When targeting pressure as the control variable, what will happen to volume with changing lung characteristics?

• A. Volume will remain constant
• B. Volume will vary
• C. Volume will be at the clinician's discretion
• D. Volume must be set higher

Answer: (B)

When pressure is set as the control variable in mechanical ventilation, the ventilator delivers breaths based on a predefined pressure level. This means that the pressure is maintained constant regardless of the patient’s lung characteristics and the compliance of the respiratory system. If lung compliance decreases (for example, due to conditions like acute respiratory distress syndrome or pulmonary edema), the volume delivered during each breath will decrease because the same amount of pressure will result in less lung expansion. Conversely, if lung compliance improves, the volume delivered may increase since the lungs can expand more easily under the same pressure.

Therefore, as lung characteristics change, particularly compliance and resistance, the volume delivered will not remain constant but will vary based on the mechanics of the lung at that moment. This dynamic interaction is crucial for understanding how to adjust ventilatory support in patients with varying respiratory conditions.