Influence of superior vena caval zone condition on cyclic changes in right ventricular outflow during respiratory support

Background Adequate fluid resuscitation in critically ill patients undergoing mechanical ventilation remains a difficult challenge, and diastolic and systolic right ventricular (RV) changes produced by positive airway pressure are important to consider in an individual patient with inadequate circulatory adaptation during respiratory support. We hypothesized that insufficient thoracic vena cava filling, predisposing to inspiratory collapse (zone 2 condition), may transiently affect RV outflow. Methods We measured beat-to-beat superior vena caval diameter and Doppler RV outflow during a routine transesophageal echocardiographic examination in 22 patients undergoing mechanical ventilation, all of whom required hemodynamic monitoring, and we calculated a collapsibility index for the superior vena cava as maximal expiratory diameter minus minimal inspiratory diameter, divided by maximal expiratory diameter. Results In 15 patients (group 1), the collapsibility index was low (17 ± 7%) and was associated with a moderate inspiratory decrease in RV outflow (25 ± 17%). However, in seven patients (group 2), we observed a high collapsibility index (71 ± 7%), which was associated with a major inspiratory decrease in RV outflow (69 ± 14%) combined with a reduced pulmonary artery flow period. A rapid volume expansion, only performed on group 2, markedly and significantly reduced both the collapsibility index (15 ± 12%) and the inspiratory decrease in RV outflow (31 ± 20%). Conclusion A major inspiratory decrease in RV outflow associated with a reduced pulmonary artery flow period in a patient undergoing mechanical ventilation reflected a high collapsibility index of the thoracic vena cava, suggesting a zone 2 condition, and may be corrected by blood volume expansion.