**2. Mechanical Alternans (MA)**

#### **2.1. History of MA**

Phenomenon of alternating weak and strong beats observed in a heart which is contracting with constant intervals between beats. It has a long history. Experimental descriptions first appeared over a century ago, and since then there has been a sustained debate among clinicians and physiologists about its origins and clinical significance. A clinical description of an alternating pulse by Traube is often quoted as appearing earlier. [2] However, careful inspec‐ tion of his figure shows alternating interbeat intervals. In fact, Traube himself commented on the alternation of intervals and used the term "bigeminus" in the title of his report, although the true nature of this arrhythmia can only be guessed at since the electrocardiograms had yet

© 2013 Hirashiki and Murohara; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

to be invented at the time when Traube reported his case. MA has been studied in the intact human and animal heart, in isolated muscle preparations, and most recently in isolated cardiac muscle cells.

atrial pacing at 120 bpm or after dobutamine infusion at 10 µg kg–1 min–1 are shown for

Dobutamine-Induced Mechanical Alternans http://dx.doi.org/10.5772/55003 97

**Figure 1.** LV pressure waveforms during atrial pacing at 120 bpm and after infusion of dobutamine at a dose of 10 µg kg–1 min–1 in representative patients of three study groups. The traces represent the lead II electrocardiogram (ECG), LV pressure, and LV *dP*/*dt*. Both LV *dP*/*dt*max and LV *dP*/*dt*min showed alternating changes with LV pressure. Strong and

There were no significant differences in age and sex among the three groups of patients (Table 1). All patients were classified as NYHA functional class I or II at the time of cardiac catheter‐ ization. The LV ejection fraction (EF) in groups P and D was significantly lower than that in group N. There were also no significant differences in plasma brain natriuretic peptide (BNP)

The abundance of phospholamban mRNA was significantly lower in group D than in group P. The SERCA2a/phospholamban mRNA ratio was significantly higher in group D than in groups N and P (Table 2). The probability of event-free survival in group D was significantly

representative patients from each group (Fig. 1).

weak beats are indicated by s and w, respectively.

or norepinephrine levels among the three groups.

lower than that in groups N or P (P = 0.002) (Fig. 2).

**4.2. Baseline clinical data**

#### **2.2. Induction of MA**

The ability to induce MA by rapid driving frequencies appears to be a fundamental property of mammalian ventricular muscle. Experimental studies have shown that by varying the pacing cycle length over a wide range, it is possible to define a critical cycle length (threshold) for the induction of sustained MA.[3] Driving the heart at cycle lengths shorter than the threshold cycle length may increase the amplitude of the beat-tobeat oscillations in contraction strength.
