Cardiac mechanical and electrical activity are inter-dependent. The free-running Purkinje fibre network is located upon the ventricular endocardial surface. When the left ventricle (LV) undergoes mechanical stimulation, in the form of acute ventricular dilation, arrhythmogenesis may occur (Hurley et al., 2023). However, it is not yet clear the extent to which the Purkinje fibre network is involved. The aim of this study was to determine whether mechanical stimulation of individual Purkinje fibres leads to the development of ectopic activations.
All work was undertaken in accordance with local ethical regulations and approval from the University of Bordeaux and in accordance with the European Parliament Directive 2010/63/EU. Surgical plane anesthesia was induced in Sheep (n=5; 50-65 kg) with 10 mg/kg sodium pentobarbital and maintained under isofluorane (2% in 100% O2) prior to euthanasia by an intravenous injection of 2000 mg sodium pentobarbital. Hearts were quickly excised and perfused with cardioplegia and heparin. LV wedges were cannulated at the ostia and coronary-perfused with Tyrode. With the endocardial surface of the heart exposed, LV wedges were electrically stimulated (1.34-1.79 Hz) at the His Bundle by external electrodes. A suture was looped underneath the midpoint of a randomly selected single free-running Purkinje fibre and attached to a force transducer. Thus, raising the force transducer stretched the Purkinje fibre and indicated the timing and level of extending force applied. Endocardial Purkinje fibres were mechanically stimulated by applying a mean extending force of 4.86 ± 0.17 g for 10s, with 10s rest between each stretch. A pseudo-ECG was recorded and the effect of stretch on the number of ectopic excitations was tested by a Wilcoxon signed-rank test.
Stretch provoked single ectopic activations which disrupted the rhythmic patterns of the pseudo-ECG. Ectopic activation occurred a minimum of 1s and maximum of 8.2s upon the initiation of stretch, with no definite distribution pattern through the 10s stretch period.
When individual Purkinje fibres were stretched, 20 ectopics occurred across 7 out of the 21 recordings. When Purkinje fibres were not stretched, a 1.8 fold decrease in ectopic activations was recorded, with only 6 out of the 21 recordings eliciting an ectopic response (mean 0.95 ± 0.33 ectopics when stretched vs. mean 0.52 ± 0.22 ectopics when at rest; ± SEM; P<0.05; n=21). In addition, when the experimental time period of stretch and rest was considered, ectopic frequency was 81% greater when Purkinje fibres were mechanically stimulated compared to at rest (1.80 ectopics/min upon stretch vs. 0.99 ectopics/min at rest; P<0.05).
The stretch of individual Purkinje fibres led to an increase in the number of ectopic activations in preparations that were already showing ectopic activity. This suggests that Purkinje fibre stretch has the potential to cause electrical destabilisation in compromised tissue. This mode of investigation has the potential to investigate the role of Purkinje fibres as a source for stretch-induced ectopics or in the maintenance of arrhythmias. However, more detailed electrical mapping of ectopic initiation sites and their conduction is necessary.