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Mechanical Ventilation: Settings, Troubleshooting & Weaning

Name: Mechanical Ventilation: Settings, Troubleshooting & Weaning
Creator: Veli Bakalov, MD

FiO2 and PEEP for oxygen, rate and volume for CO2, peak versus plateau for trouble

Emergency & Critical Care·Critical Care·5 min read·9 high-yield points·Reviewed by Veli Bakalov, MD @HemeOncBuddy

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Initial settings: volume assist-control, tidal volume 6–8 mL/kg ideal body weight, rate 12–16, PEEP 5, FiO2 titrated to SpO2 92–96% (88–92% in COPD).
Tidal volume is based on ideal body weight from height and sex, never actual weight.
ARDS: tidal volume 6 mL/kg ideal body weight, plateau ≤30 cm H2O, higher PEEP, permissive hypercapnia, prone positioning when PaO2/FiO2 <150.
Oxygenation is fixed with FiO2 and PEEP; PaCO2 is fixed with respiratory rate and tidal volume.
Sudden deterioration on the ventilator: disconnect and hand-bag with 100% O2, then DOPE (displacement, obstruction, pneumothorax, equipment).
High peak + normal plateau = resistance (mucus plug, bronchospasm, kinked or bitten tube); high peak + high plateau = compliance (pneumothorax, mainstem intubation, edema, abdominal distension).
Auto-PEEP in asthma/COPD causes hypotension: disconnect, lower the rate, lengthen expiration, bronchodilate, accept hypercapnia.
Pair a daily sedation interruption with a spontaneous breathing trial (pressure support 5–8 or T-piece); RSBI (rate ÷ tidal volume in L) <105 predicts success.
VAP prevention: head of bed 30–45°, oral care, subglottic secretion drainage, minimal sedation, and the shortest possible ventilation.

Ventilator modes

Volume assist-control. The default initial mode sets a tidal volume and a backup rate. Every breath, machine- or patient-triggered, delivers the full set volume, guaranteeing ventilation with minimal work of breathing. Peak pressure varies with the lungs, and a tachypneic patient can hyperventilate into respiratory alkalosis or stack breaths (auto-PEEP), so treat the pain or anxiety driving the rate.

Common ventilator modes

Mode	You set	What varies	Use and caution
Volume assist-control	Tidal volume, backup rate	Peak pressure	Default; alkalosis or breath stacking if tachypneic
Pressure control	Inspiratory pressure, inspiratory time, rate	Tidal volume	Caps peak pressure; volume falls silently as compliance drops
SIMV	Mandatory breaths (volume and rate)	Size of extra spontaneous breaths	Unsupported extra breaths raise work of breathing
Pressure support	Support pressure, PEEP	Rate and tidal volume	No backup rate; needs intact drive; used for breathing trials

Pressure modes. In pressure control the delivered volume depends on compliance and resistance, so stiffening lungs or an obstructed tube lower it without a pressure alarm; set low tidal- and minute-volume alarms. Its decelerating flow is comfortable for some patients. In pressure support each patient-triggered breath is boosted by a set pressure and ends as flow falls.

Initial settings and lung protection

Starting orders. Volume assist-control with a tidal volume of 6–8 mL/kg ideal body weight, rate 12–16, PEEP 5 cm H₂O, and FiO₂ 1.0 titrated down to SpO₂ 92–96% (88–92% in COPD or chronic hypercapnia). Ideal body weight comes from height and sex (50 kg for men or 45.5 kg for women, plus 2.3 kg per inch over 5 feet), because lung size does not grow with fat; using actual weight in obesity causes volutrauma. Recheck an ABG about 30 minutes after changes.

6–8 mL/kgtidal volume by ideal body weight (6 in ARDS)

≤30 cm H₂Oplateau pressure limit

<105rapid shallow breathing index predicting successful weaning

ARDS. Lung-protective ventilation uses 6 mL/kg ideal body weight (down to 4 if needed), a plateau pressure of 30 cm H₂O or less, and higher PEEP guided by FiO₂ needs; the ARDSNet trial showed that low tidal volumes lower mortality. Driving pressure (plateau minus PEEP) ideally stays below about 15 cm H₂O. Accept the resulting hypercapnia as long as pH stays above about 7.20 (permissive hypercapnia), and add prone positioning for at least 16 hours a day when PaO₂/FiO₂ is below 150.

Adjusting oxygenation and ventilation

Oxygenation. PaO₂ and SpO₂ depend on FiO₂ and mean airway pressure. FiO₂ works fast but causes oxygen toxicity if kept above about 0.6 for long. PEEP recruits collapsed alveoli and increases functional residual capacity, allowing FiO₂ to come down. Hypoxemia that barely improves on 100% oxygen implies shunt through flooded or collapsed alveoli, which responds to PEEP and recruitment rather than more oxygen.

PEEP raises intrathoracic pressure: it can reduce venous return and cause hypotension, especially in hypovolemia, and it adds barotrauma risk.

Ventilation. PaCO₂ varies inversely with minute ventilation (rate × tidal volume). To lower PaCO₂, raise the rate (preferred when tidal volume is capped) or tidal volume; to correct hypocapnia, lower them. A useful estimate: new rate ≈ current rate × current PaCO₂ ÷ desired PaCO₂. Respiratory acidosis with adequate oxygenation needs more minute ventilation, not more FiO₂ or PEEP.

Troubleshooting the crashing patient

First move. When a ventilated patient suddenly desaturates or becomes hypotensive, disconnect the ventilator and hand-bag with 100% oxygen. Improvement points to the ventilator or circuit, and bagging lets you feel compliance. Then work through DOPE:

DisplacementTube out of the trachea or in a mainstem bronchus: capnography, tube depth, breath sounds
ObstructionMucus plug, kink, or biting: pass a suction catheter, add a bite block
PneumothoraxAbsent breath sounds, hypotension, tracheal shift: decompress, then chest tube
EquipmentCircuit leak or disconnection, oxygen supply failure

Also consider breath stacking (auto-PEEP)

Peak versus plateau. Peak inspiratory pressure reflects airway resistance plus compliance; plateau pressure, measured during an inspiratory hold, reflects compliance alone.

Reading the pressures

Pattern	Problem	Causes	Fix
High peak, normal plateau	↑ Airway resistance	Mucus plug, secretions, bronchospasm, kinked or bitten tube	Suction, bronchodilators, bite block, reposition
High peak, high plateau	↓ Compliance	Pneumothorax, mainstem intubation, pulmonary edema, ARDS, atelectasis, abdominal distension	Decompress, pull tube back, treat the cause

Auto-PEEP. In asthma or COPD, a high rate, large tidal volume, or short expiratory time prevents full exhalation, causing dynamic hyperinflation. Expiratory flow fails to return to zero before the next breath, and an expiratory-hold maneuver measures the trapped pressure. Rising intrathoracic pressure cuts venous return, producing hypotension, PEA, or barotrauma. If the patient is hypotensive, disconnect the ventilator so the chest can empty. Then lower the rate, shorten inspiration with higher flow to lengthen expiration, give bronchodilators, deepen sedation (occasionally with paralysis) to slow the patient's own rate, and accept permissive hypercapnia.

Liberation and VAP prevention

Wake up and breathe. A daily spontaneous awakening trial (sedation interruption) paired with a spontaneous breathing trial shortens ventilation and ICU stay and improves survival compared with breathing trials alone.

Daily screenCause improving; FiO₂ ≤0.4–0.5 with PEEP ≤5–8; stable on minimal or no pressors; able to trigger breaths
Breathing trial30–120 minutes on pressure support 5–8 cm H₂O with PEEP 5, or a T-piece
Stop criteriaRR >35, SpO₂ <90%, HR >140 or sustained 20% change, SBP >180 or <90, agitation, diaphoresis, accessory muscle use
ExtubateAlert, adequate cough, manageable secretions

Rapid shallow breathing index. Respiratory rate divided by tidal volume in liters during a brief unassisted trial. Below 105 breaths/min/L predicts success; above 105, fast shallow breathing predicts failure (30 breaths/min at 250 mL gives 120). A failed trial means returning to full support, fixing the cause, and retrying the next day.

Extubation. After prolonged or traumatic intubation, a cuff-leak test screens for laryngeal edema; an absent leak prompts IV steroids at least 4 hours before extubation. High-risk patients (COPD, hypercapnia) go to noninvasive ventilation or high-flow nasal oxygen after extubation.

Ventilator-associated pneumonia. Pneumonia arising more than 48 hours after intubation is reduced by elevating the head of the bed 30–45°, daily oral care with toothbrushing, subglottic secretion drainage tubes for patients expected to need more than 48–72 hours of ventilation, minimal sedation with daily awakening and breathing trials, early mobility, and changing circuits only when soiled or malfunctioning.

The best VAP prevention is the shortest possible time on the ventilator.