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Thermodilution estimates blood flow from the temperature change produced by introducing fluid at a different temperature. It is a form of indicator dilution: the transported signal is a thermal deficit or excess rather than a dye concentration. An upstream catheter introduces the fluid and downstream temperature measurement records its dilution in flowing blood.

Two arrangements illustrate the principle. A finite injection produces a temperature-versus-time curve; a continuous infusion can produce an approximately steady downstream temperature change. A primary coronary-flow study used room-temperature saline, a 2.8-F infusion catheter and a 0.014-inch pressure/temperature guidewire, testing the method in five instrumented dogs and 35 patients.^[ref] The quantities observed were infusion rate, infused-fluid temperature at the catheter tip and distal mixed-blood temperature.

For an idealized finite-injection model, let $H$ be the injected thermal deficit in joules, $\kappa$ the volumetric heat capacity of the moving fluid, and $\Delta T(t)$ the downstream temperature deficit. Assume steady volume blood flow $Q$, complete passage of the signal through the detector, representative mixing and negligible heat transfer to other structures. Conservation of energy then givesThe time integral, rather than the maximum temperature change, determines this model's flow estimate. With stipulated $H=1$ J, $\kappa =4$ J/(mL K) and curve area $0.25$ K s, the inferred blood flow is $1$ mL/s, or $60$ mL/min. These values illustrate the balance, not a clinical protocol.

This ideal calculation exposes measurement assumptions. If only a fraction $\eta$ of the thermal signal reaches the detector, the observed curve area is reduced by that factor and the uncorrected estimate becomes $\hat{Q}=Q/\eta$. For example, $\eta =0.8$ would cause a $25\%$ overestimate in this model. Coronary thermodilution mixing assumptions similarly determine whether a local sensor reading represents the transported thermal signal. Accurate temperature measurement alone cannot establish those conditions.

Wilbert Aarnoudse is the named first author of a 2007 study validating direct volumetric blood flow measurement in coronary arteries by thermodilution. The method used continuous saline infusion, a 2.8-F infusion catheter and a 0.014-inch pressure/temperature-sensing guidewire. Validation included five dogs and 35 patients.^[ref]

The measurement uses a thermal tracer: a known inflow at a different temperature changes the downstream mixture temperature. An original two-stream model illustrates the principle. If blood and infused fluid have equal volumetric heat capacities, negligible heat exchange with the vessel and complete mixing,Here $Q_b,Q_i$ are volume flow rates, $T_b$ is baseline blood temperature, $T_i$ the infused-fluid temperature and $T_m$ the mixed temperature. This conservation calculation is not the clinical instrument's full calibration model.

For stipulated model inputs $Q_i=10$ mL/min, $T_b=37{}^{\circ }\mathrm{C}$, $T_i=25{}^{\circ }\mathrm{C}$ and $T_m=36.5{}^{\circ }\mathrm{C}$, the inferred blood flow is 230 mL/min. A small downstream temperature difference can therefore encode a larger flow, while also making the result sensitive to temperature offsets and incomplete mixing. Coronary thermodilution mixing assumptions matter independently of the precision of the temperature readout.

The primary study reports a canine comparison regression $Q_{\mathrm{thermo}}=0.73Q+42$ mL/min with $R^2=0.72$.^[ref] Association, calibration agreement and repeatability are different properties: a high association does not force slope one and intercept zero. The apparatus and validation comparisons make the credited work a measurement-method contribution, rather than a direct measurement of blood oxygen content or a general substitute for every coronary assessment.