Acute respiratory distress syndrome (ARDS) is a severe lung condition that affects the gas exchange process between the lungs and the bloodstream. The V/Q ratio is a useful index to describe the relationship between ventilation and perfusion in ARDS. The traditional teaching of the pathophysiology of ARDS is that it is a shunting process disease. However, ARDS is a heterogeneous disease with six mutually exclusive ventilation/perfusion compartments, each of which has unique physiological characteristics. These compartments include:
Functional exclusion: This compartment accounts for lung regions with no perfusion and no ventilation. In these regions, ventilation and perfusion are abolished due to effective hypoxic vasoconstriction or mechanical compression of blood vessels.
Shunt: This compartment refers to complete loss of ventilation with preserved perfusion. The impact of shunt on oxygenation impairment depends on the amount of perfusion in these regions. Shunt is not responsive to an increase in FiO2, but it may respond to higher PEEP levels through recruitment.
Low V/Q regions: This compartment corresponds to poorly ventilated regions with normal perfusion or normally ventilated regions with increased perfusion. This compartment contributes to oxygenation impairment and may respond to an increase in FiO2 and possibly to PEEP by reducing perfusion through mechanical vasoconstriction.
Regions with matched V/Q: This compartment corresponds to lung areas with any proportional matching of ventilation and perfusion.
High V/Q regions: This compartment corresponds to normally ventilated regions with decreased perfusion or increased ventilation in normally perfused regions.
Dead space: This compartment is associated with impaired CO2 removal, and the application of higher PEEP levels may worsen this condition.
It is important to note that any manipulation of ventilation or hemodynamics targeting one of these compartments will affect the others since they are interconnected. Therefore, a careful consideration of the effects of such interventions on each compartment is necessary to optimize treatment and improve patient outcomes in ARDS.
The majority of clinical research has centered on enhancing the V/Q ratio by increasing ventilation using PEEP and adjusting perfusion via vasodilators.
The choice of PEEP level during ventilation can significantly affect perfusion. Higher PEEP levels may lead to mechanical compression of capillaries, resulting in improved ventilation-perfusion matching in the low V/Q compartments. However, in normal V/Q regions, high PEEP levels can alter the ventilation-perfusion matching and result in higher V/Q compartments, and in hyperinflated regions, this mismatch can worsen and increase the dead space. It is important to note that changes in oxygenation may be due to redistribution of regional perfusion rather than changes in the pulmonary parenchyma.
The distribution of compartments in ARDS is not uniform, and it differs from one patient to another. As a result, the impact of the same PEEP level may differ significantly among patients.
Several drugs commonly used in critically ill patients induce pulmonary perfusion modifications, and while some are used with this purpose, others can bring such alteration unintentionally. Traditionally used and well-known vasodilators are nitric oxide (NO) donors (inhaled NO, sodium nitroprusside, and nitroglycerin). Inhaled NO has the potential of improving V/Q matching through selective vasodilation in ventilated regions only, but its impact on mortality in patients with ARDS remains unproven. Other drugs with unselective pulmonary vasodilating effects include prostacyclin, phosphodiesterase inhibitors, and endothelin antagonists. Intravenous Almitrine may also improve V/Q matching, boosting hypoxic vasoconstriction in injured lung regions.