PHysiology of Apnoeic oxygenation
The process of apnoeic oxygenation (ApOx) relies on the discrepancy between the rate at which oxygen is normally removed from the alveoli compared with that at which carbon dioxide (CO2) is delivered to them. Predominantly due to differences in their respective solubilities in the blood, it is estimated that during apnoea CO2 enters the alveoli at a rate of only approximately 10ml/min whilst oxygen is removed at a rate of approximately 250ml/min. This 240ml/min net removal of gas volume from the alveoli during periods of apnoea results in a reduction in barometric pressure in the alveoli. The sub-atmospheric pressure which develops serves as the driving force for gas to move from the upper airway to the alveoli by the process of apnoeic mass movement, provided the airway is patent. The mechanism for apnoeic mass movement is thus bulk flow down a pressure gradient, not molecular diffusion, yet is able to occur during periods of apnoea as it does not depend upon the generation of positive pressure in the upper airway nor on respiratory effort by the patient. This process has been recognised for over fifty years but has only recently started being used as a routine adjunctive technique to deliberately prolong the safe apnoea time in susceptible patients.
The requirements for effective apnoeic oxygenation are as thus as follows:
- High Oxygen Concentration: apnoeic oxygenation can only be effectively performed following preoxygenation to ensure high oxygen concentrations thoughout the respiratory tree. This is required to ensure that the a sufficient volume of gas is taken up from the alveoli and that this is replenished with gas from the upper airway that contains sufficient oxygen to maintain adequate oxygenation of pulmonary capillary blood and allow the process to continue. It should be noted that for these reasons apnoeic oxygenation is not effective in restoring the oxygen saturations of patients that have already desaturated - as during airway management desaturation usually indicates that a low oxygen concentration has developed in the alveoli.
Oxygen source: insufflation of high flow oxygen into the upper airway via the nasal or buccal routes prevents entrainment of room air so that further high concentration oxygen is available to move down the trachea to the alveoli and replace that being consumed.
Patent Airway: oxygen delivered to the upper airway can only move towards the lungs in response to the subatmospheric alveolar pressure if the airway is patent. A nasopharyngeal airway may be useful to assist nasal oxygen passing from the nasopharynx into the hypopharynx.
LIMITATIONS OF apnoeic Oxygenation
Although apnoeic oxygenation has been shown to prolong the safe apnoea time significantly this is not guaranteed and some patients may still rapidly desaturate despite these techniques. Apart from airway obstruction another factor limiting the efficacy of apnoeic oxygenation is alveolar collapse. Rather than draw further gas into the alveolus, the negative pressure generated there can potentially lead to loss of volume and ultimately collapse of the alveolus (aborption atelectasis). This can result in a pulmonary shunt and depression of the oxygen saturations. Patients with pre-existing shunt physiology and those pre-disposed to developing it during periods of apnoea are therefore less likely to benefit from apnoeic oxygenation techniques. The likelihood of developing absorption atelectasis in depends in part on the balance of expansive and retractive forces acting on the alveoli and the resistance to gas flow in the respiratory tree. Unfortunately morbidly obese patients in whom prolongation of the safe apnoea time might be most desirable are are more susceptible to developing shunt physiology during apnoea and may have the least benefit. Still, prolonged periods of sustained blood oxygenation have been demonstrated in obese patients using apnoeic oxygenation. Even obese patients, who desaturate more rapidly due to shunt, may still be demonstrating significant prolongation of their safe apnoea time relative to what it might have been in the absence of apnoeic oxygenation. This is supported by animal models where even though a higher shunt fraction has been associated with a more precipitous initial fall in SpO2 at the onset of apnoea (even with use of apnoeic oxygenation), the value at which the falling SpO2 has plateaued has been higher when apnoeic oxygenation has been implemented suggesting that a benefit is still being derived. Once shunt becomes sufficiently severe, however, the plateau SpO2 level will not longer be sufficiently high to qualify as 'safe'. A high shunt may be responsible for the inability to demonstrate a benefit from apnoeic oxygenation during airway management in critically ill patients.
Minimising atelectasis via application of CPAP and head up tilt should reduce shunt and maximise the impact of apnoeic oxygenation techniques.
The important point is to be aware that apnoeic oxygenation is a important adjunctive tool to extend the safe apnoea time but that it cannot be relied upon or used as an alternative to the definitive strategy of restoring alveolar oxygen delivery via one of the upper airway lifelines or CICO Rescue. Apnoeic oxygenation should be used to extend the time available to efficiently make attempts to restore alveolar oxygen delivery by these techniques without desaturation occurring but the maintenance of high saturations it provides should not influence decision making about the need to move on to alternative strategies where best efforts at one or more of the upper airway lifelines have not secured entry into the Green Zone.
"confirmed" alveolar oxygen delivery
Effective decision making during airway management requires the ability to rapidly confirm whether alveolar oxygen delivery has been achieved by a particular strategy. This allows an immediate judgement to be made about the need to perform optimisation manoeuvres or move on to an alternate technique. The inability to promptly identify that a technique to achieve alveolar oxygen delivery has been unsuccessful wastes valuable ‘safe apnoea time’ during which rescue strategies could have be implemented and increases the risk of exposing the patient to sustained hypoxia. Apnoeic oxygenation techniques do do not allow a prospective assessment to be made of whether they are achieving alveolar oxygen delivery or how long existing oxygen saturations are likely to be maintained before desaturation occurs. Since apnoeic methods for oxygen delivery do not typically produce a rise in existing oxygen saturations, their success or failure can only be identified in hindsight by observing when oxygen saturations begin to decline. Thus from a practical perspective, apnoeic mechanisms to deliver oxygen to the alveoli do not provide the information required to make timely decisions about whether this goal is being achieved and the need to optimise or move on to an alternate technique during airway management. As such the Vortex Approach restricts the meaning of the term "alveolar oxygen delivery" to refer only to the concept of confirmed alveolar oxygen delivery. Since alveolar oxygen delivery cannot be directly measured, real-time assessment of whether it is occurring must be inferred from other clearly definable endpoints. Real-time confirmation of alveolar oxygen delivery can thus only be achieved by confirming ventilation with oxygen or direct tracheal insufflation of oxygen via a neck airway. In most circumstances this involves obtaining a capnography trace and/or evidence of an upward trend in oxygen saturations (as opposed to sustained high saturations). The ability to confirm alveolar oxygen delivery is thus restricted to those techniques which involve cyclical inflation/deflation of the lungs as the mechanism to deliver oxygen – ventilation via the upper airway lifelines or ventilation/intermittent insufflation via neck airways. Techniques involving apnoeic mechanisms for delivering oxygen (“unconfirmed” alveolar oxygen delivery) are better considered as techniques to prolong the safe apnoea time.
As such while apnoeic oxygenation, when successful, does technically achieve "alveolar oxygen delivery" the Vortex Approach emphasises the need to achieve ‘confirmed’ alveolar oxygen delivery and uses the term "alveolar oxygen delivery" in this way.
TECHNIQUES for Apnoeic Oxygenation
NO DESAT (Nasal Oxygen During Efforts Securing A Tube)
While, if the airway is patent, apnoeic oxygenation occurs in any preoxygenated patient while a face mask is applied, the NO DESAT technique allows the benefits of apnoeic oxygenation to be yielded even during attempts at laryngoscopy. This enables apnoeic oxygenation to be achieved even in many patients in whom airway patency cannot be established via the three lifelines of face mask, supraglottic airway and endotracheal tube. This is possible because even when a patent airway cannot be achieved via one of the three lifelines, in the absence of laryngeal pathology or a foreign body, it is likely that during attempts at laryngoscopy the upper airway will still be patent and able to allow delivery of oxygen to the alveoli via apnoeic mass movement – even though a view of the larynx may not be able to be achieved or an endotracheal tube passed. Thus whilst the patent airway achieved with a laryngoscope in place does not allow for positive pressure ventilation, if a reservoir of high concentration oxygen can be created in the nasopharynx it can be drawn towards the alveoli by apnoeic mass movement.
Nasal cannulae alone provide only low inspired oxygen concentrations in spontaneously ventilating patients. This occurs because even during quiet breathing the inspiratory flow rate (in contrast to the minute ventilation which is the averaged flow rate over time derived from intermittent inspiration/expiration) of gas in and out of the lung is much higher (upto 30Lt/min) than the oxygen flow rate delivered by the cannulae. The result is that significant volumes of room air must also be entrained during inspiration to make up this difference. This low oxygen concentration gas mixes with the 100% oxygen entering the nasopharynx via the nasal prongs (as well as with low oxygen concentration gas present in the anatomical dead space at the end of the previous expiration), diluting the concentration of oxygen in the gas that is drawn into the alveoli with inspiration down to around only 30% .
The effect of oxygen delivery via nasal cannulae in the apnoeic patient is quite different, however. In the absence of ventilation, no room air or expired gas with a low oxygen concentration enters the nasopharynx to dilute the 100% oxygen insufflated by the nasal prongs. The result is that if nasal cannulae are applied with flows of 15Lt/min during preoxygenation, then once apnoea occurs they provide a reservoir of near 100% oxygen in the nasopharynx which is available to be drawn down the trachea into the alveoli by apnoeic mass movement. A second oxygen oxygen source, in addition to that being used to supply the mask being used, is required to provide oxygen flow for the nasal cannula.
Using nasal cannulae at 15L/min, apnoeic oxygenation has been demonstrated to extend the duration of apnoea without desaturation to beyond 10mins in some cases.
THRIVE (Transnasal Humidified Rapid Insufflation Ventilatory Exchange)
THRIVE employs very high flow (upto 70L/min) humidified nasal oxygen and has resulted in extension of the safe apnoea time by a mean of 14 mins and upto an hour in some patients. In addition to apnoeic oxygenation, THRIVE has the following benefits:
- Apnoeic Ventilation: limiting the rise in arterial CO2 levels during the apnoeic period (although high CO2's and respiratory acidosis are typically well tolerated in otherwise well patients). This has potential benefits when used in contexts of prolonged (> 15min) apnoea, such as some ENT surgery, but probably does not provide a significant advantage during emergency airway management when restoration of ventilation by other means would typically have been achieved well within this timeframe.
- Continuous positive airways pressure (CPAP): which can reduce atelectasis and shunting thereby assisting with maintenance of oxygen saturations. In addition this could assist with splinting open of the upper airway and maintenance of airway patency without use of an occlusive face mask.
- Increased FiO2 during PreOx: as the flow rates using THRIVE are able to exceed inspiratory flow rates, it can also be used for preoxygenation, without the need for an occlusive face mask, while avoiding the discomfort that occurs when these high flows are delivered using non-humidified oxygen sources (see below).
The equipment for nasal oxygen delivery during THRIVE involves bulky corrugated tubing which prevents being able to achieve a seal with a face mask while it is in place. As such THRIVE must be removed and replaced before and after each attempt at face mask ventilation which increases the complexity associated with its use during management of a challenging airway. THRIVE also requires dedicated equipment which is more expensive in comparison to that required for NODESAT or buccal ApOx (which utilise cheap, generic airway equipment). As such it is logical to limit use of THRIVE to the circumstances in which it offers particular advantages: prolonged apnoea, high risk of atelectasis and maintenance of a high FiO2/CPAP in situations where use of an occlusive face mask is undesirable.
Buccal Oxygen Delivery
This technique provides supplementary oxygen flow via the oral route using an adapted RAE endotracheal tube secured in the left buccal space. Like NODESAT this technique allows the benefits of ApOx to be obtained during attempts at laryngoscopy and does not interfere with the ability to face mask ventilate between these attempts (unlike THRIVE - see above). It is also simpler to apply than
It has been postulated that buccal oxygen delivery may be superior to NODESAT and THRIVE because it offers additional protection from the theoretical risk of pulmonary or gastric barotrauma with these techniques (see below), since the position of the oxygen source in the oral cavity ensures that any rise in pharyngeal pressure will be vented via the mouth. In contrast it is theoretically possible for airway pressures to rise during NODESAT if the oral cavity becomes obstructed by the tongue/soft palate. Additionally it has been suggested that the efficacy of NODESAT may be limited by the occurrence of retropalatal obstruction when obese patients are induced for airway management. Supplementary oral oxygen is not impacted by obstruction of passage between the naso- & oropharynx by the soft palate, and effective ApOx using this technique requires only that airway patency between the oro- & laryngopharyx is able to be maintained by performance of sustained laryngoscopy. Buccal oxygen supplementation is also advantageous when performing ApOx in patients in whom nasal instrumentation is contraindicated.
In obese patients, ApOx using a buccal oxygen source at 10L/min has been demonstrated to significantly reduce the risk of desaturation during apnoeic episodes in excess of 12 minutes. This flow rate (which is less than that used with NODESAT) provides a high pharyngeal oxygen concentration during apnoea, allowing ApOx to occur, but probably provides negligible CPAP and does not produce the apnoeic ventilation phenomenon of THRIVE.
Concerns with nasal oxygen insufflation
Interference with Face Mask Seal: one concern with use of the NODESAT technique is the possibility that the nasal cannula oxygen tubing might interfere with getting a good seal for face mask ventilation – though in practice this is not usually an issue, and nasal cannulae could be easily removed if this were the case. The large diameter corrugated tubing used for THRIVE does prevent a face mask seal but the THRIVE nasal cannulae can be easily removed to allow face mask ventilation and replaced again during attempts at laryngoscopy. This does provide an extra task during management of a challenging airway and it is forseeable that replacing the THRIVE cannula during laryngoscopy could be overlooked resulting in a termination of apnoeic oxygenation.
Barotrauma: the pressure generated from insufflation of gas into the upper airway is a function of the rate of gas flow and the resistance flow of this gas back to the atmosphere via the upper airway. If none of the insufflated gas were able to be vented to the atmosphere it would rapidly accumulate in the stomach or lungs leading to a rise in volume/pressure and the potential for serious complications. Gastric rupture has been reported with insufflation of relatively low flows (4L/min) of oxygen via long nasal/nasopharyngeal catheters during procedural sedation but remains a only theoretical concern with use of NO DESAT & THRIVE which utilise shorter intranasal cannulae but at much higher flow rates. THRIVE has been shown to produce elevation of pulmonary pressures equivalent to only 5cmH2O but this limit is dependent on the upper airway remaining unobstructed to allow venting of insufflated gas. Supplementary oxygen via the buccal route has been postulated to be safer a safer technique for ApOx by providing a reliable, low resistance route for egress of insufflated oxygen and avoiding this theoretical risk.
ancilliary benefits of Supplementary oxygen insufflation
In addition to allowing apnoeic oxygenation there are a number of potential ancillary benefits from using supplementary insufflation of oxygen during preoxygenation and airway management.
- Increased FiO2: the supplementary oxygen flow can compensate for room air entrainment which would otherwise prevent some oxygen delivery devices (e.g. a non-rebreather mask) providing sufficient inspired oxygen concentrations to allow for preoxygenation in spontaneously ventilating patient. Utilising supplementary nasal or buccal oxygen for this indication requires a clear understanding of the oxygen delivery capabilities of the specific devices when used in this way as the achievement of an adequate FiO2 for preoxygention is not guaranteed. Use of non-humidified gas at high flow rates during these techniques may also cause significant discomfort to fully conscious patients. Utilising an anaesthetic circuit (circle or Mapleson) or a Bag-Valve-Mask device with an expiratory port valve that provides the ability to achieve a face mask seal is preferred wherever feasible.
- Leak Compensation: supplementary oxygen flow via nasal or buccal routes may compensate for the loss of gas via small face mask leaks and improve the ability to achieve effective positive pressure ventilation via face mask in these circumstances.
- PEEP/CPAP: supplementary oxygen flow using nasal cannulae for NODESAT can assist in generating CPAP/PEEP thereby limiting atelectasis, reducing intrapulmonary shunting and optimising oxygenation of pulmonary capillary blood. In addition PEEP/CPAP increases the volume of the functional residual capacity and thus the volume of the oxygen reservoir generated during preoxygenation.
- Splinting of upper airway: based on anecdotal observation it is possible that nasal (but not buccal) oxygen flow could act to splint open the upper airway in effect creating a "pneumatic nasopharyngeal airway" that improves the ease of face mask ventilation by holding the soft palate open. This has not been confirmed in clinical trials. Conversely, as mentioned above, persistent obstruction of the nasopharynx by the soft palate, preventing nasally insufflated oxygen reaching the respiratory tree, has also been proposed as one of the factors potentially limiting the effectiveness of supplementary nasal oxygen in prolonging the safe apnoea time in obese patients (and thus an advantage of buccal oxygen supplementation).
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