Video narration
Fowler's method is used to measure anatomical dead space using the single-breath nitrogen washout technique. First, the patient exhales to residual volume and then inhales a vital-capacity breath of 100% oxygen. The patient then exhales slowly through a nitrogen analyser, which measures nitrogen concentration against expired volume. Let's understand the four phases of the nitrogen concentration curve. Phase one: Nitrogen concentration remains nearly zero because the exhaled gas comes mainly from the conducting airways, which contain the oxygen inhaled during the test. Phase two: Nitrogen concentration rises sharply as gas from the conducting airways mixes with nitrogen-rich alveolar gas. Phase three: This is the alveolar plateau, where predominantly alveolar gas is exhaled. The curve may slope slightly because different lung regions empty at different rates. Phase four: At low lung volumes, airway closure causes uneven emptying of the lungs, producing a further rise in nitrogen concentration. Now, how is anatomical dead space calculated? Fowler's method uses the equal-area principle. A vertical line is drawn through phase two so that the areas on either side of the line, relative to the idealized transition between dead-space and alveolar gas, are equal. The expired volume from the beginning of expiration to this dividing line represents the anatomical dead-space volume. Remember: Phase one represents dead-space gas, phase two is the transition, phase three is the alveolar plateau, and phase four indicates airway closure. That is Fowler's single-breath nitrogen washout method.