Limitation of measurements of expiratory tidal volume and expiratory compliance under conditions of endotracheal tube leaks

Abstract

Endotracheal tube leaks (ETTLs) occur in neonates ventilated with uncuffed tubes. Assuming that the influence of ETTLs might be neglected during expiration, only expiratory tidal volume is measured for calculation of expiratory compliance in cases of large ETTLs. However, expiratory ETTL might be substantial. Therefore, we evaluated the effect of ETTL size on expiratory tidal volume and compliance. Prospective laboratory study and retrospective clinical study. University research laboratory and neonatal intensive care unit. Sixty ventilated neonates (weight 640-2160 g, gestational age 25-33 wks) were investigated. The impact of increasing ETTLs on inspiratory and expiratory measured tidal volume (Vm), corrected tidal volume (Vc), and leak volume (Vl) was investigated in a ventilated neonatal lung model. The range of ETTLs (1% to 95%) was subdivided into five groups of 12 infants each. Furthermore, the relationships between standard ETTL size and inspiratory and expiratory ETTLs were evaluated using nonlinear regression. Standard ETTL size was defined as the difference between measured inspiratory and expiratory tidal volume (Vm) related to inspiratory Vm. The size of a standard ETTL was 40% when expiratory ETTL reached 10% and was 12% when the inspiratory ETTL reached 10%. In infants, the differences between Vm and Vc were statistically significant during inspiration in the group beginning at a standard ETTL of 41% and during expiration in the group beginning at a standard ETTL of 69% (p < .05). Results of nonlinear regression showed that the standard ETTL was 33% (95% confidence interval, 28% to 36%) when expiratory ETTL reached 10% and was 13% (95% confidence interval, 12% to 15%) when inspiratory ETTL reached 10%. Expiratory Vl has a relevant impact if a certain ETTL size is reached.