• Tiago H de Souza, Marina P Giatti, Roberto J N Nogueira, Ricardo M Pereira, Ana C S Soub, and Marcelo B Brandão.
• Pediatric Intensive Care Unit, Department of Pediatrics, State University of Campinas (UNICAMP), Campinas, São Paulo, Brazil.
• Pediatr Crit Care Me. 2020 Apr 1; 21 (4): e186-e191.

ObjectivesInferior vena cava ultrasound has been used as a predictor of fluid responsiveness in children. Two ultrasonographic modes can be used to measure the respiratory variation of inferior vena cava diameter: M-mode and B-mode. Inconsistencies in measurements between the modes can result in inaccuracies in commonly used indices that assess fluid responsiveness. Our primary objective was to determine whether there are differences in the ultrasound-based measurements between these two modes of evaluation, which would impact respiratory variation of inferior vena cava diameter calculation. Our secondary objective was to assess inferior vena cava displacements during the respiratory cycle as a possible mechanism for measurement differences between the modes.DesignProspective observational study.SettingPICU of a tertiary care teaching hospital.PatientsSeventy-three children under controlled ventilation (median age of 16 mo and weight of 10 kg).InterventionsThe inferior vena cava diameters were measured using a longitudinal view using B- and M-mode ultrasound. Two respiratory variation of inferior vena cava diameter indices were evaluated: distensibility and respiratory variation. Maximum craniocaudal and mediolateral displacements of the inferior vena cava were measured using the B-mode ultrasound.Measurements And Main ResultsMaximum diameters of the inferior vena cava were similar between the B- and M-modes (7.90 vs 7.90 mm, respectively; p = 0.326), but minimum diameters were smaller when measured by M-mode (6.36 vs 5.00 mm; p = 0.003). When calculated by data obtained from M-mode, respiratory variation of inferior vena cava diameter indices presented significantly higher values compared to B-mode measures (p ≤ 0.001, for both). Median inferior vena cava displacements were 5.00 mm (interquartile range, 3.68-6.26 mm) in the craniocaudal and 0.80 mm (interquartile range, 0.12-1.23 mm) in the mediolateral directions.ConclusionsThere is a significant difference between measurements of the minimum inferior vena cava diameter observed in M- and B-mode ultrasound during the respiratory cycle in children under controlled ventilation. This results in imprecise respiratory variation of inferior vena cava diameter indices. Displacements of the inferior vena cava during the respiratory cycle may influence the reliability of ultrasonographic measurements, particularly in M-mode.

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