Dilation of coronary microvessels by adenosine induced hypotension in dogs

An experimental model was established for fluorescence video microscopy of coronary microvessels. Nineteen dogs were anesthetized with a narcotic. Catheters were placed for hemodynamic monitoring and sampling of arterial and coronary venous blood. Myocardial perfusion was measured with radioactive microspheres. Following thoracotomy, movements of the myocardial surface area under investigation were restricted by a specially designed heart holder. Plasma was stained with FITC labelled dextran. Diameters were determined in arteriolar and venular microvessels greater than or equal to 20 microns. Measurements were performed during baseline conditions, i.e. only the basic anesthetic drug was applied, and during coronary vasodilation by continuous infusion of adenosine in a randomized sequence. Mean arterial pressure was reduced from 85 +/- 2 mmHg during baseline to 59 +/- 1 mmHg by infusion of 16.9 +/- 2.2 mg.kg-1.h-1 adenosine. Adenosine increased left ventricular blood flow by 253%, left ventricular oxygen demand remained unchanged. A total of 495 arteriolar and 170 venular diameters were measured during baseline condition and during adenosine infusion. Arteriolar diameters increased in all vessel segments between 20 and 600 microns, however, arterioles below a critical size of 100 microns had a greater dilating capacity than larger arterioles. Maximal decrease of segmental resistance occurred in 20-40 microns arterioles and amounted to 74%, which is less than the 82% decrease of total coronary resistance. Venular diameter changes, too, were more pronounced in smaller vessels.