[Conditioning factors in intrapulmonary gas trapping during controlled mechanic ventilation in anesthesia]

To establish a relation between intrapulmonary gas trapping (Vat) generated by controlled ventilation during anesthesia and the factors that are likely to condition such trapping: flow volume (VT), expiratory time (te), compliance (C), resistance (R) and a time constant in either the anesthetized patient (tau) or under conditions of forced expiration in respiratory function testing (TC 25/50).

Thirty patients undergoing surgery with general anesthesia were ventilated following 3 different patterns with te of 2.66, 2 and 1.33 sec, respectively. We measured Vat, C, R and tau for each pattern. Correlation coefficients were figured and line regressions were drawn for Vat and the conditioning factors. We also figured the correlation between mean Vat and calculated Vat based on TC 25/50.

Intrapulmonary gas trapping occurred in 93% of patients and increased significantly as te decreased. Correlations between Vat and the factors were as follows: r=0.76 for tau, r=0.62 for C, r=0.62 for R, r= 0.13 for VT and r2 = -0.92 for te/tau. The correlation coefficient of tau and TC 25/50 was 0.92.

The results obtained indicate that Vat depends above all on the ratio of te to tau and that trapping can be prevented by applying a te that is greater than 3 tau.