Influence of head-of-bed elevation on the measurement of inferior vena cava diameter and collapsibility.

PURPOSE

Inferior vena cava (IVC) diameter and variation are commonly measured in the supine position to estimate intravascular volume status of critically ill patients. Many scientific societies describe the measurement of IVC diameter in the supine position. However, critically ill patients are rarely placed supine due to concerns for aspiration risk, worsened respiratory mechanics, increases in intracranial pressure, and the time it takes to change patient position. We assessed the influence of head-of-bed (HOB) elevation on IVC measurements.

METHODS

We conducted a prospective observational study of critically ill patients undergoing critical care ultrasound. With HOB at 0°, IVC maximum (IVCmax0°) and minimum (IVCmin0°) diameters were measured. Measurements were then repeated with HOB elevated to 30° and 45°. Collapsibility index (CI), defined as (IVCmax - IVCmin)/IVCmax, was calculated for each HOB elevation. Mean differences were then compared.

RESULTS

A convenience sample of 95 patients was studied, of whom 45% were on vasopressors and 44% were spontaneously breathing. The CI did not significantly differ between the three positions. We found a significant difference (P ≤ .0001) between IVCmax at 45° (2.09 cm) and 0° (1.96 cm), IVCmin at 45° (1.75 cm) and 0° (1.59 cm), IVCmax at 45° (2.09 cm) and 30° (1.97 cm), and IVCmin at 45° (1.75 cm) and 30° (1.61 cm).

CONCLUSIONS

In a population of critically ill patients undergoing goal-directed ultrasound examinations, elevating HOB to 30° did not significantly alter IVC measurements or CI. At 45°, however, IVCmax and IVCmin diameters increased significantly, albeit with no significant change in CI. Performing ultrasound measurements of the IVC with HOB elevated to 30° is unlikely to produce clinically meaningful changes.