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The respiratory resistive properties of the normal human respiratory system are volume-dependent. The overall flow resistance (Rmax,rs) can be partitioned into airway resistance (Raw) and the additional resistance (delta Rrs) which may result from the viscoelastic properties of the respiratory system, from inequality of time constants (pendelluft), or from both. Because positive end-expiratory pressure (PEEP) increases end-expiratory lung volume and may equalize ventilation within the lungs, the effect of PEEP on Raw, delta Rrs, and their sum (Rmax,rs) was assessed in anaesthetized surgical patients without evidence of lung disease. Fifteen men were studied during paralysis and isoflow isovolume mechanical ventilation, using the end-inflation occlusion method. Ten men were studied with incremental levels of PEEP, up to 16 cmH2O (Group A). Five men were studied without PEEP (Group B). In Group A, Rmax,rs did not change with PEEP. In contrast, Raw decreased and delta Rrs increased significantly. Moreover, there was a linear relationship between PEEP and the contribution of delta Rrs to Rmax,rs. In Group B, Rmax,rs, Raw and delta Rrs, and the contribution of delta Rrs to Rmax,rs did not change. In both groups, atropine elicited a decrease in Rmax,rs, linked to a decrease in Raw, without any notable effect on the static elastance of the respiratory system (Est,rs) or on delta Rrs. We conclude that the overall flow resistance was not affected by PEEP. In contrast, PEEP clearly modified the contribution of its two components. The decrease in Raw with PEEP could have resulted, at least in part, from modification in the basal vagal tone.

The effect of positive end-expiratory pressure on respiratory resistive properties in anaesthetized paralysed humans