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.
Why Pre-oxygenation
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 [1]. 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 [2].
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 [3].
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 [4]. 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 [2]. 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).
Patient position
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 [2]. 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.