Document bydwbKw4r8DvN17bq0eK7J6Q0

Medical Evaluation for Respirator Use Philip Harber, M.D., M.P.H. Medical certification for respirator use should be based on understanding of their respiratory, cardiac, mechanical, psychological, and other effects. Work environmental factors (e.g., level of hazard and work load of the job), respirator characteristics, and persona! characteristics of the worker should determine the evaluative procedures used; a three-level scheme is suggested. Respirators (respiratory personal protective devices) are necessary for worker protection in a contaminated atmo sphere when engineering controls are impossible. An effective respiratory protection program has three com ponents: selection of the proper respirator, determination that the employee can work while wearing the respirator, and proper Lise of the respirators. This report will address selection of the worker who can wear a respirator from the occupational medical perspective, a topic that has received inadequate attention. Although a new respirator rule will be proposed, the current general Occupational Safety and Health Administration (05HA) regulation (1910.134)*1 merely states in a few lines that there should be determina tion of appropriate health conditions and "periodic (for instance, annua!)" reconsideration. There arc, however, genera! guidelines for the amount of allowable resistance in a respirator and for the permissible rebreathing of carbon dioxide (C02) (specified only for self-contained [SCBA] respirators).2 A medical certification program should be based on several considerations: the physiologic burdens that a respirator imposes, compensatory mechanisms, effects of disease, screening methods, critical limiting factors, and costs. From the Occupational Medicine Brandi and Pulmonary Divi sion, University of California at Los Angeles. Address correspondence to Department of Medicine, 37-131 Center toi the Health Sciences, University of California, Los An geles, Los Angeles, CA 90024 (Dr. Harber). I his study was supported in part by a grant Irom the California Medical Research and education Foundation. Presented' at the American Occupational Health Conference, Washington, O.C., April 24-29, 19fl.i. 490 Physiologic Loads imposed Respirators produce several important physiologic bur dens. The major physiologic effects are those on pulmonary and cardiac function as a consequence of several specific "loads" imposed by the respirator.3 First, there is resis tance to airflow, the "flow resistive load." While this may be present in both inspiration and expiration, it is pre dominantly an inspiratory load in nonpowered air-purifying respirators. The magnitude of this effect differs consider ably from respirator to respirator, and American National Standards Institute (ANSI) recommendations and OSHA regulations provide maximum limits.2 In addition, there may be a "threshhold" resistance, one that disappears after airflow begins. That is;although pressure must be generated to open a valve, once it is open, there is no effective re sistance. This is particularly true of demand-type valves, and it is generally of small significance except when there is a malfunction. The other major load is "dead space," representing the volume of exhaled air that is rebreathed from the mask with each inspiration. Respirator masks, particularly fullface masks, may be quite large, having volumes equivalent to tiie internal physiologic dead space.3'4 However, the total mask volume is not rebreathed with each breath. The rebreathing (dead space) effect requires increased ventila tion. An increased pleural pressure must be generated by the respiratory muscles (diaphragms and chest wall muscles) to overcome the airflow resistance of tiie respirator. If a respirator has significant expiratory impedance, the pres sure in the chest cavity will also be more positive during expiration. During exercise, the peak added pressure has been from 10 to 15 cm of water on the average.4'6 In creases in the pleural pressure swings have several possible effects. They require increased respiratory work and may produce dyspnea. Furthermore, pleural pressure swings may affect cardiac function hy affecting filling of the heart.7'12 The net effect of the increased resistance and increased ventilation is a significant increase in the work of breathing. This may be expressed as the integral of pressure times flow over time.13 In addition to increasing the driving pressure needed to produce any flow rate, respirators also require an Respirator Use/'Harber SWRf/Asbestos 6807 IB 030516 increase in the loial volume of air brcaihed in order to over come the dcad-spacc effect, thus further increasing the total work of breathing. The resistance of a respirator can be comparable with or even greater than the "internal resis tance" of the respiratory system (approximately .1.5 cm of water/l/s as determined by plethysmography).14 In addi tion to increasing the total work of breathing, respirators also increase the average and peak work rates. The added work may be inspiratory or expiratory or a combination of both, depending on the location of the resistance relative to the valves of the respirator. Unfortunately, it is difficult to measure directly the total work of breathing since this requires placement of a balloon catheter in the esophagus to estimate pleural pres sure. However, the added work may be more easily deter mined by measuring airflow and mouth pressure, the in tegral product of which represents the additional work required to overcome the external resistance imposed by the respirator. Normally the distribution of blood flow within the lung is matched appropriately to distribution of ventilation so that an optimal ventilation-perfusion ratio is maintained in each lung zone.15 The distribution of blood flow is depen dent on gravity and intrathoracic pressure gradients, and the regional distribution of ventilation in the lung depends on flow pattern. Thus, although not empirically verified, it is quite possible that ventilation-perfusion matching may be affected by respirator use. Physical characteristics of the device itself may be im portant. A SCBA may weigh more than 30 lb, increasing both the energy demands of the task and the cardiorespira tory demands of any particular job and contributing to back strain and to postural instability. Working in close spaces may be difficult. Mobility is limited when using an airline respirator, which may lead to unusual work postures. Providing an air-tight fit requires bands that may impede lymphatic flow. In addition, field of vision and the clarity of vision may be decreased, and there is interference with the physiology of normal speech. Other physiologic effects include decreased ability to eliminate heat, which becomes particularly important when the clothing worn is imper vious to moisture and heat.16-17 Anxiety can also produce physiologic stress. If a worker is made anxious by the use of a respirator, catecholamine levels may increase, causing heart rate, blood pressure, and respiratory rate to increase. The mask itself may produce anxiety, and the use of a respirator may signify the pres ence of "danger." In some workers, the anxiety produced by the respirator may preclude its effective utilization. Morgan18 has estimated that 10% of workers have psycho logical problems that affect respirator use. There have been several reviews16-19-21 that describe respirator loads. Compensatory Mechanisms - Faced with added loads due to the respirator, the user may employ several compen satory physiologic mechanisms. First, he may increase the respiratory work. Respiratory work may be expressed as the integral of pressure times volume over time.6 Any in crease in respiratory work is due to both the increase in inspiratory and expiratory effort measured in pressure and the increase in ventilatory volume (to compensate for the mask dead space). Normal subjects tested during progres Journal of Occupational Medicine/Vol. 26, No. 7/July 1984 sive exercise with and without respiratory loads in place were easily able to increase their respiratory work (pressure volume product) in going from submaximal to maximal ex ertion, and were able to increase the peak work rate as well as the average work rate.6 With the added inspiratory flow resistance of the respira tor, an increase in the inspiratory time will reduce the average pressure, decreasing the work rate of the inspiratory muscles. Increases in both the absolute inspiratory time and the proportion of the respiratory cycle have been demon strated at several exercise levels.3-6-22'24 Of course, increas ing inspiration leads to a decrease in the expiratory time and can cause dyspnea.24 The normal inspiratory and expiratory phases are ini tiated with high instantaneous flow rates. These early high inspiratory flows, which require marked pressure genera tion, may be decreased.6 Increases in tidal volume or frequency or both will in crease ventilation to compensate for the dead-spacc loading and the increased C02 production of exercise.25-27,5 In creases in tidal volume will be more efficient than increases in frequency, since only that portion of the breath that reaches the gas-exchanging surfaces (the alveolar ventila tion) is effective in C02 elimination. Even without a respirator, much of a normal breath is "wasted"; for ex ample, the "physiologic" dead space may be about 150 ml of each 500-ml tidal volume. Twelve breaths per minute with a tidal volume of 500 ml would produce 4,200 ml of effective alveolar ventilation. (Normal basal alveolar ventila tion is about 4.2 I.) If a mask dead space of 150 ml were added with no compensation, the effective alveolar ventila tion would fall to 2,400 ml/min, and the arterial Pco2 would then rise. To maintain the normal alveolar ventila tion simply by increasing respiratory frequency would require a respiratory rate of 21/min (fur an alveolar ventila tion of 4.2 I) with a consequent total minute ventilation of 10.5 1. Alternately, increasing the tidal volume to compen sate for the added dead space would require an additional 150 ml'per breath (new tidal volume = 650 ml) with a minute ventilation of only 7.8 I (650 ml x 12/m in). In actuality, a combination of tidal volume and frequency change occurs to deliver adequate alveolar ventilation. Workers who are unable to increase one or the other will not be able to compensate for the added dead space. If a higher than normal PC02 were tolerated by a worker, less alveolar ventilation would be necessary. Elevation of Pco2 is a potent stimulus to respiration and is accompanied by the development of tachypnea and falling arterial pH. While chronic hypercapnea may allow a metabolic compen sation, acute hypercapnea, such as would occur with tem porary respirator use, would occur too rapidly to allow such metabolic compensation; hence, there would be a large fall in the arterial pH. Therefore, tolerating a higher PC02 is not a particularly safe compensatory mechanism. However, even normal subjects do increase Pco2 when breathing through very high resistances, but they appear tachypncic and probably dyspneic.28 One additional method of adapting is to remove the respirator or to create a leak in the facial seal. Of course, this prevents the respirator from protecting the worker against airborne toxins. However, anecdotal observations and studies such as those of Smith (in cadmium workers)29 SWRf/Asbestos 6808 IB 030517 497 suggest that many workers may choose this method of adaptation. Effects of Disease Psychiatric, pulmonary, cardiovascular, and other dis eases may interfere with respirator use. Psychiatric disorders or certain personality traits may prevent safe respirator use.18 For example, certain phobias may preclude their use, and personality traits may make a worker unreliable in using respirators. Of course, workers who are unable to understand the purpose and proper use of respirators should not be permitted to use them. Pulmonary disease may be categorized as restrictive disease, obstructive disease, and respiratory control dis orders. Obstructive diseases include chronic obstructive lung disease and asthma. Persons suffering from them may have an increase physiologic dead space and characteristical ly have an increase in airway resistance. In more advanced stages, with their lungs hyperventilated, their inspiratory muscles may be inefficient; the diaphragms are low and flat, and they operate at an inefficient position on their lengthtension relationship.30'31 Individuals with obstructive disease may have dyspnea even in the absence of respirator use. However, many studies of ability to detect and scale added flow resistances suggest that persons with elevated airway resistances may be less "sensitive to" added exter nal resistances.32'39 These resistances might decrease their dyspnea but could also interfere with their ability to com pensate appropriately. Such individuals may also have right sided heart failure and be sensitive to added pleural pressure swing effects. They may be hypercapneic with exercise or even at rest. A hypercapneic worker will be particularly prone to further, more marked hypercapnea with added loads - because he has little or no respiratory reserve and has already shown a respiratory control abnormality and because any given decrease in alveolar ventolation causes a more marked elevation in PC02 Finally, persons with chronic bronchitis have frequent production of sputum in association with cough. It is not difficult to visualize how a tight mask would be poorly tolerated by a worker with a frequent productive cough. Restrictive lung disease includes many disorders such as idiopathic pulmonary fibrosis and complicated silicosis. Affected workers have decreased pulmonary compliance (i.e., the lungs are stiff) and they generally have high pleural pressure swings even in the absence of respira tor use. They also may be dyspncic, occasionally out of proportion to the apparent abnormalities on simple pul monary function tests. It would be difficult for them to increase tidal volume because of the decreased pulmonary compliance; typically, patients with restrictive lung disease breathe rapidly and shallowly. They may therefore be particularly prone to adverse effects of respirators. Respiratory control disorders may be primary (unassoci ated with other lung disease) or in association with lung disease such as chronic obstructive lung disease. Persons with such disorders do not readily adjust their ventilatory work to the needs because they have lost the normal re sponse to the stimulus of increased arterial Pc.02, hypoxe mia, or decreased arterial pH. They may be hypercapneic at rest or may be normal when at rest and while awake. Per sons with primary respiratory control disorders may be 453 difficult to detect clinically, may have normal pulmonary function tests (including normal Pc.02), and may require rather sophisticated testing to demonstrate their abnor mality.40'44 Cardiac disease must also be considered. Angina or arrhythmias may be aggravated if hypoxemia or hypercarbia occurs. Antihypertensive drugs producing postural instability (e.g., guanethidine) may increase the cardiovas cular effects of increased pleural pressure swings; fortunate ly, such drugs are used only on a limited basis. Nonselcctive |3-blocker drugs for angina or hypertension may, in sensitive individuals, increase the airway resistance. Generally, per sons who are sensitive (e.g., asthmatics) should not be treated with j3-blocker drugs even in the absence of respira tor use. The increased respiratory work will require an in crease in cardiac output,31 and persons with certain severe cardiac diseases may have a limited ability to increase car diac output. .A variety of other medical conditions may interfere with respirator use. The National Institute for Occupational Safety and Health suggests that a perforated typmpanic membrane may make respirator use ineffective.2 Structural abnormalities (congenital, traumatic, or surgical) of the face may interfere with adquate mask fit. Musculoskeletal dis orders may make it difficult to carry a heavy SCBA, and the weight of the respirator may lead to increased cardiac work load. The Critical Limiting Factors A medical certification program should be based on the detection of any critical limiting factor that may exist. Several factors may potentially be limiting. These, as well as possible medical evaluative procedures, are shown in Table 1. Ventilation may be critically limiting. It may not be possible for the worker to achieve sufficient ventilation to compensate for the added dead space. Ventilatory limita tion with the added flow resistance due to the respirator may prevent compensation. That is, some workers may be unable to sustain an adequate minute ventilation for more than a short time.45'45 Table 1 - Possible Limiting Factors and Means of Evaluation Limiting Factor Evaluation Ventilation Ventilation with resistance Hypoxemia Tidal volume increase Expiratory time compression Inspiratory muscle fatigue Pleural pressure swingsSubjective dyspnea Sensitivity to loads Cardiac output Psychological tolerance Knowledge Spirometry Spirometry Arterial blood gas analysis Exercise testing or special spi- rometry Measurement of timing Electromyogram Esophageal balloon Observation Scaling or threshold methods Measurement Interview Interview or test 1 B 03C SWRf/Asbestos 6809 Respirator Use/Harber Development of hypoxemia could limit work using respiraLors.47,48 However, there is little evidence to suggest that this is a common primary limitation. Carbon dioxide excretion may be limited, due to the inability to increase ventilation sufficiently (i.e., some persons may be unable to increase tidal volume and/or rate). Compression of expiratory lime results from prolonga tion of inspiratory time. Johnson and co-workers,23,24 in a study of normal volunteers during maximal exhausting exercise, found that expiratory time consistently appeared to predict the point at which subjects could no longer continue. In evaluating submaximal exercise, however, we found considerable reserve in expiratory time compression, suggesting ihat it is not necessarily the limiting factor under most circumstances.3,6 Inspiratory muscle fatique has been recognized in creasingly as a concomitant of respiratory failure.30,31 It can also be produced experimentally with flow resistive loads in healthy subjects; however, the magnitude of these resistance loads is much higher than that of respirators. Pleural pressure swings may be a limiting factor. Demedts and Anthonisen49 have shown that each normal person has a particular level of pleural pressure swing that he will toierate. This is probably related to the peak work rate of the respiratory muscles. The subjective sensation of dyspnea may be limiting. Dyspnea is a poorly understood sensation that has many causes other than simple mechanical factors.50-52 Pressure swings, both upper airway and intrathoracic, may con tribute to dyspnea. Distortion of J receptors may account for dyspnea in persons with pulmonary vascular and inter stitial lung disease.51 Dyspnea may be a consequence of disproportion between work output of the respiratory muscles and ventilation achieved or pressures generated; this is likely to occur in the setting of respiratory loading (i.e., Lhe resistance and additional dead space makes ventila tion less efficient). Often the abnormality in pulmonary function can be demonstrated only by evaluation during exercise. Cardiac output is also a potential limiting factor. With increasing loads, the respiratory muscles take an increasing proportion of the total cardiac output; a worker with car diovascular disease may be unable to increase total cardiac output adequately.31 Psychological tolerance to the respirator or knowledge of its care and use may prove limiting in some individuals. Certification Program Identification of workers who cannot safely and effec tively use respirators should be based on evaluative proce dures that are simple, cost-effective, truly related to ability to use respirators, and preferably "objective." Criteria should not be too stringent (excluding workers unneces sarily) and yet must provide an adequate margin of safety to ensure that no worker is injured. There are many types of respirators, they are used in many different situations, and workers may have a great variety of medical conditions. Clearly, no single system can be appropriate for all situa tions, and therefore, the choice in any individual case must be based on the three factors --job environment, type of respirator, and characteristics of the individual worker. Journal of Occupational Medicine/Vol. 26, No. 7/July 1984 A respirator may be used briefly or may be worn throughout the workday; a worker may tolerate it for a short time period but not throughout the day. The job may require minimal physical exertion or heavy physical weak with additional stresses such as heat stress. The purpose of the respirator use must be considered -- clearly more strin gent criteria are necessary for exposures that are immediate ly dangerous to life and health than for nuisance dust expo sure. The frequency of respirator use, however, should not be considered a determinant. Indeed, the case of respirator use for emergency escape only may be particularly danger ous; here, workers will not "self-select" themselves to find new jobs if they tolerate respirators poorly, and they will be using a device with which they may be unfamilar iri possible panic situations. The respirators also differ. Some have considerable in spiratory resistance, while in others this is minimal; similar ly, dead space varies considerably from respirator to respira tor. Some SCBAs are quite heavy. Characteristics of the worker must also be considered. Age, previous respirator experience, and physical conditions should determine the evaluation necessary. For example, it is possible to characterize the "risk" of an individual. Per sons who have previously experienced difficulty with respirators warrant particularly careful consideration, while previous respirator use at high exertion levels suggests that physiologic or psychological complications are unlikely to occur. Medical conditions may also constitute ``risk fac tors." Any worker with chronic respiratory symptoms or a history of recurrent pulmonary infections needs special attention. Because workers with asthma may vary from day to day, their "risk" must be assessed on a "worst case" basis, considering the most severe episode they might develop while using a respirator. Cardiac disease also con stitutes a significant risk factor for the reasons discussed earlier. Psychological assessment (done informally) may also indicate "high-risk" individuals. Ultimately, the deter mination whether a worker's personal characteristics place him/her at higher than average risk must be made on an individual clinical basis. Recognizing that several "levels" of evaluation are needed, Table 2 presents a suggested scheme for choosing the level of evaluation based on three factors. Good judg ment should always supersede any schema. A basic component of any program is observation of the worker using the respirator, preferably at an exercise level required by the job. With proper observation, this can pro vide a great deal of information. At the very least, it can help to identify those workers who require more detailed testing. The level I evaluation should include at minimum a short questionnaire ("Do you have cardiopulmonary disease? Do you have difficulty using the respirator? Do you know how to use it?"); observation of the worker using the respirator by a trained nurse or industrial hygienist; and measure ment of pulse rate, blood pressure, and respiratory rate while working with and without the respirator. A level II evaluation should include a more detailed questionnaire or interview, a limited examination of the heart and lungs by a physician or appropriately certified nurse practitioner, a spirogram, observation of the worker while using the respirator, and pulse rate, blood pressure, SWRf/Asbestos 6810 IB 030519 499 arasixaeBusi Table 2 - Choice of Evaluative Level Exertion t Duration, min "Risk" t Level Danger* H M L 0-5 6-30 >30 No Yes 1 II III IDLH X XX X X X XX X XX XX XX XXX X XX XX X XX XX X XX XXX X XX XX X X X X X X X X X MOD XX XX XX Xv X x XX XX X XX X XXX X XX VX X XX X XX XX X X SL X X XX XX All other cases X * IDLH indicates immediately dangerous to life and health; MOO, toxic atmosphere for extended exposure; SL, slight risk ^ H (high) indicates >5 METS; M (moderate, 2-5 METS; L (light), <2 METS t Age >50 or history of heart or lung disease or SCBA or "high" resistance respirator and respiratory rate measurements taken while exercising with and without the respirator. An electrocardiogram should be performed for those over 40 years of age. A level III evaluation should also include a more detailed interview and examination by a physician who is familiar with respirators. The peak inspiratory and expiratory pressures that the worker can generate should be measured; this requires only minimal equipment.53 In addition, a more forma! exercise test should be performed. While a standardized laboratory treadmill protocol54'56 would be optimal, practical considerations preclude this. However, a standardized exercise protocol can be developed for each plant involving a combination of exercise on inclined ground and stairs; comparability between plants can be pro vided by ensuring that a dozen normal volunteers achieve a pulse rate of 60% of predicted maximal. A worker should complete approximately 10 minutes of exercise without a respirator and then exercise under observation for at least 30 minutes using the respirator. Blood pressure, pulse rate, respiratory rate, and clinical assessment of respirator toler ance should be recorded. In addition, the physician should ascertain by brief interview that there are no psychological contraindications to respirator use. The criteria for determining fitness need to be defined. Table 2 provides guidelines for the selection of appropriate tests but does not specify what cutoffs should be used or biiO how the individual tests should be combined to yield a final decision. Development of such a set of specific criteria requires a coordinated effort of the occupational health professionals at each work site, integrating information about environmental and medical factors. However, an abnormal test result (e.g., outside the 95% confidence limits defined in a normal population) may define the need to consider a worker's fitness more carefully. Simple ventilatory measurements alone are unlikely to identify all workers with contraindications to respirator use. Spirometry is of limited sensitivity for detection of restrictive lung disease, respiratory control disorders, prob lems of endurance, and psychological unsuitability. Spirometric results of asthmatics may vary considerably over time, and therefore testing at one time may not reflect the physiologic state at other times. Performance of routine spirometric tests, particularly the maximal voluntary ventilation (MW), using the respirator has been sug gested,57 but several questions require further evaluation before implementation. First, the MW, even without a respirator, is an extremely variable test. Second, there is no clear and direct relationship between a 15-s MW test and a worker's ability to breathe effectively during a more pro longed work period. Third, the great variety of respirators available makes mechanical linkage to standard spirometers difficult. Fourth, the respiratory pattern during an MW maneuver is quite different from that during normal work with a respirator. Those workers who are found to be abnormal or who show subjective or objective difficulty with respirator use should be referred for more specific individualized evalua tion. For example, persons with moderate to severe ob structive lung disease'should have arterial blood gas analysis to determine if hypercapnea is present. Some workers may require formal exercise testing with measurement of gener ated pressures as well as metabolic parameters.6 Current experimental techniques include measurement of load scaling proclivity (which at this time is difficult to perform reliably) and quantitative description of respiratory pattern (which is simple to perform using only a simple spirometer). However, further research is needed to determine if these will be useful in practice. There have been several additional studies of effects of respirator use,58"66 but there is as yet no clear means of definitively identifying high-risk workers. More research and practical evaluation are necessary. In the interim, establishment of a formalized, multilevel scheme such as the one described would help ensure ade quate safety for the worker and avoid unnecessary testing. A cknowledgement Linda Huf and Karen SooHoo prepared the manuscript. References 1. Occupational Safety and Health Administration: General Industry. OSHA Safety and Health Standards, 29 CFR 1910, 134 Respiratory Protection. Washington D.C.: U.S. Dept, of Labor, 1981. 2. Dept of Health, Education, and Welfare: A guide to In dustrial Respiratory Protection, publication 76-189. Cincinnati: National Institute of Occupational Safety and Health, 1 976. 3. Harber P: Control and prevention of occupational lung disease. Clin Chest Med 2:343-356, 1981. Respirator Use/Harber 1B 030520 4. Hat bet P, Tamimic J, Bhattaiharya A, et at: Physiologic effects of' respirator deadspace and resistance loading. / Oca/p Med 24:681-684, ] 982. 5. Jones NL, Levine GB, Robertson DG, el al: The effect of added deadspace on the pulmonary response to exercise. Respira tion 38:339, 1971. 6. Ilavber P, Tamimic RJ, Bhaitacharya A, et al: Effects of exercise with industrial respirators. Am Rev Respir Dis 124(suppl): 160, 1982. 7. Rebuck AS, Pengelly LD: Development of pulsus paradoxus in the prcssence of airways obstruction. N EngI J Med 28:866-869, 1973. 8. Jackson FN, Rowland V, Corssen G: Laryngospasm-induced pulmonary edema. Chest 78:819-821 , 1980. 9. Stakup SA, Meilins RB: Mechanical forces producing pulmonary edema in acute asthma. N Engl / Med 297:592*595, 1977. 10. Buda AJ, MacKensic GW, Wiglc ED: Effect of negative intrathoracic pressure on left ventricular function. Circulation 63: 875-881, 1981. 11. Scharf SM, Brown R, Saunders N, et al: Effects of normal and loaded spontaneous inspiration on cardiovascular function. J Appi Physiol 47:582-590, 1979. 12. Robotham jL, Lixteld W, HotiandL, et al: Effects of respira tion on cardiac performance. J Appi Physio/ 44:703-709,1978. 13. Otis AB: The work of breathing. Physiol Rev 34:449-458, 1954. 14. DuBois AB, Bothelo SV, Comroe JH: A new method for measuring airway resistance in man using a body plethysmograph: Values in normal subjects and in patients with respiratory diseases. j Clin Infest 35:327-355, 1956. 15. West JB: Regional Difference in the Lung. New York: Academic Press Inc., 1977. 16. James RH: Breathing Resistance and Dead Space in Respira tory Protective Devices. Cincinnati: National Institue of Occupa tional Safety and Health, 1976. 17. Attcrbom HA, Mossman PB: Physiological effects on work performance of vapor-barrier clothing and full-face respirator. } Occup Med 20:45-52, 1978. 18. Morgan WP: Psychological problems associated with the wearing of industrial respirators: A review. Am fnd Hyg Assoc / 44:671-677, 1983. 19. Raven PB, Dodson AT, Davis TO: The physiolgical conse quences of wearing industrial respirators: A review. Am Jnd Hyg Assoc } 40: 517-534, 1979. 20. Barraclough RN: Modern concepts of respiratory protection. Ann Occup Hyg 19:351-355, 1976. 21. Cotes JE: Conference report: Advances in respiratory pro tection. Ann Occup Hyg 22:189-197, 1979. 22. Johnson AF, McCuen RH: Prediction of respiratory period on men exercising while wearing masks. Am Jnd Hyg Assoc f 42: 707-710, 1981. 23. Jchsnon AT, McCuen RH: A comparative model study of respiratory period prediction on men exercising while wearing masks. IEEE Trans Biomed Eng 102:430-439, 1980. 24. Johnson AT, Berlin HM: Exhalation time characterizing ex haustion while wearing respiratory protective masks. Am Jnd Hyg Assoc J 35:463-467, 1975. 25. Love RG, Muir DCF, Swecliand KF, et al: Tolerance and ventilatory response to inhaled C02 during exercise and with in spiratory resistive loading. Ann Occup Hyg Assoc J 22:43-53, 1979. 26. Craig FN, Blevins WV, Cummings EG: Exhausting work limited by external resistance and inhalation of certain carbon dioxide. j Appi Physio! 29:847-851, 1970. 27. Petsonk L, Boyles C, Hodous T, et al: Effects of Added Resistance to Breathing in Obstructed Lung Disease: Phase I Report. Morgantown, W. Va.: National Institute of Occupational Safely and Health, 1981. 28. Deno NS, Kamon E, Kiser DM: Physiological responses to resistance breathing during short and prolonged exercise. Am ind Hyg Assoc j 42:616-623, 1981. 29. Smith TJ, Ferrel WC, Varner MO, et al: Inhalation exposure of cadmium workers: Effects of respirator usage. Am Ind Hyg Assoc } 41:624-629, 1980. 30. Dcvcnne JP, Macklem PT, Roussos C: The respiratory Journal of Occupational Medicine/Vol. 26, No. 7/July 1984 muscles: Mechanics control and pathophysiology. Am Rev Respir Dis 118:119, 1978. 31. Roussos C, Macklem PT: The rcspiiatory muscles. /V Enul / Med 307:736-797, 1972. 32. Ailose MU, DiMarto AF, Gottfried SB, et al: The sensation of respiratory muscle force. Am Rev Respir Dis 126:807-811, 1982. 33. Killian KG, Campbell Ej, Howell JB, et al: The effect ut increased ventilation on resistive load discrimination. Am Rev Respir Dis 120:1233-1 238, 1979. 34. Killian Kj, Mahutte CK, Campbell EJM: Magnitude scaling of externally added loads to breathing. Am Rev Respir D>s 123:12, 1981. 35. Gottfried SB, Altose MD, Kelscn SG, et al: Perception of changes in airflow resistance in obstructive pulmonary disorders. Am Rev Respir Dis 124:566, 1981. 36. Hudgel DVV, Cooperson DM, Kinsman RA: Recognition of added resistive loads in asthma: The Importance of behavioral styles. Am Rev Respir Dis 126:121, 1982. 37. Zcchmnn FW, Davenport PW: The effect of increased back ground airflow resistance on ventilatory control and the threshold for detection of added resistance, abstracted. Physiologist 21: 132, 1978. 38. Burki NK, Mitchell K, Chaudhary BA, et al: The ability of asthmatics to detect added resistive loads. Am Rev Respir Dis 117: 71-75,1978. 39. Gottfriend SB, Altose MD, Kelson SG, et al: The perception of changes in airflow resistance in normal subjects and patients with chronic airway obstruction. Chest 73(suppl):2S6-288, 1978. 40. Milic-Emili J: Recent advances in clinical assessment of con trol of breathing. Lung 160:1-17, 1982. 41. Whitelaw VVA, Dercnne JP, Milic-Emili J: Occlusion pressure as a measure of respiratory center output in conscious man, Respir Physiol 23:181-199, 1975. 42. Milic-Emili J, Whitelaw WA, Dcrcnne JP: Occlusion pres sure -- a simple measure of the respiratory center's output. ,V Eng/ J Med 293:1 029-1 030, 1975. 43. Miiic-Emiii j: Clinical methods for assessing the ventilatory response to carbon dioxide and hypoxia. /V Eng! j Med 293:864* 865,1975. 44. Read DJC: A clinical method for assessing the ventilatory response to carbon dioxide. Austrafas Ann Med 16:20-32, 1967. 45. Smidt U, Worth H, Petro W: Ventilatory limitation of exer cise in patients with chronic bronchitis and/or emphysema. Bvii Ear Physiapathol Respir 15:95-101,1 979. 46. Stemler FW, Craig FN: Effects of respiratory equipment on endurance in hard work. / Appi Physio/ 42:23-32, 1977. 47. Minh VD, Light RW: Hypoxemia during exercise in patients with COPD.4/77 Rev Respir Dis 120:787, 1979, 48. Barnett TB, Rasmussen B: Ventilatory responses to hypoxia and hypercapnia with external airway resistance. Acta Physio! Scand 80:538-551,1970. 49. Demedts M, Anthonisen NR: Effects of increased external airway resistance during steady state exercise. J App! Physio! 35: 361-366, 1975. 50. Marshal! R, Stone RW, Christie RV: The relationship of dyspnea to respiratory effort in normal subjects, mitrai stenosis, and emphysema. C!in Set 1 3:625-631, 1954. 51. Fishman AP: Manifestations of respiratory disorders, in Fish man AP (Ed.): Pulmonary Diseases and Disorders. New York: McGraw-Hill Book Co., 1980. 52. Richards DW: The nature of cardiac and pulmonary dys pnea. Circulation 7:15-29, 1953. 53. Black LF, Hyatt RF: Maximal respiratory pressures: normal values and relationship to age and sex. Am Rev Respir Dis 99:696702, 1969. 54. Wasserman K, Whipp BJ: Exercise physiology in health and disease. Am Rev Respir Dis 112:219, 1975. 55. Jones NL, Campbell EJ: Clinical Exercise Testing. Philadel phia: W. 8. Saunders Co., 1975. 56. Wasserman K, Whipp Bj, Davis JA: Respiratory physiology of exercise: Metabolism, gas exchange, and ventilatory control, in Widdtcombe JG (Ed.): International Review of Physiology: Respira tory Physiology III. Ballimoie: University Park Press, 19S1, vol. 23, chap.4. SWRf/Asbestos 6812 1B 030521 501 57. Raven PB, Moss RF, Page K, et al: Clinical pulmonary func tion and industrial respirator wear. Am Ind Hyg Assoc J 42:897903,1981. 5S. Balieu E, Spindler L: Performance testing for improving the levei of respiiatory protection in a fire brigade. Arm Occup Hyg 21: 351-361,1978. 59. Kamon E, Bernard T, Stein R: Steady state respiratory responses to tasks used in federal testing of self-contained breathing apparatus. Am Ind Hyg Assoc j 36:S86-896, 1975. 60. Raven PB, Davis TO, Shafer CL, et al: Maximal stress test performance while wearing a self-contained breathing apparatus. ] Occup Med 19:802-806, 1977. 61. Meyer E: Physiologic appraisal of a new rescue respirator with positive pressure. Pneumonologic 153:61-72, 1975. 62. Love RG, Muir DCF, Swcetlnd KF, et al; Acceptable levels for the breathing resistance of respiratory apparatus: Results for men over the age of 45. Br j Ind Med 34:126-129, 1977. 63. Cumming EG, Blevins WV, Craig FN: Measurement of ex ternal deadspace with a new flowmeter. / Appl Physiol 15:741 -742, 1960. 64. White T, Schutz A, Karl-Magnus L: Effects of a welding helmet and dust respirators on respiration at rest and during exer cise. Scand l Work Environ Health 1:249-253,1975. 65. Anthonisen NR: Some steady state effects of respiratory ioads. Chest 1 (suppl):168, 1976. 66. Ballantyne B, Schnabe PM (Eds.): Respiratory Protection: Principles and Applications. London: Chapman & Hall Ltd., 1981. 67. Birkner LR: An industry viewpoint on respirator research needs. Am Ind Hyg Assoc / 41:A30, A32,A34, 1980. Editorial Comment The evaluation of a worker's ability to wear a respirator is an obviously important responsibility of the occupational physician, and certification that the worker can indeed wear a respirator is an equally important legal responsi bility. As Harber has described, the physiologic adaptation by a normal individual to the work load imposed by the respirator is a complicated response. The adaptation re quired changes as the work load increases. The physician who is asked to evaluate the worker with known heart or lung disease and to determine whether this worker can wear a respirator is faced with a problem for which there is little guidance. Dr. Harber has provided a scheme that considers the hazard of the job, the energy demands of the job, the duration of the task, and the health status of the worker. Each of these factors must be considered individually. The medical history of the worker can be obtained and physical examination, chest roentgenogram, pulmonary function tests, spirometry, and an electrocardiogram can be performed, Whether all of these tests are normal or not, a history of disease makes a decision difficult. It has been well established that a worker's ability'to do a physical job is best determined by observing him doing the job. An identical approach must be used in evaluating ability to wear a respirator. An exercise test should be required, with the energy expenditure (work output) equal to or greater than that of the job assignment. The worker should be ob served carefully for dyspnea or other signs of distress, and his pulse rate, respiratory rate, and blood pressure should be noted and recorded. If any abnormalities are detected by the test, the physician can determine whether further study of the worker is indicated or whether he can be permitted to do the job, possibly with some continued observation. Exercise testing is probably not being done as it should be because it takes time, and because physicians have not had an opportunity to observe the value of such testing. Probably more important, workers, even with cardiopul monary disease, can do the job because the work load of the job while wearing the respirator is low. Nevertheless, it should be remembered that the asthmatic may appear quite normal by examination and spirogram, but will improve in ventilatory capacity when given a bronchodilator. As Robin has stated in the Scientific American Medicine, exercise testing provides valuable information on cardiopulmonary performance, and I add that it cannot be obtained in any other manner. George Roush, jr., M.D. Life Force at the Edge The most important business of love and friendship is to hold us together as we live on the shifting edge of our differences. Friendship and love are the life force pitted directly against separation and death at the very core of true intimacy. The trembling edge of our separateness -- of our potential estrangement -- is the site at which friendship reveals its true power and does its work. Friendship draws us out of ourselves, literally forcing us to draw on faith and hope as we close the fault lines that death would claw wide open in our lelationships every day. By being friends we not only hold fast to each other, but we strengthen the forces of life, we overcome death on the battleground of everyday my stery. We are not strangers to death if we are friends with life. -- Eugene Kennedy in "On Being a Friend," published by The Continuum Publishing Company, New York, 1982. SWRf/Asbestos 6813 1B 030522 602 Respirator Use/Harfaer