The lung is influenced by internal and external factors that affect ventilation/perfusion (V A /Q) relationships, including gravity and the fractal design of the pulmonary blood vessels and airways. Efficient gas exchange requires close matching of regional ventilation and perfusion. The lung functions primarily to exchange oxygen (O 2 ) and carbon dioxide between inspired air and blood. It also produces more uniform distribution of stress and strain of lung tissue. Prone position improves V A /Q matching by more evenly distributing V A and Q. However, at high levels, it increases dead space ventilation and hypercarbia.

Ventilation/perfusion ratios less than 1 are responsible for hypoxemia in asthmatics, but no correlation exists between V A /Q mismatch and the degree of small airway obstruction.ĭuring mechanical ventilation, positive end-expiratory pressure decreases the proportion of shunt in PARDS. The primary V A /Q abnormality in pediatric acute respiratory distress syndrome (PARDS) is intrapulmonary shunt. Perfusion is affected more by gravitational forces than ventilation but is of secondary importance in Q and V A. Gravity increases pulmonary blood flow and ventilation to the base of the lung. The fractal design of the lung and gravitational effects on the flow of air and blood affect V A /Q matching. The A-a gradient can be calculated by subtracting the alveolar partial pressure of oxygen, which is calculated using the alveolar gas equation, by the arterial partial pressure of oxygen, measured with arterial blood gas (ABG).Gas exchange is optimized when pulmonary perfusion (Q) and alveolar ventilation (V A ) are tightly matched. All three of these conditions can contribute to an increase in A-a gradient and cause hypoxemia. Diffusion problems can occur when the alveolar walls are fibrosed and oxygen cannot diffuse across the alveolar wall into the blood stream. High V/Q is also called dead space and low V/Q is called shunting. V/Q mismatch means a mismatch of ventilation to perfusion (V/Q) ratio and can represent either too little perfusion due to blood flow obstruction (high V/Q) or too little ventilation due to airway obstruction (low V/Q). An increase in A-a gradient can occur in hypoxemia and the causes include V/Q mismatch such as dead space or shunting and diffusion problems. The normal A-a gradient is 10 to 15 mmHg. The A-a gradient represents the difference in partial pressure of oxygen between the alveoli and the arteries.

V/q shunt vs dead space