Cardiac Output
The heart functions primarily to pump blood around the body, providing oxygen and nutrients. Cardiac output is the measure of how effectively the heart is doing.
Cardiac output = heart rate x stroke volume.
Regulation of Cardiac Output
The Frank-Starling Mechanism
The Frank-Starling Mechanism states that the heart will pump all blood returned to it, within physiological limits.
This is true during normal periods, during times of increased contractility, or during heart failure.
Determining Cardiac Output Using the Fick Method
The Fick Method of determining CO states that oxygen concumption is related to oxygen removal from blood as it flows, such that
O2 consumption (ml/min) = O2 removed (ml/min) x blood flow (ml/min)
or
O2 consumption (ml/min) = AV O 2 difference x CO
Measuring O2 consumption in expired air, and sampling arterial and venous blodd, will provide a measure of CO.
Some Info
If preload and therefore EDV is increased, the Frank-Starling mechanism will increase stroke volume such that ESV remains the same.
If afterload is increased while keeping preload the same, however, smaller stroke volumes and increased ESVs will ensue. There is a linear relationship between afterload and ESV termed end-systolic pressure volume relation (ESPVR).
Increasing inotropy results in increased stroke volume and decreased ESV, shifting the ESPVR line upwards.
Decreased contractility causes an increase in ESV, and increased venous return will therefore also increase EDV. The Frank-Starling Mechanism will kick in to regulate this by elevating stroke volume as much as possible.
Decreased compliance decreases EDV, leading to decreased stroke volume.