Abstract
Background: In ventricular fibrillation (VF) of short duration chest compressions do not improve survival prior to defibrillation. We have previously shown that in an acute myocardial infarction (MI), frequency characteristics are markedly depressed and are poor surrogates for VF duration. We hypothesized that the VF waveform further evolves post-MI, after a myocardial scar has formed. Methods: Acute myocardial infarction was induced in nine swine by occlusion of the mid left anterior descending artery. After a two week recovery period, VF was induced and compared to VF in 16 control swine over the first five minutes. VF waveforms were analyzed for frequency content (mean, median, dominant and bandwidth), slope (the median of the absolute value of the difference in voltage every 5 ms), and AMSA (the summed product of amplitude and frequency). Results: Myocardial scar was confirmed in all MI swine postmortem. All frequency characteristics were significantly altered in post-MI swine (p<0.01). At 4 minutes, VF mean frequency in controls was 10.3 ± 0.5Hz compared to 9.1± 0.5Hz in post-MI swine (mean ±SEM, p=0.007), with similar differences for median and dominant frequencies. Bandwidth was markedly decreased and at 4 minutes was 3.2 ± 0.2Hz in controls compared to 2.1 ± 0.2Hz in post MI swine (p<0.001). However, slope and AMSA were similar in both groups. Conclusions: Frequency content of VF remains altered post MI compared to control swine, with reductions in mean, median and dominant frequencies and larger reductions in bandwidth. Amplitude based measures such as AMSA and slope, however, are not significantly altered and may serve as better predictors of time in VF.
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