Page 396 - Feline Cardiology

416 Section O: Anesthesia in the Patient with Cardiac Disease

tachycardia. Glycopyrrolate is preferred to atropine alveolar concentration of sevoflurane is approximately
because it tends to increase the heart rate less and may 1% higher than that of isoflurane, so higher require-
cause fewer arrhythmias (Mirakhur et al. 1978). The use ments for sevoflurane should be expected.
of the alpha-2 adrenergic agonist medetomidine has Balanced anesthetic techniques, most commonly
been shown to reduce left ventricular outflow tract based on utilizing an opioid infusion (fentanyl) to
obstruction in cats with left ventricular hypertrophy reduce the amount of inhalant anesthetic and improve
(Lamont et al. 2002); however, the effect has not been hemodynamics, have often been advocated in these
studied in anesthetized cats, and it is unclear whether patients. However, opioids decrease inhalant anesthetic
the hemodynamic effects of alpha-2 agonists would requirements only moderately (Ferreira et al. 2009; Ilkiw
prove beneficial during anesthesia in cats with hypertro- et al. 1997, 2002) or not at all in cats (Brosnan et al.
phic cardiomyopathy. Decreased heart rate and increased 2009). Additionally, at the doses reported to achieve
systemic vascular resistance (as produced by alpha-2 reduced inhalant requirements, significant sympathetic
agonists) are usually beneficial, as is the sympatholytic stimulation has been reported (Pascoe et al. 1997), which
effect; however, alpha-2 agonist administration does not would be detrimental to cats with most types of heart
provide good control of the intensity of these effects. disease, notably hypertrophic cardiomyopathy. Low
Moreover, alpha-2 agonists may increase arterial blood doses of opioids could be used, but benefits other than
pressure beyond desirable values, and hypertension analgesia remain to be demonstrated. Epidural mor-
increases myocardial oxygen consumption, which is phine can be used for providing analgesia, but conflict-
undesirable with ventricular hypertrophy. In summary, ing results on its effect on anesthetic requirements have
available information is insufficient to support the use been published (Golder et al. 1998; Pypendop et al.
of alpha-2 agonists routinely for premedication in all 2006). Alternatives to opioids for balanced anesthesia
cats with HCM. If necessary, reduction in heart rate can that have been studied in cats include ketamine, nitrous
be achieved by titration of a short-acting beta blocker, oxide, and lidocaine. Ketamine and nitrous oxide
such as esmolol, and increase in systemic vascular resis- produce sympathetic stimulation and may therefore not
tance by titration of a short-acting alpha-1 agonist such be good choices for cats with hypertrophic cardiomy-
as phenylephrine. Acepromazine should be avoided opathy (Bovill 2006; Ebert and Kampine 1989); lido-
because of its vasodilatory effect, and dissociative anes- caine produces significant cardiovascular depression in
thetics (e.g., ketamine) are contraindicated because of normal cats (Pypendop and Ilkiw 2005a) and is not an
the sympathetic stimulation they commonly produce. appropriate adjunct for balanced anesthesia in the cat.
Induction of anesthesia is best performed by careful Alpha-2 adrenergic agonists (medetomidine, dexme-
titration of an injectable agent to effect, because induc- detomidine) at low doses may prove useful for balanced
tion with inhalation agents causes excitement and release anesthesia in cats with hypertrophic cardiomyopathy. As
of catecholamines. Etomidate (1–2 mg/kg IV) and a ben- in other species, in normal cats, dexmedetomidine dose-
zodiazepine (midazolam 0.25 mg/kg IV or diazepam dependently decreases heart rate and inhalant require-
0.5 mg/kg IV) are preferred, particularly for severe cases ments and increases systemic vascular resistance
or those with systolic dysfunction. However, this com- (Pypendop, unpublished data), which may be beneficial
bination is not widely used or familiar in many clinical in cats with hypertrophic cardiomyopathy. In normal
settings. For mild to moderate cases, propofol (4–8 mg/ cats anesthetized with isoflurane and without any addi-
tional drug, optimal dosing of dexmedetomidine appears
kg), either alone or, preferably, in combination with a
Anesthesia benzodiazepine, is an acceptable induction combina- to be a loading dose of 0.5–1 µg/kg and a constant infu-
sion rate of 0.5–1 µg/kg/h. Local or regional anesthesia
tion. It does cause systemic vasodilation and decreased
afterload. Thiopental and dissociative agents (ketamine,
decrease anesthetic requirements, improve analgesia,
tiletamine) should be avoided, the former because of the should be used whenever possible, since it is expected to
tachycardia and ventricular arrhythmias it may induce, and be safe if properly performed. For further informa-
and the latter because of the sympathetic stimulation tion on such techniques, the reader is referred to review
produced. Anesthesia is maintained with isoflurane or articles and veterinary anesthesia textbooks (Duke 2000;
sevoflurane in oxygen. Sevoflurane may be slightly pref- Lemke and Dawson 2000; Tranquilli et al. 2007).
erable, since data suggest that it reduces systemic vascu-
lar resistance to a lesser extent than isoflurane and that Systolic Myocardial Failure and
its vasodilatory effect may reach a ceiling at low to mod- Volume Overload
erate concentrations (Pypendop and Ilkiw 2004). Both Myocardial failure is caused by decreased myocardial
agents should be carefully titrated to effect, according to contractility, and may be a primary cardiomyopathy
close monitoring of anesthetic depth. The minimum (i.e., dilated cardiomyopathy) or more commonly sec-