Define cardiac index and how it is calculated.

• A. Cardiac output per body surface area; CI = CO / BSA
• B. Stroke volume index; CI = SV / BSA
• C. Pulmonary capillary wedge pressure divided by BSA
• D. Mean arterial pressure multiplied by BSA

Answer: (A)

Cardiac index shows how much blood the heart pumps per minute relative to body size, so you can compare heart performance across people of different statures. It’s defined as cardiac output divided by body surface area: CI = CO / BSA. Since cardiac output is the volume per minute, CI is expressed as L/min per square meter (or mL/min per m^2 if you use mL for CO). Normal values are roughly 2.5–4.0 L/min per m^2.

Body surface area is typically estimated with formulas like Mosteller’s, which use height and weight. The other options don’t reflect this size-adjusted measure: one describes a stroke-volume-based index, another cites a preload parameter, and the last mixes a pressure measure with body size without reflecting actual cardiac performance per unit area.