Post-CABG Patient with Rising Plateau Pressure and Shock

Separate resistance from compliance before you do anything else.

• Driving pressure: 36 − 8 = 28 cmH2O (was 20 − 8 = 12).
• Static compliance: 400 / (36 − 8) = ~14 mL/cmH2O (was ~33).
• PIP − plateau: 44 − 36 = 8 cmH2O, which is a normal airway resistance at typical flows.

So this is a compliance problem, not a resistance problem. A mucus plug or bronchospasm large enough to triple resistance would widen the PIP–plateau gradient; it did not. Also, the patient's tidal volume on pressure support is unchanged at 400 mL, which argues against a pure resistance/secretions explanation.

Differential for an acute fall in respiratory system compliance with shock in a cardiac surgery patient:

1. Tension pneumothorax / hemothorax - especially with a chest tube that has stopped draining and stopped bubbling, which raises concern for a clotted or kinked tube.
2. Atelectasis or lobar collapse (left base findings).
3. Cardiac tamponade - does not usually raise plateau much, but it is the diagnosis that kills this patient first and is suggested by distended neck veins, hypotension, tachycardia, and a non-draining mediastinal tube.
4. Intra-abdominal process: distension, ileus, intra-abdominal hypertension.
5. Pulmonary edema, mainstem intubation, large pleural effusion.

The key judgement: the mechanics point to the lung/chest wall, but the hemodynamics point to a mechanical obstruction of venous return. Treat them as two parallel problems to be sorted out at the bedside within minutes, not sequentially.

Move in order of what changes management fastest:

1. Expiratory hold (total PEEP). Distinguishes true compliance loss from dynamic hyperinflation falsely elevating plateau.
2. Confirm tube position and patency - depth marking, capnography waveform, pass a suction catheter. Rules out mainstem migration and a mucus-plugged tube.
3. Chest tube assessment - inspect for kinks, clot, dependent loops; strip/milk only per protocol; check the drainage chamber and the water seal. A clotted mediastinal tube in an early post-CABG patient is a tamponade warning.
4. Lung and cardiac ultrasound at the bedside - absence of lung sliding with a lung point supports pneumothorax; a pericardial effusion with RV diastolic collapse supports tamponade. This is the single highest-yield test in this patient because it answers both questions at once.
5. Focused exam - tracheal position, subcutaneous emphysema, unilateral breath sounds, neck vein distension, pulsus paradoxus if measurable, skin perfusion.
6. ECG and labs - rate/rhythm, lactate, and a quick hemoglobin if bleeding is suspected.

Do not delay decompression or a call to the surgical team to complete a full set of measurements in an unstable patient. If tension pneumothorax or tamponade is suspected and the patient is deteriorating, act now and confirm afterward.

It removes air trapping from the differential and makes the plateau value trustworthy.

• Auto-PEEP is only ~1 cmH2O. Any clinician who attributed the high plateau to breath stacking would now be wrong.
• The compliance change is real: ~14 mL/cmH2O with a driving pressure of 28 cmH2O is severe and cannot be explained by ventilator settings alone.

Immediate action, in priority order:

1. Call for help and notify the surgical team. A post-CABG patient with this combination needs a surgeon at the bedside.
2. Bedside ultrasound now to look for pneumothorax and pericardial effusion.
3. Treat shock in parallel - fluid bolus and/or vasopressor as needed to maintain perfusion while the diagnosis is being established. Do not make the patient's pressure depend on finishing the diagnostic workup.
4. If tension pneumothorax is suspected on exam or ultrasound, decompress immediately, then place a chest tube; consider whether the existing mediastinal tube is clotted and needs replacement.
5. If tamponade is suspected, the surgical team must open the chest or perform pericardiocentesis.
6. Adjust the ventilator defensively: keep plateau as low as achievable with the pH, accept permissive hypercapnia, and avoid adding PEEP during active hypotension since venous return is already compromised.

The judgement point is that a normal auto-PEEP measurement does not rule out an acute life threat - it only eliminates one item and should accelerate you toward imaging and decompression, not reassure you.