M´ odulo de compresibilidad lateral en monocapas de Langmuir

In 1950, anaesthesiololgist, Peter Safar, established the concept of “advanced support of life”, keeping patients sedated and ventilated in an intensive care environment. During

the polio epidemic in Copenhagen in 1952 (where paralysed patients had to be

ventilated for prolonged period), Bjorn Ibsen set up the first Intensive Care Unit (ICU) (23). This led to an ever widening use of mechanical ventilation to treat respiratory failure. The iatrogenic lesions that resulted provided a whole new field of endeavour for surgeons. In the 1960s, scores of papers appeared in Europe and North America, describing surgical resection of post intubation strictures. Prevention of post intubation injury quickly became a priority once the origin of these lesions was evident. Initially high pressure cuffs were used and risked ischaemic injury to the mucosa and necrosis of the cartilage of the trachea. Carroll and colleagues in 1969 recommended a cuff with a large volume and low pressure which only resulted in small increases in tracheal wall pressure with over-inflation (104). Although the incidence of post intensive care unit airway stenosis is unknown and can only be approximated at between 1- 4%(105-108), a significant early injury is evident in 47% of patients (109). This is despite the use of these high volume, low pressure cuffs on endotracheal and tracheostomy tubes. This is the only paper in the literature where the trachea has been examined at the time of endotracheal tube removal. Unfortunately it does not make clear the definition of „significant injury‟.

The anatomical and pathological differences between stomal and cuff stenoses and other post intubation injuries were described at this time by Pearson, Grillo and Harley. They also stressed the importance of allowing florid inflammation to subside prior to surgical

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correction (110-113). They demonstrated that surgical resection and anastomosis produced better results than repeated dilatation, steroid injection or cryotherapy. The results of treating post intubation stenosis with resection by the same colleagues achieved an 87.5% “cure” in 200 patients in 1992. The definition of „cure‟ appears to

include patients with a suboptimal airway. Couraud and colleagues reported a 96% success rate in 217 patients in 1994, and Grillo and colleagues cited a 94% success in 503 patients in 1995 (4, 114, 115). They also described the correction of post intubation stenosis involving the subglottic area as being more difficult than lesions of the trachea. The aetiology of post intubation stenosis injuries was initially unclear. At first it was thought to be due to irritation from the materials from which tubes and cuffs were made(116). Later it became clear that pressure necrosis from tubes and cuffs leading to circumferential injury and contracture was the principal explanation(117).

The risk factors for laryngotracheal stenosis following a period of ventilation on the intensive care unit include: sizing of endotracheal tubes, excessive lateral cuff pressure due to poor cuff pressure monitoring, hypotension, local infection, duration of

intubation, use of steroids and other causes of reduced patient immunity, patient movement and agitation, tracheostomies and bilateral injuries of posterior vocal cords. The majority of patients ventilated on ICUs do not appear to develop airway stenosis. Although there are many aetiological factors, patients who tend to scar excessively following injury, may self select for airway stenosis although there is no clinical study to support this.

With respect to endotracheal tubes, modern tubes are of a high volume, low pressure design to reduce the risk of airway injury. To prevent ischaemic damage, the cuff

should not exceed a pressure greater than the capillary perfusion pressure of the mucosa. The mean capillary blood pressure is about 20 mmHg. This is 27.2 cm H2O pressure

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(118). The recommendation is that the cuff inflation pressure, of a ventilation tube, should not exceed 30 cm. Seegobin (119) studied four types of large volume, low pressure cuff types. Following periods of endotracheal intubation, photographs were taken of the circumferential trachea at the site of cuff contact. Photographs were taken for varied cuff pressures after a period of 15 minutes. The conclusion of the study was that lateral wall pressures above 30 cm of water compromise mucosal capillary blood flow leading to pressure necrosis of the adjacent mucosa and eventually the cartilage.

The other risk factors for laryngotracheal injury in a ventilated patient on the ICU (illustration 4.1) probably include gastric reflux, infection, coexisting health problems (such as diabetes mellitus and arteriopathy) and altered immunity as part of the stress response. There is, however, no research to support these as potential risk factors.

Illustration 4.1 Typical appearance of tracheal injury at extubation on the ICU

Post tracheostomy stenosis had been described as early as 1886 when Colles found four strictures in 57 patients treated for diphtheria (120). In my series of 400 adult patients with benign laryngotracheal stenosis the incidence of airway stenosis from endotracheal tubes was approximately the same as the incidence from tracheostomies. Damage to vocal fold anatomy and impairment of vocal cord mobility is more common with

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endotracheal tubes (121, 122). Surgical management of bilateral vocal fold immobility is always a compromise between voice and airway. There is very little evidence supporting early tracheostomy on ICUs (123). The TracMan study was a multicentre UK study designed to look at the timing, morbidity and mortality associated with

tracheostomies on intensive care units. Although the trial phase of this multicentre study took place in 2004, the main phase of the study failed to progress. Although this thesis cannot comment on the morbidity and mortality associated with early versus late tracheostomy, following review of the 400 patients in the database, some conclusions can be reached. There is a real risk of damage to the normal function of the glottis and vocal folds that exists with endotracheal intubation that does not occur in patients with a tracheostomy. As the surgical results of restoration of impaired glottic function remain suboptimal, early tracheostomy would minimise the risk of glottis stenosis and bilateral vocal fold mobility impairment.