Doppler flow studies focusing on the descending neither thoracic aorta may not provide a reliable measurement of the total cardiac output (for example, with epidural use), and are invalid in the presence of intra-aortic balloon pumping. Echo-Doppler cardiac output estimates vary considerably for several reasons, including difficulty in assessment of the velocity time integral, calculation error due to the angle of insonation, and problems with correct measurement of the cross-sectional area. Some training is required when using these techniques. Esophageal-Doppler techniques have been shown to be useful for optimizing fluid administration in high risk surgical patients [7,8].Simplified transesophageal Doppler techniques can be convenient as the probe is smaller than for standard esophageal echocardiography techniques.

Simplified trans-thoracic Doppler systems allow estimation of aortic blood flow and may be even less invasive; however, although these techniques can be simple to perform in healthy volunteers, access to good images may be more difficult in critically ill patients. Moreover, there is a fairly prolonged learning curve for correct use of this system [9]. These methods need further validation in critically ill patients.CO2 rebreathingCO2 rebreathing systems, based on the Fick principle, use a CO2 sensor, a disposable airflow sensor and a disposable rebreathing loop. CO2 production is calculated from minute ventilation and its CO2 content, and the arterial CO2 content is estimated from end-tidal CO2. Partial re-breathing reduces CO2 elimination and increases the end-tidal CO2.

By combining measurements taken during and without rebreathing, venous CO2 content can be eliminated from the Fick equation. However, intra-pulmonary shunting of blood and rapid hemodynamic changes affect the accuracy of the measurement, so that this technique is not considered to be reliable in acutely ill patients.Bioimpedance and bioreactanceBioimpedance is based on the fact that the conductivity of a high-frequency, low-magnitude alternating current passed across the thorax changes as blood flow varies with each cardiac cycle. These changes can be measured using electrodes placed on a patient’s chest and used to generate a waveform from which cardiac output can be calculated.

Bioreactance has developed out of bio-impedance and measures changes in the frequency of the electrical currents traversing the chest, rather GSK-3 than changes in impedance, potentially making it less sensitive to noise. These techniques are non-invasive and can be applied quickly. They have been used for physiological studies in healthy individuals and may be useful in perioperative applications [10], but are less reliable in critically ill patients [11]. Electrical interference may also occur in the ICU environment.