Cardiac outputthermodilution

Thermodilution is the technique utilised by the pulmonary artery catheter to measure right ventricular cardiac output. The principle is a modification of the Fick principle whereby a bolus of cooled 5% glucose is injected through the proximal lumen into the central circulation (right atrium) and the temperature change is detected by a thermistor at the catheter tip, some 30cm distal. A modification of the Hamilton-Stewart equation, utilising the volume, temperature and specific heat of the injectate, enables cardiac output to be calculated by an on-line computer from a curve measuring temperature change in the pulmonary artery.

Continuous thermodilution measurement uses a modified catheter that emits heat pulses from a thermal filament lying within the right ventricle and right atrium, 14-25cm from the tip. 7.5W of heat are added to the blood intermittently every 30-60s and these temperature changes are measured by a thermistor 4cm from the tip. Though updated frequently, the cardiac output displayed is usually an average of the previous 3-6min.

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