Coronary thermodilution mixing assumptions

Coronary thermodilution mixing assumptions are the conditions that make a downstream temperature reading representative of the mixture used to infer coronary blood flow. They include adequate mixing of infused fluid and blood, appropriate volumetric heat capacity values, known infusion rate and temperature, and sufficiently small unmodeled heat transfer between the fluid and surrounding structures.

The apparatus places these assumptions in a concrete setting. In a primary continuous-infusion study, a catheter supplied room-temperature saline while a pressure/temperature guidewire measured distal temperature; the infused-fluid temperature was measured at the catheter tip.^[ref] Temperature at the tip matters because fluid can exchange heat before entering the measured blood flow.

An original two-stream model makes the bookkeeping explicit. Let native blood flow be $Q_b$, infusion rate $Q_i$, baseline blood temperature $T_b$, infused-fluid temperature $T_i$ and mixed temperature $T_m$. Assume equal constant volumetric heat capacity, no external heat transfer and steady complete mixing. Conservation of energy givesand henceNative flow and total downstream mixture flow have different numerators. With constructed $Q_i=20$ mL/min, $T_b=37{}^{\circ }\mathrm{C}$, $T_i=27{}^{\circ }\mathrm{C}$ and $T_m=35{}^{\circ }\mathrm{C}$, the model gives $Q_b=80$ mL/min and $Q_{\mathrm{mix}}=100$ mL/min. This equal-capacity balance is an explanatory model, not the instrument's full calibration algorithm.

Incomplete mixing changes what the sensor observes. If a sensor sampled an unusually cold filament and read $34{}^{\circ }\mathrm{C}$ for the same other inputs, the formula would return about $46.7$ mL/min instead of 80. Repeating that local reading precisely would not correct its lack of representativeness. More generally, a mixed temperature is weighted by transported heat capacity, rather than being an unweighted spatial mean.

Finally, $Q_b$ becomes sensitive to small temperature errors when $T_b-T_m$ is small. At fixed $Q_i,T_b,T_i$,For the constructed example this is $50$ mL/(min K). The derivative describes local sensitivity within the model; it does not estimate errors from incomplete mixing or heat loss. Those require separate physical evidence.