![]() ![]() Two subclasses of second degree AV block are described, the first is Mobitz type I (more commonly called Wenckenbach arrhythmia). Second degree AV block is relatively common when anesthetic and preanesthetic drugs that enhance vagal tone are used (e.g., opioids, alpha 2 adrenergic agonists, low doses of anticholinergics). First degree block is rarely diagnosed (although likely occurs relatively frequently), and rarely is a problem because the manifestation is simply a longer delay between the normal coupling of atrial and ventricular activities. There are three main types - 1st, 2nd, and 3rd degree blocks. The most commonly identified AV-node abnormality during anesthesia is some form of AV-block. A concurrent complication is the loss of "atrial kick" reducing ventricular loading. However, if high ventricular rates cause low cardiac output blood pressure and tissue perfusion may suffer. Atrial fibrillation by itself is not necessarily going to cause severe adverse events during anesthesia. It most commonly occurs in larger breed dogs, or dogs with enlarged atria secondary to diseases such as mitral valve insufficiency. Atrial fibrillation is another disturbance that may be seen. ![]() These are usually caused by changes in the balance of sympathetic and parasympathetic nervous system efferent activity. Probably the most commonly encountered problems are sinus tachy- and brady- dysrhythmias. However there are some atrial abnormalities which can lead to adverse outcomes. If sick sinus syndrome has been diagnosed or suspected, special care and monitoring must be available and potentially pacemaker placement needed.Ītrial conduction abnormalities are not generally as likely to cause severe morbidity or mortality during anesthesia. 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. 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. This is associated with disease of the sinus node termed Sick Sinus Syndrome. However, changes in T-wave morphology with time can represent serious conditions including myocardial hypoxia and hyperkalemia, which require immediate attention.Ī 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 can take on many normal appearances. The final ECG step of the cardiac cycle is appearance of the T-wave which represents repolarization of the ventricular muscle. The different parts reflect depolarization of the different surfaces of the ventricle (e.g., intraventricular septum, free walls, etc.). Next the ventricular muscle depolarizes and creates the characteristic shape of the QRS complex. 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). This is followed by the spread of the wave across the atrial muscle (P-wave). Normally, the first step in heart depolarization is the depolarization of the SA node. Interpretation of this ECG includes determination of rate, presence of normal wave amplitudes, and correct intervals between the portions of the ECG. Normal depolarization and repolarization of the heart produces a characteristic ECG rhythm. 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. The first step to identification of rhythms is understanding the normal origins and pathways which participate in depolarization and repolarization. Rhythms seen on the ECG reflect the summation of electrical events within the heart during the cardiac cycle. 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). It provides a heart rate and a picture of the electrical activity of the heart muscle. The electrocardiogram is a useful monitoring tool, but its proper use requires training. ![]()
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