ObjectiveTo evaluate the effects of helium on the function of four ventilators commonly used in pediatrics: the Bird VIP, Bird VIP Gold, Servo 300, and Servo 900C.DesignProspective setting.SettingResearch laboratory at a university hospital.SubjectsHelium was administered as an 80:20 mixture of helium-oxygen through the air inlet of the ventilator. Delivered fraction of inspired oxygen (Fio2) was compared with the Fio2set on the blender dial. Inspiratory displayed tidal volume was recorded as an indicator of what the ventilator “believed” it had delivered and was compared with the VTdisplayed during ventilation with 100% oxygen (control). Actual delivered VTwas measured by a Neonatal Bicore connected to the side port of a “bag-in-box” spirometer, making measurements independent of inspired gas properties, and was compared with VTdelivered during ventilation with 100% oxygen.InterventionsFive gas mixtures were evaluated: Fio2= 0.2, 0.4, 0.6, 0.8, and 1.0 (balance helium).Measurements and Main ResultsDelivered Fio2was less than set Fio2on the Servo 900C and VIP ventilators. VTdisplayed was minimally altered by helium during volume-controlled ventilation but substantially decreased during pressure-controlled ventilation, particularly with the Bird ventilators. During volume-controlled ventilation, VTdelivered was substantially increased by helium with the Bird and, to a lesser degree, the Servo 900C ventilators. In contrast, VTdelivered decreased slightly in helium with the Servo 300. The same pattern, but with a decreased magnitude, was observed for VTdelivered during pressure-controlled ventilation.ConclusionsThe addition of helium has a significant effect on Fio2delivery, displayed inspiratory VT, and actual delivered VTduring both volume- and pressure-controlled ventilation in four ventilators commonly used in pediatric critical care. These effects are both ventilator specific and ventilation mode specific, mandating vigilance during helium ventilation in clinical practice.