Beware the airway filter: Deadspace effect in children under 2 years

Abstract

Background: Filters are increasingly used in breathing circuits as they protect the circuit from contamination and facilitate humidification of inspired gas. The use of filters, however, can augment the anatomical deadspace. This can be significant in children because they have much smaller tidal volumes. Methods: Following institutional ethical approval, 20 healthy children <2 years of age who required tracheal intubation were recruited. Ventilation was adjusted to achieve an endtidal carbon dioxide (PEco2) of 4.6 kPa (35 mmHg) when sampled at the tracheal tube (TT) adapter. Following a 10-min period of stabilization, an airway filter (22 ml) was introduced into the circuit. The respiratory rate (RR) was then adjusted to return PEco2 to 4.6 kPa (35 mmHg). Results: A mean increase in ventilation of 1.42 (0.38) l·min-1 was required to maintain a normal PEco 2 level. Airway pressure and respiratory rate increased by 7.9 mmHg (4.6) and 19.8 breath·min-1 (8.7) respectively. The P Eco2 and partial pressure of inspired carbon-di-oxide (PiCO2) measured from the TT adapter were higher than measured from the filter port. The mean increase was 3.6 (1.6) mmHg for PEco 2 and 5.9 (3.9) mmHg for PiCO2. Conclusion: Amplified deadspace from airway filters results in a significant increase in ventilation needed to maintain a normal PEco2 in children <2 years of age with normal lungs. Sampling of PEco2 and PiCO 2 from the filter significantly underestimates the effect of increased deadspace. The effect of increased deadspace may be predicted using a proposed mathematical model. © 2006 The Authors.