Accuracy of oscillatory pressure measured by mechanical ventilators during high frequency oscillatory ventilation in newborns.

BACKGROUND

Oscillatory pressure (ΔP) measurement during high frequency oscillatory ventilation (HFOV) is technically challenging and influenced by all the components of the measurement system.

OBJECTIVES

To evaluate the differences between the ΔP delivered at the inlet of the endotracheal tube and those displayed by commercial neonatal mechanical ventilators and monitoring devices and to characterize how the ventilator circuit and the flowmeter proximal to the patient affect these differences.

METHODS

Six devices were evaluated while ventilating three mechanical analogues representing the newborn respiratory system in different disease states. ΔP measured at different points of the ventilator circuit were compared.

RESULTS

ΔP accuracy is highly variable and decreases with increasing oscillation frequency and amplitude, independently of the mean airway pressure. At 15 Hz, a ΔP displayed by ventilators of 40 cmH O resulted in a ΔP effectively delivered at the tracheal tube ranging from 22 to 49 cmH O, depending on the ventilator model, the ventilator circuit, and the patient condition. At these settings, the errors exclusively due to the ventilator circuit and the presence of the flowmeter ranged from 6 to 9 cmH O and from 1 to 6 cmH O, respectively.

CONCLUSIONS

The ventilator model, the breathing circuit, the flowmeter, and the patient condition severely impacts ΔP measurement accuracy during HFOV, leading to highly variable performances. This prevents the possibility of using the ΔP required to normalize gas exchange as an indicator of patients' condition, complicates comparison of ventilators performances, and adds a significant element of complexity in clinical management of HFOV.