Influence of shock strength and timing on induction of ventricular arrhythmias in dogs

Abstract

We delivered strong shocks via electrodes on the left ventricular apex and the right atrium in seven dogs during the T wave of atrial pacing while recordings were made from 56 epicardial electrodes. After shocks that induced arrhythmias were given, the earliest activation occurred in the middle of the ventricles for lower-energy shocks and in the base for higher-energy shocks. For shocks late in the vulnerable period, activation was recorded soon after the shock, whereas for shocks early in the vulnerable period activation was not recorded for a mean of 70 ms (+/- 17 ms SD) after the shock. We also gave 1-J shocks during right and left ventricular pacing. For shocks early in the vulnerable period, activation initiating fibrillation arose in a focal pattern from the paced region. For shocks during the midportion of the vulnerable period, fibrillation arose by two leading circle reentrant loops rotating in opposite directions, one on the left and the other on the right ventricle. For shocks at the end of the vulnerable period, the two reentrant loops fused on the side of the heart opposite the pacing site to again form a single focal activation pattern. Thus the initial activation patterns of arrhythmias initiated by shocks, the time from the shock until earliest postshock activation, and the site of earliest postshock activation are strongly influenced by the coupling interval and strength of the shock.

Left Ventricular Apex Ablation Decreases the Upper Limit of Vulnerability

“…After shocks with the same lead configuration near the ULV 5,6 or near the defibrillation threshold, 10 the first few postshock activations almost always first appeared on the epicardium overlying this site. In the present study, subendocardial ablation was performed at this site in an attempt to remove the source of postshock activation.…”

“…3,4 Several studies suggest that VF induction by shocks during the vulnerable period occurs by the same mechanism that causes VF to recur after shocks that fail to defibrillate. 3,5 Because VF induction is probabilistic and dependent on shock strength and timing, we recently compared the activation sequences after shocks of the same strength delivered at the same time during the vulnerable period that induced VF 50% of the time, ie, those that had a 50% probability of success for the upper limit of vulnerability (ULV 50 ). The activation pattern for the first few postshock cycles after shock-induced VF episodes did not differ from that of episodes in which VF was not induced.…”

Cardiopulmonary resuscitation in a swine model of cardiac arrest

“…Στη βιβλιογραφία από τις 76 πειραματικές εργασίες στην πρόκληση KM, οι 52 έγιναν με τη μέθοδο επικάρδιας βηματοδότη ση ς. Για την πρόκληση ΚΑ, με τη μέθοδο της διαδερμικής βη-ματοδότησης με χρήση εξωτερικού απινιδιστή και με τη μέθοδο commotio cordis, είναι απαραίτητος ο συγχρο νισμός του χειροκίνητου απινιδιστή με το ηλεκτρο καρδιογράφημα του ζωικού προτύπου, καθώς η πρό κληση ΚΑ εξαρτάται από το φαινόμενο R-on-T (Herweling et al 2005) (Link et al 2003). Προηγούμενες μελέτες αναφέρουν ότι οι διάφοροι μηχανισμοί πρό κλησης KM έχουν ως αποτέλεσμα την πρόκληση KM με διαφορετικούς ηλεκτροφυσιολογικοΰς μηχανισμούς (Shibata et al 1988) (Chen et al 1988). Αλλωστε, η πρό κληση KM με τη μέθοδο έκτακτων ερεθισμάτων έχει αποδοθεί σε μηχανισμό επανεισόδου(Οΐ6η et al 1988) (Bhandari et al 1998).…”

“…Transdermal pacing, with the use of an external defibrillation and commotio cordis, requires close synchronization of the external electrical apparatus with the electrocardiogram, as induction of cardiac arrest mainly depends on the R-on-T phenomenon (Herweling et al 2005) (Link et al 2003). Prior studies suggest that various modes of VF induction may initiate VF by different electrophysio logical mechanisms (Shibata et al 1988) (Chen et al 1988). Furthermore, VF induction with the use of premature stimuli has been attributed to re-entry mechanisms (Chen et al 1988) (Bhandari et al 1998).…”

Pacing After Shocks Stronger Than the Upper Limit of Vulnerability

“…1,2 For shock strengths near the upper limit of vulnerability (ULV), the global dispersion of refractoriness immediately after the shock and the characteristics of the first postshock cycle may not be sole determinants of VF induction. 3 Instead, the number and rapidity of activation cycles soon after the shock appear to determine whether secondary reentry occurs, leading to VF.…”