Question

Physiology question about cardiac output

There are several ways how to measure cardiac output in the clinical setting. Research and describe three methods and discuss potential drawbacks of each. Finally, propose a new method to measure cardiac output (e.g. via imaging method, invasive tracing, using physiologic markers)

Answer 1

There are several methods used to measure cardiac output. They are classified as invasive and non-invasive methods. Invasive methods: Fick method: It is bsed on the principle that cardiac output is a product of stroke volume and the arteriovenous concentration difference (a.Vo). The oxygen uptake in the lungs is the product of blood flow through the lungs and the arteriovenous oxygen content difference. Therefore, cardiac output is calculated from the circulation in lungs as follows: CO- Cao-C where Vo- oxygen consumption by lungs Cao. C,o. - arteriovenous differ ence in oxygen conc. The arteiovenous oxygen difference is computed by measuringoxygen conc. Insamples. Thermodilution method: In this method, a special themistortipped catheter is inserted into the central artery from a central vein. A cold solution of nomal saline at 0 degree C is injected into the right atrium from the front end of catheter. The solution causes a drop in blood temperature which is measured b the themistor. The decrease n the temperature is nversely proportion to the dilution of the injectate. Thermal dilution method is considered a gold standard in clinical settings.

Non-invasive method: Partial CO2 rebreathing: The technique also relies on measurement of cardiac output using Fick's law. CO2 is taken as marker gas. It uses a monitor called NICO monitor which consists of a CO2 sensor, a disposable airflow sensor, and a pulse gxymeter. From the minute ventilation, CO2 concentration is calculated. The arterial carbon dioxide concentration is estimated from end-tidal CO2. The principle behind calculation of cardiac output is CO- CvCO2-CaCo vCO2 and CaCO2 re CO2 consumpti on, venous C02 concentration and areterial CO2 concentration. Electrical bioimpedance method: Low level electricity is conducted though the body thuds,j transmitted. Two sets of electrodes are placed onneckbase aonopposite side and two pairs are placed at the level of xiphoid junction. Each pair of electrodes represent a transmitting and a sensing electrode With the said electrodes, low-level electricity is measured. The derivative of dZ dt of the impedance waveform is related directly/linearly to aortic blood flow. Changes in impedance are due to altered stroke volume. Therefore, cardiac output is calculated from stroke volum e using the following fomula whereSV stroke volume ρ-resistivity of blood L = mean distance between the inner electrodes Zo-basal thoracic impedance dZjdt-the maxim um value of the first derivative during systole