Page 397 - Feline Cardiology

Chapter 27: Anesthesia in the Patient with Cardiac Disease 417

ondary to other cardiac diseases that cause significant may be beneficial by reducing afterload, the decrease in
ventricular volume overload (e.g., patent ductus arterio- myocardial contractility is undesirable, particularly in
sus, congenital or degenerative mitral or tricuspid valve cases with systolic dysfunction. Therefore, the lowest
disease). Primary myocardial failure is rare in cats. possible inhalant anesthetic concentration should be
However, systolic myocardial failure secondary to other used, and maintenance of cardiac output may require
cardiac diseases (shunts, valvular insufficiencies, severe the use of positive inotropes such as dopamine or dobu-
end stage hypertrophic cardiomyopathy, etc.) is more tamine (see “Treatment of Hypotension,” below).
commonly seen. The main feature in ventricular volume Techniques allowing a reduction of inhalant dose may
overload is eccentric hypertrophy, which is defined as an be beneficial; for example, local and regional anesthetic
increased end-diastolic ventricular volume and normal techniques are recommended as adjuncts to general
wall thickness. Maintenance of normal cardiac output anesthesia whenever feasible (Duke 2000; Lemke and
requires normal to high-normal heart rate (i.e., 140– Dawson 2000; Tranquilli et al. 2007).
180 bpm), and low to normal systemic vascular resis- As mentioned above, the use of balanced anesthesia
tance. Although maintenance of adequate preload is with opioids is often useful, but while it produces anal-
necessary (implying adequate administration of intrave- gesia, it may have limited effects on anesthetic require-
nous fluids and correction of dehydration or hypovole- ments in cats. However, sympathetic stimulation seen
mia prior to anesthesia), avoidance of fluid overload is with high doses of opioids would not necessarily be
also important (see Chapter 24). detrimental in cats with eccentric hypertrophy (ven-
Cats with myocardial failure or volume overload tricular volume overload), contrary to cats with diastolic
usually should receive an opioid such as oxymorphone, dysfunction. Systemic lidocaine (Pypendop and Ilkiw
hydromorphone, methadone, or buprenorphine, and an 2005a), and alpha-2 agonists (Pypendop, unpublished
anticholinergic such as atropine (0.02 mg/kg SQ, IM) or data) should be avoided in patients with myocardial
glycopyrrolate (0.01 mg/kg SQ, IM), unless tachycardia failure, because both dose-dependently increase sys-
(HR > 200 bpm) is present, for anesthetic premedica- temic vascular resistance. Nitrous oxide and ketamine
tion. A target for physiologically appropriate heart rates produce mild sympathetic stimulation and may be ben-
ranges from 140–180 bpm during anesthesia, with avoid- eficial. However, no data on the cardiovascular effects of
ance of heart rates under 120 and over 200 bpm. Alpha- ketamine in inhalant-anesthetized cats are available, and
2 agonists should be avoided because of their nitrous oxide appears to increase systemic vascular resis-
vasoconstrictive effect. Benzodiazepines, low doses of tance in normal cats anesthetized with isoflurane com-
acepromazine, or dissociative anesthetics (ketamine, pared to isoflurane alone (Pypendop et al. 2003).
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Telazol ) could be added if additional sedation is neces-
sary, but in the author’s experience, it is rarely the case. Decreased Preload
While an argument could be made for the potential In these diseases (e.g., significant pericardial effusion
benefit of acepromazine-induced vasodilation, the effect and cardiac tamponade, severe dehydration, mitral or
is poorly controlled, and excessive vasodilation results in tricuspid stenosis), reduction of ventricular filling leads
hypotension, which may not be easily managed in these to decreased ventricular volume and cardiac output. The
patients. Benzodiazepines may produce dysphoria rather goal is therefore to maximize preload. Although rarely
than sedation in some feline patients (Ikiw et al. 1996). clinically seen in cats, severe pericardial effusion should
Induction of anesthesia is preferably achieved using be drained prior to induction of anesthesia or immedi-
etomidate and a benzodiazepine, particularly if systolic ately thereafter in patients intolerant of the procedure
dysfunction is present. Dissociative anesthetics such as under sedation. Optimization of preload relies on judi- Anesthesia
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ketamine or Telazol may be used, but the cardiovascular cious fluid administration, and low normal (i.e., 90–
stimulation associated with their administration requires 120 bpm) heart rate to maximize diastolic filling time.
a normally functioning sympathetic nervous system, Opioids are typically used for premedication, and anti-
which may not be present in some of these patients. If cholinergics are avoided, except if moderate to severe
sympathetic stimulation does not occur, dissociative bradycardia (HR < 80 bpm) is present. Etomidate and a
anesthetics decrease myocardial contractility, which is benzodiazepine, or propofol are used for induction of
detrimental in these patients. In the absence of systolic anesthesia. Dissociative anesthetics and thiopental
dysfunction, propofol may be used with careful dose should be avoided because they increase heart rate.
titration to the lowest effective dose allowing intubation. Anesthesia is maintained with isoflurane or sevoflurane
Anesthesia is maintained with isoflurane or sevoflurane. in oxygen. High doses of opioids, nitrous oxide, and
Both agents decrease myocardial contractility and cause ketamine should be avoided during maintenance of
vasodilation. Although mild to moderate vasodilation anesthesia because they increase heart rate.