Bandwidths of respiratory gas flow and pressure waveforms in mechanically ventilated infants

The frequency content of airway pressure and gas flow in mechanically ventilated infants (MVIS) has not been adequately investigated. Pressure-cycled infant ventilators generate pressure pulses with short rise-times. Gas flow is approximately equal to the derivative of pressure when lung compliance is low, and hence contains high-frequency components. We defined bandwidth as that frequency fm below which 99.9% of the energy of the signal resided. Simulation of the measurement process using measurement systems with frequency response similar to sixth-order Bessel filters and a lung model comprising series resistance, inertance and compliance showed that measurement systems with frequency response flat +/- 10% to fm yield time domain errors less than 3% of the peak value. We digitized pressure and flow signals from 10-20 ventilator (Healthdyne 105) breaths in 33 stable MVIS. The transducers' (Gould P50, Hans Rudolph 8300 screen pneumotach) frequency responses had been measured between 1 Hz and 100 Hz and phase matched at 10 Hz. We calculated total respiratory resistance R and elastance E using multiple linear regression, and ensemble-average power spectral density using the FFT with a rectangular time window and padding to 2048 points. Power spectra were compensated for non-unity transducer and anti-alias filter responses up to 60 Hz. Measured data sequences that were not self-windowing due to spontaneous breathing efforts, that yielded regression R2 < 0.95 or that contained flow oscillations due to secretions in the airway were discarded. Satisfactory results were obtained from more than eight breaths in 18 infants. Mean bandwidths (+/- SD) of pressure and flow waveforms were 4.7 +/- 0.7, range 3.5-5.9 and 19.6 +/- 6.5, range 10.8-32.1 Hz, respectively. Flow bandwidths B correlated with the respiratory time constant tau (B = -77.2 tau + 26.8, R2 = 0.55, P < 0.0002), and with elastance E (B = 61.4E + 10.1, R2 = 0.74, P < 0.0001). We conclude that the bandwidth of the flow waveform increases with decreasing compliance and mechanical time constant. The frequency response of pressure and flow measurement systems should be flat +/- 10% at least up to 6 and 32 Hz respectively to obtain data with dynamic errors less than 3% in infants with low-compliance lung disease.