Acute Respiratory Distress Syndrome (ARDS)
6-8 mL/kg
Tidal Volume
Using IDEAL body weight
<30 cmH₂O
Plateau Pressure
Maximum safe pressure
<150
Prone Threshold
PaO2/FiO2 ratio for prone positioning
Berlin Definition
four criteria, all must be metAcute onset (within 1 week of known insult or new/worsening respiratory symptoms)
Bilateral opacities on chest imaging (not fully explained by effusions, collapse, or nodules)
Respiratory failure NOT fully explained by cardiac failure or fluid overload
PaO2/FiO2 ratio classifies severity
Mild
P/F 200-300
Moderate
P/F 100-200
Severe
P/F <100
Lung-Protective Ventilation
Tidal volume uses IDEAL body weight based on height, NOT actual body weight. This is the most commonly tested ARDS concept. An obese patient and a lean patient of the same height receive the same tidal volume.
Tidal Volume 6-8 mL/kg IBW. Prevents volutrauma to damaged alveoli
Plateau Pressure <30 cmH₂O. Prevents barotrauma
PEEP Titrate to oxygenation. Keeps alveoli open (recruitment)
FiO2 Lowest to maintain SpO2 88-95%. Prevents oxygen toxicity
Prone Positioning P/F <150, ≥16 hrs/day. Recruits dorsal lung segments
Key Values to Memorize
TV 6-8 mL/kg IBW
Plateau <30 cmH₂O
Prone if P/F <150
PAOP ≤18 (not HF)
SpO2 target 88-95%
Prone ≥16 hrs/day
you know the pathophysiology, now prove it under pressure
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Practice CCRN QuestionsExam Traps
Tidal volume uses IDEAL body weight, not actual weight
Ideal body weight is calculated from height, not the scale weight. An obese patient who is 5'4" gets the same tidal volume as a lean patient who is 5'4". Using actual weight causes volutrauma because you are ventilating excess body mass that does not have lungs.
ARDS vs cardiogenic pulmonary edema (PAOP ≤18)
Both cause bilateral infiltrates and hypoxemia. The key differentiator is the PAOP. ARDS = PAOP ≤18 mmHg (non-cardiogenic). If PAOP is elevated, the fluid is from heart failure, not ARDS. The exam loves this distinction.
Prone positioning threshold is P/F ratio less than 150
Prone positioning is recommended for moderate-to-severe ARDS when the PaO2/FiO2 ratio falls below 150 despite optimal ventilator settings. Sessions should last at least 16 hours per day.
Conservative fluid strategy improves outcomes
ARDS lungs are already waterlogged. Liberal fluid administration worsens pulmonary edema. A conservative or restrictive fluid strategy (once the patient is hemodynamically stable) reduces ventilator days.
ARDS vs Heart Failure
ARDS = PAOP ≤18 mmHg (non-cardiogenic). If PAOP is high, think heart failure, not ARDS. The bilateral infiltrates look similar but the cause is different.
Prone Positioning
Prone positioning for severe ARDS (P/F <150). It recruits dorsal lung segments and improves oxygenation. At least 16 hours per session.
Healthy lungs are like a dry, fluffy sponge that absorbs oxygen easily. ARDS makes the sponge waterlogged with inflammatory fluid. The wet, heavy sponge cannot absorb oxygen efficiently.
Low tidal volume ventilation is like gently squeezing the wet sponge instead of wringing it out. Aggressive squeezing (high TV) tears the fragile sponge fibers. 6-8 mL/kg IBW is the gentle approach.
When you lay the sponge flat, all the water pools at the bottom (dependent lung regions). Flipping the sponge over (prone positioning) lets the waterlogged bottom drain while the drier top can absorb oxygen.
PEEP is like keeping gentle pressure on the sponge so its pores stay open. Without PEEP, the water-heavy sponge collapses shut and cannot exchange gas at all.
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