Invasive and Noninvasive Measurement of the Respiratory Deadspace in Anesthetized Children with Cardiac Disease

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Abstract
To compare the magnitude of the different “invasive” and “noninvasive” dead space variables and the effect on them of ventilator setting, CO2 single breath tests (SBT-CO2) were obtained using an on-line computerized si/stem based on the Servo ventilator and CO2, Analyzer 930, in 50 children anesthetized for cardiac surgery. The variables were the airway dead space (VDaw), Bohr's dead space (VDBohr) obtained noninvasively using end-tidal PCO2 (PETCO2) foralveolar PCO2 in the dead space equation, and the physiologic dead space, VDphys. In 42 children with normal single breath tests, VDaw was two-thirds of VDBohr; in 9 children in whom phase III of SBT-CO2 (the “alveolar plateau”) was steeper than normal, it was only half of VDBohr. Steeper slopes of phase III were seen particularly in the present of left-right (LR) shunting. VDphys was very similar in magnitude to VDBohr in all children, except those with right-left (RL) shunts. VDaw was the major component of VDphys only in children with normal arterial-end-tidal PCO2 differences, i.e., those without RL shunts. When two ventilator frequencies giving the same alveolar ventilation were compared in children with normal gas exchange, VDBohr as a fraction of tidal volume was least at the lower frequency, as it also is in adults. The data confirm that noninvasive CO2 monitoring and measurement of dead-space gives useful indexes of the adequacy of ventilation in all children except those with RL shunts.

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