From the Guidelines
False. In a healthy self-ventilating adult, optimal V/Q matching does not occur in the dependent region (base) of the lungs. Instead, optimal V/Q matching occurs in the middle regions of the lungs. The dependent regions (bases) of the lungs have relatively higher perfusion than ventilation, resulting in a lower V/Q ratio [ 1 ]. This occurs because gravity causes more blood flow to the dependent regions while the increased weight of overlying lung tissue partially compresses these areas, limiting full alveolar expansion. Conversely, the non-dependent regions (apices) have higher ventilation relative to perfusion, creating a higher V/Q ratio. The middle regions of the lungs achieve the best balance between ventilation and perfusion, with a V/Q ratio closest to the ideal value of 1.0 [ 1 ]. Some key points to consider are:
- Ventilation and perfusion are both greatest at the bases of the lung in the upright position [ 1 ]
- The pulmonary vasculature maximises PaO2 by ensuring that the well-ventilated areas of the lung receive most of the pulmonary blood flow, a process called V/Q matching [ 1 ]
- This is largely achieved through a process called hypoxic pulmonary vasoconstriction (HPV) [ 1 ] This gradient of V/Q ratios from apex to base is a normal physiological phenomenon that optimizes gas exchange across the entire lung.
From the Research
V/Q Matching in the Lungs
- The statement that optimal V/Q matching occurs in the dependent region (base) of the lungs in a healthy self-ventilating adult is supported by the principles of pulmonary physiology 2.
- Ventilation-perfusion (V/A/Q) matching is crucial for efficient pulmonary gas exchange, and regional matching of fresh gas flow to deoxygenated capillary blood flow is essential 3.
- The distribution of ventilation and perfusion in the lungs is not uniform, with the dependent regions (bases) of the lungs receiving more blood flow and the non-dependent regions (apices) receiving more ventilation 2.
- This uneven distribution can lead to V/A/Q mismatch, which can impair gas exchange and result in hypoxemia or hypercapnia 4.
- The measurement of V/A/Q matching can be achieved through various techniques, including gas exchange methods and imaging techniques such as SPECT, PET, MRI, CT, and EIT 3.
- Understanding the determinants of regional ventilation and blood flow in the lungs is essential for managing critically ill patients and optimizing gas exchange 2.
Gas Exchange and V/Q Matching
- Gas exchange in the lungs is influenced by various factors, including convection, diffusion, and reaction with hemoglobin 5.
- The ventilation-perfusion ratio (V/A/Q) plays a critical role in determining gas exchange, with high V/A/Q ratios resulting in increased dead space ventilation and low V/A/Q ratios leading to shunt 4.
- The measurement of alveolar-arterial oxygen tension difference, venous admixture, and wasted ventilation can provide quantitative estimates of the effect of V/A/Q mismatch on gas exchange 4.
- A bedside gas exchange approach can be used to estimate shunt and deadspace in lung disease, using measurements of arterial and alveolar Po2 and Pco2 6.