Relation between regional electrical activation time and subepicardial fiber strain in the canine left ventricle

To determine the relation between regional electrical activation time and fiber strain, epicardial electrical activation and deformation were measured in six open-chest dogs at the left ventricular anterior free wall after 15 min of right atrial, left ventricular free wall, left ventricular apex, or right ventricular outflow tract pacing, when end-diastolic pressure was normal or elevated (volume-loading). Regional electrical activation was measured using a 192-electrode brush. Regional subepicardial fiber strain (ef) was measured simultaneously in 16 regions, using optical markers which were attached to the epicardial surface and recorded on video. When relating regional ef during the ejection phase to regional activation time, the best correlation was found when a hemodynamic time reference rather than an electrophysiological one is used. Using the moment of the maximum rate of change of left ventricular pressure as the time reference for electrical activation, regional electrical activation time (t(ea)) and the degree of ef during the ejection phase could be fitted by a linear regression equation ef = a t(ea) + b, in which a = -3.46 +/- 0.73 s-1 an b = -0.28 +/- 0.05. For electrical activation times ranging from -40 to -80 ms, fiber strain was estimated with an accuracy of +/- 0.026 (+/- SE) with this relation. During right atrial pacing, t(ea) and ef were on the average -48 ms and -0.10 respectively. On further investigation, the relation between ef and t(ea) appeared to be influenced by end-diastolic pressure. For normal (1.1 kPa) and elevated end-diastolic pressure (1.8 kPa), the slope of the linear regression line was -3.96 and -2.86 s-1, respectively. Three conclusions may be drawn. Firstly, the time interval between the moment of regional electrical activation and the moment of the maximum rate of change of left ventricular pressure is an index of regional fiber strain. Secondly, it can be concluded from the above equations that electrical asynchrony of more than 30 ms causes non-uniformities in the degree of ef of the order of mean ef during pacing from the right atrium. Finally, differences in fiber strain during asynchronous electrical activation are less pronounced at larger filling pressures.