V/Q Ratio

(Tidal Volume / Perfusion)

• Tidal volume (V): the amount of gas inhaled and exhaled from the lungs in a given time period, usually one minute.
  • adult female is about 4 liters per minute
  • adult male is about 6 liters per minute
  • in general practice we use the 4L tidal volume for both genders
• Perfusion (Q) is total volume of blood reaching the pulmonary capillaries in one minute is about 5 liters per minute.
  • Normal V/Q ratio is 4/5 or 0.8.
  • When the V/Q is higher than 0.8, it means ventilation exceeds perfusion.
  • When the V/Q is < 0.8, there is a VQ mismatch caused by poor ventilation.

The V/Q ratio provides a useful measure of average gas exchange occurring in the lungs. When the V/Q of an adult female individual with normal alveolar ventilation of 4 L/min and total pulmonary blood flow 5 L/min is calculated, the V/Q ratio is 0.8. In this individual a resting V/Q ratio of 1 may indicate hyperventilation, and a V/Q ratio of .7 may indicate hypoxia.

Consider, V/Q is not equal across all lung fields. Due to the effect of fluid density and gravity, the lung apices have, on average, a greater ventilatory value while the lung bases have  a greater blood perfusion value. Therefore, air flow obstruction in the upper portions of the lung can have a more significant effect on V/Q than similar amount of airflow obstruction in the lower lung.  Conversely, impaired perfusion in the bases of the lungs can have a greater effect on V/Q than a similar amount of impairment in the apices.

Application

An actual V/Q assessment requires specialized equipment and cannot currently be performed at the bedside. 

Initial Bedside Assessment for V/Q Mismatch

• Use a stethoscope to listen for abnormal breath sounds like wheezing or crackles that may suggest ventilation problems.
• Pulse Oximetry: Measure blood oxygen saturation (SpO2). A low SpO2 can be a sign of a perfusion V/Q mismatch, especially if breath sounds are clear.
• Chest X-ray: can sometimes reveal underlying lung conditions that might affect V/Q balance.

Document your findings and correlate them with the patient's history and symptoms. If there are concerning signs like significant hypoxia or respiratory distress, escalate care and notify a physician promptly. Arterial blood gas analysis may be ordered for further evaluation.

These steps, can play a crucial role in the initial assessment of patients with suspected V/Q mismatch and contribute to timely diagnosis and management.

Reference

Mirza H, Hashmi MF. Lung Ventilation Perfusion Scan (VQ Scan) [Updated 2023 Jan 13]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK564428/

Powers KA, Dhamoon AS. Physiology, Pulmonary Ventilation and Perfusion. [Updated 2023 Jan 23]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK539907/

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