Video narration
DEAD SPACE Not all the air we breathe participates in gas exchange. The volume that does not take part is called dead space. Let's understand its four important types. First, anatomical dead space. This is the air within the conducting airways, where gas exchange does not occur. According to the traditional Weibel model, the conducting airways extend from generation zero, the trachea, through generation sixteen, the terminal bronchioles. The respiratory zone begins at generation seventeen. Anatomical dead space in adults is approximately two millilitres per kilogram. It can be measured using Fowler's single-breath nitrogen washout method. Second, alveolar dead space. Here, air reaches the alveoli, but blood flow is absent or insufficient for effective gas exchange. A classic example is pulmonary embolism, where blood flow to ventilated alveoli is obstructed. Alveolar dead space can be estimated by subtracting anatomical dead space from physiological dead space. Third, physiological dead space. This is the total volume of ventilation that does not participate effectively in gas exchange. It includes anatomical dead space plus alveolar dead space. It can be calculated using the Bohr equation. Clinically, the Enghoff modification, which uses arterial carbon dioxide tension, is commonly used. Finally, apparatus dead space. This is the volume of gas within equipment between the patient and the breathing system that can be rebreathed. For example, consider a face mask. Its internal volume can be estimated by filling the mask cavity with water and measuring the volume of water required. That gives an estimate of the mask's internal volume, which contributes to apparatus dead space. Apparatus dead space is especially important in infants and patients receiving small tidal volumes. So remember: anatomical is the conducting airways; alveolar is ventilation without adequate perfusion; physiological is anatomical plus alveolar; and apparatus dead space comes from equipment.