Sunita R Balla and Prof Cyprian Mendonca look at the common practises, benefits and controversies surrounding pre-oxygenation
From the early years of anaesthetic training, every trainee is taught the ritual of preoxygenating patients before inducing general anaesthesia. Not only is it important for critically ill and high-risk patients or those undergoing rapid sequence induction, but for every patient. As we can’t perfectly predict the difficulty in airway management, there is always a possibility of unexpected difficult airway where every bit of oxygen left in the lungs holds up the oxygen saturation for the patient while the struggle to intubate is won. In this article, we look at the common practices, benefits and controversies surrounding preoxygenation.
In a healthy anaesthetized patient, there is continuous consumption of oxygen at rate of 250 ml/min. Hence, with each passing minute, there will be a fall in partial pressure of alveolar oxygen (PA O2 ) and consequently in partial pressure of arterial oxygen (Pa O2 ). SaO2 will decrease only when PaO2 is 6-7kPa hence oximetry can be misleading to detect critical hypoxia . Any further apnoea which leads to SaO 2 falling below 70 per cent will be detrimental as it can then cause brain damage, dysrhythmia and death. The fall in oxygen saturation will be faster in patients with high metabolism, critically ill patients, children, pregnant and the obese. Safe apnea, hence, is defined as the time until a patient reaches a saturation level of 88-90 per cent, to allow for placement of a definitive airway .
Where is the oxygen reserve?
The most important method to delay the desaturation during apneic period is to make sure that there are enough oxygen reserves in the body by giving oxygen to the patient before induction of anaesthesia. During the process of pre-oxygenation, nitrogen in the lungs is replaced by oxygen. Hence pre-oxygenation can also be defined as denitrogenation of the functional residual capacity (FRC) of the lungs. In a healthy adult, the FRC amounts to 30 ml/kg, totalling 2100 ml in a 70 kg adult. By the alveolar gas equation, when breathing air, PAO 2 is 13.2 kPa, which is equivalent to 13 per cent (273ml) of oxygen in a FRC of 2100ml. As oxygen consumption at rest is 250ml/min, 273 ml of O2 will last about 70 seconds if a patient is breathing room air .
However, by breathing 100 per cent oxygen if we increase the fraction of oxygen in the alveoli to 90 per cent, we would have 1800 ml of oxygen in the FRC. This should last for seven minutes of oxygen consumption.
How do we pre-oxygenate?
Preoxygenation is noted to be complete when all compartments, namely, alveoli, arteries, veins and tissues are fully saturated with oxygen . This is achieved by the traditional method of 3-5 minutes of normal tidal ventilation or the four deep breaths over 30 seconds or eight vital capacity breaths. There is no significant difference between the arterial oxygen tension (PaO 2 ) achieved at the end of the traditional or four deep breath methods.
True nonrebreather masks set at 15 L/minute for patients with normal ventilatory patterns are capable of delivering near 90 per cent FiO2 . Given adequate time for pre-oxygenation, SaO2 should be more than 95 per cent. If not, we must consider the presence of a shunt. Any alveoli which is not being ventilated and yet has a blood supply becomes a shunt like in disease processes where there is lung consolidation or edema. Hence, increasing amount of oxygen delivered to these alveoli will not be useful. The answer lies in increasing the mean airway pressure which will then produce the necessary results. This is brought about by using CPAP mask, non-invasive positive pressure ventilation or PEEP valves on a bag-valve- mask device . In hypoxemic patients, PEEP during pre-oxygenation, intermittent ventilation after paralysis and passive oxygenation during tracheal intubation will all help create a conducive environment for intubation. The positive pressure breaths during facemask ventilation should be low volume (6-7 ml/kg), low rate ventilations (6-8 breaths/min) and should be given slowly over 1-2 seconds. It is important to keep the inspiratory pressures less than 25cm H2O so as not to force open the esophageal sphincter. The application of CPAP of 5cm H2O using a Mapleson “A” circuit with a fixed positive end expiratory pressure device during 5 min of pre‑oxygenation with 100 per cent oxygen prior to the induction of anaesthesia has shown to delay desaturation significantly (5).
Another factor to consider apart from the duration and devices used, which can play a pivotal role in preoxygenation is positioning of the patient. A head-elevated position will enhance the FRC by pushing the diaphragm away. For patients with suspected spinal injury, reverse Trendelenburg position will help achieve the same effect as a head up position of twenty to thirty degrees . Ear-to–sternal notch positioning will maximise airway patency. Nasal airways and jaw thrust also contribute to keep a continuous passage for oxygen to flow from pharynx to glottis.
There has been some debate regarding the choice of muscle relaxant. Rocuronium can bring about ideal intubating conditions same as succinylcholine when a higher dose of 1.2 mg/kg is used . The fasciculation brought on by succinylcholine can theoretically increase oxygen consumption. Hence, rocuronium usage can help give a safer margin with respect to apneic times when the risk of desaturation is higher.
Newer methods of preoxygenation are paving their way in routine anaesthetic practice. The Optiflow delivers high concentration humidified oxygen at 10-70 L/min using nasal prongs. Its efficacy has been shown by measurements of end tidal oxygen and transcutaneous oxygen monitoring. A recent study, comparing efficiency of face mask and Optiflow method of preoxygenation has shown that high flow humidified nasal oxygenation for 3 minutes with the mouth closed was as effective as three minute face mask oxygenation at 10 L/min . Hence the Optiflow can be put to use in patients with respiratory failure, during difficult airway management and awake fibreoptic intubation. Irrespective of which method is being used, the key is to achieve a FiO 2 of 1.0 which quite often is a failing target simply because the seal for the mask is loose which causes room air entrainment . If a close-fitting face mask is intimidating for the patient, then there is always an option of using a mouth piece  which is so commonly used in obstetrics by women in labour to breath the laughing gas. Once the patient is anaesthetized, the mouthpiece can be changed over to a standard mask.
The other novel concept of apnoeic oxygenation has come up recently which means oxygen supplementation during laryngoscopy. This is easily done using nasal cannula as soon as the patient presents to the anaesthetic room. Prior to induction of anaesthesia, 3-4 L/min oxygen is administered and soon after induction, oxygen flow can be increased to 10L/ min. It is important to maintain a patent airway during apnoeic period. Use of jaw thrust and nasal airways with high-flow O2 from a non-rebreathing mask, will allow continued apnoeic oxygenation . The other benefit of such noninvasive ventilation is that alveoli can be maintained open which otherwise would have collapsed because of absorption atelectasis. The Hungarian Air Ambulance have devised their own contraption of cutting the oxygen tubing to the reservoir bag and insert it 3-5 cm deep into the nasopharyngeal airway, thus allowing oxygen insufflation to continue unimpeded during laryngoscopy. This reduced their episodes of desaturation while intubating .
An emerging practice to allow preoxygenation in patients who will not tolerate a NRB mask or NIV is Delayed Sequence Intubation . The methodology involves administering sedative agents, which do not blunt spontaneous ventilations or airway reflexes like ketamine. The patient is then preoxygenated safely till saturation of 100 per cent or nearer is obtained and then a muscle relaxant is given.
Of all the anaesthetic practices, preoxygenation remains a technique, which has stood the test of time and prevented many distressing consequences during induction of anaesthesia. The reason behind this is that this technique works well if conducted in an appropriate manner. Newer methods to improve its efficiency are making their foray into current practices in emergency medicine and intensive care; however, their role in elective procedures cannot be undermined. Positive pressure ventilation, apneic preoxygenation, Optiflow and delayed sequence intubation are all developments to make intubation a safer process.
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Dr Sunita R Balla, FRCA, Specialty trainee, Department of Anaesthesia, University Hospitals Coventry and Warwickshire, Coventry, United Kingdom
Prof Cyprian Mendonca, MD, FRCA, Consultant anaesthetist, Department of Anaesthesia, University Hospitals Coventry and Warwickshire, Coventry, United Kingdom
Dr Sunita R Balla, Department of Anaesthesia, University Hospitals Coventry and Warwickshire, University Hospital, Clifford Bridge Road, Coventry, CV2 2DX, United Kingdom.