SPECIFIC ARRHYTHMIAS - sinus nodal rhythm disturbances



Normal sinus rhythm refers to impulse forma­tion beginning in the sinus node and, in adults, having a rate of between 60 and 100 beats per minute. The P wave is upright in leads 1,2, and aV> and negative in lead aVR. The rate of sinus nodal discharge is under autonomic control and in­creases with sympathetic and decreases with par­asympathetic stimulation. Sinus tachycardia re­fers to a tachycardia of sinus origin with a rate exceeding 100 beats per minute. Sinus tachycar­dia occurs with stresses such as fever, hypoten­sion, thyrotoxicosis, anemia, anxiety, exertion, hypovolemia, pulmonary emboli, myocardial is­chemia, congestive heart failure, shock, drugs (e.g., atropine, catecholamines, thyroid, alcohol, caffeine), or inflammation. Therapy should be fo­cused on the cause of the tachycardia. If the sinus tachycardia must be treated directly, propranolol may be used. Sinus bradycardia refers to sinus node discharge at a rate less than 60 beats per min­ute. The P wave contour is normal, but sinus ar­rhythmia is often present. Sinus bradycardia fre­quently occurs in young adults, especially well-trained athletes, and is common at night. Sinus bradycardia can be produced by a variety of con­ditions, including eye manipulation, increased intracranial pressure, myxedema, hypothermia, sepsis, fibrodegenerative changes, vagal stimula­tion, or vomiting, and the administration of par­asympathomimetic drugs, beta-adrenergic block­ing drugs, or amiodarone. It occurs commonly in the acute phase of myocardial infarction, espedaily inferior myocardial infarction. Treatment of asymptomatic sinus bradycardia is usually not necessary. If cardiac output is low or tachyar­rhythmias occur owing to the slow heart rate, atro­pine or, if necessary, isoproterenol may be effec­tive. There is no drug that effectively and safely increases the heart rate over a long period of time, and therefore electrical pacing is the treatment of choice chronically if symptomatic sinus brady­cardia is present.

Sinus arrhythmia refers to phasic variation in the sinus cycle length by greater than 10 per cent. P wave morphology is normal. Respiratory sinus arrhythmia occurs when the PP interval shortens during inspiration as a result of reflex inhibition of vagal tone and lengthens during expiration. Nonrespiratory sinus arrhythmia refers to sinus arrhythmia not associated with the respiratory cycle. Symptoms are unusual and treatment not necessary.

In sinus pause (sinus arrest) and sinoatrial exit block, a sudden unexpected failure of a P wave occurs. In sinoatrial exit block, the PP interval sur­rounding the absent P wave is a multiple of the P to P intervals, implying that the sinus impulse was generated but did not propagate through the perinodal tissue to the atrium. If no such cycle relationship can be found, the term sinus pause or sinus arrest is employed. Acute myocardial in­farction, degenerative fibrotic changes, digitalis toxicity, or excessive vagal tone can produce sinus arrest or exit block. Therapy involves searching for the underlying etiology. Patients are not treated if they are asymptomatic. If they are symptomatic and the arrhythmia is not reversed by correcting the underlying etiologies, pacing is employed.
Wandering atrial pacemaker involves a transfer of the dominant pacemaker from the sinus node to latent pacemakers in other atrial sites or in the AV junction. The change from one pacemaker focus to another occurs gradually, associated with a change in the RR interval, PR interval, and P wave morphology. Treatment is usually not nec­essary except if symptoms occur from bradyar-rhythmias.
The hypersensitive carotid sinus syndrome is characterized by cessation of atrial activity due to sinus arrest or sinoatrial exit block with light pres­sure over the carotid baroreceptors. In addition, AV block may be observed. Adequate junctional or ventricular escape complexes may not occur. Cardioinhibitory carotid sinus hypersensitivity is arbitrarily defined as ventricular asystole exceed­ing three seconds during carotid sinus stimula­tion. Vasodepressor carotid sinus hypersensitiv­ity is defined as a fall in systolic blood pressure of 30 to 50 mm Hg without cardiac slowing, usu­ally with reproduction of a patient’s symptoms. The treatment in symptomatic patients is pace­maker implantation (to include at least a ventric­ular lead, since the sinus node slowing is usually also associated with AV block). Neither atropine nor pacing prevents the vasodepressor manifes­tations of carotid sinus hypersensitivity. Severe vasodepressor carotid sinus hypersensitivity oc­casionally requires denervation of the carotid sinus.
The sick sinus syndrome is applied to a variety of sinus nodal and AV nodal abnormalities that occur alone or in combination. They include [1) persistent spontaneous sinus bradycardia not caused by drugs and inappropriate to the physi­ological circumstances, (2) sinus arrest or exit block, (3) combinations of sinus and AV conduc­tion disturbances, and (4) alternation of parox­ysms of atrial tachyarrhythmias with periods of slow atrial and ventricular rates (bradycardia/ tachycardia syndrome]. The sick sinus syndrome may be associated with AV nodal or His-Purkinje conduction disturbances. If symptoms are present from bradyarrhythmias, pacemaker implantation is appropriate. Pacing for the symptomatic brady-arrhythmia combined with drug therapy for the tachyarrhythmia is often needed.
Sinus nodal re-entrant tachycardia accounts for 5 to 10 per cent of paroxysmal supraventric­ular tachycardias. Its mechanism is presumed to be re-entry within the sinus node and the peri-nodal tissues, giving rise to a tachycardia, usually with a rate of 130 to 140 beats per minute and containing P waves very similar to sinus P waves. AV block may occur without affecting the tachy­cardia. Vagal activation may slow and then abruptly terminate the tachycardia by its action on sinus nodal tissue. Tachycardia may be induced and terminated at electrophysiological study with premature atrial stimulation. Treatment with pro­pranolol, verapamil, or digitalis is effective ther­apy