Work of breathing during spontaneous ventilation in anesthetized children: A comparative study among the face mask, laryngeal mask airway and endotracheal tube

Work of breathing (WOB) increases during general anesthesia in adults, but such information has been limited in pediatric patients. We studied WOE in 24 healthy children (mean age 2 +/- 1.9 yrs), during elective urogenital surgery under 1 minimum alveolar anesthetic concentration halothane-nitrous oxide anesthesia with a caudal block while breathing spontaneously. WOE was measured with an esophageal balloon, miniature flowmeter, and a computerized (Bicore) system. In each patient, WOE was computed under four conditions: a mask without oral airway (-AW), a mask with oral airway (I-AW), a laryngeal mask airway (LMA), and an endotracheal tube (ETT). With each apparatus WOE was studied both with continuous positive airway pressure (CPAP) (5-6 cm H(2)O) and without CPAP (or zero end-expiratory pressure [ZEEP]). Under ZEEP, WOB (g.cm/kg) among the four apparatus were (mean +/- SEM): mask (-AW) (64 +/- 19.2) > mask (+AW) (44 +/- 17.2), LMA (42 +/- 15.6) > ETT (25.4 +/- 12.4) (P < 0.05). WOE with CPAP significantly (P < 0.05) decreased from WOE with ZEEP in three groups (mask [-AW], mask [+AW], and LMA), but not in the ETT group. Tidal volume (both ZEEP and CPAP) and end-tidal P(CO2) (with CPAP only) were significantly (P < 0.05) decreased only in the ETT group, whereas no significant difference was found in respiratory rate or minute volume among the four airway apparatus groups, either with or without CPAP. The reduction in WOB, when breathing through ETT was primarily attributable to decreases in tidal volume and volume work. The finding that WOB decreases with CPAP in all groups except for the ETT group suggests that the decrease is a result of improved patency of the upper airway rather than of increases in functional residual capacity and lung compliance.