The respiratory system anatomy is designed to make the process of respiration as effective as possible. The nose, mouth and pharynx humidify air as it is conducted to the larynx where the vocal apparatus is situated. Air is then transported down the trachea towards the carina at the level of T4, where it splits into the two main bronchi. The right bronchus divides at a much sharper incline than the left, and thus increases the likelihood of obstruction caused by inhalation of foreign bodies or unintended single-bronchusendobronchial intubation.
The bronchi each divide 23 times (23 generations) to maximise the volume-to-surface area ratios for gas exchange at the distal bronchioles. The generations are split into two functional categories:
During ventilation, about a third of air does not participate in gas exchange, and is known as physiological dead space, a combination of both the anatomical dead space and another dead space known as the alveolar dead space.
Alveolar dead space occurs when the alveolar are being ventilated but not perfused resulting in a V/Q mismatch. Large areas of alveolar dead space are common in conditions like pulmonary embolism, plural effusion and pneumonia.
Lung volumes vary according to age, sex, height and weight, as well as the altitude lived at.
Lung capacity refers to any volume added together.
FRC is the body’s oxygen store. Typically it will contain 2200ml of 21% oxygen in an average healthy adult. Considering body oxygen consumption over a minute is approximately 250ml, this store of oxygen would last for just short of 2 minutes should the patient become apnoeic.
Important concept to consider:
FRC generally holds 2200ml of gas.
The gas typically contains 21% (or 2200ml x 0.21 = 462ml) of oxygen; and 79% of nitrogen
With apnoea, the FRC oxygen store is used as there is no new oxygen to utilise.
Providing the body requires 250ml of oxygen per minute.
The store of oxygen would therefore last = 462/250 = under 2 minutes!
This highlights the vital principle of pre-oxygenation prior to intubation, i.e. to maximize the FRC oxygen store and to reduce the nitrogen levels within the alveoli by 'de-nitrogenation' (in simple terms, to replace nitrogen with oxygen in the lungs). The pre-oxygenation delays desaturation in healthy patients prior to intubation.
Lack of supply of oxygenated blood to the brain and other vital organs is the quickest killer in injured and unwell patients. To optimise transfer of oxygen to tissues, a patent, protected airway and adequate ventilation are required to deliver oxygen to the gas exchange surface in the alveoli.
This is the first principle of the ABCDE approach to the patient during the primary survey. Be aware that c-spine immobilisation should also be initiated at "A" when neck, spinal or multi- trauma is suspected.
A compromised airway may occur suddenly or progressively. It may be the result of altered consciousness- head injuries, intoxication with drugs or alcohol, seizures- and also trauma or inflammation (e.g. inhalational injury, anaphylaxis, ENT, infections) to the airway as well as mechanisms such as foreign body aspiration, suffocation, poisoning and drowning. Regurgitation of gastric contents may be aspirated so be aware and be prepared to suction and rotate the patient to the lateral position. Equally, blood and mucus may pool in the pharynx and require suctioned.
Signs of airway obstruction:
For effective gas exchange, adequate ventilation is required. Inadequate ventilation may be caused by airway obstruction as described above. Patients with thorax trauma may breathe shallowly with tachypnoea due to dyspnoea from rib fractures etc. Patients with chronic lung disease, neuromuscular disorders, metabolic disturbances and deformities of the chest wall and spine etc. may also have ventilation issues. Any injury or illness that causes CNS depression can affect ventilation.
Signs of inadequate ventilation:
*End tidal CO2 can be measured using capnography, which must be used to monitor ventilation during moderate and heavy sedation. There are various adjuncts which can be used to measure CO2, from simple Litmus paper samplers to more complex monitors which display a trace on a ventilator. The CO2 trace represents the inhaled and exhaled concentration of CO2.
Having a system for assessment of the airway, and being well prepared with equipment and assistance, is vital to optimise management of the airway.
Should problems be identified or even suspected during the airway assessment, it is necessary to take immediate measures to manage these. Airway management should occur with c-spine immobilisation in any patient suspected of having an injury or where c-spine clearance has not been established.
Supplementary oxygen via a facemask should be used before and after airway management measures to oxygenate the dead space and maximise the oxygen store during apnoea. For critically unwell patients, use 15 litres/ minute of oxygen via a reservoir or bag-valve mask. Depending on the clinical state of the patient, oxygen flow rate can be adjusted upto 15 litres/ minute and the FiO2 (inspired fraction of oxygen) can be manipulated by using different devices.
As previously mentioned, a rigid suction device should be ready to use.
Patients with reduced conscious levels may obstruct their pharynx as the tongue falls backwards into it*. This can be corrected with a simple airway manoeuvre.
*The usual sites of upper airway obstruction include the tongue, soft palate and epiglottis.
The two commonest manoeuvres are:
Be cautious of excessive head tilt in patients with known neck arthritis as atlanto-axial joint instability may transect the spinal cord and cause sudden death.
To maintain an airway, a simple airway adjunct can be used.
The oropharyngeal airway (OPA or Guedel) can be used as a temporary and simple measure to aid ventilation in an unconscious patient. If used in a conscious patient, it may instigate the gag reflex and cause vomiting and aspiration. The first step is to select the correct sized OPA. Measure from the incisor teeth to the angle of the jaw for best fit. Insert the airway upside down so the concavity points toward the palate. Slide into the mouth up to the soft palate, and then rotate 180o. The device should sit behind the tongue with the concavity pointing down towards the oropharynx and not be pushing the tongue backwards. A bag-mask may then be used to ventilate the patient.
The nasopharyngeal airway (NPA) can be inserted into an unobstructed nostril and passed into the oropharynx. They are better tolerated in conscious patients than OPAs and are relatively simple to insert. They should be used with caution in a patient with suspected basal skull fractures or cribiform injury (although this teaching is based on two case studies). It should be used with caution in those with epistaxis or a history of nasal obstruction (e.g. from nasal polyps). The nostrils must be inspected first to ensure there is no obstruction. Then select the correct sized NPA. Measure from the nostril to the tragus of the ear for best fit. Ensure the NPA fits the nostril too. Remember to lubricate the edges of the tube, but do not obstruct the airway by lubricating over the distal end. Insert into the nostril, directing it posteriorly. Insert with a slight rotating motion until the flange is level with the nostril. A bag-mask device may then be used to ventilate the patient.
The laryngeal mask airway (LMA) is an established device for patients with a difficult airway or in scenarios where endotracheal intubation has failed where it is used as a rescue device. It does not totally prevent aspiration of gastric contents but may provide protection against aspiration of blood etc produced above the device. However, the LMA is not a definitive airway and it is generally not well tolerated unless the patient is anaesthetised or sedated. Additionally, the seal may not be sufficient to optimise adequate ventilation.
The first step is to ensure adequate preoxygenation occurs and that all equipment is ready. Select the correct sized LMA- size 3 for small women, 4 for larger women and men and 5 for large men. The LMA cuff should be inflated to check it does not leak. The cuff should then be completely deflated and lubricated ready for insertion. With the c-spine immobilised, and holding the LMA like a pen in the dominant hand with the cuff opening pointing down over the tongue, pass the LMA into the mouth. Push the LMA into position slowly but firmly. Once satisfied, inflate the cuff and check the position by bag-mask to tube ventilations whilst observing the equal rise and fall of the chest.
The laryngeal tube airway (LTA) is similar to the LMA. It has orophayngeal and oesophageal cuffs with ventilation outflow between the two. The distal tip sits in the oesophagus, so when air flows down the tube it flows out and into the trachea.
To insert the LTA, provide adequate preoxygenation, ensure the tube is properly sterilised and is free from obstruction, and select the LTA size. Inflate both cuffs to check there is no leak before deflating them. Lubricate the distal tip. Ensure the patient is appropriately anaesthetised. Introduce the LTA into the mouth at a 45-90o rotated angle and advance behind the tongue. Rotate the tube back to the midline at the pharynx and advance the LTA. Inflate the cuffs, and begin ventilation with bag-mask to tube. Withdraw the tube until ventilation is easy and take note of the depth of the tube. Confirm the location by checking with auscultation, chest expansion and end-tidal CO2 with capnography. Readjust cuff pressure to ensure seal. Secure LTA.
The i-Gel is in principle similar to the LMA; however its cuff is made from a thermoplastic elastomer gel which moulds to the airway shape when in situ. It does not require inflation. The tube of the i-Gel has a bite block and an oesophageal drain tube. It is easy and simple to insert.
A definitive airway consists of a secured tube placed in the trachea with an inflated cuff and the tube connected to an oxygen source to improve ventilation. There are three different definitive airways- see diagram.
Indications for a definitive airway can be split into:
A LEMON assessment checks for likelihood of a difficult intubation.
Orotracheal intubation is the commonest route used. After assessing the LEMON criteria, ensure adequate preoxygenation. Then check the cuff of the endotracheal tube. Connect the laryngoscope blade to the handle and check the light works. With an assistant- often the ODP- immobilising the head and neck in any suspected c-spine injury, hold the laryngoscope in the left hand and insert into the right side of the patient’s mouth. This displaces the tongue to the left. Visualise the epiglottis and the vocal cords and insert the tube into the trachea. Inflate the cuff to provide the seal, and attach to the bag-mask to tube (N.B. uncuffed tube may be used in some small children). Assess placement of the tube by checking the rise and fall of the chest, looking for misting in the tube, auscultating for breath sounds and ensuring no borborygmi on inspiration (rumbling or gurgling noises in the epigastrium suggesting oesophageal intubation) whilst looking at end-tidal CO2 (the partial pressure or maximum concentration percentage of CO2 at the end of exhalation, normal values are 5-6% or 35-45mmHg CO2). Secure the tube in place.
If efforts to intubate fail at first, remove equipment, and apply ventilation with the bag-mask device to optimise preoxygenation.
Nasotracheal intubation requires spontaneous breathing and is contraindicated in apnoea and relatively contraindicated in patients with cranial fractures.
For rapid intubations with a risk of gastric reflux, a rapid sequence intubation (RSI) should be performed by a skilled doctor trained in the technique. There are two main methods for RSI- the traditional method and the drug titration method. There are no standard guidelines on RSI but the overall sequence is similar:
*Caution in patients with myasthenia gravis
Succinylcholine has been the drug of choice for the last fifty years, due to it's rapid onset of action and short period of action. However it can result in potentially lethal hyperkalaemia. There has recently been increasing interest in the use of rocuronium, which has been shown to be equal to succinylcholine in terms of failed intubations, severe oxygen desaturations, and duration of oxygen desaturations. Rocuronium has a longer length of action compared to succinylcholine.
If the trachea cannot be intubated, then a surgical airway will be the airway management of choice. For emergency airways, a surgical cricothyroidotomy is preferred to a tracheostomy as it is easier to perform, has less bleeding and is quicker to do.
Both methods carry risks of complications including aspiration of blood, oesophageal laceration, surgical emphysema and haematoma.
This involves insertion of a needle into the cricothyroid membrane into the trachea, however it is a short-term intervention to provide oxygenation until an alternative airway can be established. The jet insufflations technique is performed by inserting a large bore cannula through the cricothyroid membrane into the trachea below the obstruction. This is connected to oxygen at 15 litres/ minute with a side hole cut out of the oxygen tubing. Intermittent insufflations are performed by placing the thumb over the hole with one second on and four seconds off.
This airway management technique is performed by making a transverse incision through the cricothyroid membrane, which is then dilated open, so an endotracheal or tracheostomy tube may be passed directly into the trachea.
The airway decision scheme on the right is a useful flowchart to summarise the above discussion on intubation in emergency situations.
Airway assessment and management is the first step in the primary survey and therefore takes priority over all other aspects (N.B. situation may differ in military cases and in life-threating haemorrhages). Begin with simple manoeuvres and then move to simple adjuncts before considering laryngeal airways, and finally definitive airways.
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