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Decreased CaO2, or arterial oxygen content, in the context of anaemic hypoxia primarily indicates a decrease in hemoglobin concentration. Hemoglobin plays a crucial role in transporting oxygen from the lungs to the tissues, and any reduction in the amount of hemoglobin reduces the blood's overall capacity to carry oxygen, even if the oxygen saturation and partial pressure of oxygen in the blood remain normal.

In anaemic hypoxia, the hemoglobin level is insufficient to transport adequate amounts of oxygen to meet the tissue demands, leading to decreased CaO2. Thus, when hemoglobin concentration is lowered due to conditions such as anemia, it directly results in decreased CaO2.

While factors like blood flow and lung volume are important in the overall oxygenation process, they do not directly affect the CaO2 as much as hemoglobin concentration does. Therefore, the indication of decreased CaO2 specifically points to the reduction in hemoglobin, reflecting the impact of anemia on oxygen delivery to the tissues.