Pulmonary hypertension (PH) itself is not a disease and does not specify any unique pathophysiology. PH refers to elevated pulmonary artery (PA) pressure.
The underlying physiology is variable with a major distinction between PH caused by elevated pulmonary venous (distal) pressure, elevated pulmonary vascular resistance (PVR), and high flow (Figure 9-1).1
Pulmonary hypertension can be due to any combination of these 3 underlying physiologies in various types of congenital heart disease. Marked pulmonary vascular remodeling does not necessarily translate into elevated pulmonary artery pressure in the setting of low flow. The presence of dynamic shunting complicates interpretation. An acute increase in flow itself causes recruitment and dilation of pulmonary vessels, resulting in decreased pulmonary vascular resistance.
While the equation for PVR (PVR = ΔP/CO) may suggest that PVR is equivalent at any flow (eg, if you double CO then ΔP will double and therefore PVR will remain stable), the relationship is not linear (Figure 9-2). While this phenomenon can be important in the absence of congenital heart disease (CHD), its appreciation is critical in interpretation of hemodynamic data in patients with shunting.
Further, distinct underlying etiologies often produce equivalent physiology and indistinguishable pulmonary vascular pathology.
Pulmonary arterial hypertension (PAH) is defined as PH with high PVR without pulmonary venous hypertension.2
This chapter will review PH in adults with congenital heart disease from the perspective of the underlying physiology followed by several cases which highlight dynamic interplay between the component parts of "pulmonary pressure" and finally a discussion of special cases of right heart pathology requiring special consideration.
The hemodynamic underpinnings of elevated pulmonary artery pressure. Elevated left-sided filling (or distal) pressure is the most common reason for elevated pulmonary artery pressure in the general population, and is a contributor in a significant subset of patients with congenital heart disease. High pulmonary vascular resistance is the hemodynamic cause of elevated pulmonary pressure in pulmonary arterial hypertension (PAH) of various causes. Increased pulmonary flow is seen in patients with left-to-right shunting, and defining the extent of flow is critical in the assessment of shunt lesions. There is important overlap and the findings are flow-dependent and may be dynamic for a given patient depending on the context. That is, distal pressure and pulmonary vascular resistance may vary at different levels of pulmonary flow.
The relationship between mean pulmonary artery pressure and mean flow (ie, pulmonary vascular resistance) is not equivalent at all levels of flow as a result of vessel recruitment and distention with higher flow. Therefore, resistance (the ratio of pressure to flow) is lower at higher flow. PAP, pulmonary artery pressure.
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