Regional timing of myocardial shortening is related to prestretch from atrial contraction: assessment by high temporal resolution MRI tagging in humans.

ABSTRACT

Earlier studies have shown substantial nonuniformity in normal left ventricular (LV) myocardial function concerning both the degree of shortening and timing of shortening. We hypothesized that nonuniform LV function may be related to nonuniform prestretch induced by atrial contraction. Eleven healthy human subjects were studied using MRI myocardial tagging and strain analysis. The amount of circumferential prestretch was assessed in 30 LV segments. Prestretch was defined as the difference in strain between end diastole (at ECG R wave) and diastasis. Furthermore, both the degree of shortening (quantified as peak circumferential shortening, peak systolic shortening rate, and amount of postsystolic shortening) and timing of shortening (quantified as the onset time of shortening and time to peak shortening) were assessed. LV prestretch was found to be nonuniform, with the highest values in the lateral wall. The amount of segmental prestretch correlated significantly with peak shortening (r = 0.79), peak shortening rate (r = 0.50), amount of postsystolic shortening (r = 0.67), onset time of shortening (r = -0.57), and time to peak shortening (r = 0.71) (P < 0.001 for each of these relations). These relations may be explained by regional differences in wall stress or by a regional Frank-Starling effect. The correlation between timing of shortening and prestretch demonstrates that mechanical timing is not determined by electrical phenomena alone. In conclusion, regional variation in LV function correlates with the nonuniform prestretch from atrial contraction.