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The subglottic space refers to the portion of the airway between the vocal cords and the inferior border of the cricoid cartilage. Attention to surgical pathology in this area was overshadowed for many years by the advent of successful surgery for strictures and neoplasms in the rest of the trachea. Surgical forays into this anatomic area was restricted to trauma and the management of neoplasms, wherein the ablative procedure resulted in significant impairment in glottic function. With the introduction of compliant cuffs on endotracheal tubes, the incidence of cuff strictures after endotracheal intubation decreased significantly. In addition, there has been an increasing tendency to maintain oral or nasal endotracheal intubation for longer periods of time before reverting to tracheotomy. Although more prolonged endotracheal intubation has not been causally linked to subglottic stenosis, the latter appears to have increased in incidence over the last 15 years. Before 1974, resection of the subglottic area involved the sacrifice of the recurrent laryngeal nerves because surgeons performed standard sleeve resections of the airway even in this high position. In order to resect higher into the subglottic compartment, Dr D.P. Bryce described the resection of the anterior arch of the cricoid, sparing the posterior plate and, thus, preserving the recurrent laryngeal nerves.
It is, however, at the level of the posterior cricoid plate that the greatest degree of scarring is noted secondary to endotracheal intubation. Because of the curvature of the endotracheal tube, it's point of maximal contact and pressure with the tracheal mucosa is the posterior cricoid plate. In order to overcome this difficulty, Dr F.G. Pearson described subperichondrial resection of the posterior plate in order to permit removal of the scar and at the same time preserve the recurrent nerves.
The understanding of this technique is dependent on a thorough appreciation of the anatomy of the area.
Before considering surgical technique, it is important to understand the alternatives to surgical therapy based on the type of stricture. Fibrous strictures in the subglottic region are either inflammatory, idiopathic, or iatrogenic. Inflammatory strictures are most commonly seen with Wegener's granulomatosis and less commonly with amyloid and collagenosis. It may respond to dilation and drug therapy, but in the later fibrous stage it will usually require resection. Idiopathic strictures occur primarily in middle-aged women
It is the iatrogenic stricture secondary to endotracheal intubation that accounts for the majority of stenoses. This too may respond to dilatation and/or stenting in the early stages, but by the time a fibrous stricture has occurred, surgical therapy is the rule. Laser therapy has been described in the past. However it is the experience of the investigators that it offers little over simple dilation and may indeed prolong the period of inflammatory change. Certainly, it is rarely definitive therapy for fibrous subglottic strictures, which frequently are associated with chondritis and calcification of the cricoid cartilage. Under these circumstances, laser therapy cannot be expected to provide lasting improvement and indeed may increase the magnitude of the injury if there is failure to control the depth of coagulation. The use of silastic stents bears particular mention. In patients who present contraindications to surgical resection, a silastic “T” tube provides a stable airway. The upper limb of the tube is positioned above the vocal cords. In this position, the patient maintains voice and the ability to swallow without aspiration.
In cases of acute subglottic injury, it may provide definitive therapy if positioned before the establishment of a mature scar.
The required understanding of this regional anatomy begins with the cricoid cartilage. It is the only complete cartilage in the airway and because it is in the immediate subglottic area, it encircles the narrowest portion of the trachea (Figure I). It consists of a shallow anterior arch and a thick, deep posterior plate. The superior border of the cartilage articulates with the inferior cornua of the thyroid cartilage. It is immediately posterior to this joint that the recurrent laryngeal nerves pass as they enter the airway, proceeding superiorly to innervate the muscles controlling cord function. The thickness of the posterior plate of the cricoid and this position of the recurrent nerves are the major anatomic determinants of sleeve resection of the airway at this level. Superiorally and posteriorally, the cricoid articulates with the arytenoid cartilages, which attach by its vocal processes to the posterior limb of the vocal cords. This fact defines the limit of posterior dissection and resection. Although interarytenoid scarring can be excised and patched with a tongue of tracheal mucosa (see Surgical Technique), subperichondrial resection of the posterior plate can interfere with glottic function if allowed to proceed to the upper limit of the cartilage. Fortunately, the latter is not necessary in benign disease. The cricothyroid ligament provides attachment to the thyroid cartilage superiorally and anteriorally. It is a stout structure that holds anastomotic suture material well.
The vagus nerve supplies the glottic and subglottic area through the superior and recurrent laryngeal nerves. The former has two branches. It's first branch, the interior laryngeal nerve is sensory to the glottis above the vocal cords. It enters the larynx near the articulation of the thyroid and hyoid cartilages at the superior thyroid cornua. As a result, it is rarely in jeopardy during subglottic resection. It is more likely to be damaged during the infrahyoid laryngeal release.
The recurrent nerves are both motor and sensory. They innervate the intrinsic laryngeal muscles and provide sensation to the subglottic mucosa. The nerves enter the larynx posterior to the cricothyroid joint deep to the inferior pharyngeal constrictor.
A thorough bronchoscopic assessment of the airway by the operating surgeon is essential. This permits an evaluation of vocal cord function and of concomitant laryngeal injury. The surgical plan may be altered with prior knowledge of vocal cord paralysis or laryngeal damage such as posterior interarytenoid scarring (Figure II). The latter suggests the involvement of an otolaryngologist, as well as the fact that laryngofissure and laryngeal stenting may be necessary accompaniments of the subglottic resection. The bronchoscopy also permits dilation of the airway, which will facilitate the placement of an endotracheal tube by the anesthetist. When there is inflammation and/or ulceration or granulation in the stricture, a biopsy specimen for culture is prudent. In such circumstances, the airway may not only be colonized with pathogenic bacteria, but there may be associated bacterial chondritis. A positive culture warrants preoperative and postoperative antibiotic therapy. The entire airway should be examined to rule out associated disease, which is seen particularly in Wegener's granulomatosis and collagenosis of the airway.
A computed axial tomography (CAT) scan of the affected area delineates the extent of involvement of the tissues abutting the inner surface of the airway and may also delineate destruction of the cartilage when chondritis is present. When neoplasms are present, the external extent of the tumor will influence the surgical approach. A preoperative chest radiograph is particularly important because many of these patients either have or have had pulmonary sepsis that may be active, or in cases of chronic obstruction, be associated with bronchiectasis.
Dilation of the stricture at the time of the resection facilitates intubation. We prefer an armored endotracheal tube. The anesthetist should also have available an uncuffed no. 5 or 6 endotracheal tube that may be required at the end of the procedure (see Surgical Technique). On the operative set, another armored tube no. 7 is necessary for ventilation across the surgical field when division of the airway has taker place. There are frequent periods of apnea during the placement of anastomotic sutures that will over time lead to the development of atelectasis. Because intermittent positive pressure with positive end expiratory pressure appears to reinflate areas of atelectasis and thus, prevent the onset of hypoxemia, we do not recommend using jet ventilation. During the completion of the anastomosis, intermittent ventilation with 100% oxygen permits up to 2 minutes of apnea for suture placement. The anesthetist generally requires only one ventilation system because the endotracheal tube inserted by the surgeon upon division of the airway can be passed under the drapes to the anesthetist who simply uses the same system he/she employed for the initial oral endotracheal tube.
As far as monitoring is concerned, oximetry and end tidal carbon dioxide measurements are routine. Both are important when intermittent apnea is employed. During division of the airway, oral re-intubation is facilitated by the placement of a suture through the eye of the oral endotracheal tube by the surgeon. This permits the anesthetist to safely withdraw the oral tube with the assurance that it can be easily reinserted by traction on the suture.
We attempt to have the patient breathing spontaneously at the end of the procedure to avoid the need for ventilatory support. If it is anticipated that spontaneous ventilation will not be possible for awhile then the following is advisable:
In the absence of a stent or a tracheotomy, an uncuffed no. 6 endotracheal tube is placed nasally in the airway. The nasal position is more comfortable than the oral route and is more reliably secured. It can be left in place for several days with the patient breathing spontaneously, should there be significant postoperative swelling in the glottis or the anastomosis. It can also be used for ventilatory support. Often it obturates the airway, but even in the presence of a leak in the tube, ventilation is adequate. It is preferable to the use of a large cuffed tube, which may only add to airway damage.
If a “T” tube is in place, ventilation can be provided by one of two routes. First, one can ventilate through the horizontal limb of the “T” tube. If the leak out of the proximal limb is too large, it can be occluded with a bronchus blocker inserted orally through the larynx. The blocker is secured by a suture tied about it's end, brought out through the horizontal limb, and held in place by the cork of the tube or a hemostat. Second, one can insert a small oral endotracheal tube into the upper limb of the “T” tube. Again it is held in place by a suture tied into it's eye and brought out through the horizontal limb in the same fashion as noted for the bronchus blocker.
If the patient experiences airway obstruction in the early postoperative period, it may be alleviated with the inhalation of heliox (70% helium, 30% oxygen) and racemic epinephrine. At 1 week, bronchoscopy is performed. Postoperative dilation or subsequent stent insertion are uncommon but are guided by this surveillance endoscopy. If the patient has a stent in place, it is removed at approximately 6 weeks, although the timing is variable depending on the disease in the subglottis and/or glottis.
The management of laryngeal stenosis by resection and direct anastomosis.