The quality of breathing and capnography during laryngeal mask and facemask ventilation

Clinical measures of ventilation and the relationship between arterial and end-tidal carbon dioxide tensions were studied during inhalational anaesthesia in 18 patients using a laryngeal mask airway or a facemask. Tidal volumes were similar in both groups but expired minute volume, respiratory rate and physiological deadspace ventilation were significantly increased in the facemask group. Both end-tidal and arterial carbon dioxide tensions were higher in the laryngeal mask group. Mean arterial to end-tidal carbon dioxide tension differences ranged from 0.13 to 4.13 kPa in the facemask group and from 0-1.73 kPa with the laryngeal mask airway. Pooled data analysis revealed a better correlation between arterial and end-tidal carbon dioxide tensions during laryngeal mask ventilation as compared to facemask breathing. With both techniques the arterial to end-tidal carbon dioxide tension difference was related to respiratory rate and physiological deadspace ventilation. Estimation of arterial carbon dioxide partial pressure by monitoring end-tidal carbon dioxide tension is more reliable with the laryngeal mask airway than during facemask breathing, in particular at small tidal volumes.