Supplemental inhaled gases alter tidal volume delivery and measurement

Supplemental inspired nitrogen (N(2)) or carbon dioxide (CO(2)) is commonly used to balance pulmonary blood flow in patients with single-ventricle physiology. The objective of this study was to assess if supplemental inspired gas alters delivery or measurement of tidal volume (V(T)) by a ventilator.

Prospective, experimental study.

Respiratory Care Laboratory, Cincinnati Children's Hospital.

Using a test lung, expired V(T) measurements from Servo 300 ventilators were compared with actual delivered V(T) (true V(T)) at baseline and during supplemental N(2) or CO(2) administration to mimic clinical use in single-ventricle patients. At compliance settings simulating normal and compromised lung function, true V(T) was determined by the test lung and inline Pneumotach. True and measured V(T) were compared by repeated-measures analysis of variance with significance defined as p < .05.

With normal lung compliance, supplemental gas administration increases both true and measured V(T), and expired V(T) measurements remain accurate. With poor lung compliance, supplemental gas flow disproportionately affects V(T) measurement. Poor lung compliance reduces true V(T) markedly (p < .001), causing a large discrepancy between true and measured V(T). Supplemental gas administration amplifies this discrepancy because the additional gas flow in the circuit erroneously augments expired V(T) measurements by the ventilator (p < .001). The discrepancy is greatest with higher-set V(T) and greater supplemental gas flow.

The addition of supplemental inspired gas directly into the ventilator circuit can alter tidal volume delivery or measurement by a ventilator. The extent and magnitude of the alterations are determined by lung compliance. Variable effects of supplemental gas administration may confound ventilator management of patients with single-ventricle physiology.