Physiological Considerations in the Mechanically Ventilated Patient

The volume of the upper airway is approximately 72 mL in the adult subject.64 An endotracheal tube of 8 mm internal diameter cuts down this volume by 55-60 mL or by approximately 1 mL/kg body weight.26 By thus reducing the upper airway volume - and the dead-space - this can increase the alveolar ventilation. In health, it appears that the volume of the upper airway can change by as much as 50% by mere changes in head position. Therefore, the diminution in airway volume that occurs when an endotracheal tube is placed may not be greatly beyond the physiological changes that occur in the innate airway.64 In fact, the interposition of a Y-connector adds approximately 75 mL of dead-space to the circuit, and so the impact of the endotracheal tube in reducing the dead-space is largely negated.