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The volume of air that participates in gas exchange because it is in contact with perfused alveoli is the alveolar ventilation ( V ˙ A = V ˙ E − V ˙ D physiological ). Physiological dead space may be increased with lung disease, due to an increase in the alveolar component. The physiological dead space ( V D) and wasted ventilation ( V ˙ D ) are usually expressed as a fraction of the tidal volume ( V D/ V T) and total ventilation ( V ˙ D / V ˙ E ), respectively. The symbols V D and V ˙ D without further qualifiers are used herein to signify the physiological dead space and its ventilation. V D physiological = V D anatomic + V D alveolar It is small in young healthy subjects but increases with age, and is particularly large in the presence of obstructive lung disease. The volume expired after the start of this secondary increase in nitrogen is the closing volume. This is because the airways in the more compliant regions of the lung (bases) have closed and the exhalate comes from the apical part, which, because of its lower compliance, received less oxygen during the initial inspiration.
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Towards the end of the exhalation the nitrogen concentration may show a secondary rise. A steeper rise during the plateau phase can be used as a measure of uneven ventilation. As the breath continues, alveolar gas is expired if ventilation is even, the plateau rises only slowly. There is then a mixing phase anatomic dead space is determined by bisecting the line of mixing. Initially, the expired gas comes from the dead space and is nitrogen free. The subject then exhales to residual volume into the spirometer, during which continuous recordings of nitrogen concentration and volume are made. The procedure is for the subject to make a maximal exhalation to atmosphere, then take a vital capacity inspiration of 100% oxygen. The method requires a rapidly responding nitrogen meter, a recording spirometer, and an X–Y plotter to relate the expired nitrogen concentration to the expired volume. These values can be determined using the technique described by Fowler, which involves a single breath of pure oxygen. It is the sum of closing volume and residual volume. Closing capacity is the volume of gas within the lungs at the point at which airways closure begins. Closing volume is the volume towards the end of a forced expiration, after which some airways have effectively closed and more of the expired gas comes more from the relatively poorly ventilated regions of the lung. Roger Hainsworth, in Foundations of Anesthesia (Second Edition), 2006 ANATOMIC DEAD SPACE AND CLOSING VOLUMEĪnatomic dead space is defined as the volume of the conducting airways, where no gas exchange takes place ( Fig. This approach is radically different from the assumption of a constant dead space which is subtracted from the tidal volume, the difference then is multiplied by the respiratory frequency to indicate the alveolar ventilation. V d/ V t = 0.3), then the alveolar ventilation will be 70% of the minute volume. Thus if the physiological dead space is 30% of the tidal volume (i.e. It is, therefore, generally more useful to use the V d/ V t ratio: the alveolar ventilation will then be (1 − V d/ V t) × the respiratory minute volume. Physiological dead space remains a fairly constant fraction of the tidal volume over a wide range of tidal volumes. Today it is universally defined by the Bohr mixing equation with substitution of arterial P co 2 for alveolar P co 2 as described later. Physiological dead space is the sum of all parts of the tidal volume that do not participate in gaseous exchange. Andrew B Lumb MB BS FRCA, in Nunn's Applied Respiratory Physiology (Eighth Edition), 2017 Physiological Dead Space