Conventionally, measurement of cardiac output was done invasively. Initially with dye dilution techniques and later by thermodilution techniques. Both needed pulmonary artery catheterization, which in turn was likely to cause morbidity if used for continuous monitoring. Almost all non-invasive cardiac imaging modalities like echocardiography, computed tomography, magnetic resonance imaging and nuclear imaging can measure cardiac output. But these are not suitable for bedside monitoring, though echocardiography may be used for intermittent bedside assessment.
Assessment of cardiac output though desirable, was most often time consuming and hence often it was replaced by blood pressure monitoring in the intensive care setting. An adequate blood pressure need not mean good tissue perfusion. Hence the role of non-invasive cardiac output monitoring.
Several technologies with varying accuracy exist for non-invasive or minimally invasive monitoring of cardiac output in the intensive care unit, emergency department and operation theatre. Some of the methods are pulse wave transit time, non-invasive pulse contour analysis, thoracic electrical bioimpedance, esophageal Doppler and partial carbon dioxide rebreathing .
Though non-invasive monitoring is more convenient, studies have not shown equivalence with invasive monitoring. A meta-analysis by Peyton PJ et al showed that the percentage of error ranged from 41.3% to 44.5% . This was much higher than the acceptable 30% limit.
Bioimpedance method uses a low amplitude high frequency current passed across the chest and picks up the voltage developed to calculate the impedance. Thoracic impedance decreases when there is more blood in the aorta as blood contains water and iron. Partial carbon dioxide rebreathing technique uses a mask with facial seal. An extra loop of ventilatory circuit creates transient partial carbon dioxide rebreathing. Cardiac output is calculated using end tidal carbon dioxide values using Fick principle.
Pulse wave transit time is estimated from the time interval between R wave on the ECG and the peripheral pulse. Pulse wave transmit time is inversely correlated to the stroke volume. Special algorithms are used to calculate cardiac output from pulse wave transit time. Esophageal Doppler uses velocity time integral of aortic flow and aortic valve cross section area as in continuity equation to calculate the cardiac output . This measurement is usually not continuous, but intermittent. Aortic valve area can be either predetermined or calculated using age and body weight.
Each of the techniques used has its own limitations in the intensive care setting with highly variable cardiac output. Most devices have been standardized using measurements in those with near normal cardiac output. The lack of accuracy in comparison with thermodilution technique is a major problem. Even thermodilution technique has its own limitations in the intensive care setting, so that there is no true gold standard for comparison of the other techniques.