1984 Volume 4 Issue 3 Pages 284-290
A new and simple method was established to measure the tidal volume of preterm and term neonates ventilated with a Servo 900C ventilator on pressure control pattern. The expiratory tidal volume (Vte) measured by the ventilator consists of the true tidal volume (Vt) and the compressible volume (Vc) in the patient circuit. If Vte is correct and Vc is known, Vt is calculated as Vte-Vc.
The accuracy of Vte of a Servo 900C in small tidal volume area was tested injecting known volume of air (Vinj) into the expiratory inlet of the ventilator during the expiratory phase; Vinj=0.73×Vte-1.01 (γ=0.997). On the ventilator at the ordinary working condition (pressure control pattern, f=40b/m, insp. time%=50%) with closing the Y-piece of the patient circuit, the relation between Vc measured as Vte on the ventilator and the end inspiratory and end-expiratory pressure (EIP and EEP, respectively) on a mingograph was studied; without PEEP Vc=0.31×(EIP-EEP)-1.19 (γ=0.995), and with PEEP Vc=0.26×EIP=0.99 (γ=0.992). These results suggest that Vt can be calculated with Vte, EIP and EEP. Measurement of tidal volume was performed on a test lung model of premature baby ventilated with the ventilator, and the result was compared with that of the simultaneously performed bodypletysmography. Vt measured by our method varied from 4.1-13.2ml and the difference between paired measurements was 0.02±0.51ml (mean±SD) with a muximum difference of 1.0ml.
It is concluded that using our method, the small tidal volume measurement can be performed with a satisfactory accurancy and without disturbing the ordinary ventilation nor the safety of the baby.
