The effect of continuous positive airway pressure on inferior vena cava collapsibility as measured by bedside ultrasound

Objectives The purpose of this study was to determine the impact of progressively increasing continuous positive airway pressure (CPAP) on measurements of the caval index (CI) using bedside ultrasound at the 3 common inferior vena cava (IVC) evaluation sites. Methods This was a prospective, observational trial that included 165 healthy adults over 18 years old enrolled between February 2015 and May 2018. Measurements of the IVC were obtained during normal tidal respirations from the subxiphoid area in the long and short axis and from the right mid‐axillary line in the long axis. Measurements were obtained in each of these locations at atmospheric pressure and with CPAP at 5, 10, and 15 cmH2O. The CI was then calculated for each of the 3 selected locations at each level of pressure. Results As CPAP pressures increased from 0 to 15 cmH2O the CI measurements obtained at the lateral mid‐axillary line did not show any statistically significant variation. There was a statistically significant difference (P 0.05). Conclusion When evaluating the IVC in a spontaneously breathing patient, measurements from an anterior orientation are preferred as the lateral mid‐axillary view can underestimate CI calculations.