High Peak Pressure With an Unchanged Plateau
A 63-year-old man is ventilated in volume-control mode after an open abdominal procedure: set tidal volume 500 mL, rate 14/min, PEEP 5 cm H2O, FIO2 0.40, high-pressure limit 40 cm H2O. Over 20 minutes the high-pressure alarm sounds repeatedly. Peak inspiratory pressure has climbed from 24 to 37 cm H2O. An inspiratory hold maneuver gives a plateau pressure of 21 cm H2O (it was 20 cm H2O earlier). Exhaled tidal volume is 410 mL on the last three breaths. Breath sounds are coarse over the trachea and diminished at both bases. The patient is intermittently biting the tube, and secretions are visible in the airway. SpO2 is 93% (was 97%) and HR is 118/min.
1. Using the pressure data, what changed - airway resistance or lung compliance? Show the arithmetic.
Read the worked answer
Resistance increased. Compliance did not change meaningfully.
- Driving pressure across the airway = peak − plateau. Earlier: 24 − 21 = 3 cm H2O? No - earlier peak 24 with plateau 20 gives a gradient of 4 cm H2O. Now: 37 − 21 = 16 cm H2O. That four-fold rise in the peak-to-plateau gradient is the signature of increased resistive load (secretions, tube biting/kinking, bronchospasm, mucus plug).
- Plateau pressure reflects alveolar distending pressure and therefore compliance. It moved only 20 → 21 cm H2O, so compliance is essentially unchanged; this is not a pneumothorax, atelectasis, or abdominal-distension pattern.
- Conclusion: the problem is in the airway/proximal to the alveoli, not in the lung parenchyma. That directs the intervention to the tube and airways rather than to ventilator settings.
2. List your actions in priority order, and state why simply raising the high-pressure limit would be wrong.
Read the worked answer
Priority actions:
- Look at the patient first - inspect for biting, coughing, tube position, and check the capnogram/SpO2 trend. Biting alone can generate this gradient.
- Suction the airway (the secretions are visible), using pre-oxygenation and limiting each pass to about 10–15 seconds.
- Insert a bite block or oral airway if the patient is biting; consider whether sedation/analgesia is inadequate.
- Assess for bronchospasm (wheeze, rising gradient) and treat with a bronchodilator if indicated.
- Check the circuit and tube for kinks, water accumulation, or a partially obstructed tube; evaluate the need for tube exchange if obstruction persists.
- Re-measure peak and plateau, and reassess SpO2/HR after each step.
Why not just raise the alarm limit: the alarm is a symptom, not a setting error. Raising the limit silences the warning while the patient continues to breathe against a rising resistive load - that means increased work of breathing, possible incomplete exhalation and air trapping (note the exhaled volume has already fallen to 410 mL), and risk of unrecognized tube occlusion. The high-pressure limit is a safety boundary for the patient's airway, not a nuisance to be adjusted away.
3. The exhaled tidal volume is 410 mL against a set 500 mL. Should you increase the set volume to compensate?
Read the worked answer
No - correct the cause first.
Reasoning:
- In volume-control, the ventilator delivers the set volume; a low exhaled volume means gas is being lost or compressed before it is exhaled. With pressures this high, compressible volume in the circuit accounts for part of the loss, and the resistive problem may be preventing full delivery or causing a leak around the tube.
- Raising the set tidal volume would raise peak pressures further, increase the resistive work imposed on the patient, and increase the risk of volutrauma and air trapping - without addressing the obstruction.
- Once secretions, biting, and tube patency are managed, re-measure. If the delivered volume still lags the set volume at acceptable pressures, then evaluate for a circuit or cuff leak, and only then adjust the set volume.
- Watch plateau pressure as the safety target (generally keep ≤30 cm H2O); the peak pressure alone should not drive the volume decision.