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The importance of these findings lies in the rather striking difference between the volume-outcome relationship found for operations for abdominal aortic aneurysms and congenital cardiac defects and the lack of such a relationship for coronary artery bypass grafting. This observation may be largely explained by the quality improvement program in New York State for bypass operations since 1989. If so, these results have important implications for expanding the scope of quality improvement efforts in New York State.

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We studied the ultrastructure and electrophysiology of an anterior mitral valve leaflet obtained from the heart of a recipient of a cardiac transplant. The anterior leaflet of this human mitral valve contained atrial muscle in direct continuity with the left atrial myocardium. The muscle fibers were 3-6 , in diameter and contained atrial-specific granules. They were arranged in longitudinal bands of two or three muscle cells with intercalated discs at the cellular poles. These groups of cells were separated from one another by abundant loose connective tissue. The muscle fibers had maximum diastolic potentials of -61 ± 8 mV and action potentials with slow upstrokes (Vmax <15 V/sec) and low amplitudes (66 ± 6 mV) occurred during electrical stimulation of the valve. In the absence of electrical stimulation, valve muscle had phase 4 depolarization and initiated automatic impulses at a slow rate. The rate of impulse initiation was increased by epinephrine. During electrical stimulation, epinephrine also caused the appearance of delayed afterdepolarizations. Triggered sustained rhythmic activity could be elicited. Delayed afterdepolarizations and triggered activity were abolished by acetylcholine and verapamil. Our results indicate that cardiac muscle in the human mitral valve can initiate impulses and might be a site of origin of arrhythmias in the human heart. ATRIAL MUSCLE EXTENDS into the leaflets of the mammalian mitral valve, including the valve of humans.1`4 We have previously reported that in the dog and in the monkey, the transmembrane action potentials of this valve muscle are quite different from ordinary atrial muscle,2' 5, I although the ultrastructure of valve muscle is similar to ordinary atrial muscle.2 The transmembrane potentials of valve muscle are similar to atrioventricular (AV) nodal potentials.7 Furthermore, canine and simian valve muscle can initiate nondriven impulses in the presence of catecholamines, which led us to suggest that the valve may be a site of origin of some atrial arrhythmias.5 6 Another study on the electrophysiologic characteristics of cardiac fibers in the rabbit mitral valve indicates that they are similar to cardiac fibers in the canine and simian valve.8 Because action potential characteristics sometimes vary from one species to another,7 we recorded action potentials from the cardiac muscle in the human mitral valve leaflet to determine whether they are the same as those in the valves of the other mammals described above and, in particular, whether nondriven impulses can be initiated in the human valve. Our results show that action potentials of normal human valve muscle also resemble AV nodal potentials and suggest that human mitral valve From the fibers can initiate both automatic and triggered9 impulses in the presence of catecholamines. Therefore, the mitral valve leaflet may be a source of ectopic atrial impulses in the human heart. Methods The Clinical CaseWe obtained the anterior mitral valve leaflet from the heart of a recipient of a cardiac transplant. The patien...