Related Experiment Video
Updated: Aug 6, 2026

Monitoring Lung Function with Electrical Impedance Tomography in the Intensive Care Unit
Published on: September 6, 2024
Effect of humidifying devices on the measurement of tidal volume by mechanical ventilators
Yasuki Fujita1, Hideaki Imanaka, Yuji Fujino
1Intensive Care Unit, Osaka University Hospital, 2-15 Yamadaoka, Suita, 565-0871, Japan.
Purpose:
We hypothesized that expiratory tidal volume was underestimated, because a heat-moisture exchanger traps the expired vapor. We, therefore, designed patient and bench studies to investigate the accuracy of tidal volume monitoring.
Methods:
In a patient study, applying two humidifying systems (a heat-moisture exchanger and a heated humidifier) and two tidal volumes (12 and 6 ml x kg(-1)) with a Servo ventilator 300, we recorded the displayed expiratory tidal volume and thoracic volume displacement, measured by respiratory inductive plethysmography. Temperature, relative humidity, and absolute humidity were measured at the airway opening and at the end of the expiratory limb. Using a model lung, we also tested three different ventilators (Puritan-Bennett 7200ae, Evita 4, and Servo ventilator 300) to investigate whether the effects of the heat-moisture exchanger and the heated humidifier on monitored tidal volume varied according to the brand of ventilator.
Results:
With the use of the heat-moisture exchanger, the displayed expiratory tidal volume was significantly smaller, by 12%-14%, than that with the heated humidifier, although thoracic volume displacement was identical in the two systems. The temperature and absolute humidity at the end of the expiratory limb were significantly lower with the heat-moisture exchanger than with the heated humidifier. In the model lung study, we investigated the effects of different brands of ventilator on the expiratory tidal volume. A similar degree (8%-14%) of underestimation of tidal volume was observed with the heat-moisture exchanger, regardless of ventilator brand.
Conclusion:
Monitored expiratory tidal volume was underestimated by approximately 10%, when using a heat-moisture exchanger.
Related Concept Videos
Mechanical Ventilation II: Invasive Ventilation
Negative-Pressure Ventilators
Negative-pressure ventilators create a vacuum around the chest or body to draw air into the lungs, simulating breathing. This method does not require an...
Mechanical Ventilation I: Indication and Settings
Respiratory Volumes
Tidal Volume (TV) Tidal volume (TV) is the air inhaled or exhaled in a...
Ventilatory Modes
There are three ventilatory modes: full support, partial support, and spontaneous. These are described below.
Full Support Modes
Full support modes include controlled mechanical ventilation, continuous mandatory...
Assessment of Ventilation I: Respiratory Rate
A Ventilation assessment is critical for monitoring a patient's health status. Respiration, one of the most accessible vital signs, provides insights into the function of numerous body systems and can indicate serious health issues, such as brainstem injuries from head trauma.
Critical Guidelines for Assessing Ventilation:
Factors Affecting Pulmonary Ventilation
Alveolar Surface Tension
The alveolar fluid lines the luminal surface of the alveoli and exerts a force called surface tension. This force is caused by the polar water molecules in the liquid being more strongly attracted to each...
