Anesthesia for dogs and cats with cardiac or renal disease (Proceedings)

General anesthesia for patients with cardiac disease

Cardiac diseases occur frequently in small animal patients. It is often necessary to anesthetize these animals for routine procedures (dental prophylaxis, OVH, neuter), emergency procedures (GDV, fracture repair) or for the cardiac condition itself (PDA correction, balloon valvuloplasty, pacemaker implantation). Additionally, degenerative conditions such as endocardiosis resulting in reverse blood flow across heart valves are commonly found and require consideration in animals undergoing general anesthesia. These conditions increase the risk of anesthesia due to the potential for cardiopulmonary decompensation and subsequent death. Changes in myocardial function, secondary effects of cardiac disease on other organs, interaction with currently administered drugs, and altered drug pharmacokinetics and phramacodynamics should be taken into account when organizing and anesthetic plan in dogs and cats with cardiac disease. Increasing the chances for a favorable outcome can be accomplished by:

• considering cardiac conditions during preoperative examination and management

• treating the primary condition

• treating secondary complications

• understanding the pathophysiology of the conditions associated with the cardiac abnormalities and selecting appropriate analgesic drugs, anesthetics, and therapies that reduce risks

• vigilant appropriate monitoring throughout the peri- and intra-operative periods

• formulate an anesthetic plan that includes actions for emergency situations

Using techniques that ensure adequate delivery of oxygenated blood (DO2) that meet or exceed the requirements of the body (VO2) are paramount in anesthetizing these types of patients. Considering and correcting factors that affect myocardial oxygen balance (m(D-V)O2) can be performed using a variety of techniques throughout the anesthesia period.

Factors affecting mDO2 include:

• blood oxygen tension (PaO2)

• hemoglobin concentration

• coronary blood flow

• arterial blood pressure

• heart rate

Factors affecting mVO2 include:

• heart rate

• systolic wall tension and after load

• contractility

• basal metabolic rate

Various cardiopulmonary pathologies and anesthetic agents affect these parameters in a variety of ways and will be discussed.

General anesthesia for patients with renal disease

Practitioners are often presented patient with some form of renal disease that need anesthesia for diagnostic or therapeutic purposes. Geriatric dogs and cats with renal insufficiency make up a significant portion of a practice case load. When anesthetizing patients with renal compromise, three main factors need to be considered:

• the effect of drugs on renal function

• the effect of renal disease on drug metabolism

• fluid and electrolyte balance

In unanesthetized patients, renal blood flow is maintained through an autoregulation mechanism that is independent of systemic blood pressure when mean pressure is between 80 to 180 mmHg. In hypovolemic or low pressure states, renal blood flow may decrease and therefore reduce glomerular filtration rate (GFR). Patients with compensated preexisting renal disease that become hypotensive from anesthesia may decompensate postoperatively and have further renal complications. Therefore, the goals of anesthesia of the patient with renal disease are:

• maintain normal fluid balance

• maintain adequate depth of anesthesia

• avoid drugs that affect renal function

• avoid hypoxia and hypercapnia

Patients with renal and/or urinary tract disease may have severe metabolic, acid-base, and electrolyte abnormalities. Uremic animals may have an impaired level of consciousness and require a smaller dose of anesthetic. Additionally, azotemia causes bradyarrhythmias and increases myocardial sensitivity to anesthetics. Elevated potassium levels can also accompany renal disease. Hyperkalemia can cause arrhythmias and worsen acidosis. Patients with a potassium greater than 5.5 mmol/l should not be anesthetized unless in an emergency situation. Potassium levels greater than 6.0 mmol/l are likely to cause dysrhythmias. Monitoring and maintaining appropriate electrolyte balance should be initiated prior to anesthesia and should continue throughout the operative and post-operative periods.

Anesthetic management may be different for patients with acute renal disease compared with those with chronic renal insufficiency. Acute renal disease may be due to hypovolemia, drug or toxin induced nephropathy, or idiopathic. These patients may present with anuria, oliguria, or polyuria and fluid balance and electrolyte abnormalities must be taken into account when formulating an anesthetic plan. Patients with chronic renal disease usually have a reduction in glomerular filtration rate. Hypertension may also be present and needs to be managed appropriately.

Using anesthetic techniques that minimize further renal impairment, maintain fluid and electrolyte balance, and maintain normotension is essential for a successful outcome.