M20Reading the ventilator: waveforms, trigger, flow and dyssynchrony

Clinical overview

The ventilator draws a picture of the patient's respiratory system several times a minute. Learning to read it converts a box of numbers into a monitor that shows resistance, compliance, effort and trapped gas in real time — usually before the blood gas catches up.

This module is about that skill, and about the single most consequential misreading in intensive care: calling a distressed patient "agitated" or "fighting the ventilator" and reaching for sedation, when the machine is delivering breaths that do not match what she is asking for. In an obstetric unit this misreading has an extra cost, because deepening sedation to settle a mismatch delays waking, delays mobilisation, delays lactation decisions and prolongs separation from a newborn.

Core knowledge

The three waveforms and what each answers

• Pressure–time: how much pressure the system needed. Compare the peak (resistance plus compliance) with the plateau on an inspiratory hold (compliance alone). A high peak with a normal plateau is an airway/tube problem — secretions, bronchospasm, kinking, biting. A high peak and a high plateau is a lung or chest-wall problem — oedema, consolidation, pneumothorax, overdistension, or in late pregnancy the abdomen itself.
• Flow–time: the most useful and most ignored trace. Expiratory flow that never returns to baseline before the next breath means breath stacking and auto-PEEP — gas trapped in the chest, rising intrathoracic pressure, falling venous return.
• Volume–time: the exhaled volume returning to zero. A trace that fails to return signals a leak — circuit, cuff or bronchopleural.

Tap to enlarge

Figure M20.1 — Pressure–time separates resistance from compliance through the peak–plateau comparison, flow–time exposes incomplete exhalation and auto-PEEP, and volume–time reveals a leak when exhaled volume fails to return to zero.

Trigger and flow: the settings nobody adjusts

Two comfort settings explain a large share of "the patient needs more sedation":

• Trigger sensitivity. Too insensitive and she must generate a large effort to be noticed — visible as effort with no delivered breath, and exhausting. Too sensitive and the circuit self-triggers on water, cardiac oscillation or leak, delivering unwanted breaths. Flow triggering generally reduces work compared with pressure triggering.
• Inspiratory flow rate and pattern. A patient with a high respiratory drive — sepsis, acidosis, pain, anxiety, and physiologically pregnancy itself — wants gas fast. Set flow too low and she pulls against the machine for the whole breath; the pressure trace scoops downward on inspiration, and she looks agitated because she is being starved of flow.