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indirect videolaryngoscopes ( vls ) make endotracheal intubation possible without extensive experience . since their introduction , the singular goal is optimal visibility of the pharyngolarynx . an underestimated aspect of the design is lighting .
when illumination is poor , positioning the endotracheal tube is difficult , potentially traumatizing soft tissue , especially when using a rigid stylet .
the aim of this study was to compare the intensity of illumination from the light source between different vls .
devices were compared in a photographer 's dark room , an operating theatre and outside - in bright sunlight .
the intensity of light was measured at the oral cavity opening of a laerdal intubation trainer .
the board decided that formal review was not necessary as this study did not involve any patients nor participants other than the investigators . in this in vitro study , using a manikin ( airway trainer , laerdal stavanger , norway ) , fourteen different vls were compared , readily available in the institution where both authors were employed at the time of the study ( respectively as resident and professor in anaesthesiology):coopdech vlp-100 ( daiken medical , tokyo , japan ) , c - mac ( 8402 zx monitor ) and c - mac pm ( karl storz , tuttlingen , germany ) , glidescope ranger ( ranger single use system ) and glidescope cobalt ( gvl system ) ( verathon medical , bothell , wa , usa ) , king vision channelled and mac ( king systems , noblesville , in , usa ) , mcgrath series 5 and mac edl ( aircraft medical , edinburgh , uk ) , venner ap advance channelled and mac ( venner medical , kiel , germany ) , airtraq a-011 ( prodol meditec , guecho , spain ) , pentax aws - s100 ( hoya corporation , tokyo , japan ) and a classic direct laryngoscope ( brite blade led ) ( karl storz , tuttlingen , germany ) .
measurements were performed at three different locations ; the photographers ' dark room , the operating theatre and outside , in bright sunlight ( copying endotracheal intubation in a prehospital setting ) .
the investigator placed the vls in the manikin 's mouth , achieving the best possible view of the glottis . once achieved
, width and depth of vision were analyzed by scoring the cormack and lehane grade .
the cormack and lehane grade was scored to make the quality of laryngoscopy as uniform as possible ; it was not used as a way to score oral illumination .
hereafter , a professional photographer , knowledgeable in measuring light intensity , measured light intensity from the opening of the oral cavity , using a sekonic l-758cine digital master light meter ( sekonic corporation japan , tokyo , japan ) , set to cd / m .
the light meter was placed inside the oral cavity . due to the size of the light meter , this was within 2 cm of the mandibular dental margin .
all vls had fully loaded new batteries inserted , or were used while directly connected to mains electricity .
light intensity was measured three times per vls and defined as luminance ( cd / m ) , the amount of light emitted or re - emitted per unit area of a surface in a given direction . at the operating theatre , all ceiling lamps were lit along with one satellite lamp . this satellite lamp ( kls martin
, tuttlingen , germany ; vac 50/60 hz , 24 vdc , 300 va ) was directed towards the thorax of the same laerdal manikin , resembling normal endotracheal intubation conditions .
after the experiment , we analysed the illumination by the different vls by comparing the values of luminance ( cd / m ) .
a cormack and lehane grade 1 was achieved with every laryngoscopy . as depicted in tables 1 and 2
, the measurements of luminance differed substantially among devices . in the photographer 's dark room ,
luminance ranged from 1.2 cd / m ( pentax aws ) to 73.2 cd / m ( c - mac ) .
light intensity ( luminance cd / m ) per non - channelled ( video ) laryngoscope ( macintosh design blade ) light intensity ( luminance cd / m ) per channelled video laryngoscope in the operating theatre , large differences in luminance were measured among the vls , ranging from 2.7 cd / m ( mcgrath series 5 ) to 110.0 cd / m ( c - mac ) .
all vls , except mcgrath series 5 and mcgrath mac , produced substantial higher luminance values in a well - illuminated environment compared to the dark photographer 's room [ figures 1 and 2 ] .
light intensity measured in non - channelled ( video ) laryngoscopes light intensity measured in channelled ( video ) laryngoscopes our experiments outside - in bright sunlight - were interfered by direct sunlight penetrating through the synthetic material of the manikin .
this caused the light meter to measure the intensity of sunlight instead of light intensity of the laryngoscopes .
the existing vls mainly differ whether they use : ( 1 ) channelled or non - channelled blades ; ( 2 ) classic macintosh curved , or more acutely angled blades ; and ( 3 ) whether the illumination results from halogen , xenon bulbs or from led light origin .
all of the above have major implications for the anaesthesiologist . a clear visibility of the oropharynx and laryngeal entrance and the availability of substantial space in the mouth for manoeuvring an endotracheal tube and other adjuncts ( e.g. , gum elastic bougie , stylets , oro / nasogastric tube , magill forceps ) are keys to success .
so although less than optimal illumination does not have to result in unsuccessful endotracheal intubation , the risk of trauma remains .
indeed , it is important that we clearly see what we do during any manipulation in the mouth to avoid traumatising tissue .
blind spots is known to cause palatal , palatopharyngeal and tonsillar trauma . for direct laryngoscopy
, the minimum required luminance is reported to be 100 cd / m at the centre of the light field .
the optimum dimensions of the light field hereby were however not reported . to the best of our knowledge ,
although we can not determine the minimum required amount of illumination to prevent oral trauma , when the amount of illumination is 40 times less than the minimum required for direct laryngoscopy , the caregiver should be well - aware of this . over the years
, illumination of the oral cavity during direct laryngoscopy was improved significantly by changing the filament bulbs , and later by changing the halogen or xenon bulbs into the more solid - state led lights .
compared to conventional bulb - on - blade laryngoscope lighting systems ( using halogen light ) , led bulbs use very little energy , operate with less heat , produce a brilliant light and have a much longer lifespan than the earlier bulbs ( 15,000 - 30,000 h compared to approximately 1000 h ) , thereby eliminating the need for bulb replacement . improvement in quality and longevity of batteries and using the mains , further improved illumination options and visibility for both direct and vls .
one can argue if the reported minimum required luminance of 100 cd / m for direct laryngoscopy also holds true for vls . indeed
, the techniques are not simply interchangeable . on the other hand , the importance of seeing what one is doing while passing an endotracheal tube ( especially one with a rigid stylet in situ ! ) is applicable to both techniques . when applying this limit of 100 cd / m to this study , illumination in all tested vls ( except the c - mac ) is poor when used in the operating theatre .
our finding , that due to extensive glare on the monitor screen of different vls endotracheal intubation in bright sunlight while looking at the screen is severely hampered , needs further investigation .
the material of the manikin is penetrated by direct sunlight , in contrast to human skin . as a consequence
, we can not draw conclusions from this part of our study , but we can conclude that studies in bright sunlight involving manikins have severe limitations .
prehospital endotracheal intubation has highly variable success rates and can be challenging due to patient immobilisation , entrapment , air medical evacuation , constrained access to the patient , poor patient positioning and variable ambient lighting .
our finding ( hampered endotracheal intubation due to extensive glare ) could , however , have major implications for the caregiver in the prehospital setting ( e.g. , ambulance , air - ambulance services and war zone circumstances ) and should be taken into account when choosing a device .
vls , using channelled vls , may impair direct visibility of the oral cavity by the significant additional volume of the blade itself or the acute angle of the blade , whereby the light produced at the top illuminates more the anterior part of the laryngeal entrance , and not necessarily the whole mouth .
this is in contrast with vls that uses a macintosh design blade whereby a larger part of the oral cavity is illuminated .
we believe that the triad : ( 1 ) less room to manoeuvre in the mouth ; ( 2 ) distraction by  looking at the monitor screen outside the mouth : and ( 3 ) the use of a sharp , rigid styletted endotracheal tube combined with less optimal illumination possibly resulting in 
blind spots , may pose a greater risk to harm the patient ( oesophageal intubation and trauma ) .
the goal of the study was to compare the intensity of illumination between different vls . to compare different devices in exactly the same circumstances ,
it would have been necessary to place all the different vls in the same persons ' mouth .
light intensity in humans may be different due to absorption and/or reflection by the manikin .
comparing maybe one or two different devices could perhaps be done in a group of patients / volunteers .
second , background light ( in the operating theatre ) could possibly have influenced the measurements more than the light emission per se .
however , we performed a baseline recording when we compared illumination by the different devices in a photographer 's dark room . this way , we got a primary idea on the amount of light emitted by the vls , and when using a vls , virtually there always is background light .
when comparing illumination , we chose to place the light meter at the opening of the oral cavity .
the eye of the operator registers the amount of light from the opening of the oral cavity when manoeuvring the endotracheal tube before looking at the video screen . as stated by vincent and colleagues 
it is important to advance the endotracheal tube beyond the uvula before directing one 's attention to the video monitor. precisely , the fact that the amount of light emitted and the amount of light reaching the eye of the caregiver may significantly differ could pose an extra risk to the patient . when the image on the screen is well - lit , but there is little light reaching the eye when one is looking into the oral cavity and not looking at the video screen , one is still poking in the dark , which poses a subsequent risk to the patient ! even when the amount of light emitted is enough to ( easily ) intubate a patient ( although maybe not the preferred 100 cd / m ) , suboptimal illumination poses a risk to the patient , especially that of palatopharyngeal trauma .
we wanted to investigate the relationship between light emission by vls and oral cavity illumination .
the relationship between light emission at the oral cavity and light emission at the laryngeal entrance is not well - described in current literature .
furthermore , a search for suitable equipment small enough to measure light emission from the laryngeal entrance learned that it is not available .
third , the synthetic material of the manikin may not only be penetrated by direct sunlight but possibly also by the light of the lamps in the operating theatre .
when this is true , the amount of penetration would have been the same for all devices for all measurements and thus does not influence the comparison between devices . the fact that in the operating theatre for the mcgrath series 5 and mcgrath mac the amount of light measured is less ( mcgrath series 5 ) and the same ( mc grath mac ) as in the photographer 's dark room , contradicts penetration of the synthetic material of the manikin .
it is also possible that the material of the manikin reflected the light of the devices .
when this is true , this may result in a different level of illumination when the vls are used in humans , but again this is valid for all devices , which makes comparison still possible .
fourth , we did not investigate if there are differences when a device is connected to mains instead of working on a ( fully charged ) battery .
possibly , the light output is better when a vls is connected to mains . finally , we used a led lit direct laryngoscope ( karl storz , tuttlingen , germany ) , as which was the standard in hospital use for endotracheal intubation
. it would have been interesting to know how a direct laryngoscope using a conventional light bulb would perform , compared to different vls .
illumination is best when using the c - mac vls . comparing these results with the recommendations made previously for direct laryngoscopy ,
illumination is poor with all vls tested , except for the c - mac when used in the operating theatre .
background and aims : insufficient illumination of the oral cavity during endotracheal intubation may result in suboptimal conditions . 
 consequently , suboptimal illumination and laryngoscopy may lead to potential unwanted trauma to soft tissues of the pharyngeal mucosa . 
 we investigated illumination of the oral cavity by different videolaryngoscopes ( vls ) in a manikin model.methods:we measured light intensity from the mouth opening of a laerdal intubation trainer comparing different direct and indirect vls at three occasions , resembling optimal to less - than - optimal intubation conditions ; at the photographer 's dark room , in an operating theatre and outdoors in bright sunlight.results:substantial differences in luminance were detected between vls . 
 the use of led light significantly improved light production . 
 all vls produced substantial higher luminance values in a well - luminated environment compared to the dark photographer 's room . 
 the experiments outside - in bright sunlight - were interfered with by direct sunlight penetration through the synthetic material of the manikin , making correct measurement of luminance in the oropharynx invalid.conclusion:illumination of the oral cavity differs widely among direct and indirect vls . 
 the clinician should be aware of the possibility of suboptimal illumination of the oral cavity and the potential risk this poses for the patient .