Final answer:
Decreased ETCO₂ is commonly caused by increased ventilation, such as hyperventilation, or metabolic conditions leading to decreased PCO₂ as a compensation mechanism. Enzymatic activity and metabolic acidosis, due to conditions like diabetes ketoacidosis, also contribute to lower end-tidal CO2 levels.
Step-by-step explanation:
The causes of decreased end-tidal CO2 (ETCO₂) can be attributed to various physiological and pathological conditions. One of the primary causes is increased ventilation, which can occur due to hyperventilation. This leads to a greater expulsion of CO₂ from the lungs, resulting in lower levels of ETCO₂. Additionally, conditions that lead to metabolic acidosis, such as diabetes with ketoacidosis, can initially present with normal PCO₂ levels. However, as the body compensates, PCO₂ may decrease to maintain the acid-base balance. Factors such as the decreased activity of enzymes involved in glycolysis, like phosphofructokinase, and those involved in the citric acid cycle, like isocitrate dehydrogenase, can affect the production of CO₂ and hence ETCO₂ levels.
Conversely, conditions leading to decreased CO₂ production and conditions such as respiratory alkalosis can also cause lowered ETCO₂. This may occur during states of mild starvation with ketosis, where the body metabolizes fat rather than carbohydrates, leading to a decreased respiratory quotient (RQ) and therefore lowered CO₂ output.