#11 The Physiologically Difficult Airway

AirwayHemodynamics
Written By Nick Mark

Airway management in the critically ill can be one of the most challenging, stressful and rewarding elements of critical care medicine. While the anatomically challenging airway is often the first place our minds go when discussing difficult airways, the physiologically difficult airway is both more common and often requires more savvy to overcome. Join Cyrus & Nick as they take a pragmatic, case-based approach to this topic to equip you with the tools you need to successfully tackle a physiologically difficult airway, or support an airway operator charged with this task!

Quick Take Home Points:

  1. Airways can be anatomically difficult, physiologically difficult, or a combination of the two!

  2. Physiologically difficult airways are often harder to spot/predict and harder to manage.

  3. Intubation presents a number of physiologic challenges to a critically ill patient that must be overcome through deliberate thought and planning.

  4. Using non-invasive positive pressure ventilation and aggressive pre-oxygenation are almost never bad ideas when it comes to providing as much slack as possible prior to inducing and intubating.

  5. Call for help, have your team ready, have the tools you are most comfortable with and have a plan!

  6. Try to avoid intubating patients in absolute extremis if possible - if you can start resuscitation or otherwise support the patient prior to induction and intubation, that is usually the best practice.

  7. In general, you cannot go wrong with ketamine for induction and rocuronium for paralysis in critically ill patients given that you won’t have much in the way of hemodynamic consequences (i.e. shock) and you won’t have to worry about transient rises in potassium or oxygen utilization secondary to fasciculations from succinylcholine.


Infographic:

Infographic for understanding the 4 mechanisms that contribute that can contribute to a physiologically difficult airway.


Show Notes:

What is a “difficult airway”?

    • A difficult airway is an umbrella-term which we agree can be applied to a scenario where a skilled operator has difficulty safely intubating a patient

      • This can be due to anatomic factors, physiologic factors, or often a combination of both

      • Could alternatively consider the term complex airway

        • Complexity is secondary to a number of factors including human factors, experience of the operator, patient factors, etc.

    • For context/references the 2022 ASA Practice Guidelines for Management of the Difficult Airway define the difficult airway as:

      • “…the clinical situation in which anticipated or unanticipated difficulty or failure is experienced by a physician trained in anesthesia care, including but not limited to one or more of the following: facemask ventilation, laryngoscopy, ventilation using a supraglottic airway, tracheal intubation, extubation, or invasive airway.”

    • Anatomically difficult airway: anatomical characteristics hinder BVM ventilation, visualization of the cords, or passage of the endotracheal tube

    • Physiologically difficult airway: physiological characteristics make it difficult to safely induce the patient and transition to positive pressure

    • Anatomically difficult airways are generally easier anticipate (and may be easier to manage) in comparison to physiologically difficult airways for a variety of reasons:

      • Validated, predictive tools exist to assist in assessing anatomically difficult airways:

        • Mallampati (I-IV)

        • 3-2-2 Rule 

          • Looks at 3 external measurements; has an AUC of 0.83 for predicting anatomically difficult airway

            • 3+ fingers between the upper and lower teeth

            • 3+ fingers from the anterior tip of the mandible to the anterior neck (submandibular space, also know as the hyomental distance)

            • 2+ fingers between the floor of the mandible and the thyroid notch on the anterior neck

            • The thyromental distance can also be assessed - distance from the anterior chin to the thyroid cartilage when the patient’s neck partially extended should be 7+ cm or 4+ finger breadths 

        • LEMON Scale uses 5 factors (each score with 2 points) to predict the an anatomically difficult airway.

        • MACHOCA: Scores airways using 7 factors on a 0-12 point scale, 12 very difficult, 0 easy - useful for ICU intubations / intubations done by non-anesthetists. The 7 factors include:

          • Mallampati score III or IV (5)

          • OSA (2)

          • Reduced cervical mobility (1)

          • Limited mouth opening <3 cm (1)

          • Coma (1)

          • Severe hypoxemia with SpO2 < 80% (1)

          • Non-anesthesiologist (1)

        • Selected Others:

          • Assessment of atlanto-occipital extension: assessment of voluntary neck flexion & extension

          • Combined assessment of mandibular space

          • Warning Sign of Delikan: place an index finger under the chin, one under the inferior occipital prominence, have patient fully extend the head and neck voluntarily, if the “chin-finger” is higher than the “occipital finger” the intubation should proceed with relative ease.

          • Prayer Sign: patients unable to approximate the palmar surfaces of their phalangeal joints while pressing their hands together commonly lack cervical spinal mobility

            • Seen in patients with advanced diabetes, for example

          • Wilson Risk Sum Score: weight, head & neck movement, jaw movement, receding mandible, “buck teeth”

          • BMI, history of OSA, neck size, presence of excessive facial hair (which obviously can be mitigated but not emergently)

      • Operator technical skill & experience can be used to mitigate many of the challenges of an anatomically difficult airway (ex: opting for awake bronchoscopic intubation) 

    • In contrast, physiologically difficult airways may not be readily apparent, or even if they are, there are not myriad scores that can be used to assess them specifically nor are there well outlined algorithms to guide management

    • ED and ICU admissions - for largely physiologic reasons - are demonstrably challenging compared to those that are done in the OR under controlled circumstances

  • Intubating critically ill patients is dangerous:

    • In the 2021 INTUBE study of almost 3000 critically ill patients undergoing intubation, a major adverse event such as severe hypotension or hypoxemia occurred in 45% of intubations.

      • Cardiac arrest occurred in 3.1% of people in the peri-intubation period.

    • Understanding physiological hazards of intubation is essential to minimize risk.


The FOUR Physiologic Derangements that occur with Intubation

    • Physiology of induction and intubation

      During the process of inducing, intubating and placing a patient on mechanical ventilation, there are 4 key elements with major physiologic ramifications

      1. 1. Administration of induction agents

        • Induction agents (sedative/hypnotic + paralytic) → loss of sympathetic tone → vasodilation → hypotension (decreased SVR & decreased CO)

        • This can be somewhat mitigated by induction strategy but in the ICU, patients are often in extremis with little physiologic reserve yet so recognize that even the most gentle approach to induction may prompt profound hypotension - be ready for this!

      2. 2. Apneic interval

        • Induction medications → apnea → hypoxemia & hypercapnia → worsening acidosis + increased PVR

        • Acidosis → worsening CO & decreased SVR

      3. 3. Airway manipulation

        • Airway stimulation during laryngoscopy may increases SBP and ICP appreciably which can be problematic - especially in the critically ill neuro patient in whom there are already ICP/CCP concerns.

        • Neuroprotective intubation strategies should be considered in these patients

      4. 4. Transition to positive pressure ventilation

        • Positive pressure ventilation → increased intrathoracic pressure during breaths which decreases preload and ultimately can decrease CO

        • Volume status and preload dependence are crucial variables



Some Illustrative Cases:

    • Cases! For a comprehensive discussion of these patients, we encourage you to listen to the Podcast! This is a quick reference discussing the primary issue and our recommended mitigation strategies.



Case #1: The “under-resuscitated intubation”

    • Case: 55M with cirrhosis admitted with a brisk UGIB, in shock with Hgb of 5 gm/dL who has yet to receive blood products when he suddenly has a marked decrement in his mental status, a drop in his SpO2 and requires a secure airway.

      • Physiological Problem: You need to do everything in your power to temporize/provide some resuscitation (i.e. defend the MAP) prior to intubation

        • This is a patient who will otherwise be significantly impacted - i.e. likely arrest - as a function of the impact of induction and positive pressure ventilation when they are markedly intravascularly depleted due to active hemorrhage

      • What do we do?

        • Call for help! You are going to want to have a lot of hands and eyes on the patient to make sure you are getting real-time updates once you are focused on the airway.

        • Fix the fundamental physiologic derangement: Initiate your institutions MTP (massive transfusion protocol)

        • IV access: Short & stocky peripheral IVs STAT if access is not already present, consider IO access if an IV cannot be quickly placed (which is a common problem in  under-resuscitated patients who are volume depleted) 

        • Attempt oxygenation via non-rebreather, perhaps with an OPA, maybe a jaw thrust - this will help washout nitrogen and enrich the air in the patient’s anatomic dead space

        • Consider having the patient lay flat in the bed, and then angle the bed so the head is up relative to the feet (reverse trendelenburg) 

        • This isn’t the best position to get airway access in general, however, it will help keep blood in the stomach and out of the airway.

        • While awaiting blood:

          • Start some crystalloid - not a lot mind you, perhaps a few 100ccs - just enough to support the patient’s MAP without diluting their oxygen carrying capacity / clotting factors. Start a vasopressor - remember these also provide venoconstriction which will help augment preload.Once blood is going and measures to support the patient’s hemodynamics are in place

      • Consider induction with ketamine or etomidate (ketamine being the preferred agent owing to some concerns regarding adrenal suppression, persistent/post-intubation hypotension, morality in sepsis and etomidate)

        • Use rocuronium for paralysis (or succinylcholine, although the risks of succinylcholine in the MICU patient are significant and should not be overlooked - big potassium shifts, oxygen consuming fasciculations)

          • Use a standard geometry VL or DL blade

            • A hyperangulated VL blade requires video visualization to be functional and it’s very easy to lose that in a contaminated environment!

            • A standard geo VL blade allows you to have the added benefits of video laryngoscopy, but you can fall back on using it like a traditional mac blade if blood obscures your view 

          • Make sure to have two working suction devices ready to go

            • Make sure to have push-dose pressors readily available, and if not done, have norepinephrine running in the background so it can easily be titrated up if need be while giving a push dose of something to support the MAP acutely.


Case #2: the “Acidemic Airway”

    1. Case: 31 M with DKA and a pH of 7.1 with a PCO2 of 15 who is stable initially, but suddenly decompensates with a repeat pH of 7.0, PCO2 of 45, lactate of 7 while looking like he is “tiring out.”

      1. Physiologic Problem: Severe Acidosis places this patient at high risk during intubation because the apneic interval can cause a low pH to drop to perilous levels.

        1. Usually we try very hard NOT to intubate these patients - they can typically compensate without mechanical ventilation and ventilators cannot provide as much ventilation as a person can breathing spontaneously.

      2. What do we do?

        1. Call for help, again! Get RT support and Bi-Level in the room - this will help you augment the patient’s respirations / decrease some WoB while preparing for next steps

          1. This may even allow you to weather the storm, although you probably don’t want to hang your hat on that “certainty”

          2. Try to talk the patient through the Bi-Level and orient them to the mask interface, consider finding an interface - such as a SCUBA-style mask or similar - that is often more tolerable than a tight fitting face mask

          3. Consider giving an anti-emetic to help reduce the risk of vomiting while using the mask 

            1. Recognize we don’t usually use Bi-Level in the altered, nauseous patient - this is a case where you do not leave the room

          4. Make sure the Bi-Level is not set to an adaptive mode but rather it provides reliable, consistent support with a high back-up rate to achieve a high minute ventilation in the setting of acidosis (18-22 RR)

        2. Volume - these folks are usually volume depleted - give a 500-1000 cc bolus of crystalloid to help support the blood pressure while preparing for likely intubation

        3. Bicarbonate - maybe yes, maybe no. There is not strong data to move the needle one way or another. There are theoretical concerns that giving bicarbonate could increase intracellular lactic acid, however, if we support the patient with PAP and need to support the pH acutely (i.e. its close to or less than 7) giving an amp of bicarbonate and assessing the response is reasonable

        4. Assuming the patient has not corrected and needs to be intubated:

          1. Consider a hemodynamically neutral induction agent like ketamine

          2. Avoid K+ shifts - definitely use rocuronium for a patient like this (succ could be fatal)

          3. The apneic interval is not your friend - these patients can go from a minute ventilation of well over 10L to 0, resulting in marked worsening of their acidosis

            1. Ensure the airway operator is experienced - not the time to staff an intern on their first airway 

            2. Keep the Bi-Level on as long as possible, even once the paralytic is pushed but increase their rate to 28-30

            3. Use the tool you are most comfortable with to get the airway as fast as possible - this will usually be VL with a hyperangulated (or standard geometry) blade 

            4. Post-intubation settings: This is not the time for a RR of 12 - consider a TV of 8-10 cc/kg IBW and a rate of 30+, get a gas in 15-20 min and adjust from there

Case #3: the “extremely hypoxemic airway”

    1. Case: 36F 22 weeks pregnant with COVID hypoxic respiratory failure was doing okay with a nasal cannula, then upgraded to high-flow and is now on big-dose high flow support at 40 LPM and 100%. She failed CPAP.

      1. Physiologic Derangement: This is the “hypoxemic patient” who need oxygen support fast and has failed non-invasive modes

        1. Pregnancy introduces some real challenges, both physiologic and anatomic: pregnant women have decreased oxygen reserves due to increased consumption and decreased lung volumes, specifically suffering from a reduced FRC due to the gravid uterus

      2. What do we do?

        1. Trust but verify: confirm that the patient cannot tolerate or failed PAP - try different interfaces and try coaching her through it. Recognize that facial anatomy during pregnancy changes and sometimes getting a good mask seal could be difficult

          1. Medications like dexmedetomidine or low-dose ketamine can help get an anxious patient comfortable and make them work with the mask rather than fight it

        2. Preoxygenation: this is a great time to consider aggressive preoxygenation - giving someone effective breaths at 100% FiO2 is essentially giving them oxygen that is enriched 5-fold allowing for nitrogen washout and permitting a longer/safer apneic interval during intubation - consider adding a NRV to the HHFNC

        3. Preparing to intubate: This isn’t heart failure or COPD - we cannot likely fix this quickly and thus, intubation will likely be necessary

          1. Alert OB so they know what’s happening (if they don’t already) - for patients that are farther along, they may want to monitor the fetus or move the patient to the OR in case an emergent C-Section is necessary

          2. If not on NIPPV, try to transition the patient to this temporarily to buy time and more effectively oxygenate while preparing to intubate - ketamine and dexmedetomidine can be friends here

          3. Remember the physiology of pregnancy 

            1. A gravid uterus means reverse trendelenburg might help keep the weight of the abdomen down as oppose to up (towards the airway) and reduce risk of aspiration

            2. Vocal cords and the upper airway may be edematous

              1. Use a smaller tube (6.0 or 6.5)

              2. Have a bougie ready 

              3. Good time for VL (probably with a hyperangulated blade)

Case #4: The “RV Failure Airway”

    1. Case: 42M with acute PE - originally classified as submissive that then develops into massive with onset of shock and BP of 80/40, complicated by respiratory distress and worsening mental status.

      1. Physiologic Derangements: This is a case of intubating in the setting of RV failure. Many aspects of intubation can worsen RV failure potentially catastrophically.

        1. The cardinal problem with PE is acute-onset RV failure

          1. Induction meds will cause vasodilation, decreasing preload, which the failing RV depends upon.

          2. Apnea will increase cause worsening hypoxemia, hypercarbia, and acidosis. All three factors increase PVR, which increases the RV afterload. PVR also changed dynamically with lung volume and too low or too high a lung volume increases PVR.

          3. The transition to positive pressure ventilation will further decrease RV preload.

        2. Preload management is challenging: too little and there isn’t enough blood flow to support the coronaries, the LV and the systemic circulation… too much and the failing RV gets overwhelmed quickly

          1. If possible, consider interventions followed by RV directed POCUS

      2. What do we do?

        1. NIPPV may be helpful as a bridge, although recognize that positive pressure will have untoward consequences relating to decreasing venous return & preload

        2. Pulmonary vasodilators - if RV afterload is a problem, why not try to reduce it? Medications like inhaled epoprostenol or nitric oxide can help in this and improve hemodynamics as evidenced by improved oxygenation, cardiac output and MAP

        3. Vasopressor support: Consider vasopressin early in these patients as you get the two prongs of systemic vasoconstriction with pulmonary vascular vasodilation

          1. Can augment with push-dose epinephrine or an epinephrine drip which may support CO and SVR more equally than norepinephrine - although there is no strong evidence or guideline to dictate the approach here

        4. Once stable, would proceed with intubation similarly to the other cases - the tool you are most comfortable with, most experienced operator, minimize the apneic interval, consider oxygenating right up until the tube goes in


Case 5: “The High ICP Airway”

  • Case: 65M with TBI, initially doing well, but becomes unresponsive and is not protecting his airway while manifesting vitals concerning for a Cushing Reflex.

  • This is the case of the High ICP airway

    1. These patients have very deranged physiology and can be very sick in the absence of needing an airway

    2. Remember that MAP-ICP = CCP

    3. Laryngeal stimulation increases BP abruptly, but also increased ICP which can be bad as far as worsening an intracranial bleed and worsening CCP depending on the specific situation

  • What do we do?

    1. Consider premedication with fentanyl or something similar to try and reduce the big bumps you could see in SBP or ICP

    2. Propofol may be a good option given the profound hypertension, but you can over do it

    3. Ketamine has been shown to be safe in these patients - thus, this may be a good time for a 50/50 mix of ketamine and propofol to give you the best of both worlds

    4. You could consider giving succinylcholine to regain your neuro exam ASAP… but you could also give rocuronium followed by sugammadex and avoid the potential risks of succinylcholine

    5. Another time when VL with hyperangulated geometry can be helpful - this allows you to reduce pressure on the larynx / jaw, apply minimal force, and get the tube in place safely


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Nick Mark
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