There are many situations in which the performance of ventilators or similar respiratory care equipment must be tested. Perhaps the most common is in the Biomedical Engineering department of a hospital, where preventative maintenance testing is routinely performed for a wide variety of medical equipment. But there are many stages in a device’s life cycle at which testing is warranted. Development engineers need to test new ideas and mechanisms as new ventilator technologies are developed, and Production and Quality Control engineers must calibrated and assure the performance of the machines during manufacture. As wear and tear from use take their toll or problems otherwise develop, service personnel need to troubleshoot the systems in order to locate any problems and effect appropriate repairs.
In practice, many different types of equipment are used to test or otherwise assess the performance of ventilators and similar respiratory care equipment. These range from simple flow measuring devices, to electronically instrumented, flow meter based “ventilator analyzers”, to fixed and variable volume test lungs, and finally all the way to instrumented, computer controlled, spontaneously breathing lung simulators(1). While each has their own advantages and disadvantages in terms of capabilities, convenience and cost, for most testing applications it is essential that at least the basic parameters of pulmonary compliance and resistance are realistically simulated.
A realistic compliance load, in particular, has profound effects on gas flow, the action of many ventilator components and on the performance of ventilators and similar devices in general. In fact, standard-setting organizations like ASTM and ISO mandate that ventilators be tested under conditions simulating actual use, and using realistic compliance and resistance loads in particular (2). Gas is a compressible substance, and even at the relatively low pressures used in Respiratory Care, volume and flow rate measurements are very sensitive to changes in pressure. This characteristic affects ventilator testing in a number of ways; here are a few examples:
- In order to test the accuracy and reliability of flow and volume sensors that are so frequently part of modern ventilators, pressures and changes in pressures must be realistically simulated during testing of such machines.
- Gas output as part of a breath delivered by a ventilator, but which ends up compressed in the breathing circuit, never makes it to the patient. While many ventilators include sensors and software systems designed to compensate for this common phenomenon, complex mechanisms like this must be tested frequently to ensure their continued integrity and patient safety. Only by realistically simulating the compliance and resistance characteristics of use conditions is it possible to test such systems.
- Many components of ventilators, and especially many components of breathing circuits used in conjunction with ventilators, are designed to work with and respond to specific flow and pressure conditions in order to function properly. Only by realistically simulating the conditions under which these components are designed to operate can they be made to work correctly. Testing under any other conditions will cause these components to function differently, which can affect the apparent overall performance of the ventilator, and which denies the technician the ability to accurately assess the performance of the components themselves.
Pulmonary compliance changes throughout the inhalation and exhalation phases of a breath; this is as true for test lungs as it is for real, human lungs. The Training and Test lung and PneuView system (both produced by Michigan Instruments) correctly and accurately model the dynamic compliance characteristic, in addition to upper and lower airway resistances. Each lung is calibrated individually, and each installation of the software system is tuned to specifically match the lung it is connected with.
The Michigan Instruments, Inc. TTL is designed to simulate the pulmonary characteristics of patients of all ages, sizes and conditions. With the addition of the Breath Simulation Module, these devices may also be used to simulate the spontaneously breathing patient. The package of hardware, instrumentation and software provides an unparalleled capacity for testing all types of respiratory care equipment under conditions simulating those of actual use.
In a clinical setting, the concept of “ventilator performance” takes on a larger meaning. Even ventilator systems functioning exactly to specifications perform differently depending on the particular patient or other use conditions presented in various real life situations. Even within a specific ventilator system many different modes of operation may be available, and the modes most appropriate for use in a given situation may be subject, at least in part, to certain patient or use conditions. Realistically simulated patient loads can be invaluable in these situations, and care givers can make effective use of test lungs to allow the widest possible range of treatments to be investigated before application to the patient.
For training and teaching applications, use of a realistically simulated patient is also essential. Training the use of ventilators and the employment of various respiratory care techniques to medical professionals requires fidelity of the training scenarios, equipment and materials to the real-world, clinical setting. Training and test lungs provide the ability to simulate a huge variety of normal and pathologic pulmonary conditions, and cover the range from very small infant/pediatric to very large adult settings. Training and test lungs can also be used to simulate the spontaneously breathing patient; it is only through such simulation that some of the more advanced modes of ventilation, such as assist/control, IMV/SIMV and pressure support can be taught.
(1) Anderson, JA: Ventilator Testing Comes of Computer Age. 24x7, June 2003, 35-39.
(2) ASTM: F1100-1999 Minimum Performance Specification for Critical Care Ventilators.