One of the most promising techniques to gauge fluid responsiveness has been ultrasound measurement of respiratory induced changes in the inferior vena cava diameter. The technique works because as one breathes the shifting diaphragm causes intrathoracic pressure gradients to arise. In spontaneously breathing people, the diaphragm pulls down during inspiration, creating a significant negative pressure change in the upper half of the body and a positive pressure change in the lower half of the body. This relationship is reversed in the case of positive pressure ventilation, where a positive intrathoracic pressure accompanies inspiration. As a very compliant blood vessel, the inferior vena cava is affected greatly
by this abdominal pressure changes, collapsing under negative pressures (inspiration during spontaneous breathing, expiration under mechanical ventilation) and expanding under positive pressures (expiration during spontaneous breathing, inspiration under mechanical ventilation). The extent of collapse (or expansion) is a function of the amount of blood present in the vessel, itself a function of the amount of blood returning to the heart, and thus of the fluid status itself.
Differing from the single static measure of the inferior vena cava diameter (previously discussed), the collapsibility of the inferior vena cava has been shown to reliably guide fluid therapy [68, 69, 70, 71]. The current line of training suggests that a collapse of less than 50% in response to inspiration or a sniff suggests a normovolemic or fluid overloaded state, whereas a collapse of greater than 50% indicates fluid responsiveness. However, recent evidence suggests 40% may be the most predictive threshold . In any case, such a measurement can be taken quickly and easily at the bedside, providing actionable data in the time it takes to take a breath. Because of the relative ease in taking the measurement (usually with subcostal echocardiography), its predictive power of fluid responsiveness, and the rise of point-of-care ultrasound system, the caval index as a guide for fluid therapy has become increasingly used clinically.
Where the technique falls short is its reliance on a clinicians skill and a patients habitus. Little training and large patients make for poor results. A further limitation of the method stems from the discretization of information, where the volume status is measured only in spot checks, failing to identify trends.