Bradycardia/atrioventricular block caused

The most important toxic effects reported for calcium channel blockers are direct extensions of their therapeutic action. Excessive inhibition of calcium influx can cause serious cardiac depression, including cardiac arrest, bradycardia, atrioventricular block, and heart failure. These effects have been rare in clinical use. 

Ranitidine is generally well tolerated but may occasionally cause diarrhoea and other gastrointestinal disturbances, altered liver function tests, headache, dizziness, rash and tiredness. Other rare side-effects include acute pancreatitis, bradycardia, atrioventricular block, confusion, depression and hallucinations, particularly in the very ill or elderly. 

Amiodarone is contraindicated in patients with sick sinus syndrome and may cause severe bradycardia and second-and third-degree atrioventricular block. Amiodarone crosses the placenta and will affect the fetus, as evidenced by bradycardia and thyroid abnormalities. The drug is secreted in breast milk. 

In terms of its potential for inducing cardiac dysrhythmias, cannabis is most likely to cause palpitation due to a dose-related sinus tachycardia. Other reported dysrhythmias include sinus bradycardia, second-degree atrioventricular block, and atrial fibrillation. Also reported are ventricular extra beats and other reversible electrocardiographic changes. 

Lithium may cause cardiac effects including T-wave flattening or inversion (up to 30% of patients), atrioventricular block, and bradycardia. If a patient has preexisting cardiac disease, a cardiologist should be consulted and an electrocardiogram obtained at baseline and regularly during therapy. 

The most common cardiac effects are atrioventricular block, sinus bradycardia, and ventricular extra beats. Occasionally serious dysrhythmias occur (SEDA-17, 219), including ventricular fibrillation (15). ATP can cause transient atrial fibrillation (16). Chest pain occurs in 30-50% of patients and dyspnea and chest discomfort in 35-55%. Chest pain can occur in patients with and without coronary artery disease, and the symptoms are not always tjrpical of cardiac pain. 

Clonidine causes sinus bradycardia and atrioventricular block, as illustrated by two cases, one a 10-year-old boy (6) and the other a 71-year-old woman (7), who developed Wenckebach s phenomenon. Clonidine was also studied in seven patients subjected to electrophysiological studies after 5 weeks of therapy (8). It slowed the sinus rate and increased the atrial pacing rate, producing Wenckebach s phenomenon, indicating depressed function of the sinus and AV nodes. 

A careful haseline physical examination, ECG, and laboratory work-up are essential. Underlying ECG changes (U waves, prolonged QT interval, or flattened T waves) secondary to hypokalemia or bradycardia and atrioventricular block from starvation may be present. AU antidepressants can cause seizures thus a careful risk-benefit assessment is warranted if the patient has predisposing factors such as a personal or family history of seizures, cerebrovascular disease, or alcohol or sedative-hypnotic withdrawal. 

The main effect of atropine on the heart is the alteration of the rate. At low doses, the rate is slowed (bradycardia) without a change in blood pressure or cardiac output. Higher doses cause an increase in pulse rate (tachycardia). Atropine may be used in the initial treatment of a myocardial infarction or high-grade atrioventricular block. 

TABLE M. SELECTED DRUGS AND TDXINS CAUSING BRADYCARDIA OR ATRIOVENTRICULAR BLOCK ... 

A. Calcium is a cation necessary for the normal functioning of a variety of enzymes and organ systems, including muscle and nerve tissue. Hypocalcemia, or a blockade of calcium s effects, may cause muscle cramps, tetany, and ventricular fibrillation. Antagonism of calcium-dependent channels results in hypotension, bradycardia, and atrioventricular block. 

Patients with impaired function of the sinus node or impaired atrioventricular conduction can develop sinus bradycardia, sinus arrest, heart block, hypotension and shock, and even asystole, with verapamil (139) or diltiazem. These drugs should not be given to patients with aberrant conduction pathways associated with broad-complex tachydysrhythmias, and they can cause severe conduction disturbances in hypertrophic cardiomyopathy. 

The main toxins of the yew species are the alkaloids taxine A and taxine B, which are present in all parts of the shrub except the fleshy part of the berry. These compounds are capable of causing symptoms similar to digitalis poisoning including hypotension, bradycardia, and depressed myocardial contractility and conduction delay. The mechanism appears to involve a block of the distal part of the conduction tissue of the heart, which can result in fatal arrhythmias. Atrioventricular conduction is particularly susceptible to yew alkaloids. 

A. Ataxia, nystagmus, ophthalmoplegia, movement disorders (dyskinesia, dystonia), mydriasis, and sinus tachycardia are common with mild to moderate overdose. With more serious intoxication, myoclonus, seizures (including status epilepticus), hyperthermia, coma, and respiratory arrest may occur. Atrioventricular (AV) block and bradycardia have been reported, particularly in the elderly. Based on Its structure similarity to tricyclic antidepressants, carbamazepine may cause QRS and QT interval prolongation and myocardial depression however, in case reports of overdose, QRS widening rarely exceeds 100-120 msec and Is usually transient. 

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