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Ventilation–perfusion coupling

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Ventilation-perfusion coupling

Lead paragraph

Physiology

Real-time magnetic resonance imaging (MRI) showing the thoracic cavity volume change during ventilation

Ventilation

Ventilation (or breathing) is the flow of moving air in and out of the lungs to facilitate gas exchange between the lungs and the atmospheric air. The air flows into the lungs through inhalation (inspiration) and out of the lungs through exhalation (expiration)[1]. During ventilation, the air movement is the air pressure gradient between the atmosphere and the lungs produced by the contraction of thoracic muscles and the diaphragm. The air is forced in and out of the lungs as the air flows from the region with higher pressure to the region with lower pressure. During inhalation, the diaphragm contraction causes the increase of the thoracic cavity volume. This results in the decrease of the pressure inside the lungs, forcing the air to flow into the lungs. During exhalation, the diaphragm relaxation causes decrease of the thoracic cavity volume. The increased pressure inside the lungs pushes the air out of the lungs[2]. The primary function of ventilation is the replacement of the stale gases in the lungs with fresh air through the removal of carbon dioxide for oxygenation of the blood[3]. The oxygen is then supplied to the entire body by the circulatory system.

Exchange of carbon dioxide and oxygen between alveolus and pulmonary capillaries during perfusion

Perfusion

Perfusion is the delivery of oxygen-rich blood to the body's tissues through the lymphatic system or circulatory system[4]. The primary function of perfusion is the efficient removal of cellular waste and nutrition supply during gas exchange. Perfusion occurs during heart contraction when the oxygen-rich blood is pumped into the arteries. The arteries deliver the blood to the capillary bed of the tissues, where the oxygen is removed by the process called diffusion[5]. Oxygen in the alveoli moves down the concentration gradient, diffusing into the blood through the pulmonary capillaries. Once oxygen enters the blood, it dissolves in plasma by binding to hemoglobin (Hb) of red blood cells and transported to body tissues[6]. Then the deoxygenated blood returns to the heart via veins, and perfusion begins again after the blood is re-oxygenated through the ventilation process.

Anatomy

Ventilation-perfusion ratio

Ventilation-perfusion coupling is the relationship between ventilation and perfusion, represented by the ventilation-perfusion ratio(V/Q). Ventilation rate (V) is the total volume of gas that enters and leaves the alveoli in a given amount of time, usually in a minute. Ventilation rate is calculated by multiplying the tidal volume (volume of gas either inhaled or exhaled gas during the normal breath) by the frequency of breaths per minute, represented by the formula, tidal volume (L) x breath per minute (breath/min) = L/min[7]. Perfusion (Q) is the total volume of blood that enters the alveolar capillaries in a given amount of time during the gas exchange. Therefore, the ventilation-perfusion ratio represents the volume of gas that enters the alveoli per minute compared to the volume of blood that enters the alveoli per minute.

The ideal V/Q ratio is 1, the most efficient state of pulmonary function when the amount of oxygen entering the lungs equals the amount of oxygen being delivered to the body. Adequate achievement of the ventilation and perfusion matching is essential as it ensures the continuous supply of oxygen and removal of waste products from the body. Thus, strict regulation of ventilation and perfusion is needed for efficient gas exchange.

On average, four liters of oxygen (V) and five liters of blood (Q) pass through the alveoli in a minute, thus the normal V/Q ratio is 0.8[8]. It is considered abnormal when the ratio is greater or smaller than 0.8. When the V/Q ratio is above 0.8, it indicates that ventilation exceeds perfusion. This might have been caused by blood clotting, heart failure, pulmonary emphysema, or damage in alveolar capillaries. On the other hand, the V/Q ratio below 0.8 is an indication of excess perfusion, which may have been caused by pneumonia, pulmonary edema, asthma, or the blockage of the bronchus[9].

While the ideal V/Q ratio is 1, the ratio in the normal lungs of the healthy individual is approximately 0.8, meaning that the ventilation and perfusion do not equal. Due to gravity, lower lungs have a relatively greater amount of blood, and upper lungs have a relatively greater amount of air. Thus, the blood in the lower lungs is not fully oxygenated, and the oxygen of air in the upper lungs is not fully extracted, decreasing the V/Q ratio[10].

Regional variations

The V/Q ratio differs within the lungs, depending on the region of the lungs concerned. The ventilation rate is 50% greater at the base than the lungs' apex. The V/Q ratio in the apex is approximately 3.3 and in the base is 0.63, which indicates that perfusion is greater than ventilation towards the base, and the ventilation rate is greater than perfusion towards the apex[11].

Towards the base of the lungs, the fluid volume in the pleural cavity increases due to gravity, resulting in increased intrapleural pressure. As a result, alveoli expand less and become more compliant at the base, increasing ventilation. Perfusion also increases as gravity pulls down the blood towards the base. Overall, both ventilation and perfusion increase towards the base of the lungs, but perfusion increases more, resulting in decreased V/Q ratio. Towards the apex of the lungs, the hydrostatic pressure is decreased due to gravity, which results in decreased blood flow, thus decreased perfusion. Since ventilation exceeds perfusion, the V/Q ratio is increased at the apex of the lungs[12].

Clinical significance

References

  1. ^ "Mechanics of Ventilation | SEER Training". training.seer.cancer.gov. Retrieved 2022-03-27.
  2. ^ "Mechanics of Ventilation | SEER Training". training.seer.cancer.gov. Retrieved 2022-03-27.
  3. ^ Lei, Yuan. Lung Ventilation: Natural and Mechanical. Oxford University Press. doi:10.1093/med/9780198784975.001.0001/med-9780198784975-chapter-3. ISBN 978-0-19-182718-1.
  4. ^ "Perfusion - an overview | ScienceDirect Topics". www.sciencedirect.com. Retrieved 2022-03-27.
  5. ^ Ricketts, Patricia L.; Mudaliar, Ashvinikumar V.; Ellis, Brent E.; Pullins, Clay A.; Meyers, Leah A.; Lanz, Otto I.; Scott, Elaine P.; Diller, Thomas E. (2008-11-01). "Non-Invasive Blood Perfusion Measurements Using a Combined Temperature and Heat Flux Surface Probe". International journal of heat and mass transfer. 51 (23–24): 5740–5748. doi:10.1016/j.ijheatmasstransfer.2008.04.051. ISSN 0017-9310. PMC 2701710. PMID 19885372.
  6. ^ "Respiration: Ventilation, Diffusion and Perfusion | Ausmed". www.ausmed.com. Retrieved 2022-03-27.
  7. ^ "The Pulmonary System and Exercise". web.cortland.edu. Retrieved 2022-03-27.
  8. ^ "Ventilation/Perfusion Ratio - an overview | ScienceDirect Topics". www.sciencedirect.com. Retrieved 2022-03-27.
  9. ^ Sarkar, Malay; Niranjan, N; Banyal, PK (2017). "Mechanisms of hypoxemia". Lung India : Official Organ of Indian Chest Society. 34 (1): 47–60. doi:10.4103/0970-2113.197116. ISSN 0970-2113. PMC 5234199. PMID 28144061.{{cite journal}}: CS1 maint: unflagged free DOI (link)
  10. ^ Powers, Kyle A.; Dhamoon, Amit S. (2022), "Physiology, Pulmonary Ventilation and Perfusion", StatPearls, Treasure Island (FL): StatPearls Publishing, PMID 30969729, retrieved 2022-03-27
  11. ^ West, J. B. (1962-11-01). "Regional differences in gas exchange in the lung of erect man". Journal of Applied Physiology. 17 (6): 893–898. doi:10.1152/jappl.1962.17.6.893. ISSN 8750-7587.
  12. ^ Powers, Kyle A.; Dhamoon, Amit S. (2022), "Physiology, Pulmonary Ventilation and Perfusion", StatPearls, Treasure Island (FL): StatPearls Publishing, PMID 30969729, retrieved 2022-03-27
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