Ventilation–perfusion mismatch

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In the respiratory system, ventilation/perfusion (V/Q) mismatch refers to the pathological discrepancy between ventilation (V) and perfusion (Q) resulting in an abnormal ventilation/perfusion (V/Q) ratio. Ventilation is a measure of the amount of inhaled air that reaches the alveoli, while perfusion is a measure of the amount of deoxygenated blood that reaches the alveoli through the capillary beds. [1] Under normal conditions, ventilation-perfusion coupling keeps ventilation (V) at approximately 4 L/min and normal perfusion (Q) at approximately 5 L/min. Thus, at rest, a normal V/Q ratio is 0.8. [2] Any deviation from this value is considered a V/Q mismatch. Maintenance of the V/Q ratio is crucial for preservation of effective pulmonary gas exchange and maintenance of oxygenation levels. A mismatch can contribute to hypoxemia and often signifies the presence or worsening of an underlying pulmonary condition. [3]

Contents

Examples

In a condition such as pulmonary embolism, the pulmonary blood flow is affected, thus the ventilation of the lung is adequate, however there is a perfusion defect. Gas exchange thus becomes highly inefficient leading to hypoxemia, as measured by arterial oxygenation. A ventilation perfusion scan or lung scintigraphy can be used to diagnose areas of lungs being ventilated but not adequately perfused. This results in a raised Alveolar-arterial (A-a) gradient which is responsive to supplemental oxygen.

In conditions with right to left shunts, there are also ventilation perfusion defects with high A-a gradients. Hypoxemia is difficult to correct with supplemental oxygen and is associated with a widened A-a gradient. In cases of right to left shunts more of deoxygenated blood mixes with oxygenated blood from the lungs and thus to a small extent the condition might neutralize the high A-a gradient with pure oxygen therapy. Patient with parenchymal lung diseases will have an increased A-a gradient with moderate response to oxygen therapy.

A patient with hypoventilation will have complete response to 100% oxygen therapy.

Diagnosis

To differentiate between a ventilation perfusion mismatch or not, a lung scan is performed. The ventilation and perfusion are measured separately. If both scans are done simultaneously then it is called as V/Q scan. Ventilation Scan is done first as it is easy to wash out tracer gas from lungs with the help of hyperventilation than clearing the tracer radioactive material from blood.

Indication: It is most commonly done in suspected case of pulmonary embolism.

  1. Ventilation Scan: Takes 15 to 20 minutes. A radioactive tracer gas is inhaled and then this radioactive tracer gas is traced (looked) by taking pictures from specialised camera which capture the route and location of tracer gas. These pictures show us the part of lung devoid of tracer gas and then we correlate it with different types of lung pathology.
    • By using a mouth piece while closing nose with the help of clip, person is asked to inhale the radioactive tracer gas for few minutes and pictures are taken at regular intervals.
    • Then person is asked to hold the breath and remain still for at least 10 seconds and final picture is taken.
    • Then by looking upon the picture, the poorly ventilated or overvented area is noted.
  2. Perfusion scan: It is done by injecting a radioactive tracer into a peripheral vein usually in arms and then following the path of tracer by camera as the tracer moves to lungs. It tells us about the area of lung poorly perfused or any type of obstruction to the flow like in case of pulmonary embolism which will be seen on image as an area devoid of tracer element distal to the obstruction.

Results:

  1. Ventilation Scan is abnormal but perfusion scan is normal indicating abnormal airway suggesting COPD or asthma.
  2. Ventilation Scan is normal but perfusion is abnormal indicating any obstruction to the blood flow (perfusion), may be because of the pulmonary embolism obstructing the flow.
  3. Both scans are abnormal. It may be found in pneumonia or COPD.

Results are reported in following way:

  1. Normal - No abnormality found.
  2. Low Probability - Chances of blood clot is low. Needs further and more testing.
  3. Intermediate Probability - Scan shows possibility of blood clot. Needs more testing.
  4. High Probability - High chances of clot. Needs treatment.

Management

Through ventilation and perfusion scans, the abnormal area of lung may be localized. A provisional diagnosis of COPD, asthma or pulmonary embolisms may be made. Treatment of these underlying conditions may address ventilation perfusion mismatch.[ citation needed ]

Management of the condition may vary. If ventilation is abnormal or low, increasing the tidal volume or the rate may result in the poorly ventilated area receiving an adequate amount of air, which ultimately leads to an improved V/Q ratio.[ citation needed ] Conversely, if perfusion scan is of low quality showing low perfusion to lung as in case of hypovolemia, treatment of the conditions is by giving it fluid and using inotropes in case of shock. [ citation needed ]

Related Research Articles

<span class="mw-page-title-main">Hypoxia (medicine)</span> Medical condition of lack of oxygen in the tissues

Hypoxia is a condition in which the body or a region of the body is deprived of adequate oxygen supply at the tissue level. Hypoxia may be classified as either generalized, affecting the whole body, or local, affecting a region of the body. Although hypoxia is often a pathological condition, variations in arterial oxygen concentrations can be part of the normal physiology, for example, during strenuous physical exercise.

<span class="mw-page-title-main">Respiratory failure</span> Inadequate gas exchange by the respiratory system

Respiratory failure results from inadequate gas exchange by the respiratory system, meaning that the arterial oxygen, carbon dioxide, or both cannot be kept at normal levels. A drop in the oxygen carried in the blood is known as hypoxemia; a rise in arterial carbon dioxide levels is called hypercapnia. Respiratory failure is classified as either Type 1 or Type 2, based on whether there is a high carbon dioxide level, and can be acute or chronic. In clinical trials, the definition of respiratory failure usually includes increased respiratory rate, abnormal blood gases, and evidence of increased work of breathing. Respiratory failure causes an altered state of consciousness due to ischemia in the brain.

Diffusing capacity of the lung (DL) measures the transfer of gas from air in the lung, to the red blood cells in lung blood vessels. It is part of a comprehensive series of pulmonary function tests to determine the overall ability of the lung to transport gas into and out of the blood. DL, especially DLCO, is reduced in certain diseases of the lung and heart. DLCO measurement has been standardized according to a position paper by a task force of the European Respiratory and American Thoracic Societies.

<span class="mw-page-title-main">Acute respiratory distress syndrome</span> Respiratory failure due to widespread inflammation in the lungs

Acute respiratory distress syndrome (ARDS) is a type of respiratory failure characterized by rapid onset of widespread inflammation in the lungs. Symptoms include shortness of breath (dyspnea), rapid breathing (tachypnea), and bluish skin coloration (cyanosis). For those who survive, a decreased quality of life is common.

<span class="mw-page-title-main">Generalized hypoxia</span> Medical condition of oxygen deprivation

Generalized hypoxia is a medical condition in which the tissues of the body are deprived of the necessary levels of oxygen due to an insufficient supply of oxygen, which may be due to the composition or pressure of the breathing gas, decreased lung ventilation, or respiratory disease, any of which may cause a lower than normal oxygen content in the arterial blood, and consequently a reduced supply of oxygen to all tissues perfused by the arterial blood. This usage is in contradistinction to localized hypoxia, in which only an associated group of tissues, usually with a common blood supply, are affected, usually due to an insufficient or reduced blood supply to those tissues. Generalized hypoxia is also used as a synonym for hypoxic hypoxia This is not to be confused with hypoxemia, which refers to low levels of oxygen in the blood, although the two conditions often occur simultaneously, since a decrease in blood oxygen typically corresponds to a decrease in oxygen in the surrounding tissue. However, hypoxia may be present without hypoxemia, and vice versa, as in the case of infarction. Several other classes of medical hypoxia exist.

<span class="mw-page-title-main">Scintigraphy</span> Diagnostic imaging test in nuclear medicine

Scintigraphy, also known as a gamma scan, is a diagnostic test in nuclear medicine, where radioisotopes attached to drugs that travel to a specific organ or tissue (radiopharmaceuticals) are taken internally and the emitted gamma radiation is captured by gamma cameras, which are external detectors that form two-dimensional images in a process similar to the capture of x-ray images. In contrast, SPECT and positron emission tomography (PET) form 3-dimensional images and are therefore classified as separate techniques from scintigraphy, although they also use gamma cameras to detect internal radiation. Scintigraphy is unlike a diagnostic X-ray where external radiation is passed through the body to form an image.

<span class="mw-page-title-main">Respiratory acidosis</span> Decrease in blood pH due to insufficient breathing

Respiratory acidosis is a state in which decreased ventilation (hypoventilation) increases the concentration of carbon dioxide in the blood and decreases the blood's pH.

Hyperoxia occurs when cells, tissues and organs are exposed to an excess supply of oxygen (O2) or higher than normal partial pressure of oxygen.

<span class="mw-page-title-main">Hypoxemia</span> Abnormally low level of oxygen in the blood

Hypoxemia is an abnormally low level of oxygen in the blood. More specifically, it is oxygen deficiency in arterial blood. Hypoxemia has many causes, and often causes hypoxia as the blood is not supplying enough oxygen to the tissues of the body.

Airway obstruction is a blockage of respiration in the airway that hinders the free flow of air. It can be broadly classified into being either in the upper airway (UPA) or lower airway (LOA).

A right-to-left shunt is a cardiac shunt which allows blood to flow from the right heart to the left heart. This terminology is used both for the abnormal state in humans and for normal physiological shunts in reptiles.

<span class="mw-page-title-main">Ventilation/perfusion scan</span> Medical imaging to evaluate circulation of air and blood in the lungs

A ventilation/perfusion lung scan, also called a V/Q lung scan, or ventilation/perfusion scintigraphy, is a type of medical imaging using scintigraphy and medical isotopes to evaluate the circulation of air and blood within a patient's lungs, in order to determine the ventilation/perfusion ratio. The ventilation part of the test looks at the ability of air to reach all parts of the lungs, while the perfusion part evaluates how well blood circulates within the lungs. As Q in physiology is the letter used to describe bloodflow the term V/Q scan emerged.

In medicine, hepatopulmonary syndrome is a syndrome of shortness of breath and hypoxemia caused by vasodilation in the lungs of patients with liver disease. Dyspnea and hypoxemia are worse in the upright position.

In respiratory physiology, the ventilation/perfusion ratio is a ratio used to assess the efficiency and adequacy of the ventilation-perfusion coupling and thus the matching of two variables:

A pulmonary shunt is the passage of deoxygenated blood from the right side of the heart to the left without participation in gas exchange in the pulmonary capillaries. It is a pathological condition that results when the alveoli of parts of the lungs are perfused with blood as normal, but ventilation fails to supply the perfused region. In other words, the ventilation/perfusion ratio of those areas is zero.

The Alveolar–arterial gradient, is a measure of the difference between the alveolar concentration (A) of oxygen and the arterial (a) concentration of oxygen. It is a useful parameter for narrowing the differential diagnosis of hypoxemia.

Persistent fetal circulation is a condition caused by a failure in the systemic circulation and pulmonary circulation to convert from the antenatal circulation pattern to the "normal" pattern. Infants experience a high mean arterial pulmonary artery pressure and a high afterload at the right ventricle. This means that the heart is working against higher pressures, which makes it more difficult for the heart to pump blood.

In some individuals, the effect of oxygen on chronic obstructive pulmonary disease is to cause increased carbon dioxide retention,

Medical gas therapy is a treatment involving the administration of various gases. It has been used in medicine since the use of oxygen therapy. Most of these gases are drugs, including oxygen. Many other gases, collectively known as factitious airs, were explored for medicinal value in the late eighteenth century. In addition to oxygen, medical gases include nitric oxide (NO), and helium-O2 mixtures (Heliox). Careful considerations and close monitoring needed when medical gases are in use. For the purpose of this article only gas mixtures are described.

<span class="mw-page-title-main">Ventilation–perfusion coupling</span> Relationship between respiratory and cardiovascular processes

Ventilation–perfusion coupling is the relationship between ventilation and perfusion processes, which take place in the respiratory system and the cardiovascular system. Ventilation is the movement of gas during breathing, and perfusion is the process of pulmonary blood circulation, which delivers oxygen to body tissues. Anatomically, the lung structure, alveolar organization, and alveolar capillaries contribute to the physiological mechanism of ventilation and perfusion. Ventilation–perfusion coupling maintains a constant ventilation/perfusion ratio near 0.8 on average, while the regional variation exists within the lungs due to gravity. When the ratio gets above or below 0.8, it is considered abnormal ventilation-perfusion coupling, also known as a ventilation–perfusion mismatch. Lung diseases, cardiac shunts, and smoking can cause a ventilation-perfusion mismatch that results in significant symptoms and diseases, which can be treated through treatments like bronchodilators and oxygen therapy.

References

  1. ↑ Magder, Sheldon; Malhotra, Atul; Hibbert, Kathryn A.; Hardin, Charles Corey (2021-09-01). Cardiopulmonary Monitoring: Basic Physiology, Tools, and Bedside Management for the Critically Ill. Springer Nature. ISBN   978-3-030-73387-2.
  2. ↑ Petersson, Johan; Glenny, Robb W. (2014-10-01). "Gas exchange and ventilation–perfusion relationships in the lung". European Respiratory Journal. 44 (4): 1023–1041. doi: 10.1183/09031936.00037014 . ISSN   0903-1936. PMID   25063240.
  3. ↑ Powers, Kyle A.; Dhamoon, Amit S. (2023), "Physiology, Pulmonary Ventilation and Perfusion", StatPearls, Treasure Island (FL): StatPearls Publishing, PMID   30969729 , retrieved 2023-11-11
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