Terminology compendium

Positive End-Expiratory Pressure

Pressure maintained in the airway at end-expiration. Recruits collapsed alveoli, improves oxygenation, and prevents atelectrauma. High PEEP can reduce venous return and cause hypotension.

Fraction of Inspired Oxygen

The percentage of oxygen in the delivered gas (21% = room air). Titrate to the target SpO2 — use the lowest FiO2 that achieves it to avoid oxygen toxicity.

Tidal Volume

The volume of gas delivered per breath. Set on predicted (ideal) body weight, never on the weight on the stretcher — 6 mL/kg as the default for every ventilated patient, with 4–8 mL/kg the range, stepped down in ARDS while the plateau and driving pressures stay inside their limits.

Tidal Volume per Ideal Body Weight

Tidal volume normalized to ideal body weight. Hamilton expresses tidal volume this way so you set mL/kg rather than an absolute volume.

Minute Ventilation

Volume of gas moved per minute = tidal volume × respiratory rate. The primary determinant of CO2 clearance.

Peak Airway Pressure

Highest pressure during inspiration. Reflects airway resistance plus lung compliance; a rising Ppeak with a normal plateau suggests a resistance problem (secretions, bronchospasm, kinked tube).

Plateau Pressure

Alveolar pressure at end-inspiration during a pause. Reflects lung compliance — keep ≤ 30 cmH2O to limit barotrauma.

Pplat − PEEP

The pressure distending the lung each breath. Strongly associated with mortality in ARDS — keep ≤ 15 cmH2O.

Lung Compliance

Change in volume per unit pressure (mL/cmH2O). Low compliance = stiff lungs (ARDS, pulmonary edema, pneumothorax).

Airway Resistance

Opposition to airflow (cmH2O/L/s). Rises with bronchospasm, secretions, and small-bore tubes.

Inspiratory to Expiratory Ratio

Time spent inspiring versus exhaling. Normally ~1:2; lengthened toward 1:4 in obstructive disease to allow full exhalation.

Inspiratory Time

Duration of the inspiratory phase in seconds. Longer TI raises mean airway pressure; shorter TI shortens the time available to exhale.

Inspiratory Flow

Rate at which gas is delivered (L/min). Higher flow shortens inspiratory time — useful in obstructive disease.

Sensitivity

How much effort the patient must generate to start a breath. Set too sensitive → auto-triggering; too insensitive → the patient fights the vent.

Pressure Support

Extra inspiratory pressure added to spontaneous breaths to overcome tube resistance and reduce work of breathing.

Inspiratory Pressure

Pressure applied above PEEP in pressure-control ventilation. Determines the tidal volume delivered.

Inspiratory Positive Airway Pressure

The higher pressure in bilevel (BiPAP) therapy — supports inspiration and augments tidal volume.

Expiratory Positive Airway Pressure

The lower pressure in bilevel therapy; functionally equivalent to PEEP — it recruits and splints the upper airway.

Continuous Positive Airway Pressure

A single constant pressure throughout the breath while the patient breathes spontaneously. Splints the airway and recruits alveoli.

Bilevel Positive Airway Pressure

Two pressures (IPAP/EPAP) with spontaneous triggering. First-line NIV for COPD exacerbation, pulmonary edema and obesity hypoventilation.

Assist-Control

Every breath — patient-triggered or time-triggered — receives full ventilator support. Guarantees minute ventilation at the cost of hyperventilation if the patient triggers rapidly.

Synchronized Intermittent Mandatory Ventilation

A set number of mandatory breaths are synchronized with effort; spontaneous breaths in between receive pressure support. Largely a weaning mode.

Adaptive Support Ventilation

The ventilator continuously adjusts rate and tidal volume to a target minute ventilation using the patient's measured compliance and resistance.

Airway Pressure Release Ventilation

A high continuous pressure with brief timed releases. Recruits and maintains lung volume while allowing spontaneous breathing.

Pressure-Regulated Volume Control

A pressure breath whose pressure is adjusted breath by breath to deliver the tidal volume you set. It reconciles the comfort of pressure control with the guaranteed volume of volume control — you set the volume, the machine works out the pressure from the patient's own compliance.

Intrinsic PEEP

Trapped gas raising end-expiratory alveolar pressure above the set PEEP. Common in obstructive disease; detect with an expiratory hold.

Physiologic Dead Space

Ventilated lung that does not participate in gas exchange (~2 mL/kg, roughly 150 mL in adults). It dilutes the effective alveolar ventilation.

End-Tidal Carbon Dioxide

CO2 measured at end-expiration; approximates PaCO2. Guides ventilation targets and confirms endotracheal tube placement.

Peripheral Oxygen Saturation

Oxygen saturation estimated by pulse oximetry. Affected by poor perfusion, motion and dyshemoglobinemias.

Low Tidal Volume Strategy

6 mL/kg predicted body weight (4–8 mL/kg range), plateau ≤ 30, driving pressure ≤ 15, and enough PEEP to minimize volutrauma, barotrauma and atelectrauma. This is the default for every ventilated patient, not only for ARDS — prehospital and emergency ventilation is characteristically injurious, and ventilator-induced lung injury develops early.

Tolerated Hypercapnia

Accepting an elevated CO2 (pH ≥ 7.20) in exchange for protecting the lung with low tidal volumes. It is expected with a low tidal volume, but it has two exceptions: a raised ICP, where CO2 dilates the cerebral vessels and raises the pressure inside the skull, and a sickle cell crisis, where acidosis promotes sickling. Neither patient gets a permissive CO2 target.

Deterioration Mnemonic

Displacement of the tube, Obstruction, Pneumothorax, Equipment failure, Stacking of breaths — the checklist for sudden deterioration on a ventilator.

Rapid Sequence Intubation

Simultaneous sedation and neuromuscular blockade to secure the airway quickly and reduce aspiration risk.

Bag-Valve-Mask

Manual ventilation with a self-inflating bag. A bridge to a definitive airway — it cannot provide controlled PEEP or precise volumes.

Intrapulmonary Shunt

Blood that crosses the lung without picking up oxygen. It is why a flooded or consolidated lung stays hypoxemic however high the FiO2 goes — the answer is opening the alveoli up, not adding oxygen.

Average Pressure Over the Breath

The average pressure in the airway across a whole breath. It recruits alveoli and improves oxygenation, but it is also the pressure that opposes venous return — raise it carefully in a patient with low preload.

PaO2 ÷ FiO2

Arterial oxygen divided by the fraction of inspired oxygen — the standard measure of how severe the oxygenation failure is. Under 300 suggests lung injury; under 100 is severe.

IBW — Ideal Body Weight

The weight calculated from height and sex, and the basis for every tidal volume. It is not the patient's actual weight — a 150 kg patient and a 60 kg patient of the same height get the same tidal volume.

Over-distension Injury

Lung injury from too large a tidal volume stretching the open alveoli. This is why tidal volume is set on predicted body weight rather than the patient's actual size.

Pressure Injury

Lung injury from excessive airway pressure — air leaks into the pleural space and a pneumothorax follows. Plateau pressure is the value to watch.

Repeated Opening Injury

Injury from alveoli opening and collapsing with every breath. Enough PEEP holds them open and prevents it.

Sustained Inflation

A deliberate sustained inflation — a breath held at a high pressure for 30–40 seconds — intended to open collapsed alveoli. Prolonged recruitment maneuvers are no longer recommended: they do not improve survival and can cause harm, including barotrauma and hypotension. Open the lung with PEEP and let the FiO2 come down instead.

Ventral Position

Turning the patient face down. In moderate–severe ARDS, prone positioning for more than 12 hours a day improves survival — it redistributes the lung's blood flow toward the dorsal units that stay open and lets the ventral units drain. It is an escalation for a P/F ratio that will not hold on PEEP and FiO2 alone, and it is not a transport intervention.