A calculation: tubing compliance CT 2.5 mL/cm H2O; tidal volume at the exhalation port is 550 mL; peak inspiratory pressure is 28 cm H2O. The volume delivered to the patient is which of the following?

• A. 550 mL
• B. 480 mL
• C. 520 mL
• D. 430 mL

Answer: (B)

Tubing has mechanical compliance, so part of the tidal volume measured at the exhalation port is used to expand the tubing itself rather than reaching the patient’s lungs. The volume that actually reaches the patient equals the exhalation-port tidal volume minus the volume stored in the tubing at peak pressure. That stored volume is CT times the pressure across the tubing. With a peak inspiratory pressure of 28 cm H2O and tubing compliance of 2.5 mL/cm H2O, the tubing stores 2.5 × 28 = 70 mL. Assuming no PEEP (so the pressure across the tubing is 28 cm H2O), the delivered volume is 550 − 70 = 480 mL. Therefore, the volume delivered to the patient is 480 mL.