One equation, built up until you can name any mode and read any waveform.
At every instant, the pressure from the patient's muscles plus the ventilator must overcome the elastic load (elastance E = 1/compliance, times volume) and the resistive load (resistance × flow).
Modes, waveforms, and patient–ventilator interaction all read off this one balance. [Mireles-Cabodevila 2025]
Pressure, flow and volume are not three separate pictures — they are the one equation plotted over time.
Learn the normal shapes; every finding is a deviation from them.
Whatever you set becomes a clean square wave and tells you nothing about the patient. Setting volume alone, or flow alone, is not volume control.
A breath is one cycle of positive (inspiratory) then negative (expiratory) flow. Inspiration is started by a trigger and ended by a cycle.
Who triggers and who cycles is what classifies the breath.
| Breath | Trigger | Cycle | Meaning |
|---|---|---|---|
| Spontaneous | Patient | Patient | The patient both starts and ends it — independent of the machine's timing settings (e.g. pressure support) |
| Mandatory | Ventilator or patient | Ventilator | The ventilator triggers and/or cycles it; assisted by definition (e.g. a time-triggered, time-cycled A/C breath) |
Trigger and cycle are the two switches. A breath the patient starts but the ventilator ends is still mandatory. Only when the patient owns both is it spontaneous.
Continuous mandatory. Spontaneous breaths are not possible between mandatory breaths; set frequency is a minimum.
Intermittent mandatory. Spontaneous breaths are possible between mandatory ones; set frequency is a maximum. Five types.
Continuous spontaneous. Every breath is patient-triggered and patient-cycled.
Control variable (2) × breath sequence (3) → five patterns: VC-CMV, VC-IMV, PC-CMV, PC-IMV, PC-CSV. VC-CSV is impossible — volume control implies ventilator cycling, which makes every breath mandatory. [Chatburn 2014]
| Scheme | What it does | Example |
|---|---|---|
set-point (s) | Operator sets every parameter of the waveform; fixed settings | Conventional VC, PC, PSV |
dual (d) | Switches between volume and pressure control within one breath | VAPS |
bio-variable (b) | Randomly varies pressure/VT to mimic natural breathing variability | Variable / noisy PS |
servo (r) | Output follows a varying input — pressure proportional to effort | PAV, NAVA, ATC |
adaptive (a) | Sets one target between breaths (pressure) to hit another (average VT) | PRVC, VC+, AutoFlow |
optimal (o) | Adjusts targets to minimize work rate of breathing | ASV |
intelligent (i) | Adjusts targets with AI (rule-based, fuzzy logic, neural nets) | SmartCare/PS, IntelliVent-ASV |
We elaborate on set-point, servo, adaptive, and optimal (ASV) next. [Chatburn 2014; Mireles-Cabodevila 2013]
Drive the patient's effort up and watch the airway pressure in each scheme.
These display signatures let you name the targeting scheme at the bedside. [Chatburn 2026]
Optimal targeting adjusts the whole ventilatory pattern to minimize an overall cost — here the work rate of breathing. For a required alveolar ventilation, very slow breaths cost elastic work (big VT) and very fast breaths cost resistive work; a rate in between costs the least.
ASV picks the rate at the minimum and sets VT to match, then re-optimizes breath to breath.
Illustrative model of the optimal-targeting principle; ASV is optimal targeting on a PC-IMV chassis. [Chatburn 2014]
PRVC / VC+ / AutoFlow / “volume guarantee” set a tidal-volume target but control pressure breath-to-breath, adjusting it between breaths to hit an average VT.
By behaviour they are PC-CMVa — pressure control, CMV, adaptive — not the volume mode the brand name suggests.
You cannot read the waveforms until you know what the ventilator is actually programmed to do. The TAG = control variable + breath sequence + targeting tells you.
PC-CMVa[Mireles-Cabodevila 2022; Majumdar 2025]
[Chatburn 2014, ten fundamental maxims]
You set volume & flow →
read the load off PRESSURE
You set pressure →
read the load off FLOW & VOLUME
Whatever you set is a flat square and carries no information; the patient lives in the other waveform. [Mireles-Cabodevila 2022]
In VC the inspiratory flow is set — it stays square regardless of mechanics. The load shows in the pressure:
Pressure is the square wave, so the patient shows up in flow & volume. Peak inspiratory flow = ΔP / R — set by the pressure and resistance, independent of compliance.
Change compliance and the peak does not move: what changes is the decay slope (τ = R·C) and the tidal volume it fills to. Flow decays to zero over about 3τ.
Expiration is passive: flow decays on the time constant. If the next breath starts before the lung empties (Te < ~3τ), the expiratory flow does not return to zero — gas is trapped and auto-PEEP builds.
R and C are fixed here, so the peak flow and the decay slope don’t change — only the expiratory time shrinks. Watch the flow fail to reach zero and auto-PEEP appear. Fix: slow the rate, lengthen expiration.
PIP high & Pplat high
→ compliance (elastic) problem
Pneumothorax, edema/ARDS, atelectasis, mainstem intubation, abdominal pressure, breath stacking.
PIP high & Pplat normal
→ resistance problem
Kinked or bitten tube, secretions / mucus plug, bronchospasm.
The peak-to-plateau gap is the resistive pressure — the same R·V̇ term from the equation of motion.
Safe gas exchange on the compliant part of the P–V curve. [Mireles-Cabodevila 2013; El-Khatib 2024]
The reference signal is the patient's effort, Pmus (from esophageal pressure or EAdi, or inferred from the waveforms). Interaction has two axes:
In VC, effort scoops the pressure concave — flow starvation.
When the patient and ventilator share the work, the tell depends on the control variable:
Work shifting is not inherently abnormal and can occur with perfect timing. [Mireles-Cabodevila 2026]
Δt = tvent − tmus: negative = early, zero = synchronous, positive = late. [Mireles-Cabodevila 2026; Liu 2024]
A companion to the mode maxims — same equation, different subject. [Mireles-Cabodevila 2026]
Name the mode
as a TAG — control + sequence + targeting
Read the load
off the waveform opposite the control variable
Diagnose interaction
phase by phase: trigger → inspiration → cycle → expiration
4 · Intervene. Pick the single primary goal — safety, comfort, or liberation — then adjust settings, change the mode, or do nothing. Many interactions are temporary and harmless. [Mireles-Cabodevila 2022]
Four virtual patients. In your groups, read the waveforms and vitals, choose a management step, and see how the patient responds — then reach a debrief. ARDS · severe obstruction · dyssynchrony · the post-intubation crash.
Scenario console → vent-megacode.pages.dev
Everything in this hour, written to read top-to-bottom — plus the interactive simulator the figures come from.
vent-waveforms.pages.dev
Figures and concepts adapted from the published mechanical-ventilation work of R.L. Chatburn & E. Mireles-Cabodevila. Educational reference; not a medical device; not for clinical use.