Pulmonary · Topic 5 of 28

Mechanical Ventilation: Modes & Initial Settings

Pulmonary · mechanical ventilation

Mechanical Ventilation: Modes & Initial Settings

Start assist-control at 6–8 mL/kg ideal body weight, FiO₂ 1.0 and PEEP 5; fix oxygen with FiO₂ and PEEP, fix CO₂ with rate and tidal volume.

Key numbers

Start at 6–8 mL/kg ideal body weightVT 6–8 mL/kg IBW

01 · DoStart at 6–8 mL/kg ideal body weight

Typical start: assist-control, tidal volume 6–8 mL/kg ideal body weight (6 in ARDS), FiO₂ 1.0 weaned to the lowest effective level, and PEEP 5 cmH₂O to limit atelectasis.

02 · The ruleFiO₂/PEEP fix O₂; rate/VT fix CO₂

Oxygenation is adjusted with FiO₂ and PEEP; ventilation (PaCO₂) with minute ventilation — respiratory rate × tidal volume. Raise rate or volume to lower CO₂, within plateau and auto-PEEP limits.

03 · What it isVolume control: set volume, pressure varies

Assist-control volume ventilation delivers a set tidal volume at a guaranteed minimum rate, and every patient-triggered breath gets the full volume; airway pressure varies with compliance and resistance.

What it is

Core idea

Pressure control: set pressure, volume varies must knowPressure control delivers a set inspiratory pressure for a set inspiratory time at a minimum rate, and every triggered breath gets the full pressure; tidal volume varies with lung mechanics, so monitor exhaled tidal volume.

SIMV: extra breaths are unassistedSIMV gives mandatory breaths at a set rate and volume synchronized to effort, but breaths above the set rate are unassisted; it does not lower mortality, shorten ICU stay or speed liberation.

Pressure support: no set ratePressure support has no backup rate: every breath is patient-triggered and supported until inspiratory flow falls, so it needs a reliable respiratory drive; it is the usual mode for liberation trials.

Diagnosis & monitoring

Work-up

PEEP 5 for most; more for edema, ARDSPEEP 5 cmH₂O suits most ventilated patients; higher PEEP improves oxygenation in ARDS and cardiogenic pulmonary edema and allows a lower FiO₂, but excess PEEP can cause hypotension, RV strain and overdistension.

Ventilate: RR > 30, SpO₂ < 90%Signs that mechanical ventilation is needed: RR above 30, SpO₂ below 90% despite high-flow or NIV, pH below 7.25, or PaCO₂ above 50 (unless chronic); judge clinically and do not wait for extremis.

Ideal body weight from height and sexIdeal (predicted) body weight: men 50 + 2.3 × (height in inches − 60); women 45.5 + 2.3 × (height in inches − 60). Never set tidal volume from actual weight.

Peak above 25: check plateau pressurePeak pressure above about 25 cmH₂O calls for an inspiratory hold: peak − plateau > 10 means airway resistance (kinked or plugged tube, bronchospasm); plateau − PEEP > 10 means stiff lungs or chest wall.

Do right now

In this order

First moves

Ventilate: RR > 30, SpO₂ < 90%

→

Start at 6–8 mL/kg ideal body weightVT 6–8 mL/kg IBW

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FiO₂/PEEP fix O₂; rate/VT fix CO₂

Why each step

Start at 6–8 mL/kg ideal body weightTypical start: assist-control, tidal volume 6–8 mL/kg ideal body weight (6 in ARDS), FiO₂ 1.0 weaned to the lowest effective level, and PEEP 5 cmH₂O to limit atelectasis.

FiO₂/PEEP fix O₂; rate/VT fix CO₂Oxygenation is adjusted with FiO₂ and PEEP; ventilation (PaCO₂) with minute ventilation — respiratory rate × tidal volume. Raise rate or volume to lower CO₂, within plateau and auto-PEEP limits.

Management

Orders

ARDS: 6 mL/kg, plateau ≤ 30In ARDS use tidal volume 6 mL/kg ideal body weight, keep plateau pressure at or below 30 cmH₂O, raise the rate up to 35/min and accept hypercapnia to pH about 7.15 (see the ARDS topic).

COPD: long exhalation, watch auto-PEEPIn COPD start assist-control at 5–7 mL/kg and rate 20–24 (lower if auto-PEEP rises), with high inspiratory flow, I:E 1:4 or longer, and external PEEP ≤ 85% of intrinsic PEEP.

Status asthmaticus: slow rate, permissive hypercapniaIn status asthmaticus limit hyperinflation, not CO₂: rate 8–10/min, inspiratory flow 70–120 L/min, keep plateau under 30–35 cmH₂O and intrinsic PEEP under 15, and accept pH above 7.15.

Hypoxemic on the vent: raise FiO₂, then PEEPFor hypoxemia on the ventilator raise FiO₂ to restore saturation, then titrate PEEP up in 2–3 cmH₂O steps so FiO₂ can come down to 0.6 or less.

Red flags & don'ts

Avoid harm

Wean FiO₂ below 0.6 complicationStart FiO₂ at 1.0 and lower it to the least that keeps oxygenation adequate; prolonged FiO₂ ≥ 0.6 injures the lung, so add PEEP rather than leaving FiO₂ high.

Volume control or pressure control?

Commonly confused

You set	Tidal volume and rate
What varies	Airway pressure
Watch for	Rising peak and plateau pressures
Patient-triggered breath	Full set tidal volume
Outcome in ARDS	No proven difference at equal volumes

More detail

Reference

Show 6 reference facts

Other default settings: trigger sensitivity about −2 cmH₂O, inspiratory flow about 60 L/min (up to 120 in airflow obstruction), and an I:E ratio of 1:3 with normal mechanics.

Cardiogenic pulmonary edema on the ventilator: assist-control, tidal volume 6–8 mL/kg, rate about 25, FiO₂ 1.0 and PEEP 5–8 titrated up in 2.5 cmH₂O steps; positive pressure lowers preload and LV afterload.

Pressure control has no proven outcome advantage over volume control in ARDS; at the same delivered volume the distending pressure is the same.

Auto-PEEP from short expiratory time or high minute ventilation (> 20 L/min) raises work of breathing and cuts venous return, causing hypotension; reduce the rate or shorten inspiratory time.

Prefer light sedation over deep sedation in ventilated adults; check triglycerides about every 48 hours when propofol runs longer than 24–48 hours.

Prone positioning (at least 12 hours a day in moderate–severe ARDS) is contraindicated with spinal instability or raised intracranial pressure.

Pearl

Oxygenation is FiO₂ and PEEP; ventilation is rate times tidal volume — change the knob that matches the problem.

Sources · Merck Manual Professional — Overview of Mechanical Ventilation · Merck Manual Professional — Acute Hypoxemic Respiratory Failure (AHRF), Including ARDS · Merck Manual Professional — Hypoxia in Hospitalized Patients (oxygen toxicity) · Fan E et al. ATS/ESICM/SCCM Clinical Practice Guideline: Mechanical Ventilation in Adult Patients with ARDS. Am J Respir Crit Care Med 2017;195:1253–1263 · Merck Manual Professional — Ventilatory Failure (status asthmaticus, acute-on-chronic failure) · NIH NHLBI ARDS Clinical Network — Mechanical Ventilation Protocol Summary (predicted body weight formula, plateau ≤ 30) · Starter sheet built from the verified AGACNP fact map (facts reviewed 2026-10-15).