Dead space ventilation in volume controlled versus pressure controlled mode of mechanical ventilation

Dead space ventilation (VD) is one of the important measurements that indicates the ventilatory efficiency of a patient who requires mechanical ventilation. However, VD is not constant and can change according to the pathology in the lungs, ventilatory patterns, perfusion and ventilation-perfusion matching. The objective of this study was to measure and compare the dead space in pediatric patients who were using pressure controlled and volume controlled modes of mechanical ventilatory by measuring the difference between arterial PCO₂ and end-tidal PCO₂ [P(a-ET)CO₂]. From November 1996 to March 1997, 12 patients who were admitted to the pediatric intensive care unit and needed ventilator support for various reasons, were enrolled in the study. Their ages ranged from 2 to 15 years. The mechanical ventilator (Benett 7200 or Servo 900C) setting during VD measurement i.e. tidal volume, inspiratory time and positive end expiratory pressure were kept constant between changing from pressure controlled to volume controlled mode or vice versa for twenty minutes in order to allow adequate time for equilibration. The P(a-ET)CO₂ between volume controlled and pressure controlled mode were 3.1 and 2.6 torr (p=0.5) and peak inspiratory pressure were 20.0 and 17.3 torr (p=0.01), respectively; whereas mean airway pressure, PaO₂, O₂ saturation and heart rate revealed no significant difference between these two modes. The authors concluded that VD in pressure controlled mode from the present study was not significantly different from VD when using volume controlled mode of mechanical ventilation in the same patient. However, VD will change according to the pathophysiologic change in respiratory system and can be used for monitoring of ventilatory pattern of patients in the pediatric intensive care unit.