Page 395 - Feline Cardiology

Chapter 27: Anesthesia in the Patient with Cardiac Disease 415

cardiac ultrasound are recommended. Cardiac ultra- in hypertensive patients. In addition, the primary cause
sound should be performed in all cats with abnormal should be treated when possible.
cardiac auscultation (murmur, gallop, or arrhythmia) Cats with hypertrophic cardiomyopathy have decreased
prior to general anesthesia, since most cats with ventricular compliance, and maintenance of cardiac
cardiac disease are asymptomatic (Paige et al. 2009; output requires a large ventricular volume and mainte-
Gundler et al. 2008; Rush et al. 2002), and thoracic nance of (slow) sinus rhythm, which enhances diastolic
radiographs may be unremarkable if the disease has filling. The atrial contribution to filling is important to
not resulted in moderate or severe cardiac enlargement cats with diastolic dysfunction. In cats with systolic ante-
and/or congestive heart failure. Normal auscultation rior motion of the mitral valve, the determinant of the
does not exclude cardiac disease in cats, and clinical physical resistance to left ventricular outflow is not sys-
experience suggests that anesthetic death in apparently temic vascular resistance but the severity of the left ven-
healthy cats is often associated with asymptomatic tricular outflow tract obstruction.
hypertrophic cardiomyopathy. Increased suspicion (e.g., In patients with moderate or severe left ventricular
breeds at risk; immediate relative has cardiomyopathy) outflow tract obstruction (diagnosed by echocardiogra-
should prompt a discussion with the owner of the phy), a decreased afterload, increased myocardial con-
potential for an increased risk despite normal ausculta- tractility, or decreased preload may worsen the severity
tion, which further justifies the consideration of tho- of the obstruction. For example, in a cat with severe left
racic radiographs and/or echocardiography before ventricular outflow tract obstruction, avoidance of posi-
anesthesia. Furthermore, in such patients, even if tive inotropes such as dobutamine, excessive vasodila-
testing is not undertaken, anesthetic and monitoring tion (e.g., excessively deep anesthetic plane), and
protocols used in cats with heart disease may be excessively low preload (e.g., dehydration) are impor-
preferred. tant to avoid increasing the obstruction. In contrast,
Anemia can be exacerbated by anesthesia (i.e., hemo- factors such as bradycardia, mild myocardial depression,
dilution or blood loss) and should be corrected if sig- increased systemic vascular resistance, and avoidance of
nificant (e.g., hematocrit < 20%). Because some cardiac volume underload to the ventricle (e.g., ensuring normal
patients are sensitive to fluid overload, correction of hydration) typically improve systolic function and
anemia is best carried out prior to anesthesia, using slow cardiac output in these cases. The degree of systolic ante-
administration of packed red blood cells and careful rior motion (SAM) of the mitral valve, and therefore
patient monitoring. Cats receiving loop diuretics may obstruction to outflow from the left ventricle, is often
have electrolyte imbalances that should be corrected thought to decrease with anesthesia based on the reduc-
prior to anesthesia. Medications such as inotropes, tion of intensity of a heart murmur when some cats are
diuretics, beta-adrenergic blockers and calcium-channel under anesthesia. However, cats with physiologic
blockers should be administered the day of anesthesia. murmurs (structurally normal hearts) also routinely
However, it may be preferable to withhold angiotensin experience a decrease in murmur intensity under anes-
converting enzyme inhibitors, since these drugs have thesia. A possible mechanism for both situations is a
been associated with severe, refractory hypotension in decrease in cardiac output associated with general anes-
anesthetized humans. A study in cats showed that enala- thesia, resulting in less turbulence at the level of the
pril significantly worsens isoflurane-induced hypoten- dynamic obstruction. Therefore, reduction in intensity
sion (Ishikawa et al. 2007). (or disappearance) of a murmur under anesthesia
cannot be used for concluding that there is or is not
ANESTHESIA structural heart disease in a given cat. Anesthesia
Cats with hypertrophic cardiomyopathy are com-
Diastolic Dysfunction monly premedicated with an opioid such as oxymor-
Hypertrophic cardiomyopathy is the most common phone, hydromorphone, methadone, or buprenorphine.
cardiac disease in cats (Paige et al. 2009; Kittleson and Butorphanol may be satisfactory for sedation prior to a
Kienle 1998b). Other causes of diastolic dysfunction, nonpainful procedure, but its analgesic effect is likely
including pressure overload due to systemic hyperten- weak, and µ-agonists (full or partial) are preferred for
sion or aortic or pulmonic stenosis, RCM, UCM, HCM, painful procedures. Midazolam (IM, SQ, IV) or diaze-
and concentric ventricular hypertrophy due to hyper- pam (IV) may be added to improve sedation but may
thyroidism, are managed similarly from an anesthetic result in dysphoria in some cats. Glycopyrrolate may
standpoint, except that drugs known to raise arterial be added if the heart rate is excessively low (i.e., less
blood pressure (e.g., alpha-2 agonists such as medeto- than 90 bpm), but it is not routinely used in premedica-
midine and dexmedetomidine) may be contraindicated tions because administering it pre-emptively may cause