Pharmacological vasodilatation in the assessment of pressure-derived collateral flow index

Left Ventricular Unloading Increases the Coronary Collateral Flow Index Before Reperfusion and Reduces Infarct Size in a Swine Model of Acute Myocardial Infarction

Background Unloading the left ventricle and delaying reperfusion reduces infarct size in preclinical models of acute myocardial infarction. We hypothesized that a potential explanation for this effect is that left ventricular (LV) unloading before reperfusion increases collateral blood flow to ischemic myocardium. Methods and Results Acute myocardial infarction was induced by balloon occlusion of the left anterior descending artery for 120 minutes in adult swine, followed by reperfusion for 180 minutes. After 90 minutes of occlusion, animals were assigned to 30 minutes of continued occlusion (n=6) or to 30 minutes of support with either an Impella CP (n=4) or venoarterial extracorporeal membrane oxygenation (n=5) with persistent occlusion. The primary end point was measures of microcirculatory blood flow including the collateral flow index (CFI) during left anterior descending artery occlusion as (P_w-RA)/(P_a-RA), where P_a, P_w, and RA are aortic, coronary wedge, and right atrial pressure, respectively. Infarct size was quantified using triphenyltetrazolium chloride. Compared with continued occlusion, Impella, not venoarterial extracorporeal membrane oxygenation, reduced infarct size relative to the area at risk. Before reperfusion, Impella reduced LV stroke work by 25% and increased the CFI by 75%, but venoarterial extracorporeal membrane oxygenation did not. Among all groups, the change in CFI between 90 and 120 minutes correlated inversely with the change in LV stroke work (r²=0.44, P=0.01) and infarct size (r²=0.41, P=0.02). Conclusions We report for the first time that 30 minutes of LV unloading during coronary occlusion increases the CFI, which correlates inversely with LV stroke work and infarct size. Venoarterial extracorporeal membrane oxygenation failed to increase the CFI and did not reduce infarct size.

Pathophysiology of coronary collaterals

While the existence of structural adaptation of coronary anastomoses is undisputed, the potential of coronary collaterals to be capable of functional adaptation has been questioned. For many years, collateral vessels were thought to be rigid tubes allowing only limited blood flow governed by the pressure gradient across them. This concept was consistent with the notion that although collaterals could provide adequate blood flow to maintain resting levels, they would be unable to increase blood flow sufficiently in situations of increased myocardial oxygen demand.

However, more recent studies have demonstrated the capability of the collateral circulation to deliver sufficient blood flow even during exertion or pharmacologic stress. Moreover, it has been shown that increases in collateral flow could be attributed directly to collateral vasomotion.

This review summarizes the pathophysiology of the coronary collateral circulation, ie the functional adapation of coronary collaterals to acute alterations in the coronary circulation.

Quantitative Assessment of Myocardial Blood Flow with SPECT

The quantitative assessment of myocardial blood flow (MBF) and coronary flow reserve (CFR) may be useful for the functional evaluation of coronary artery disease, allowing judgment of its severity, tracking of disease progression, and evaluation of the anti-ischemic efficacy of therapeutic strategies. Quantitative estimates of myocardial perfusion and CFR can be derived from single-photon emission computed tomography (SPECT) myocardial perfusion images by use of equipment, tracers, and techniques that are available in most nuclear cardiology laboratories. However, this method underestimates CFR, particularly at high flow rates. The recent introduction of cardiac-dedicated gamma cameras with solid-state detectors provides very fast perfusion imaging with improved resolution, allowing fast acquisition of serial dynamic images during the first pass of a flow agent. This new technology holds great promise for MBF and CFR quantification with dynamic SPECT. Future studies will clarify the effectiveness of dynamic SPECT flow imaging.

Assessment of coronary flow reserve using single photon emission computed tomography with technetium 99m-labeled tracers

The quantitative assessment of coronary flow reserve (CFR) may be useful for the functional evaluation of coronary artery disease (CAD), allowing judgment of its severity, tracking of disease progression, and evaluation of the anti-ischemic efficacy of therapeutic strategies. Invasive techniques, such as intracoronary Doppler ultrasound and the pressure-derived method, which directly assess CFR velocity and fractional flow reserve, have been used for the evaluation of the physiologic significance of coronary lesions. Considerable progress has been made in the improvement of technologies directed toward the noninvasive quantification of myocardial blood flow and CFR. Positron emission tomography has emerged as an accurate technique to quantify CFR. The absolute measurements obtained with this noninvasive approach have been widely validated. Nevertheless, it has not been applied to routine studies because of its high cost and complexity. On the other hand, technetium 99m-labeled tracers have been largely used for the evaluation of myocardial perfusion with single photon emission computed tomography (SPECT) imaging in patients with suspected or known CAD. Recently, attempts to estimate CFR with SPECT tracers have been made to obtain, with noninvasive methods, data for quantitative functional assessment of CAD. This review analyzes the relative merit and limitations of CFR measurements by cardiac SPECT imaging with Tc-99m-labeled tracers and describes the potential clinical applications of this technique.

Coronary Collaterals Remain Recruitable After Percutaneous Intervention

Background— Rapid loss of collateral support has been reported after percutaneous coronary intervention (PCI), leaving the myocardium susceptible to subsequent infarction. However, well-developed collaterals have been found in normal hearts, suggesting that collaterals exist even in the absence of an ischemic stimulus. We assessed the plasticity and determinants of collateral supply after PCI.

Methods and Results— Collateral flow index (CFI) was calculated in 60 patients as (P w −P v )/(P a −P v ) by measurement of aortic (P a ), central venous (P v ), and coronary wedge (P w ) pressures. CFI was reassessed during transient balloon occlusion 5 minutes and 24 hours after PCI in the first 29 patients and at 6 months in the subsequent 25 patients. We also evaluated the relationship between collateral supply, lesion characteristics, and circulating hemopoietic cells numbers before and after successful PCI. CFI at baseline was 0.23±0.10, with no change 5 minutes and 1 day later (0.21±0.12, P =0.62; and 0.22±0.11, P =0.96, respectively). At 6 months, CFI was 0.14±0.07 or 63±27% of the baseline value ( P <0.001). CFI was proportional to severity of the coronary lesion at baseline ( r =0.63, P <0.0001) but not 6 months after PCI ( r =−0.04, P =0.87). The number of circulating CD133 + and CD34 + cells was associated with CFI 6 months after PCI (CD133, r =0.59, P =0.035; CD34, r =0.63, P =0.037).

Conclusions— Coronary collateral flow remains undiminished for at least 24 hours after successful PCI. Functional collateral support subsequently declines but does not regress completely.