Page 214 - Feline Cardiology

Chapter 18: Arrhythmias and Other Electrocardiographic Abnormalities 219

QRS

P T

Figure 18.4. Sinus bradycardia. This cat sustained a tarsal fracture and was receiving opiate analgesia (hydromorphone), which in-
creases vagal tone. Heart rate = 100 beats/minute. 25 mm/sec, 1 cm = 1 mV. Tracing courtesy of Dr. Cate Creighton, Atlantic Veterinary
College, UPEI, Canada.

causing cardiac sympathetic denervation or spinal cord dia and second- or third-degree AV block, and the ECG
trauma, and central nervous system disturbances causing is diagnostic. Further diagnostic testing in sinus brady-
massive central vagal discharge (Ornato and Peberdy cardia is indicated only for identifying its underlying Arrhythmias
1996). Massive release of cardiac nucleosides has been cause if it is not apparent and accounted for by the
documented in experimental myocardial hypoxia in cats history and physical examination.
(Berne 1963), and such nucleosides, notably adenosine, Treatment of sinus bradycardia is almost never neces-
can decrease myocardial contractility, pacemaker auto- sary, inasmuch as treatment should concentrate on cor-
maticity, and AV nodal conduction (Ornato and Peberdy rection of the underlying cause. Therapy that alters the
1996). The lack of responsiveness to atropine by heart rate (e.g., parasympatholytic drugs such as atro-
adenosine-mediated bradycardias, but successful in pine) may be indicated if sinus bradycardia is associated
vitro response with methylxanthine drugs (Ornato and with hypotension and the underlying cause is already
Peberdy 1996), helps explain the common prescription being addressed; the most common clinical example is
of drugs such as theophylline in an attempt to treat sinus probably an excessively deep plane of general anesthesia,
bradycardia when it is part of the sick sinus syndrome where reduction of inhalant anesthetic is followed by a
(sinus node dysfunction). Such treatment has not been lag period, and sinus bradycardia causing hypotension
evaluated objectively in cats with hypotensive shock- is treated with glycopyrrolate or atropine to temporarily
associated bradycardia (nor sick sinus syndrome), however. raise the heart rate while the level of anesthesia
As noted previously, a relative sinus bradycardia may normalizes.
be said to exist in a cat that has normal sinus rhythm
(e.g., at a heart rate of 130/minute) in a context that Respiratory Sinus Arrhythmia
would be expected to produce sinus tachycardia (e.g., A cyclical variation in heart rate characterized by faster
noisy treatment area). Therefore, the textbook definition heart rate during inspiration and slower heart rate
of sinus bradycardia involves a rate of <110 bpm, but the during expiration, but otherwise normal ECG charac-
practical clinical concept of sinus bradycardia—and the teristics of normal sinus rhythm, is termed respiratory
desire to investigate a surprisingly low heart rate with sinus arrhythmia (RSA; Figure 18.5). Respiratory sinus
ECG—is more fluid and must factor in the influences of arrhythmia is primarily vagally mediated, with negative
environment and medical status. intrathoracic pressure of inspiration causing less pres-
The history and physical exam of cats with sinus bra- sure on the paired vagus nerves and therefore lower
dycardia essentially always reflect the underlying cause vagal tone (heart rate increases), and the reverse sequence
rather than the arrhythmia itself. A rare exception is the causing a decrease in heart rate during expiration.
cat with sinus node dysfunction (sick sinus syndrome), Additional factors involved in generating RSA include
which is a very uncommon disorder in this species (see the Bainbridge reflex (atrial stretch stimulates receptors
below). in the atrial wall, increasing heart rate via vagal inhibi-
Physical examination of cats with sinus bradycardia tion; countermechanism to baroreceptor reflex), the
reveals a slow, regular rhythm with a synchronous pulse, Bezold-Jarisch reflex (venous pooling causing arterial
the strength of which varies depending on the underly- hypotension, which triggers an increase in heart rate via
ing cause of the bradycardia. In a cat with a low heart baroreceptor activation), respiratory muscle stretch
rate, the main differential diagnoses are sinus bradycar- reflexes, and interaction of respiratory and cardiac