Reversible blockade of myelinated and non-myelinated cardiac afferents in cats by instillation of procaine into the pericardium

Pericardial local anaesthesia was instituted in 33 chloralosed cats through catheters implanted into the pericardium. The activity of 1531 single fibres teased from either vagus in the neck was identified and classified according to source. Random testing with 2% procaine instilled into the pericardium revealed that only cardiac afferents could be blocked. Arterial and other receptors were unaffected. Blocking concentrations were determined for 38 non-myelinated cardiac afferents (mean conduction velocity 1.3 m.s-1) and for 43 myelinated fibres (mean conduction velocity 19 m.s-1). Blocking concentrations varied widely but equally for both fibre categories between 0.05% and 2.0% procaine. Non-myelinated fibres required a mean concentration of 0.26% procaine for block, whereas myelinated fibres needed a mean concentration of 0.45% (P less than 0.01). The large overlap in the frequency distributions of the blocking concentrations for both sets of fibres, would not, however, allow the blocking of one fibre category without affecting the other. Additionally two cardiac sympathetic afferents were blocked at 0.25 and 0.5% procaine. Mean times for block onset and recovery for non-myelinated fibres were 3.05/1.98 min and for myelinated fibres 1.4/0.59 min, respectively. Pericardial local anaesthesia is selective for cardiac afferents but does not allow for differential block of a given fibre category. A 1% solution of procaine will eliminate the heart as a reflexogenic area.