- Airway management (AM) is one of cornerstones of general anaesthesia, critically ill patients in ICU and emergencies inside or outside hospitals .It is a critical skill which has to be executed carefully and with attention to details
- . The objectives are to continuously maintain oxygen supply, carbon dioxide(CO2 ) elimination and to protect the airway. For a trained person, especially anaesthesiologist and emergency physician, achieving these objectives are not difficult and associated with minimal complications in patients with normal airway and without any other risk factors.
- However, when the airway is recognized as an actual or potential difficult airway (DA) or when difficulty is encountered unanticipated (more so when it is complicated by other factors), AM becomes progressively difficult.
Importance of airway management in general and difficult airway management lies in the fact that any apnoea or obstruction or both during or due to difficulty in airway management leads to interruption in oxygen supply and results in hypoxia very rapidly. It’s a well-known fact that hypoxia persisting for more than 2-3 min can cause irreversible neurological damage. Hence any plan for difficult airway management should have specific and practicable plan for maintaining adequate oxygenation during the procedure.
|Intubation and extubation are the most critical parts of airway management predisposing for hypoxia due to inadequate oxygenation.|
Different mechanisms contributing to hypoxia during airway management:
- Difficult mask ventilation
- Prolonged laryngoscopy
- Repeated intubation attempts
- Airway obstruction
- Airway oedema
Among those patients with difficult airway, presence of significant comorbidities increases the risk of hypoxia should there be prolongation of apnoea during AM.
- Obstructive sleep apnoea
- ASA 3 and 4 PS
- Poor preoperative pulmonary function
- Emergency procedures
- Preoperative airway obstruction
- Oropharyngeal malignancy
- Congenital heart disease
- Extremes of age
Principles of prevention of hypoxia during difficult airway management
The fundamental principle is a) achieving optimal oxygen reserves before the airway management is started, b) continuous oxygen administration, both active and passive (apnoeic oxygenation), during airway management, c) Optimizing hemodynamic status and other factors which influence oxygen transport d) identifying and managing rare but life threatening complications like pneumothorax, e) immediate confirmation of proper placement of endotracheal/tracheostomy tube inside trachea by capnography/ultrasound/both (SpO2 can continue to be normal for some time even when it is esophageal intubation) and f)prevention of hypoxia during extubation by proper planning and technique.
- Optimal preoxygenation
This is the first step in prevention of hypoxia. By optimal preoxygenation, denitrogenation is achieved and oxygen reserve is increased. This will prolong the safe apnoea period as well as increase the effectiveness of apnoeic oxygenation techniques. In fact, adequate preoxygenation is a prerequisite before apnoeic oxygenation process is initiated. Clinically it goes beyond merely achieving 100% oxygen saturation on monitor. SpO2 reflects only the saturation of haemoglobin in the blood and tissue oxygen reserves.
Preoxygenation is described in terms of different end points:
1) end tidal O2 more than 0.9
2) Time: 30 seconds to 3 minutes depending on the type of breathing
3) Type of breathing and number of breaths: Different recommendations in this regard include a) 4 vital capacity breaths preceded by deep expiration b) 8 deep breaths within 60 seconds and c) 3-minute tidal volume breathing at a flow of 10L.Different studies have shown different values for prolongation of apnoea and differences in PaO2 values. Vital capacity breaths are commonly preferred in emergency rapid sequence induction and the tidal volume breathing in elective procedures.
The other factors important in preoxygenation are the breathing system, oxygen flow rate and position of the patient. Bain’s coaxial circuit is most commonly preferred with a tight-fitting face mask with a flow of 10L per minute. Closed circle system can also be used for preoxygenation, with a flow of 5L/min after filling the circuit with 100% Oxygen. In obese and pregnant patients, a 30-degree head up position is preferred. Upper airway dimensions are maximum when the patient is positioned head up in such a way that the external auditory meatus is in a horizontal plane as the sternal notch.
Inadequate preoxygenation or preoxygenation failure:
There are several situations where preoxygenation becomes inadequate in improving oxygenation and different strategies for ventilation and equipment have been described for them. These patients include a) those in whom mask fit is difficult to achieve. An ill-fitting mask can cause a reduction of 20% in inspired O2 concentration b) restless or uncooperative patients c) Obesity and obstructive sleep apnoea d) those requiring rapid sequence intubation. The strategies useful in these situations are a) Head up position b) Use of continuous positive airway pressure (CPAP) or positive end expiratory pressure(PEEP) which prevents alveolar collapse by splinting them, c) Pressure support ventilation and c) delayed sequence intubation (DSI).
PEEP valve: adjustable, can be used to provide CPAP
Pressure support ventilation when combined with PEEP significantly prolongs safe apnoea period in obese patients.
DSI is a recently described technique, especially in emergency departments where in patients where preoxygenation is difficult (difficulty with mask fit or lack of patient cooperation), a very small dose of sedative, especially low dose ketamine is used to achieve patient cooperation and oxygen is administered to achieve a good oxygen reserve.
The main concern with 100% oxygen administration during preoxygenation is risk of absorption atelectasis which, however, is of less concern in a difficult airway situation.
- Oxygenation during intubation
It’s very important in difficult airway management to give constant attention to maintain oxygenation. Different techniques include:
- Low flow nasal oxygen with a nasal catheter or nasal prongs
- Administration of oxygen through the working channel of intubating fiberscope
- Use of oxyport blade
- Intubation through the supraglottic airway device while continuing ventilation using a swivel connector
- NO DESAT (Nsal Oxygenation During Attempts to Secure Tracheal Tube) technique
- THRIVE (Transnasal Humidified Rapid Insufflation Ventilatory Exchange) with use of high flow of upto 75L/min of oxygen using special equipment
- Buccal oxygen
- Trans tracheal catheters
- Wei’s Jet Tube
Apnoeic Oxygenation based techniques
Apnoeic oxygenation is the technique of passive administration of oxygen even during apnoea. Though benefits of passive nasal oxygen administration were observed in early 19th century, recently there has been a resurgence in the apnoeic oxygenation techniques with better understanding of physiology and availability of specialized equipment. Available scientific evidence also is clearly in favour of apnoeic techniques. NO DESATS (Nasal oxygen during efforts securing a tube), THRIVE (trans nasal humidified rapid-insufflation ventilatory exchange) and buccal oxygen administration are examples of apnoeic techniques.
Physiologically, during apnoea period, alveoli continue to take up oxygen and deliver it to the tissues through circulation, without any diaphragmatic or lung movements, provided upper airway remains patent. Movement of oxygen from the mouth or nasal cavity is due to generation of a mass flow of oxygen due to a negative pressure of up to 20cmH2O generated in the lungs. The latter is due to the difference between the rates of oxygen removal (200-250ml/min) and Carbon dioxide delivery to the alveoli from the tissues.
- NO DESATS
This technique utilizes nasal cannula with flows of 5-15 litres.NO DESATS, as a concept was proposed by Levitan as a strategy for prevention of hypoxia in emergency departments .It can be started before preoxygenation, under the face mask and left in place during intubation attempt. This prolongs the duration of apnoea by 3-5 min. However, a high flow of dry oxygen into the nose can be associated with discomfort, pain,
dryness and nasal bleeds. Low flow nasal oxygen prolongs apnoea period by 3-5 min.
Here a high flow up to 35- 70 litres of humidified oxygen is delivered through the nasopharynx with specialized equipment which produces a continuous positive airway pressure (CPAP) effect reducing atelectasis and also produces apnoeic ventilation in addition to apnoeic oxygenation. This high flow also results in flow dependent dead space flushing. Warmed humidified oxygen is better tolerated by patients than the dry gas.
Optiflo system for THRIVE
Prerequisite of a patent upper airway for successful apnoeic oxygenation cannot be overemphasized.
THRIVE, when preceded by preoxygenation can significantly prolong safe apnoea period up to 30 min. Since the technique also provides apnoeic ventilation accumulation of CO2 is slower compared simple low flow oxygen administration.
C. Buccal Oxygenation: Another recently published technique of apnoeic oxygenation is oxygen insufflation through a nasal RAE tube placed close to the buccal mucosa during oral intubation attempts.
Jet ventilation for oxygenation
Conventionally, jet ventilation has been used for transtracheal oxygen administration. Inherently, jet ventilation delivers oxygen in jets under high pressure, manually or automatically, exposing the patient to risk of barotrauma. Hence it’s role is mainly in a “Can’t intubate Can’t ventilate situation(CICV).
Recently Huafen Wei et al have successfully designed and used 2 devices for jet ventilation in anticipated difficult airway, both electively and to manage can’t intubate can’t ventilate situations. They are a) Supraglottic Jet Oxygenation and Ventilation (SJOV) through “Wei Nasal Jet Tube” and b) modified endotracheal tube “Wei Jet “for supraglottic jet ventilation.
- Wei Nasal jet tube: is a modification of a nasal airway with provision for jet ventilation and monitoring of ETCO2.The authors have described a case of CICV wherein use of SJOV saved the patient from the need for surgical airway. It should be remembered that this is a short term measure till either patient wakes up or a definitive airway is established.
- Wei jet for supraglottic ventilation: here the jet ventilation is delivered through a Cook airway exchange catheter inserted through the endotracheal tube in such a way the its tip is at the same level of tip of ETT.Its particularly useful when the glottic opening can’t be identified or when vocal cord lesions are present. When jet ventilation is provided through the catheter with the tip of ETT is near the glottic opening it helps in maintaining oxygenation and also helps in directing the tube into the trachea.
- Continuous oxygen insufflation through side port of modified Macintosh blade
- Oxygen through transtracheal catheters
- Mask ventilation in between intubation attempts
- Optimal preoxygenation, as indicated by FeO2 more than 0.9 is the first step in maintaining oxygenation during difficult airway management. Not in all patient’s oxygen reserves are adequately built up and these include obese patients, parturient, those with poor mask fit etc. CPAP/PEEP, pressure support ventilation and head up are additional strategies in these patients.
- Apnoeic oxygenation techniques are NO DESAT, THRIVE, Buccal oxygenation and administration through the intubating devices. THRIVE requires a specialized equipment and high flow of oxygen. It provides apnoeic oxygenation and apnoeic ventilation.
- Jet ventilation either through a modified nasal airway or through endotracheal tube also can helps in maintaining oxygenation.
- Delayed sequence intubation involved low dose ketamine sedation, adequate oxygenation before intubation in uncooperative patients.
None of the above strategies are replacement for meticulous planning and execution of airway techniques using appropriate airway devices.
- Dziewit JA, Wei H (2011) Supraglottic Jet Ventilation Assists Intubation in a Patient with Difficult Airway Due to Unrecognizable Supraglottic Structures. J Anesth Clinic Res 2:141. doi: 10.4172/2155-6148.1000141
- Wei HF. A new tracheal tube and methods to facilitate ventilation and placement in emergency airway management. Resuscitation. 2006; 70:438–44
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- Anil D Patel. Nasal ventilation: Oxygenation, NO DESATS and THRIVE Anaesthesiology News 2016
- Heard A, Toner AJ et al, Anesth analg. 2017 April 24(4)