PETCO2  measured via ET tube                                PETCO2 measured via LMA

During spontaneous ventilation in adults, the mean difference between PaCO2 and PETCO2 measured via LMA is similar to that measured via endotracheal tube.1 However, in children, PETCO2 measured via LMA does not accurately reflect PaCO2 in spontaneously breathing children.2 On the other hand, infants and children weighing less than 10 kg who are mechanically ventilated via the LMA, PETCO2 is as accurate an indicator of PaCO2 as when ventilated via LMA.3

References:
1. Hicks I, Soni N, Shephard J. Comparison of end-tidal and arterial carbon dioxide measurements during anaesthesia with laryngeal mask airway.  Br J Anaesth 1993;71:734-5.

2. Spahr-Schopfer IA, Bissoonnette B, Hartley EJ. Capnometry and the pediatric laryngeal mask airway. Can J Anaesth 1993;40:1038-43.

3. Chhibber AK, Kolano JW, Roberts WA. Relationship between end-tidal and arterial carbon dioxide with laryngeal mask airways and endotracheal tubes in children. Anesth Analg 1996;82:247-50.]