Volume and pressure during transient added resistance

Total respiratory resistance (Rrs) can be computed from measurements of flow before and during a brief period of added resistance. This "added resistance' method does not require measurement of total driving pressure (Pt). One of the assumptions of this method is that the change in Pt (delta Pt) is negligible during the time of increased resistance. To evaluate this assumption we measured both the volume exhaled during a brief imposition of an external resistor, and the total respiratory compliance (Crs) of 7 healthy volunteers. The mean volume exhaled was 18.6 ml, and the mean Crs was 88 ml/cm H2O. The volume exhaled divided by Crs gives the change in total static elastic recoil pressure (delta Pel, rs), which is one component of delta Pt. The mean delta Pel, rs was 0.22 cm H2O. Since the pressure exerted by contraction of inspiratory muscles (Pmus,I) opposes Pel, rs, we also estimated delta Pmus,I and subtracted it from delta Pel, rs to obtain delta Pt. The computed mean delta Pt during the time of added resistance was 0.045 cm H2O. This value is quite small in relation to Pt during a spontaneous expiration at rest. We also studied 13 subjects with Chronic Obstructive Pulmonary Disease and found no significant effect of transient added resistance on intraesophageal pressure. We conclude that the assumption of negligible delta Pt should not prevent the added resistance method from being very useful, particularly when simplicity of equipment and testing procedure are important.