Cardiac index ($\mathrm{CI}=Q/\mathrm{BSA}$)

Cardiac index is cardiac output divided by body surface area:When $Q$ is in L/min and area in m${}^2$, its units are L/(min m${}^2$). Despite the word index, it is not dimensionless. A primary comparison of thermodilution and estimated Fick principle measurements explicitly indexed output to body surface area for its analyses.^[ref]

An original comparison shows how normalization can change ranking. Output 5 L/min with area 2 m${}^2$ gives 2.5 L/(min m${}^2$). Output 4 L/min with area 1.5 m${}^2$ gives about 2.67 L/(min m${}^2$). The second output is lower in absolute terms but higher after this area normalization. Neither statement contradicts the other.

The denominator is often estimated rather than directly measured. If the Mosteller formula is chosen, with numerical height $h_{\mathrm{cm}}$ in centimetres and body mass $m_{\mathrm{kg}}$ in kilograms, thenThe unit-specific area equation is documented in the official VA technical manual.^[ref] Choosing an area model is consequently part of defining the reported index.

For small input errors, $\delta \mathrm{CI}/\mathrm{CI}\simeq \delta Q/Q-\delta \mathrm{BSA}/\mathrm{BSA}$. An overestimated area can lower the index without changing the underlying output measurement. Area normalization also does not by itself account for every difference in metabolism or body composition. The quantity is a standardized way of expressing flow relative to one body-size measure, not an additional independent observation of how much blood the heart pumps.