Pulses Alternans - Definition, Etiology and Mechanism : Internal medicine : #Usmle Cardiology
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PULSUS ALTERNANS
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โข Pulsus alternans (also termed mechanical alternans) is a variation in pulse amplitude occurring with alternate beats due to changing systolic pressure. It is best appreciated by applying light pressure on the peripheral arterial pulse, and can be confirmed by measuring the blood pressure. When the cuff pressure is slowly released, phase I Korotkoff sounds are initially heard only during the alternate strong beats; with further release of cuff pressure, the softer sounds of the weak beat also appear. The degree of pulsus alternans can be quantitated by measuring the difference in systolic pressure between the strong and the weak beat.
โข Etiology of pulsus alternans โ The most important cause of pulsus alternans is left ventricular failure. In clinical practice, true pulsus alternans is rarely seen in the absence of significant left ventricular myocardial failure, and should prompt further investigation to determine the severity and cause of left ventricular myocardial dysfunction. Although pulsus alternans almost exclusively occurs in systolic heart failure (low left ventricular ejection fraction), it can occur rarely in diastolic heart failure (preserved ejection fraction).
Pulsus alternans also may be evident in the following situations:
โLeft ventricular pulsus alternans without systemic arterial pulsus alternans has been observed in patients with hypertrophic cardiomyopathy and a significant rest or provocable outflow gradient . The mechanism remains unclear; abolition of left ventricular alternans occurs after successful myomectomy.
โPulsus alternans is rarely encountered in patients with cardiac tamponade.
โIt can occur in the presence of marked tachypnea when the respiratory rate is one-half the heart rate due to an inspiratory decrease in the pulse amplitude. The pulse abnormality disappears when respiration is held transiently.
โIt may be seen in patients with severe aortic regurgitation. It is, however, rare in the absence of left ventricular systolic dysfunction.
โPulsus alternans is frequently precipitated by ectopic beats; apparent pulsus alternans may be observed in patients with a bigeminal rhythm. In the latter situation, the premature beats are usually out of phase with the normal beats and postectopic pauses are appreciated. Simultaneous auscultation of the sequence of the heart sounds and palpation of the arterial pulse can differentiate between true pulsus alternans and apparent pulsus alternans due to bigeminy.
Pulsus alternans should not be diagnosed when the cardiac rhythm is irregular. Pulsus alternans is more common with faster heart rate. Pacing induced tachycardia can precipitate sustained pulsus alternans in patients with idiopathic dilated cardiomyopathy and is associated with worse prognosis.
Mechanism of pulsus alternans โ The precise mechanism for pulsus alternans remains unclear; alternating preload (Frank-Starling mechanism) and incomplete relaxation have been proposed . Changes in afterload, which is lower before the strong beat because of the lower output during the weak beat, may also contribute.
It also has been suggested that a change in ventricular contractility is the primary mechanism. Changes in the activity of the sarcoplasmic calcium pump with alternate strong and weak beats appears to be the mechanism for changes in contractility. In experimental animals, acute myocardial ischemia is associated with regional pulsus alternans, leading to the hypothesis that alternating potentiation and attenuation or deletion of contraction accounts for the pulse abnormality . Thus, pulsus alternans may result primarily from an alternating contractile state of the ventricle. The magnitude of the alteration of pressure and stroke volume during pulsus alternans, indices of pump function, reflects the interaction of an alternating contractile state with changes in preload and afterload.
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