REGIONAL ALTERNANS IN RELAXATION AND THE ONSET OF PULSUS ALTERNANS INTHE HEART OF THE ANESTHETIZED PIG

1. The factors leading to the alternation in myocardial contractility believed primarily responsible for pulsus alternans are not known. We examine regional and global contraction patterns in the in situ heart at stimulation rates just below the threshold for pulsus alternans to determine if events occurring in the transition to alternans can give clues to cellular mechanisms. 2. Twelve pigs were anaesthetized, the c hest wall removed and regional contraction measured in three areas of the left ventricle using tripodal strain gauges. We analysed regional and global dynamics during right atrial pacing at cycle lengths 50-150 ms greater than the threshold for pulsus alternans. 3. At pacing cycl e lengths 50 ms greater than that required to produce pulsus alternans seven of twelve pigs showed alternans in the maximum rate of ventricu lar pressure decay but none showed alternans in the maximum rate of pr essure rise. Pigs showing alternans in global relaxation were more lik ely to show alternans in regional contractility (P<0.05). 4. Twenty-si x of the thirty-six areas sampled showed alternans in end-diastolic le ngth at pacing rates below the threshold for pulsus alternans. In fift een of these areas alternation in end-diastolic length occurred in the absence of alternans in measures of contractility 5. Alternans in glo bal measures of relaxation may simply be a manifestation of regional a lternans in contractility. It is therefore not appropriate, from globa l haemodynamic data, to suppose that alternans in relaxation is the pr imary abnormality in the generation of pulsus alternans. However, anal ysis of regional behaviour does suggest that regional alternans in rel axation can occur in the absence of regional alternans in contractilit y.