Hemodynamic effects of PEEP

In most of the previously mentioned studies., hemodynamics were compared between zero end-expiratory pressure (ZEEP) and one or two distinct levels of PEEP. Linear relations between cardiac output and PEEP were often assumed and hemodynamic analyses were based on them. In our studies hemodynamics were analysed at all levels of PEEP between 0 and 15 cmH20 except for cardiac output which was measured at intervals of 0.33 cmH,O. For this study PEEP was applied as a ramp. i.e. as a slow and continuous rise. We performed initially a methodological study which compared two thermodilution techniques for the estimation of cardiac output during IPPV and PEEP (Chapter II). Subsequently the hemodynamic changes to PEEP applied as a ramp were analysed to evaluate the mechanisms influencing cardiac output (Chapter Ill). In Chapter IV a hypothesis suggested by the first PEEP study was tested. The hypothesis was that a lung stretch depressor reflex was involved as an additional mechanism in the negative cardiac output responses to PEEP. In Chapter V the contribution of the lung stretch depressor reflex and cardiovascular compensatory mechanisms was studied at different levels of circulatory load