CO can be measured through noninvasive and invasive methods. The gold standard CO monitor remains the PAC. Cardiogenic shock is an indication for placement of PAC to assess trends and optimization of CO medically or by mechanical support. The PAC can measure CO using thermodilution or the Fick principle. Thermodilution involves rapid injection of a crystalloid bolus at a known temperature into the superior vena cava and measuring temperature change at the tip of the PAC. The temperature change in the PA has an inverse relationship with CO. The Fick equation measures CO based on oxygen consumption. The Fick equation states that the ratio of oxygen consumption divided by the difference between arterial and venous oxygen content is equal to CO. The Fick equation requires measurement of oxygen consumption which is not possible in a critically ill patient and leads to significant error when estimates are employed. Thermodilution remains the primary method of determining CO.

The most common noninvasive methods include transthoracic echocardiography (TTE) and automated peripheral CO monitors. Doppler-derived CO by TTE usually requires calculation by a trained echocardiographer. CO can vary depending on two-dimensional or three-dimensional echocardiographic techniques, and the values obtained may not be interchangeable with PAC thermodilution. However, they can be useful to determine trends.

Peripheral CO monitors utilize bioimpedance (NCCom), bioreactance (NICOM), partial CO₂ rebreathing (INNOCOR), pulse wave transit time (EsCCO), or ultrasonic (USCOM) techniques to calculate CO. Peripheral CO monitors are not the standard of care in cardiogenic shock as they have a wide array of potential errors, including being rhythm and rate dependent, and are affected by patient body habitus and edema (NICOM). In several trials these continuous noninvasive monitors have shown poor agreement with PAC measurements. However, the trend in CO is still considered useful and often employed in patients where the risks of invasive monitors outweigh the potential benefits.

References

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