Advancements in Critical Care

Ventilator performance, criteria for comparison

Karl Cornelius-Lorenz, Dipl. Ing
Technologie-Institut Medizin (TIM) GmbH
Göttingen, Germany

The article also available in PDF: 1390KB

[Editor’s note: This paper accompanies the Clinical Window presentation at the 16th ESICM Annual Congress in Amsterdam, Netherlands, 5-8 October 2003. See also CWWJ’s Internet podium area, where you will find the author’s presentation slides.]

Characterizing the patient-relevant performance of ventilators

Ventilation in an ICU today involves more than just inflating and deflating the patient’s lungs. New ventilation strategies need adequate high-tech equipment. Every modern ventilator tries to meet the user’s requirements but the technical data of the machines given in the operation manuals are not sufficient to determine how adaptable the machine is to patient needs in practice. To assess this, we need to establish objective criteria that characterize the “patient-relevant” performance of the ventilators.

Impact on the patient’s work of breathing

The most important parameter for characterizing patient-relevant performance is how sensitively the ventilator responds to the patient’s effort during inspiration and expiration as this has a direct impact on the patient’s work of breathing. Trigger delay and flow dynamics influence the ventilator response time. The quality of ventilator support gives an idea how the work of breathing is shared between the patient and machine.

Measurement using a lung model

To measure a ventilator's performance in a laboratory setting, we utilize technology based on a spontaneously breathing lung model (Figure 1). This approach makes it possible to measure important ventilator related parameters, such as the resulting "alveolar" pressure (Table 1). The behavior of different ventilators can then be compared when we repeat measurements while keeping the lung model settings fixed.
It must also be taken into account that the accessories used for ventilation such as breathing circuits, humidification systems, filters, HME and endotracheal tubes may have a large influence on the performance of the ventilators.

Figure 1: Principle of the lung model used as test equipment. Measurements of pressure, flow and volume are required to test the ventilator’s “patient-relevant” performance.

How to test ventilator performance?

We use special test equipment to determine the relevant data. This includes computer-controlled spontaneously breathing lung models with physiological humidification, systems for automatically measuring the airway resistance, and full-scale patient simulators for generating special situations.

Ergonomics and availability of different ventilation modes

Other possible criteria for comparison include the available ventilation modes, the operating range (suitability for non-invasive ventilation, patient transport or MRI-environment), monitoring parameters, and suitability for patients of different type and size.

Besides these clinical features there is always the requirement of ease-of-use. A comparison for this parameter can be made using a simulation with a patient model and measuring the time users need to overcome a simulated critical situation, managing a routine situation, or preparing the machines (Figure 2-3).

Figure 2. Managing a simulated critical situation.

Figure 3. Test methods using patient simulators can acquire new relevant data.

Summary

Modern ventilation strategies require ventilators that do not restrict the patient’s spontaneous breathing. Hence, safe and easy-to-use machines are important for clinical practice.

In our opinion there is a need for new comparative criteria since these are not provided in the information provided by manufacturers. Test methods using patient simulators can acquire new relevant data. Lung model values that correspond to alveolar pressure appear to give significant new information on how the ventilator reacts to the patient’s demand. The relevance of the user interface can be estimated by observing scenarios using a full-scale patient simulator.

Last updated: 1 November 2003Created
Legal notice	© GE Healthcare 2008 ISSN 1795-6269 (Web) ISSN 1795-6277 (CD)	Webmaster