Читать книгу Practical Cardiovascular Medicine - Elias B. Hanna - Страница 183

Coronary blood flow constitutes ~5% of the total cardiac output and may increase up to 5 times with exercise. Normally, the coronary microcirculatory resistance constitutes the only resistance to myocardial flow; the epicardial vessels are just conductance vessels that offer no resistance to myocardial flow. In the presence of a functionally significant stenosis, classically a 70% diameter stenosis, the trans-stenotic flow drops during exertion; at a 90% diameter stenosis, the trans-stenotic flow drops at rest. During exercise or adenosine infusion, exten- sive microvascular dilatation occurs, requiring an extensive increase in flow to fill the dilated circulation; since the flow cannot increase across a flow-limiting stenosis, ischemia occurs.¹

Supply ischemia is typically caused by ≥50% diameter stenosis of the left main coronary artery or ≥70% diameter stenosis of the major epicardial vessels. However, a 40–70% stenosis may be functionally significant, i.e., may impede maximal coronary flow during stress. The functional significance of a fixed lesion depends not only on the luminal narrowing, but also on:¹

 The size of the territory supplied by the vessel: a 50% proximal LAD stenosis is often significant, whereas a 50% diagonal or distal LAD stenosis may not be. A larger flow translates into a larger percentage of flow drop across the stenosis.

 Lesion length, as resistance across a stenosis correlates with (viscosity × length)/radius⁴ (Poiseuille law).

 Amount of viable myocardium.

 Degree of coronary distensibility at the lesion site. As such, for the same luminal stenosis, a large necrotic core or positive remodeling increases the functional significance.²

Therefore, stress imaging may be useful to assess the functional significance of a borderline lesion. Also, in the cath lab, fractional flow reserve (FFR), i.e., the relative drop in flow across a lesion, may be invasively measured. FFR consists of assessing the pressure drop across a lesion using a coronary pressure wire; this pressure drop corresponds to a flow drop in patients with maximal microcirculatory hyperemia that exhausts autoregulation (flow = pressure/microvascular resistance). A flow drop ≥20%, i.e., FFR flow ratio ≤0.80, implies functional significance. In the FAME trial of multivessel PCI, 35% of 50–70% stenoses, 80% of 70–90% stenoses, and almost all stenoses >90% were functionally significant (FFR ≤0.80).³ This highlights the limitations of angiography even for stenoses of 70–90%.