Ventilator Dyssynchrony After Sedation Bolus
A 58-year-old man with ARDS (P/F ratio 140 on PEEP 12 cm H2O, FiO2 0.55) is ventilated in volume-control, 6 mL/kg predicted body weight (420 mL), rate 22, inspiratory time 0.9 s. He was calm, but 10 minutes after a propofol bolus for a line placement his respiratory drive dropped. Total rate now reads 10/min, peak pressure fell from 29 to 21 cm H2O, and the flow-time scalar shows the expiratory limb never returning fully to baseline before the next mandatory breath. Plateau pressure is 24 cm H2O. SpO2 91%, EtCO2 38 mmHg. The nurse asks whether to increase the set rate, add pressure support, or change something else.
1. Identify the most likely cause of the low total rate and the flow-time abnormality, and state your first action.
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What the numbers say. Set rate 22 but total rate 10 means the patient has essentially stopped triggering; nearly every breath is mandatory. Peak pressure dropped 8 cm H2O while plateau stayed at 24, so the drop is in resistive/flow-related pressure, not compliance. The key clue is the expiratory flow not returning to zero before the next breath: that is air trapping / incomplete exhalation, not a patient problem.
Why it happened. With a 0.9 s inspiratory time and a rate of 22, expiratory time is short. When the patient's own drive disappeared after the propofol bolus, the mandatory rate alone does not allow enough expiratory time at this I-time, so gas stacks. Rising auto-PEEP reduces effective triggering and can suppress inspiratory effort.
First action. Do not reflexively increase the rate or add pressure support. Shorten inspiratory time (e.g., to 0.6–0.7 s) or reduce the set rate to lengthen exhalation, then reassess total rate, auto-PEEP (end-expiratory hold), and hemodynamics. Increasing rate or adding support would worsen trapping. Also re-evaluate sedation depth and airway/ventilator disconnection as contributors.
2. The team shortens I-time and the total rate rises to 20. Two hours later peak pressure is 34 and plateau 30 cm H2O. SpO2 is 88% on FiO2 0.55. What is the priority interpretation and next step?
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Interpretation. Driving pressure = 30 − 12 = 18 cm H2O, and plateau rose 6 cm H2O from baseline. This is a compliance deterioration, not an airway/secretions problem (peak-plateau gradient is only 4). In ARDS with a falling P/F and rising plateau, consider progression of lung injury, atelectrauma, or a new process such as pneumothorax, mucus plugging with derecruitment, or abdominal distension.
Next step. Do not simply raise FiO2 and walk away. Perform a focused bedside assessment: bilateral breath sounds, chest wall/abdomen exam, suction, and check for auto-PEEP again. Verify plateau with an inspiratory hold. If unilateral findings or sudden deterioration, get immediate imaging for pneumothorax. Meanwhile titrate PEEP/FiO2 to target SpO2 88–95%, keep plateau ≤30 cm H2O and driving pressure low, and consider lung-protective adjustments. Escalate to the provider for possible recruitment assessment or further workup.
3. The patient stabilizes but remains on FiO2 0.55 with PEEP 12 and SpO2 92%. A colleague suggests a spontaneous breathing trial now. What factors determine readiness, and what would you check before agreeing?
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Readiness criteria to verify. Underlying cause improving or resolved; P/F adequate (typically PaO2/FiO2 >150–200 on PEEP ≤8–10 and FiO2 ≤0.4–0.5 for a true SBT); hemodynamic stability without escalating vasopressors; adequate mental status; manageable secretions; no ongoing need for deep sedation; afebrile; and acceptable respiratory mechanics (e.g., RSBI <105, NIF adequate).
This patient does not yet meet typical criteria. FiO2 0.55 and PEEP 12 are above usual SBT thresholds, and he had a recent compliance deterioration. Attempting an SBT now risks failure and re-intubation.
What to check first. Confirm the cause of the earlier deterioration is treated, wean FiO2 toward ≤0.4–0.5 and PEEP toward ≤8 while maintaining SpO2 88–95%, verify adequate cough and secretion clearance, assess sedation and delirium, and review hemodynamics and nutrition. Only then perform a supervised SBT with close monitoring and clear failure criteria (RR >35, SpO2 <90%, tachycardia, distress).