Pre-hospital emergency cricothyrotomy in dogs part 2: Airway sealing and ventilation using cricothyrotomy tubes

Abstract

Cricothyrotomy (CTT) has been recommended for use in the pre-hospital setting for military working dogs and Operational K9s during airway emergencies. Although the CTT can establish a patent airway for spontaneous ventilation, the ability to seal the airway and provide positive pressure ventilation (PPV) using tubes designed for humans has not been determined. Using various CTT tubes placed in cadaver dog airways, this study aimed to determine: (1) Whether the tube cuff could create a functional airway seal with safe intra-cuff pressures; (2) The magnitude of delivered tidal volume (TV) loss during a standard breath to assess the possibility of delivering an adequate tidal volume with a bag-valve device (BVM); (3) The best performing tubes for either test; (4) The reasons behind the findings using observations from upper airway endoscopy, dissection, and measurements. Cadaver dogs of similar weights to MWD and Operational K9 breeds had various CTT tubes placed including three from commercial kits, a standard endotracheal tube, and a tracheostomy tube. The minimum occlusive volume technique was used to inflate the tube cuff and a pressure ≤ 48 cm H₂O with an adequate seal was considered successful. Individual TVs were calculated for each dog and added to the volume lost during delivery of a standard breath from an ICU ventilator. Endoscopy and airway dissection were performed to assess the relationship between tubes cuffs and the airway. The tubes from the CTT kits performed poorly with regards to producing an airway seal with the H&H tube failing to seal the airway all tests. Tracheal dimensions were significantly associated with successful airway sealing (P = 0.0004). Tidal volume loss could be compensated using a BVM in 34/35 tests with the H&H tube in cadaver 8 the only to fail. Tracheal airway sealing is influenced by airway anatomy when tube cuffs are inflated to a target pressure and larger tubes do not always provide a better seal. The CTT tubes tested have the potential to facilitate ventilation with a BVM under the conditions set in this study. The 8.0 mm endotracheal tube performed the best and the H&H the worst in both tests.

Keywords: CICO; H&H; Portex; airway—obstruction; cricothyrotomy; military working dog; operational K9; tracheostomy.

Figure 1

Caudal view of the larynx, with the ventral aspect toward the top of the image. The shape of the caudal cricoid is more circular when compared to the ovoid trachea (highlighted by the blue ring). The internal diameter at the narrowest point of the cricoid ring is smaller laterally, which is opposite to the trachea (white arrows).

Figure 2

Bar chart of successful airway seal (%) using the minimum occlusion volume technique with intra-cuff pressure ≤ 48 cm H₂O in five tubes used for cricothyrotomy in ten cadaver dogs. Success (%) describes a successful airway seal between tube cuff and trachea as defined as absence of audible air leak at a PIP of 15 cm H₂O and a cuff pressure of ≤ 48 cm H₂O. Tube type definitions: PCK–Portex^® PCK 6.0 mm I.D.; Melker–Melker^® cricothyrotomy tube 5.0 mm I.D.; H&H–H&H^® cricothyrotomy tube 6.0 mm I.D.; 8.0 mm–Generic, unbranded polyvinylchloride endotracheal tube 8.0 mm I.D.; 10.0 mm–Portex^® cuffed blueline ultra^® suctionaid 10.0 mm I.D.

Figure 3

Scatterplot of percent of successful airway sealing for all tubes by lateral to dorsoventral tracheal ratio with cadaver dog ID next to diamond markers, with line of best fit.

Figure 4

Line plot of (A) tidal volume leak (ml) at 15 cm H₂O PIP and 20 cm H₂O IC pressure by tube type. (B) Sum of calculated tidal volume and leak volume with red dashed line at 785 ml indicating potential for breath delivery using BVM (dogs 4–10).

Figure 5

Predicted tidal volume leak (ml) by tube type (dogs 4–10). Point estimates, whiskers = 95% CI.

Figure 6

Ventral is toward the top of the image. (A) The Melker tube is shown in situ in cadaver 2. Air can escape lateral to the cuff, particularly in tracheas with higher lateral to dorsoventral ratios. (B) The Melker tube is shown in a dissected portion of the trachea from cadaver 3. Contact on the dorsal and ventral trachea occurs first when inflated to a pressure of 20 cm H₂O. (C) Hyperinflated tube cuff from Melker tube, pulled proximally to lodge at the caudal cricoid ring in Cadaver 1. An effective seal can be observed as the spherical cuff conforms to the circular boundary of the caudal cricoid ring.

Figure 7

Image depicting the low volume, high pressure cuff of the H&H cricothyrotomy tube in cadaver dog 2. (A) Inflated to 20 cm H₂O, (B) inflated as much as possible >99 cm H₂O. The tube cuff contacts the airway at a single point and the IC pressure is unlikely to be transferred to the mucosa.

Figure 8

(A) The tube cuff of the 10.0 mm tracheostomy tube inflated to 20 cm H₂O conforms to the ovoid shape of the trachea in cadaver dog 3. (B) Folding of the 10.0 mm tube cuff within the relatively small trachea of cadaver dog 9, allowing channels for air leak. Cadaver 9 had air leak with the larger 10.0 mm tube but not the smaller 8.0 mm tube.