Systemic filling pressure in the intact circulation determined with a slow inflation procedure

In eight mechanically ventilated, anaesthetized pigs weighing 10.3 +/- 0.8 kg (mean +/- SD) we studied the effect of the inflation time of the lung on the estimation of the mean systemic filling pressure (Psf) from the changes in venous return and central venous pressure during inflation of the lung. For this purpose we applied slow inflation procedures (SIP) to the lung with inflation times of 2.4, 4.8, 7.2, 9.6 and 12 s at tidal volumes (VT) of 15 and 30 ml/kg. The data were compared with the values of Psf obtained from inspiratory pause procedures (IPPs). A linear regression between venous return and central venous pressure applied during a SIP underestimated Psf compared with the value obtained with IPPs. An exponential fit through the values of Psf obtained from the different SIPs predicted an inflation time of about 15 s for an estimation of Psf that is not different from the Psf (IPP). The advantage of the SIP method is that the Psf can be determined much faster than with the method based on IPPs. However, due to the rather long inflation time needed, the method may be only applicable under circumstances where neurohumoral control mechanisms are suppressed as during intensive care and anaesthesia.