- Email: [email protected]
Effect of alcohol on the efficacy of excimer laser power
Effect of alcohol on the efficacy of excimer laser power Arthur C.K. Cheng, MRCS, Ricky W.K. Law, FRCS, Alvin L.M. Young, FRCS, Geoffrey C.P. Chu, Dennis S.C. Lam, FRCS, FRCOphth Purpose: To evaluate the effect of various alcohol concentrations on excimer laser power. Setting: University Eye Clinic, The Chinese University of Hong Kong, Hong Kong, China. Methods: The fluency of the Technolas 217z laser (Bausch & Lomb) was calibrated according to the standard calibration procedure. The effect of ethanol 20%, 40%, 60%, and 80% and sodium hypochlorite 0.06% solutions on laser fluency was assessed in 5 experiments. Results: Ethanol concentrations of 20% and 40% had no significant effect on laser fluency. Ethanol concentrations of 60% and 80% reduced fluency to 97.6%. Sodium hypochlorite had no effect on laser fluency. Conclusions: A high concentration of ethanol affected laser fluency and should be avoided in the operating theater. Ethanol 20%, which is commonly used during laser-assisted subepithelial keratectomy, did not affect laser fluency. J Cataract Refract Surg 2004; 30:1545–1548 2004 ASCRS and ESCRS
T
he use of alcohol in the operating theater has become popular with the advance of laser-assisted subepithelial keratectomy (LASEK). Laser-assisted subepithelial keratectomy involves dehiscence of the epithelial flap with alcohol and replacement of the epithelium at the end of the procedure. A high concentration of alcohol is also commonly used as a disinfectant before a procedure. However, volatile compounds such as perfumes, oil-based paint, and hairspray reduce the laser
Accepted for publication March 10, 2004. From the Department of Ophthalmology & Visual Sciences, The Chinese University of Hong Kong, Hong Kong Eye Hospital, Kowloon, Hong Kong, China. Supported in part by the Action for Vision Eye Foundation, Hong Kong, China. None of the authors has a financial or proprietary interest in any material or method mentioned. Reprint requests to Dr. Arthur C.K. Cheng, Assistant Professor, Department of Ophthalmology & Visual Sciences, The Chinese University of Hong Kong, 3F, Hong Kong Eye Hospital, 147K Argyle Street, Kowloon, Hong Kong. E-mail: [email protected]. 2004 ASCRS and ESCRS Published by Elsevier Inc.
beam power over time.1 Thus, it is important to assess the effect of alcohol on excimer laser power.
Materials and Methods Solution Preparation In this study, 4 alcohol concentrations were used: 20%, 40%, 60%, and 80%. These were prepared by diluting commercially available ethanol 99.5% (200 proof, Aldrich Chemical) with an appropriate amount of distilled water in a separate equipment room. There was no direct communication between the equipment room and the operating theater; that is, the ethanol was not prepared in the latter. Ethanol 20% was chosen because it is the concentration used in LASEK, and 80% was chosen because it is the concentration used to disinfect the operating suite. Sodium hypochlorite is an alternative to alcohol 80% that is used to disinfect the operating suite; its effect on laser energy was also tested. Sodium hypochlorite solution was prepared by diluting the 6% solution with distilled water in a 1:99 ratio.
Excimer Laser Power Measurement The Technolas 217z laser (Bausch & Lomb) was used. Excimer laser energy can be assessed during the calibration 0886-3350/04/$–see front matter doi:10.1016/j.jcrs.2004.03.033
LABORATORY SCIENCE: EFFECT OF ALCOHOL ON EXCIMER LASER EFFICACY
Table 1. Mean number of pulses for each ethanol concentration at each given time. Mean Number of Pulses (effective Laser Power) ⫾ SD Time (Min)
No Ethanol
20% Ethanol
40% Ethanol
60% Ethanol
80% Ethanol
P Value*
Hypo6
0
65.0 ⫾ 0.0
65.0 ⫾ 0.0
65.0 ⫾ 0.0
65.0 ⫾ 0.0
65.0 ⫾ 0.0
NA
65.0 ⫾ 0.0
1
65.0 ⫾ 0.0
65.2 ⫾ 0.45
65.0 ⫾ 0.45
65.8 ⫾ 0.83
66.2 ⫾ 0.84‡
.025
65.0 ⫾ 0.0
2
65.0 ⫾ 0.0
65.2 ⫾ 0.45
65.0 ⫾ 0.45
65.8 ⫾ 0.45
66.4 ⫾ 1.14‡
.011
65.0 ⫾ 0.0
3
65.0 ⫾ 0.0
65.2 ⫾ 0.45
65.6 ⫾ 0.55
66.0 ⫾ 0.71
‡
66.2 ⫾ 0.45
.004
65.0 ⫾ 0.0
4
65.0 ⫾ 0.0
65.4 ⫾ 0.55
65.8 ⫾ 0.45
65.8 ⫾ 0.45
66.4 ⫾ 0.89
‡
.013
65.0 ⫾ 0.0
5
65.0 ⫾ 0.0
65.2 ⫾ 0.45
65.6 ⫾ 0.55
66.2 ⫾ 0.45‡
66.6 ⫾ 1.14‡
.003
65.0 ⫾ 0.0
6
65.0 ⫾ 0.0
65.2 ⫾ 0.45
65.8 ⫾ 0.45
66.2 ⫾ 0.45
‡
66.4 ⫾ 0.89
.001
65.0 ⫾ 0.0
7
65.0 ⫾ 0.0
65.0 ⫾ 0.0
65.4 ⫾ 0.55
66.0 ⫾ 0.71
‡
66.0 ⫾ 0.71
.007
65.0 ⫾ 0.0
8
65.2 ⫾ 0.45
65.0 ⫾ 0.0
65.6 ⫾ 0.55
66.4 ⫾ 0.55‡
66.2 ⫾ 0.84
.002
65.0 ⫾ 0.0
9
65.0 ⫾ 0.0
65.2 ⫾ 0.45
65.4 ⫾ 0.55
66.6 ⫾ 0.55‡
66.2 ⫾ 0.45‡
.000
65.0 ⫾ 0.0
10
65.2 ⫾ 0.45
65.0 ⫾ 0.0
65.2 ⫾ 0.45
66.2 ⫾ 0.45
‡
66.6 ⫾ 0.55
.000
65.0 ⫾ 0.0
11
65.0 ⫾ 0.0
65.0 ⫾ 0.0
65.2 ⫾ 0.45
66.0 ⫾ 0.71
‡
‡
66.2 ⫾ 0.45
.000
65.0 ⫾ 0.0
12
65.0 ⫾ 0.0
65.0 ⫾ 0.0
65.4 ⫾ 0.54
65.8 ⫾ 0.45
66.0 ⫾ 0.71‡
.005
65.0 ⫾ 0.0
13
65.0 ⫾ 0.0
65.0 ⫾ 0.0
65.2 ⫾ 0.45
65.4 ⫾ 0.55
66.0 ⫾ 0.71
14
65.0 ⫾ 0.0
65.0 ⫾ 0.0
65.2 ⫾ 0.45
65.2 ⫾ 0.45
15
65.0 ⫾ 0.0
65.0 ⫾ 0.0
65.0 ⫾ 0.0
.6
.17
.12
P value†
‡
‡ ‡
‡
.039
65.0 ⫾ 0.0
66.0 ⫾ 0.71
‡
.007
65.0 ⫾ 0.0
65.9 ⫾ 0.65
66.0 ⫾ 0.71
‡
.002
65.0 ⫾ 0.0
.01
.29
—
.42
Hypo 6 ⫽ sodium hypochlorite 0.06%; NA ⫽ not applicable *Analysis of variance; indicates the level of significance among various concentration groups for each time interval. † Analysis of variance; indicates the level of significance among various time intervals for each concentrating group. ‡ Statistically significant (P⬍.05) when compared with no ethanol at the same time interval.
process, during which a sequence of pulses is fired at a “fluency plate” with a fixed thickness. This plate, provided by the manufacturer, is composed of a metal layer and a plastic layer. According to the manufacturer, exactly 65 pulses are required to perforate the metal layer to set the fluency of the laser beam at 120 mJ/cm2. When more pulses are required to penetrate the metal layer, it indicates a lower fluency and vice versa. The amount of energy delivered by each laser pulse will not be adjusted automatically. It must be readjusted manually based on the number of pulses required to perforate the fluency plate. In this experiment, the laser energy level was not readjusted during testing.
Experimental Procedure During the study, 2 pieces of cotton gauze soaked with 10 mL of testing solution were placed at 14 cm on either side of the fluency plate. This distance was chosen because it is within the proximity of where the alcohol solution is mixed for LASEK. It also covers the area within which the entire surface would be disinfected with alcohol. The study was started with dry cotton gauze and repeated with cotton gauze soaked with testing solutions in the sequence of ethanol 20%, 40% 60%, and 80% and sodium 1546
hypochlorite. An interval of 15 minutes between each test is needed to allow elimination of ethanol in the operating room. This was confirmed with a pulse of 65 mJ/cm2 at the beginning of each trial. The pulse reading of 65 mJ/cm2 at the beginning of each trial without readjusting the laser energy also proved there was no natural laser energy drift. The operating area was equipped with 4 heavy-duty dehumidifiers to achieve a room humidity of 50%. The same dehumidifiers helped remove the ethanol in the air. Fluency tests were performed at 1-minute intervals until 15 tests (15 minutes) were completed. Fifteen minutes was chosen because it is the average time required for 1 procedure. The number of pulses required in each test was recorded. The whole procedure was repeated 5 times on 5 separate days. Statistical analysis was performed using the 1-way analysis of variance (ANOVA) and independent sample t test in SPSS version 11.
Results Effect of Ethanol Concentration The effects of ethanol concentration on laser fluency were significant at all times (Table 1). The maxi-
J CATARACT REFRACT SURG—VOL 30, JULY 2004
LABORATORY SCIENCE: EFFECT OF ALCOHOL ON EXCIMER LASER EFFICACY
Figure 1. (Cheng) Effect of ethanol 20% on the number of pulses
Figure 2. (Cheng) Effect of ethanol 40% on the number of pulses
required to perforate the fluency plate against time.
required to perforate the fluency plate against time.
mum number of pulses increased to a mean of 65.4 ⫾ 0.55 (SD) at 4 minutes with a 20% ethanol concentration (Figure 1), 65.8 ⫾ 0.45 at 4 minutes with a 40% concentration (Figure 2), 66.6 ⫾ 0.55 at 9 minutes with a 60% concentration (Figure 3), and 66.6 ⫾ 1.14 at 5 minutes with an 80% concentration (Figure 4). An independent sample t test comparing ethanol and no ethanol showed no statistical difference between ethanol 20% and no ethanol or between ethanol 40% and no ethanol. At 3 minutes and 5 to 11 minutes, the difference between ethanol 60% and no ethanol was significant. There was also a significant difference between ethanol 80% and no ethanol at 1 to 12 and 14 to 15 minutes.
Effect of Time There was a significant increase in the number of pulses with time for each alcohol concentration, after which the number regressed toward normal. However, with ethanol 60% and 80%, the number of pulses did not return to normal within 15 minutes. Changes along the time sequence were significant for ethanol 60% only (P ⫽ .01). There was no change in the number of pulses with sodium hypochlorite throughout the testing period.
Figure 3. (Cheng) Effect of ethanol 60% on the number of pulses
Figure 4. (Cheng) Effect of ethanol 80% on the number of pulses
required to perforate the fluency plate against time.
required to perforate the fluency plate against time.
Discussion Although LASEK is becoming a more popular option in refractive surgery, the use of ethanol in the
J CATARACT REFRACT SURG—VOL 30, JULY 2004
1547
LABORATORY SCIENCE: EFFECT OF ALCOHOL ON EXCIMER LASER EFFICACY
operating room is also growing. Volatile compounds such as perfumes, oil-based paint, and hairspray reduce laser beam power over time. Thus, it is important to assess the effect of alcohol on excimer laser power.1 Our results show that when no ethanol is used, excimer laser energy delivery is very consistent. However, when the ethanol concentration increases, the number of pulses increases and there is a reduction in fluency. The change in the number of pulses required to perforate the fluency plate was statistically significant at all time intervals, indicating that ethanol concentration has a direct effect on laser energy delivery. At an ethanol concentration of 20%, the maximum number of pulses required to perforate the fluency plate was 65.4. At 40%, 60%, and 80%, the maximum number was 65.8, 66.6, and 66.6. This translates to a reduction in fluency to 99.4% for ethanol 20%, 98.8% for ethanol 40%, and 97.6% for ethanol 60% and 80%. An independent sample t test showed no statistically significant difference between no ethanol and ethanol 20% at all time intervals. As this is the most commonly used concentration in LASEK, the exposure of ethanol 20% immediately before laser ablation should not effect negative treatment outcomes caused by unpredictable laser energy delivery. At higher concentrations, the effect of ethanol on laser energy could be significant and long lasting. Our results show that ethanol 40% can reduce laser energy to 98.8% and that the effect subsides within 15 minutes. Ethanol 60% and 80% further reduced the laser energy to 97.6%, and the number of pulses failed to return to normal after 15 minutes. Disinfecting the operating suite with a high concentration of ethanol immediately before the operation should be avoided. Sodium hypochlorite can be used a substitute as it does not have an effect on laser energy.
1548
The reduction in laser fluency represents a reduced amount of energy delivered to a specified area. The beam profile and ablation pattern created by every pulse will be different from the original design. Thus, more pulses are required to penetrate a metal plate of the same thickness. As most new laser systems use scanningspot technology, in which many small-spot laser ablations are overlapped to generate the overall profile, changing the individual beam profile will create fundamental problems in how these small laser ablations overlap. A changed beam profile may not generate an overall smooth ablation surface because the beam profile may no longer be Gaussian. This problem cannot be overcome by adjusting the nomogram alone. Adjusting the nomogram can only change the total amount of pulses delivered to the cornea; however, if the laser pulses are different from the original design, they will not be able to overlap properly from the beginning. Theoretically, ethanol can reduce laser energy delivery in 2 ways.1 It can absorb laser energy along the beam path. This is particularly important in systems in which the beam paths are not flushed with nitrogen and in systems with a long focusing distance. With repeated use, ethanol can also interact with the coatings of the laser optics, causing degradation of the optical elements. Based on the our findings, we suggest the following: Avoid preparing ethanol inside the operating room, avoid a high concentration of ethanol as a disinfectant in the operating room and consider using sodium hypochlorite instead, and inspect laser optics more frequently when it is noticed that the laser power is reduced during calibration.
Reference 1. Van Horn SD, Hovanesian JA, Maloney RK. Effect of volatile compounds on excimer laser power delivery. J Refract Surg 2002; 18:524–528
J CATARACT REFRACT SURG—VOL 30, JULY 2004
T
he use of alcohol in the operating theater has become popular with the advance of laser-assisted subepithelial keratectomy (LASEK). Laser-assisted subepithelial keratectomy involves dehiscence of the epithelial flap with alcohol and replacement of the epithelium at the end of the procedure. A high concentration of alcohol is also commonly used as a disinfectant before a procedure. However, volatile compounds such as perfumes, oil-based paint, and hairspray reduce the laser
Accepted for publication March 10, 2004. From the Department of Ophthalmology & Visual Sciences, The Chinese University of Hong Kong, Hong Kong Eye Hospital, Kowloon, Hong Kong, China. Supported in part by the Action for Vision Eye Foundation, Hong Kong, China. None of the authors has a financial or proprietary interest in any material or method mentioned. Reprint requests to Dr. Arthur C.K. Cheng, Assistant Professor, Department of Ophthalmology & Visual Sciences, The Chinese University of Hong Kong, 3F, Hong Kong Eye Hospital, 147K Argyle Street, Kowloon, Hong Kong. E-mail: [email protected]. 2004 ASCRS and ESCRS Published by Elsevier Inc.
beam power over time.1 Thus, it is important to assess the effect of alcohol on excimer laser power.
Materials and Methods Solution Preparation In this study, 4 alcohol concentrations were used: 20%, 40%, 60%, and 80%. These were prepared by diluting commercially available ethanol 99.5% (200 proof, Aldrich Chemical) with an appropriate amount of distilled water in a separate equipment room. There was no direct communication between the equipment room and the operating theater; that is, the ethanol was not prepared in the latter. Ethanol 20% was chosen because it is the concentration used in LASEK, and 80% was chosen because it is the concentration used to disinfect the operating suite. Sodium hypochlorite is an alternative to alcohol 80% that is used to disinfect the operating suite; its effect on laser energy was also tested. Sodium hypochlorite solution was prepared by diluting the 6% solution with distilled water in a 1:99 ratio.
Excimer Laser Power Measurement The Technolas 217z laser (Bausch & Lomb) was used. Excimer laser energy can be assessed during the calibration 0886-3350/04/$–see front matter doi:10.1016/j.jcrs.2004.03.033
LABORATORY SCIENCE: EFFECT OF ALCOHOL ON EXCIMER LASER EFFICACY
Table 1. Mean number of pulses for each ethanol concentration at each given time. Mean Number of Pulses (effective Laser Power) ⫾ SD Time (Min)
No Ethanol
20% Ethanol
40% Ethanol
60% Ethanol
80% Ethanol
P Value*
Hypo6
0
65.0 ⫾ 0.0
65.0 ⫾ 0.0
65.0 ⫾ 0.0
65.0 ⫾ 0.0
65.0 ⫾ 0.0
NA
65.0 ⫾ 0.0
1
65.0 ⫾ 0.0
65.2 ⫾ 0.45
65.0 ⫾ 0.45
65.8 ⫾ 0.83
66.2 ⫾ 0.84‡
.025
65.0 ⫾ 0.0
2
65.0 ⫾ 0.0
65.2 ⫾ 0.45
65.0 ⫾ 0.45
65.8 ⫾ 0.45
66.4 ⫾ 1.14‡
.011
65.0 ⫾ 0.0
3
65.0 ⫾ 0.0
65.2 ⫾ 0.45
65.6 ⫾ 0.55
66.0 ⫾ 0.71
‡
66.2 ⫾ 0.45
.004
65.0 ⫾ 0.0
4
65.0 ⫾ 0.0
65.4 ⫾ 0.55
65.8 ⫾ 0.45
65.8 ⫾ 0.45
66.4 ⫾ 0.89
‡
.013
65.0 ⫾ 0.0
5
65.0 ⫾ 0.0
65.2 ⫾ 0.45
65.6 ⫾ 0.55
66.2 ⫾ 0.45‡
66.6 ⫾ 1.14‡
.003
65.0 ⫾ 0.0
6
65.0 ⫾ 0.0
65.2 ⫾ 0.45
65.8 ⫾ 0.45
66.2 ⫾ 0.45
‡
66.4 ⫾ 0.89
.001
65.0 ⫾ 0.0
7
65.0 ⫾ 0.0
65.0 ⫾ 0.0
65.4 ⫾ 0.55
66.0 ⫾ 0.71
‡
66.0 ⫾ 0.71
.007
65.0 ⫾ 0.0
8
65.2 ⫾ 0.45
65.0 ⫾ 0.0
65.6 ⫾ 0.55
66.4 ⫾ 0.55‡
66.2 ⫾ 0.84
.002
65.0 ⫾ 0.0
9
65.0 ⫾ 0.0
65.2 ⫾ 0.45
65.4 ⫾ 0.55
66.6 ⫾ 0.55‡
66.2 ⫾ 0.45‡
.000
65.0 ⫾ 0.0
10
65.2 ⫾ 0.45
65.0 ⫾ 0.0
65.2 ⫾ 0.45
66.2 ⫾ 0.45
‡
66.6 ⫾ 0.55
.000
65.0 ⫾ 0.0
11
65.0 ⫾ 0.0
65.0 ⫾ 0.0
65.2 ⫾ 0.45
66.0 ⫾ 0.71
‡
‡
66.2 ⫾ 0.45
.000
65.0 ⫾ 0.0
12
65.0 ⫾ 0.0
65.0 ⫾ 0.0
65.4 ⫾ 0.54
65.8 ⫾ 0.45
66.0 ⫾ 0.71‡
.005
65.0 ⫾ 0.0
13
65.0 ⫾ 0.0
65.0 ⫾ 0.0
65.2 ⫾ 0.45
65.4 ⫾ 0.55
66.0 ⫾ 0.71
14
65.0 ⫾ 0.0
65.0 ⫾ 0.0
65.2 ⫾ 0.45
65.2 ⫾ 0.45
15
65.0 ⫾ 0.0
65.0 ⫾ 0.0
65.0 ⫾ 0.0
.6
.17
.12
P value†
‡
‡ ‡
‡
.039
65.0 ⫾ 0.0
66.0 ⫾ 0.71
‡
.007
65.0 ⫾ 0.0
65.9 ⫾ 0.65
66.0 ⫾ 0.71
‡
.002
65.0 ⫾ 0.0
.01
.29
—
.42
Hypo 6 ⫽ sodium hypochlorite 0.06%; NA ⫽ not applicable *Analysis of variance; indicates the level of significance among various concentration groups for each time interval. † Analysis of variance; indicates the level of significance among various time intervals for each concentrating group. ‡ Statistically significant (P⬍.05) when compared with no ethanol at the same time interval.
process, during which a sequence of pulses is fired at a “fluency plate” with a fixed thickness. This plate, provided by the manufacturer, is composed of a metal layer and a plastic layer. According to the manufacturer, exactly 65 pulses are required to perforate the metal layer to set the fluency of the laser beam at 120 mJ/cm2. When more pulses are required to penetrate the metal layer, it indicates a lower fluency and vice versa. The amount of energy delivered by each laser pulse will not be adjusted automatically. It must be readjusted manually based on the number of pulses required to perforate the fluency plate. In this experiment, the laser energy level was not readjusted during testing.
Experimental Procedure During the study, 2 pieces of cotton gauze soaked with 10 mL of testing solution were placed at 14 cm on either side of the fluency plate. This distance was chosen because it is within the proximity of where the alcohol solution is mixed for LASEK. It also covers the area within which the entire surface would be disinfected with alcohol. The study was started with dry cotton gauze and repeated with cotton gauze soaked with testing solutions in the sequence of ethanol 20%, 40% 60%, and 80% and sodium 1546
hypochlorite. An interval of 15 minutes between each test is needed to allow elimination of ethanol in the operating room. This was confirmed with a pulse of 65 mJ/cm2 at the beginning of each trial. The pulse reading of 65 mJ/cm2 at the beginning of each trial without readjusting the laser energy also proved there was no natural laser energy drift. The operating area was equipped with 4 heavy-duty dehumidifiers to achieve a room humidity of 50%. The same dehumidifiers helped remove the ethanol in the air. Fluency tests were performed at 1-minute intervals until 15 tests (15 minutes) were completed. Fifteen minutes was chosen because it is the average time required for 1 procedure. The number of pulses required in each test was recorded. The whole procedure was repeated 5 times on 5 separate days. Statistical analysis was performed using the 1-way analysis of variance (ANOVA) and independent sample t test in SPSS version 11.
Results Effect of Ethanol Concentration The effects of ethanol concentration on laser fluency were significant at all times (Table 1). The maxi-
J CATARACT REFRACT SURG—VOL 30, JULY 2004
LABORATORY SCIENCE: EFFECT OF ALCOHOL ON EXCIMER LASER EFFICACY
Figure 1. (Cheng) Effect of ethanol 20% on the number of pulses
Figure 2. (Cheng) Effect of ethanol 40% on the number of pulses
required to perforate the fluency plate against time.
required to perforate the fluency plate against time.
mum number of pulses increased to a mean of 65.4 ⫾ 0.55 (SD) at 4 minutes with a 20% ethanol concentration (Figure 1), 65.8 ⫾ 0.45 at 4 minutes with a 40% concentration (Figure 2), 66.6 ⫾ 0.55 at 9 minutes with a 60% concentration (Figure 3), and 66.6 ⫾ 1.14 at 5 minutes with an 80% concentration (Figure 4). An independent sample t test comparing ethanol and no ethanol showed no statistical difference between ethanol 20% and no ethanol or between ethanol 40% and no ethanol. At 3 minutes and 5 to 11 minutes, the difference between ethanol 60% and no ethanol was significant. There was also a significant difference between ethanol 80% and no ethanol at 1 to 12 and 14 to 15 minutes.
Effect of Time There was a significant increase in the number of pulses with time for each alcohol concentration, after which the number regressed toward normal. However, with ethanol 60% and 80%, the number of pulses did not return to normal within 15 minutes. Changes along the time sequence were significant for ethanol 60% only (P ⫽ .01). There was no change in the number of pulses with sodium hypochlorite throughout the testing period.
Figure 3. (Cheng) Effect of ethanol 60% on the number of pulses
Figure 4. (Cheng) Effect of ethanol 80% on the number of pulses
required to perforate the fluency plate against time.
required to perforate the fluency plate against time.
Discussion Although LASEK is becoming a more popular option in refractive surgery, the use of ethanol in the
J CATARACT REFRACT SURG—VOL 30, JULY 2004
1547
LABORATORY SCIENCE: EFFECT OF ALCOHOL ON EXCIMER LASER EFFICACY
operating room is also growing. Volatile compounds such as perfumes, oil-based paint, and hairspray reduce laser beam power over time. Thus, it is important to assess the effect of alcohol on excimer laser power.1 Our results show that when no ethanol is used, excimer laser energy delivery is very consistent. However, when the ethanol concentration increases, the number of pulses increases and there is a reduction in fluency. The change in the number of pulses required to perforate the fluency plate was statistically significant at all time intervals, indicating that ethanol concentration has a direct effect on laser energy delivery. At an ethanol concentration of 20%, the maximum number of pulses required to perforate the fluency plate was 65.4. At 40%, 60%, and 80%, the maximum number was 65.8, 66.6, and 66.6. This translates to a reduction in fluency to 99.4% for ethanol 20%, 98.8% for ethanol 40%, and 97.6% for ethanol 60% and 80%. An independent sample t test showed no statistically significant difference between no ethanol and ethanol 20% at all time intervals. As this is the most commonly used concentration in LASEK, the exposure of ethanol 20% immediately before laser ablation should not effect negative treatment outcomes caused by unpredictable laser energy delivery. At higher concentrations, the effect of ethanol on laser energy could be significant and long lasting. Our results show that ethanol 40% can reduce laser energy to 98.8% and that the effect subsides within 15 minutes. Ethanol 60% and 80% further reduced the laser energy to 97.6%, and the number of pulses failed to return to normal after 15 minutes. Disinfecting the operating suite with a high concentration of ethanol immediately before the operation should be avoided. Sodium hypochlorite can be used a substitute as it does not have an effect on laser energy.
1548
The reduction in laser fluency represents a reduced amount of energy delivered to a specified area. The beam profile and ablation pattern created by every pulse will be different from the original design. Thus, more pulses are required to penetrate a metal plate of the same thickness. As most new laser systems use scanningspot technology, in which many small-spot laser ablations are overlapped to generate the overall profile, changing the individual beam profile will create fundamental problems in how these small laser ablations overlap. A changed beam profile may not generate an overall smooth ablation surface because the beam profile may no longer be Gaussian. This problem cannot be overcome by adjusting the nomogram alone. Adjusting the nomogram can only change the total amount of pulses delivered to the cornea; however, if the laser pulses are different from the original design, they will not be able to overlap properly from the beginning. Theoretically, ethanol can reduce laser energy delivery in 2 ways.1 It can absorb laser energy along the beam path. This is particularly important in systems in which the beam paths are not flushed with nitrogen and in systems with a long focusing distance. With repeated use, ethanol can also interact with the coatings of the laser optics, causing degradation of the optical elements. Based on the our findings, we suggest the following: Avoid preparing ethanol inside the operating room, avoid a high concentration of ethanol as a disinfectant in the operating room and consider using sodium hypochlorite instead, and inspect laser optics more frequently when it is noticed that the laser power is reduced during calibration.
Reference 1. Van Horn SD, Hovanesian JA, Maloney RK. Effect of volatile compounds on excimer laser power delivery. J Refract Surg 2002; 18:524–528
J CATARACT REFRACT SURG—VOL 30, JULY 2004