Transmural coronary vasodilator reserve, and flow distribution during tachycardia in conscious young swine with right ventricular hypertrophy

Regional distribution of myocardial blood flow (MBF) was examined in eight normal and nine pulmonary artery banded (PAB) pigs before and during pacing induced tachycardia (heart rates: 175, 225 and 275 beats X min-1) as well as during iv adenosine infusion (1.5 mg X kg-1 X min-1; maximal coronary vasodilatation) using radionuclide labelled 15 micron diameter microspheres that were injected into the left atrium. It was observed that MBF per unit myocardial mass in the hypertrophied right ventricle (RV) of PAB pigs was similar to that in the RV of normal pigs. Also, minimal coronary vascular resistance per unit myocardial mass was similar between the two groups of pigs for RV as well as left ventricle (LV). This suggests that the increase in RV myocardial mass of young PAB swine was attended by appropriate adjustments in functional cross-sectional area of the RV coronary vascular bed. Despite similarity of maximal coronary vasodilator capacity in the two groups of swine, during pacing induced tachycardia MBF in the hypertrophied RV subendocardium as well as the right and middle layers of the interventricular septum in PAB pigs increased to a significantly lower level than in normal pigs and the RV endo:epi perfusion ratio, unlike in normal pigs, decreased to near unity. Increments in transmural LV MBF of PAB pigs were also attenuated during pacing at 225 and 275 beats X min-1 but the LV endo:epi perfusion ratio for the two groups of pigs remained similar. These findings suggest a possible overall depression of myocardial function in PAB swine.