Electrical activation during ventricular fibrillation in the subacute and chronic phases of healing canine myocardial infarction.

BACKGROUND: Little information is available regarding the effects of myocardial infarction on the characteristics of ventricular fibrillation (VF). Epicardial activation during VF can be characterized by the cycle length and by the characteristics of activation wave fronts. METHODS AND RESULTS: VF was induced by programmed stimulation in 6 dogs with subacute healing (1 week) myocardial infarction (MI), 5 dogs with chronic (8 week) healing MI, and 6 dogs without MI. Using a plaque electrode array with a 2.5-mm interelectrode distance, 112 electrograms were recorded and 91 vector loops were created for each cycle of VF from either the anterior (infarcted) or lateral (noninfarcted) wall. Direction of maximum epicardial activation was determined at each site for the first 10 cycles of VF (early) and for 10 cycles after 5 seconds of VF (late). Wave front size was determined based on a similarity in epicardial activation directions within a given area and by a statistical analysis that determined the degree of spatial linking at varying distances over the recording plaque. VF cycle length was defined as the mean interval of 10 consecutive local activation times. Differences among groups and differences between the anterior and posterolateral walls were determined by ANOVA. The mean wave front area was significantly larger in the presence of subacute MI (97 +/- 4 mm2, early; 78 +/- 3 mm2, late) or chronic MI (94 +/- 5 mm2, early; 78 +/- 5 mm2, late) than in noninfarcted animals (73 +/- 5 mm2, early; 61 +/- 3 mm2, late). The degree of linking of epicardial activation directions was similar in the three groups at distances of 2.5 and 5.0 mm but was lower at a distance of 7.5 mm among animals without infarction, confirming a smaller wave front size and suggesting less organization of activation. VF cycle length was significantly longer in the presence of infarction (98 +/- 5 ms, normal control animals; 121 +/- 13 ms, subacute MI; 127 +/- 13 ms, chronic MI). VF cycle length was significantly longer over the anterior than the lateral wall in the presence of subacute MI (131 +/- 8 ms, anterior; 109 +/- 5 ms, lateral) or chronic MI (136 +/- 9 ms, anterior; 119 +/- 6 ms, lateral) but not in noninfarcted animals (99 +/- ms, anterior; 97 +/- 5 ms, lateral). The prolongation of VF cycle length among animals with infarction was associated with slower estimated conduction velocities during VF. CONCLUSIONS: During VF, in animals with subacute or chronic healing MI, (1) the size of activation wave fronts is larger, (2) the cycle length of VF is longer, (3) the conduction velocities are slower, and (4) the degree of organization is greater than in control animals. Thus, the characteristics of VF throughout the heart are altered by the presence of regional myocardial infarction. The implications of these findings for the initiation and maintenance of VF in the presence of different underlying myocardial substrates require further study.

Cognitions and stress: direct and moderating effects on depressive versus externalizing symptoms during the junior high school transition.

This study examined direct and stress-moderating effects of attributional style and global self-worth on depressive and externalizing symptoms in adolescents. Attributional style, perceived self-worth, depressive symptoms, and externalizing behaviors were assessed in 371 students in the spring of 6th grade. After the transition to 7th grade, they again completed measures of depression and externalizing symptoms as well as measures of negative life events and school hassles. Stressors around the transition predicted both depressive and externalizing behaviors. Perceived self-worth predicted depressive symptoms, but not externalizing behaviors. Attributional style directly and in interaction with stressors predicted depressive symptoms and did not predict externalizing behavior. A 3-way interaction between stress, attributional style, and self-worth suggested that level of perceived self-worth may moderate the effects of attributional style in times of stress.

Spatial and temporal linking of epicardial activation directions during ventricular fibrillation in dogs. Evidence for underlying organization.

BACKGROUND: It remains controversial as to whether electrical activation during ventricular fibrillation (VF) is organized. To detect the presence of organization in VF, the direction of epicardial activation (EA) at multiple sites was examined by using vector mapping. If VF is not a random process, EA direction at a given site should be related to adjacent sites and prior beats. METHODS AND RESULTS: Thirteen dogs with healing myocardial infarction (MI) and four dogs without MI had VF induced by programmed stimulation. Using a plaque electrode array with a 2.5-mm interelectrode distance, 91 vector loops were created for each "beat" of VF. Direction of maximum EA was determined at each site for the first 10 consecutive beats of VF and for 10 consecutive beats recorded 5 seconds after VF was established. Spatial and temporal linking of EA directions was evaluated by the ability of activation direction at a given site to be predicted by activation directions at eight adjacent sites for the index beat and at eight adjacent sites and the site of interest for the preceding beat using stepwise linear regression. The strength of the model as reflected by the correlation coefficient (r) indicated the degree of linking. We determined 1) changes in the degree of linking over time during a given episode of VF (using a paired-difference t test), 2) differences in the degree of linking between the anterior and posterolateral walls in animals with (n = 4) and without (n = 4) MI (using two-way ANOVA), and 3) the effect of repeated inductions (n = 10) on the degree of linking (using one-way ANOVA with repeated measures). During 57 episodes of VF, r for each model ranged from 0.64 to 0.88 during the transition to VF to 0.39-0.78 during established VF (p < 0.0001 for the difference). The presence of MI, the site of recording, and repeated inductions did not affect the degree of linking. For each episode, spatial linking was more prominent than temporal linking. CONCLUSIONS: Electrical activation during VF is organized. The degree of linking of EA directions during VF is not affected by the presence of MI, the site of recording, or repeated inductions of VF. During the first 5 seconds of VF, the degree of linking decreases.