Resistance(R) and Time Constant(t) Resistance measured in presence of flow o R=AP/F where f= flou elastic z=RxC⊥△P△FCmH2O sec F△Pml/ sec cmh2O Where t= time constant tiime it takes for 64% of volume to be exhaled. normally r-2cmh2o per ml of air per sec and c=0.1 liter/ cmH2O so that t=2 sec. About 4t(0. 8 sec) needed for complete expiration Assumption: single unit emptying Endotracheal tubes and inhomgeneous ventilation can greatly slow emptying- multiple compartments. Anything which increases R or C increases exhalation time(both occur in obstructive lung disea R = ΔP/F where F = flow  PLung = Presistance + Pelastic Where τ = time constant tiime it takes for 64% of volume to be exhaled. Normally R=2cmH2O per ml of air per sec and C= 0.1 liter/cmH2O so that τ = .2 sec. About 4 τ (0.8 sec) needed for complete expiration. Assumption: single unit emptying. Endotracheal tubes and inhomgeneous ventilation can greatly slow emptying – multiple compartments. Anything which increases R or C increases exhalation time (both occur in obstructive lung disease.) sec /sec 2 2           cmH O ml ml cmH O P V F P  R C Resistance measured in presence of flow