DL is the creation of a direct line-of-sight to the larynx using a rigid blade. The two primary structures that must be manipulated in order to directly expose the larynx are the tongue and the epiglottis. Anatomic variables that can affect laryngoscopic exposure include mouth opening, dentition, and cervical spine positioning.

DL is fundamentally dependent on the quality of illumination and the operator’s visual acuity. Proper positioning, a stepwise approach to tongue and epiglottis control, and an overall understanding of mechanics are required for procedural success.

Straight laryngoscopy blades displace a smaller amount of tissue than do curved blades. They are best used with a paraglossal approach, keeping the proximal portion of the blade in the right corner of the mouth, and not impacting on the central dental arch or teeth. The small flange size of most straight blades does not allow sweeping of the tongue. The epiglottis is elevated directly with the tip of the blade. Tube delivery should be from the extreme right side of the mouth, avoiding the barrel of the blade, which would block the line-of-sight. The Miller design, created in 1941, is the most common of the straight blades. It has a small flange height and a flattened D-shaped barrel. Straight blades are commonly used in infants and children who tend to have a relatively small displacement space. Because of the very small tube size relative to the blade lumen size, tube delivery with straight blades is easier in children compared to adults.

In 1943, Macintosh observed that direct visualization of the larynx could sometimes be obtained with a Boyle-Davis mouth gag, an instrument commonly used by otolaryngologists. Its curved laryngoscope blade design has a natural fit against the curvature of the tongue. It lifts the epiglottis by indirect pressure on the hyoepiglottic ligament at the base of the vallecula.

Intubation via DL can be broken down into three components: (1) epiglottoscopy; (2) laryngeal exposure; and (3) tube delivery.

The epiglottis is roughly halfway between the tongue and the glottic opening. Unlike any other laryngeal landmark, it has a horizontally oriented edge. Head positioning, specifically elevation of the head until the ear and sternal notch are aligned, and keeping the face plane parallel to the ceiling, are critical to avoid pivoting the tongue base and epiglottis against the hypopharyngeal posterior wall. Atlantooccipital extension impedes epiglottis elevation, narrows the space for laryngeal exposure, and prevents maximal mouth opening. In a supine orientation, head-elevated positioning is fundamental for the mechanics of laryngoscopy and vital for proper preoxygenation and passive apneic oxygenation, since it reduces alveolar collapse and improves functional residual capacity (FRC). The most useful mechanism of improving laryngeal view, after the epiglottis has been identified, is for the operator to apply direct pressure on the larynx. This bimanual laryngoscopy technique pushes the larynx down into the operator’s line-of-sight. When using a curved blade, bimanual manipulation also optimizes the tip position in the vallecula and more effectively transmits force to the hyoepiglottic ligament. A second technique to improve the view after epiglottis identification, is to increase head elevation. This improves mouth opening, shortens the distance to the glottis, and aligns the laryngoscopic view and cervical trachea with the natural forward inclination of the thoracic trachea.