Methods of measuring cardiac output

Methods of measuring cardiac output

Cardiac output is the volume of blood pumped out by the heart in a minute. It is the product of stroke volume and heart rate. With a nominal stroke volume of 70 ml and a nominal heart rate of 72/min it comes to around 5 liters per minute. Cardiac output can vary widely depending on the need of the body. For a forty year old person, heart rate can rise up to 180/minute with exercise. Stroke volume also rises with exercise, but in untrained persons, the rise plateaus off at about 40% of the maximum oxygen consumption.

Non-invasive monitoring of cardiac output has been dealt with earlier. Hence this discussion will be mostly on conventional methods of measurement of cardiac output. The time old method is the direct Fick’s principle utilizing the measured oxygen consumption. It was described way back in 1870! It assumes that lungs does not consume oxygen and hence dividing the oxygen consumption by arteriovenous oxygen content difference will give the cardiac output. Mixed venous sample is obtained by pulmonary artery catheterization. Oxygen consumption was measured by breathing into a Douglass bag earlier. Now there are spirometer circuits with sensors which can measure it.

Partial carbon dioxide rebreathing is another related method used in commercially available cardiac output monitors. It uses intermittent carbon dioxide rebreathing. A disposable rebreathing loop with carbon dioxide infra-red light absorption sensor does the measurement. In this method, the cardiac output is calculated from the dilution of carbon dioxide in blood instead of oxygen in case of direct Fick’s principle [1].

Indicator dilution technique was another conventional method, using Indocyanine Green as the dye. A refined technique is pulse dye densitometry using Indocyanine Green [2]. After a bolus injection, Indocyanine Green distributes exclusively in the intravascular space. Hepatic elimination occurs with a half-life of 4.1 minutes. The machine auto-calibrates for repeated measurements. Blood concentration is measured with a pulse dye densitogram analyser which is connected to a clip device attached to the patient’s forefinger.

Other modalities using the principle of indicator dilution technique which are used commercially are pulmonary artery catheter thermodilution methods and transpulmonary lithium bolus dilution method. Pulmonary artery thermodilution methods are thermodilution with bolus injection of cold saline, continuous thermodilution method and transpulmonary thermodilution methods. All indicator dilution techniques assume complete mixing of indicator in blood with no loss of indicator between the place of injection and place of detection.

References

1. Geerts BF, Aarts LP, Jansen JR. Methods in pharmacology: measurement of cardiac output. Br J Clin Pharmacol. 2011 Mar;71(3):316-30. doi: 10.1111/j.1365-2125.2010.03798.x. PMID: 21284692; PMCID: PMC3045542.
2. Baulig W, Bernhard EO, Bettex D, Schmidlin D, Schmid ER. Cardiac output measurement by pulse dye densitometry in cardiac surgery. Anaesthesia. 2005 Oct;60(10):968-73. doi: 10.1111/j.1365-2044.2005.04296.x. PMID: 16179040.