In pulsus alternans, beats occur at regular intervals with a regular alternation of the systolic height of the pressure pulses188489 (see Fig. 10-42.fi). Rarely, pulsus alternans is so marked that the weaker pulses are not felt at all. When pulsus alternans is noticed first after a premature beat, the extent of the difference in systolic pressure in alternating beats may decline for several cycles until the pulse amplitude is again constant. The initiation of post-premature ventricular beat pulsus alternans is probably related to the increased duration of LV filling after the premature beat, resulting in a greater end-diastolic volume and hence increased contractile force due to the Frank-Starling mechanism.
Sustained pulsus alternans (see Chap. 20) is seen in severe depression of LV performance with an alteration in aortic flow, systolic LV pressure, aortic systolic pressure, LV dP/dt, and LV end-diastolic pressure. Sustained pulsus alternans likely is due to alteration of the contractile state of at least part of the myocardium, which may be caused by the failure of electromechanical coupling in some cells during the weaker contraction.189 A subsequent stronger contraction would then represent contraction of all cells, some of which were potentiated.190
Pulsus alternans may be better appreciated when palpating a distal artery, which normally has a slightly wider pulse pressure than the carotid artery. The patient's respiration should be held, since the small changes in arterial pressure caused by normal respiration may obscure the recognition of pulsus alternans. Pulsus alternans can be confirmed by using a sphygmomanometer and is usually
Hurst's the Heart; Chapter 10: THE HISTORY, PHYSICAL EXAMINATION, AND CARDIAC AUSCULTATION, Page 4
associated with a LV third heart sound.
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