Effect of helium–oxygen mixture on time constant inequalities in COPD patients during controlled ventilation

The respiratory system of the COPD patients is characterised by regional differences in mechanical properties. When mechanical ventilation is applied, this inhomogeneous lung causes regional dynamic hyperinflation. The aim of the present study was to compare the time constant inequalities obtained with air–oxygen and helium–oxygen mixtures in 14 COPD patients. The local ethic committee approved the protocol and consent was obtained from next of kin. The patients were sedated and paralysed for the duration of the protocol. Controlled mechanical ventilation was performed with constant flow. A Fleisch pneumotachograph calibrated for the use of helium and a differential pressure transducer were inserted between the endotracheal tube and the Y piece. The patients were ventilated successively with air–oxygen and helium–oxygen mixture in a random order. The time constant inequalities at the end of inspiration were calculated by the difference between P₁ and plateau pressure (obtained by an occlusion of 5 s). The time constant inequalities at the end of expiration were calculated by the difference between static intrinsic PEEP (PEEPistat) and dynamic intrinsic PEEP (PEEPidyn). A Wilcoxon test was used to compare the data.

The time constant inequalities at end inspiration and end expiration were reduced (Figs 1 and 2) with the helium–oxygen mixture (15 and 40% respectively).

Figure 1

Figure 2

A modification of the viscoelastic behaviour of the system could explain these results. The inspired volume is distributed more homogeneously to the different unit, with helium–oxygen mixture. During expiration, the regions with long time constant empty faster which could explain the decrease in regional dynamic hyperinflation. In conclusion, helium–oxygen mixture reduces the time constant inequalities of the respiratory system in COPD patient during controlled ventilation.