Abstract
We present a case of an intermediate right coronary artery lesion with a fractional flow reserve (FFR) of 0.93 despite a myocardial perfusion scan demonstrating reversible ischemia in the inferior wall. An assessment of microvascular resistance was performed using the index of microcirculatory resistance (IMR). This was consistent with elevated resistance and may explain the discordance between the high FFR result, the positive perfusion study, and the lesion appearance on angiography. FFR assumes stable and minimal resistance at hyperemia; however, there may be situations where this does not occur, contributing to discrepant values. An understanding of the underlying physiological principles that underpin FFR is required for all physicians using this increasingly utilized technique.
1
Introduction
Following Gruentzigs’ first percutaneous balloon angioplasty in 1977, the growth of percutaneous coronary interventions (PCI) with the use of stents has been exponential. Figures from Europe show the number of stents implanted increased from 3000 in 1992 to 777,000 in 2004 . The discrepancies of coronary angiography for determining the hemodynamic importance of an epicardial stenosis are well established with recent data from the FAME study further corroborating this discordance showing that in lesions assessed to be between 71% and 90% in severity, 20% were not functionally significant . The inadequacies of coronary angiography have thus led to a rapid development of technologies, both invasive and non-invasive, in order to qualify a hemodynamically significant lesion.
2
Fractional flow reserve
Fractional flow reserve (FFR; refer to Fig. 1 ) is one such technique that is based on the ratio of maximal myocardial blood flow in the presence of a stenosis to blood flow if theoretically there was no stenosis present . This can be derived with the use of either the Volcano WaveWire (Volcano, Inc., Rancho Cordova, CA, USA) or with the coronary PressureWire (St. Jude Medical, Minnesota, USA). The basic physiological principle utilized by both wires is that coronary pressure is equal to aortic pressure in the absence of a stenosis and that microvascular resistance is constant and minimal during hyperemia induced by intravenous or intracoronary adenosine . An extrapolation of Ohm’s law states that coronary flow is equal to change in pressure divided by resistance. Change in pressure reflects coronary arterial pressure minus outflow pressure for which we substitute central venous pressure. If we assume that resistance is minimal and venous pressure is negligible at hyperemia, then coronary flow will be proportional to coronary pressure. The pressure distal to a stenosis will therefore represent flow in the presence of a stenosis, whereas aortic pressure represents flow in the absence of a stenosis. This is shown graphically in Fig. 1 . The use of an FFR-guided stenting strategy is well established and associated with improved outcomes in both single and multivessel disease, predicting prognosis following PCI when compared to a conventional angiography-guided strategy .
2
Fractional flow reserve
Fractional flow reserve (FFR; refer to Fig. 1 ) is one such technique that is based on the ratio of maximal myocardial blood flow in the presence of a stenosis to blood flow if theoretically there was no stenosis present . This can be derived with the use of either the Volcano WaveWire (Volcano, Inc., Rancho Cordova, CA, USA) or with the coronary PressureWire (St. Jude Medical, Minnesota, USA). The basic physiological principle utilized by both wires is that coronary pressure is equal to aortic pressure in the absence of a stenosis and that microvascular resistance is constant and minimal during hyperemia induced by intravenous or intracoronary adenosine . An extrapolation of Ohm’s law states that coronary flow is equal to change in pressure divided by resistance. Change in pressure reflects coronary arterial pressure minus outflow pressure for which we substitute central venous pressure. If we assume that resistance is minimal and venous pressure is negligible at hyperemia, then coronary flow will be proportional to coronary pressure. The pressure distal to a stenosis will therefore represent flow in the presence of a stenosis, whereas aortic pressure represents flow in the absence of a stenosis. This is shown graphically in Fig. 1 . The use of an FFR-guided stenting strategy is well established and associated with improved outcomes in both single and multivessel disease, predicting prognosis following PCI when compared to a conventional angiography-guided strategy .
3
Index of microcirculatory resistance
The primary wire used in most laboratories for measuring FFR is the coronary PressureWire. This wire is unique in that it has a distal pressure sensor to measure distal coronary pressure and also a thermistor that is able to detect changes in temperature. Utilizing the pressure wire, DeBruyne et al. demonstrated, using the thermodilution principle, that a surrogate of coronary blood flow could be derived following the intracoronary injection of 3 ml of saline. The inverse of this transit time of saline had an excellent correlation with absolute coronary flow in an in vitro model and Doppler-derived measures in animals . Using this specific property of the pressure wire, and again based upon Ohm’s law, we are able to derive the minimal microcirculatory resistance (IMR) assuming maximal hyperemia induced by intravenous adenosine. As resistance is equal to the pressure change across the circulation divided by coronary flow, and as we are able to derive both measures utilizing the coronary pressure wire, we can calculate minimal microvascular resistance. More simply, multiplying distal coronary pressure by the mean transit time of 3 ml of saline can derive the index of microvascular resistance . Fearon et al. validated this technique against true microvascular resistance in an animal model and found good correlation between IMR and true microcirculatory resistance . IMR has been shown to correlate with viability in patients presenting with ST-segment myocardial infarction and to be a more reliable marker of microcirculatory integrity than other established measures .
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