Tubing Compressibility: The volume that leaves the ventilator's outlet is not the volume that enters the patient's lungs. During inspiration, positive pressure builds up in the patient circuit, resulting in expansion of the patient circuit and compression of some of the gas in the circuit . The compressed gas never reaches the patient's lungs.

 

In most adult ventilator circuits, about 2 to 3 mL of gas is lost to tubing compressibility for every 1 cm H2O that is measured by the airway pressure sensor. If high pressures are required to ventilate a patient, as much as 200 mL of gas may be compressed in the circuit and never reaches the patient's lungs. Conversely, a patient whose lung compliance is improving, can be ventilated at lower pressures; therefore less volume is lost to circuit compressibility.

 

System Leaks: System leaks are another reason a delivered volume may be less than the set volume. The ventilator may be unable to recognize or compensate for leaks. A leak can be detected by using an exhaled volume monitor. If the measured volume from the patient is less than that delivered by the ventilator, a leak is present.