For this and other reasons anatomic dead space is now most frequently measured using CO2. Theoretically, the Fowler dead space measurement could be performed by any system capable of measuring single-breath nitrogen washouts but it is actually performed most frequently by volumetric capnography systems used to monitor ICU patients and patients undergoing anesthesia. Numerous gases have been studied for this purpose (helium, oxygen, nitrogen, argon, nitrous oxide, carbon dioxide) and they all produce similar curves and with the possible exception of helium they all produce reasonably similar values for dead space. Fowler back-extrapolated the phase III slope and using a planimeter (a mechanical device that measures area) he found the mid-point of Phase II that was defined by equal areas (A1, A2). Phase II is the S-shaped transition between pure deadpace gas and alveolar gas. In this method, after an inhalation of oxygen, the nitrogen concentration in an individual’s exhalation is plotted against exhaled volume.ĭuring exhalation, phase I is where nitrogen-free gas is exhaled from the airways. The most commonly used method for measuring anatomic dead space in a research setting is the single-breath technique developed by Fowler in 1948. The physiological component of the respiratory dead space cannot be determined without knowing the anatomical dead space.Īnatomic dead space is usually considered to be the physical volume of the airways but static measurements of airway volume do not take into consideration the dynamic aspects of respiration. An accurate estimate of this volume is important because respiratory dead space ( Vd/Vt, discussed previously) is composed of both anatomical and physiological dead space. The official definition of anatomic dead space is that it is that part of the inhaled volume that remains in the airways at the end of inhalation and does not participate in gas exchange.
It’s a relatively simple test but assessing its results as well as the results of alternate dead space measurement techniques turns out to be more complicated than I had remembered. I’d spent some time researching single-breath tests a while back and of course ran across the Fowler method for measuring anatomic dead space.
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