In the spirit of the new academic year (and 1 year anniversary of CriticalCareNow!), let us now join you, the newly minted second year resident. You’ve got your new white coat (finally got a new, clean one!) Since you’ve developed an appetite for critical care medicine, your first rotation is in the MICU overnight. You are called to the bedside to help the new fellow admit a brand new patient to the ICU.
The patient is a 73 year old female with a past medical history of type II diabetes, hypertension, and hyperlipidemia. She’s here with what appears to be septic shock secondary to pneumonia. She is on 10 mcg/min of norepinephrine to maintain her mean arterial pressure > 65 mmHg, and the fellow has just finished intubating the patient. The fellow asks you what your next steps would be.
A) Say “well, she sounds pretty sick…” while backing away slowly. The fellow can handle this!
B) Her problem is that she needs more oxygen, right? Sick lungs need oxygen… RIGHT? Give her more!!
C) Ask for some more information before making any decisions.
D) Let the intern handle this admission. She’s experienced enough to do these admits now.
The answer is C (to no great surprise). So, let’s get more information!
You sneak a look at her ventilator settings, and notice that she’s set to volume control mode with a tidal volume of 600 mL, a rate of 12, PEEP of 10, and FiO2 of 100%. You ask for an ABG, but the fellow asks you if there is something you can do right now that’ll make an impact on this patient’s chances of survival before even getting the ABG.
A) Anoint the patient in essential oils. Oil of Lavender is quite good for ARDS.
B) Maybe she needs to breathe deeper? I read an article by some guy on some website that mentioned a deep breath… increase the tidal volume!
C) Ask for her height and what her 6 mL/kg predicted body weight tidal volume is
Unless this patient is a giant, there is no way that 600mL is 6mL/kg tidal volume based on predicted body weight (PBW). A 6’6” male’s 6 mL/kg is only 550 mL. Why 6 mL/kg? This was based on one of the most important trials in the critical care literature, the ARMA trial, which was published in May 2000. ARMA looked at lung protective ventilation at 6 mL/kg vs “traditional” ventilation of 12 mL/kg, and demonstrated that lung protective ventilation conferred a mortality benefit and lowered the number of ventilator dependent days for mechanically ventilated patients with ARDS.
ARMA-ed with this knowledge (ok, bad joke), you ask the RT to lower the tidal volume to 6 mL/kg based on this patient’s PBW (full disclosure: none of the RT’s I’ve ever worked with have ever started a patient out on this high tidal volume; this is exaggerated purely for this scenario!). You also ask for an ABG to be sent off in 30 min to see where our baseline is. The fellow nods her approval, and you have some time to consider your next moves. You decide to start the patient on broad spectrum antibiotics to treat the underlying pneumonia while waiting for the gas to come back. Luckily, our patient’s hemodynamics are not worsening so far.
The ABG finally comes back, and shows a pH of 7.30, PaO2 of 180 mmHg, PCO2 of 50 mmHg and a HCO3 of 24. What would your next moves on the ventilator be?
A) The higher the PaO2, the better, right? More oxygen is good! Let’s just not touch anything.
B) PEEP? Let’s give PEEP? Can we add PEEP?
C) We should be able to drop the FiO2 to 80% (maybe more) and recheck a gas in 1 hour (but collaborate with your RT and make a plan!)
C is the best choice here. In this example, the P/F with the PaO2 being 180 mmHg and the FiO2 being 100% is 180. The ABG values here have been exaggerated to demonstrate a point, but let’s imagine these are the real values. You should be able to drop the FiO2 without any issue (which will lower the PaO2, but will also aid in your overall lung protective strategy in terms of preventing hyperoxic lung injury).
Granted, there are a ton of finer details that we can go over in terms of checking Peak Inspiratory Pressures (PIPs), Plateau Pressures, and Driving Pressure (DP) to make sure we are not causing pressure induced lung injury, allowing permissive hypercapnia in severe ARDS, when to prone, etc. but this article is focusing on the very basics of ARDS management.
- Lung protective strategy has been proven to reduce mortality and increase ventilator free days in mechanically ventilated patients with ARDS.
- The key number to remember is 6 mL/kg of predicted body weight (which is based on the patient’s height).
In our next article, we will follow up the repeat ABG and see what our interventions did, and we will learn about more advanced techniques/therapies that we can employ in order to do our best to protect our patients’ lungs while they heal.