An Automated breathing metabolic simulator (ABMS) simulates human breathing and metabolism through mechanical means respiration. ABMS technology is used as a platform for qualification and evaluation of Respiratory protective equipment like a Closed-Circuit breathing apparatus or N95 mask, usually by government or commercial entities. In the US, this regulatory function is performed through the National Institute for Occupational Safety and Health (NIOSH) and National Personal Protective Technology Laboratory (NPPTL) (Kyiazi, 1986)
Requirements
To simulate human respiration and metabolism, an automated breathing metabolic simulator needs to consume and produce the inputs and outputs of human respiration. These include: Carbon dioxide production Oxygen consumption Inhalation/exhalation pressures Carbon dioxide percentage Temperatures Respiratory frequencies while monitoring oxygen percentage (Wischhoefer, 1984)
History
Respiratory researchers have long desired an accurate method of simulating the breathing and metabolic processes of a human in a laboratory setting. Simulators have become a vital resource for research institutes, regulatory centers, testing houses, and manufacturers due to their ability to generate reproducible data without the risk of direct human exposure to a tested device. Prior to the early 1980s, accurately simulating human breathing in a controlled and repetitive manner was not possible due to the lack of technological development and human knowledge of the respiratory system.
With the first models of human respiration, basic Breathing Metabolic Simulators (BMS) were devised. These relied upon a manual pump that a technician would use to simulate human breathing. These BMS were criticized for testing inaccuracies which led to unexpected respiratory protective device risks. The first BMS was developed by U.S. Bureau of Mines in 1973. The U.S. Bureau of Mines continued funding the development of BMS's until 1985, when funding stalled. These efforts led to a design that is currently used in parts of NIOSH-NPPTL and the U.S. Navy (Sinkule, 2013).
The first BMS was designed under the following guidelines:
The simulations produced by the BMS were to be as physiologically appropriate as possible. The construction was to employ low cost methods using standard, commercially available items wherever possible. Operation was to be simple and easily learned. Complex computer programs were to be avoided. The BMS was to be capable of manual as well as automatic operation. All data inputs into the computer were to be paralleled with analog outputs suitable for general purpose laboratory recorders (Development Of An Automated Breathing Metabolic Simulator, 1984).
Current manufacturers of BMS's include: CSE, Dräger (Kyriazi, 2011), and Ocenco (Kyriazi 2011).
Automation
Automated simulators use the same mechanical principles as the BMS, updated with electronic components, thus offering more accurate control. The first ABMS contained three modules designed to be able to function independently from each other or work as a unit: a Breathing Simulator Module, a Gas Analysis Module, and a Supervisory Controller (Wischhoefer, L.L., & Reimers, 1984).
The first automated breathing simulator concepts used a bellows design. After numerous prototypes, bellows were found to be inconsistent in generating precision volumes of human breathing. Subsequently, bellows were replaced with a rigid piston design. The rigid piston solved the problem of repeatability and proved that controlled breathing waveforms were possible, subject to the precision of motor drivers.
Oxygen consumption has been simulated through various methods over the history of BMS development. Older methods of oxygen consumption include catalytic conversion, which produces hydrogen. This method saw limited use. ABMS manufacturers eventually began to prefer to remove a mixture of the breathing media, analyze the sample for the oxygen content, and then adjust the flow to contain the correct number of liters required for the correct oxygen consumption rate. At various points in the breathing cycle, CO2 represents a small percentage of the gas mix withdrawn to "consume" oxygen. This gas is generally analyzed separately and then algorithmically re-injected to compensate for artifacts of measurement.