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Reading ventilator waveforms
Three traces, one equation. Once you can see which term is changing, most ventilator problems announce themselves before the numbers do.
The equation underneath
Every pressure the ventilator shows you is the sum of three things.
The first term is elastic: the cost of distending the lung and chest wall, and it is what the plateau pressure measures. The second is resistive: the cost of driving gas through the tube and airways, and it exists only while gas is moving. The third is the baseline you set.
Nearly all waveform interpretation is working out which of those three changed.
Pressure against time
In volume control the shape is set by the flow pattern, and the height is the patient. Peak pressure is reached while gas is still flowing, so it carries both the elastic and the resistive load. Hold the breath at end-inspiration for half a second, flow stops, the resistive term disappears, and the pressure falls to the plateau.
The gap between them is the resistive load, and it should be under 5 cmH2O. Above 10, suspect resistance.
| What moved | What it means | Usual causes |
|---|---|---|
| Peak up, plateau unchanged | Resistance has risen | Bronchospasm, secretions, a kinked or bitten tube, a mucus plug |
| Peak and plateau both up | Compliance has fallen | Atelectasis, edema, worsening ARDS, pneumothorax, mainstem intubation, abdominal distension |
| Both down | Compliance improving, or a leak | Disease resolving, or a circuit disconnect or cuff leak. Check the returned tidal volume to tell them apart |
| Plateau below PEEP | Impossible | A measurement error, or unmeasured auto-PEEP |
Flow against time
The flow trace is where trapping and effort show up. The single most useful question to ask of it: does expiratory flow reach zero before the next breath starts? If it does not, gas is being stacked, and there is auto-PEEP.
A scooped, concave inspiratory limb in volume control means the set flow is below what the patient is asking for. They are pulling against the ventilator, and the fix is more flow or a faster rise time rather than more sedation.
Volume against time
Volume is the integral of flow, so it carries no information flow does not, but it makes one thing obvious: whether the volume that went in came back out. A trace that does not return to baseline is a leak, around the cuff or somewhere in the circuit.
Patterns worth knowing on sight
| Pattern | Reading |
|---|---|
| Expiratory flow truncated by the next breath | Auto-PEEP |
| Scooped inspiratory pressure limb | Flow starvation: the patient wants more than is set |
| A small pressure dip with no breath delivered | Ineffective triggering, usually from auto-PEEP or a trigger set too insensitive |
| Two breaths back to back with no pause | Double triggering, usually a tidal volume or inspiratory time below what the patient wants |
| A sharp spike at end-inspiration | Delayed cycling: the patient is exhaling while the ventilator is still inspiring |
Before you touch the settings
A new waveform abnormality in a deteriorating patient is a reason to run DOPES first: displacement of the tube, obstruction, pneumothorax, equipment failure, stacked breaths. Sedating a patient who is fighting the ventilator because of a pneumothorax removes the only sign you had.
Nobody ever mastered a ventilator by reading about one.
Change resistance and compliance one at a time on a live patient and watch which trace responds. The Ventilator Workbook puts a live ventilator in front of you with a simulated patient behind it, so you can break things that cannot be harmed. Three modules and time on the simulator are free, and no card is needed.