Page 73 - Basic Monitoring in Canine and Feline Emergency Patients

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Fig. 3.17. Lead II ECG (25 mm/s, 10 mm/mV) showing sinus node dysfunction. The average heart rate is
approximately 70 bpm. There is an underlying sinus arrhythmia with rhythm pauses exceeding two typical R–R
intervals (periods of sinus arrest) terminated by ventricular escape complexes. The last two QRS complexes
in this strip are not preceded by P waves and occur at an instantaneous heart rate of ~215 bpm, consistent
with supraventricular premature complexes (SVPCs). This alternation of bradycardia/sinus arrest with SVPCs/
supraventricular tachycardia is suggestive of bradycardia–tachycardia syndrome.

Fig. 3.18. Lead II ECG (25 mm/s, 10 mm/mV) showing atrial standstill in a cat with hyperkalemia. The heart rate
is approximately 110 bpm. No P waves are visible. The QRS complexes are wide and bizarre, reflecting abnormal
ventricular conduction. T waves are high amplitude. This rhythm can be distinguished from VT based on the heart rate
(bradycardia) and complete absence of any atrial depolarization (no P waves).

depolarization; however, the atrial myocytes them- This leads to a suite of ECG findings including sinus
selves do not depolarize. For this reason, atrial stand- bradycardia, diminished P wave amplitude, pro-
still is also sometimes called a ‘sinoventricular longed P–R interval, widened QRS complex, and
rhythm.’ Atrial standstill is sometimes confused for a increased T wave amplitude (‘tall tented T waves’).
ventricular rhythm because QRS complexes are wide Eventually, hyperkalemia ‘paralyzes’ sodium channels
and bizarre in both cases; however, recall that most in atrial myocytes, leading to complete absence of
ventricular rhythms are tachycardic, while heart rate P waves. The second and less common cause of
in atrial standstill will be a relative bradycardia for atrial standstill is a primary atrial myopathy, believed
that patient. Additionally, dissociated P waves are to be an inherited cardiomyopathy in English Springer
frequently visible in VT (see Fig. 3.9), while P waves Spaniels and Labrador Retrievers.
are completely absent in atrial standstill. Atrial standstill is a medical emergency. If
There are two major causes of atrial standstill. caused by hyperkalemia, calcium gluconate is
The most common cause is severe hyperkalemia, administered initially to re-establish the relation-
for example, as seen in cats with urethral obstruction ship between resting membrane potential and
or dogs with hypoadrenocorticism. Hyperkalemia threshold potential in cardiomyocytes. This treat-
causes the resting membrane potential within cardiac ment is ‘cardioprotective’ and will improve the
myocytes to be less negative, which affects the ability ECG abnormalities, but does not directly treat the
of sodium channels to trigger an action potential. electrolyte imbalance itself. Directed treatments to

Electrocardiography 65