Atrioventricular nodal block

Severely depressed conduction may result in simple block, eg, atrioventricular nodal block or bundle branch block. Because parasympathetic control of atrioventricular conduction is significant, partial atrioventricular block is sometimes relieved by atropine. Another common abnormality of conduction is reentry (also known as "circus movement"), in which one impulse reenters and excites... 

Cardiovascular System. Atropine is sometimes used to block the effects of the vagus nerve (cranial nerve X) on the myocardium. Release of acetylcholine from vagal efferent fibers slows heart rate and the conduction of the cardiac action potential throughout the myocardium. Atropine reverses the effects of excessive vagal discharge and is used to treat the symptomatic bradycardia that may accompany myocardial infarction.4 Atropine may also be useful in treating other cardiac arrhythmias such as atrioventricular nodal block and ventricular asystole. 

Atrioventricular block was common (42%) first-degree, 14% second-degree, 17% and complete, 11%). However, first-degree heart block (that is prolongation of the PR interval) without higher degrees of atrioventricular nodal block can occur in the absence of digitalis intoxication. 

Digitalis can cause supraventricular extra beats or tachycardia. The combination of such dysrhythmias with atrioventricular block is particularly suggestive of digitalis toxicity and carries a high mortality rate (3,36). Rarely atrial fibrillation (37) and atrial flutter (38) may be attributed to digitalis toxicity. The frequency of atrioventricular nodal block is mentioned above. 

In a 37-year-old woman with atrial flutter with 1 1 conduction and partial right bundle branch block, intravenous dofetihde 5 micrograms/kg given over 5 minutes not only suppressed the atrioventricular nodal block to 2 1 or 3 1 but also caused complete right bundle branch block and QT interval prolongation (58). 

Variables for sinoatrial and atrioventricular nodal tissue only. 0 lso has sodium, calcium, and /3-blocking actions. 

A decision to refer for permanent pacemaker implantation for loss of atrioventricular synchrony due to first degree AV nodal block is challenging. In the setting of normal conduction in the ventricles, the improvement that may be gained by improving atrioventricular synchrony must be weighed against... 

Verapamil blocks both activated and inactivated L-type calcium channels. Thus, its effect is more marked in tissues that fire frequently, those that are less completely polarized at rest, and those in which activation depends exclusively on the calcium current, such as the sinoatrial and atrioventricular nodes. Atrioventricular nodal conduction time and effective refractory period are invariably prolonged by therapeutic concentrations. Verapamil usually slows the sinoatrial node by its direct action, but its hypotensive action may occasionally result in a small reflex increase of sinoatrial nodal rate. 

Verapamil can induce atrioventricular block when used in large doses or in patients with atrioventricular nodal disease. This block can be treated with atropine and 3-receptor stimulants. 

Propranolol and sotalol are non-selective (1-ad renoceptor antagonists that differ in the relative contributions of their (h-blocking (decreased atrioventricular nodal conduction, propranolol) and direct membrane (prolonged... 

C. Clinical Use and Toxicities Calcium channel blockers are effective for converting atrioventricular nodal reentry (also known as nodal tachycardia) to normal sinus rhythm. Their major use is in the prevention of these nodal arrhythmias in patients prone to recurrence. These drugs are orally active verapamil is also available for parenteral use (Table 14—2). The most important toxicity of verapamil is excessive pharmacologic effect, since cardiac contractility, AV conduction, and blood pressure can be significantly depressed. See Chapter 12 for additional discussion of toxicity. Amiodarone has moderate calcium channel-blocking activity. 

Verapamil s cardiotoxic effects are dose-related and usually avoidable. A common error has been to administer intravenous verapamil to a patient with ventricular tachycardia misdiagnosed as supraventricular tachycardia. In this setting, hypotension and ventricular fibrillation can occur. Verapamil s negative inotropic effects may limit its clinical usefulness in diseased hearts (see Chapter 12 Vasodilators the Treatment of Angina Pectoris). Verapamil can lead to atrioventricular block when used in large doses or in patients with atrio-ventricular nodal disease. This block can be treated with atropine and -receptor stimulants. In patients with sinus node disease, verapamil can precipitate sinus arrest. 

ADVERSE EEEECTS AND PRECAUTIONS The /3 adrenergic blocking agents should be avoided in patients with asthma, with sinoatrial or atrioventricular (AV) nodal dysfunction, or in combination with other drugs that inhibit AV conduction, such as verapamil. Patients with type 1 diabetes mellitus also are better treated with other drugs e.g., ACE inhibitors). 

A. The presence of nodal or ventricular rhythms in the setting of third-degree atrioventricular or intraventricular block. These are usually reflex escape rhythms that may provide lifesaving cardiac output, and abolishing them may result in asystole. 

In a review of 47 patients who had ingested mad honey 0.5-9 (mean 2.8) hours before presentation, the heart rate was 30-77 (mean 47) per minute and the systolic blood pressure was 50-140 (mean 47) mmHg [105"]. Cardiac rhythms on arrival were sinus bradycardia (n = 37), nodal rhythm (6), sinus rhythm (3), and complete atrioventricular block (1). AU were given atropine 0.5-2 mg. 

In a prospective study of 42 patients (33 men median age 49 years) who had been hospitalized with mad honey intoxication, all had nausea, vomiting, dizziness, fainting, and sweating five had syncope [106"]. The mean blood pressure was 73/52 mmHg and the mean heart rate 38/minute 18 had sinus bradycardia, 15 had complete atrioventricular block, and nine had nodal rhythm. None needed temporary pacing and all were discharged without complications. 

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