Author(s): Kroeker CA, Tyberg JV, Beyar R
Left ventricular (LV) twist has been defined as the counterclockwise rotation of the ventricular apex with respect to the base during systole. We recently showed that, since base rotation is minimal, measurement of apex rotation reflects the dynamics of LV twist. Since ischemia is known to affect endocardial and epicardial fiber force and shortening and therefore the transmural balance of torsional moments, we hypothesized that ischemia has a significant effect on apex-rotation amplitude and on untwisting during the isovolumic relaxation (IVR) period.
With an optical device coupled to the LV apex, apex rotation was recorded simultaneously with LV pressure, ECG, LV segment length, and minor-axis diameters in 16 open-chest dogs. Ischemia was caused by a 1- to 2-minute snare occlusion of either the left anterior descending (LAD) or circumflex (LCx) arteries. LAD ischemia had a pronounced effect on apex rotation: an increase in apex-rotation amplitude attributed to subendocardial dysfunction at 10 seconds of ischemia; maximum apex rotation occurring later (during the IVR period) throughout the ischemia; a paradoxical relaxation pattern of initial untwisting followed by twisting and untwisting during the IVR period with ischemia; and a decrease in the amplitude of apex rotation with ischemia, possibly due to transmural dysfunction. LCx occlusion had similar effects on apex rotation, except that apex-rotation amplitude was not increased at 10 seconds of occlusion and the amplitude of apex rotation did not decrease with severe ischemia. Under control preischemic conditions, a linear relationship between apex rotation and segment length was observed during ejection and a different, steeper relationship during IVR. With regionally ischemic segments, this relationship became nonlinear for both ejection and IVR.
Both LAD and LCx ischemia had profound effects on the dynamics of apex rotation. A paradoxical relaxation pattern occurred with ischemia. We suggest that these observations are due to changes in the dynamic transmural balance of torsional moments that determine LV twist.