Final answer:
The phenomenon of increased PaCO2 and decreased etCO2 is most likely caused by hypoventilation, which leads to the accumulation of CO2 in the blood due to impaired respiratory functions.
Step-by-step explanation:
When a patient exhibits an increase in PaCO₂ (partial pressure of arterial carbon dioxide) and a decrease in etCO₂ (end-tidal carbon dioxide), the most likely contributing factor is b) Hypoventilation. Hypoventilation leads to an accumulation of CO₂ in the bloodstream (hypercapnia), which increases PaCO₂. The lower etCO₂ suggests that less CO₂ is being exhaled, supporting the presence of hypoventilation.
This condition can be caused by anything that impairs respiratory function such as certain drugs, respiratory muscle weakness, or lung diseases like pneumonia and congestive heart failure. Hyperventilation, on the other hand, would lead to a decrease in PaCO₂ due to the excessive exhalation of CO₂ leading to hypocapnia; thus, hyperventilation does not align with the described trend of gas levels. Metabolic acidosis and metabolic alkalosis are changes in blood pH due to metabolic disturbances and would not directly cause the described changes in PaCO₂ and etCO₂ levels, although they could lead to compensatory respiratory responses.