Presented in part at the Annual Meeting of the Association of Paediatric Anaesthetists, Norwich; 2005.
Flexibility and light emission of disposable paediatric Miller 1 laryngoscope blades*
Article first published online: 6 JUL 2006
Volume 61, Issue 8, pages 792–799, August 2006
How to Cite
Goodwin, N., Wilkes, A. R. and Hall, J. E. (2006), Flexibility and light emission of disposable paediatric Miller 1 laryngoscope blades. Anaesthesia, 61: 792–799. doi: 10.1111/j.1365-2044.2006.04721.x
- Issue published online: 6 JUL 2006
- Article first published online: 6 JUL 2006
- Accepted: 30 May 2006
With the emergence of Creutzfeldt-Jakob disease and the discovery of prions in tonsillar material, there has been an increase in the number of available disposable laryngoscope blades. This has led to non-conformity over many aspects of blade design. Miller 1 disposable blades have been produced in both metal and plastic and appear to have different properties of rigidity. We examined the rigidity of 11 disposable Miller 1 blades in three different axes of force. There was a significant difference in flexibility between metal and plastic blades in both primary and torsional axis (p = 0.006). We also studied the blades' light intensity and angle of light emission, finding up to an eightfold difference in the level of illumination provided at a distance of 10 mm from the tips of the blades. The area of maximal illumination varied, with some blades providing narrow beams of light, and others provided a more dispersed field of illumination. In addition, the angle of maximal illumination varied between the blade types from a central position to one directed to the right-hand side.
Miller developed a straight laryngoscope blade  that was modified for paediatric use in 1946 . He described a straight blade with a shallow vertical portion and a small flange facing to the right, curved near the distal end. The blade has the advantage that it may be used to lift the floppy epiglottis of an infant. With the discovery of variant Creutzfeldt-Jakob disease in tonsillar material , there has recently been an increase in the number of disposable blades on the market , including many describing themselves as ‘Miller 1’, their manufacturers having made alterations to the original design (Fig. 1).
Disposable adult laryngoscopes have been investigated both in patients  and in the laboratory [4, 6, 7] with varying results. Blade design often changes during the manufacturing process [8, 9], and therefore blades from different manufacturers may have different characteristics. Disposable blades have been manufactured from both plastic and metal , and their flexibility appears to vary. In the first part of this laboratory-based study, we investigated the flexibility of 11 different disposable Miller 1 blades when tested in three different directions of force.
The illumination produced by a variety of adult blades has been studied [10–14] but no studies have looked at paediatric blades. Some have looked at the luminescence (a measure of reflected light) produced by blades rather than the light intensity [10–12]. Whilst the area of illumination has been investigated from a variety of blades [11, 14] this previously has been studied as the widest area illuminated rather than specifically looking at the angle of illumination. In the second part of this study, we tested both the light emission and the angle of that light from disposable Miller 1 blades.
|1. ProAct Metal Max GS 90™||Metal||Fibreoptic||ProAct Medical Ltd, Great Oakley, Northamptonshire, UK|
|2. ProAct Metal Max 90™||Metal||Bulb|
|3. ProAct Metal Max 100™||Metal||Bulb|
|4. Timesco Callisto™||Metal||Fibreoptic||Timesco of London Ltd, London, UK|
|5. Timesco Europa™||Metal||Bulb|
|6. Timesco Freeway Optima™||Plastic||Fibreoptic|
|7. Intersurgical™||Plastic||Fibreoptic||Intersurgical Ltd, Wokingham, UK|
|8. Heine XP™||Plastic||Fibreoptic||Albert Waeschle, Creekmoor, Poole, UK|
|9. Penlon Crystal™||Plastic||Fibreoptic||Penlon Ltd, Abingdon, UK|
|10. Truphatek Liteblade Slim™||Plastic||Bulb||ProAct Medical Ltd, Great Oakley, Northamptonshire, UK|
|11. Vital Signs Vital View™||Plastic||Fibreoptic||Vital Signs Ltd, Littlehampton, UK|
Each blade was tested in three different planes of force: primary, lateral and torsional axes. The primary axis was perpendicular to the handle and is the axis in which force is most likely to be directed during intubation. The lateral axis placed force at right angles to the blade. Torsional axis was a twisting force applied to the tip of the blade and was included, as force in this direction may be applied after the blade is inserted in the mouth in order to improve the view. To test the primary axis the laryngoscope handle was clamped in an adaptor on a pneumatically powered rig (Fig. 2a). A sling was placed between the tip of the blade and the rig pull shaft. A gradually increasing force up to 150 N was applied to the tip of the blade by activating a piston. Force was detected using a force transducer (AFG 500 N, McMesin Ltd, Horsham, UK). Extension data were collected using a data acquisition program (LabView 5.1, National Instruments, Austin, TX). Extension due to the rig and string itself was measured, analysed and deducted from the final results. The lateral axis was tested with the laryngoscope handle fixed vertically (Fig. 2b). A sling was again positioned between the tip of the blade and the rig pull shaft. A gradually increasing force up to 50 N was applied and data collected as for the primary axis. To test the torsional axis, an additional adapter was clipped on to the tip of the blade and torsional force applied over an accurately measured distance so that the actual force applied at the tip could be calculated (Fig. 2c). Force up to 5 N was gradually applied and data collected and analysed as described above.
To study light emission the blades were attached to a handle mounted above a template over which illumination was measured (Fig. 3). A green ringed Intersurgical laryngoscope handle was used for all the fibreoptic blades, and a Truphatek handle for the standard blades. Vital View blades have a non-standard fitting and so a Vital Signs handle was used for these blades. The cells in the handles (all size C) were tested before and after each assessment to ensure that they provided over 3 V and that this dropped by < 0.1 V during each individual test (Fluke 77 series Multimeter, John Fluke Mfg. Co. Inc., Seattle, WA). A ‘daisy’ design template with a central 40-mm diameter disc surrounded by six overlapping discs (seen drawn on the horizontal base of the apparatus in Fig. 3) was used to measure illumination. The laryngoscope was positioned above the template using a retort stand so that the light from the blade was perpendicular to the centre of the daisy. Light emission was detected at 10 and 20 mm from the tip of the blade to the centre of the light sensor (Testo 545, Lux fc, Testo Ltd, Alton, Hants., UK). Further light measurements were noted at all positions around the daisy template to help interpret the spread of light. The study was performed in a darkened room and background illumination was measured at the start and end of each test, with averaged background illumination subtracted from the test readings.
Results for blade flexibility were analysed using linear regression analysis on Statistical Package for the Social Sciences (11.5). Differences between metal and plastic blades were assessed using the Mann–Whitney U-test. The effect of the different blades on illumination was assessed using the Kruskal–Wallis test, the effect of placing the lightmeter at the different positions on the ‘daisy’ target on illumination was assessed using the Friedman test, the effect of the type of blade (fibreoptic or bulb) on illumination was assessed using the Mann–Whitney U-test, and the effect of distance (10 or 20 mm) on illumination was assessed using the Wilcoxon signed-rank test. A p value < 0.05 was considered to indicate a significant effect for all comparisons.
There was a large variation in flexibility of the blades in the primary axis (Table 2). Metal blades were less flexible in the primary axis than plastic blades (p = 0.006). At 150 N this correlated to a range in movement of 5.4 mm for the ProAct Metal Max GS 90 blade to 38 mm for the Truphatek LiteBlade. The Vital View blades were the most flexible but they were not able to withstand an application of the test standard of 150 N: all three Vital View blades broke before the maximum force was reached (at 111 N, 123 N and 129 N) (Fig. 4). No blades broke when testing to the maximum force of 50 N in the lateral axis (Table 2) and the difference between metal and plastic was not statistically significant (p = 0.068). Some blades appeared to move at the connection between handle and blade, but this was not quantified separately. There was a large variation in the results between different blade types, with the ProAct Metal Max 100 blade being particularly rigid. No blades broke in the torsional axis test (Table 2). The adaptor for this test was 4 cm from the blade to the string, so torque was 0.04 × force in Nm. As the piston pulled the string along a constant direction, but the adaptor turned, the torque changed as the angle changed. This changing force was the same for all blades tested, and therefore the values were not subtracted in this paper. These values may therefore only be used as comparison between the blades and not as exact results. The difference between metal and plastic blades was statistically significant (p = 0.006) in the torsional axis. For some blades it was apparent that there was a degree of movement at the connector between the handle and blade as well as a twisting movement of the blade. This was not formally analysed.
|Primary axis||Lateral axis||Torsional axis|
|1. ProAct Metal Max GS 90||0.036 (0.032–0.04)||0.11 (0.104–0.125)||1.252 (0.385–2.94)|
|2. ProAct Metal Max 90||0.038 (0.03–0.046)||0.113 (0.094–0.132)||1.106 (0.697–1.48)|
|3. ProAct Metal Max 100||0.042 (0.038–0.046)||0.012 (0.009–0.035)||0.547 (0.251–0.843)|
|4. Timesco Callisto||0.055 (0.051–0.06)||0.192 (0.173–0.21)||1.3 (0.95–1.649)|
|5. Timesco Europa||0.058 (0.045–0.072)||0.155 (0.135–0.174)||1.275 (0.758–1.792)|
|6. Timesco Freeway Optima||0.114 (0.092–0.136)||0.136 (0.114–0.158)||3.443 (3.162–3.731)|
|7. Intersurgical||0.146 (0.134–0.096)||0.134 (0.109–0.159)||2.693 (2.509–2.84)|
|8. Heine XP||0.149 (0.137–0.16)||0.173 (0.154–0.192)||2.956 (2.625–3.288)|
|9. Penlon Crystal||0.175 (0.169–0.18)||0.287 (0.268–0.305)||1.95 (1.737–2.162)|
|10. Truphatek Liteblade||0.255 (0.244–0.266)||0.253 (0.236–0.27)||2.83 (2.568–3.092)|
|11. Vital View*||0.275* (0.211–0.339)||0.278 (0.239–0.29)||3.762 (3.1–4.424)|
There was a large variation in the illumination produced by the different blades (Table 3). The level of illumination was reduced as the light source was moved from 10 mm to 20 mm (p = 0.0001). In addition, there was a large range in illumination within the samples of each blade (Table 3). The illumination measured by the lightmeter varied at the different positions of the daisy target (p < 0.001) and the degree of spread and angle of maximal illumination was also variable (Fig. 5). The order from the highest to lowest illumination was positions 1, 5, 6, 7, 2, 3 and 4 (the central position being 1), although the Timesco Callisto had its brightest areas of illumination to the right (in circle 5) instead of the central circle (Fig. 5(4)). The background level of illumination was always < 3 lx. The type of blade (fibreoptic or bulb) did not have an effect on illumination at either 10 or 20 mm (p = 0.20 and 0.22, respectively).
|Illumination; lx||Range of illumination|
|10 mm||20 mm||10 mm||20 mm|
|ProAct Metal Max GS 90||509 (128–665)||172 (70–376)||106%||178%|
|ProAct Metal Max 90||676 (431–803)||527 (368–575)||55%||39%|
|ProAct Metal Max 100||833 (615–833)||611 (473–630)||26%||26%|
|Timesco Callisto||530 (507–672)||465 (403–530)||21%||27%|
|Timesco Europa||4178 (3057–4367)||1743 (930–2616)||31%||97%|
|Timesco Freeway Optima||1421 (1406–1922)||1154 (995–1222)||36%||20%|
|Intersurgical||1197 (903–1505)||818 (786–903)||50%||14%|
|Heine XP||1363 (1344–1407)||1079 (882–1109)||5%||21%|
|Penlon Crystal||1930 (1764–2126)||1339 (1289–1599)||19%||23%|
|Truphatek Liteblade||4077 (3498–4369)||3707 (3157–3721)||15%||21%|
|Vital View*||807 (727–1084)||548 (268–583)||44%||57%|
Miller originally described a straight blade with a right-sided flange  and then made several sizes suitable for the paediatric market . He specified neither the strength nor the illumination levels required for his blades. We found a large variation in the degree of flexibility between Miller 1 disposable blades, with plastic blades being more flexible generally. The difference between metal and plastic blades was most pronounced in the primary and torsional axes. There is also a large difference in the level of illumination provided by different disposable Miller 1 blades. In addition, the direction and angle of spread is inconsistent, some blades providing a narrow beam of light and others a more dispersed light, either in the midline or angled to the right.
There has recently been a large increase in the number of disposable blades on the market. Variant Creutzfeldt-Jakob disease results from a prion infection . Prions may be found in lymphoreticular tissue during the asymptomatic incubation period  and Hirsh et al. found that 30% of laryngoscope blades were contaminated with lymphocytes after intubation. Prions resist routine sterilisation  and without the use of either single-use equipment or blade sheaths there is a risk of passing prion infection from patient to patient. There has therefore been a move to single use laryngoscope blades, not only for tonsillectomies, but for routine use .
This paper describes 11 different blades that all purport to be ‘Miller 1’. Many do not conform to the original design. This seems to be common practice when disposable equipment is available from more than one manufacturer. Jones et al. describe how the original Cardiff blade has been altered when manufactured as a disposable version without testing or discussion with the original inventor . Macintosh's original design has also undergone many changes during its commercial development . It is crucial, however, that any modified design is fully tested before it enters the mass market.
There are no standards set as to the degree of flexibility that is considered acceptable . A recent draft proposal from the International Organisation for Standardization (ISO) has suggested that blades should not flex more than 10 mm with a vertical force of 65 N (A.R. Wilkes, personal communication). The Penlon, Truphatek and Vital Signs blades would not have reached this standard. We postulate that most anaesthetists probably would prefer a relatively rigid blade. This would allow force exerted by the anaesthetist's hand to be transferred accurately to the tip of the laryngoscope. This is especially important for difficult intubations.
This study looked at flexibility in three different axes. Most anaesthetic literature tests blade strength and forces applied in the primary axis only [7, 20, 21] or during laryngoscopy as a composite measurement . Although the majority of the force exerted during laryngoscopy would be exerted in the primary axis, it is important that the blades should withstand both lateral and twisting forces that may occur whilst trying to improve the view of the larynx. It was surprising that the blades varied so much in these directions. It may be that some of the variation, particularly between the metal blades, was due to the strength of the connection with the handle rather than excessive blade bending. The ProAct Metal Max 100 has metal bearings in the connection with the handle, whilst the Metal Max 90 and GS 90 both have plastic in this connection. This could explain why the Metal Max 100 showed markedly less movement in the lateral and torsional axes.
In the primary axis, we demonstrated large variation between the flexibility of the different blades. In particular, the plastic blades bent much more than the metal blades. Testing to the maximal force of 150 N this correlated to a sevenfold difference in the amount of movement between the best and worst performing blades. We chose to test the blades to 150 N. This may be a high limit to set for a paediatric blade and it is to be hoped that anaesthetists generally would not exert this degree of force. Caesar and Scott found that 15 experienced anaesthetists using an adult blade felt comfortable applying a maximal force of 70–350 N in an unexpected difficult intubation, with a mean value of 151 N . They argue that with these values a test of 150 N may be inadequate . Bishop et al. reported peak forces of 58.3 N (SD 4.7 N) in a manikin study , whereas up to 80 N has been recorded in human studies . However, during a difficult intubation, or with a relatively inexperienced paediatric anaesthetist, the forces exerted may be higher than anticipated . All three Vital View blades broke before reaching the proposed test standard of 150 N . In addition, they were the most flexible blades before they snapped. The manufacturer has stated that all batches are tested to 100 N . It is disappointing that these blades broke at values only slightly higher than the manufacturer's own test standard, and that they flexed to such a degree during the test. Vital Signs argues that there have been only four reported incidents of blade breakage, and none in the Miller 1 blades. This argument may reflect the fact that these high forces are not often used, but does not have any impact upon the degree to which the blades bent during use. The Medicines and Healthcare products Regulatory Agency (MHRA) has commented that despite reports of failing equipment in the anaesthetic literature [27, 28] it has not had cases reported to it . Without cases being reported it remains difficult for equipment to be investigated, standards to be set and improvements to be made to equipment. Cases can be reported at any time on the MHRA website at http://www.mhra.gov.uk.
Several studies have looked at the light intensity from various laryngoscope blades [10–14]. Skilton et al. and Crosby and Cleland  measured the luminance provided by Macintosh 3, and Macintosh 3 and 4 blades, respectively. However, they used the candela as their unit of measurement, which is a measure of luminance rather than direct light. Luminance is the amount of reflected light and it is therefore not possible to compare the results of earlier studies directly with our results. They did, however, find that bulbs in handles produced higher luminance values than fibreoptic lights. Tousignanat and Tesler  also found that the light from incandescent bulbs provided more illumination than fibreoptic sources. This was not supported by the present study, which found no effect of blade type on the level of brightness. This may be due to advances in fibreoptic design and standards. In addition, we found that most blades, both bulb and fibreoptic, showed a wide range between different samples of the same blade, suggesting poor quality control in the manufacturing process.
Tousignanat and Tesler  photographed the area illuminated by laryngoscope blades to ascertain the degree of spread of light, but did not comment on the direction of spread. We have quoted the lux value in the central circle of our daisy pattern for consistency, but it is clear from the spread of light patterns that this was not always the brightest area. However, we feel that it was important to have a standard position in which to measure light intensity to be able to make accurate comparisons between the blades. The Timesco Callisto blades in particular showed their brightest areas to be to the right of the central disc. Some may argue that it is preferable to have increased light to the right of the blade in order to illuminate the advancing tracheal tube. However, we feel that it is probably more important to have the brightest light directly on the glottis.
There are no standards set at present for the degree of illumination that is required for laryngoscope blades. The Health and Safety Executive has suggests that 500 lx is the minimum for work requiring perception of fine detail . In a draft standard presently under discussion from the ISO it has been suggested that 700 lx at a distance of 20 mm for 10 min should be the minimum standard (A. R. Wilkes, personal communication). Many of the blades tested in this study did not meet that requirement. It is also not known what the optimal level of illumination is, as too bright a light may cause glare from the mucosal surfaces. We measured light emission at both 10 and 20 mm as we were studying paediatric blades. The distance from the tip of the blade to the glottis is less in this population, but the actual distance remains unknown. We used a 40-mm disc over which to measure illumination. This value was the same as that used by Skilton et al. and relates to the size of the adult glottis. The glottis is of course smaller in children, but we kept the same sized disc primarily as it was the same size as the light measuring device, and also to be able to compare many different blade types in future studies.
In conclusion, it is not straightforward to combine the flexibility and illumination results to produce a ‘best blade’. If this were possible, doubtless all manufacturers would conform. Ours was a laboratory-based study and did not take into account individual anaesthetists' preferences. However, it may be stated that a very bright blade with minimal flexibility was the Timesco Europa. Whether or not that makes this the best blade or not remains open to debate.
ARW was previously funded by the Medicines and Healthcare products Regulatory Agency.
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- 29Health and Safety Executive. Lighting at Work. HS/G 38. London: Her Majesty's Stationary Office, 1987.