Pulmonary Vascular Resistance Curve
When treating patients sensitive to changes in pulmonary vascular resistance (PVR), such as those with right heart failure or pulmonary hypertension, it's crucial to understand how lung volumes affect PVR.
To clarify, we can categorize the pulmonary vasculature into two types: intra-alveolar and extra-alveolar vessels.
Intra-alveolar vessels are found within alveolar ducts and walls, where gas exchange occurs. As the lungs inflate, the capillaries in the alveolar septal walls become compressed, increasing intra-alveolar PVR.
Extra-alveolar vessels, which are connected to lung parenchyma, distend or elongate during lung inflation. An important factor here is hypoxic vasoconstriction: in regions of atelectasis, the lack of oxygen leads to vessel constriction, causing intra-pulmonary shunting. This reduces gas exchange efficiency and raises PVR.
The overall PVR curve follows a U-shape, with the optimal balance occurring at functional residual capacity (FRC). At this point, intra-alveolar vessels are minimally compressed, and extra-alveolar vessels are adequately distended.
Clinical application: For example, when treating a patient with severe pulmonary hypertension, a chest X-ray revealed hyperinflation (10 ribs expanded) and darkened hilar regions. Recognizing that hyperinflation could place the patient at the high end of the PVR curve, we decreased PEEP, which led to an immediate increase in SpO2 from the low 90s to 100%. This allowed us to gradually reduce nitric oxide and FiO2 as well.