Final answer:
When arterial PO₂ cannot raise ventilation, it likely indicates that carbon dioxide levels are abnormally low due to hyperventilation leading to a condition known as hypocapnia.
Step-by-step explanation:
If a decrease in arterial PO₂ can no longer increase ventilation in a patient, it is most likely that the carbon dioxide levels are abnormally low. When hyperventilation occurs, there is an increased ventilation rate that leads to hypocapnia, which is the condition of abnormally low blood levels of CO₂, along with a consequent increase in blood pH becoming more alkaline.
Therefore, the body's drive to breathe, which is partially regulated by carbon dioxide levels, may be diminished if CO₂ levels are already low. This can become a compensatory effect during situations such as metabolic acidosis, where the body attempts to reestablish the proper ratio of bicarbonate and carbonic acid/CO₂ by lowering the CO₂.