Many of us may work in an area that is not designed specifically for the care of children undergoing anesthesia. In this setting it might prove tempting to use an adult breathing circuit. Is this appropriate? How might it affect patient outcome?
Drs. Matthew F. Pearsall and Jeffrey M. Feldman, General Anesthesia Division, Children’s Hospital of Philadelphia, Perelman School of Medicine / University of Pennsylvania, Philadelphia, Pennsylvania, demonstrate the effect of apparatus dead space on CO2 elimination and ventilation requirements in children. Their analysis appears in this month’s edition of Anesthesia & Analgesia in the technical communication “When Does Apparatus Dead Space Matter for the Pediatric Patient?”
The authors present a mathematical model of the relationship between increases in breathing circuit apparatus dead space volume and alveolar CO2. The model describes the change in minute ventilation needed to keep alveolar CO2 constant as dead space increases. The authors used circuit filters and connectors commonly used in their institution. They determined volumes either through package insert documentation or by measuring the mls of water needed to fill the internal volume. In their model, patient weights varied from 2-17 kg. They used the formula FACO2 =VCO2/VA where FACO2 is the fraction of alveolar CO2, VCO2 is metabolic minute production of CO2, and VA is the difference between total minute ventilation and dead space ventilation.
Their model demonstrated that increasing apparatus dead space increases PACO2, requiring an increase in RR to maintain normocapnia. They also showed that Vd/Vt increases more rapidly for smaller patients as dead space increases. Lastly, they showed that the relationship between PACO2 and apparatus dead space is exponential.
This technical communication confirms what pediatric anesthesiologists realize intuitively: additions of various airway devices on the patient end of the Y-piece can have substantial effects on the efficiency of CO2 elimination. If not countered in an effective way, devices that increase dead space can lead to serious hypercarbia and even hypoxemia. The model described by Drs. Pearsall and Feldman’s provides significant insight into the standard textbook description that dead space increases ventilatory requirements.