Ventilationperfusion ratio VAQ

If the distributions of ventilation and perfusion are known, VA/Q can be estimated. The vertical distributions of ventilation and blood flow in a healthy subject, both awake and anesthetized, are summarized in the upper panels of Fig, 3. As discussed earlier, both ventilation and blood flow increase down the lung, with the fractional increase of blood flow being larger than that of ventilation. The average VA/Q is approximately unity, with higher ratios in upper lung regions and lower ratios in lower lung regions. Thus VA/Q will vary from approximately 0.5 to 5. VA/Q can also be assessed without direct measurement of the spatial distributions of ventilation and blood flow.

Fig. 3 Schematic ventilation V and perfusion Q distributions from top to bottom of the lung in a healthy subject when awake and anesthetized, as well as the ventilation-perfusion ratios VA/Q that fit the vertical distributions of ventilation (open circles) and blood flow (solid circles). Note the increasing ventilation and blood flow down the lung in the awake subject, with a rather good match of VA/Q at all vertical levels, which is also evidenced by the narrow distribution of VA/Q ratios centered upon VA/Q= 1. Anesthesia lowers FRC and causes collapse of dependent lung regions, and may promote airway closure, impeding ventilation in lower lung regions. Increased alveolar pressure, as may occur during mechanical ventilation, impedes perfusion of the uppermost lung regions. The ensuing VA/Q distribution differs from the awake situation with a shunt ( VA/Q= 0) and perfusion of poorly ventilated lung regions ( VA/Q< 0.1). The poor perfusion in upper regions may also cause high VA/Q ratios (VA/Q> 10). It can be anticipated that much of the anesthesia-induced VA/Q mismatch may also develop in acute respiratory failure.

Fig. 3 Schematic ventilation V and perfusion Q distributions from top to bottom of the lung in a healthy subject when awake and anesthetized, as well as the ventilation-perfusion ratios VA/Q that fit the vertical distributions of ventilation (open circles) and blood flow (solid circles). Note the increasing ventilation and blood flow down the lung in the awake subject, with a rather good match of VA/Q at all vertical levels, which is also evidenced by the narrow distribution of VA/Q ratios centered upon VA/Q= 1. Anesthesia lowers FRC and causes collapse of dependent lung regions, and may promote airway closure, impeding ventilation in lower lung regions. Increased alveolar pressure, as may occur during mechanical ventilation, impedes perfusion of the uppermost lung regions. The ensuing VA/Q distribution differs from the awake situation with a shunt ( VA/Q= 0) and perfusion of poorly ventilated lung regions ( VA/Q< 0.1). The poor perfusion in upper regions may also cause high VA/Q ratios (VA/Q> 10). It can be anticipated that much of the anesthesia-induced VA/Q mismatch may also develop in acute respiratory failure.

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