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Hazardous Sound Levels Produced by Extracorporeal Shock Wave Lithotripsy
0022-5347/87 /1376-1113$02.00/0 Vol. 137, June
THE JOURNAL OF UROLOGY
Printed in U.S.A.
Copyright© 1987 by The Williams & Wilkins Co.
HAZARDOUS SOUND LEVELS PRODUCED BY EXTRACORPOREAL SHOCK WAVE LITHOTRIPSY RODNEY P. LUSK* AND RICHARD S. TYLER From the Department of Otolaryngology-Head and Neck Surgery, University of Iowa Hospitals and Clinics, Iowa City, Iowa
ABSTRACT
Sound emitted from the Dornier system GmbH lithotriptor was found to be of sufficient intensity to warrant concern about noise-induced sensorineural hearing loss. The patients were exposed to impulses of 112 dB. peak sound pressure level. Operating room personnel were exposed to sounds of less intensity, although the number of impulses they were exposed to was much greater, thereby increasing the risk of hearing loss. Hearing protection is recommended for patients and operating room personnel. Extracorporeally generated shock waves recently have been used to fragment stones in the upper urinary tract. 1- 3 The shock wave is generated by vaporizing water with a high voltage electrical discharge. Manipulation of the patient over an ellipsoid reflector focuses the shock wave into a designated area and depth within the patient. The ambient sound produced by the shock wave is noticeably intense. Therefore, it is important to determine if these sounds are potentially loud enough to cause sensorineural hearing loss in the patient or operating room personnel. METHODS
The sound emitted from the Dornier system GmbH lithotriptor was measured in the clinical environment. The lithotriptor is located within a hard-walled room with dimensions of 21 feet 8 inches x 20 feet 7½ inches with an 8 foot 5 inch ceiling. The walls are prepared specially to shield the x-rays and to prevent sound emission from the room. However, the walls are not designed to dampen noise within the room itself. A Quest 215 sound level meter with a peak-hold module PH35 was used to measure sound intensity. This linear impact meter was used to measure the true peak and conforms to the Occupational Safety and Health Administration (OSHA) impact limitation measurement guidelines of rise time less than 50 µsec. 4 Acoustic recordings were made at various locations within the room. The measurements were taken at the head of the patient, at the normal position of the anesthesiologist, 8 feet from the tank and in the corner of the room where the urologist operated the control panel. RESULTS
The waveform and spectrum of the shock wave are shown in figures 1 and 2. The waveform shows a rapid onset followed by an exponential decay that continues for about 180 msec. Smoorenburg suggested that the effective duration of impulse sound is measured best from the onset to the point at which the envelope has decayed by 10 dB. from the peak level. 5 The shock wave produced by the lithotriptor was 63 msec. in duration. The spectrum shows energy peaks around 250 and 3,000 Hz. Because the instrument is located in a sound-reflecting room the sound pressure levels varied in intensity throughout the room. The peak sound pressure level at various locations ranged from 103 to 112 dB. The most intense measurements (112 dB. sound pressure level) were located at the head of the patient and the anesthesiologist was exposed to intensities of 108 dB. Accepted for publication January 9, 1987. * Requests for reprints: Department of Pediatric Otolaryngology, St. Louis Children's Hospital, Washington University School of Medicine, St. Louis, Missouri 63110.
sound pressure level. The least intense sound was at 103 dB. peak sound pressure level, which was located at an 8-foot radius from the tank. The control panel is located in the corner and the sound intensity was 110 dB. peak sound pressure level. This increased intensity probably is secondary to reflected sound. It appears that most facilities are equipped similar to our hospital. Chaussy and associates described a room that apparently was not treated in any way to dampen sound reflection.' A review of the literature did not reveal instruments placed in a specially prepared sound dampening room. Measurements were made on 2 consecutive days and the sound intensity was averaged. The number of patients and number of impulses per patient were recorded and averaged during a 4-week period. These data were used to derive the average number of impulses per patient and the total number of impulses per day. The average number of shocks per patient was 1,585 and the average number of daily shocks delivered to operating room personnel was 8,721. DISCUSSION
Audiometric reference levels for sound intensity can be measured as sound pressure levels. In general, sound intensity is measured in decibels. Intensity is a physical attribute of sound, which can be measured with appropriate electronic equipment. The psychological correlate of intensity is loudness, although they are not related on a 1-to- l basis. The ear actually detects loudness by comparing the ratios of pressure rather than the actual differences. A logarithmic system using decibels has been adopted by acoustic scientists and engineers to measure sound intensity. Noise intensity is measured in decibels of sound pressure level. Noise exposure can be constant, or in the form of an impulse or impact noise. The ear has a protective mechanism to protect it from constant loud noises. When a sound is greater than 85 dB. hearing loss an acoustic reflex is elicited. The acoustic reflex arc is a protective feedback loop that starts in the cochlea, goes through the spinal cord and back out to the stapedius muscle. Contraction of the stapedius muscle impedes mobility of the ossiculum chain and, therefore, dampens the sound transmitted to the cochlea. This protective mechanism is not as effective for impulse noise because the reflex is initiated after exposure to the sound. The acoustic reflex impedes constant sound effectively but impulse sound elicits a reflex after each impulse and, therefore, it is less protective. The hazardous effects of impulse noise on hearing depends on the number of exposures, sound intensity and time between impulses. Safe limits for continuous and intermittent noise have been discussed for many years but controversy remains regarding safe levels of impulse noise. 5- 7 There are no current
1113
1114
LUSK AND TYLER
20msec
FIG. 1. Waveform of shock wave
10k
20k
LOG FREQUENCY (Hz)
FIG. 2. Spectrum measured with Bruel and Kjaer spectrum analyzer (model 2033) with flat weighing.
dB. at the level of equivalent energy in a continuous 8-hour period for patients and personnel. The OSHA standard 1910.95 on occupational noise exposure states only that "exposure to impulsive or impact noise should not exceed 140 dB peak SPL". 4 The Committee on Hearing, Bioacoustics and Biomechanics proposed limited impulse guidelines based on limited data obtained from subjects exposed to gunfire. 8 The noise produced by the lithotriptor does not exceed the OSHA limits, although it does exceed more conservative guidelines based on data produced in Holland5 and West Germany. 7 Therefore, noise protection is recommended. The operating room personnel are at high risk for hearing loss because they are exposed to the noise on a daily basis. It is obvious that some machines may be busier than others and the exposure to the operating room personnel will differ. An additional concern is related to the sound intensity being delivered to the patient. This sound intensity at the head of the patient was measured at 112 dB. peak sound pressure level for airborne sound. However, the shock wave is propagated through water and it vibrates the entire body directly, possibly resulting in further stimulation of the cochlea by bone conduction. The equivalent sound pressure level produced by this bone conduction stimulation is not known but it may be more intense than that conveyed by airborne sound. It is conceivable that an earplug placed in the ear canal of the patient could prevent the dissipation of the sound energy produced by bone conduction and actually increase the potential noise damage. For this reason earmuffs would appear to be the best ear protectors for the patient. Sound impulse produced by the lithotriptor is intense enough to result potentially in noise-induced sensorineural hearing loss. In certain environments, particularly small rooms, the noise levels may be higher than those measured at our hospital. Therefore, we recommend that patients and operating room personnel use hearing protection. Unprotected operating room personnel are at particular risk if exposed for prolonged periods. Further work is needed to establish the intensity of sound delivered to the patient through bone conduction. REFERENCES
American or international standards for the safe limits of impulse noise. When the ear is exposed to loud impulse noises the hearing threshold becomes poorer because of auditory fatigue. The ear usually recovers in 1 hour to 2 weeks, depending on the intensity of the noise exposure. This phenomenon is known as a temporary threshold shift. In a recent discussion of the topic Smoorenburg suggested that a relationship existed among the temporary threshold shift, peak noise level and total noise duration. 5 The effective total noise duration of impulse with the lithotriptor is 0.063 seconds multiplied by the number of impulses. In our facility the lithotriptor presented an impulse at 1second intervals. During the 4-week study period an average of 1,585 shocks were delivered per patient. The operating room personnel were exposed to an average of 8,721 shocks per day. The patients were exposed to peak impulses of 112 dB. peak sound pressure level for a total of 99.9 seconds (0.063 second times an average of 1,585 shocks). The operating room personnel were exposed to 110 dB. peak sound pressure level impulses for a total of 549.4 seconds (0.063 second times 8,721 shocks per day). This amount of exposure results in values close to 85
1. Chaussy, C. and Schmiedt, E.: Shock wave treatment for stones in
the upper urinary tract. Urol. Clin. N. Amer., 10: 743, 1983. 2. Chaussy, C., Brendel, W. and Schmiedt, E.: Extracorporeally induced destruction of kidney stones by shock waves. Lancet, 2: 1265, 1980. 3. Chaussy, C., Schmiedt, E., Jocham, D., Brendel, W., Forssmann, B. and Walther, V.: First clinical experience with extracorporcally induced-destruction of kidney stones- by shock waves. J. Urol., 127: 417, 1982. 4. Occupational Safety and Health Standards for General Industry (29 CFR Part 1910). Chicago: Commerce Clearing House of Chicago, 1984. 5. Smoorenburg, C. F.: Damage risk criteria for impulse noise. In: New Perspectives on Noise-Induced Hearing Loss. Edited by R. P. Hamernik, D. Henderson and R. Salve. New York: Raven Press, 1982. 6. Kryter, K. D.: Impairment to hearing from exposure to noise. J. Acoust. Soc. Amer., 53: 1211, 1973. 7. Pfander, F.: Damage risk criteria with and without ear protection for impulse noise with high intensities regarding ear, larynx and lungs. Scand. Audio!., suppl. 16, pp. 41-48, 1982. 8. Guidelines for Preparing Environmental Impact Statements on Noise, Report of Working Group 69. Committee on Hearing, Bioacoustics and Biomechanics. Contract #N00014-75-C0406, National Research Council/1977. Washington, D. C.
THE JOURNAL OF UROLOGY
Printed in U.S.A.
Copyright© 1987 by The Williams & Wilkins Co.
HAZARDOUS SOUND LEVELS PRODUCED BY EXTRACORPOREAL SHOCK WAVE LITHOTRIPSY RODNEY P. LUSK* AND RICHARD S. TYLER From the Department of Otolaryngology-Head and Neck Surgery, University of Iowa Hospitals and Clinics, Iowa City, Iowa
ABSTRACT
Sound emitted from the Dornier system GmbH lithotriptor was found to be of sufficient intensity to warrant concern about noise-induced sensorineural hearing loss. The patients were exposed to impulses of 112 dB. peak sound pressure level. Operating room personnel were exposed to sounds of less intensity, although the number of impulses they were exposed to was much greater, thereby increasing the risk of hearing loss. Hearing protection is recommended for patients and operating room personnel. Extracorporeally generated shock waves recently have been used to fragment stones in the upper urinary tract. 1- 3 The shock wave is generated by vaporizing water with a high voltage electrical discharge. Manipulation of the patient over an ellipsoid reflector focuses the shock wave into a designated area and depth within the patient. The ambient sound produced by the shock wave is noticeably intense. Therefore, it is important to determine if these sounds are potentially loud enough to cause sensorineural hearing loss in the patient or operating room personnel. METHODS
The sound emitted from the Dornier system GmbH lithotriptor was measured in the clinical environment. The lithotriptor is located within a hard-walled room with dimensions of 21 feet 8 inches x 20 feet 7½ inches with an 8 foot 5 inch ceiling. The walls are prepared specially to shield the x-rays and to prevent sound emission from the room. However, the walls are not designed to dampen noise within the room itself. A Quest 215 sound level meter with a peak-hold module PH35 was used to measure sound intensity. This linear impact meter was used to measure the true peak and conforms to the Occupational Safety and Health Administration (OSHA) impact limitation measurement guidelines of rise time less than 50 µsec. 4 Acoustic recordings were made at various locations within the room. The measurements were taken at the head of the patient, at the normal position of the anesthesiologist, 8 feet from the tank and in the corner of the room where the urologist operated the control panel. RESULTS
The waveform and spectrum of the shock wave are shown in figures 1 and 2. The waveform shows a rapid onset followed by an exponential decay that continues for about 180 msec. Smoorenburg suggested that the effective duration of impulse sound is measured best from the onset to the point at which the envelope has decayed by 10 dB. from the peak level. 5 The shock wave produced by the lithotriptor was 63 msec. in duration. The spectrum shows energy peaks around 250 and 3,000 Hz. Because the instrument is located in a sound-reflecting room the sound pressure levels varied in intensity throughout the room. The peak sound pressure level at various locations ranged from 103 to 112 dB. The most intense measurements (112 dB. sound pressure level) were located at the head of the patient and the anesthesiologist was exposed to intensities of 108 dB. Accepted for publication January 9, 1987. * Requests for reprints: Department of Pediatric Otolaryngology, St. Louis Children's Hospital, Washington University School of Medicine, St. Louis, Missouri 63110.
sound pressure level. The least intense sound was at 103 dB. peak sound pressure level, which was located at an 8-foot radius from the tank. The control panel is located in the corner and the sound intensity was 110 dB. peak sound pressure level. This increased intensity probably is secondary to reflected sound. It appears that most facilities are equipped similar to our hospital. Chaussy and associates described a room that apparently was not treated in any way to dampen sound reflection.' A review of the literature did not reveal instruments placed in a specially prepared sound dampening room. Measurements were made on 2 consecutive days and the sound intensity was averaged. The number of patients and number of impulses per patient were recorded and averaged during a 4-week period. These data were used to derive the average number of impulses per patient and the total number of impulses per day. The average number of shocks per patient was 1,585 and the average number of daily shocks delivered to operating room personnel was 8,721. DISCUSSION
Audiometric reference levels for sound intensity can be measured as sound pressure levels. In general, sound intensity is measured in decibels. Intensity is a physical attribute of sound, which can be measured with appropriate electronic equipment. The psychological correlate of intensity is loudness, although they are not related on a 1-to- l basis. The ear actually detects loudness by comparing the ratios of pressure rather than the actual differences. A logarithmic system using decibels has been adopted by acoustic scientists and engineers to measure sound intensity. Noise intensity is measured in decibels of sound pressure level. Noise exposure can be constant, or in the form of an impulse or impact noise. The ear has a protective mechanism to protect it from constant loud noises. When a sound is greater than 85 dB. hearing loss an acoustic reflex is elicited. The acoustic reflex arc is a protective feedback loop that starts in the cochlea, goes through the spinal cord and back out to the stapedius muscle. Contraction of the stapedius muscle impedes mobility of the ossiculum chain and, therefore, dampens the sound transmitted to the cochlea. This protective mechanism is not as effective for impulse noise because the reflex is initiated after exposure to the sound. The acoustic reflex impedes constant sound effectively but impulse sound elicits a reflex after each impulse and, therefore, it is less protective. The hazardous effects of impulse noise on hearing depends on the number of exposures, sound intensity and time between impulses. Safe limits for continuous and intermittent noise have been discussed for many years but controversy remains regarding safe levels of impulse noise. 5- 7 There are no current
1113
1114
LUSK AND TYLER
20msec
FIG. 1. Waveform of shock wave
10k
20k
LOG FREQUENCY (Hz)
FIG. 2. Spectrum measured with Bruel and Kjaer spectrum analyzer (model 2033) with flat weighing.
dB. at the level of equivalent energy in a continuous 8-hour period for patients and personnel. The OSHA standard 1910.95 on occupational noise exposure states only that "exposure to impulsive or impact noise should not exceed 140 dB peak SPL". 4 The Committee on Hearing, Bioacoustics and Biomechanics proposed limited impulse guidelines based on limited data obtained from subjects exposed to gunfire. 8 The noise produced by the lithotriptor does not exceed the OSHA limits, although it does exceed more conservative guidelines based on data produced in Holland5 and West Germany. 7 Therefore, noise protection is recommended. The operating room personnel are at high risk for hearing loss because they are exposed to the noise on a daily basis. It is obvious that some machines may be busier than others and the exposure to the operating room personnel will differ. An additional concern is related to the sound intensity being delivered to the patient. This sound intensity at the head of the patient was measured at 112 dB. peak sound pressure level for airborne sound. However, the shock wave is propagated through water and it vibrates the entire body directly, possibly resulting in further stimulation of the cochlea by bone conduction. The equivalent sound pressure level produced by this bone conduction stimulation is not known but it may be more intense than that conveyed by airborne sound. It is conceivable that an earplug placed in the ear canal of the patient could prevent the dissipation of the sound energy produced by bone conduction and actually increase the potential noise damage. For this reason earmuffs would appear to be the best ear protectors for the patient. Sound impulse produced by the lithotriptor is intense enough to result potentially in noise-induced sensorineural hearing loss. In certain environments, particularly small rooms, the noise levels may be higher than those measured at our hospital. Therefore, we recommend that patients and operating room personnel use hearing protection. Unprotected operating room personnel are at particular risk if exposed for prolonged periods. Further work is needed to establish the intensity of sound delivered to the patient through bone conduction. REFERENCES
American or international standards for the safe limits of impulse noise. When the ear is exposed to loud impulse noises the hearing threshold becomes poorer because of auditory fatigue. The ear usually recovers in 1 hour to 2 weeks, depending on the intensity of the noise exposure. This phenomenon is known as a temporary threshold shift. In a recent discussion of the topic Smoorenburg suggested that a relationship existed among the temporary threshold shift, peak noise level and total noise duration. 5 The effective total noise duration of impulse with the lithotriptor is 0.063 seconds multiplied by the number of impulses. In our facility the lithotriptor presented an impulse at 1second intervals. During the 4-week study period an average of 1,585 shocks were delivered per patient. The operating room personnel were exposed to an average of 8,721 shocks per day. The patients were exposed to peak impulses of 112 dB. peak sound pressure level for a total of 99.9 seconds (0.063 second times an average of 1,585 shocks). The operating room personnel were exposed to 110 dB. peak sound pressure level impulses for a total of 549.4 seconds (0.063 second times 8,721 shocks per day). This amount of exposure results in values close to 85
1. Chaussy, C. and Schmiedt, E.: Shock wave treatment for stones in
the upper urinary tract. Urol. Clin. N. Amer., 10: 743, 1983. 2. Chaussy, C., Brendel, W. and Schmiedt, E.: Extracorporeally induced destruction of kidney stones by shock waves. Lancet, 2: 1265, 1980. 3. Chaussy, C., Schmiedt, E., Jocham, D., Brendel, W., Forssmann, B. and Walther, V.: First clinical experience with extracorporcally induced-destruction of kidney stones- by shock waves. J. Urol., 127: 417, 1982. 4. Occupational Safety and Health Standards for General Industry (29 CFR Part 1910). Chicago: Commerce Clearing House of Chicago, 1984. 5. Smoorenburg, C. F.: Damage risk criteria for impulse noise. In: New Perspectives on Noise-Induced Hearing Loss. Edited by R. P. Hamernik, D. Henderson and R. Salve. New York: Raven Press, 1982. 6. Kryter, K. D.: Impairment to hearing from exposure to noise. J. Acoust. Soc. Amer., 53: 1211, 1973. 7. Pfander, F.: Damage risk criteria with and without ear protection for impulse noise with high intensities regarding ear, larynx and lungs. Scand. Audio!., suppl. 16, pp. 41-48, 1982. 8. Guidelines for Preparing Environmental Impact Statements on Noise, Report of Working Group 69. Committee on Hearing, Bioacoustics and Biomechanics. Contract #N00014-75-C0406, National Research Council/1977. Washington, D. C.