Frequency dependence of elastance and resistance in ventilated children with and without the chest opened.

Measurements of respiratory mechanics in mechanically-ventilated children are potentially useful for understanding the pathogenesis and progress of diseases resulting in respiratory failure. Measurement techniques that can be simply and noninvasively employed model the respiratory system as a single compartment. The frequency dependence of elastance and resistance, both of the total respiratory system and of the lungs, was investigated in eight children (aged 2-56 months) undergoing open-heart surgery. The children were studied whilst anaesthetized, paralysed and mechanically-ventilated. Dynamic elastance (Ers) and resistance (Rrs) of the respiratory system and of the lung (EL, RL) were calculated using a multilinear regression technique, with the chest wall intact, prior to the commencement of surgery, and with the chest wall opened via a mid-line sternotomy. Measurements were repeated after brief (60 s) changes in ventilation frequency. The total respiratory system and the lungs demonstrated frequency-dependent behaviour, with elastance increasing and resistance decreasing with frequency. The pattern of the frequency-dependent behaviour was essentially the same, whether the chest wall was intact or opened, suggesting that the chest wall was not solely responsible for this behaviour in these children. These data are consistent with a linear viscoelastic model containing a Kelvin body. When using measurements of respiratory mechanics to follow the progress of respiratory disease in mechanically-ventilated children, this frequency-dependent behaviour must be taken into account.