Emergency Medicine/Endotracheal Intubation
Endotracheal intubation describes the technique of passing a tube through the vocal cords. A cuff is then usually inflated to provide a seal allowing positive pressure ventilation, and to protect the airway from the aspiration of gastric contents or upper airway material.
Indications for endotracheal tube (ETT) placement include: severe hypoxaemia requiring PEEP; hypercapnia with acidosis; inability to protect the airway; to control ventilation in the presence of raised intracranial (or intraocular) pressure; to facilitate certain diagnostic or therapeutic procedures; to allow administration of neuromuscular blockade
Endotracheal placement is a painful and stimulating procedure and requires induction of anaesthesia and muscular relaxation for facilitation. In cooperative patients topical anaesthesia alone can be used.
The classical technique in a starved patient undergoing elective anaesthesia describes the administraion of a hypnotic (sleep inducing) drug, gentle ventilation to check for airway patency, followed by administration of a muscle relaxant. A rapid onset opiate may also be administered to blunt sympathetic response to intubation. The vocal cords are then visualised with the aid of a laryngoscope and the endotracheal tube visualised passing through them. Confirmation of endotracheal placement is sought by 3-point ausculation (both lung fields and the stomach), and capnography. The optimum position for endotracheal intubation is described as 'sniffing the morning air'. This aligns the axes of the oral cavity, pharynx and trachea. The neck is flexed (on a pillow) and the head extended. When intubating it is important not to push the laryngoscope against the persons teeth which could cause discomfort or dental deformations, and can lead to malpractice.
In the practice of emergency medicine, ideal intubating conditions as described above are rare. Patients are almost always unstarved or have other factors which may contribute to delayed gastric emptying. Additionally many patients are unstable or hypoxaemic. A technique known as 'rapid-sequence induction' is often used.
Rapid Sequence Induction
Here the step of checking airway patency prior to administration of muscle relaxation is omitted with the aim of securing the airway as quickly as possible. A pre-calculated dose of hypnotic, followed immediately by a rapidly aciting muscle relaxant, are administered. Cricoid pressure by a trained assistant is applied. As soon as adequate intubation conditions are deemed to have been obtained, direct laryngoscopy followed by endotracheal intubation is performed. In patients with suspected cervical spine injury, patients are generally removed from their hard-collars just prior to induction, and the spine is immobilised by manual in-line stabilisation. This can impair the view of the larynx with conventional laryngoscopy.
Because the ability to ventilate prior to administration of muscle is not determined, the 'can't intubate, can't ventilate' scenario is always possible. This represents an immediate threat to life, and it is important that one is familiar with a 'failed intubation drill'.
Rapid Sequence Induction Technique
Rapid sequence intubation refers to the pharmacologically induced sedation and neuromuscular paralysis prior to intubation of the trachea. The technique is a quicker form of the process normally used to induce general anesthesia. A useful framework for describing the technique of RSI is the "seven Ps".
The patient is assessed to predict the difficulty of intubation. Continuous physiological monitoring such as |ECG and pulse oximetry is put on the patient. The equipment and drugs for the intubation are planned, including the endotracheal tube size, the laryngoscope size, and drug dosage. Drugs are prepared in syringes. Intravenous access is obtained to deliver the drugs, usually by placing one or two IV cannulae.
The aim of preoxygenation is to replace the nitrogen that forms the majority of the functional residual capacity with oxygen. This provides an oxygen reservoir that will delay the depletion of oxygen in the absence of ventilation (after paralysis) for up to 8 minutes (to an oxygen saturation of 90%) in the healthy adult. This time will be significantly reduced in obese patients, ill patients and children. Preoxygenation is usually performed by giving 100% oxygen via a tightly fitting face mask.
Pretreatment consists of the medications given to specific groups of high-risk patients 3 minutes before the paralysis stage with the aim of protecting the patient from the adverse effects of introducing the laryngoscope and endotracheal tube. Intubation causes increased sympathetic activity, an increase in intracranial pressure and bronchospasm. Patients with reactive airway disease, increased intracranial pressure, or cardiovascular disease may benefit from pretreatment.
Paralysis with induction
With standard intravenous induction of general anesthesia, the patient typically receives an opioid, and then a hypnotic medication. Generally the patient will be manually ventilated for a short period of time before a neuromuscular blocking agent is administered and the patient is intubated. During rapid sequence induction, the person still receives an IV opioid. However, the difference lies in the fact that the induction drug and neuromuscular blocking agent are administered in rapid succession with no time allowed for manual ventilation.
Commonly used hypnotics include thiopental, propofol and etomidate. Commonly used neuromuscular blocking agents used include succinylcholine and rocuronium. The neuromuscular blocking agents paralyze all of the skeletal muscles, most notably and importantly in the oropharynx, larynx, and diaphragm. Opioids such as fentanyl may be given to attenuate the responses to the intubation process (accelerated heart rate and increased intracranial pressure). This is supposed to have advantages in patients with ischemic heart disease and those with brain injury (e.g. after traumatic brain injury or stroke). Lidocaine is also theorized to blunt a rise in intracranial pressure during laryngoscopy, although this remains controversial and its use varies greatly. Atropine may be used to prevent a reflex bradycardia from vagal stimulation during laryngoscopy, especially in young children and infants. Despite their common use, such adjunctive medications have not been demonstrated to improve outcomes.
This phase consists of appropriately positioning the patient to facilitate intubation. The optimal position to open up the airway is usually described as a "sniffing" position, with head extension and flexion of the neck on the body. This is usually done by placing a pillow under the head of the patient and extending the head. However, some evidence exists that head extension alone results in similar ease of intubation.
Sellick's maneuver, or cricothyroid pressure, may also be used in this phase.
Placement of tube
During this stage, laryngoscopy is performed to visualize the glottis. The endotracheal tube is then passed in between the vocal cords, and a cuff is inflated around the tube to hold it in place and prevent aspiration of stomach contents.
The position of the tube in the trachea can be confirmed in a number of ways, including observing increasing end tidal carbon dioxide, aspiration, chest movement and misting of the tube.
Malpositioning of the endotracheal tube (in a bronchus, above the glottis, or in the oesophagus) should be excluded by performing a chest X-ray.