Cardiac pacemaker cells generate rhythmic discharge of action potentials. Parasympathetic activity slows, while sympathetic activity accelerates, the rate of discharge. Neuronally released transmitters activate different receptors on pacemaker cells from those activated by exogenous transmitters. Neuronally activated "junctional" receptors selectively decrease and increase inward current during diastole.
Obstruction to the outflow of the right ventricle may be at the pulmonary valve, the infundibular region, or both, and may be associated with defects in the septa. The normal and pathological anatomy of this area has been fully described by Brock (1957), and various surgical techniques have been suggested and practised for dividing the valve or relieving the infundibular stenosis.(1) Transventricular approach using a valvotome and splitter to open the valve and a punch to remove the infundibular thickening-with intact circulation (Sellors, 1948;Brock, 1948).(2) Transarterial approach using a valvotome to cut the valve-with intact circulation (Potts et al., 1950; S0ndergaard, 1952;Pettersen, 1954 This blind approach to the pulmonary valve was abandoned in favour of the direct visual exposure through the pulmonary artery and this has been used in 61 cases. Not only can the valve be seen directly and be accurately divided to the valve ring, but the finger can be passed through it into the right ventricle to ensure that there is no true infundibular stenosis. If such a stenosis is found, it can then be approached through the divided valve and the amount of material punched out controlled by finger palpation. Although this approach to an infundibular stenosis cannot be considered ideal, it is less disturbing to the heart than a transventricular approach and has the advantage that a tactile check of the residual opening can be made. There is little doubt that if the presence of an infundibular stenosis is demonstrated or suspected at the pre-operative catheterization, its complete removal can be guaranteed only by a direct visual approach. The time required for this together with any plastic repair on the outflow tract of the right ventricle demands, at present, the use of a cardiopulmonary by-pass. It has been argued that in pulmonary valve stenosis, the evidence obtained at pre-operative catheterization can not always guarantee that some degree of fibrotic infundibular stenosis or a small ventricular septal defect will not be found, and 472
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