Ventilator Basics
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Direct answer
Set every tidal volume from predicted body weight, never actual weight, and you have internalised the single most consequential rule in mechanical ventilation. A ventilator breath is defined by three answers — what triggers it (time or patient effort), what limits it (volume or pressure), and what cycles it off (volume, time or flow) — while the baseline settings are tidal volume, rate, PEEP, FiO2 and inspiratory time. Lung-protective ventilation, straight from the ARDSNet trial, means 6 mL/kg of predicted weight, plateau pressure no higher than 30 cm H2O and driving pressure kept low; everything else in ventilator management is titration and troubleshooting around that scaffold.
What you must remember
- Predicted body weight: male 50 kg plus 0.91 kg for each cm over 152 cm; female 45.5 kg plus 0.91 kg per cm over 152 cm — tidal volume 6 mL/kg (range 4-8) of this weight, not the measured weight.
- Usual starting rate 12-20 breaths per minute (higher in ARDS, lower in obstruction); adjust on pH and CO2, watching for auto-PEEP when expiration is incomplete.
- PEEP 5 cm H2O is the standard floor; it recruits alveoli and prevents atelectrauma, with higher values guided by ARDS tables or oesophageal manometry.
- Plateau pressure (measured with an inspiratory hold, no flow) stays at or below 30 cm H2O; driving pressure (plateau minus PEEP) is best kept under 15.
- Peak pressure reflects airway resistance; plateau reflects compliance — peak high with a normal plateau means a resistance problem (secretions, kinking, biting, bronchospasm), not a stiff lung.
- Trigger: pressure or flow sensing of patient effort; cycling off: time in pressure control, volume in volume control, and flow (at about 25% of peak inspiratory flow) in pressure support.
- I:E ratio begins at 1:2; obstructed patients need 1:3 or 1:4 with lower rates to empty, accepting permissive hypercapnia.
- Alarms and drills: high pressure (suction, kink, biting, pneumothorax, worsening compliance), low pressure or low volume (disconnection, cuff leak), apnoea (backup ventilation must be armed).
- Complications: ventilator-induced lung injury (barotrauma, volutrauma, atelectrauma, biotrauma), ventilator-associated pneumonia (head-up 30-45 degrees, oral chlorhexidine care, subglottic suction where available), reduced venous return and cardiac output at high intrathoracic pressures.
Settings for one ARDS patient
Walk through a 35-year-old woman, 165 cm tall, with ARDS from pneumonia. Predicted weight: 45.5 plus 0.91 times 12.6 gives about 57 kg — so tidal volume 340 mL at 6 mL/kg. Rate 24, PEEP 10, FiO2 started high and weaned to keep SpO2 88-95%. An inspiratory hold reads plateau 28 — acceptable; but if driving pressure (plateau minus PEEP) had exceeded 15, the volume would come down further and the rate up, tolerating a higher CO2 in exchange for a protected lung.
Now the alarm drill on the same patient. High pressure alarms at 38 cm H2O: disconnect, hand-ventilate with 100% oxygen via a self-inflating bag — the manoeuvre is both diagnostic and therapeutic — then look, listen and feel. Bilateral air entry with improving compliance points to secretions or biting (suction, sedation, or a bite block). Absent left-sided sounds with hypotension and tracheal shift is tension pneumothorax: needle decompression, then a chest drain. The order of that sentence is the exam answer.
Where students slip
Using actual weight for tidal volume is the error that does the most harm: a 120 kg obese man has the lungs of a 60 kg man, and 6 mL/kg of measured weight delivers a tidal volume that volutraumatises him. Confusing peak with plateau pressure is the second classic: one inspiratory-hold manoeuvre separates a tube problem from a lung problem in five seconds, and candidates who describe the manoeuvre earn the viva. Third, chasing oxygenation with FiO2 alone when the recruitable lung needs PEEP, or chasing normocapnia in ARDS when permissive hypercapnia is the protective strategy. Keep the monitoring straight: SpO2 and PaO2 report oxygenation; ETCO2 and PaCO2 report ventilation — no ventilator change fixes both with one dial.
Frequently asked questions
Calculate the tidal volume for a 170 cm ventilated woman.
Predicted weight 45.5 + (0.91 × 17.6) is about 61 kg; at 6 mL/kg, roughly 370 mL.
Why is predicted rather than actual body weight used?
Lung size tracks height and sex, not mass — in obesity the excess weight is not lung, so actual-weight volumes overdistend.
What is the plateau pressure limit and how is it measured?
At or below 30 cm H2O, read during an inspiratory hold at zero flow; it reflects alveolar, not airway-resistance, pressure.
How do peak and plateau pressures differ?
Peak includes airway resistance; plateau excludes it — a high peak with normal plateau means resistance (secretion, kink, bite), while both high means poor compliance.
What does PEEP accomplish?
Keeps alveoli open at end-expiration, improving oxygenation and preventing cyclic collapse-reopening injury (atelectrauma).
What is atelectrauma?
Lung injury from repeated alveolar collapse and reopening at low end-expiratory volume — the reason a PEEP floor exists.