The influence of the internal compliance of a respirator on the alveolar gas distribution.

The occurrence of intrapulmonary redistribution "pendelluft" during the plateau phase of a respirator, with and without internal compliance, was studied in a lung-model which simulated an obstructive inhomogeneity. Internal compliance was achieved by adding a flask, with a variable volume, to the patient circuit of a respirator which had a low compressible volume (Engström Care System ECS 2000, Junger Instrument AB, Sweden). The amount of redistributed volume was found to be dependent upon the internal compliance. Without additional compressible volume, the ventilator produced up to 18% redistribution which under all conditions could be brought below the 1% level when an internal compliance was introduced. A highly significant correlation between the amount of redistributed volume (pendelluft) and the discharge time-constant of the plateau (taupl = CI (RI + Rtr) could be established. The results of mathematical consideration gained from an electrical analogue coincided with our experimental findings. From this, some consequences could be derived for the design of an internal compliance for use with a respirator.