Ventilator Modes and Settings

In depth

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Respiratory mechanics: the equation of motion

Every ventilator breath obeys one equation: airway pressure = (Vt ÷ compliance) + (flow × resistance) + total PEEP. The terms are the elastic load (lungs and chest wall), the resistive load (tube and airways) and end-expiratory pressure. A breathing patient's own muscle pressure adds to the ventilator's, so pressures and volumes vary breath to breath.

An inspiratory hold stops flow, so the resistive term drops out: peak − plateau reflects resistance and plateau − PEEP (driving pressure) the elastic load. This holds only on a passive patient receiving a constant-flow volume breath.

The time constant governs exhalation. One τ empties 63% of the inspired volume, two τ 86%, three τ 95%. With resistance 10 cm H₂O/L/s and compliance 0.05 L/cm H₂O, τ is 0.5 s and 1.5 s of exhalation is enough. A COPD patient with resistance 25 and compliance 0.08 has τ of 2 s and needs about 6 s, which a rate of 20 cannot provide; this is the arithmetic behind auto-PEEP.

Positive pressure also changes the circulation. Raising intrathoracic pressure lowers venous return (preload) and lowers left ventricular transmural pressure (afterload), so it helps a failing, volume-loaded left ventricle and hurts a hypovolemic patient or a failing right ventricle. Pulmonary vascular resistance is lowest near functional residual capacity, so excessive PEEP or Vt can precipitate right ventricular failure.

Each mode in depth: trigger, target, cycle

Chatburn's taxonomy describes every breath by three variables: what starts it (trigger), what the ventilator controls during it (target), and what ends it (cycle). A pressure trigger is usually set at −1 to −2 cm H₂O and a flow trigger at 1–3 L/min; flow triggering costs the patient less work.

Volume control guarantees minute ventilation but fixes flow, so a hungry patient pulls against it (flow starvation). Pressure control lets flow follow demand, but Vt falls when compliance falls or resistance rises. PRVC (Maquet's name; VC+, AutoFlow and Adaptive Pressure Ventilation are similar) delivers a test breath, calculates compliance, then adjusts inspiratory pressure in small steps (up to 3 cm H₂O per breath on Servo ventilators) to reach the target Vt, capped 5 cm H₂O below the high-pressure alarm.

SIMV delivers a set number of mandatory breaths synchronized to patient effort; breaths above that rate are spontaneous, usually pressure supported. Brochard 1994 and Esteban 1995 found it the slowest weaning method. PSV is entirely patient-triggered with no backup rate (apnea ventilation is the safety net); each breath ends when flow decays to a set fraction of peak, typically 25–30% by default and adjustable, so COPD patients with slow exhalation often need a higher cycle-off percentage (40–50%).

APRV holds a high CPAP level (P-high, often started near the plateau, 20–30 cm H₂O) for a long T-high (4–6 s) with brief releases to P-low (often 0) for a T-low of 0.2–0.8 s. In the time-controlled adaptive ventilation approach, T-low is set so the release ends when expiratory flow has fallen to 50–75% of its peak, leaving deliberate intrinsic PEEP to keep alveoli open. Patients breathe spontaneously throughout, which preserves diaphragm activity and may improve V/Q matching; Vt depends on compliance and the release, so it can exceed 6 mL/kg PBW. Evidence is limited to small trials (Zhou 2017, n = 138, single centre: more ventilator-free days); no guideline recommends it routinely.

NAVA reads the diaphragm's electrical activity (Edi) through a nasogastric catheter with electrodes; the ventilator triggers on a rise in Edi, delivers pressure proportional to Edi multiplied by the set NAVA level, and cycles when Edi falls to a set fraction of its peak. PAV+ measures resistance and compliance every few breaths with a brief end-inspiratory pause and supplies a set percentage (5–95%) of the work the patient generates. Both follow the patient's own pattern and reduce asynchrony, need an intact respiratory drive, and have limited outcome data.

Initial settings by condition

Start on full support (AC/VC or AC/PC), FiO₂ 1.0 weaned quickly, and Vt from predicted body weight; check an ABG 20–30 min after settings settle. PReVENT 2018 (n = 961, patients without ARDS) found no difference in ventilator-free days between 4–6 and 8–10 mL/kg PBW, so 6–8 mL/kg is the usual range for normal lungs.

ARDSNet starts at 8 mL/kg PBW and steps down by 1 mL/kg every 2 h or less to 6 mL/kg, raising the rate (maximum 35) to hold minute ventilation. In obstructive disease the goal is time to exhale: low rate, high inspiratory flow, and accepted hypercapnia (pH ≥7.15–7.20 is commonly tolerated). External PEEP in passive asthma adds to trapping; in spontaneously breathing COPD, PEEP up to about 80% of measured auto-PEEP lowers the effort needed to trigger.

After brain injury, the Brain Trauma Foundation (4th ed., 2016) recommends against prophylactic hyperventilation to PaCO₂ ≤25 and advises avoiding hyperventilation in the first 24 h, when cerebral blood flow is already low. Brief hyperventilation (PaCO₂ 30–35) is a bridge for signs of herniation only. PEEP of 5–10 has little effect on ICP when MAP is maintained.

Adjusting oxygenation and ventilation; PEEP strategy

Oxygenation depends on FiO₂ and mean airway pressure; mean airway pressure rises with PEEP, inspiratory time and inspiratory pressure. Ventilation (PaCO₂) depends on alveolar minute ventilation, which is rate × (Vt − dead space). In ARDS, Vt is capped, so rate is the main lever, followed by cutting dead space (a heated humidifier instead of an HME, shorter circuit tubing) and accepting hypercapnia. A widening PaCO₂–EtCO₂ gap (normally 2–5 mm Hg) signals rising dead space: low cardiac output, pulmonary embolism or overdistension.

No single PEEP method has shown a mortality benefit on its own. ALVEOLI 2004 (n = 549) found no difference between the tables (mortality 27.5% higher vs 24.9% lower PEEP). LOVS 2008 (n = 983) combined higher PEEP with recruitment and allowed plateau up to 40: hospital mortality 36.4% vs 40.4% (not significant) with less refractory hypoxemia. EXPRESS 2008 (n = 767) set PEEP to reach a plateau of 28–30: 28-day mortality 27.8% vs 31.2% (not significant) with more ventilator-free days. The Briel 2010 individual-patient meta-analysis of these three trials found lower hospital mortality with higher PEEP when P/F was ≤200 (34.1% vs 39.1%), which underlies the ATS 2024 conditional recommendation for higher PEEP in moderate–severe ARDS.

ART 2017 (n = 1,010) tested a staircase recruitment maneuver with PEEP titrated to best compliance and found higher 28-day mortality (55.3% vs 49.3%) and more barotrauma; the protocol was softened mid-trial after cardiac arrests. EPVent-2 2019 (n = 200) titrated PEEP to esophageal pressure and found no outcome difference from the empiric higher-PEEP table. Individualized PEEP remains a research question.

Severe ARDS: recruitment, proning, paralysis, ECMO

Recruitment maneuvers (for example CPAP 30–40 cm H₂O for 30–40 s) open collapsed alveoli briefly and can cause hypotension, desaturation and pneumothorax. ATS 2024 and ESICM 2023 recommend against prolonged high-pressure maneuvers; a brief maneuver after a disconnection or suctioning is common practice, although ESICM 2023 suggests against routine brief high-pressure maneuvers too (weak, high certainty). Watch BP and SpO₂ throughout and abort for hypotension or desaturation.

PROSEVA 2013 (n = 466) enrolled patients with P/F <150 on FiO₂ ≥0.6, PEEP ≥5 and Vt about 6 mL/kg after 12–24 h of stabilization, and proned them for at least 16 consecutive hours a day. 28-day mortality fell from 32.8% to 16.0% and 90-day mortality from 41.0% to 23.6%. Sessions stopped once P/F stayed ≥150 with PEEP ≤10 and FiO₂ ≤0.6 for 4 h after returning supine. The benefit comes from more uniform lung stress, not the P/F gain alone.

• Before turning: tube secured and depth noted, suction, preoxygenate, feeds held, lines and drains routed, ECG electrodes moved to the back
• Team of 4–5 with one person managing the head and airway; capnography continuous
• Protect face, eyes, breasts, genitals, knees; reposition head and arms (swimmer's position) every 2 h
• Contraindications vary by protocol: unstable spine, open chest or abdomen, raised ICP, massive hemoptysis
• Arrest while prone: compress over the thoracic spine between the scapulae if a turn is not immediately possible

ACURASYS 2010 (n = 340, P/F <150) gave cisatracurium for 48 h and found lower adjusted 90-day mortality (hazard ratio 0.68). ROSE 2019 (n = 1,006, P/F <150 on PEEP ≥8) compared the same regimen with light sedation and stopped for futility: 90-day mortality 42.5% vs 42.8%. ATS 2024 gives a conditional recommendation for neuromuscular blockade in early severe ARDS, while ESICM 2023 recommends against routine continuous infusion, so the guidelines disagree. During paralysis, confirm deep sedation first, protect the eyes, keep the head of bed raised and remember that a paralyzed patient cannot show distress or seizure.

VV-ECMO referral fits patients still failing after optimized ventilation, proning and paralysis. EOLIA 2018 (n = 249) enrolled patients with P/F <50 for >3 h, P/F <80 for >6 h, or pH <7.25 with PaCO₂ ≥60 for >6 h despite FiO₂ ≥0.8, PEEP ≥10 and RR 35. 60-day mortality was 35% vs 46% (p = 0.09); CESAR 2009 (referral to an ECMO center; death or severe disability 37% vs 53%) also favors referral. Relative contraindications are more than 7 days of high-pressure, high-FiO₂ ventilation, no anticoagulation and non-recoverable disease. Call the ECMO center early, before the patient meets EOLIA thresholds, because transfer takes hours.

Alarms and troubleshooting

Set the high-pressure limit about 10 cm H₂O above the usual peak and the low exhaled-volume and low minute-volume alarms 10–20% below the patient's values. When an alarm sounds, look at the patient (color, chest rise, SpO₂, EtCO₂ waveform, BP) before the screen.

• ETT tip 3–5 cm above the carina on CXR; typical depth at the teeth about 21 cm women, 23 cm men
• Cuff pressure 20–30 cm H₂O (below about 30 to protect tracheal mucosa, above 20 to limit microaspiration)
• Normal EtCO₂ 35–45 mm Hg; a sudden loss of waveform means tube displacement, disconnection or arrest

Patient-ventilator asynchrony

Asynchrony is a mismatch between the patient's neural breath and the machine breath. Thille 2006 found an asynchrony index above 10% in about a quarter of patients and linked it to longer ventilation. Most types show on the pressure and flow waveforms.

Complications, VAP/VAE bundle and daily care

Ventilator-induced lung injury includes barotrauma, volutrauma (overdistension), atelectrauma (repeated opening and closing) and biotrauma (inflammation spreading to other organs). Other harms are hypotension, oxygen toxicity, ventilator-induced diaphragm dysfunction, ICU-acquired weakness, delirium, tracheal stenosis from cuff overinflation, sinusitis, stress ulcers, venous thromboembolism and pressure injuries from tubes and proning.

The CDC surveillance definition replaced VAP with ventilator-associated events. After at least 2 days of stable or improving settings, a ventilator-associated condition (VAC) is a rise in the daily minimum PEEP of ≥3 cm H₂O or in the daily minimum FiO₂ of ≥0.20 sustained for ≥2 days. An infection-related VAC (IVAC) adds temperature >38 °C or <36 °C or WBC ≥12,000 or ≤4,000 plus a new antimicrobial continued ≥4 days; possible VAP adds laboratory evidence: purulent secretions with an organism identified, a culture above the quantitative threshold, or a specific test such as Legionella antigen (NHSN 2026).

• Head of bed 30–45° unless contraindicated
• Daily paired sedation interruption and SBT; light sedation targets (RASS −2 to +1)
• Toothbrushing daily; SHEA/IDSA/APIC 2022 no longer recommends chlorhexidine oral care
• Subglottic secretion drainage tube if ventilation is expected beyond 48–72 h
• Cuff pressure 20–30 cm H₂O; change circuits only when soiled or malfunctioning
• Early mobility, VTE prophylaxis, stress ulcer prophylaxis if at risk
• Prefer NIV or HFNC when they can avoid intubation

Document each shift: settings, measured Vt, rate, peak and plateau, tube depth, cuff pressure, secretions, RASS and CAM-ICU. For transport, bring a bag with a PEEP valve, oxygen for the trip with margin, suction, capnography and the airway kit.

Liberation: SBTs, extubation and tracheostomy

Liberation starts with a daily screen and a paired awakening trial. Girard 2008 (ABC trial, n = 336) paired daily spontaneous awakening and breathing trials and cut 1-year mortality from 58% to 44%. ATS/CHEST 2017 suggests the first SBT use inspiratory pressure augmentation (PS 5–8 cm H₂O) rather than T-piece or CPAP, protocolized sedation minimization and preventive NIV for high-risk patients. Subirà 2019 (n = 1,153) found more successful extubations after 30 min on PS 8 than after 2 h on T-piece (82.3% vs 74.0%).

Passing an SBT shows the patient can breathe; extubation also needs airway protection (cough strength, secretion volume, mental status) and a patent upper airway. Extubation failure, reintubation within 48–72 h, occurs in about 10–20% and carries higher mortality. Post-extubation support depends on risk: Hernández 2016 found HFNC reduced reintubation in low-risk patients (4.9% vs 12.2%), and HIGH-WEAN 2019 found NIV alternating with HFNC beat HFNC alone in high-risk patients (11.8% vs 18.2%).

Weaning is simple (first SBT succeeds), difficult (up to 3 SBTs or 7 days) or prolonged (beyond that) by the 2007 consensus. TracMan 2013 (n = 909) compared tracheostomy at day 1–4 with day 10 or later and found no difference in 30-day mortality (30.8% vs 31.5%); about 55% of the late group never needed one (44.9% received a tracheostomy). SETPOINT2 2022 (stroke, n = 382) found no better 6-month functional outcome with tracheostomy by day 5. Most centres consider tracheostomy after 7–14 days when liberation is not in sight, and practice varies.

Keep the obturator, a spare tube one size smaller and a bag at the bedside of every tracheostomy. A speaking valve goes on only with the cuff fully deflated.

Exam pearls

• ARMA 2000 (n = 861): Vt 6 vs 12 mL/kg PBW and plateau ≤30 cut mortality from 39.8% to 31.0%
• Auto-PEEP: measure with an end-expiratory hold; fix by lowering rate and lengthening exhalation
• PSV cycles off on flow (often 25% of peak); it has no set rate, so apnea backup matters
• PRVC and pressure modes: falling compliance shows as falling Vt and a low-volume alarm
• PROSEVA: prone ≥16 h/day for P/F <150 halved 28-day mortality (32.8% → 16.0%)
• ROSE: routine 48-h cisatracurium did not lower mortality versus light sedation
• ART: staircase recruitment raised mortality; avoid prolonged high-pressure maneuvers
• VAC = PEEP up ≥3 or FiO₂ up ≥0.20 for ≥2 days after 2 stable days
• Daily SAT plus SBT (ABC trial) improves survival; RSBI <105 favors success