Mechanical Ventilation — Initial Settings (ARDSNet)
Biological sex
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Mechanical ventilation is life support for patients who cannot maintain adequate gas exchange independently — whether due to respiratory failure, reduced consciousness, haemodynamic instability, or the requirement for general anaesthesia. Optimising ventilator settings is both an art and a science, critically informed by the landmark ARDSNet trial (ARMA, 2000), which demonstrated that using a tidal volume of 6 mL/kg ideal body weight (IBW) rather than the then-conventional 12 mL/kg reduced 28-day mortality in ARDS from 40% to 31%. This lung-protective strategy — low tidal volumes, limited plateau pressure, appropriate PEEP, and permissive hypercapnia — is now the global standard for managing any patient at risk of ventilator-induced lung injury (VILI). Ideal body weight (IBW) rather than actual body weight is used for tidal volume calculation because the number and size of alveoli correlates with the skeleton's size (height) rather than total body mass — a fact particularly important in obese patients who would otherwise receive dangerously large tidal volumes. Key ventilator parameters include: tidal volume (Vt), positive end-expiratory pressure (PEEP), respiratory rate (RR), fraction of inspired oxygen (FiO2), inspiratory-to-expiratory ratio (I:E), plateau pressure (Pplat), and driving pressure (Pplat − PEEP). Driving pressure is emerging as the most powerful predictor of VILI and mortality — a target of below 15 cmH2O is widely recommended, with some evidence suggesting that <13 cmH2O provides the greatest survival benefit. Ventilator management requires a systematic approach: setting initial parameters based on IBW, then titrating FiO2 and PEEP using the ARDSNet FiO2-PEEP table, monitoring driving pressure, and continually reassessing oxygenation (P/F ratio) and ventilation (PaCO2) from serial ABGs.
Tidal Volume (Vt) = 6 mL/kg × IBW IBW Male (kg) = 50 + 2.3 × (height in inches − 60) IBW Female (kg) = 45.5 + 2.3 × (height in inches − 60) Driving Pressure = Plateau Pressure − PEEP Target Driving Pressure: <15 cmH2O (ideally <13) Target Plateau Pressure: ≤30 cmH2O Target SpO2: 92–96% Target PaCO2 (permissive hypercapnia): 45–60 mmHg acceptable in ARDS RR: 12–20 breaths/min (up to 35 in severe ARDS)
- 1Calculate the patient's ideal body weight (IBW) using height in inches: IBW males = 50 + 2.3 × (height(in) − 60); IBW females = 45.5 + 2.3 × (height(in) − 60).
- 2Set initial tidal volume at 6 mL/kg IBW (may start at 8 mL/kg if no ARDS present; reduce to 6 in ARDS or ARDS risk).
- 3Set respiratory rate 12–20 breaths/min and adjust to maintain pH 7.30–7.45; in ARDS, allow permissive hypercapnia (PaCO2 up to 60 mmHg).
- 4Set initial FiO2 at 1.0 (100%) and titrate down using ARDSNet FiO2-PEEP table targeting SpO2 92–96% or PaO2 55–80 mmHg.
- 5After 30 minutes, perform an inspiratory hold to measure plateau pressure. If Pplat >30 cmH2O, reduce Vt by 1 mL/kg IBW increments (minimum 4 mL/kg).
- 6Calculate driving pressure = Pplat − PEEP. If >15 cmH2O, reduce Vt or adjust PEEP.
- 7Monitor P/F ratio from ABG. If P/F <150 mmHg after 12–24 hours of optimal ventilation, initiate prone positioning for 16+ hours/day.
Monitor Pplat and driving pressure; target SpO2 92–96%; reassess in 1–2 hours with ABG
Using IBW (73 kg) rather than actual body weight ensures the tidal volume is sized to the alveolar surface area. At 440 mL per breath and RR of 18, minute ventilation is ~7.9 L/min. PEEP is titrated per the ARDSNet low-PEEP table at this FiO2 level. Serial ABGs guide ongoing FiO2 and PEEP adjustments.
Using actual weight (110 kg) would give Vt = 660 mL — twice the appropriate volume, causing VILI
This example illustrates the critical importance of IBW. Using actual body weight in an obese patient (110 kg) would prescribe 660 mL tidal volumes — nearly double the lung-protective target. The lungs of a 5'4" female hold the same number of alveoli regardless of her total weight; overventilation causes barotrauma and VILI.
Driving pressure >15 cmH2O predicts VILI and mortality; reduce by adjusting PEEP or Vt
Driving pressure of 18 cmH2O exceeds the safe threshold. The Vt per kg IBW is already at target (5.7 mL/kg). Increasing PEEP from 10 to 12 cmH2O might recruit atelectatic alveoli, increase compliance, and reduce driving pressure — if the lung is recruitable. If not, further Vt reduction to 5 mL/kg IBW is necessary.
If SBT tolerated (RR <30, SpO2 >90%, no distress), extubate; post-extubation HFNO or NIV if high risk
Weaning readiness criteria include haemodynamic stability without vasopressors, adequate oxygenation on low support, neurological recovery, and ability to protect airway. A 30-minute SBT on minimal support (CPAP 5 cmH2O or T-piece) is the gold-standard test. Success predicts extubation success in approximately 80% of cases.
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Neonatal and paediatric ICU ventilation using weight-adjusted lung-protective parameters adapted for children's smaller lung volumes and higher compliance.
Asthma — Risk of Auto-PEEP
Status asthmaticus may require intubation. Severe bronchospasm causes prolonged expiratory flow, risking auto-PEEP and dynamic hyperinflation. Key settings: low RR (8–12/min), long expiratory time (I:E ratio 1:4 or 1:5), minimal PEEP (0–5 cmH2O). Aggressive bronchodilator therapy via nebulisation through the ventilator circuit is the priority. If cardiovascular collapse occurs, brief apnoea ('release the gas') while disconnecting from the ventilator may relieve acute hyperinflation.
Traumatic Brain Injury
In TBI, permissive hypercapnia is contraindicated as CO2 causes cerebral vasodilatation and raises intracranial pressure. Target PaCO2 35–45 mmHg. PEEP may reduce venous return and cerebral venous drainage, raising ICP — use minimal PEEP while maintaining oxygenation. SpO2 target is ≥95% (higher than ARDS target) to prevent secondary brain hypoxia.
Unilateral Lung Disease — Differential Ventilation
In conditions such as unilateral pneumonia, massive haemothorax, or bronchopleural fistula, one lung may be severely diseased while the other is normal. Standard Vt ventilation overdistends the healthy lung while under-ventilating the diseased one. Differential lung ventilation using a double-lumen endotracheal tube allows each lung to be independently ventilated with appropriate settings.
Prone Positioning Logistics
Prone positioning requires a dedicated team of at least 4–6 trained personnel to safely rotate a ventilated patient. All lines, tubes, and drains must be secured before turning. Common complications include facial oedema, pressure sores (face, anterior chest, knees), unplanned extubation, and line dislodgement. A prone turn checklist is used in experienced centres to standardise the process and prevent adverse events.
| Parameter | Initial Setting | Target | Action if Outside Target |
|---|---|---|---|
| Tidal Volume | 6–8 mL/kg IBW | 6 mL/kg IBW in ARDS | Reduce by 1 mL/kg if Pplat >30 |
| Respiratory Rate | 14–18/min | 12–20/min; up to 35 in severe ARDS | Increase if acidotic; reduce if auto-PEEP |
| FiO2 | 1.0 initially | Minimum to maintain SpO2 92–96% | Use PEEP-FiO2 table |
| PEEP | 5 cmH2O initial | Per ARDSNet table; usually 5–20 | Titrate with FiO2 to oxygenation target |
| Plateau Pressure | Measure after setting Vt | ≤30 cmH2O | Reduce Vt if exceeded |
| Driving Pressure | Calculate: Pplat − PEEP | <15 cmH2O (ideally <13) | Adjust Vt or PEEP |
Why is ideal body weight used instead of actual body weight for tidal volume?
Lung volume and alveolar count correlates with body frame (reflected by height and sex) rather than total body mass. Adipose tissue does not increase alveolar number. Using actual body weight in obese patients would deliver tidal volumes far exceeding what the existing lung tissue can accommodate, causing ventilator-induced lung injury (VILI) through overdistension. IBW standardises Vt to lung size.
What is the ARDSNet low-PEEP table?
The ARDSNet PEEP-FiO2 table provides recommended PEEP levels paired with FiO2 to maintain oxygenation targets (SpO2 88–95%). The low-PEEP table pairs lower PEEP (5–24 cmH2O) with higher FiO2; the high-PEEP table pairs higher PEEP with lower FiO2. The choice of table (high vs low PEEP strategy) depends on patient-specific lung recruitability and haemodynamics. Most centres start with the low-PEEP table.
What is permissive hypercapnia?
Permissive hypercapnia is the intentional acceptance of elevated PaCO2 (typically 45–60 mmHg, pH ≥7.20) in order to maintain low tidal volumes. In ARDS, reducing Vt to 6 mL/kg IBW may cause CO2 retention if minute ventilation is reduced. This is tolerated because the survival benefit of low-Vt lung protection outweighs the harm of mild-moderate hypercapnia, which is generally well tolerated. Permissive hypercapnia is contraindicated in raised intracranial pressure.
What is driving pressure and why is it important?
Driving pressure (Pplat − PEEP) reflects the stress placed on aerated lung tissue with each tidal breath. It represents the pressure required to deliver Vt above PEEP. Amato et al. (2015) showed that driving pressure is more strongly associated with survival in ARDS than either Vt or Pplat independently. A driving pressure >15 cmH2O is associated with significantly higher mortality; the target is <15 cmH2O (ideally <13 cmH2O).
When should prone positioning be initiated?
PROSEVA trial evidence supports prone positioning (16+ hours/day) for moderate-to-severe ARDS (P/F ratio <150 mmHg) on optimal ventilator settings. Prone positioning improves V/Q matching, recruits dorsal atelectasis, and reduces driving pressure. It should be initiated early (within 48 hours of ARDS diagnosis) in eligible patients without contraindications (haemodynamic instability, recent sternotomy, open abdomen, facial trauma).
What are the common modes of mechanical ventilation?
Volume-controlled (VC): delivers a fixed Vt; pressures vary with compliance. Pressure-controlled (PC): delivers a fixed driving pressure; Vt varies with compliance. Pressure support (PS): patient-triggered mode where each breath is supported by a set pressure; used during weaning. CPAP (continuous positive airway pressure): provides a constant elevated airway pressure without ventilatory support; used for weaning and non-invasive ventilation. SIMV (synchronised intermittent mandatory ventilation) is now less used as evidence does not support it over pressure support for weaning.
What is a spontaneous breathing trial (SBT) and how is it done?
An SBT assesses whether a mechanically ventilated patient can sustain independent breathing before extubation. The patient is placed on minimal ventilator support (CPAP 5 cmH2O or T-piece/flow-by) for 30 minutes. Success criteria: SpO2 >90%, respiratory rate <30 breaths/min, heart rate and blood pressure stable, no significant distress or use of accessory muscles. Passing the SBT predicts successful extubation in about 80% of cases.
What causes auto-PEEP and how is it managed?
Auto-PEEP (intrinsic PEEP) occurs when the expiratory time is insufficient for complete lung emptying, causing air trapping and progressive lung hyperinflation. It is common in obstructed airways disease (asthma, COPD). Auto-PEEP can cause haemodynamic compromise by reducing venous return. Management includes increasing expiratory time (by reducing RR or I:E ratio), reducing bronchospasm with bronchodilators, and applying external PEEP slightly below auto-PEEP to reduce inspiratory trigger work.
专业提示
Calculate and document driving pressure (Pplat − PEEP) at least once every 12 hours on all ventilated ARDS patients. If driving pressure exceeds 15 cmH2O, do not simply accept it — systematically address the modifiable causes: reduce Vt further (minimum 4 mL/kg IBW), increase PEEP to recruit atelectasis (if recruitable), treat bronchospasm, and drain pleural effusions. Driving pressure reduction is the most impactful ventilator intervention for survival in ARDS.
你知道吗？
The ARDSNet ARMA trial (2000) that established 6 mL/kg IBW as the lung-protective standard is one of the most consequential trials in critical care medicine. Before this trial, many ICUs were routinely using tidal volumes of 10–12 mL/kg, inadvertently injuring patients' lungs with each breath. The trial was stopped early by its safety monitoring board — not because of harm, but because the benefit of low tidal volume was so dramatic that continuing with high tidal volumes in the control group was deemed unethical.
参考资料
- ›ARDSNet. Ventilation with lower tidal volumes as compared with traditional tidal volumes (ARMA trial). N Engl J Med. 2000;342(18):1301-1308.
- ›Amato MB et al. Driving pressure and survival in the acute respiratory distress syndrome. N Engl J Med. 2015;372(8):747-755.
- ›Guérin C et al. Prone positioning in severe ARDS (PROSEVA). N Engl J Med. 2013;368(23):2159-2168.
- ›Fan E et al. An Official ATS/ESICM/SCCM Clinical Practice Guideline: Mechanical Ventilation in Adult Patients with ARDS. Am J Respir Crit Care Med. 2017;195(9):1253-1263.