8.2.1 What Are the Principal Design Components of Laryngoscopy Blades and How Do They Work to Facilitate Endotracheal Intubation?
Laryngoscope blade design, light, and battery systems affect procedural performance since they impact on illumination, laryngeal exposure, and endotracheal tube (ETT) delivery. This holds true for both straight and curved laryngoscope blade designs, but because these designs function differently, there are different considerations (see below).
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 (Figure 8-1), a fluid-filled noncompressible structure, to the side of the mouth and into the mandibular space (the space below the tongue). This concept of mandibular space volume is particularly important in clinical practice as the practitioner evaluates for difficult laryngoscopy and intubation (see Chapter 1).
Design of the laryngoscope blades. (Modified with permission from Dorsch JA, Dorsch SE: Laryngoscopes. In: Dorsch JA, Dorsch SE (Eds). Understanding Anesthesia Equipment, 2nd ed. Williams & Wilkins; Baltimore, 1984)
Straight blades, such as the Magill blade (see Chapter 1, Figure 1-1), were originally designed to pick up the epiglottis and elevate it directly, while the curved Macintosh-type blades are intended to be advanced into vallecula and indirectly elevate the epiglottis by applying pressure to the hyoepiglottic ligament. These factors illustrate two distinguishing features of these blade designs: that straight blades are inserted more deeply than curved blades; and that curved blades have an atraumatic tip design to reduce the risk of vallecular injury.
Because laryngoscopy was originally an operative technique where the practitioner needed their dominant right hand (85% of the population is right-hand dominant) to be free to operate, the laryngoscope became by default a left-handed instrument.
Laryngoscopy and intubation are performed through the right side of the mouth. The left hand is used to insert the laryngoscope blade into the mouth to expose the glottis. The right hand is then free to perform a variety of tasks including the insertion of an intubation aid (eg, Eschmann Tracheal Introducer, [ETI]), manipulate the larynx, lift the head, suction the airway, and ultimately, pass the tube.
Early pioneers employing straight blades recognized that tongue displacement to one side facilitated laryngeal visualization, particularly if the blade of the laryngoscope was inserted in the corner of the mouth and along the paraglossal gutter. Importantly, it was recognized that this paraglossal or retromolar technique optimized the laryngeal view mostly because it shortened the distance between the teeth and the larynx (ie, the molars are closer to the larynx than the incisors). The other benefit of right paraglossal laryngoscopy is that the rigid laryngoscope blade impacts the molar teeth rather than the relatively more fragile central incisors. The flange of the laryngoscope remains a threat to dentition and should never be leveraged backward against the teeth.
Straight blade laryngoscopes tend to have smaller displacement volumes (defined by the dimensions of the spatula and flange) than curved designs. It is logical, therefore, that straight blades (and a paraglossal approach) are favored in patients who have a small mandibular volumes into which the tongue is displaced (or compressed) 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.
8.2.2 What Are the Variables that Determine the Illumination Created by a Laryngoscope Blade?
Illumination is critically important for direct laryngoscopy. Bright light is necessary for tissue edge and color discrimination, and the identification of tissues and structures. This is particularly important in preterm infants where the appreciation of subtle color differences is critical to intubation success.
Laryngoscope lighting systems can be divided into those with a light source mounted directly on the blade (bulb-on-blade), and those in which the light source is at the top of the handle (bulb-on-handle).
Bulb-on-blade designs (sometimes referred to as conventional blades) have a simple electrical connection between the bulb socket on 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. These removable bulbs can usually be replaced if they fail. This feature confers the risk that should the bulb become loose it may flicker during operation, or worse yet, become dislodged and lost into the patient.5,6 To eliminate this risk, some manufacturers fuse the bulb to the blade rendering the bulb nonreplaceable.
Laryngoscope bulbs for both designs are of several types: incandescent filament (tungsten with halogen gas), xenon gas, and light-emitting diodes (LED). The bulb itself can have either a frosted or clear lens, and may incorporate a 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. All of the newer intubation devices (video laryngoscopes, mirror laryngoscopes, chip-on-stick CMOS-imaging devices, etc) use LED lights.
With bulb-on-handle systems, a light-conducting fiber, made of either 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 nondisposable fiber-lit blades use glass fiber bundles. In the United States, 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 (commonly referred to as a green-line handle). It is important for practitioners to appreciate that fiber-lit blades and handles and conventional blades and handles are not interchangeable.
A critical and essentially unexamined area of laryngoscope illumination involves batteries.7
Alkaline batteries have a gradually declining discharge curve. Failure to appreciate and rectify this declining illumination may compromise direct laryngoscopy by low light, heralded by a difficult or failed intubation.
Lithium batteries have a much flatter, higher discharge curve than alkaline batteries, but fail precipitously once the energy output falls below a certain threshold. Lithium batteries are much more expensive and 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 these systems at a fraction of their cost, draw little energy, and are offered in a single-use, disposable, bulb-on-blade design.
Regardless of the type of light, the intensity of light reaching the distal end of a laryngoscope blade is dependent on the distance the light must travel. This phenomenon is governed by the inverse square law of physics, that is, 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. So, generally, blade designs with shorter light-to-tip distances create more intense distal light. This produces substantial variability in the amount of light emitted by different combinations of blades and handles used in clinical practice. In a study conducted in emergency departments, there was a 500-fold difference in light output between the best and worst blade-handle combinations.8
Few clinical settings monitor the light output with light meter testing. Lighting standards in dentistry or surgery recommend 5000 lux.9 While there is no well-accepted light intensity standard for the laryngoscopes, the International Organization for Standardization has suggested 700 lux as a minimum light output for laryngoscopes.10 In the presence of blood, secretions, and vomitus, common to emergency airways, more light is needed to discriminate landmarks.10
8.2.3 What Are the Distinguishing Features of Commonly Used Curved (Macintosh) Blade Designs?
The term "Macintosh blade" is generally used to mean any curved blade. However, since Macintosh's original description in 19434 several variations have been produced that are distinguishable by their flange height, flange shape, light position, and light type. These designs are commonly designated by their geographic manufacturing origins, that is, American (commonly known as "Standard"), English (commonly known as "Classic"), and German designs. The common features are a gently curved spatula and a large reverse Z-shaped flange (Figure 8-1).
American blades closely follow Macintosh's original description, that is, 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, and 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. A large rectangular-shaped 5-mm glass fiber bundle is incorporated in the flange. The English and German designs have a much shorter light-to-tip distance than the American design (Figure 8-2). Most American designs use a frosted bulb, while most English designs have a clear lens. American and English designs now offer fiber illumination options. Numerous manufacturers around the world now offer American, English, and German curved blades, and many blades have a mix of features.
Design of the curved Macintosh laryngoscope blades. The German design has a much shorter light-to-tip distance than the American design.
It is interesting to note that Macintosh envisioned one adult size for his blade (corresponding to approximately a Macintosh size 3). Market demand lead to the current variety of pediatric and adult sizes available. Size selection is largely a matter of patient size and practitioner choice. No matter the size chosen, it should be noted that the most common error of the novice is inserting the blade too deeply and into the upper esophagus before visualization is performed. The shorter light-to-tip distance (and light source common to German or English designs) also provides better illumination relative to the American design.
A recent variation of the Macintosh design is the McCoy (also known as Corazelli-London-McCoy [CLM]) levering laryngoscope blade (Figure 8-3). 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 United Kingdom (where it originated), but published clinical investigations have reported mixed results.11-16
The McCoy (also known as Corazelli-London-McCoy [CLM]) levering laryngoscope blade.
8.2.4 What Variations Exist between Miller Blade Laryngoscope Designs?
Robert Miller's straight blade design in 1941 adapted the straight shape of early laryngoscopes, in particular that of Magill (see Figure 1-1), but added a slightly upturned distal tip and narrower flange.3 The flange had a compressed D-shape (when viewed longitudinally) with a height large enough to accept a 37 French Argyle tube. Compared to tubular shaped blades (eg, Jackson-Wisconsin) the much shallower proximal flange was intended to minimize dental injury (Figure 8-4). The light was situated at the distal tip on the right side of the spatula, opposite the flange side, and tilted toward midline (Figure 8-1).
Different designs of the straight laryngoscope blades with different tubular shapes.
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). Designs with light sources located on the exposed edge of the left flange are less preferred by some since a light at this location can become embedded in the tongue with resultant poor illumination. Most Miller designs currently made for adults cannot accommodate an adult-sized cuffed endotracheal tube down the barrel. In addition, tube passage down the barrel 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 (which often requires manual retraction by an assistant) must be used for tube delivery. Alternatively, an ETI can be employed. Another challenge of the narrow-flange straight blades is that it makes landmark recognition down the barrel difficult.
8.2.5 What Is the "Straight Blade Paradox?"
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 an ETT.
Straight blade designs with larger flanges (and spatulas) than the Miller design include the Phillips (a two-third small C-shaped flange), Wisconsin (a higher, nearly full C-shaped flange), and the Guedel (a very large, sideways U-shaped flange and spatula) (Figure 8-4).
The Henderson straight blade has small incomplete two-third C-shaped flange that is large enough for tube delivery. It also has a uniquely visible distal tip (a knurled edge at the distal blade tip is visible when viewed down the barrel), and a large, recessed, fiber bundle light source (Figure 8-5).
The Henderson laryngoscope blade.
8.2.6 Apart from the McCoy and Henderson Blades Already Mentioned, Are There Other Recent Blade Designs that Might Be of Use?
The Dorges universal blade is intended to replace Macintosh size 2 to 4 blades with one blade for all patients from age 1 to adult.17 The curve is much reduced, the spatula is tapered from proximal end to distal tip, and the proximal flange height is very short (15 mm), allowing it to be used with children and those with limited mouth opening, while at the same time permitting deeper insertion in larger adults owing to its length (Figure 8-6).
The Dorges universal laryngoscope blade.
The Grandview blade is an emergency blade for adult patients that is available in two sizes.18 It combines a very wide spatula with a slight overall curve and a narrow proximal flange (Figure 8-7). The resulting blade can be used to lift the epiglottis directly or indirectly.
The Grandview universal laryngoscope blade.