FLOW AND VOLUME DEPENDENCE OF RESPIRATORY SYSTEM MECHANICS DURING CONSTANT FLOW VENTILATION IN NORMAL SUBJECTS AND IN ADULT RESPIRATORY-DISTRESS SYNDROME

Seven control subjects and seven patients with adult respiratory distress syndrome (ARDS) were artificially ventilated and flow, volume, and tracheal pressure were monitored. Respiratory system resistance (R(rs,max)) was partitioned into its homogeneous (R(rs,min)) and uneven (R(rs,u)) components. Respiratory system elastance (E(rs)) was also measured. In both groups E(rs) did not vary with different inspiratory flows and volumes, but was significantly higher in ARDS. With increasing volume (isoflow maneuvers), R(rs,max) and R(rs,u) increased but R(rs,min) remained unaltered in ARDS. In control patients, however, resistances did not vary but R(rs,max) and R(rs,u) were smaller and R(rs,min) equaled their corresponding values in ARDS. Hence, stress relaxation seems to be increased in ARDS. During isovolume maneuvers R(rs,max) and R(rs,u) decreased with increasing flows (both groups), although they were significantly higher in ARDS. R(rs,min) was not modified by different flows and was similar in both groups. Thus, pendelluft is also increased in ARDS. In conclusion, the mechanical profile of ARDS is characterized by increased E(rs) and R(rs,max), the latter being secondary to augmented mechanical unevenness within the system.