. Effect of passive myocardium on the compliance of porcine coronary arteries. Am J Physiol Heart Circ Physiol 285: H653-H660, 2003; 10.1152/ajpheart.00090.2003.-The objective of this study was to determine the effect of passive myocardium on the coronary arteries under distension and compression. To simulate distension and compression, we placed a diastolicarrested heart in a Lucite box, where both the intravascular pressure and external (box) pressure were varied independently and expressed as a pressure difference (⌬P ϭ intravascular pressure Ϫ box pressure). The ⌬P-cross-sectional area relationship of the first several generations of porcine coronary arteries and the ⌬P-volume relationship of the coronary arterial tree (vessels Ͼ0.5 mm in diameter) were determined using a video densitometric technique in the range of ϩ150 to Ϫ150 mmHg. The vasodilated left anterior descending (LAD) coronary artery of six KCl-arrested hearts were perfused with iodine and 3% Cab-O-Sil. The intravascular pressure was varied in a triangular pattern, whereas the absolute cross-sectional area of each vessel and the total arterial volume were calculated using video densitometry under different box pressures (0, 50, 100, and 150 mmHg). In the range of positive ⌬P, we found that the compliance of the proximal LAD artery in situ (4.85 Ϯ 3.8 ϫ 10 Ϫ3 mm 2 /mmHg) is smaller than that of the same artery in vitro (16.5 Ϯ 6 ϫ 10 Ϫ3 mm 2 /mmHg; P ϭ 0.009). Hence, the myocardium restricts the compliance of the epicardial artery under distension. In the negative ⌬P range, the LAD artery does not collapse, whereas the same vessel readily collapses when tested in vitro. Hence, we conclude that myocardial tethering prevents collapse of large blood vessel under compression. digital subtraction angiography; video densitometry; mechanical properties; mechanics; tethering THE MECHANICAL PROPERTIES of coronary vessels play an essential role in understanding the physiological functions of the vessels, and as a result, have direct clinical implications in the diagnosis and treatment of patients with coronary artery disease and atherosclerosis (22,27). Anatomically, the coronary arteries originate from the aortic ostia, just above the aortic valve, and continue along the surface of the heart as they penetrate into the myocardium where they deliver blood throughout the thickness of the heart (16). The posterior aspect of the proximal coronary artery is partially embedded into the myocardium, whereas the anterior portion is surrounded by the serous visceral pericardium. As the coronary artery descends along the ventricle, it becomes fully embedded into the myocardium.A number of previous studies (1, 2, 9, 10, 13, 15, 21) have examined the mechanical properties of coronary arteries under in vitro conditions; i.e., after dissection of the vessels from the myocardium. For example, Patel and Janicki (21) determined the in vitro pressure-diameter relationship for isolated segments of the dog's left circumflex. The pressure-diameter relationship of excised coro...
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