The mean difference between new equal area method and the Fletcher area method was -0.09 ml (-1.52 to 1.34 ml). The mean difference (limits of agreement) between the physiologic dead spaces calculated by the new equal area method and Bohr-Enghoff equation was -0.07 ml (-1.27 to 1.13 ml). The new graphical equal area method for calculating physiologic dead space is shown analytically to be identical to the Bohr-Enghoff calculation. Dead space was varied by varying tidal volume, end-expiratory pressure, inspiratory-to-expiratory ratio, and inspiratory hold in each patient. Physiologic dead spaces of 1,200 carbon dioxide expirograms obtained from 10 ventilated patients were calculated by the Bohr-Enghoff equation, the Fletcher area method, and the new graphical equal area method and were compared by Bland-Altman analysis. This study introduces a graphical method that uses similar principles for measuring and displaying anatomical, physiologic, and alveolar dead spaces.Ī new graphical equal area method for estimating physiologic dead space is derived. Alveolar dead space is calculated as the difference between anatomical dead space estimated by the Fowler equal area method and physiologic dead space. V d V t = P A C O 2 − P e C O 2 P A C O 2 Ī common step is to then presume that the partial pressure of carbon dioxide in the end-tidal exhaled air is in equilibrium with that gas' tension in the blood that leaves the alveolar capillaries of the lung.Physiologic dead space is usually estimated by the Bohr-Enghoff equation or the Fletcher method. The original formulation by Bohr, required measurement of the alveolar partial pressure P A. The Bohr equation is used to quantify the ratio of physiological dead space to the total tidal volume, and gives an indication of the extent of wasted ventilation. It differs from anatomical dead space as measured by Fowler's method as it includes alveolar dead space. This is given as a ratio of dead space to tidal volume. The Bohr equation, named after Danish physician Christian Bohr (1855–1911), describes the amount of physiological dead space in a person's lungs. Not to be confused with the Bohr model or the Bohr effect.
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