Endotracheal tube intubation has its own set of complications. It was William McEwen of England who first performed orotracheal intubation in 18781. The tube which was used for this purpose was made of brass. Endotracheal intubation to administer general anesthesia became really common during the first world war. Magill and Macintosh further refined the process of general anesthesia. It was in 1964 PVC cuffed endotracheal tube was introduced. Initial tubes used were low volume high pressure cuffed tubes. These tubes took in lot of air to inflate the cuff. This increased the cuff pressure which traumatized the laryngeal mucosa leading on to various complications. High-volume low-pressure cuffs were introduced in 1970. These tubes to some extent reduced the complications of endotracheal intubation.
Ischemic changes occur in the tracheal mucosa if the pressure exerted by the cuff of endotracheal tube exceeds the capillary perfusion pressure. In high-volume low-pressure cuff, the cuff inflates at a low pressure. The inflation rate is more for the given amount of pressure exerted. The incidence of tracheal ischemia can be reduced by maintaining the intracuff pressure lower than that of the capillary perfusion pressure. The cuff is usually inflated until it just prevents an air leak during mechanical ventilation. This is also known as the just seal pressure.
This just seal pressure increases linearly 2 as the peak inflation pressure in the airway increases. There is considerable variation in the just seal pressure particularly under high air way pressures. As the cuff of the tracheal tube is inflated the airway pressure proximal to the cuff decreases towards the atmospheric pressure, while the pressure distal to the cuff remains unchanged. This causes air to move away from distal to the proximal portion of the cuff. This redistribution of air inside the cuff creates a self-sealing action despite its lower cuff pressure.
Two factors could cause the cuff pressure to exceed the airway pressure. They are:
Overstretching of the cuff – The intracuff pressure of high-volume, low pressure cuff may act as a low-pressure cuff when the diameter of the cuff is equal to or larger than that of the tracheal diameter. The cuff may not at as low-pressure high volume one if the diameter of the trachea is larger than the cuff diameter 3. This scenario is common during positive pressure ventilation when the tracheal diameter increases during inspiration. The cuff in this scenario needs to overstretch by high cuff pressure to maintain air seal.
Wrinkles present in the cuff – Cuff of endotracheal tube may contain wrinkles which could create channels for air to escape around the cuff. The cuff pressure in this case needs to be increased in order to obliterate all these wrinkles to create a better air seal.
Endotracheal intubation lasting for more than 2 weeks can be considered as prolonged intubation. The first long term sequelae due to prolonged intubation was described by Bergstrom in 1962. His observations came from managing patients with Barbiturate poisoning.
Pathogenesis off intubation injuries:
The endotracheal tube lies in the posterior portion of larynx. The structures vulnerable to injuries in this area are:
- Mucous membrane and muco perichondrium covering the medial surface of arytenoid cartilages and their vocal processes
- Cricoarytenoid joints and adjoining parts of cricoid cartilage below
- Posterior glottic and Interarytenoid regions
- Supraglottic structures (false vocal cords may be oedematous) reversible
- Over inflation of cuff of the endotracheal tube may cause mucosal injury and ciliary loss in the trachea
- Subglottic stenosis
Classification of intubation injuries:
Acute intubation changes:
General / Nonspecific – Inflammation / edema / protrusion of ventricular mucosa
Pressure necrosis / loss of mucosa / cartilage loss / injury to muscles
Flaps of granulation tissue
Ulceration of vocal process
Ulcerated troughs and intact median strip
Posterior ulceration without intact median strip
Bilateral posterolateral mid cricoid ulceration
Annular inferior cricoid ulceration
ET tube in the posterior glottis
Figure showing the types of laryngeal injuries
Image showing various types of granulations
Chronic long-term complications:
Healed fibrous nodule
Posterior glottic adhesion
Posterior glottic stenosis
Posterior subglottic stenosis
Circumferential subglottic stenosis
Factors responsible for laryngeal injuries:
- Diameter / shape / contour of endotracheal tube: Safe sizes of endotracheal tubes that can be used are 8 mm in males and 7 mm in females.
- Plastic tubes of siliconized rubber tubes are smooth walled and less irritating
- Duration of intubation – adults 5-7 days children 7-14 days
- High volume low pressure cuffed tubes minimize risk of laryngeal trauma
- Guyton D, Banner MJ, Kirby RR. High-volume, low-pressure cuffs. Are they always low pressure? Chest 1991; 100: 1076-1081.
- Griscom NT, Wohl MEB. Tracheal size and shape: effects of change in intraluminal pressure. Radiology 1983; 149: 27-30