流量传感器。 近端流量和压力测量

Many ventilators measure expired tidal volume (VT) without compensation either for the compliance of the ventilator circuit or for variations in the circuit setup. We hypothesized that the exhaled VT measured with a conventional ventilator at the expiratory valve would differ significantly from the exhaled VT measured with a pneumotachometer placed at the endotracheal tube. To investigate this we studied 98 infants and children requiring conventional ventilation. We used linear regression analysis to compare the VT obtained with the pneumotachometer with the ventilator-measured volume. An additional comparison was made between the pneumotachometer volume and a calculated effective VT. For infant circuits (n = 70), our analysis revealed a poor correlation between the expiratory VT measured with the pneumotachometer and the ventilator-measured volume (r(2) = 0.54). Similarly, the expiratory VT measured with the pneumotachometer did not correlate with the calculated effective volume (r(2) = 0.58). For pediatric circuits (n = 28), there was improved correlation between the expiratory VT measured with the pneumotachometer and both the ventilator-measured volume and the calculated effective VT (r(2) = 0.84 and r(2) = 0.85, respectively). The data demonstrate a significant discrepancy between expiratory VT measured at a ventilator and that measured with a pneumotachometer placed at the endotracheal tube in infants. Correcting for the compliance of the ventilator circuit by calculating the effective VT did not alter this discrepancy. In conventionally ventilated infants, exhaled VT should be determined with a pneumotachometer placed at the airway.

OBJECTIVES

Tidal volume (VT) delivered to infants' airways are overestimated and pressure underestimated when measured in the ventilator and not at the Y piece. This study aimed at evaluating the influence of respiratory system impedance on expiratory VT (VTE) and pressure measurement difference.

DESIGN

Prospective observational study.

SETTING

Pediatric intensive care unit at a university hospital.

PATIENTS

Data were collected between February 2000 and October 2001 for 30 infants (range, 1-23 months) ventilated in the pressure-controlled or volume-controlled mode.

INTERVENTIONS

Measurements of VTE, pressure obtained at the same time at the Y piece and on the ventilator Servo 300, were collected in ventilated infants. Respiratory system impedance was calculated from data obtained at the Y piece. Circuit compliance was measured in vitro. VTEs were corrected for compressible volume.

MEASUREMENTS AND RESULTS

VTEs were overestimated by the Servo 300 in the pressure-controlled and volume-controlled modes (from 5% to 62% of the value displayed on Servo 300). Maximal inspiratory pressures were underestimated by the Servo 300 in the pressure-controlled mode (difference from -2 to +19 cm H(2)O). Measurement difference increased with increasing respiratory system impedance. Ventilator VTE corrected for circuit compliance did not offer a sufficiently accurate estimation of VTE at the Y piece.

CONCLUSIONS

VT and pressure measurements must be performed at the Y piece, especially in infants with increased respiratory system impedance (i.e., decreased respiratory system compliance or increased resistance). Correcting VTE for circuit compliance cannot replace measurement of VT at the Y piece.

INTRODUCTION

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.

METHODS

The 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.

RESULTS

In 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.

CONCLUSION

Previous 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.