Laryngoscope Blade Design

Overview

Design Components of Laryngoscope Blades

Laryngoscope Light Systems

Batteries, Energy Draw, and the Inverse-Square Law 

Laryngoscope Light Intensity in Actual Use

Curved Laryngoscope Blade Design

Straight Laryngoscope Blade Design

BUY LARYNGOSCOPY EQUIPMENT

Overview

Equipment is an important component of patient safety and best practice in airway management.  Equipment failure and poor quality laryngoscopes are especially hazardous in emergency airways and following rapid sequence intubation. The clinical scenarios common to emergency airway management, i.e., blood, vomitus, secretions, distorted anatomy, cervical spine precautions, dynamically deteriorating patients, etc. all add technical challenges to any intubation technique, including laryngoscopy. 

Laryngoscope blade design, light, and battery systems affect procedural performance since they impact on laryngeal exposure, tracheal tube delivery, and illumination.  This holds true for both straight and curved laryngoscope blade designs, but because these designs function differently, there are different considerations.

Design Components of Laryngoscope Blades

The principal components of a laryngoscope blade are the spatula (that passes over the lingual surface of the tongue) and the flange that is used to direct the tongue toward the left side of the mouth (contacting the patient’s right buccal tongue surface). Laryngoscopy, both in terms of laryngeal visualization and also tube delivery, is performed down the right side of the mouth. Laryngoscopes are a left-handed instrument, with the operator’s right hand used to pass the tube. Ever since the early pioneers described the procedure it was appreciated that tongue displacement to one side would facilitate reaching the larynx. Since an operator would want their dominant hand free for instrumentation, the laryngoscope became, by default, a left-handed instrument (appropriate since 85% of the population is right hand dominant). The other benefit of right paraglossal blade placement, target visualization, and tube delivery is that the curvature of the upper dentition favors the placement of a rigid instrument (laryngoscope, bronchoscope, etc.) away from the patient’s central incisors, which would otherwise limit advancement. In the right corner of the mouth there is room to insert and tilt the laryngsocope backward (for lifting epiglottis with a straight blade, for example) without impacting on the teeth. In modern elective anesthesia dental injury is also a major concern, and another reason why operators should pay close attention to keeping the flange away from the central incisors. The flange of the laryngoscope should never be leveraged backward against the teeth.

In general, straight blade laryngoscopes have a smaller displacement volume (defined by the dimensions of the spatula and flange) and lift the epiglottis directly, while curved blades have a larger displacement volume (spatula and flange) and lift the epiglottis indirectly. This makes intuitive sense, as straight blades must be passed deeper, under the epiglottis, while curved blades need only reach the vallecula and apply pressure to the underlying hyoepiglottic ligament. Viewed in this manner, it is also appropriate that straight blades are favored in patients who have a small displacement space (namely a small distance from the chin to the thyroid cartilage), since this is the area the tongue gets pushed into during direct laryngoscopy. Examples of such patients are small children (below the age of 8, but especially below age 5) and adults who have a receding chin.

Laryngoscope Light Systems

Laryngoscope lighting systems can be divided into those with a light source mounted directly on the blade, and those in which the light source is at the top of the handle.

Bulb-on-blade designs (sometimes referred to as conventional blades) have a simple electrical connection between the blade and the handle (with enclosed batteries). This connection is very robust and less subject to malfunction than the spring-loaded, on-off lights used with bulb-on-handle systems. A removable bulb allows replacement if the bulb fails. Conversely, it may become loose and flicker during operation. Some manufacturers fuse the bulb on the blade eliminating this risk, but thereby making the bulb non-replaceable. Laryngoscope bulbs can be made with an incandescent filament (tungsten with halogen gas), xenon gas, or from a light emitting diode (LED). The bulb itself can have either a frosted or clear lens, and sometimes also includes a specialized reflector (common with bulb-on-handle designs).

Compared to other light producing systems, LED bulbs use very little energy, operate with less heat, and have a much longer life span, thereby eliminating bulb replacement as a major concern. They now can be produced at less cost than other bulbs and produce brilliant light. The light from an LED tends to be whiter and bluer than traditional bulbs, and the author subjectively believes this helps with tissue edge discrimination. It is not coincidental that all of the newer devices for intubation (video laryngoscopes, mirror laryngoscopes, chip-on-stick CMOS imaging devices, etc.) all use LED lights.

With bulb-on-handle systems a light conducting fiber, either made of glass or plastic, conveys the light from the top of the handle to the distal portion of the blade. Although such blades are often called “fiber-optic,” they have no optical fibers, per se, and a more appropriate term is “fiber-lit”. Glass fibers conduct light more efficiently, but cost significantly more. Disposable blades commonly use a light-conducting bundle made of plastic, whereas non-disposable fiber-lit blades all use glass fiber bundles. In the U.S. any blade or handle that uses fiber illumination has a green dot on the blade base and a green circle at the top of the handle. It is important for clinicians to appreciate that fiber-lit blades and handles and conventional blades and handles are not interchangeable.

Fiber-lit and conventional handles and blades are not interchangeable. In the U.S. fiber-lit blades have green dot on base and fiber handles have green ring near top. Fiber-lit laryngoscopes use glass or acrylic fiber bundles to conduct light from the handle, along the blade, and to the distal tip.

Excellent disposable fiber-lit laryngoscope blades are now available in both plastic and stainless steel.
Original plastic blades were too slippery and too flexible. Heine XP disposable fiber lit blades (top) have great strength and excellent illumination via acrylic light bundle. Disposable, stainless steel, fiber-lit blades with acrylic stems are now produced by many manufacturers.

Batteries, Energy Draw, and the Inverse-Square Law

A critical and essentially unexamined area of laryngoscope illumination involves batteries. Alkaline batteries have a gradually declining discharge curve.  Unfortunately, in a high-intensity device, like a laryngoscope, the energy output from used batteries can be a fraction of the energy output (and resultant light) with new batteries. This may be unappreciated by clinicians, who generally only open the blade and see if the light comes on.  Lithium batteries have a much flatter, higher discharge curve than alkaline batteries, but die precipitously once the energy output falls.  Lithium batteries are much more expensive and also generate more heat than alkaline batteries. Some manufacturers, especially those producing high quality fiber-lit blades, offer nickel-metal-hydride rechargeable battery systems, which produce very intense light when combined with a xenon bulb and glass fibers.  While the light output from these high-end fiber-lit systems is impressive, they are very expensive.  Newer LED technology has the potential to rival the light output of such a system at a fraction of the cost, with little energy draw, in a single-use, disposable, bulb-on-blade design.

Regardless of the light type, the intensity of light reaching the distal end of a laryngoscope is dependent on the distance from the light to the distal tip. This phenomenon is governed by the inverse-square law of physics; if the distance from the light source to an object is doubled, the resultant amount of light energy reaching the object is reduced to one quarter of the original amount.  Blade designs with a shorter light-to-tip distance create more intense distal light.

Laryngoscope Light Intensity in Actual Use

 Few clinical settings monitor the light output of their laryngoscopy equipment with light meter testing.  Unlike in dentistry or surgery, where lighting standards are defined (5000 lux is recommended), there is no light intensity standard for the distal end of laryngoscopes.  In the presence of blood, secretions, and vomitus, common to emergency airways, more light is needed to discriminate landmarks.  Because most laryngoscopes use alkaline batteries, that have a long and slow discharge curve, the light may continue to turn on when tested but have very low light output. In a study of 17 Philadelphia area emergency departments, there was a 500 fold difference in light output using a light meter between the best and worst laryngoscope handle and blade pairs. A large percentage of laryngoscopes tested did not produce enough light at the distal tip to meet even ambient light standards for an emergency department:

Levitan RM, Kelly JJ, Kinkle WC, Fasano C. Light intensity of curved laryngoscope blades in Philadelphia emergency departments. Ann Emerg Med. 2007; 50: 253-7.

Curved Laryngoscope Blade Design

The term “Macintosh blade” is generally used to mean any curved blade, however since Macintosh’s original description in 1943 there are several commonly produced Macintosh styles that are distinguishable by their flange height, flange shape, light position, and light type (Macintosh RR.  A new laryngoscope.  Lancet 1943; 1:205.).

These designs are commonly labeled by their geographic manufacturing origins, i.e., American (a.k.a. “Standard”), English (a.k.a. “Classic”), and German designs.  The common features are a gently curved spatula and a large reverse Z-shaped flange.

American blades closely follow Macintosh’s original description, i.e., a large vertical, square-shaped, proximal flange that does not extend to the distal tip, coupled with a bulb-on-blade illumination system.  The English design has a smaller, curvilinear proximal flange that runs all the way to the distal tip, but also uses a conventional light.  Heine of Germany developed a fiber-lit blade that follows the English contour in terms of a short proximal flange and complete flange.  It uses a large rectangular-shaped 5mm glass fiber bundle.  The English and German designs each have a much shorter light-to-tip distance than the American design.  Most American designs use a frosted bulb, while most English designs have a clear lens.  Given the variety of manufactured blades and the desire of manufacturers to offer different lighting systems, there are now American and English designs offered with fiber illumination. Numerous manufacturers around the world now offer “American”, “English” and “German” curved blades, and many blades have a mix of features.

American design has large proximal flange height and relatively long distance between bulb (light) and tip of blade.  German design has smaller proximal flange and smaller distance from end of fiber bundle to tip of blade.  G-Mac flange goes all the way to tip, unlike American blade.  Small flange of G-Mac permits using #4 blade in any adult. American Mac #4 flange may be problematic in smaller adults, patients with large upper teeth, or limited mouth opening.  English Mac (not shown) has G-Mac blade shape with bulb.

It is interesting to note that Macintosh envisioned one adult size for his blade (corresponding to approximately a Macintosh size 3). Because some practitioners requested a longer blade, a larger size was made, and then subsequently smaller sizes as well (for pediatric use). In the author’s opinion, a Macintosh 4 is the preferred blade for emergency use in adults, assuming a German of English style flange is chosen. This allows greater depth insertion if needed without any difference in flange height between the size 3 and 4 blades. The author also favors the shorter light-to-tip distance (and light source common to German or English designs) since they provide better illumination relative to the American design.
	Size of fiber bundle significantly affects light transmission; Larger bundle on G-Mac (5 mm bundle on right vs. 3 mm bundle on left) transmits more light. Glass fibers transmit light better than acrylic.
A recent variation of the Macintosh design is the McCoy or CLM levering laryngoscope blade. This blade is a Macintosh design with an articulating distal tip that when activated is intended to elevate the tissue at the base of the tongue (improving epiglottis lift and laryngeal exposure). This blade has become quite popular in the U.K. (where it originated) but published clinical investigations have reported mixed results.
Straight Laryngoscope Blade Design Robert Miller’s straight blade design in 1941 adapted the straight shape of early laryngoscopes but added a slightly upturned distal tip and narrow flange (Miller RA. A new laryngoscope. Anesthesiology. 1941; 2: 318–20.).  The flange had a compressed D-shape (when viewed longitudinally) but the height was large enough to accept a 37 French Argyle tube down the barrel.  Compared to tubular shaped blades (Jackson-Wisconsin, for example) the much shallower proximal flange minimized dental impaction problems.  The light was placed at the distal tip on the right side of the spatula, opposite the flange and tilting toward midline.   Since Miller’s original description various manufacturers have compressed the flange height, and some have changed the bulb location (to the left flange edge, or recessed within the flange).  Most Miller designs currently made for adults cannot accept an adult sized tube down the barrel. While passing a tube down the barrel is not ideal, since it blocks the line of sight to the target, the very narrow design of modern Miller blades necessitates careful paraglossal placement (the small flange cannot sweep the tongue) and the extreme right corner of the mouth must be used for tube delivery.  Another challenge to using narrow flange straight blades is that it creates a small area for landmark recognition down the barrel.   Landmark recognition and ease of tube delivery improves as the flange height and spatula size of a straight blade is increased.  Paradoxically, it gets harder to introduce the blade alongside the tongue, and reach the larynx, as the displacement volume of the blade increases.  This was known to Miller, who shortened his flange height, but left the resulting D-shaped barrel large enough to accept a tracheal tube.     The upturned distal tip of the Miller blade is shown at top.  Inset left shows the narrow flange height of modern Miller designs.  There is great variation in current Miller designs; the original Miller design had light on right side of flange when looking down blade (far right).  Poorly designed Miller blades (far left in group) have the light on the leading edge of the left flange (the light embeds in the tongue and produces very poor illumination). Straight blade designs with larger flanges (and spatulas) than the Miller design include the Phillips (a 2/3 small “C shape flange), Wisconsin (a higher, nearly full “C” shaped flange) and the Guedel (a very large, sideways “U” shaped flange and spatula).   Straight blade paradox – larger lumen blades provide a larger working area and easier tube passage, but are harder to get to larynx (i.e., need to displace more tissue). From left to right with increasing blade volume and higher flanges: Cranwall, Miller, Phillips, Wisconsin, and Guedel blades. An ideal straight blade is the relatively recently introduced Henderson blade (Karl Storz Endoscopy).  This straight blade has small incomplete 2/3 “C” shaped flange, but is large enough for tube delivery if needed.  It also has a uniquely visible distal tip (a knurled edge at the distal blade tip, visible when viewed down the barrel), and a large, recessed, fiber bundle light source. Barrel of Henderson blade fits an adult tube (left).  Flange height of Henderson (right) is a 2/3 C-shape, with a distal visible knurled tip (visible in laryngoscopy image, center).