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The estimation of alveolar ventilation is primarily assessed through the measurement of PaCO2 (partial pressure of carbon dioxide in arterial blood). This parameter reflects the balance between carbon dioxide production and its elimination via ventilation. When alveolar ventilation is adequate, a normal PaCO2 range is maintained, typically between 35 to 45 mmHg.

If alveolar ventilation decreases, carbon dioxide accumulates in the blood, leading to an increase in PaCO2, indicating hypoventilation. Conversely, if ventilation is excessive, as in hyperventilation, the PaCO2 level decreases. Therefore, monitoring PaCO2 provides direct insight into the effectiveness of ventilation, making it a crucial parameter for estimating alveolar ventilation.

In contrast, while PaO2 (partial pressure of oxygen) and the alveolar-arterial oxygen gradient can provide information about oxygenation status and diffusion across the alveolar-capillary membrane, they do not directly reflect ventilation. pH is influenced by the levels of carbon dioxide and bicarbonate in the blood but is not a direct measure of ventilation itself. Thus, the correct choice, PaCO2, effectively serves as a critical indicator of alveolar ventilation status.