What is the primary risk associated with severe untreated hypercapnia?

The primary risk associated with severe untreated hypercapnia, which is an elevated level of carbon dioxide (CO2) in the blood, is CO2 narcosis and possible coma. When CO2 levels rise significantly, it depresses the central nervous system, leading to symptoms such as confusion, drowsiness, and lethargy. As the condition worsens, it can progress to respiratory acidosis, which further exacerbates the neural effects. If adequate treatment is not administered, there is a risk of the individual slipping into a coma due to the profound impact of high CO2 levels on brain function.

The other options, while related to respiratory conditions, do not encapsulate the immediate and severe neurologic threat posed by hypercapnia as directly as CO2 narcosis. Cerebral dilation may occur as a secondary effect, increased pulmonary arterial pressure may arise from chronic respiratory issues, and acute respiratory distress syndrome is a separate condition that can be the result of various factors, but does not specifically highlight the dangers of rising CO2 levels in the way that CO2 narcosis does. Thus, recognizing the direct and immediate dangers posed by severe hypercapnia underscores the importance of prompt intervention to prevent progression to coma.