CORRESPONDENCE: LETTER TO THE EDITOR
Human Ventricular Action Potential Duration Restitution
Raja J. Selvaraj, MD and
Vijay S. Chauhan, MD, FRCPC*
* PMCC 3-522, Toronto General Hospital 150 Gerrard St. W., Toronto, Ontario M5G 2C4, Canada (Email: vijay.chauhan{at}uhn.on.ca).
We read with interest the paper by Narayan et al. (1) that related right ventricular (RV) action potential duration (APD) restitution slopes to the presence of T-wave alternans (TWA) and ventricular tachycardia (VT) inducibility in patients with left ventricular (LV) dysfunction (LV ejection fraction [EF] 28 ± 8%). Standard restitution was measured by plotting APD90 as a function of the preceding diastolic interval (DI) during extrastimulus testing (S1S2). The prevalence of steep restitution slopes (>1) was similar between patients with positive or negative TWA, and mean restitution slopes were no different between those with inducible or noninducible VT.
Both these findings are contrary to a study reported by our group in a similar patient population (LVEF 30 ± 8%) (2). We feel this discrepancy can be accounted for by considering differences in the interpretation and measurement of the minimum DI which can significantly influence the steepness of the standard restitution slope. We determined activation recovery interval (ARI) restitution using a pacing protocol similar to Narayan et al. (1), but at short S1S2 coupling interval where local activation of S2 occurs before repolarization of the preceding S1 the DI is negative and can only be derived by measuring the ARI of the preceding beats of the S1 drive train (2,3). With this approach, the minimum DI (–22 ± 14 ms) is typically negative (2,3), unlike the positive minimum DIs reported by Narayan et al. (19 ± 13 ms) (1). It is not clear how the authors measured DI at the shorter S1S2 coupling intervals, though it is apparent that APD90 of the last drive train beat was not reached before the S2 upstroke (Fig. 1C in Narayan et al. [1]). The presence of a negative DI is consistent with effective refractory period/APD ratios less than unity in humans. Without correcting DI at the shorter S1S2 coupling intervals, negative DIs will be under-recognized, which in turn may overestimate and equalize the steepest restitution slope between patient groups.
It is also important to consider the spatial heterogeneity of restitution slopes, which has been implicated in the pathogenesis of alternans and ventricular arrhythmias in experimental and human studies. Although Narayan et al. (1) did not find differences in restitution slopes between the RV apex and outflow track, nor between the RV and LV endocardium, we have reported steeper ARI restitution slopes at the RV apex compared with the RV base in a similar patient population with impaired LV function (2). Steeper ARI restitution slopes have also been found in the human RV endocardium versus LV endocardium (3).
We would be interested in the authors' comments on whether the lack of difference in steepest restitution slopes between patient groups or between recording sites could have been influenced by the method used to measure DI at the shorter S1S2 coupling intervals.
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References
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- Narayan S, Franz M, Lalani G, Kim J, Sastry A. T-wave alternans, restitution of human action potential duration, and outcomes J Am Coll Cardiol 2007;50:2385-2392.[Abstract/Free Full Text]
- Selvaraj R, Picton P, Nanthakumar K, Chauhan V. Steeper restitution slopes across the right ventricular endocardium in patients with cardiomyopathy at high risk of ventricular arrhythmias Am J Physiol Heart Circ Physiol 2007;292:H1262-H1268.[Abstract/Free Full Text]
- Yue AM, Franz MR, Roberts PR, Morgan JM. Global endocardial electrical restitution in human right and left ventricles determined by noncontact mapping J Am Coll Cardiol 2005;46:1067-1075.[Abstract/Free Full Text]
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- Sanjiv M. Narayan and Michael R. Franz
J. Am. Coll. Cardiol. 2008 51: 1722-1723.
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