Letter by Knaapen et al Regarding Article, “Hemodynamic Effects of Long-Term Cardiac Resynchronization Therapy: Analysis by Pressure-Volume Loops”
To the Editor:
With great interest we read the recent work by Steendijk et al1 that investigated the effects of long-term cardiac resynchronization therapy (CRT) on hemodynamics as evaluated by pressure-volume loops. That study elegantly demonstrates that the previously observed favorable short-term hemodynamic effects of CRT are maintained over time. Among other findings, the authors report a significant increase in end-systolic elastance (Ees). Ees is indeed widely used as a reliable index of ventricular contractility and can be obtained by calculating the slope of the connecting end-systolic points of a family of pressure-volume loops that are acquired under varying loading conditions.2 Steendijk et al,1 however, did not use this method and in their estimation of Ees fixed the volume axis intercept at zero despite the fact that this approach can result in substantial inaccuracies, especially in the dilated heart.2 Although it is justifiable that the investigators refrained from such potential hazardous and cumbersome measurements, the so-called single beat Ees estimation algorithms could have been used to more accurately define Ees.3 Moreover, the potential errors in estimating Ees were in turn incorporated into the quantification of total mechanical energy as determined by the total pressure-volume area and in the calculation of ventricular-arterial coupling. The reported effects of CRT on these parameters, therefore, should be interpreted with caution.
Furthermore, the authors1 refer to mechanical efficiency as the energy transfer ratio of pressure-volume area to stroke work, whereas traditionally this term is reserved for the ratio of generated mechanical work to myocardial consumed oxygen.2 The authors also appear to have interchanged the definition of mechanical efficiency, given their subsequent comparison with equivalents of traditionally defined mechanical efficiency, ie, left ventricular work parameters relative to oxidative metabolism, reported by Nelson et al4 and Sundell et al.5 In our opinion, it would be important to maintain a more stringent use of the aforementioned definitions.
Steendijk P, Tulner SA, Bax JJ, Oemrawsingh PV, Bleeker GB, van Erven L, Putter H, Verwey HF, van der Wall EE, Schalij MJ. Hemodynamic effects of long-term cardiac resynchronization therapy: analysis by pressure-volume loops. Circulation. 2006; 113: 1295–1304.
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Senzaki H, Chen CH, Kass DA. Single-beat estimation of end-systolic pressure-volume relation in humans: a new method with the potential for noninvasive application. Circulation. 1996; 94: 2497–2506.
Nelson GS, Berger RD, Fetics BJ, Talbot M, Spinelli JC, Hare JM, Kass DA. Left ventricular or biventricular pacing improves cardiac function at diminished energy cost in patients with dilated cardiomyopathy and left bundle-branch block. Circulation. 2000; 102: 3053–3059.
Sundell J, Engblom E, Koistinen J, Ylitalo A, Naum A, Stolen KQ, Kalliokoski R, Nekolla SG, Airaksinen KE, Bax JJ, Knuuti J. The effects of cardiac resynchronization therapy on left ventricular function, myocardial energetics, and metabolic reserve in patients with dilated cardiomyopathy and heart failure. J Am Coll Cardiol. 2004; 43: 1027–1033.