketoacidotic patients should be stabilized with insulin therapy and rehydration prior to anesthesia. This minimizes the potential for post-operative complications that can severely impact recovery for these patients that were compromised prior to anesthesia.

Other considerations

There are some other conditions that precautions should be taken in order to minimize the potential for post-operative complications. This is meant to be a quick reference for what may need to be considered in some patients. It is by no means a complete list:

     • Obesity: Drug doses should be based on lean body weight since some anesthetic agents can be stored in fat reserves, thus prolonging recovery. These patients should be ventilated more often due to increased pressure on the diaphragm causing hypoventilation.

     • Pregnant/Nursing: Be aware of drugs that cross the placenta. If feasible, premedicate the patient with a low dose of a partial opioid agonist such as Buprenorphine to supply some analgesia to the dam and minimize respiratory depression to the litter. Induce with Propofol. After the pups/kits are retrieved, administer a pure opioid agonist such as Hydromorphone to provide more analgesia to the dam. Avoid Ketamine and barbituates. The engorged uterus can place additional pressure on the diaphragm causing hypoventilation; therefore these patients should be ventilated often. The blood pressure should be closely monitored after the uterus is externalized as the removal of pressure from the aorta can cause a rapid drop in blood pressure as the body's hemodynamics adjust. Epidural analgesia, if possible, is preferred as this greatly minimizes exposure to anesthesia for both the dam and her litter.

     • Brachycephalic: Anticholinergics should be administered due to a high vagal tone. Preoxygenate before induction and monitor closely during recovery and after extubation. These patients can have an elongated soft palate that can swell and cause an airway obstruction after being intubated.

     • Respiratory disease: Do not stress, preoxygenate, ventilate often, and monitor closely during recovery.

     • Hepatic disease: Run a complete biochemistry before anesthesia, reduce the amount of drugs used, and monitor closely for clotting abnormalities.

     • Renal disease: Check for electrolyte imbalances such as hyperkalemia, hypokalemia, hyperphosphatemia, and metabolic acidosis before anesthesia. The blood pressure should be checked prior to anesthesia and monitored throughout the procedure. Overall, drug doses should be reduced and caution used when administering barbiturates. These patients should be hydrated prior to anesthesia.

     • Trauma/shock: Perform a thorough physical examination and workup with bloodwork and thoracic and abdominal radiographs if feasible. Support, stabilize, and manage pain as much as possible before anesthesia. Watch closely for arrhythmias. These patients will need intensive supportive care and monitoring post-operatively.

References

Grubb, Tamara. Anesthesia and Analgesia of the Young. Proceedings International Veterinary Emergency and Critical Care Symposium 2008.

Kittleson, Mark, Rishniw, Mark, Stein, Robert. Anesthetic Considerations for the Cardiac Patient. Veterinary Information Network. 1/11/2006.

McKelvey, Diane, Hollingshead, Wayne. Veterinary Anesthesia and Analgesia, Third Edition. Mosby. 2003.

Sereno, Robin. Anesthetic Complications in Internal Medicine Patients. Proceedings American College Veterinary Internal Medicine Forum 2006.

Sisak, Donna. Anesthesia Considerations: Pediatric & Geriatric Patients. Proceedings International Veterinary Emergency and Critical Care Symposium 2007.