Effective ventilation at conventional rates with tidal volume below instrumental dead space: a bench study.

OBJECTIVE: The authors previously showed that 48% of infants <800 g were ventilated with tidal volume (VT) < dead space (DS) using volume guarantee (VG) ventilation. Here, The authors sought to confirm those findings under the rigorous conditions of a bench study.

DESIGN AND METHODS: The authors measured the time to wash out CO2 from a 45-ml test lung using end-tidal CO(2) monitor (ETCO(2)). The test lung was filled with 100% CO(2), then ventilated using VG at VT ranging from DS+2 ml to DS-1.5 ml. With ventilation, ETCO(2) declined exponentially as CO(2) was washed out, the rate being proportional to VT - effective instrumental DS. The time from initiation of ventilation to threshold of accurate detection was determined in triplicate.

RESULTS: Halving the theoretical 'alveolar ventilation' (DS+2 ml to DS+1 ml) only increased the elimination time by 26%, not the 100%, as predicted by conventional physiology. CO(2) washout was less efficient, but still occurred even at VT=DS and VT=DS-1.5 ml. Halving the theoretical 'alveolar ventilation' by decreasing respiratory rate from 80 to 40 breaths/min only increased elimination time by 35%, not 100%, as predicted by conventional physiology. Twenty minutes of continuous positive airway pressure prior to ventilation did not alter the elimination time, verifying that CO(2) did not diffuse or leak out of the test lung. Size of the endotracheal tube (ETT; 2.5, 3.0 and 3.5 mm) flow rate (4, 6 and 10 l/min) and inspiratory time (0.25 vs 0.35 s) did not affect the results.

CONCLUSIONS: Contrary to conventional physiology, effective CO(2) elimination appears to be possible with VT<DS even at conventional rates. With small ETT a spike of fresh gas likely penetrates through the DS, rather than pushing it ahead.