The electrocardiogram is a useful monitoring tool, but its proper use requires training. It provides a heart rate and a picture
of the electrical activity of the heart muscle. The anesthetist should be trained to recognize many commonly encountered
intraoperative arrhythmias (e.g., multifocal and unifocal ventricular premature complexes, atrioventricular blockade, ventricular
tachycardia, etc.) and the veterinarian should be prepared to treat arrhythmias when they occur (if necessary).
Rhythms seen on the ECG reflect the summation of electrical events within the heart during the cardiac cycle. The first step
to identification of rhythms is understanding the normal origins and pathways which participate in depolarization and repolarization.
General locations of the origins of ectopic beats or blocks can then be identified and decisions made by the anesthetist about
the probable impact these dysrhythmias will have on cardiovascular function.
Normal sinus rhythm
Normal depolarization and repolarization of the heart produces a characteristic ECG rhythm. Interpretation of this ECG includes
determination of rate, presence of normal wave amplitudes, and correct intervals between the portions of the ECG. Normally,
the first step in heart depolarization is the depolarization of the SA node. This is followed by the spread of the wave across
the atrial muscle (P-wave). The wave of depolarization enters the AV node and starts the relatively long process of AV node
depolarization, bundle of His depolarization, and right and left bundle branch depolarization (appears as the delay between
P-wave and beginning of QRS complex). Next the ventricular muscle depolarizes and creates the characteristic shape of the
QRS complex. The different parts reflect depolarization of the different surfaces of the ventricle (e.g., intraventricular
septum, free walls, etc.). The final ECG step of the cardiac cycle is appearance of the T-wave which represents repolarization
of the ventricular muscle. This can take on many normal appearances. However, changes in T-wave morphology with time can
represent serious conditions including myocardial hypoxia and hyperkalemia, which require immediate attention.
A potentially life-threatening dysrhythmia that is occasionally seen during anesthesia, especially in breeds such as miniature
schnauzer or West Highland white terriers is sinus bradycardia or arrest. This is associated with disease of the sinus node
termed Sick Sinus Syndrome. Some animals will have bradycardia or sinus pauses noticed during the preanesthetic physical,
however some animals will be asymptomatic until after administration of anesthetic drugs. The danger of this disease is sinus
arrest and subsequent ventricular asystole the occurs during anesthesia, exacerbated by anesthetic drug associated electrical
depression and opioid-associated increased vagal tone. If sick sinus syndrome has been diagnosed or suspected, special care
and monitoring must be available and potentially pacemaker placement needed.
Atrial conduction abnormalities are not generally as likely to cause severe morbidity or mortality during anesthesia. However
there are some atrial abnormalities which can lead to adverse outcomes. Probably the most commonly encountered problems are
sinus tachy- and brady- dysrhythmias. These are usually caused by changes in the balance of sympathetic and parasympathetic
nervous system efferent activity. Atrial fibrillation is another disturbance that may be seen. It most commonly occurs in
larger breed dogs, or dogs with enlarged atria secondary to diseases such as mitral valve insufficiency. Atrial fibrillation
by itself is not necessarily going to cause severe adverse events during anesthesia. However, if high ventricular rates cause
low cardiac output; blood pressure and tissue perfusion may suffer. A concurrent complication is the loss of "atrial kick"
reducing ventricular loading.