CARDIAC OUTPUT COMPUTER

Typical Number in Hospital: 3

Cost Bands: 4

References: 3,9

The rate at which the heart pumps blood can be calculated from the concentration which a known quantity of an inert indicator reaches when injected into the blood stream close to the heart. The concentration of a dye (usually indocyanine green - ICG) can be detected in the pulmonary artery (i.e. leaving the heart) when it has been injected through a catheter opening into the right atrium. The concentration rises and then falls, and the cardiac output is calculated by computer from this curve using information about the quantity of dye injected and the calibration constant of the densitometer. This is an invasive procedure since the catheter must be fed up to the heart through an artery, usually the brachial artery. The correct position of the injection catheter eye and the collecting eye may be identified using X-ray screening (during cardiac catheterization) or by monitoring the pressure waveform at each point through the catheter itself.

An alternative to using a dye is to inject a bolus of cold saline and calculate the cardiac output from the dip in temperature sensed in the pulmonary artery. The temperature profile will follow the same course as the dye concentration. Thermal dilution has the advantage that the procedure may be repeated many times without loading the blood with toxic material, and is therefore sometimes applied in the intensive care unit.

The apparatus consists of an injection pump (usually), a special catheter which may be expensive and intended for single use only, and the computer itself.

Another method of calculating cardiac output during cardiac catheterization which does not involve an injection is the Fick method. In this case the indicator is oxygen. Consumption is measured by a spirometer in which exhaled carbon dioxide is absorbed to allow calculation of the quantity of oxygen used (a typical figure is 0.25 l/min). Samples of blood are taken through catheters, one in an artery of the arm or leg, and another in the pulmonary artery. The flow rate is then calculated from the formula:

F = (dm/dt) / (Ca - Cv)

where F is the blood flow in litres/minute, dm/dt is the consumption of oxygen in litres/minute, Ca is the arterial concentration of oxygen in litres/litre, and Cv is the venous concentration of oxygen in litres/litre.

Blood oxygen concentration is normally calculated by an optical method (see Oximeter).

There is also a method of calculating cardiac output using electrical impedance measurements on the chest but this has not been widely accepted.