Inferior vena cava dynamics during triggered breaths under different levels of pressure support and PEEP: A randomized crossover physiologic study

BACKGROUND: Inferior vena cava (IVC) distensibility is widely used to assess fluid responsiveness (FR) during passive positive pressure ventilation. However, interpretation of IVC dynamics during triggered breaths under assisted ventilation remains challenging. This study aimed to evaluate the effects of different pressure support ventilation (PSV) and positive-end expiratory pressure (PEEP) levels on IVC dynamics during triggered breaths. METHOD: This randomized crossover physiologic study included critically ill patients undergoing weaning from mechanical ventilation. Participants were exposed to four randomized sets of 12 PSV conditions combining different levels of pressure support (PS) and PEEP.IVC behavior during inspiration was assessed using ultrasound. The IVC collapsibility index (cIVC) and distensibility index (dIVC) were calculated. Respiratory effort was assessed using airway occlusion pressure at 100 milliseconds (P0.1). RESULTS: Sixty ICU patients were included. Inspiratory IVC collapse occurred in 85% of observations, particularly at lower PS/PEEP levels, whereas IVC distension predominated at higher settings. Among observations with IVC collapse, cIVC values were highest at lower PS/PEEP levels. Each 1-cmH2O increase in P0.1 was associated with an approximately 3.5%-point increase in cIVC, and mediation analysis showed that P0.1 accounted for nearly 40% of the association between PS settings and cIVC (38.5% [95% CI 19.2 to 57.7]). CONCLUSION: During PSV, inspiratory IVC collapse is common. The use of cIVC during patient-triggered breaths primarily reflects ventilator-patient interaction rather than preload alone and should be interpreted with caution. Future studies should account for respiratory effort when using cIVC to access FR under assisted ventilation.