The left circumflex artery is instrumented by a perivascular (perivasc) flow probe and occluder. A 2.8-F infusion catheter is advanced over a 0.014-inch pressure/temperature (temp) sensor-tipped guidewire through a Y-connector (Y₁) and positioned with its tip just proximal to the flow probe and occluder. The infusion catheter is connected to an infusion pump by a second Y-connector (Y₂), enabling continuous infusion of saline at room temperature (8 to 25 ml/min). The sensor-tipped guidewire is connected to the interface (Radi-Analyzer) as routinely done in coronary pressure measurement, and distal coronary pressure (P_d) and temperature (T) are displayed on the interface. The aortic pressure (P_a), measured at the tip of the guiding catheter, is recorded by a regular pressure transducer and displayed on the interface. (Inset) Set-up in humans was identical to that in dogs, with the exception of the perivascular flow probe and occluder. The tip of the infusion catheter is positioned proximal to the stenosis (if present) and with its side holes in a segment without major side branches.

In the upper part of the screen, aortic pressure (P_a) and distal coronary pressure (P_d) are shown. The blue curve in the lower part of the screen shows intracoronary temperature. On the left, blood temperature at steady-state hyperemia (T_b) is set to 0. Next, infusion of saline is started at a rate of 20 ml/min (yellow arrow). During steady-state saline infusion, temperature in the coronary artery (T) decreases by 0.85°C. Next, the sensor is pulled back to measure blood temperature to the tip of the infusion catheter (T_i) (_*), which is 5.57°C below initial blood temperature. Therefore, calculated coronary blood flow = 20 x (5.57/0.85) x 1.08 = 147 ml/min (true flow measured by the flow probe was 154 ml/min in this case).

Circles indicate a proximal (prox) sensor position, squares indicate a more distal (dist) sensor position (3 and 6 cm from the tip of the infusion [inf] catheter, respectively). Closed points indicate a high infusion rate of saline (15 to 25 ml/min), open points indicate a low infusion rate (8 to 15 ml/min). The line indicates the line of identity. In Dog 1, only part of the measurements could be performed because of a rather severe stenosis at the site of the instrumentation. Q = true blood flow; Q_thermo = blood flow as calculated by thermodilution.

(A) Q_thermo versus Q. (B) Second versus first measurement. (C) High versus low saline infusion rate. (D) Distal versus proximal location of sensor. Abbreviations as in Figure 3.

The tip of the infusion catheter is indicated by the long arrow. The side holes for saline infusion are located proximal to this tip. The sensor of the pressure wire (for pressure and temperature measurement) is indicated by the bold arrow.

In the upper part of the screen, aortic and intracoronary pressures at steady-state hyperemia are shown: aortic pressure (P_a) = 68 mm Hg, distal pressure (P_d) = 59 mm Hg, and fractional flow reserve (FFR) = 0.86. The blue curve in the lower part of the screen shows intracoronary temperature. On the left side of the lower panel, blood temperature (T_b) is set to 0 when steady-state hyperemia is achieved. Approximately 30 s later, the infusion of saline at room temperature is started with an infusion rate (Q_i) of 25 ml/min. Consequently, distal blood temperature decreases and quickly reaches a plateau at T = –0.97°C. Steady-state distal temperature (T) is recorded for another 30 s, after which the sensor is pulled back into the infusion catheter (_*) to measure the temperature of the infused saline as it enters the coronary artery at the tip of the infusion catheter: T_i = –7.1°C. The absolute blood flow in this coronary artery can now easily be calculated by: Q_b = 25 x (–7.1/–0.97) x 1.08 = 198 ml/min.

In the upper part of the screen, intracoronary pressures at steady-state hyperemia are shown: aortic pressure (P_a) = 101, distal pressure (P_d) = 70 and fractional flow reserve (FFR) = 0.69. The blue curve in the lower part of the screen shows intracoronary temperature. On the left side of the lower panel, blood temperature (T_b) is set to 0 when steady-state hyperemia is achieved. Thirty seconds later, the infusion of saline at room temperature is started with an infusion rate (Q_i) of 15 ml/min. Consequently, distal blood temperature decreases and reaches a plateau at T = –0.90°C, which is recorded for another 30 s. Thereafter, the sensor is pulled back into the infusion catheter (_*), where the temperature of the infused saline is measured as it enters the coronary artery: T_i = –4.5°C. The absolute blood flow in this coronary artery can now be calculated by: Q_b = 15 x (–4.5/–0.90) x 1.08 = 81 ml/min.

(A) Ratio of thermodilution flow (Q_thermo) before and after percutaneous coronary intervention versus ratio of coronary fractional flow reserve (FFR_cor) before and after percutaneous coronary intervention, and corresponding Bland-Altman diagram of the relative difference. (B) Reproducibility of all measurements. (C) High versus low infusion rate. (D) Distal versus proximal location of sensor.