Airway Management: General Principles
Airway assessment is best done without removing the patient's surgical mask if there are no overt features suggestive of difficult intubation. Patients with a history or features suggestive of difficult intubation will require more extensive assessment. The primary plan and subsequent rescue techniques should be formulated and clearly communicated to all team members including the runners in the anteroom. The need for postoperative ventilatory support should be established. A timely elective or semielective intubation will avoid the additional risks posed to staff by an emergency intubation.
Preoxygenation and General Anesthesia Induction/Intubation Strategies
Preoxygenation Considerations. A minimum of 5 minutes preoxygenation is required. The choice of anesthetic circuit is between a circle circuit or a hand-held circuit such as the Mapleson C with the minimum necessary gas flow (≤6 L O2 minute−1).[13,18] A tight-fitting facemask applied with a 2-handed "vice grip" technique should be used with either option. A T-piece or any other semiopen anesthetic circuit without a viral filter should not be used.
It is imperative that a viral filter is fitted between the manual ventilation device and facemask to reduce circuit contamination and reduce aerosolization risks from expired gases. A heat and moisture exchange (HME) filter should be directly attached between the elbow connector and the facemask to reduce the number of connections between the mask and the filter. An additional HME filter should be placed at the expiratory limb of the anesthetic circuit toward the machine end. At one of the author's institutions, viral filters are used on both the inspiratory and expiratory limbs of the circuit at the machine end; as a practice change implemented during the outbreak (Supplemental Digital Content, Figure 1, https://links.lww.com/AA/D131). This is intended to prevent erroneous placement of a single viral filter on the inspiratory limb instead of the expiratory limb.
Preoxygenation aiming for an expired O2 concentration (Eto2) of 90% is achieved with 100% O2 in with a tight mask fit with flow rates limited to 6 L·minute−1. Videolaryngoscopes (VL) should be used when available, as they have been shown to reduce the number of failed intubations. A recent airway simulation study showed that VL nearly doubled the mouth-mouth distance from the operator to patient compared with direct laryngoscopy. VL also placed the operator's face above the line of sight to laryngeal inlet. A device with a disposable blade and a separate screen can reduce droplet transmission.
Anesthesia Induction and Tracheal Intubation. A slow titrated administration of opioids and an antisialagogue is suggested before induction, taking into consideration that first time exposure to fentanyl derivatives including alfentanil can induce cough soon after administration. Judicious use of either rocuronium 1.5 mg·kg−1 or succinylcholine 1.5 mg·kg−1 is recommended to ensure adequate paralysis. Rocuronium is preferable as it can prolong the neuromuscular blockade, thereby reducing aerosol generation in comparison to the possibility of early cough response with succinylcholine. Ample time should be allowed before laryngoscopy to ensure complete muscle relaxation so as to avoid patient movement. Rapid sequence intubation is recommended to limit aerosol spread from bag-mask ventilation (BMV). Use of cricoid pressure may cause the operator to move closer to the patient's airway while also precipitating a cough response secondary to its stimulating nature. Therefore, the risks and benefits of cricoid pressure must be weighed up carefully and will vary for each patient. Manual ventilation at small tidal volumes with a 2-hand mask seal is reserved only for oxygen desaturation. Assistance should be sought for bagging while utilizing as low flows as possible. For rescue BMV, further head end elevation ramping, together with the use of oropharyngeal airway with low flow rates are recommended.
Oxygen flow should be ceased during intubation and ventilation should commence only after cuff inflation. The use of a stylet or bougie may increase droplet spread, and if required, care should be taken with their removal and subsequent disposal. Successful tube placement is better confirmed with end-tidal CO2. Intubation encounters from Wuhan, China, reveal that oxygen saturations did not increase immediately after intubation in critically ill patients as a result of impaired gas exchange. Auscultation to confirm correct endotracheal tube (ETT) placement is not advised as it is likely to be difficult with PPE in place. Further, it can contaminate the stethoscope and the operator. The ETT cuff pressure should be measured to ensure no leak when it is appropriate. If ETT suction is necessary, a closed in-line suction system should be used. If suction use is thought to be very likely, it may be advisable to attach this to the anesthetic circuit at the time of preoxygenation. However, the utility of this must be balanced against the need to have a simple circuit and reduce the possibility of a circuit disconnection.
Whenever there is a need to disconnect, to prevent aerosolization, the adjustable pressure limiting (APL) valve should be opened fully, fresh gas flows turned-off and positive pressure ventilation (PPV) should be ceased, and the ventilator bellow should be at end-expiration. Mechanical ventilation should only be restarted once the circuit is reconnected and a closed system is reestablished. Clamping the ETT during disconnections has been suggested, but accidental damage to the tube and the pilot balloon assembly is an unwelcome possibility. A simpler alternative is to disconnect the circuit proximal to the viral filter. If a disconnection distal to the filter at the patient end becomes inevitable (eg, attaching an in-line suction later during the procedure), it is recommended to clamp the ETT close to the tube connector over a gauze or tape. This method can preserve a decent length of the ETT in the event of an accidental damage.
If the primary intubation attempt fails, a second-generation supraglottic airway (SGA) that allows direct adult ETT insertion guided by a fiberoptic bronchoscope (eg, LMA® Protector™, i-gel [Intersurgical, Wokingham, UK], AmbuAuraGain [Ambu, Ballerup, Denmark], Air-Q [Salterlabs, Lake Forest, IL]) should be preferred.[38,39] Repeated intubation attempts could potentiate virus aerosolization; hence, SGA should be considered early. A second-generation SGA offers better seal pressure and reduces virus aerosolization during PPV if there is a need to prolong oxygenation. Devices with a small laryngeal lumen, for example, LMA® Supreme™ are not suitable. Although airway exchange catheter-guided tracheal intubation is an option, it increases the contamination risk as it involves multiple steps: SGA removal, ETT insertion, and subsequent airway exchange catheter removal before connecting the ventilator. Notably, for devices that allow direct placement of ETT, only a few steps are involved and the SGA can be left in situ.[38,39] This process is much quicker and there is less chance of contamination. When performed in a fully paralyzed patient without PPV or suction through the bronchoscope, this procedure should not generate any aerosols. For urgent front of neck access, a scalpel-bougie technique as described by the Difficult Airway Society guidelines is recommended in view of aerosol generation with high-pressure oxygen delivery related to cannula techniques.[13,18,32] Avoid any attempts to deliver oxygen above the site of tracheal puncture during a cricothyroidotomy procedure to avoid aerosolization of virus-containing fluid. A nasogastric tube (if required) should be inserted after intubation to avoid any further close contact with the patient's airway. Sites other than the nasopharynx should be chosen for temperature monitoring to avoid further contamination risk from the upper airway.
Role of Airway Tents/Aerosol Boxes/Airway Shields
During the SARS outbreak, special airway intervention and resuscitation (AIR) tents have been described to reduce droplet and aerosol transmission to health care workers. Various adaptations (aiming to reduce aerosolization) of tents, boxes with arm holes, shield, and tent combinations and a negative pressure intubating tent (where a suction port with an attached viral filter is left inside the tent) have been described. Their merits have been discussed across various online COVID-19 airway discussion forums and social media. A clear plastic sheet attached over a frame/right angle extension bar/surgical tray, or a rectangular screen or rigid box with fixed shapes and arm holes are some of the more popular modalities. Currently, the evidence for these strategies in limiting aerosol spread is limited to benchtop studies. While it might offer extra protection, these interventions can add an extra layer of complexity and pose an infection risk. To be successful, it must be a simple system which allows good visibility, seal, free arm movements, and poses no contamination risk either during or after its use. A recent simulation study assessed the efficacy of specialist anesthetists performing intubation using both early (2 holes for the person doing intubation) and late generation (with extra holes for assistant's hands, and for bougie insertion). The results revealed that aerosol boxes increased intubation times and pose infection risk to clinicians as a result of breaches of their PPE. First-hand experiences from Wuhan emphasize intubating using VL under a transparent disposable sheet. It should be replaced after securing the airway, but it can be left over the patient's head, neck, and upper part of chest during the procedure.
Extubation may pose an even greater aerosolization risk and the same principles that guide intubation must apply to extubation. There is an additional consideration that some patients may become agitated in the periextubation period, thereby increasing the risk of droplet spread. Antiemetics are strongly recommended to prevent postprocedure droplet spread from vomiting (Figure 4). Deep extubation may help in attenuating the airway response but leaves the issue of an anesthetized patient and an unprotected airway. Recent consensus guidelines on this topic do not suggest SGA exchange as a first-line option in view of the need for a second procedure and the likelihood of airway difficulty after its insertion. If sugammadex reversal is used, practitioners should be aware that the sudden return of the upper airway tone has induced laryngospasm or straining on the ETT in some patients. Pharmacological measures such as intravenous lidocaine, opioids, and dexmedetomidine can be considered to reduce the cough response. Applying a head-up tilt with the operators positioned behind the head end of the patient may assist in reducing droplet exposure. A new set of masks and airway supporting devices are recommended. Extubating and transitioning to facemasks under clear plastic sheets may be a viable approach to reduce droplet/aerosol dispersion. In a recent manikin model, clear plastic sheets have been shown to reduce cough-induced droplet spread. An anesthetic facemask with an attached viral filter can be held over the airway as the ETT is withdrawn. To further reduce droplet spread, a soft cloth/gauze can be used to wipe any secretions from the ETT while it is being removed and disposed of. When it is appropriate, patients can be transitioned to either a standard facemask or low-flow nasal cannula. Facemasks with in-built viral filters are good options if available. As a minimum, a surgical mask (preferable with ear loops) should be placed over the supplemental oxygen delivery device. It is prudent not to attempt use of any unfamiliar techniques and pharmacological agents.
Airway management for known or suspected COVID-19 patients. COVID indicates coronavirus disease; Etco2, end-tidal CO2 concentration; Eto2, expired O2 concentration; ETT, endotracheal tube; HEPA, high-efficiency particulate air; IV, intravenous; PPE, personal protective equipment; PPV, positive pressure ventilation; RSI, rapid sequence induction; SGA, supraglottic airway.
Anesth Analg. 2020;131(3):677-689. © 2020 International Anesthesia Research Society