Page 209 - Feline Cardiology

214 Section F: Arrhythmias and Other Electrocardiographic Abnormalities

reverse the cause of the bradycardia (e.g., reduce the
concentration of inhalant anesthetic), and if not suf-
ficient, treatment to increase the heart rate directly
Cardiac Output (ml/min) (e.g., glycopyrrolate for sinus bradycardia under anes-
thesia, or pacemaker implantation for AV block and a
low ventricular escape rate) are essential in order to
promptly raise the heart rate and return the cardiac
output to a level compatible with life.
• A physiologic maximum exists for the sinus heart rate
(peak heart rate), beyond which the heart rate cannot
reach even under maximal sympathetic stimulation.
≈90 ≈280
This innate maximum heart rate limit (red arrow)
Heart rate (beats/min) varies from one individual to another but broadly
exists at a point at which cardiac output is maximized.
Figure 18.1. Effect of heart rate on cardiac output. As heart
rate increases, cardiac output increases, because the volume of Examples of reaching this peak heart rate are sinus
blood ejected by the heart to the circulation per minute depends tachycardia during extreme physical activity or in a
Arrhythmias times the number of beats per minute (cardiac output = stroke heart failure. In most cats this maximum is likely
cat with life-threatening pulmonary edema/congestive
exactly on the amount of blood in the ventricles for each beat
between 260 and 300 beats/minute.
volume × heart rate). This direct relationship is constructive up
to a point (red arrow), after which further increases in heart rate
lar filling to a disproportionately greater degree
lead to a decrease in cardiac output. Yellow box: range compat- • Too high a heart rate compromises diastolic ventricu-
ible with life (hemodynamic stability and adequate perfusion of than the benefit of the rise in heart rate, resulting
vital organs in the cat). in a fall in cardiac output. That is, the benefit of the
cardiac output relationship (cardiac output = heart
rate × stroke volume) is compromised by a lower
tachycardias that exceed this maximum rate may force stroke volume when ventricles are “told to contract
the heart to beat at a rate that leaves the ventricles insuf- faster than they can fill.” This range of heart rates
ficiently filled from one beat to the next. In this way, exceeds the maximum sinus rate (red arrow), which
ventricular tachycardia at 360 beats/min is expected to means it only occurs due to an arrhythmia (classically
produce a lower cardiac output than ventricular tachy- atrial tachycardia, atrial fibrillation, or ventricular
cardia at 260 beats/minute (Figure 18.1). The clinical tachycardia). Therein lies the risk associated with
implication is that a cat exhibiting clinical signs such as these arrhythmias, since they are not confined to the
suspected or confirmed syncope needs to have ECG con- normal range of heart rate and may produce very rapid
firmation of the heart rhythm during those clinical tachycardias that markedly reduce cardiac output (the
signs, in order to identify whether a bradycardia or a descending limb on the right side of the curve within
tachycardia is responsible, and choose a treatment that the yellow box). This region of the curve illustrates
corrects, rather than adds to, the problem. that tachycardias generally produce negative hemody-
A number of clinically important points can be namic effects that are more severe when the heart rate
understood from the relationship in figure 18.1: is higher. Therefore, arrhythmias warrant treatment
with an antiarrhythmic more promptly (or at all) if the
• An increase in heart rate triggered by physical exercise heart rate they produce is very high, whereas a less
or by sympathetic stimulation to maintain an indi- aggressive treatment approach is more often taken
vidual in an asymptomatic state despite cardiomyopa- when the arrhythmia produces a heart rate that is not
thy, for example, increases the cardiac output (desirable much different from the normal range of heart rate.
effect). This is represented by the ascending limb of The curve also shows that similar reductions in cardiac
the curve in the left part of the yellow box. output (bottom of the yellow box) may occur with
• Too low a heart rate, even when the myocardium is severe tachycardias, on the right, or severe bradycar-
healthy, is inadequate for hemodynamic stability. dias, on the left; thus, a patient with a presenting
Examples include marked sinus bradycardia during complaint of syncope, reflective of cerebral hypoper-
general anesthesia, or third-degree AV block and a fusion, requires an ECG diagnosis to identify the
slow ventricular escape mechanism. This is repre- arrhythmia because exactly the same clinical signs
sented by the tail of the curve in the bottom left, can arise from tachycardias requiring treatments that
outside the yellow box. In this instance, measures to reduce the heart rate (bottom right of yellow box) and