Abstract
IntroductionThe objective of this laboratory study was to measure the effect of decreased lung compliance and endotracheal tube (ETT) leakage on measured exhaled tidal volume at the airway and at the ventilator, in a research study with a test lung.MethodsThe subjects were infant, adult and pediatric test lungs. In the test lung model, lung compliances were set to normal and to levels seen in acute respiratory distress syndrome. Set tidal volume was 6 ml/kg across a range of simulated weights and ETT sizes. Data were recorded from both the ventilator light-emitting diode display and the CO2SMO Plus monitor display by a single observer. Effective tidal volume was calculated from a standard equation.ResultsIn all test lung models, exhaled tidal volume measured at the airway decreased markedly with decreasing lung compliance, but measurement at the ventilator showed minimal change. In the absence of a simulated ETT leak, calculation of the effective tidal volume led to measurements very similar to exhaled tidal volume measured at the ETT. With a simulated ETT tube leak, the effective tidal volume markedly overestimated tidal volume measured at the airway.ConclusionPrevious investigators have emphasized the need to measure tidal volume at the ETT for all children. When ETT leakage is minimal, it seems from our simulated lung models that calculation of effective tidal volume would give similar readings to tidal volume measured at the airway, even in small patients. Future studies of tidal volume measurement accuracy in mechanically ventilated children should control for the degree of ETT leakage.
Highlights
The objective of this laboratory study was to measure the effect of decreased lung compliance and endotracheal tube (ETT) leakage on measured exhaled tidal volume at the airway and at the ventilator, in a research study with a test lung
We proposed that the discrepancy between effective VT and VT measured at the ETT in children was due mainly to ETT leakage around uncuffed ETTs, and that in situations with minimal ETT leakage there would be minimal difference between the effective VT and VT measured at the airway
In the absence of ETT leakage the difference between effective VT and VT measured at the ETT was less than 10% across the range of lung compliances with a standard deviation of 0–0.2 ml/kg for all sets of measurements
Summary
The objective of this laboratory study was to measure the effect of decreased lung compliance and endotracheal tube (ETT) leakage on measured exhaled tidal volume at the airway and at the ventilator, in a research study with a test lung. Three investigators have reported that tidal volume (VT) in children is inaccurate when measured at the ventilator, even when effective VT is used [1,2,3]. Cannon and colleagues [1] studied 98 infants and children and found a significant discrepancy between expiratory VT measured at the ventilator and that measured with a pneumotachometer. Castle and colleagues [2] studied 56 intubated children and found that exhaled VT displayed by the Servo 300 significantly overestimated VT measured at the airway by between 2% and 91%.
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