A volume-time curve on a ventilator shows decreasing delivered volume with each breath, suggesting a circuit problem. What is the most likely cause?

• A. Pneumothorax
• B. System leak
• C. Atelectasis
• D. Pulmonary edema

Answer: (B)

Decreasing delivered volume across breaths on a volume-time curve points to a loss of gas somewhere after the ventilator has generated the breath—that is, a circuit leak. When the circuit has a leak, part of the inspired gas escapes before reaching the patient, so the ventilator ends up delivering less volume with each successive breath. This pattern is a hallmark of a circuit integrity problem rather than a primary change in the patient’s lungs. Lung problems like pneumothorax, atelectasis, or pulmonary edema alter lung mechanics and pressures, but they don’t produce a progressive drop in delivered tidal volume due to a circuit defect. So the most likely cause is a leak in the ventilator circuit, and you’d want to check connections, tubing, and seals for leakage.