Non-Invasive Cardiometry and Ultrasound Guided Inferior Vena Cava Collapsibility Index in Assessing Fluid Responsiveness

Based on its precedent electrical impedance. 4 electrodes are placed on the patient: 2 on the left of the neck and the other 2 on the left lower chest . A low magnitude (2 mA), high frequency (30-100 KHz) alternating electrical current (AC) of constant amplitude is applied through the outer electrodes, and the resulting voltage is received by the inner electrodes. The ratio of the detected voltage to the applied current is the bio-impedance. The principle on which this is based is that during systole, red blood cells flow in a parallel manner, which allows the electrical current to flow easily thereby improving the electrical velocity and decreasing impedance. While during diastole, RBCs are randomly arranged, consequently hindering the electrical current (increased impedance) and decreasing electrical velocimetry. The changes of impedance over time are integrated in a complex algorithm that allows to measure CO and the other parameters.

A low-frequency phased array transducer (3.5-5 MHz) will be used to assess the IVC, which lies in the retroperitoneum, to the right of aorta. At or near the junction with the hepatic veins, we will measure the IVC diameter. To properly visualise the IVC, the probe will be inserted in the subxiphoid 4-chamber position with the probe marker oriented vertically to find the right ventricle and atrium. We will see the convergence of the IVC with the right atrium as the probe is progressively aimed towards the spine. We will then follow the IVC inferiorly, to detect the meeting of the hepatic veins with the IVC. M-mode Doppler sonography of the IVC will be used to graphically document the absolute size and dynamic changes in the calibre of the vessel during inspiration and expiration. After the visualisation of the IVC, we will freeze the US screen, and using the caliper function on the US machine, maximum and minimum diameters of the IVC will be documented.