Effects of calcium and sodium concentrations on atrioventricular conduction: experimental study in rabbit hearts with clinical implications on heart block and slow calcium channel blocking agent usage.

Effects of electrolyte concentrations on atrioventricular (AV) conduction were studied in isolated, perfused rabbit hearts by recording the His bundle electrogram. Initial calcium (Ca++), sodium (Na+), and potassium (K+) concentrations were 2.4, 144.8, and 4.5 mM, respectively. Lowering of Ca++ to 0.8 mM slightly prolonged the AH interval, whereas elevation of Ca++ to 4.8 or 7.2 mM more markedly prolonged this interval, often causing intranodal block. High Ca++-induced depression of intranodal conduction was antagonized by high K+ (7.5 mM). Verapamil (0.5 to 1.0 mg/L) produced second-degree intranodal block. High Na+ (172 mM) restored 1:1 conduction, whereas high Ca++ did not. These results suggest that: (1) an optimal Ca++ concentration for intranodal conduction exists; (2) high K+ counteracts high Ca++-induced intranodal block; (3) verapamil effect on AV node is antagonized by high Na+; and (4) slow Na+ current may play a role in AV nodal action potentials.