Capillary growth and changes in heart performance induced by chronic bradycardial pacing in the rabbit.

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Abstract
To determine whether chronic bradycardial pacing could change myocardial capillary density and heart performance, transvenous right atrial pacing was used to reduce the heart rate of rabbits to about 55% of normal for up to 52 days. Control animals, animals which had been chronically paced and sham-operated animals were compared. The chronically paced animals showed no cardiac hypertrophy [heart:body weight ratio (H:BR) was unchanged]. All animals paced for longer than 10 days showed increased myocardial capillary density (CD) when compared with controls of similar heart weight. The increase in CD was correlated with the duration of pacing (r = 0.830) and reached levels up to 70% greater than those of controls. The increase in stroke volume seen during acute bradycardial pacing was more than twice as great in animals which had been chronically paced as in the controls, which had not previously been paced (84% vs. 38%). Resting minute work and stroke work were similar in all groups, but maximum values achieved after norepinephrine administration were much greater in animals that had been chronically paced. Minute work increased by 59.1 .+-. 9.6% in controls, by 62.1 .+-. 16.3% in sham-operated animals and by 107.1 .+-. 8% (means .+-. SE) in paced animals. Stroke work increased by 64.9 .+-. 9.8% in controls, by 75.4 .+-. 19.5% in sham-operated animals and by 121.8 .+-. 7.6% in paced animals. Although the greater CD of most of the chronically paced hearts may be partly responsible for their improved performance, this cannot be the only factor, since animals paced for short periods showed improved performance but normal CD.