Thermodilution-Based Invasive Assessment of Absolute Coronary Blood Flow and Microvascular Resistance: Quantification of Microvascular (Dys)Function?

Abstract

During the last two decades, there has been a sharp increase in both interest and knowledge about the coronary microcirculation. Since these small vessels are not visible by the human eye, physiologic measurements should be used to characterize their function. The invasive methods presently used (coronary flow reserve (CFR) and index of microvascular resistance (IMR)) are operator-dependent and mandate the use of adenosine to induce hyperemia. In recent years, a new thermodilution-based method for measurement of absolute coronary blood flow and microvascular resistance has been proposed and initial procedural problems have been overcome. Presently, the technique is easy to perform using the Rayflow infusion catheter and the Coroventis software. The method is accurate, reproducible, and completely operator-independent. This method has been validated noninvasively against the current golden standard for flow assessment: Positron Emission Tomography-Computed Tomography (PET-CT). In addition, absolute flow and resistance measurements have proved to be safe, both periprocedurally and at long-term follow-up. With an increasing number of studies being performed, this method has great potential for better understanding and quantification of microvascular disease.

Figure 1

The evaluation of ANOCA patients in a flowchart. ANOCA: Angina with Nonobstructive Coronary Arteries, CT: computed tomography, FFR: fractional flow reserve, IMR: index of microvascular resistance, and CFR: coronary flow reserve.

Figure 2

Pressure wire and infusion catheter position. In the left panel, a normal circumflex artery is shown. The middle panel shows the pressure wire X, which is placed in the distal coronary artery, and the Rayflow catheter in the proximal artery. The Rayflow is visible by a radiopaque dot at the tip. In this position, the measurement starts. The right panel shows the position of the pressure wire when it is pulled back towards the inner side holes of the Rayflow catheter. Now the infusion temperature is assessed. T_i: the infusion temperature of the saline as measured at the infusion holes of the Rayflow catheter; T: the distal coronary temperature after complete mixing of blood and saline measured by the pressure wire after calibration to body temperature.

Figure 3

Difference in infusion between normal catheter and Rayflow. In the upper panel, an in vitro setup of a coronary artery is shown. The regular infusion catheter is placed in the “vessel” filled with saline and the infusate is dyed black with ink. It is visible that there is incomplete and variable mixing in case of the regular infusion catheter (arrow indicates tip of infusion catheter). In the lower panel, the Rayflow is used. Here immediate and complete mixing is shown, starting directly at the infusion holes (indicated by the arrow).

Figure 4

Measurement screen, step-by-step. This figure shows the software screen during the flow and resistance measurement. Panel 1: the temperature measured by the pressure wire is zeroed, which means it is calibrated at body temperature. Panel 2: the infusion of saline starts at 20 ml/min in this case; the fast decrease in temperature is visible here. Panel 3: steady-state maximum hyperemia has been reached here and T is recorded. Panel 4: the pressure wire is pulled back to the tip of the Rayflow to measure the infusion temperature of the saline. This pullback is indicated by the fast decrease in temperature and sudden increase in distal pressure. Panel 5: the infusion temperature measurement reaches steady state and T_i is calculated. Panel 6: the infusion pump is stopped and the temperature of the blood reaches starting values within 30 seconds. Further, the timeline in the figure indicates the time in seconds. Here, it can be appreciated that it takes approximately 20 seconds for hyperemia to occur. Red tracing: aortic pressure; green tracing: distal coronary pressure; blue tracing: coronary temperature. The numerical values of all relevant parameters are displayed in the right side of the Coroventis screen. T_i: the infusion temperature of the saline as measured at the infusion holes of the Rayflow catheter; T: the distal coronary temperature after complete mixing of blood and saline measured by the pressure wire after calibration to body temperature.

Figure 5

Equipment. All equipment needed for the thermodilution-based assessment of absolute blood flow and resistance is displayed here. The Rayflow catheter is shown in the upper panel, clearly indicating the 4 infusion holes at 0°, 90°, 180°, and 270°. The middle panel shows the infusion pump. The lower panel shows the pressure wire with the necessary software.