Figure 3. Illustration, in one patient, of the effects of increasing the level of positive pressure applied at the airway (P_mask).

Effects on the absolute value of the k₁/k₂ ratio (k₁ and k₂, coefficients of the V̇_I = k₁P_d + k₂P_d² equation describing the flow-pressure relationship up to maximal flow, see text for details) are shown. The peak of the k₁/k₂ curve corresponds to optimal upper airway stability. From the pressure-flow response to phrenic nerve stimulation established at various CPAP levels, it is thus possible to calculate what would theoretically be the efficient pressure P_eff (P_eff,stim) in a given individual. In this patient, P_eff,stim was 5.3 cmH₂O, whereas the P_eff conventionally determined during a titration sleep recording was 7 cmH₂O.