[End inspiratory lung volumes as the limiting factor in PEEP ventilation]

In 17 patients requiring artificial ventilation expiratory pressure-volume curves (VPE) were plotted all over the inspiratory capacity (IC) between total lung capacity (TLC) and functional residual capacity (FRC). Then patients were ventilated with PEEP of 0, 5, 10 and 15 cmH2O for 30 min. The volume excursions with known PEEP were displayed graphically into the VPE to determine the position of end-inspiratory volume within the IC. The steep, quasi-linear part of each VPE was flattening exponentially against TLC in the upper part of the curve. This "overdistension range" was found to be identical with the upper 40% of IC. Heavy side-effects of PEEP ventilation were never seen before the inspiratory volume excursion had reached this upper 40% of IC. If this was the case, the following observations were made: 1. Static compliance (Cst) decreased by more than 10% in all cases. 2. Pulmonary vascular resistance (PVR) increased by more than 20% in 12 cases. 3. Cardiac index (CI) decreased by more than 15% in 12 cases. 4. The alterations observed in Cst, PVR and CI increased rapidly, when the lungs were further distended, and were well correlated parabolically to the position of end-inspiratory volume within IC. --5 patients failed to show important circulatory alterations. It is concluded that overdistension-related side-effects of PEEP on respiratory mechanics and circulation do not occur before the linear part of the pressure-volume curve has been exceeded by hyperinflation. For the first adjustment of controlled ventilation at least the upper third of IC should be avoided.