Medical Effects of Wearing Self-Contained Breathing Apparatus
SELF-CONTAINED BREATHING APPARATUS
Decause of the hostile environment in which firefighters must work, self-contained breathing apparatus (SCBA) was universally adopted for fire service use.
The number of toxic gases present under fire conditions has also been well documented. Carbon monoxide, the most wellknown product of combustion, has been considered the primary cause of smoke related deaths. Also, ordinary combustibles, found in most fires, can generate temperatures of several hundred degrees at floor level. Inhalation of air at these temperatures will cause serious thermal injury to the lungs.
The majority of oxygen used by the body’s cells is transported by the circulatory system. Carbon monoxide binds to hemoglobin with an affinity 210 times greater than does oxygen. This attachment of carbon monoxide to hemoglobin greatly reduces the total oxygen content in the blood, and, in the most severe cases, causes death. In the body, carbon monoxide concentrations of 12,800 ppm will cause immediate unconsciousness and possible death. And with the ever-increasing use of plastics and synthetics, the list of deadly toxins continues to grow.
On July 1, 1983, an Occupational Safety and Health Administration (OSHA) regulation took effect requiring positive pressure SCBA to be used in all structural firefighting. Statistics for that year showed that smoke and gas inhalation accounted for 18.6% of all fireground injuries. However, smoke and gas inhalation accounted for only 5.5% of all injuries during training sessions. This indicates that incidents of this type are largely preventable, and fireground management and training have been cited as the areas needing improvement.
Included in this training should be a knowledge of the capabilities and limitations of SCBA. To help us define these capabilities and limitations, we must examine the medical effects of wearing SCBA. Our discussion here will focus on the more common open-circuit (positive pressure) SCBA.
When the wearer of a demand type SCBA inhales, he creates a slight negative pressure (as related to atmospheric conditions) in the facepiece. This negative pressure will open a demand valve in the regulator and allow air to flow into the facepiece. Once the lungs are filled, the facepiece pressure changes to positive and the demand valve closes. As the wearer exhales, the facepiece pressure remains positive and the air is vented out through the exhalation valve (see Figure 1 on page 45). It is during the inhalation phase, when the facepiece pressure is negative, that problems can develop. If the facepiece fit is not perfect, the negative pressure will allow the outside atmosphere to enter the facepiece and contaminate the inhaled air.
It was because of this potential problem that positive pressure SCBA was developed. The operation of positive pressure SCBA is much the same as the operation of a demand type SCBA with only some minor alterations, that allow for the facepiece pressure to remain positive during both inhalation and exhalation (see Figure 2 on page 45). These alterations include having a small amount of air flowing constantly into the facepiece and a spring loaded exhalation valve. These two elements create a constant “back-pressure” in the system, which translates to approximately 1 to 1 1/2 inches water column height of positive pressure.
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Photo by Warren Fuchs
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An important point to remember is that the pressure is not only positive in the facepiece itself, but is also positive through the body’s airways and down into the lungs. Therefore, in the positive pressure mode, the body’s entire pulmonary system, all the way down to the smallest unit of the lung, called alveoli, are maintained at that same positive pressure. In the hospital, this condition of maintaining positive pressure in the lungs is known as continuous positive airway pressure (CPAP).
The mechanics for positive pressure SCBA and CPAP are similar. Both conditions are created by the application of an exhalation resistance to breathing, along with an available airflow within the system to keep the pressure positive. The only difference is the purpose, keeping out toxic contaminants vs. medical treatment of respiratory disorders. Since much research has been done on the physiological effects of CPAP, we can draw on these studies, along with those done on firefighters, to examine the effects SCBA may have.
Using positive pressure SCBA has several advantages. First is the obvious increase in protection offered by the positive pressure keeping out the toxic atmosphere.
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Second, a side effect of this positive pressure, is that the amount of air left in the lungs at the end of each breath (called functional residual capacity) is increased because of the added pressure constantly maintained in the lungs. When the SCBA wearer breathes out against the respiratory resistance, a greater volume of air is left in the lungs and they become hyperinflated. This increase in functional residual capacity allows for a better distribution of air in the lungs. Normally, not all areas of the lungs are ventilated evenly. Some receive slightly less volumes of air because of the position of the lungs or damage from disease. With a larger volume of air remaining, more overall ventilation occurs and there is an increase in the oxygen levels of the blood.
Studies also suggest that positive pressure may promote better gas exchange in the lungs and, therefore, get more oxygen into the blood. This may explain an observation that was made during experiments in which maximum exercise tests were conducted with SCBA in demand and positive pressure modes. The tests were continued until the test subjects could not go any further. In the demand mode, eight subjects listed the lack of air as one of the reasons for termination of the test. In the positive pressure mode, no complaints were made. This effect may be related to the greater amount of oxygen levels afforded by the increase in functional residual capacity and improved gas exchange.
Of course, SCBA also has some negative effects. First, we shall consider the increase of “dead space” imposed by the facepiece. Respiration involves the intake of oxygen and the excretion of carbon dioxide, the waste product of metabolism. During exhalation, air rich in carbon dioxide flows from the lungs. When wearing a full facepiece, as is the case with SCBA in the fire service, some of the air and carbon dioxide is trapped within the mask and then is rebreathed in the next breath. This partial rebreathing of air will cause an increase in the carbon dioxide blood levels. Since the body normally monitors carbon dioxide levels in the blood to regulate respiration, this increase of dead space causes an increase in respiration.
Secondly, though the reason is unclear, it has been demonstrated that the application of continuous positive pressure will cause an increase in the depth of each breath.
Both the deeper breaths, up to seven times greater than normal, and an increase in the rate of breathing means more work for the body. This process starts to set up a cycle where more breathing causes more work, and more work causes more breathing.
How does the firefighter deal with this? The answer may be found in studies of firefighter heart rates while performing simulated and actual fireground activities. These studies have shown that firefighting requires a very high level of energy expenditure.
Under stress, firefighters quickly reach and maintain 85%-100% of their maximum predicted heart rates, whether they are using no SCBA; new, light-weight SCBA; or heavy, older SCBA. This would indicate that firefighters adjust their work output when they start to exceed their maximum level of exertion. They keep themselves at this maximum level of exertion regardless of the physical limitations imposed. The weight of SCBA alone causes approximately 20% reduction in work performance. Continuous positive airway pressure has been shown to cause adverse cardiac effects and drops in blood pressure. However, this occurred at much higher levels of pressure than those used in SCBA, and these effects occurred in people who already had a marginal cardiovascular status.
One myth that needs to be dispelled concerning positive pressure SCBA is that positive pressure SCBA assures a positive pressure in the facepiece at all times.
Government regulations require that the regulator of open-circuit SCBA be able to flow 200 liters per minute (1pm). Most manufacturers supply units that have a capacity to flow 400 to 500 1pm. However, during maximal work, inhalation flow rates in excess of 720 1pm have been found, greatly exceeding the capabilities of most regulators used today. The breathing pattern starts with normal respiration, and as the work load increases, the wearer needs the flow coming into the facepiece to be greater and greater. Finally, the flow is not enough to keep the facepiece positive, and a negative pressure is created (see Figure 3 below). It is now that toxic contaminants can enter the facepiece, even though the positive pressure mode is being used.
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Photo by Warren Fuchs
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If this lack of available air does occur, what is the effect on the wearer? As stated before, the wearer is limited by the restrictions placed on him. Therefore, if the flow does not meet his inhalation needs, then the work output is decreased to adjust for this restriction. This may be partly why new personnel sometimes complain of not being able to get enough air from their SCBA. They have not learned how to adjust their work output to deal with the limitations.
It can be seen that SCBA can offer both advantages and disadvantages to the wearer. However, while the reduction of work capacity and other factors may appear significant, they do not outweigh the benefits gained.
All SCBA wearers need comprehensive training in theory and application of this equipment—because the time to discover the limitations of SCBA is not during operations in a toxic atmosphere.
More work needs to be done to study the effects of SCBA on firefighters. Current National Institute of Occupational Safety and Health (NIOSH) regulations do not address firefighter requirements sufficiently. Their basis of testing SCBA for duration of service, flow capabilities, and use by personnel do not consider the conditions under which firefighters work. Service time for open-circuit SCBA is currently determined by simulating a breathing volume of 40 1pm (this is the actual volume of air breathed, not the rate at which the air flows into the facepiece). Depending on the individual, maximum work loads may require firefighters to breathe up to 86 1pm of air. This reduces actual service time to just under 15 minutes for a 30-minute rated SCBA.
Until such issues can be addressed, firefighters must continue to be aware of SCBA limitations and apply the recent advances in SCBA technology to their benefit.
A Fire Service Guide for the Selection, Use, Care, and Maintenance of SCBA. National Fire Protection Association.
Self-Contained Breathing Apparatus, 1st edition. International Fire Service Training Association.
“Minimum Protection Factors for Respiratory Protective Devices for Firefighters.” W. A. Burgess, et al. American Industrial Hygiene Association Journal, January 1977, pages 18-23.
“The Effect of Positive End-Expiratory Pressure on Regional Ventilation and Perfusion in the Normal and Injured Primate Lung.” J. W. Hammon, W. G. Wolfe, J. F. Moran, et al. Journal of Thoracic Cardiovascular Surgery, 1976, pages 72 and 680.
“Cardiac Output and Gas Exchange During Heavy Exercise with a Positive Pressure Respiratory Protective Apparatus.” M. Arborelius, G. Dahlback, et al. Scandinavian Journal of Work Environmental Health, September 1983, pages 471-477.
“Maximal Stress Test While Wearing a Self-Contained Breathing Apparatus.” P. B. Raven, T. O. Davis, et al. Journal of Occupational Medicine, December 1977, pages 802-806.
“Medical Evaluation for Respirator Use.” P. Harber. Journal of Occupational Medicine, July 1984; pages 496-502.
“Effects of Positive End-Expiratory Pressure on Breathing Patterns of Normal Subjects and Intubated Patients with Respiratory Failure.” M. J. Tobin. Critical Care Medicine, November 1983, pages 859 -867.
“Heart Rates in Firefighters Using Light and Heavy Breathing Equipment: Similar Near-Maximal Exertion in Response to Multiple Work Load Conditions.” Manning and T. R. Griggs. Journal of Occupational Medicine, March 1983, pages 215-218.
“Heart Rate and E.C.G. Response of Firefighters.” R. J. Barnard and B. S. Duncan. Journal of Occupational Medicine, April 1975, pages 247-250.