Cardiac outputother invasive Dye dilution

Mixing of a given volume of indicator to an unknown volume of fluid allows calculation of this volume from the degree of indicator dilution. The time elapsed for the indicator to pass some distance in the cardiovascular system yields a cardiac output value, calculated as:

...where I is the amount of indicator injected, Cm is the mean concentration of the indicator and t is the total duration of the curve. The traditional dye dilution technique is to inject indocyanine green into a central vein followed by repeated sampling of arterial blood to enable construction of a time-concentration curve with a rapid upstroke and an exponential decay. Plotting the dye decay curve semilogarithmically and extrapolating values to the origin produces the cardiac output. The COLD-Pulsion device measures the concentration decay directly from an indwelling arterial probe, thus computing cardiac output. Alternatively, this device may use the thermodilution approach, avoiding pulmonary artery catheterisation. The LiDCO device is based on a similar principle using lithium as the 'dye'.

Advantages

Reasonably accurate, less invasive than pulmonary artery catheter placement.

Disadvantages

Invasive, recirculation of dye prevents multiple repeated measurements, lengthy, underestimates low output values. Inaccurate with moderate/ severe valvular regurgitation. Use of paralysing agents may interfere with lithium measurement.

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