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A clinical evaluation of anoperational postanesthesia care unit source control system
A Clinical Evaluation of an Operational Postanesthesia Care Unit Source Control System J. MICHAEL BADGWELL, MD Various air safety hazards in the PACU and a number o f attempts to cope with the hazards have been addressed (see J Peri Anesth NuTs 11:207, 1996). This article presents a clinical evaluation o f an operational source control system developed specifically for use in the PACU. The criterion for evaluation was the degree to which the source control system could reduce the concentration of waste anesthetic gases released into the environment by the patient. The N20 molecule is a thousand times smaller than droplet nuclei that carry infectious respiratory disease. Thus, containment of waste gases m a y also indicate containment o f pathogens. Twenty-two postsurgica] patients were studied. The control group was given routine care with supplemental oxygen by nasal prong. The experimental group was given supplemental oxygen and had exhalent scavenged via the source-control system. Waste gas concentrations were monitored, and a criterion was applied to the data to determine the effectiveness o f the source control group when compared to nasal prong group. The nasal prong group exceeded the compliance criterion 58% of the time. The source control group exceeded the compliance criterion at no time during the study. From these results, the source control system is effective at reducing concentrations of waste anesthetic gases allowed into the atmosphere of a room. Application o f the source control to the PACU environment could prove valuable in addressing air safety hazards.
9 1997 by American Society of PeriAnesthesia Nurses.
HE PACU is a high risk environment with regard to air safety issues for both patients and health care workers because of a unique combination of circumstances found only in the PACU. The PACU is a general ward with a high patient census and a rapid patient turnover. Patients in the PACU often have inadequate histories taken and diagnoses made at the time of treatment. Many patients have a suppressed immune state and are particularly vulnerable to infection. 1'2 Most patients undergo cough-inducing procedures that generate drops and droplet nu-
T
Journal of PeriAnesthesia Nursing, Vol 12, No 2 (April), 1997: pp 73-81
clei. Because of the open room and presence of ventilation systems designed to promote mixing of air, nuclei that are introduced into the PACU have the potential to be well distributed and reJ. Michael Badgwell, MD is a Professor of Anesthesiology and Pediatrics, Texas Tech University Health Sciences Center, Lubbock, TX. Address correspondence to J. Michael Badgwell, MD, Texas Tech University Health Sciences Center, 3601 4th St, Room 1C-282, Lubbock, TX 79430. 9 1997 by American Society of PeriAnesthesia Nurses. 1089-9472/97/1202-0002503.00/0
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main in the air indefinitely. 3 By necessity, caregivers work in close proximity to patients' faces during cough-inducing procedures and at the time when the patient is breathing off relatively high concentrations of waste anesthetic gases. 4'5 Because of the P A C U ' s nature as a general ward with high patient census and rapid patient turnover, patients are exposed to one another and caregivers are exposed to a high number of patients over the course of a workday. Cross-exposure of patients is of concern because patients often have inadequate histories taken at the time of their PACU stays. Some patients may have infectious respiratory or bloodborne diseases that have been undiagnosed. Consideration must be given to this point because other patients in the same ward may be immunocompromised and may mount an inadequate immune response to a challenging pathogen. Most patients undergo cough-inducing procedures that produce drops and droplet nuclei that have the potential to be vectors of infection. The combination of coughinducing procedures with the presence of patients having undiagnosed illnesses combine to heighten the risk of transmission of infectious respiratory diseases in the PACU. 6,v Recent patient endotracheal intubation and extubation often lead to the appearance of gross blood in the expectorated fluids from postsurgical patients and increases the likelihood that caregivers will be exposed to blood-contaminated fluids. 8 It is necessary for caregivers to be in close proximity to patients' faces to perform airway maintenance at the time that the patient is emerging from anesthesia. Most patients in the PACU exhale concentrations of waste anesthetic gases for several hours after anesthesia. 9 These concentrations of waste anesthetic gases can exceed the gas concentrations found in scavenged operating rooms ~~ and can be above National Institute for Occupational Safety and Health (NIOSH) recommended exposure limits] 3-~5 In summary, air safety risks in the PACU can be divided into the following three categories: exposure of patients and caregivers to infectious respiratory diseases, exposure of caregivers to bloodborne pathogens, and chronic exposure of caregivers to waste anesthetic gases. Nowhere else in the hospital does such a combination of conditions prevail. It is well established that infectious pathogens
J. M I C H A E L B A D G W E L L and waste anesthetic gases are released into the PACU environment via the exhaled breath of postsurgical patients. 6'8'16 The air safety risks present in the PACU mentioned previously were once present in the OR (particularly the high concentrations of waste anesthetic gases caused by the unscavenged administration of anesthesia). However, source control implementation in the OR protects OR caregivers from these air safety risks. The patient's airway is sealed with a cuffed tracheal tube, and the patient's exhalations are removed by the anesthesia machine's scavenging system. Thus, waste anesthetic gases and exhaled pathogens are significantly reduced in the OR environment thus lessening the potential for contamination. OR personnel are now protected from occupational exposure and the related adverse health consequences because of the implementation of source control as the standard of care. However, in the PACU there have been no previous attempts to provide source control of air safety risks. Although the American Thoracic Society, NIOSH, and the Center of Disease Control have urged the development of s o u r c e c o n t r o l , 17'18 no such technology has previously existed for use in the PACU. Present methods of addressing air safety hazards in the PACU are administrative controls, work practice specifications, engineering controls, and the use of personal protective equipment. All of these methods of providing air safety have limitations. A more effective solution needed to be explored.
in the PACU there have been no previous attempts to provide source control of air safety risks
PURPOSE
This study clinically evaluated the effectiveness of a newly developed source control system. The source control system studied was designed specifically for use in the PACU. The source control system is based on the sealed airway principle used by the scavenging system of the anesthesia machine used in the OR. Instead of the cuffed endotracheal tube used in the OR, an ex-
SOURCE CONTROL MAINTAINS PACU AIR SAFETY ternal airway seal in the form of a close-fitting facemask with special features that allow for airway maintenance was used. The mask was connected to a flow-controlling Airator (Apotheus Laboratories Lmtd, Lubbock, TX) that provides the patient oxygen and scavenges patient exhalations. (for a full description, see Appendix A.) The key criterion was how effectively the system controlled the amount of waste anesthetic gases that the patient introduced into the room environment after general anesthesia. The anesthetic gases measured were isoflurane and nitrous oxide. A nitrous oxide molecule is 0.148 nm in diameter, whereas the vectors for infectious respiratory disease-droplet nuclei range from 100 to 500 nm. If the system can contain gas molecules of less than 0.2 nm, then it can also contain droplet nuclei that are a thousand times larger. Therefore, the measure of effectiveness of preventing the escape of exhaled waste anesthetic gases after anesthesia is also a measure of how well the system can prevent the introduction of respiratory and bloodborne pathogens into the PACU environment via patient expectorations. METHODS
The study was conducted in a medium-sized hospital with 365 beds where 8,295 surgeries were performed per year. With the approval of the Institutional Review Board and informed consent, patients 18 to 70 years of age who underwent pelvic, abdominal, or orthopedic surgery were studied. Patients with facial wounds, facial deformities, or facial hair were excluded. To eliminate cross-contamination of waste gases from neighboring patients, all studies were conducted in an isolation room adjacent to the PACU. All patients included in the evaluation were cared for by the PACU staff. The PACU nurses participating in the study received 2.5 hours of training by research staff on the source control system before use. The attending nurses were not made aware of the effectiveness criteria nor were they able to influence in any way the measurements of waste anesthetic gas concentrations. Anesthesia was induced with thiopental sodium (1 to 4 mg/kg) and fentanyl (---<2#g/kg), and was maintained with isoflurane and nitrous oxide titrated to clinical endpoints as determined by the anesthesiologist who was unaware of the patient's group assignment9 End-tidal concentrations of
7s
isoflurane and nitrous oxide were measured by mass spectrophotometry and recorded during anesthesia. The patients' tracheas were extubated in the OR when patients could open their eyes on command and could perform a sustained head lift. Within 10 minutes of tracheal extubation, patients were transported to the isolation room. The patients were randomly assigned to one of two groups. The control group was given supplemental oxygen by nasal prong (nasal prong group). The experimental group was given oxygen and had exhaled gases scavenged by the source control system (source control group). A sampling site had to be selected and standardized to duplicate sampling conditions for all subjects. A location 53 cm from the patients' mouths was chosen for the following two reasons: (1) close proximity to the source of the measured waste anesthetic gases facilitates sensitive detection and (2) the NIOSH has stated that sampling should be representative of the breathing zone of workers as they go about their duties.14 Fifty-three cm is an approximation of the distance between the nurse and patient during bedside care. The distance was determined from observations of nurses caring for patients. 16 Concentration of waste anesthetic gases was monitored using a Miran IB2 Infrared Spectrophotometer (Foxboro, East Bridgewater, MA) for isoflurane and a DEO Nitrous Oxide Monitor (Dynatech Electro-Optics Corp, San Luis Obispo, CA) for nitrous oxide. Four hundred eighty measurements were acquired each minute and stored electronically for later analysis. For each case, the data were analyzed over the interval beginning 3 minutes after patient arrival in the isolation room and ending 2 minutes before departure from the isolation room. Oxyhe* The reason for beginning data analysis 3 minutes after patient arrival in the isolation room was to provide time for the placement of the source eontrol mask. As indicated earlier, the effectiveness measure--how well the source control system prevented emission of waste anesthetic gas into the isolation r o o m - - w a s also used as an indicator as to how well the source control system can control the release of drops and droplet nuclei. A key point was that the source control system cannot work until it is placed on the patient (the "source"). The current protocol did not allow r the placement of the mask on the patient before arrival in the isolation room; however, it is functionally possible to place the mask on the patient before the patient leaves the operating room. Thus, it is simply a matter of procedure to determine at what point the source control system will be placed into operation.
76 moglobin saturation was monitored by pulse oximetry. Waste anesthetic gas concentrations were measured and recorded; however, a standard of analyzation was needed to facilitate comparisons between the nasal prong group and the source control group. The NIOSH recommended exposure limits 15 were chosen, * with one adjustment imposed. The NIOSH recommended exposure limit for nitrous oxide is a time-weighted average of 25 ppm over the time of exposure. 17 (A time weighted average shows the average exposure over a given period of time.) We desired a more stringent criterion to determine efficacy of waste gas control. We therefore treated the 25 ppm nitrous oxide limit as a ceiling for purposes of comparison of the two groups. (To indicate the level of compliance of the nasal prong group to NIOSH recommended exposure limits, nitrous measurements were evaluated as a time-weighted average as well as on a minute-by-minute basis.) For isoflurane, the NIOSH recommended exposure limit of 2.0 ppm is was used as the standard of comparison. Concentrations greater than 2.0 ppm would be "out of compliance" with our imposed standard. A total of 22 patients were studied, with 10 enrolled in the nasal prong group, and 12 in the source control group.* Patients' morphometric characteristics were compared between the groups. For isoflurane and nitrous oxide, the duration of surgery and concentration of anesthetic ? With regard to waste anesthetic gases, the NIOSHprinciple is clear: NIOSH is unable to identify a safe level of exposure for waste anesthetic gases. Therefore, it recommends that the risk be minimized by reducing exposures to the greatest extent possible. 14 The recommended exposure limit Jbr halogenated anesthetic agents is a 2 ppm ceiling (rather than a time weighted average that gives an average level of exposure to waste anesthetic gas over the period of exposure). The recommended exposure limit f o r nitrous oxide is 25 ppm as a time weighted averageJ s NIOSH has set the recommended exposure limit at the concentration achievable through the use of current technology. r The study was initially designed to have 12 subjects' in the control group, but preliminary analyses with 10 control subjects showed that, even if the dependent variable were to be at its minimum possible value (0%) f o r two additional control subjects, the conclusions of the study would remain unchanged. The experiment was thus terminated with 10 subjects' in the control group.
J. MICHAEL B A D G W E L L agent administration were recorded and used to calculate MAC-hours (minimal alveolar concentration or volatile agent • number of hours). RESULTS
Patient morphometrics, duration of anesthesia, and MAC-hours for isoflurane and nitrous oxide were not statistically different between the two groups (Table 1) when tested by a two-tailed Student's t-test. The two groups were comparable with a 95% confidence interval (P < 0.05) when compared by multivariate testing (Appendix B). Nine of 10 patients in the nasal prong group exhaled concentrations above the NIOSH recommended exposure limit. None of the source control group patients exhaled concentrations above the NIOSH recommended exposure limit at any time. Patients in the nasal prong group exceeded the NIOSH limit 58% of the cumulative time of study. No patient in the source control group exceeded the limit at any time during the study. It should be kept in mind that this was not a study designed to measure waste anesthetic gas concentrations in the PACU, but rather to determine the efficacy of the source control system. Stating that waste anesthetic gas concentrations were above NIOSH recommended exposure limits is not an implication of excessive waste anesthetic gas concentrations in the PACU. However, it is a comparison showing the marked reduction in release of waste anesthetic gases to the room atmosphere by a recovering patient when the source control system was used. The release is also indicative of the potential release of bloodborne and respiratory pathogens by patients not wearing the source control system.
this was a study designed to measure the efficacy of the source control system The differences in waste gas concentrations between the two groups were statistically determined by using multivariate testing (Appendix B). For isoflurane, the data for the nasal prong group show a 99% confidence interval of 58.61 +_ 43.51. Because the lower boundary of the confidence interval does not include zero (58.61-
SOURCE CONTROL MAINTAINS PACU AIR SAFETY
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Table 1. Characteristics of the Patients in the Two Study Groups Characteristic
Nasal Prong (N - 10)
Source Control (N - 12)
P-value*
A g e (years)
38,4 _+ 13.1
38.4 _+ 13,0
.69
Weight (kilograms)
84.5 + 13.0
74,0 _+ 13.5
.08
134.2 -- 49.7
120,3 + 36.8
.46
Nitrous o x i d e MAC-hr (hr)
1.25 +_ 0.39
1,06 _+ 0,30
.21
Isoflurane Mac-hr {hr)
2.01 _+ 0.90
2.00 * 0.94
.98
Duration o f PACU stay (min)
67.5 _+ 17.9
68,3 -+ 19.9
.93
Duration o f Anesthesia (min)
Note: Data are means (+ standard deviation). * Significant difference if P -< .05,
43.51 > 0), concentrations of waste anesthetic gas detected from the source control group is considered statistically different (lower) than the nasal prong group (P < .01). In plain English, the two groups are different, and the source control group allowed significantly less (99% confidence level) waste isoflurane to be emitted into the room from the patient. For nitrous oxide concentrational comparison between the two groups, the differences between the two groups using the 25 ppm of nitrous oxide as a time weighted average was determined. Analysis of both isoflurane and nitrous oxide was also performed. Waste anesthetic gas levels were determined to be "out of compliance" if either isoflurane concentrations were greater than 2.0 ppm, or if nitrous oxide concentrations were greater than a 25 ppm time-weighted average. This comparison yielded a 99% confidence interval of 0.9 _+ .24. Once again, because the lower boundary of the confidence interval does not include zero (0.9-.24 > 0), concentrations of waste anesthetic gas emissions detected from the source control group are considered statistically different (lower) than those of the nasal prong group (99% confidence level). When considering the total time that the NIOSH recommended exposure limit is exceeded, a specific case with a time-weighted average greater than the NIOSH recommended exposure limit is considered out of compliance for its' entire duration. Using this criteria the total outof-compliance time for the nasal prong group increases from 58% to 87%, and that of the source control group still remains at zero. DISCUSSION Data from the present study affirm data from previous studies that indicate that patients recov-
ering from general anesthesia exhale high concentrations of waste anesthetic gases. 4's However, the question arises as to the significance of the fact that PACU nurses breathe in the exhaled waste anesthetic gases of their patients while providing bedside care. Most research dealing with waste anesthetic gases shows an association between exposure and a variety of adverse health consequences. The strongest and most consistent associations link chronic, occupational exposure to such gases with adverse reproductive outcomes (infertility, spontaneous abortions, and congenital anomalies). 19-21
most research dealing w i t h waste anesthetic gases shows an association b e t w e e n exposure and a variety of adverse health consequences The data presented in this study suggest that source control technology is effective in reducing air contamination hazards in the PACU. It is the only method presently available that simultaneously addresses all three air safety risks in the PACU environment. The source control system shows a high level of effectiveness in lowering the concentrations of waste anesthetic gases emitted by postoperative patients. Waste anesthetic gas containment is of greater importance when we consider that the degree of containment of waste anesthetic gases is the key marker for efficacy in the containment of airborne and bloodborne pathogens. The placement of the sample site at 53 cm from the patient's mouth
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J. M I C H A E L B A D G W E L L
provided a rigorous and reproducible test of the effectiveness of the source control system at reducing waste anesthetic gases emitted by the patient. The technology used made it possible to accurately measure and record waste gas concentrations continuously and in real time. Because concentrations change rapidly as the patient eliminates the anesthetic gases during recovery, intermittent sampling technologies (samples taken at various times in various locations) are less likely to provide an accurate assessment of the effectiveness of a source control system. Other factors may have influenced the exhaled waste anesthetic gases concentrations such as the patients' body composition, concentration of anesthetic used, and anesthetic depth on arrival to the isolation room. However, patient weight, duration and MAC-hours of anesthesia, and duration of recovery were similar between the two groups. Therefore, these factors are not likely to have confounded our results. Both groups were statistically similar in physical characteristics as well as concentrational duration of anesthesia. In conclusion, this study corroborates earlier findings showing that scavenging systems can effectively reduce concentrations of waste anesthetic gases in the room environment, t~ Application of the source control system in the PACU should result in a concurrent reduction of waste anesthetic gases, airborne disease, and bloodborne pathogens. APPENDIX A
The AirCare Source Control System by Apotheus Laboratories, Ltd
Description. The AirCare Source Control System is modeled after the anesthesia machine in the recovery mode that, by virtue of an inflatable tracheal cuff, seals a patient's airway, supplies oxygen, and scavenges patient exhalent through the hospital's vacuum system. The AirCare system seals the airway externally by means of a special mask with an adhesive rim. The mask possesses a closed-system suction port which allows for patient airway maintenance without disconnecting the system. The mask is connected to the patient's breathing circuit (the same one used by the patient in the OR), which is connected to the Airator. The Airator delivers hospi-
tal oxygen, which flows from the wall supply into an oxygen reservoir bag. When the patient inspires, oxygen flows across a primary inspiratory valve, down the inspiratory limb of the breathing circuit, and into the lungs. When the patient exhales, expired gases flow down the expiratory limb of the breathing circuit, across a primary expiratory valve, and into an exhalational reservoir bag. As on OR anesthesia machines, the exhaled gases in the reservoir bag are removed through a hose connected to the hospital's vacuum supply. The breathing circuit is not a circle system; that is, exhaled gases do not flow back through the inhalational side of the system (Fig 1). Both the oxygen and exhalational reservoirs are equipped with positive and negative pressurerelief valves. On the inhalational side of the system, if oxygen is not supplied to the system, the negative pressure-relief valve opens and the patient breathes room air. On the exhalational side of the system, if vacuum is not supplied to the system, the positive pressure-relief valve opens and the patient exhales into the room. Access to the patient's airway is obtained by means of a closed system suction port on the mask that can be opened without disconnecting the system. It accommodates a suction catheter while maintaining the integrity of the sealed system. Limitations--The source control system is not indicated for use with patients who have facial wounds, deformities, or excessive facial hair that could affect the integrity of the seal of the adhesive mask on the patient's face. It is intended as an adjunct to, and not as a replacement for, appropriate isolation of patients with known or suspected respiratory infections. Nor is it a replacement for proper design and maintenance of an HVAC (heating, ventilation, air conditioning) system in the PACU, or appropriate work practice and administrative controls. Factors that determine the effectiveness of the system include the length of time the patient is recovered using the system, the efficacy of the mask seal, the proper operation of the system, and the proper operation of the hospital's oxygen and vacuum systems. Availability and Cost--The AirCare system has recently been approved by the Food and Drug Administration. Price has not been determined. How-
SOURCE CONTROL MAINTAINS PACU AIR SAFETY
79 - HOSPITAL VACUUM
o27
INHALED 02~
\ EXHALED GASES
ADHESIVE SEAL
5
Fig 1. The AirCare Source Control System.
ever, it is expected that the per patient cost to the hospital would be comparable to the current cost of a single-use humidifier and vend m a s k - - a system commonly used by hospitals in the U.S. APPENDIX B
Statisical Analysis
Comparability of the Experimental and Control Groups--To ensure comparability of the control group to the source control group, the two groups were compared on the following six dimensions: age, weight, duration of anesthesia, nitrous oxide MAC-hours, isoflurane MAChours, and duration of PACU stay (Table 1). The General Linear Model (GLM) procedure in SAS provided a comparison of differences in patient morphometrics and anesthetic regimens between study groups. This procedure tests the hypothesis of no group effect, ie, that the cases are drawn from a single population. The P-values reported are the estimated probabilities of observing mean differences as large as those observed when the
null hypothesis (the group means are equal) is true. A P-value greater than .05 was considered an indication that the two groups were from the same population. This test was performed on each of the six variables separately, and as a multivariate (MANOVA) test on the six-element dependent vector variable simultaneously in order to control the risk of type-I error at the desired overall level. The model tested w a s : Yijk = #k + Oj -}- eijk, where Yijk is the score of subject (i) in group (j) on dependent variable (k), #k is the overall mean on variable (k), 0j is the effect of group (j), and eijk is the error term, assumed to be normally and independently distributed with constant variance. In the event that such assumptions might be violated, nonparametric analyses were conducted as well. The results of the Multivariate Analysis of Variance (MANOVA) are as follows: M A N O V A Test Criteria and Exact F Statistics for the Hypothesis of no Overall G R O U P Effect, H = Type III SS&CP Matrix for GROUP, E = Error SS& CP Matrix
Statistic
Value
F
Num DF
Den DF
Pr > F
Wilks' Lambda
0.68365421
1.1568
6
15
0.3787
J. M I C H A E L B A D G W E L L
8O
Although the difference in average weight between the groups approaches statistical significance when considered individually, we are able to conclude that, in an overall multivariate sense, there is no evidence to allow us to reject the hypothesis of no difference between the groups.
Statistical Significance of the Differences in the Results of the Experimental and Control Groups--The key criterion to judge the effectiveness of the source control system was the percentage of the time a patient was in the isolation room during which concentrations of waste anesthetic gases exceeded the criteria of 2 ppm and 25 p p m respectively for isoflurane and nitrous oxide, when measured at the 53 cm point for both the control group and the source control group. Since the source control group contained zero minutes in excess of the criteria and therefore had zero variability, our statistical test involves testing the hypothesis that the control group mean also equals zero, within sampling error. The procedure is to compute a 99% confidence interval around the control group mean, using the formula: M _+ t.99*cr/SQRT(N) where M equals mean of the control group; t.99 equals the critical value of the t-Distribution for a 99%
confidence interval with N-1 degrees of freedom; cr equals the estimated standard deviation of the control group, and N = sample size. However, as indicated above, measurements were compared to 25 p p m for nitrous oxide both as a ceiling and as a time weighted average. For the latter, the normal approximation to the binomial distribution needs to be calculated since the time weighted average will yield a proportion of cases that comply or do not comply with the time weighted average REL from NIOSH. Thus, the following formula was used to calculate the 99% confidence limits: P + Z99*SQRT[(P*(1-P))/N)] where P equals the proportion of cases out of compliance with the NIOSH REL, Z99 = the critical value of the normal approximation to the binomial distribution at the 99% confidence level with N-1 degrees of freedom (2.576), and N equals the sample size. ACKNOWLEDGMENT The author thanks the following from Apotheus Laboratories for furnishing the source control systems and measurement equipment necessary for the conduct of the product evaluation: Brett Moore, Stephanie Walton, and Scott Wehmeyer.
REFERENCES 1. Chandra RK: Nutrition, immunity, and infection: Present knowledge and future directions. Lancet 1:688-691, 1983 2. Moudgil GC: Immunosuppression in the postoperative period. SA Rice, KJ Fish (eds): Anesthetic Toxicity. New York, NY, Raven Press, 1994, pp 241-66 3. Austin PR, Austin PJ: Measurement of nitrous oxide in a simulated post anesthesia care unit environment. J Peri Anesth Nurs 11:259-268, 1996 4. Wehmeyer MS, Moore B, Walton S, et al: Exhaled isoflurane from PACU patients may produce ambient concentrations exceeding NIOSH standards one hour after surgery. Anesthesiology 87:A361, 1996 5. Moore B, Wehmeyer MS, Walton S, et al: Exhaled N20 from PACU patients may produce ambient concentrations exceeding NIOSH standards one hour after surgery. Anesthesiology 87:A427, 1996 6. Griffith DE, Kerr CM: Tuberculosis: Disease of the past, disease of the present. Peri Anesth Nurs 11:240-245, 1996 7. Centers for Disease Control: Guidelines for preventing the transmission of mycobacterium tuberculosis in health care facilities 1994, Morbidity and Mortality Weekly Report 43 (Supplement RR- 13, 1994) 8. Csete M: Special considerations for bloodborne pathogens in the PACU. Peri Anesth Nurs 11:223-230, 1996 9. Otomo N, Tsutomu I, Kitayama M, et al: Excessive nitrous oxide exhalation by postoperative patients in the recovery room. Masui 42:353-357, 1993
10. Nikki P, Pfaffli P, Ahlman K: End-tidal and blood halothane and nitrous oxide in surgical personnel. Lancet 2:490-491, 1972 11. Nikki P, Pfaffli P, Ahlman K, et al: Chronic exposure to anaesthetic gases in the operating theatre and recovery room. Ann Clin Res 4:266-272, 1972 12. Pfaffli P, Nikki P, Ahlman K: Halothane and nitrous oxide in end-tidal air and venous blood of surgical personnel. Ann Clin Res 4:273-277, 1972 13. Kant I, van Rijssen-Moll M, Borm P: Simulation of nitrous oxide concentrations in operating and recovery rooms. Ann Occup Hyg 34:575-583, 1990 14. Department of Health, Education, and Welfare, Public Health Service, Center for Disease Control, National Institute for Occupational Safety and Health: NIOSH A Recommended Standard for Occupational Exposure to Waste Anesthetic Gases and Vapors. Government Printing Office, Washington, DC, Pub No. 77-140, 1994 15. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health: NIOSH Pocket Guide to Chemical Hazards, DHHS (NIOSH) Publication No. 94-116, US Government Printing Office, Washington, DC, 1994 16. Allen A, Badgwell JM: The post anesthesia care unit: Unique contribution, unique risk. J Peri Anesth Nuts 11:248258, 1996 17. American Thoracic Society, Medical Section of the
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MAINTAINS
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AIR SAFETY
American Lung Association: Control of tuberculosis in the United States. Am Rev Respir Dis 146:1623-33, 1992 18. Bierbaum PJ, Lippmann M (eds): Proceedings of the Workshop on Engineering Controls for Preventing Airborne Infections in Workers in Health Care and Related Facilities. July 14-16, 1993, Cincinnati, Ohio. U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health. DHHS (NIOSH) Publication No. 94-106
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19. Rowland A, Baird D, Weinberg C, et al: Reduced fertility among women employed as dental assistants exposed to high levels of nitrous oxide. N Engl J Med 327:993-997, 1992 20. Rowland A, Baird D, Shore D, et al: Nitrous oxide and spontaneous abortion in female dental assistants. Am J Epidemiol, 141:531-538, 1995 21. Guirguis S, Pelmear P, Wong R: Health effects associated with exposure to anaesthetic gases in Ontario hospital personnel. Br J Ind Med, 47:490-497, 1990
9 1997 by American Society of PeriAnesthesia Nurses.
HE PACU is a high risk environment with regard to air safety issues for both patients and health care workers because of a unique combination of circumstances found only in the PACU. The PACU is a general ward with a high patient census and a rapid patient turnover. Patients in the PACU often have inadequate histories taken and diagnoses made at the time of treatment. Many patients have a suppressed immune state and are particularly vulnerable to infection. 1'2 Most patients undergo cough-inducing procedures that generate drops and droplet nu-
T
Journal of PeriAnesthesia Nursing, Vol 12, No 2 (April), 1997: pp 73-81
clei. Because of the open room and presence of ventilation systems designed to promote mixing of air, nuclei that are introduced into the PACU have the potential to be well distributed and reJ. Michael Badgwell, MD is a Professor of Anesthesiology and Pediatrics, Texas Tech University Health Sciences Center, Lubbock, TX. Address correspondence to J. Michael Badgwell, MD, Texas Tech University Health Sciences Center, 3601 4th St, Room 1C-282, Lubbock, TX 79430. 9 1997 by American Society of PeriAnesthesia Nurses. 1089-9472/97/1202-0002503.00/0
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74
main in the air indefinitely. 3 By necessity, caregivers work in close proximity to patients' faces during cough-inducing procedures and at the time when the patient is breathing off relatively high concentrations of waste anesthetic gases. 4'5 Because of the P A C U ' s nature as a general ward with high patient census and rapid patient turnover, patients are exposed to one another and caregivers are exposed to a high number of patients over the course of a workday. Cross-exposure of patients is of concern because patients often have inadequate histories taken at the time of their PACU stays. Some patients may have infectious respiratory or bloodborne diseases that have been undiagnosed. Consideration must be given to this point because other patients in the same ward may be immunocompromised and may mount an inadequate immune response to a challenging pathogen. Most patients undergo cough-inducing procedures that produce drops and droplet nuclei that have the potential to be vectors of infection. The combination of coughinducing procedures with the presence of patients having undiagnosed illnesses combine to heighten the risk of transmission of infectious respiratory diseases in the PACU. 6,v Recent patient endotracheal intubation and extubation often lead to the appearance of gross blood in the expectorated fluids from postsurgical patients and increases the likelihood that caregivers will be exposed to blood-contaminated fluids. 8 It is necessary for caregivers to be in close proximity to patients' faces to perform airway maintenance at the time that the patient is emerging from anesthesia. Most patients in the PACU exhale concentrations of waste anesthetic gases for several hours after anesthesia. 9 These concentrations of waste anesthetic gases can exceed the gas concentrations found in scavenged operating rooms ~~ and can be above National Institute for Occupational Safety and Health (NIOSH) recommended exposure limits] 3-~5 In summary, air safety risks in the PACU can be divided into the following three categories: exposure of patients and caregivers to infectious respiratory diseases, exposure of caregivers to bloodborne pathogens, and chronic exposure of caregivers to waste anesthetic gases. Nowhere else in the hospital does such a combination of conditions prevail. It is well established that infectious pathogens
J. M I C H A E L B A D G W E L L and waste anesthetic gases are released into the PACU environment via the exhaled breath of postsurgical patients. 6'8'16 The air safety risks present in the PACU mentioned previously were once present in the OR (particularly the high concentrations of waste anesthetic gases caused by the unscavenged administration of anesthesia). However, source control implementation in the OR protects OR caregivers from these air safety risks. The patient's airway is sealed with a cuffed tracheal tube, and the patient's exhalations are removed by the anesthesia machine's scavenging system. Thus, waste anesthetic gases and exhaled pathogens are significantly reduced in the OR environment thus lessening the potential for contamination. OR personnel are now protected from occupational exposure and the related adverse health consequences because of the implementation of source control as the standard of care. However, in the PACU there have been no previous attempts to provide source control of air safety risks. Although the American Thoracic Society, NIOSH, and the Center of Disease Control have urged the development of s o u r c e c o n t r o l , 17'18 no such technology has previously existed for use in the PACU. Present methods of addressing air safety hazards in the PACU are administrative controls, work practice specifications, engineering controls, and the use of personal protective equipment. All of these methods of providing air safety have limitations. A more effective solution needed to be explored.
in the PACU there have been no previous attempts to provide source control of air safety risks
PURPOSE
This study clinically evaluated the effectiveness of a newly developed source control system. The source control system studied was designed specifically for use in the PACU. The source control system is based on the sealed airway principle used by the scavenging system of the anesthesia machine used in the OR. Instead of the cuffed endotracheal tube used in the OR, an ex-
SOURCE CONTROL MAINTAINS PACU AIR SAFETY ternal airway seal in the form of a close-fitting facemask with special features that allow for airway maintenance was used. The mask was connected to a flow-controlling Airator (Apotheus Laboratories Lmtd, Lubbock, TX) that provides the patient oxygen and scavenges patient exhalations. (for a full description, see Appendix A.) The key criterion was how effectively the system controlled the amount of waste anesthetic gases that the patient introduced into the room environment after general anesthesia. The anesthetic gases measured were isoflurane and nitrous oxide. A nitrous oxide molecule is 0.148 nm in diameter, whereas the vectors for infectious respiratory disease-droplet nuclei range from 100 to 500 nm. If the system can contain gas molecules of less than 0.2 nm, then it can also contain droplet nuclei that are a thousand times larger. Therefore, the measure of effectiveness of preventing the escape of exhaled waste anesthetic gases after anesthesia is also a measure of how well the system can prevent the introduction of respiratory and bloodborne pathogens into the PACU environment via patient expectorations. METHODS
The study was conducted in a medium-sized hospital with 365 beds where 8,295 surgeries were performed per year. With the approval of the Institutional Review Board and informed consent, patients 18 to 70 years of age who underwent pelvic, abdominal, or orthopedic surgery were studied. Patients with facial wounds, facial deformities, or facial hair were excluded. To eliminate cross-contamination of waste gases from neighboring patients, all studies were conducted in an isolation room adjacent to the PACU. All patients included in the evaluation were cared for by the PACU staff. The PACU nurses participating in the study received 2.5 hours of training by research staff on the source control system before use. The attending nurses were not made aware of the effectiveness criteria nor were they able to influence in any way the measurements of waste anesthetic gas concentrations. Anesthesia was induced with thiopental sodium (1 to 4 mg/kg) and fentanyl (---<2#g/kg), and was maintained with isoflurane and nitrous oxide titrated to clinical endpoints as determined by the anesthesiologist who was unaware of the patient's group assignment9 End-tidal concentrations of
7s
isoflurane and nitrous oxide were measured by mass spectrophotometry and recorded during anesthesia. The patients' tracheas were extubated in the OR when patients could open their eyes on command and could perform a sustained head lift. Within 10 minutes of tracheal extubation, patients were transported to the isolation room. The patients were randomly assigned to one of two groups. The control group was given supplemental oxygen by nasal prong (nasal prong group). The experimental group was given oxygen and had exhaled gases scavenged by the source control system (source control group). A sampling site had to be selected and standardized to duplicate sampling conditions for all subjects. A location 53 cm from the patients' mouths was chosen for the following two reasons: (1) close proximity to the source of the measured waste anesthetic gases facilitates sensitive detection and (2) the NIOSH has stated that sampling should be representative of the breathing zone of workers as they go about their duties.14 Fifty-three cm is an approximation of the distance between the nurse and patient during bedside care. The distance was determined from observations of nurses caring for patients. 16 Concentration of waste anesthetic gases was monitored using a Miran IB2 Infrared Spectrophotometer (Foxboro, East Bridgewater, MA) for isoflurane and a DEO Nitrous Oxide Monitor (Dynatech Electro-Optics Corp, San Luis Obispo, CA) for nitrous oxide. Four hundred eighty measurements were acquired each minute and stored electronically for later analysis. For each case, the data were analyzed over the interval beginning 3 minutes after patient arrival in the isolation room and ending 2 minutes before departure from the isolation room. Oxyhe* The reason for beginning data analysis 3 minutes after patient arrival in the isolation room was to provide time for the placement of the source eontrol mask. As indicated earlier, the effectiveness measure--how well the source control system prevented emission of waste anesthetic gas into the isolation r o o m - - w a s also used as an indicator as to how well the source control system can control the release of drops and droplet nuclei. A key point was that the source control system cannot work until it is placed on the patient (the "source"). The current protocol did not allow r the placement of the mask on the patient before arrival in the isolation room; however, it is functionally possible to place the mask on the patient before the patient leaves the operating room. Thus, it is simply a matter of procedure to determine at what point the source control system will be placed into operation.
76 moglobin saturation was monitored by pulse oximetry. Waste anesthetic gas concentrations were measured and recorded; however, a standard of analyzation was needed to facilitate comparisons between the nasal prong group and the source control group. The NIOSH recommended exposure limits 15 were chosen, * with one adjustment imposed. The NIOSH recommended exposure limit for nitrous oxide is a time-weighted average of 25 ppm over the time of exposure. 17 (A time weighted average shows the average exposure over a given period of time.) We desired a more stringent criterion to determine efficacy of waste gas control. We therefore treated the 25 ppm nitrous oxide limit as a ceiling for purposes of comparison of the two groups. (To indicate the level of compliance of the nasal prong group to NIOSH recommended exposure limits, nitrous measurements were evaluated as a time-weighted average as well as on a minute-by-minute basis.) For isoflurane, the NIOSH recommended exposure limit of 2.0 ppm is was used as the standard of comparison. Concentrations greater than 2.0 ppm would be "out of compliance" with our imposed standard. A total of 22 patients were studied, with 10 enrolled in the nasal prong group, and 12 in the source control group.* Patients' morphometric characteristics were compared between the groups. For isoflurane and nitrous oxide, the duration of surgery and concentration of anesthetic ? With regard to waste anesthetic gases, the NIOSHprinciple is clear: NIOSH is unable to identify a safe level of exposure for waste anesthetic gases. Therefore, it recommends that the risk be minimized by reducing exposures to the greatest extent possible. 14 The recommended exposure limit Jbr halogenated anesthetic agents is a 2 ppm ceiling (rather than a time weighted average that gives an average level of exposure to waste anesthetic gas over the period of exposure). The recommended exposure limit f o r nitrous oxide is 25 ppm as a time weighted averageJ s NIOSH has set the recommended exposure limit at the concentration achievable through the use of current technology. r The study was initially designed to have 12 subjects' in the control group, but preliminary analyses with 10 control subjects showed that, even if the dependent variable were to be at its minimum possible value (0%) f o r two additional control subjects, the conclusions of the study would remain unchanged. The experiment was thus terminated with 10 subjects' in the control group.
J. MICHAEL B A D G W E L L agent administration were recorded and used to calculate MAC-hours (minimal alveolar concentration or volatile agent • number of hours). RESULTS
Patient morphometrics, duration of anesthesia, and MAC-hours for isoflurane and nitrous oxide were not statistically different between the two groups (Table 1) when tested by a two-tailed Student's t-test. The two groups were comparable with a 95% confidence interval (P < 0.05) when compared by multivariate testing (Appendix B). Nine of 10 patients in the nasal prong group exhaled concentrations above the NIOSH recommended exposure limit. None of the source control group patients exhaled concentrations above the NIOSH recommended exposure limit at any time. Patients in the nasal prong group exceeded the NIOSH limit 58% of the cumulative time of study. No patient in the source control group exceeded the limit at any time during the study. It should be kept in mind that this was not a study designed to measure waste anesthetic gas concentrations in the PACU, but rather to determine the efficacy of the source control system. Stating that waste anesthetic gas concentrations were above NIOSH recommended exposure limits is not an implication of excessive waste anesthetic gas concentrations in the PACU. However, it is a comparison showing the marked reduction in release of waste anesthetic gases to the room atmosphere by a recovering patient when the source control system was used. The release is also indicative of the potential release of bloodborne and respiratory pathogens by patients not wearing the source control system.
this was a study designed to measure the efficacy of the source control system The differences in waste gas concentrations between the two groups were statistically determined by using multivariate testing (Appendix B). For isoflurane, the data for the nasal prong group show a 99% confidence interval of 58.61 +_ 43.51. Because the lower boundary of the confidence interval does not include zero (58.61-
SOURCE CONTROL MAINTAINS PACU AIR SAFETY
77
Table 1. Characteristics of the Patients in the Two Study Groups Characteristic
Nasal Prong (N - 10)
Source Control (N - 12)
P-value*
A g e (years)
38,4 _+ 13.1
38.4 _+ 13,0
.69
Weight (kilograms)
84.5 + 13.0
74,0 _+ 13.5
.08
134.2 -- 49.7
120,3 + 36.8
.46
Nitrous o x i d e MAC-hr (hr)
1.25 +_ 0.39
1,06 _+ 0,30
.21
Isoflurane Mac-hr {hr)
2.01 _+ 0.90
2.00 * 0.94
.98
Duration o f PACU stay (min)
67.5 _+ 17.9
68,3 -+ 19.9
.93
Duration o f Anesthesia (min)
Note: Data are means (+ standard deviation). * Significant difference if P -< .05,
43.51 > 0), concentrations of waste anesthetic gas detected from the source control group is considered statistically different (lower) than the nasal prong group (P < .01). In plain English, the two groups are different, and the source control group allowed significantly less (99% confidence level) waste isoflurane to be emitted into the room from the patient. For nitrous oxide concentrational comparison between the two groups, the differences between the two groups using the 25 ppm of nitrous oxide as a time weighted average was determined. Analysis of both isoflurane and nitrous oxide was also performed. Waste anesthetic gas levels were determined to be "out of compliance" if either isoflurane concentrations were greater than 2.0 ppm, or if nitrous oxide concentrations were greater than a 25 ppm time-weighted average. This comparison yielded a 99% confidence interval of 0.9 _+ .24. Once again, because the lower boundary of the confidence interval does not include zero (0.9-.24 > 0), concentrations of waste anesthetic gas emissions detected from the source control group are considered statistically different (lower) than those of the nasal prong group (99% confidence level). When considering the total time that the NIOSH recommended exposure limit is exceeded, a specific case with a time-weighted average greater than the NIOSH recommended exposure limit is considered out of compliance for its' entire duration. Using this criteria the total outof-compliance time for the nasal prong group increases from 58% to 87%, and that of the source control group still remains at zero. DISCUSSION Data from the present study affirm data from previous studies that indicate that patients recov-
ering from general anesthesia exhale high concentrations of waste anesthetic gases. 4's However, the question arises as to the significance of the fact that PACU nurses breathe in the exhaled waste anesthetic gases of their patients while providing bedside care. Most research dealing with waste anesthetic gases shows an association between exposure and a variety of adverse health consequences. The strongest and most consistent associations link chronic, occupational exposure to such gases with adverse reproductive outcomes (infertility, spontaneous abortions, and congenital anomalies). 19-21
most research dealing w i t h waste anesthetic gases shows an association b e t w e e n exposure and a variety of adverse health consequences The data presented in this study suggest that source control technology is effective in reducing air contamination hazards in the PACU. It is the only method presently available that simultaneously addresses all three air safety risks in the PACU environment. The source control system shows a high level of effectiveness in lowering the concentrations of waste anesthetic gases emitted by postoperative patients. Waste anesthetic gas containment is of greater importance when we consider that the degree of containment of waste anesthetic gases is the key marker for efficacy in the containment of airborne and bloodborne pathogens. The placement of the sample site at 53 cm from the patient's mouth
78
J. M I C H A E L B A D G W E L L
provided a rigorous and reproducible test of the effectiveness of the source control system at reducing waste anesthetic gases emitted by the patient. The technology used made it possible to accurately measure and record waste gas concentrations continuously and in real time. Because concentrations change rapidly as the patient eliminates the anesthetic gases during recovery, intermittent sampling technologies (samples taken at various times in various locations) are less likely to provide an accurate assessment of the effectiveness of a source control system. Other factors may have influenced the exhaled waste anesthetic gases concentrations such as the patients' body composition, concentration of anesthetic used, and anesthetic depth on arrival to the isolation room. However, patient weight, duration and MAC-hours of anesthesia, and duration of recovery were similar between the two groups. Therefore, these factors are not likely to have confounded our results. Both groups were statistically similar in physical characteristics as well as concentrational duration of anesthesia. In conclusion, this study corroborates earlier findings showing that scavenging systems can effectively reduce concentrations of waste anesthetic gases in the room environment, t~ Application of the source control system in the PACU should result in a concurrent reduction of waste anesthetic gases, airborne disease, and bloodborne pathogens. APPENDIX A
The AirCare Source Control System by Apotheus Laboratories, Ltd
Description. The AirCare Source Control System is modeled after the anesthesia machine in the recovery mode that, by virtue of an inflatable tracheal cuff, seals a patient's airway, supplies oxygen, and scavenges patient exhalent through the hospital's vacuum system. The AirCare system seals the airway externally by means of a special mask with an adhesive rim. The mask possesses a closed-system suction port which allows for patient airway maintenance without disconnecting the system. The mask is connected to the patient's breathing circuit (the same one used by the patient in the OR), which is connected to the Airator. The Airator delivers hospi-
tal oxygen, which flows from the wall supply into an oxygen reservoir bag. When the patient inspires, oxygen flows across a primary inspiratory valve, down the inspiratory limb of the breathing circuit, and into the lungs. When the patient exhales, expired gases flow down the expiratory limb of the breathing circuit, across a primary expiratory valve, and into an exhalational reservoir bag. As on OR anesthesia machines, the exhaled gases in the reservoir bag are removed through a hose connected to the hospital's vacuum supply. The breathing circuit is not a circle system; that is, exhaled gases do not flow back through the inhalational side of the system (Fig 1). Both the oxygen and exhalational reservoirs are equipped with positive and negative pressurerelief valves. On the inhalational side of the system, if oxygen is not supplied to the system, the negative pressure-relief valve opens and the patient breathes room air. On the exhalational side of the system, if vacuum is not supplied to the system, the positive pressure-relief valve opens and the patient exhales into the room. Access to the patient's airway is obtained by means of a closed system suction port on the mask that can be opened without disconnecting the system. It accommodates a suction catheter while maintaining the integrity of the sealed system. Limitations--The source control system is not indicated for use with patients who have facial wounds, deformities, or excessive facial hair that could affect the integrity of the seal of the adhesive mask on the patient's face. It is intended as an adjunct to, and not as a replacement for, appropriate isolation of patients with known or suspected respiratory infections. Nor is it a replacement for proper design and maintenance of an HVAC (heating, ventilation, air conditioning) system in the PACU, or appropriate work practice and administrative controls. Factors that determine the effectiveness of the system include the length of time the patient is recovered using the system, the efficacy of the mask seal, the proper operation of the system, and the proper operation of the hospital's oxygen and vacuum systems. Availability and Cost--The AirCare system has recently been approved by the Food and Drug Administration. Price has not been determined. How-
SOURCE CONTROL MAINTAINS PACU AIR SAFETY
79 - HOSPITAL VACUUM
o27
INHALED 02~
\ EXHALED GASES
ADHESIVE SEAL
5
Fig 1. The AirCare Source Control System.
ever, it is expected that the per patient cost to the hospital would be comparable to the current cost of a single-use humidifier and vend m a s k - - a system commonly used by hospitals in the U.S. APPENDIX B
Statisical Analysis
Comparability of the Experimental and Control Groups--To ensure comparability of the control group to the source control group, the two groups were compared on the following six dimensions: age, weight, duration of anesthesia, nitrous oxide MAC-hours, isoflurane MAChours, and duration of PACU stay (Table 1). The General Linear Model (GLM) procedure in SAS provided a comparison of differences in patient morphometrics and anesthetic regimens between study groups. This procedure tests the hypothesis of no group effect, ie, that the cases are drawn from a single population. The P-values reported are the estimated probabilities of observing mean differences as large as those observed when the
null hypothesis (the group means are equal) is true. A P-value greater than .05 was considered an indication that the two groups were from the same population. This test was performed on each of the six variables separately, and as a multivariate (MANOVA) test on the six-element dependent vector variable simultaneously in order to control the risk of type-I error at the desired overall level. The model tested w a s : Yijk = #k + Oj -}- eijk, where Yijk is the score of subject (i) in group (j) on dependent variable (k), #k is the overall mean on variable (k), 0j is the effect of group (j), and eijk is the error term, assumed to be normally and independently distributed with constant variance. In the event that such assumptions might be violated, nonparametric analyses were conducted as well. The results of the Multivariate Analysis of Variance (MANOVA) are as follows: M A N O V A Test Criteria and Exact F Statistics for the Hypothesis of no Overall G R O U P Effect, H = Type III SS&CP Matrix for GROUP, E = Error SS& CP Matrix
Statistic
Value
F
Num DF
Den DF
Pr > F
Wilks' Lambda
0.68365421
1.1568
6
15
0.3787
J. M I C H A E L B A D G W E L L
8O
Although the difference in average weight between the groups approaches statistical significance when considered individually, we are able to conclude that, in an overall multivariate sense, there is no evidence to allow us to reject the hypothesis of no difference between the groups.
Statistical Significance of the Differences in the Results of the Experimental and Control Groups--The key criterion to judge the effectiveness of the source control system was the percentage of the time a patient was in the isolation room during which concentrations of waste anesthetic gases exceeded the criteria of 2 ppm and 25 p p m respectively for isoflurane and nitrous oxide, when measured at the 53 cm point for both the control group and the source control group. Since the source control group contained zero minutes in excess of the criteria and therefore had zero variability, our statistical test involves testing the hypothesis that the control group mean also equals zero, within sampling error. The procedure is to compute a 99% confidence interval around the control group mean, using the formula: M _+ t.99*cr/SQRT(N) where M equals mean of the control group; t.99 equals the critical value of the t-Distribution for a 99%
confidence interval with N-1 degrees of freedom; cr equals the estimated standard deviation of the control group, and N = sample size. However, as indicated above, measurements were compared to 25 p p m for nitrous oxide both as a ceiling and as a time weighted average. For the latter, the normal approximation to the binomial distribution needs to be calculated since the time weighted average will yield a proportion of cases that comply or do not comply with the time weighted average REL from NIOSH. Thus, the following formula was used to calculate the 99% confidence limits: P + Z99*SQRT[(P*(1-P))/N)] where P equals the proportion of cases out of compliance with the NIOSH REL, Z99 = the critical value of the normal approximation to the binomial distribution at the 99% confidence level with N-1 degrees of freedom (2.576), and N equals the sample size. ACKNOWLEDGMENT The author thanks the following from Apotheus Laboratories for furnishing the source control systems and measurement equipment necessary for the conduct of the product evaluation: Brett Moore, Stephanie Walton, and Scott Wehmeyer.
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10. Nikki P, Pfaffli P, Ahlman K: End-tidal and blood halothane and nitrous oxide in surgical personnel. Lancet 2:490-491, 1972 11. Nikki P, Pfaffli P, Ahlman K, et al: Chronic exposure to anaesthetic gases in the operating theatre and recovery room. Ann Clin Res 4:266-272, 1972 12. Pfaffli P, Nikki P, Ahlman K: Halothane and nitrous oxide in end-tidal air and venous blood of surgical personnel. Ann Clin Res 4:273-277, 1972 13. Kant I, van Rijssen-Moll M, Borm P: Simulation of nitrous oxide concentrations in operating and recovery rooms. Ann Occup Hyg 34:575-583, 1990 14. Department of Health, Education, and Welfare, Public Health Service, Center for Disease Control, National Institute for Occupational Safety and Health: NIOSH A Recommended Standard for Occupational Exposure to Waste Anesthetic Gases and Vapors. Government Printing Office, Washington, DC, Pub No. 77-140, 1994 15. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health: NIOSH Pocket Guide to Chemical Hazards, DHHS (NIOSH) Publication No. 94-116, US Government Printing Office, Washington, DC, 1994 16. Allen A, Badgwell JM: The post anesthesia care unit: Unique contribution, unique risk. J Peri Anesth Nuts 11:248258, 1996 17. American Thoracic Society, Medical Section of the
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AIR SAFETY
American Lung Association: Control of tuberculosis in the United States. Am Rev Respir Dis 146:1623-33, 1992 18. Bierbaum PJ, Lippmann M (eds): Proceedings of the Workshop on Engineering Controls for Preventing Airborne Infections in Workers in Health Care and Related Facilities. July 14-16, 1993, Cincinnati, Ohio. U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health. DHHS (NIOSH) Publication No. 94-106
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19. Rowland A, Baird D, Weinberg C, et al: Reduced fertility among women employed as dental assistants exposed to high levels of nitrous oxide. N Engl J Med 327:993-997, 1992 20. Rowland A, Baird D, Shore D, et al: Nitrous oxide and spontaneous abortion in female dental assistants. Am J Epidemiol, 141:531-538, 1995 21. Guirguis S, Pelmear P, Wong R: Health effects associated with exposure to anaesthetic gases in Ontario hospital personnel. Br J Ind Med, 47:490-497, 1990