Distribution of coronary blood flow across the canine left ventricular free wall during reactive hyperemia

Miniature silicon diode matrix-detector for in vivo measurement of 133xenon disappearance in the canine myocardium following local tissue injection

After local tissue depositioning of 133Xenon (133Xe) the regional washout is usually registered by a NaI(Tl) detector. The residual radioactivity of 133Xe is usually measured at its 81 keV photopeak. However, using small Silicon (Si) photodiodes it is feasible to measure only the low-energy activity in the X-ray energy range. In the myocardium of open chest dogs 133Xe washout measurements by a matrix of Si diodes composed in a 4 x 4 array and a conventional NaI(Tl) detector were carried out simultaneously. Fourteen separate pairs of measurements were performed in 3 dogs. When the Si-diodes in the matrix were selected individually in accordance to the position with reference to the diode with maximum count rate or pooled, comparisons could be made between the corresponding washout rate constants measured by the reference detector. In the correlation between the rate constants the intercepts with the y axis were not significantly different from zero allowing the correlation lines to be fitted through (0.0). The slope of the correlation line was close to unity. The registration of the low-X-ray energy of 133Xe by the Si-detectors is an alternative to the conventional high energy activity recording appearing from the gamma-energy of the photopeak. The detector matrix concept allows elimination of motion artefacts and indicator distribution in the myocardial tissue. Due to the uniformity and low cost of Si-diodes the perspective may be the introduction as a disposable transducer useful during cardiac surgery for example.

Transmural gradients of coronary flow reserve with physiologically and morphometrically defined stenoses in dogs

Coronary angiography permits identification of stenotic lesions but underestimates their severity and does not provide information regarding their physiologic significance. Evaluation of coronary flow reserve by means of selective coronary artery Doppler flow catheters or quantitative arteriography has been proposed to obtain this information. However, these techniques may not accurately reflect transmural gradients in flow. We evaluated the relationship between flow reserve defined with an epicardial Doppler flow probe and the transmural gradient of flow measured with radiolabeled microspheres in 21 dogs with graded stenoses and correlated results with coronary artery geometry measured morphometrically. Four groups of dogs were studied. In five control dogs without stenosis, reactive hyperemia after 20 seconds of complete coronary occlusion was 4.5 +/- 1.5 (mean +/- SD) times resting flow with an endocardial/epicardial flow ratio at peak flow of 1.0 +/- 0.2. When reactive hyperemia was blunted (without affecting resting flow) by 50% (n = 6), 75% (n = 5), or was abolished completely (n = 5) by coronary stenosis, the endocardial/epicardial flow ratio at peak flow was 1.0 +/- 0.3, 0.7 +/- 0.2, and 0.5 +/- 0.1, respectively. Cross-sectional area of the stenosed segment was reduced by 85.6 +/- 3.5%, 91.1 +/- 2.2%, and 92.8 +/- 4.3% in these groups, respectively. Thus in dogs with stenoses exceeding 86% of the cross-sectional area, endocardial flow reserve is compromised disproportionately compared with epicardial flow reserve, suggesting that clinical measurements of coronary flow reserve may underestimate the physiologic significance of coronary stenoses.

Transmural myocardial perfusion

The predilection for subendocardial underperfusion and ischemia is great and must be considered in the management of any patient, especially if there is coronary artery disease or ventricular hypertrophy. Although the mechanisms of subendocardial ischemia remain to be fully defined, they are clearly associated with the transmural distribution of intramyocardial systolic pressures. Even though almost all the myocardium is perfused in diastole, a reduction of diastolic perfusion pressure or duration will result in subendocardial ischemia. The factors that produce subendocardial ischemia are all associated with a reduction or loss of coronary flow reserve, and as our ability to measure flow reserve in humans improves, it is likely that we will be able to select medical or surgical therapy that will minimize or abolish subendocardial ischemia. For example, it will someday become possible to choose a time for valve replacement in an asymptomatic patient to obtain maximal protection of the myocardium or to select the right combination of therapies for the immediate post-operative period so that as much myocardium as possible will be spared. The more we learn to understand the mechanisms of subendocardial ischemia, the sooner will we be able to achieve these desired ends.