The TFC is a simple clinical tool for microcirculation assessment and was suggested by C. Michael Gibson [4]. It is defined as the number of cineframes required for contrast to reach a standardized distal coronary landmark in the culprit vessel. The first frame used for TFC is the first frame in which dye fully enters the artery (Fig. 30.1) [4]. The last frame counted is that in which contrast enters a distal landmark. Full opacification of the branch is not required. The number is expressed based upon a cinefilming rate of 30 frames/s. Therefore, a frame count of 30 would mean that 1 s was required for dye to traverse the artery. The TIMI frame count is counted using an electronic frame counter.

These landmarks are as follows: the distal bifurcation in the left anterior descending artery, the distal branch of the lateral left ventricular wall artery furthest from the coronary ostium in the circumflex system, and the first branch of the posterolateral artery in the right coronary artery [4]. In general, the TFC for the left anterior descending (LAD) and the circumflex arteries is assessed in a right anterior oblique projection with caudal angulation (RAO caudal view) and the TFC for the right coronary artery in a left anterior oblique projection with cranial angulation (LAO cranial view).

The TIMI frame count was not significantly affected by increasing the dye injection rate or by changing catheter size. However, the use of intracoronary nitrates significantly increased the TIMI frame count in normal and diseased coronary arteries. In addition, TIMI frame count varies with body size, systemic arterial pressure, age, or gender [6]. The impact of the mechanical force of injection may also impact on TIMI frame count although it was small and insignificant.

CFR is the magnitude of the increase in coronary flow that can be measured as the ratio of coronary flow during maximal microvascular dilation and basal coronary flow. CFR can be measured by intracoronary Doppler wire or thermodilution technique. Using intracoronary Doppler wire, CFR can be calculated as the ratio of hyperemic average peak velocity (hAPV) during maximal hyperemia induced by adenosine or others and baseline average peak velocity (bAPV) (hAPV/bAPV) (Fig. 30.2).

Besides, by thermodilution technique , it is possible to measure pressure and to estimate coronary artery flow simultaneously with a single pressure-temperature sensor-tipped coronary wire (PressureWire Certus, St. Jude Medical, MN, USA) [9, 10]. For measurement of CFR, coronary flow under basal conditions was determined by intracoronary administration of 3 ml of room-temperature saline three times in succession manually (3 ml/s). Maximal hyperemia was then induced, and three additional room temperature saline boluses of 3 ml were administered intracoronarily to determine peak coronary flow presented as peak mean transit time. In this method, the mean transit time (Tmn) of room-temperature saline injected down a coronary artery can be determined and has been shown to correlate inversely with absolute flow [11]. CFR was calculated based on the ratio of the mean transit times during hyperemia and at baseline (Fig. 30.3). A thermodilution-based CFR can be derived that has been shown to correlate well with Doppler velocity wire-derived CFR both in their experimental model and in humans [9, 10]. A CFR less than 2.5 was considered to be abnormal [12, 13].

CFR interrogates the entire coronary system, including the epicardial artery and microcirculation. A normal CFR indicates that epicardial and minimally achievable microvascular bed resistances are low and normal. However, CFR is unable to differentiate which component is affected when it is abnormal. Furthermore, CFR was largely affected by the baseline coronary flow velocity (CFV) associated with heart rate, preload, contractility, and after percutaneous coronary intervention (PCI) [14, 15]. Therefore, using a CFR to evaluate the microcirculation has a few limitations.

With recent technological advances, coronary microcirculation can be measured simultaneously with the same pressure wire by calculating the IMR using this thermodilution technique . By using this wire and modified software, it is able to calculate the mean transit time (Tmn) of room-temperature saline injected down a coronary artery. The inverse of the hyperemic Tmn has been shown to correlate with absolute flow. For measure of IMR, a 0.014 coronary temperature and pressure-sensing guidewire (PressureWire Certus, St. Jude Medical, MN, USA) was calibrated for the pressure recording. It was then equalized with aortic pressure (Pa) in the guiding catheter. The tip pressure sensor was advanced across the stented segment and beyond the mid-to-distal portion of the culprit vessel. In a simplified form, assuming coronary flow and myocardial flow are equal and that the contribution of collateral flow is negligible, then IMR was calculated as mean distal coronary pressure multiplied by the thermodilution-derived hyperemic Tmn (Fig. 30.4) [11].