 Thermodilution

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.