APPLICATION OVERVIEW
There are many situations in which the performance of ventilators or similar respiratory care equipment must be tested. One of 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 calibrate 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 affects on gas flow, the action of many ventilator components and on the performance of ventilators and similar devices in general. In fact, standards-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:
1. 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.
2. 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.
3. 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 the TTL test lungs as it is for real, human lungs. The TTL test lungs and PneuView system 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(s) it is connected with.
The Michigan Instruments, Inc. TTL/PneuView training and test lung systems are 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. None of the “flow meter” based devices marketed as “ventilator analyzers” can do this, including devices such as the “RT200”, “RespiCal”, “QA-VTM”, “VT Plus” and the “Certifier FA”.
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 model(s) 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.
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SYSTEM OVERVIEW & THEORY OF OPERATION
The total PneuView system is comprised of precisely engineered mechanical test lung (or lungs), an electronic sensor and signal conditioning package and a host computer. The test lung is designed to mimic compliance and resistive loads representative of the range human pulmonary physiology, including normal and extreme pathologic conditions. The sensor and signal conditioning board includes a set of pressure transducers and a precision time base. Lung and airway pressure are measured continuously and digitized to 12-bit precision at a sampling frequency of 100 Hz. The board includes a serial interface circuit that creates the data stream transmitted using an RS-232 protocol to the host computer (through an available serial, or “COM” port). A block diagram representation of the Sensor/Interface board is shown in Figure 1. The PneuView software package running on the host computer manages and records the data stream, calculates and analyzes a wide variety of breath parameters and presents the results in various ways to the user or saves reports for later retrieval.
FIGURE 1.
PneuView System Sensor/Interface Board
(click to view)
The software package includes approximately 100 modules containing the algorithms that acquire, condition and store the data stream, parse the breath pattern, calculate defined breath parameters, perform analyses of trends, and finally present the myriad results to the user, numerically and/or graphically, either on the screen or in printed form. Two key elements of the system are the calculation module, which are responsible for converting the raw data into real-time data streams representing airway and lung pressures, lung volume and flow rate, and the breath parsing module, which converts these streams into the logical phases of a breath pattern. Once the breath phases are identified, useful parameters such as Breath Rate, Tidal Volume, and Peak Airway Pressure are calculated relative to the appropriate phase of each breath. Both the real time and breath related parameters are made available for presentation or documentation, either as numeric values or in the form of graphs. A separate module is used to analyze the measured parameters over time, allowing the system to quantify and present trends over comparatively long periods of time.
Like many mechanical and gas systems, the physical characteristics of the test lung may be conveniently modeled as a set of mathematical differential equations. The mechanical and gas dynamics of the device are well understood, quantifiable, and very repeatable, thanks to the built in calibration adjustments that are part of its design. A system of second order differential equations, then, is defined for use as a mathematical representation of the physical test lung within the software. The second order model allows the system to account for inertial, damping and elastic properties of the physical system. Inertial effects include the mass of the lung plates and other moving parts; damping effects include shaft friction and hysteresis in the bellows; elastic effects include the compliance and counterbalance springs. The state variables for the equations are lung and airway pressure, and the first and second derivatives with respect too time of each. The coefficients for each equation are calculated from a set of fourth order polynomial correlation functions, one set for each compliance setting. These correlation functions are produced individually for each lung based on data taken at calibration time.
FIGURE 2.
PneuView System Correlation Functions
(click to view)
Figure 3. presents a block diagram of the software modules involved in the calculation of the real time and breath parameters.
FIGURE 3.
PneuView System Breath Parameter Calculation Engine Overview
(click to view)
The raw data comes to the calculation module as measurements of lung pressure, airway pressure and time. From this data stream, algorithms scale and condition the measurements, and then calculate the first and second time derivatives of the pressures. The next step is to calculate physical lung volume by solving the set of differential equations defining the physical test lung. Once physical volume is known, gas law relationships are used to convert volume to the specified reference system (e.g., NTPD). From here, flow rate is calculated and the data set is prepared for breath parsing.
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CALIBRATION
Factory Calibration
Factory calibration encompasses all aspects of sensor, electronic, mechanical and software tuning to ensure maximum accuracy of all measurements and simulated parameters provided by TTL/PneuView systems. Calibration procedures include setting of the compliance and resistance characteristics of the lungs and setting offset and gain characteristics for each of the pressure transducer channels. Multiple data points are taken at different compliance settings, which are mathematically analyzed and then used to produce a Compliance Characteristic Calibration Table individually for each lung. It is these calibration tables that the volume calculation algorithms described in the previous section use to determine the coefficients to use for the real time solution of the dynamic equations.
Figure 4. Outlines the factory calibration procedure.
FIGURE 4.
Test Lung Calibration Procedure Overview
(click to view)
Calibration Verification by the User
Pressure and volume calculation accuracy may be easily verified with the aid of a calibrated syringe and independent pressure measurement device. All versions of the PneuView system ship with pre-formatted templates to aid the user with the verification of calibration accuracy. These templates include step-by-step instructions, which should be followed periodically to ensure maintenance of calibration accuracy or anytime accuracy comes into question, such as if the unit suffers some type of physical damage.
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BREATH PARSING SETTINGS -
Overview and Recommended Practice for Adjustment
In the PneuView system, "breath parsing" is the term used to encompass the time and frequency domain analysis techniques used to identify the various phases of the breath (e.g., baseline, inspiration, inspiratory hold, expiration). As with any waveform analysis system, there are tradeoffs between sensitivity to detail events and stability in identifying larger features.
The Breath Parsing dialog box accessible from within the PneuView system using the <Shift><F6> key combination was originally designed for only internal use as we ran experiments to determine an optimal compromise between sensitivity and stability.
It became clear, however, that there was the opportunity to have it both ways - keep sensitivity high and minimize the amount of filtering needed on the data stream, but allow the user to adjust sensitivity in special cases or for particular applications. Thus, the Breath Parsing dialog remains a 'feature', albeit a largely undocumented one, of the program.
The first thing to understand is that adjustments made to the breath parsing settings can affect the values calculated for the Measured Parameters during a test. This is not necessarily a bad thing, however, and long as the adjustments are appropriate for the type of real world waveform undergoing analysis. The settings determine when the system will start to recognize that the breath pattern may be entering a new phase. The three different rules sets essentially cast 'votes' for the breath phase each thinks the breath pattern is in. A vote occurs every 10ms.
For example, the "Flow Threshold" settings determine the level, in increments of 0.1LPM, the system considers either positive enough of negative enough to indicate the breath pattern is entering the inspiratory phase or expiratory phase, respectively. This means that the instantaneous flow rate calculated must achieve at least that threshold before the Flow Rule will cast more than half its votes for a change of state.
For flow waveforms exhibiting a fast rise time (e.g., 'square' and 'sinusoidal' flow waveforms), these settings may be adjusted to large magnitudes without adversely affecting the calculation of Measured Parameters. Raising the Inspiratory Flow Threshold can help eliminate 'false triggers' caused by the lung top plate coming down hard against its rest in low airway resistance, low compliance conditions with no significant baseline pressure.
The Differential Pressure and Volume rules function similarly, except that the former looks at the difference in pressure across the simulated airway and the latter looks for a change in physical lung volume. At high airway resistances the Differential Pressure threshold settings may be adjusted quite high without adversely affecting the calculation of Measured Parameters. Such adjustments may be useful if the flow waveform is spiky or erratic because of 'noisy' valve actions or other mechanical or acoustic interference in the system being tested.
In cases where the breath is very shallow and the flow waveform is not steep, it can be helpful to turn the Flow Rules OFF and rely primarily on the Volume Rules for breath parsing.
This can be especially important when airway resistance is low (e.g., Rp5), in which case it might help to turn the Differential Pressure Rules OFF as well.
It should be added that, unless you explicitly turn a rule OFF, it might cast votes for a certain breath state even if a corresponding threshold is not completely met. For example, if the Inspiratory Flow Threshold is set to 10 (1.0L/min) then it will start casting some of its votes for entering the Inspiratory phase when the calculated instantaneous flow rate reaches +0.5L/min.
This way, even if no other rule picks up the slight but present beginnings of inspiration, the system can recognize the start of the Inspiratory phase more quickly than it otherwise might if it were required to rise high enough to exceed the threshold absolutely.
The best way to select breath parsing settings is to begin by considering the forgoing discussion to select which rule(s) should be adjusted. Make adjustments incrementally until the breath is being satisfactorily parsed. To ensure that the changes are not adversely affecting the calculation of Measure Parameters, note the measurements (or save a sample in a Data Table) and continue incrementing the settings in the same direction until you see a change in measurements.
As long as the settings can be changed two or more clicks without significantly affecting the measurements, one can be reasonably assured that the breath parsing settings are valid and are not causing meaningful information to be filtered from the data stream.
Having done this, system sensitivity and stability have also been optimized for the current test conditions.
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TYPICAL SETTINGS FOR SIMULATING VARIOUS PULMONARY PHYSIOLOGIES
The following conditions are based on a “standard” adult human patient who might normally be expected to exhibit pulmonary characteristics as follows:
Dual Adult Lung Simulation
Compliance: 0.05 L/cmH2O in each lung (0.10 L/cmH2O total compliance)
Resistance: Upper airway: Rp5 Lower airway: Rp20 to each lung
Single Adult Lung Simulation
Compliance: 0.10 L/cmH2O
Resistance: Rp20
Diseases affecting the airways, like Chronic Obstructive Pulmonary Disease (COPD)
These conditions are characterized by increased resistance to airflow, particularly in the lower airways. Depending on the severity and duration of the disease, pulmonary compliance may be slightly depressed, and upper airway resistance may be increased if the simulated patient is assumed to be intubated.
Dual Adult Lung Simulation
Compliance: 0.04 L/cmH2O in each lung (0.08 L/cmH2O total compliance)
Resistance: Upper airway: Rp20 Lower airway: Rp50 to each lung
Single Adult Lung Simulation
Compliance: 0.80 L/cmH2O
Resistance: Rp50
Diseases affecting Lung Compliance, like Emphysema
These conditions are characterized by decreased pulmonary compliance (increased lung stiffness). Airway resistance is typically unaffected by the disease, but may be increased if the simulation supposes the patient is intubated.
Dual Adult Lung Simulation
Compliance: 0.02 L/cmH2O in each lung (0.04 L/cmH2O total compliance)
Resistance: Upper airway: Rp5 ; Lower airway: Rp20 to each lung
Single Adult Lung Simulation
Compliance: 0.02 L/cmH2O to 0.05 L/cmH2O
Resistance: Rp20
Acute Asthma Attack
Characterized by greatly increased airway resistance, with generally normal pulmonary compliance. Compliance will decrease, however, as the duration of the simulated attack increases.
Dual Adult Lung Simulation
Compliance: 0.05 L/cmH2O in each lung (0.10 L/cmH2O total compliance)
Resistance: Upper airway: Rp5Lower airway: Rp50 to each lung
Single Adult Lung Simulation
Compliance: 0.10 L/cmH2O
Resistance: Rp50
Collapsed Lung
Characterized by drastically reduced compliance in the affected lung(s), with normal airway resistance values. If the simulated cause of the collapse includes a blocked airway, use a higher resistance value for that portion of the airway (e.g., replace Rp20 with Rp50).
Dual Adult Lung Simulation
Compliance: 0.01 L/cmH2O in affected lung(s) (0.05 L/cmH2O in the normal lung)
Resistance: Upper airway: Rp5 Lower airway: Rp20 to each lung
Single Adult Lung Simulation
Compliance: 0.01 L/cmH2O to 0.05 L/cmH2O
Resistance: Rp20
Pneumothorax / Hemothorax
Similar to the Collapsed Lung scenario, but decrease in pulmonary compliance may not be as marked.
Dual Adult Lung Simulation
Compliance: 0.02 L/cmH2O in affected lung(s) (0.05 L/cmH2O in the normal lung)
Resistance: Upper airway: Rp5 ; Lower airway: Rp20 to each lung
Single Adult Lung Simulation
Compliance: 0.02 L/cmH2O to 0.05 L/cmH2O
Resistance: Rp20
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USE OF THE PNEUVIEW SOFTWARE WITH 3rd PARTY EQUIPMENT MANAGEMENT PROGRAMS
Several biomedical test equipment manufacturers develop and market systems used to manage the preventative maintenance (PM) testing and service for all manner of biomedical equipment.
These programs typically manage a database of equipment information, test schedules, maintenance procedures and test results. Such systems are widely used to manage large fleets of medical equipment. The programs are meant to be used with a variety of specialized test apparatus, most of which would be manufactured by the same company. Such equipment includes electrical safety testers, electro-surgery testers, ultrasound testers, ECG simulators, etc. Many of these systems may be used as stand-alone equipment management databases or in conjunction with other, intermediary an often more portable, devices that interface directly with the various test apparatus during testing, but which then interface with the host computer system later so that test data may be assimilated into the central database management system.
The PneuView system includes facilities allowing for simple compatibility with equipment management systems like those just described. These are designed to be straightforward and intuitive to any user of such a database system.
There are at least two ways of linking test data taken by the PneuView system to such 32 database systems: one applicable if the user also employs an intermediary data collection device, and another if he/she instead interfaces directly with a computer hosting the equipment management system.
If an intermediary device is used, PM and service templates for testing ventilators (for example) set up using the central equipment management program can be set to include a separate task item calling for a specific PneuView test to be performed as part of the maintenance or service work order (another task might be an electrical safety test). Such a test takes the form of a PneuView Template, designed by either the user or MII, specific to that ventilator. The technician can perform the electrical safety tests (for example) prompted by intermediary device, and then perform tests specified by the PneuView template(s).
The intermediary device generates its results text file, and the data export facilities included with PneuView do the same. When the Sentinel 32 system is later used to close that work order, both tasks are closed and the appropriate results text file associated with each respective task is included with the respective test record.
In cases where an intermediary device is not used (the norm for ventilator testing, in most cases), the scenario is simpler. Templates stored with the central equipment management system for ventilator tests include a task calling for tests using the PneuView system and detail using PneuView templates. After the user performs the required tests, PneuView generates a results text file, which the user would in turn associate with the test record when that task/work order is later closed using the equipment management system software.
In either case, the procedure is straightforward. Equipment management programs generally include a facility for attaching at least a text file to each “Task” itemized on the work orders they are designed and used to generate. While attaching the text results file created by most intermediary devices is automatic, it is simple for the user to attach a separate text file to a task when closing a work order.
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