Page 399 - Feline Cardiology

Chapter 27: Anesthesia in the Patient with Cardiac Disease 419

TREATMENT striction (e.g., cats with diastolic dysfunction such as
HCM patients), dopamine is preferred because it
Treatment of Hypotension
increases blood pressure more consistently than dobu-
Universal techniques for treatment of hypotension in all tamine, allowing easier titration to effect. However, in
types of heart disease include reducing inhalant dose patients with congestive heart failure and severe systolic
(i.e., decreasing anesthetic depth with or without using dysfunction, increases in afterload are poorly tolerated,
anesthetic adjuncts) and correction of hypovolemia if and because dopamine administration carries the risk of
present, using fluids. Treatment of hypovolemia should arterial constriction via stimulating alpha-1 adrenergic
be done cautiously, since most patients with cardiac receptors, dobutamine is preferred. Both dopamine and
disease tolerate fluid loading poorly. With the exception dobutamine increase heart rate, but this effect usually is
of massive fluid loss (e.g., major hemorrhage), small seen at doses higher than those needed to increase
fluid boluses (e.g., 5 ml/kg) should be given in incre- cardiac output and/or blood pressure.
ments until hypovolemia is corrected, or signs of fluid
overload are observed. In anesthetized patients, fluid Treatment of Arrhythmias
overload is usually detected as a decrease in oxygenation Treatment of bradycardia (see Chapter 18) and first- and
(based on blood gas analysis or pulse oximetry), due to second-degree atrioventricular blocks in anesthetized
development of pulmonary edema. Arterial blood gas cats usually relies on the use of anticholinergics (e.g.,
analysis will provide an earlier warning than pulse oxim- atropine, glycopyrrolate). If anticholinergics are ineffec-
etry but requires catheterization or direct puncture of tive, or in cases of third-degree atrioventricular block, a
an artery. Fluid overload should be treated immediately beta-1 adrenergic agonist such as dopamine can be used.
with diuretics and/or vasodilators. The dose required to produce a positive chronotropic
Appropriate treatment of hypotension when it is not effect (increased heart rate) is usually higher than that
related to hypovolemia and after the inhalant dose has producing a positive inotropic effect (increased ventricu-
been reduced as much as possible is determined by the lar contractility). The author titrates dopamine to effect,
type of cardiac disease. In cases of concentric hypertro- starting with an infusion of 10 µg/kg/min. Alternatively,
phy, the administration of vasoconstrictors (phenyleph- isoproterenol has been used. However, isoproterenol is
rine, norepinephrine) is indicated, and the administration more arrhythmogenic than dopamine, and while it
of positive inotropes (dobutamine, dopamine) is contra- increases heart rate more consistently than dopamine, it
indicated; the opposite is true for eccentric hypertrophy is a nonselective beta-agonist. The effect on beta-2 adren-
or myocardial failure. In cases of decreased preload, if ergic receptors produces vasodilation and potentially
optimizing preload is difficult or impossible (e.g., mitral severe hypotension. It is therefore usually avoided in
or tricuspid valve stenosis), vasoconstrictors are likely to anesthetized cats. In some cats with third-degree AV
be more effective than positive inotropes. If a vasocon- block, neither drug effectively increases heart rate. The
strictor is used, phenylephrine may be preferable to nor- most effective approach to bradycardia associated with
epinephrine, because of the former’s lack of effect on third-degree atrioventricular block is the placement of a
beta-1 adrenergic receptors, although a study in healthy temporary pacemaker, and patients should be referred
cats suggested an increase in stroke volume at high dose, prior to anesthesia if this procedure is unavailable.
which may reflect a positive inotropic effect (Pascoe et The need for treatment of supraventricular tachycar-
al. 2006). Once euvolemia is achieved, the author typi- dia in normovolemic anesthetized cats is dictated by the
cally titrates phenylephrine to obtain adequate blood arrhythmia’s hemodynamic effects and the underlying
pressure, starting with an infusion of 0.5 µg/kg/min. condition. If the underlying cause cannot be identified Anesthesia
Dopamine and dobutamine are commonly used for their or treated, beta-adrenergic antagonists are usually rec-
positive inotropic effect. Both can be titrated to produce ommended. Esmolol (0.1 mg/kg/min IV) is commonly
adequate blood pressure, starting with an infusion of used in anesthetized patients because of its short dura-
5 µg/kg/min. Higher doses (i.e., >10 µg/kg/min) of dopa- tion of action. Alternatively, drugs that increase vagal
mine may cause vasoconstriction and should be avoided tone can be used and will not affect myocardial contrac-
in cases of eccentric hypertrophy, particularly if systolic tility. The author uses edrophonium, an acetylcholines-
dysfunction is present. Dobutamine appears to cause terase inhibitor, at the dose of 0.5–1 mg/kg IV.
mild vasodilation, which may mask its positive inotropic Management of marked, persistent tachycardia (i.e.,
effect as indirectly assessed by blood pressure measure- heart rate >180 bpm) in patients with concentric hyper-
ment, despite an overall increased cardiac output (Pascoe trophy during anesthesia relies on the use of beta block-
et al. 2006). The positive inotropic effect of both drugs ers such as esmolol or propranolol. The former is usually
is similar. In animals that would tolerate mild vasocon- preferred because of its short duration of action. Calcium