Fig. 6

Brain curve (red line), unsupported (dashed black line) and supported with PS (green line) ventilation curves, relatively late in the course of critical illness of the patient of Fig. 5. A High PS, stable breathing. B Unstable breathing with increasing PS. Point 1: PaCO₂ during unsupported spontaneous breathing; Point 2: RCO/min corresponding to desired V’_E with unsupported spontaneous breathing; Point 3: actual PaCO₂ with PS during stable breathing (stable ventilation); Point 4: RCO/min corresponding to desired V’_E with PS ventilation (stable ventilation); closed circles: apneic threshold; Point 5: Actual V’_E that results in apnea; Notice that with PS ventilation curve is shifted to the left of brain curve. See text for further explanation. PS: pressure support; RCO/min: respiratory centers output per min; V’_E: minute ventilation