AGE-RELATED DIFFERENCES IN THE RESPONSE TO ACIDOSIS, HYPOXIA, AND HYPERKALEMIA IN CANINE CARDIAC PURKINJE-FIBERS

Objective: In the clinical setting, the response of adult and neonatal cardiac rhythms to hypoxia and to acidosis differs, the former leadin g to tachyarrhythmias and the latter to bradyarrhythmias. In this stud y, the aim was to determine whether a cellular electrophysiological su bstrate could be identified to explain the clinical observation. Metho ds: Conventional microelectrode techniques were used to study the elec trophysiological responses of automatic Purkinje fibres to acidosis, h ypoxia, and hyperkalaemia individually. Results: Adult Purkinje fibres showed decreases in maximum diastolic potential, activation voltage, and automaticity as pH was decreased from 7.3 to 6.1. Triggered activi ty due to early after depolarisations developed in 70% of adult Purkin je fibres at pH 6.1. Neonatal Purkinje fibres showed decreased automat icity as pH decreased and, in contrast to adults, quiescence occurred at pH 6.1 At PO2 <2.9 kPa automaticity decreased significantly in adul t Purkinje fibres, whereas neonatal fibres were unaffected. The effect of raising [K+]o was comparable at both ages. Conclusions: The respon se to acidosis and to hypoxia differs significantly between neonatal a nd adult Purkinje fibres and may explain some developmental difference s in the expression of cardiac arrhythmias.