# What is the ventilation-perfusion ratio?

Last update25th Nov 2020

An important determinant of arterial oxygen tension is the effectiveness of coupling of lung ventilation to lung perfusion. Not all parts of the lung are equally ventilated or perfused. The relationship between ventilation and perfusion in a lung region is expressed as the ventilation-perfusion ratio (V/Q). The modest imbalance between ventilation and perfusion in normal individuals accounts for the small alveolar-arterial oxygen gradient routinely measured with arterial blood gas testing.

## Equal ventilation and perfusion

When breathing room air at an FIO2 of 0.21, an alveolus with one unit of ventilation and one unit of perfusion has a ventilation-perfusion ratio of one, a PAO2 of 100 mmHg, and a PACO2 of 40 mmHg.

Figure 1. Equal ventilation-perfusion ratio (V/Q = 1.0) occurs when the breathing air has an FIO2 of 0.21, a partial pressure of oxygen (PAO2) of 100 mmHg, and a partial pressure of carbon dioxide (PACO2) of 40 mmHg.

### Perfused, not ventilated

In one extreme of ventilation-perfusion mismatch, an alveolus is perfused, but not ventilated; in other words, it has a ventilation-perfusion ratio of zero. Since no air enters the alveolus as alveolar gas equilibrates with mixed venous blood returning to the lungs, the alveolar gas tensions are those of mixed venous blood: PAO2 of 40 mmHg and PACO2 of 45 mmHg.

Figure 2. A ventilation-perfusion ratio of zero (V/Q = 0.0) occurs when the alveolus is perfused but not ventilated. Since no air enters the alveolus, the alveolar gas pressure is the same as the mixed venous blood returning to the lungs.

### Ventilated, not perfused

In another extreme case of ventilation-perfusion mismatch, the alveolus is ventilated, but not perfused; in other words, the ventilation-perfusion ratio is infinity. In the absence of blood flow to the unit, the alveolar gas tensions are those of inspired air: PAO2 of about 150 mmHg and PACO2 of nearly 0 mmHg.

Figure 3. A ventilation-perfusion ratio (V/Q) of infinity occurs when the alveolus is ventilated but not perfused. Since there is an absence of blood flow to the unit, the alveolar gas tension is the same as inspired air.

#### Become a great clinician with our video courses and workshops

There actually is a spectrum of ventilation-perfusion relationships throughout the lung, created by normal physiologic relationships that dictate regional perfusion and ventilation

Figure 4. The ventilation, perfusion, and the ventilation-perfusion ratio spectrums throughout the lungs, created by normal physiology that dictate regional perfusion and ventilation.

In the upright lung, more ventilation goes to the lung base than the lung apex. This arises because there are more alveoli at the larger bases. In addition, the basilar alveoli are less stretched than the apical ones and can “give more” with inflation (i.e., they are more compliant).

Figure 5. Ventilation of the lung decreases as the rib number decreases to the apex lung. This arises because there are more alveoli at the larger bases, and basilar alveoli have larger inflation.

In the upright lung, more perfusion goes to the lung base than the lung apex because there are more alveoli and pulmonary blood vessels in the larger bases, and because gravitational effects on pulmonary blood flow favor perfusion to the bases.

Figure 6. Blood flow in the lung decreases as the rib number decreases to the apex lung. This arises because there are more alveoli and pulmonary blood vessels in the larger bases.

Although the apical-basal gradients for ventilation and perfusion are in the same direction, the magnitudes of changes in each from apex to base are different. The slope of the perfusion curve is steeper than that for ventilation. As a result, the ventilation-perfusion ratio decreases from apex to base.

Figure 7. The slope of the ventilation-perfusion ratio decreases from apex to base. This arises from the slope of the perfusion curve being steeper than that of the ventilation slope.

In disease states, ventilation-perfusion relationships throughout the lung are altered, creating abnormal gas exchange, especially for oxygen. In particular, regions of the lung characterized by ventilation-perfusion ratios of less than one contributes to hypoxemia and widening of the alveolar-arterial oxygen gradient.

That’s it for now. If you want to improve your understanding of key concepts in medicine, and improve your clinical skills, make sure to register for a free trial account, which will give you access to free videos and downloads. We’ll help you make the right decisions for yourself and your patients.

• Grippi, MA. 1995. “Gas exchange in the lung”. In: Lippincott's Pathophysiology Series: Pulmonary Pathophysiology. 1st edition. Philadelphia: Lippincott Williams & Wilkins. (Grippi 1995, 137–149)
• Grippi, MA. 1995. “Clinical presentations: gas exchange and transport”. In: Lippincott's Pathophysiology Series: Pulmonary Pathophysiology. 1st edition. Philadelphia: Lippincott Williams & Wilkins. (Grippi 1995, 171–176)
• Grippi, MA and Tino, G. 2015. “Pulmonary function testing”. In: Fishman's Pulmonary Diseases and Disorders, edited by MA, Grippi (editor-in-chief), JA, Elias, JA, Fishman, RM, Kotloff, AI, Pack, RM, Senior (editors). 5th edition. New York: McGraw-Hill Education. (Grippi and Tino 2015, 502–536)
• Tino, G and Grippi, MA. 1995. “Gas transport to and from peripheral tissues”. In: Lippincott's Pathophysiology Series: Pulmonary Pathophysiology. 1st edition. Philadelphia: Lippincott Williams & Wilkins. (Tino and Grippi 1995, 151–170)
• Wagner, PD. 2015. The physiologic basis of pulmonary gas exchange: implications for clinical interpretation of arterial blood gases. Eur Respir J45: 227–243. PMID: 25323225