A Curious Case of Refractory Hypoxemia

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Matt Siuba
Matt Siuba
Zentensivist. ARDS, Mechanical Ventilation, RV, & Shock Enthusiast.

The Pre-brief

A young adult male is transferred to your facility due to reported ARDS and severe hypoxemia related to pancreatitis. He was intubated one day prior to transfer for encephalopathy and suspected aspiration. 

He arrives to your ICU, hemodynamically stable, on 100% O2, PEEP 12, and tidal volume 8 ml/kg, with saturations in the mid 80%. You are called to bedside due to the hypoxemia. No additional data is available at this point.

Physical exam is otherwise unremarkable.

What’s your next step?

  1. Prone position ventilation
  2. VV-ECMO
  3. Examine the equipment (vent, tubing, etc)
  4. Increase PEEP

It’s too early to make any treatment decisions without more data. As such, you examine the ventilator and equipment. Pressure waveform with plateau pause is shown below:

Just like “one view is no view”, one waveform is no waveform! Let’s examine the volume curve:

What is happening?

  1. Reassuring findings
  2. There’s a leak!
  3. This is an airway resistance problem
  4. This is a compliance problem

The difference between plateau and PEEP was suspiciously small, so you were concerned about a leak in the system, which was reflected in the incomplete return to baseline of exhalation in the volume waveform (see the arrow below). As you are describing your findings, the nurse mentions “oh yeah, transport said they had to keep adding air to his endotracheal tube cuff, and have been since yesterday(!)”.

The endotracheal tube is exchanged, and examination of the old ETT shows the cuff was indeed torn! Unfortunately, this change did not fix the hypoxemia, and the vitals are roughly unchanged. Chest x-ray has been paged and has not yet arrived at bedside.  

Back to the ventilator:

Being the intensivist you are, you naturally attempt to increase the PEEP. Here is the data:

Titration attempts are labeled 1, 2, 3, and 4 on the graph. Unfortunately every increase in PEEP lead to successive decreases in saturation. 

What is the next step?

  1. Prone positioning
  2. VV-ECMO
  3. Chest US and Echo
  4. Just turn down the PEEP!
  5. More than one of the above

PEEP was returned to 12 while you roll in the ultrasound machine. 

Chest US is notable for predominant A-line pattern bilaterally with lung sliding present. Focused cardiac ultrasound shows normal RV and LV size and function. Bubble study shows no intracardiac shunt.

Chest XR finally arrives at bedside!

In the interim, you have titrated the PEEP downwards, all the way to 5; tidal volume is dropped to 6 ml/kg. O2 is able to be weaned to 30% within the hour! He is uneventfully extubated the following day.

This case highlights key features of diagnostic reasoning, particularly avoidance of premature closure. The combination of history from the referring facility (“ARDS” with plausible mechanisms of aspiration pneumonitis as well as pancreatitis) plus our “normal” behaviors created a mental trap. New patients need to be evaluated de novo despite working diagnoses of others. 

Increasing PEEP was clearly detrimental; the question was why? Key differential diagnoses include:

(1) Intracardiac shunt (PFO or similar) which would have worsened with increased pulmonary vascular resistance. This was ruled out by the bedside bubble study.

(2) Overdistention of the lung causing dead space physiology. This was a bit harder to detect in this case because there were no clear ventilator signs of overdistention. The driving pressure (Pplat – PEEP) remained constant during PEEP titration. This is likely due to very compliant alveoli compressing the adjacent capillaries (see image below), as the chest x-ray and lung ultrasound did not suggest any significant lung pathology. The lung fields are rather small on x-ray, perhaps as a consequence of abdominal distention related to pancreatitis.

The Debrief

  1.  Start with the simple things. Equipment checks are vital!
  2. Consider shunt physiology (of many forms) when evaluating the patient with hypoxemia out of proportion to other findings.
  3. Given the inexpensive and non-invasive nature of POCUS, it is reasonable to perform echo and lung ultrasound on all unexplained cases of respiratory failure. It is part of my physical exam.
  4. Refractory hypoxemia deserves a careful, systematic approach to its evaluation. For an approach to hypoxemia in ARDS, see the following post: https://criticalcarenow.com/2020/05/21/how-to-manage-refractory-hypoxemia

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