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CHAPTER 7
Physiologic Assessment and Guidance in the Cardiac Catheterization Laboratory
Allen Jeremias, Sukhjinder Nijjer, Justin Davies, and Carlo DiMario
Why to use physiology
The purpose of angiography and revascularization is to improve blood flow and thereby reduce myocardial ischemia. However, angiographic assessment of coronary stenosis has limited value in determining the degree of ischemia imposed and the viability of the subtended myocardium. Angiographic parameters also have limited predictive value of clinical outcome measures, and often, even mild and moderate stenoses can be important and underappreciated [1]. Furthermore, while many patients undergo non‐invasive functional testing prior to angiography, the findings can be at odds with the coronary appearances.
International guidelines recommend coronary physiology for angiographically moderate coronary stenoses (diameter stenosis 50–90%) where non‐invasive functional information is lacking [2,3]. Additionally, in many cases, invasive physiology provides additive information over and above non‐invasive functional testing. Intracoronary pressure wires offer an established and rapid solution for assessing the significance of coronary disease, while also assessing myocardial viability and offering insights into the best treatment strategies [4]. Pressure wires provide greater spatial localization of ischemia, not only to the vessel but also to the lesion level. This is particularly pertinent in multi‐vessel disease where relative perfusion changes can be matched leading to underestimation of ischemia.
There are complex pressure‐flow relationships that determine the measurable pressure gradient across a stenosis (Figures 7.1–7.3) [5], however these can be simplified to produce pressure‐only measurements that are simple to use in the cardiac catheterization laboratory. Multiple research studies have validated these pressure‐only measures of coronary physiology against non‐invasive ischemia testing [6–10] and major international outcomes studies have demonstrated that stenoses of all severities can be managed efficiently by pressure‐only indices such as instantaneous wave‐free ratio (iFR) or Fractional Flow Reserve (FFR) (Figure 7.4) [11–18]. These are now included in international guidelines both in North America and Europe [2,19–21]. Safe deferral of lesions that are non‐ischemic will reduce stent related events. In patients with confirmed ischemia, the pressure wire can determine interventional approaches and, following PCI, can be assessed to validate the hemodynamic success of the intervention [22,23] and identify those at higher risk of later events [24–38].
Figure 7.1 Behavior of resting and hyperemic flow in relation to stenosis severity. Resting flow is remarkably preserved despite worsening stenosis severity – upto 85% diameter stenosis by formal measurement. Hyperemic flow starts falling when stenoses are 30% but falls significantly once stenoses are 50%. Adapted from Gould, Lipscomb and Hamilton 1974.
Figure 7.2 The flow of blood across a coronary stenosis. Pressure gradients across stenoses are due to both viscous and separation losses. Pressure is lost owing to viscous friction along the stenosis (Poiseuille’s law). Flow also accelerates as it passess through the narrowed segment, causing pressure loss as pressure is converted into kinetic energy (Bernoulli’s law). Flow separation and the formation of eddies prevent complete pressure recovery at the exit of the stenosis. Measurement of intracoronary haemodynamics includes proximal perfusion pressure (Pa), coronary pressure and flow velocity distal to the stenosis (Pd and Vd, respectively), and the venous pressure (Pv), which is typically assumed to be negligible. ΔP is the difference between Pd and Pa. Normal diameter (Dn), stenosis diameter (Ds), proximal velocity (Vn), and stenosis velocity (Vs) are indicated. From van de Hoef et al (2013).
Figure 7.3 The curvilinear relationship between pressure and flow is unique for each stenosis. Pressure drop across a stenosis is determined by ΔP = Fv + Sv2