Guide — Mechanical Ventilation
Ventilator Waveforms & Graphics
Ventilator graphics turn the numbers into a picture you can read in seconds. This guide builds the two skills that matter most — separating an airway-resistance problem from a stiff lung, and spotting trapping, leaks, and dyssynchrony — using the same scalars the waveform quiz drills.
11 min read · Mechanical Ventilation
Written by Apex Respiratory Editorial Team
Educational use only. This material supports respiratory therapy education and exam review. It is not medical advice and is not a substitute for clinical judgment, institutional protocols, or physician orders. Always follow facility policies and current provider orders, and verify calculations independently before clinical use.
Overview
Ventilator graphics are the real-time drawings of what the breath is doing. There are two families: the scalars — pressure, flow, and volume each plotted against time — and the loops, which plot two of those variables against each other (pressure–volume and flow–volume). A single pressure number tells you something is wrong; the waveform tells you what, often before an alarm sounds or the next gas comes back.
Key Concepts
The mode sets which variable is fixed. In volume control the flow is set (classically a square wave) and pressure is the dependent variable that climbs to the peak; in pressure control the pressure is set (square) and flow is dependent, decelerating through the breath. A decelerating flow in pressure control is normal, not a problem — always read a waveform against its mode.
The inspiratory hold splits peak from plateau. Peak inspiratory pressure (PIP) is measured while gas is moving, so it carries both the resistive pressure (overcoming the airway and tube) and the elastic pressure (inflating the lung). Add a brief inspiratory pause and flow stops; the pressure falls to the plateau (Pplat), which reflects elastance — compliance — alone. The difference is the story:
The loops add two more views.The pressure–volume (P-V) loop shows compliance as its slope — it flattens toward the pressure axis when the lung stiffens, develops a flattened “beak” at the top with overdistension, and fails to close when there is a leak. The flow–volume (F-V) loop shows a scooped expiratory limb with expiratory flow limitation (COPD/asthma) and a sawtooth pattern with secretions.
Assessment & Findings
Read the scalars in the same order every time — leak, then flow, then the pressure morphology — because the abnormality lives in exactly one channel. These are the six patterns the waveform quiz drills:
| Pattern | Channel | Waveform finding | Common cause | First move |
|---|---|---|---|---|
| Normal | All three | Peak ≈ plateau (small gap); flow returns to zero; volume returns to baseline | Healthy mechanics on the set breath | No action — this is the reference picture |
| Increased airway resistance | Pressure | High peak, normal plateau — wide peak-to-plateau gap | Secretions, bronchospasm, kinked or bitten tube, mucus plug | Suction, check for kinks/biting, treat bronchospasm |
| Decreased compliance | Pressure | Peak and plateau both elevated — narrow gap; plateau > 30 | ARDS, pulmonary edema, atelectasis, pneumothorax, abdominal distension | Reduce tidal volume (plateau > 30 risks barotrauma); treat the cause |
| Auto-PEEP (air trapping) | Flow | Expiratory flow does not return to zero before the next breath | Too little expiratory time — high rate, COPD/asthma, long inspiratory time | Lengthen expiratory time (lower rate or I:E), treat obstruction, cut minute ventilation |
| Circuit / cuff leak | Volume | Expiratory volume limb does not return to baseline; P-V loop fails to close | Loose circuit connection, cuff or pilot-balloon leak, chest-tube air leak | Check connections, ET cuff and pilot balloon, chest-tube system (expect low VTE / MV alarm) |
| Flow starvation (air hunger) | Pressure | Concave (scooped) rising pressure limb in volume control | Patient inspiratory demand exceeds the set flow | Increase inspiratory flow or switch to decelerating-flow / pressure mode; reassess sedation |
Two bedside maneuvers make these reliable. An inspiratory hold (in a passive patient) gives a true plateau for the resistance-vs-compliance call; an end-expiratory hold reads total PEEP, and total minus set PEEP is the auto-PEEP.
RT Priorities & Interventions
- High peak, normal plateau → clear the airway. Suction, check for a kinked or bitten tube, and treat bronchospasm. The resistive pressure is the problem, not the lung; this triggers a high-peak-pressure alarm.
- High peak and plateau → protect the lung. A plateau above 30 cmH₂O risks barotrauma. Reduce tidal volume toward 6 mL/kg (range 4–8) of predicted body weight and treat the cause of the stiffness.
- Auto-PEEP → give time to exhale. Lengthen expiratory time by lowering the rate or the I:E ratio, treat the obstruction, and reduce minute ventilation. In COPD, adding set PEEP up to ~80% of the auto-PEEP can ease triggering without adding trapping.
- Leak → find where gas escapes. Trace the circuit connections, the ET tube cuff and pilot balloon, and any chest-tube system. Expect a low exhaled-tidal-volume or low-minute-ventilation alarm.
- Flow starvation → feed the demand. Increase the inspiratory flow, or switch to a decelerating-flow or pressure mode that delivers what the patient asks for, then reassess sedation and the underlying drive.
Common Pitfalls
- Acting on a high peak pressure as if the lung were stiff. Always check the plateau first — a wide peak-to-plateau gap is an airway problem, not a compliance problem, and the fix is different.
- Reading a plateau in an actively breathing patient. A spontaneous effort during the hold distorts the value; a true plateau needs a passive, relaxed patient.
- Missing auto-PEEP because the set PEEP looks normal. Set PEEP says nothing about trapping — the flow scalar and an end-expiratory hold do.
- Calling a leak a drop in compliance. The volume scalar settles the question: a leak fails to return to baseline; decreased compliance still returns, with both pressures elevated.
- Sedating a flow-starved patient instead of fixing the flow setting. The concave pressure limb is a settings problem first.
Board Exam Pearls
- The single most tested graphic: PIP up with Pplat normal (wide gap) = increased airway resistance → suction / bronchodilator / check the tube; PIP and Pplat both up (narrow gap) = decreased compliance.
- Plateau pressure > 30 cmH₂O → reduce tidal volume (lung-protective; barotrauma risk).
- Expiratory flow not returning to baseline before the next breath = auto-PEEP → increase expiratory time.
- Volume scalar not returning to baseline, or a P-V loop that won’t close = a leak (low exhaled VT).
- A concave (scooped) inspiratory pressure limb in volume control = flow starvation → increase the flow.
- Mode shapes the picture: volume control sets flow (square) and lets pressure vary; pressure control sets pressure (square) and lets flow decelerate.
FAQ
What is the difference between peak and plateau pressure?
Peak inspiratory pressure (PIP) is measured during gas flow, so it reflects both airway resistance and lung-chest wall elastance. Plateau pressure (Pplat), measured during an inspiratory hold with no flow, reflects elastance (compliance) alone. The peak-to-plateau gap is the resistive pressure: a wide gap with a normal plateau is an airway problem; a high plateau is a stiff-lung (compliance) problem.
How do I tell a circuit leak from decreased compliance?
Look at the volume scalar and the P-V loop. With a leak, the expiratory volume limb never returns to baseline (exhaled volume is less than delivered) and the P-V loop fails to close. With decreased compliance the volume still returns to baseline, but both peak and plateau pressures are elevated and the P-V loop flattens (rotates toward the pressure axis).
How do I detect auto-PEEP on the waveforms?
On the flow scalar, expiratory flow does not return to zero before the next breath begins — gas is still leaving the lungs when the ventilator re-inflates. The waveform shows the trapping; to measure total PEEP, perform an end-expiratory hold in a passive patient and read the pressure, then subtract the set PEEP to get the auto-PEEP.
Why does the pressure waveform sometimes look scooped or concave?
In volume control with a fixed inspiratory flow, a concave (dished-in) rising pressure limb means the patient's inspiratory demand exceeds the set flow — flow starvation, or air hunger. The patient is pulling airway pressure down. Increase the inspiratory flow (or switch to a decelerating-flow or pressure mode) and reassess sedation and demand.
Do these patterns depend on the ventilator mode?
Yes. In volume control the flow is set (often a square wave) and pressure is the dependent variable that rises to PIP; in pressure control the pressure is set (square) and flow is dependent, decelerating through the breath. The figures here show a volume-control breath with an inspiratory pause, which is where peak-vs-plateau and flow starvation read most clearly.
Put it to work
Drill these six patterns on fresh scalars every round — the quiz draws the same pressure, flow, and volume traces and asks you to make the call, with the alarm each one triggers.
Open the Ventilator Waveform Quiz →Related Resources
Sources
- Kacmarek RM, Stoller JK, Heuer AJ. Egan's Fundamentals of Respiratory Care. 12th ed. Elsevier; 2021. Ventilator graphics and waveform analysis.
- Hess DR, Kacmarek RM. Essentials of Mechanical Ventilation. 4th ed. McGraw-Hill; 2019.
- Cairo JM. Mosby's Respiratory Care Equipment. 11th ed. Elsevier; 2022. Graphics monitoring.