Everything you need to know about the ventilation / perfusion (V / Q) ratio

Not all parts of the lung are equally ventilated and perfused. In this video, from our Blood Gas Analysis Essentials course, we'll cover the anatomical and physiological factors that determine how well different parts of the lung are ventilated, the key equations you need to know that describe this heterogeneity, and how this affects your clinical decision-making.

Master arterial blood gas analysis based on an understanding of relevant physiological principles. You’ll cover the crucial factors that determine the oxygenation of blood in the lungs, as well as oxygen transport and delivery to peripheral tissues. Learn about the interplay between blood gas and acid-base analysis, and how carbon dioxide affects arterial pH. This course complements our Acid-Base Essentials course.

A critically important determinant of the arterial oxygen tension is the effectiveness of coupling of lung ventilation to lung perfusion. But not all parts of the lung are equally ventilated and perfused. The relationship between ventilation and perfusion in a lung region is expressed as the ventilation perfusion ratio expressed as v dot slash q dot.

When breathing room air at an FIA O two of 0.21 and alveolus with one unit of ventilation, and one unit of perfusion has a v q of one and alveolar oxygen tension of 100 and an alveolar carbon dioxide tension of 40. Now let's imagine one extreme of ventilation perfusion mismatch and alveolus is perfused, but not ventilated, that is, has a v q of zero.

Here since no external air can enter in the alveolar gas equilibrates with mixed venous blood in the capillary, the alveolar gas pressures are the same as in mixed venous blood returning to the lungs alveolar oxygen tension of 40 millimeters of mercury and alveolar carbon dioxide tension of 45 millimeters of mercury. In another extreme case of a ventilation perfusion mismatch, the alveolus is ventilated, but not perfused.

That is, v q is infinity. In the absence of blood flow to the unit, the alveolar gas pressures are the same as inspired air. That is an alveolar oxygen tension of about 150 millimeters of mercury, and an alveolar carbon dioxide tension of nearly zero. It's useful to think about a range of v Q relationships throughout the 300 million alveoli in the normal lung.

There actually is a spectrum of v Q relationships throughout the lung, created by normal physiologic relationships that dictate regional blood flow, or perfusion and ventilation. It's the gradients for ventilation and perfusion in the normal lung that create variation in these variables. It's useful to understand how ventilation and perfusion gradients arise in the lung, and contribute to adverse effects on gas exchange in disease.

In the upright lung, more ventilation goes to the lung base than to the lung apex. As another way of looking at this, we can plot the relationship between ventilation and ribs number in regions of the lung corresponding to lower rib numbers, that is more apical regions, ventilation is less than in Basilar regions.

This arises for two reasons. One, there are more alveoli at the larger lung bases and to the basilar alveoli are less stretch than the apical ones, and can give more with inflation. That is to say they are more compliant. In the upright lung, more perfusion goes to the lung base than the lung apex. Again, we can plot the blood flow or perfusion against rib number to get a better sense of this relationship.

In the regions of the lung corresponding to lower rib numbers, that is more apical regions, perfusion is less than in the basilar regions. This arises for two reasons. One, there are more alveoli and pulmonary blood vessels at the larger lung bases. And two, gravitational effects on pulmonary blood flow favour perfusion at the lung bases.

As we've just seen, the apical basal gradients for ventilation and perfusion are in the same direction with greater ventilation and perfusion at the bases. However, the magnitudes of changes in each from base to apex are different with the slope of the perfusion curve steeper than that for ventilation. So there is more perfusion and ventilation at the bases and there is greater ventilation and perfusion at the APCs.

So if we now plot the v Q Ratio against rib number, we can see the ratio increases from base to apex producing the distribution of a alveolar oxygen tension based on this distribution of v Q ratios with higher P alveolar OTU in apical regions and lower p alveolar OTU. In basal regions.

The modest imbalance between ventilation and perfusion in normal individuals accounts for the small alveolar arterial oxygen gradient routinely measured with an arterial blood gas analysis. In disease states, v Q relationships throughout the lung may be profoundly altered, creating abnormal gas exchange, especially for oxygen.

In particular, regions of the lung characterized by a v q of less than 1.0 contribute to hypoxemia and widening of the alveolar arterial oxygen gradient. In fact, the impact of disruption in the relationship between ventilation and perfusion on arterial oxygen tension in lung disease is significantly greater than the effects of other pathophysiologic arrangements, for example, diffusion block or hypo ventilation.

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