Ultrasound Assessment of Inferior Vena Cava Collapsibility Is Not a Valid Measure of Preload Changes During Triggered Positive Pressure Ventilation: A Controlled Cross-Over Study

Abstract

Respiratory changes in the diameter of the inferior vena cava (IVC) have been validated as a measure of volume status and preload responsiveness during spontaneous breathing and mechanical ventilation. However, many intensive care patients are ventilated with triggered positive pressure ventilation (PPV). In this setting, there is no evidence regarding IVC collapsibility (IVCc) as a surrogate for preload. We aimed to elucidate the effects of increasing levels of triggered PPV and of varying preload conditions on the IVCc. 10 healthy volunteers were connected to a ventilator through a tight-fitting mask and exposed to 6 different levels of positive end-expiratory pressure (PEEP) and pressure support (PS) after a baseline reading. All ventilator settings were performed at neutral preload (horizontal position), low preload (reverse-Trendelenburg) and high preload (Trendelenburg position with an intravenous fluid bolus). At each ventilator setting, the IVC was imaged throughout at least 1 respiratory cycle using 3 commonly used ultrasound techniques including sagittal M-mode and 2-dimensional echocardiography in both sagittal and transverse views. Increasing PS diminished IVCc (p = 0.01) in the reverse-Trendelenburg position, and increasing PEEP caused a higher IVCc in the Trendelenburg position (p = 0.02). In the horizontal position, no significant effects of increasing PS, PEEP or a combination of the two were seen. Overall ANOVA analysis showed that IVCc was not independent of preload. During PPV, IVCc was highest at neutral preload at most ventilator settings, IVCc was lowest at low preload, while high preload generally facilitated an IVCc between neutral and high preload. In addition, sagittal M-mode and transverse 2-dimensional echocardiography overestimated IVCc as compared to sagittal 2-dimensional echocardiography. The compiled results of this study show that IVCc cannot be held as a valid measure of preload status during PPV. This may be explained by systematic alterations in other determinants for IVCc. Comparison of methods encourages the use of sagittal 2-dimensional echocardiography for dynamic imaging of the IVC. Sagittal M-mode and transverse 2-dimensional echocardiography overestimate IVCc as compared to sagittal 2-dimensional echocardiography.