Hence, the V/Q in non-dependent regions rises to 10. Lung overinflation compresses the capillaries and reduces blood flow to those units. The dependent lung units are inflated normally but the non-dependent units are now overinflated. Let’s take the example of a patient with ARDS who is set on a high PEEP and high tidal volume. Again, this is not truly alveolar dead space since the V/Q ratio is not infinity. >= 10), this can cause a ‘dead space effect’. If a substantial proportion of lung units have a very high V/Q (e.g. Normally, there is only mild heterogeneity of V/Q ratios but this is increased in various pathological states. This is the most difficult to grasp but also perhaps the most important factor in many disease states. A V/Q of infinity is the definition of alveolar dead space. Since the perfusion of the lung unit reduces to zero, the V/Q ratio for this unit is infinity (V÷0 = ∞). Consider a scenario where a small embolus blocks all perfusion to a lung unit. Normally, ventilation and perfusion of respiratory zones (lung units) are nicely matched (normal V/Q ~0.9). The dead space in an average adult has been reported to be ~150 cc or 2cc/kg ideal body weight. Anatomical dead space is thus defined as the volume of the conducting zone (Figure 1). The remaining circuit: respiratory bronchioles to alveolar sacs (generation 23) participate in gas exchange and is called the respiratory zone. The entire airway circuitry all the way from mouth to the terminal bronchioles (~generation 14-16) is the conducting zone of the respiratory system.
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