Fig 6Ventilator and test lungClick a box · ↑ ↓ turn · H holdVC-AC
Bellows platen: rises by the volume delivered; springs are complianceCircuit: insp port, heated humidifier, insp limb, Y, flow sensor, orifice, lung; exp limb back to the exhalation valveO₂ and air from the wall panel to the blender inletsAn ICU ventilator on a therapist's bench. Gas from the wall O₂ and air outlets is mixed to the set FiO₂, pushed out the inspiratory port, warmed and wetted in the humidifier, and carried down the inspiratory limb to the Y-piece, the flow sensor and an orifice that stands for airway resistance. The bellows lung rises against four springs that stand for compliance. Exhaled gas returns down the expiratory limb, through a filter, to the exhalation valve, which holds PEEP. The screen draws airway pressure; the gauge on the lung reads alveolar pressure; the gap between them is resistance × flow.
Keys Patient
Settings the therapist sets
How a breath is delivered, what starts it and what ends it. VC-AC: every breath is a set volume. PC-AC: a set pressure for a set time. PRVC: pressure control that retunes itself, at most 3 cmH₂O per breath, to hit a target volume. SIMV: a set number of volume breaths; extra breaths get pressure support. PSV: the patient starts every breath and the machine adds pressure. CPAP: constant pressure, the patient does all the work.
Tidal volume, the gas in each breath. Set from predicted body weight (height and sex, not actual weight): 6 to 8 mL/kg, 4 to 6 in ARDS.
Set rate: the minimum number of machine breaths per minute. With VT it sets minute ventilation, and that sets CO₂.
Positive end-expiratory pressure: what the exhalation valve holds at the end of every breath. Keeps alveoli open and raises oxygenation.
Fraction of inspired oxygen, 21 to 100 %. The blender mixes the wall O₂ and air lines; the O₂ cell measures what actually arrives.
Inspiratory pressure above PEEP in pressure control. The volume that follows depends on compliance, resistance and time.
Inspiratory time for pressure breaths. With the rate it sets the I:E ratio and how long the lung has to exhale.
Peak inspiratory flow for volume breaths. Faster flow, shorter Ti and a higher peak pressure.
Flow pattern. Square holds constant flow; decelerating ramps flow down as the lung fills, which lowers peak pressure.
Pressure support: extra pressure on each patient-started breath to offload breathing work and the resistance of the tube.
Rise time: how fast pressure reaches target. Too slow starves a hungry patient; too fast overshoots.
Expiratory sensitivity: a support breath ends when flow falls to this percent of its peak. Raise it for COPD so breaths end sooner.
Flow trigger: how much inspiratory flow from the patient starts a breath. Too sensitive and it autotriggers; too stiff and efforts are missed.
High-pressure limit. Above it the alarm sounds and the breath is cut short to protect the lung.
The patient (the test lung)
Compliance, the springs on the test lung: litres of volume per cmH₂O. Healthy and ventilated 50 to 80; ARDS 20 to 30.
Resistance, the orifice wheel on the lung inlet. Intubated and normal 5 to 10; bronchospasm 20 to 50.
The patient's own inspiratory muscle pressure (Pmus). Zero is a sedated, passive patient.
The patient's own breathing rate when awake.
What the ventilator measures
PpeakcmH₂OHighest airway pressure in the breath: resistance plus elastic recoil plus PEEP.
PplatcmH₂OPressure in the alveoli at end-inspiration, read during an inspiratory hold. Keep under 30.
Driving pressurecmH₂OPplat − PEEP = VT ÷ compliance. Keep under 15.
PmeancmH₂OAverage airway pressure over the breath. Tracks oxygenation.
Auto-PEEPcmH₂OTrapped pressure when the lung cannot empty: total PEEP from an expiratory hold minus set PEEP.
VTemLExhaled tidal volume, measured on the way back through the expiratory valve.
MVeL/minExhaled minute volume: VTe × total rate.
fTotal/minTotal breathing rate, machine plus patient breaths.
RSBIRate ÷ VT in litres. Under 105 on a breathing trial predicts extubation success.
FiO₂ measured%O₂ cell reading. Lags the setting by a few seconds.
The respiratory therapist
The RT owns the ventilator at the bedside. They build and leak-test the circuit, pick the mode and first settings with the physician from the patient's predicted body weight and disease, then keep checking: plateau and driving pressure with an inspiratory hold, auto-PEEP with an expiratory hold, the shape of every waveform, the blood gas. They change one thing at a time and watch what the lung does.
On rounds they also check the airway: tube position and cuff pressure (20 to 30 cmH₂O), suction, humidity (the heater keeps gas at 37 °C, fully saturated, because the tube bypasses the nose), and nebulized drugs given in-line. When an alarm goes off they work it from the patient back to the machine: tube Displaced, Obstructed, Pneumothorax, Equipment (DOPE), bagging the patient by hand if needed. And they lead weaning: a daily spontaneous breathing trial, RSBI, and extubation.
Alarms on this machine
High pressure: Paw over Pmax. The breath is cut short. Think cough, biting, secretions, kinked tube, bronchospasm, stiffer lung.
Apnea · backup: no breath for 20 s in PSV or CPAP. The machine starts volume breaths at the set rate until the patient triggers twice.
Low minute volume under 4 L/min, Low VTe under 250 mL: leak, disconnect, weak patient, too little support.
High rate over 35, High minute volume over 15 L/min: distress, pain, acidosis, autotrigger.
Auto-PEEP: the lung starts the next breath before it has emptied. Slow the rate, shorten Ti, treat bronchospasm.
Pplat over 30, Driving pressure over 15: advisories after a hold. Lower VT.
Keyboard
Focus the figure. ← → choose a setting, ↑ ↓ turn the dial (Shift for ×5), M mode, H insp hold, E exp hold, B manual breath, O O₂ 100 %, S silence, 1 to 4 patient presets. Click a box on the screen to select it, the dial to turn it, the orifice wheel or the knob on top of the lung to change the patient.
Teaching model, not for clinical use. One-compartment lung, Paw = R·flow + V/C − Pmus. Dead space 150 mL, VCO₂ 200 mL/min, PBW 70 kg.