The Physiologically Difficult Airway

Why this matters

Anatomy is what you were tested on. Physiology is what kills patients in the ambulance. A physiologically difficult airway is a patient who will desaturate, drop their blood pressure, or arrest during airway management even when the view of the cords is easy. Collapse around the tube is common in the critically ill. The bag raises pressure in the chest and reduces blood returning to the heart. When you stop their breathing, you also stop the fast breaths an acidotic patient was using to stay alive. Your job is not a faster tube. It is oxygen, blood pressure, and a plan for the two minutes after the tube.

Clinical sequence

1. If they need a tube, say why they may arrest. Say the sat, the blood pressure, the acidosis story, or the right ventricle (RV).
2. Buy time. Give oxygen. Use noninvasive ventilation or delayed sequence intubation only if your protocol and the picture fit. Give fluids or a push-dose vasopressor only per protocol. Put the defibrillator pads on. See the preoxygenation lesson.
3. Plan the first pass. Keep the time without a breath short. Use video laryngoscopy if you have it. Your partner watches the sat and the blood pressure, not the view.
4. Once the tube is in, confirm the waveform, then bag to match the patient. Acidotic patients were breathing fast on purpose. A hard, fast bag drops the blood pressure in a hypotensive patient.

Teaching points

• Four problems can turn RSI into pulseless electrical activity. They are low oxygen, low blood pressure, severe metabolic acidosis, and right-ventricular failure.
• The bag and a low blood pressure do not mix. Positive pressure cuts blood returning to the heart. A dry patient, or a patient with dilated blood vessels, can lose their pressure. Treat the pressure first when you have seconds to do it.
• Acidosis is a ventilation problem. They were compensating with a very high minute ventilation. A long pause without breaths, or a slow bag, lets carbon dioxide and acid rise again. Match their effort. Do not slow them to 8 quiet breaths if that was not how they were breathing.
• An easy view is not a safe airway. Physiology is its own kind of difficult airway. A memory list such as HEAVEN can help if that is what your service teaches. It mixes low oxygen, extremes of size, anatomy, vomit, blood, or fluid, bleeding out, and neck mobility. Still name the physiology out loud.
• Your protocol owns the induction drug. Induction and paralysis remove the tone that was keeping the blood pressure up. Have a blood-pressure plan before you push the drug.

Common errors

• Starting RSI because the view will be easy, while the sat is 86% and the blood pressure is 78.
• Bagging a patient in diabetic ketoacidosis as if they were a stable elective case.
• Calling arrest around the tube “they were going to arrest anyway,” without naming the physiology you did not treat.
• Skipping fluids or a protocol vasopressor so you can just get the tube.

Practice

Related: Preoxygenation & Apneic Oxygenation, Delayed Sequence Intubation, Difficult Airway Anticipation.

Sources & Further Study

Named guidelines for further study. They are not clinical orders and do not replace your protocol or medical direction.

• NASEMSO National Model EMS Clinical Guidelines (2022) — U.S. EMS model language; follow your protocol
• Mosier et al. — The Physiologically Difficult Airway (WestJEM 2015) — hypoxemia, hypotension, metabolic acidosis, RV failure
• Society for Airway Management 2021 consensus — physiology as its own difficult-airway class
• INTUBE (JAMA 2021) — cardiovascular collapse around the time of intubation is common in the critically ill