What are the basic principles of mechanical ventilation for a patient with respiratory distress?

Basic Principles of Mechanical Ventilation

Initial Mode Selection

Start with volume-cycled Assist-Control (AC) ventilation when initiating mechanical ventilation, as this provides complete ventilatory support immediately after intubation and prevents central apneas. 1, 2, 3

• AC mode guarantees a preset number of mandatory breaths per minute while allowing patient-triggered breaths, with all breaths delivering identical preset tidal volume. 1, 3
• Synchronized Intermittent Mandatory Ventilation (SIMV) can achieve similar respiratory support and may be used as an alternative. 4, 3
• Pressure Support Ventilation (PSV) should not be used as the initial mode but may be considered during weaning or for prolonged ventilation in stable patients. 3

Critical Initial Ventilator Settings

Set tidal volume at 6 mL/kg predicted body weight (PBW)—not actual body weight—to reduce mortality in ARDS and sepsis-induced respiratory failure. 4, 2, 3, 5

Calculating Predicted Body Weight:

• Men: 50 + 2.3 × (height in inches - 60) 2, 3
• Women: 45.5 + 2.3 × (height in inches - 60) 2, 3

Pressure Limits:

• Maintain plateau pressure ≤30 cmH₂O to prevent alveolar overdistension and ventilator-induced lung injury (VILI). 4, 2, 3, 5
• Monitor driving pressure (ΔP = Plateau pressure - PEEP), keeping it <15 cmH₂O, as this predicts outcomes better than any other ventilatory parameter. 4

Oxygenation Targets:

• Target PaO₂ 70-90 mmHg or SaO₂ 92-97%—avoid both hypoxemia and hyperoxia. 4
• Hyperoxia increases lung inflammation, adversely affects microcirculation, and is associated with increased mortality. 4

PEEP Strategy

Apply PEEP appropriately based on gas exchange, hemodynamic status, lung recruitability, end-expiratory transpulmonary pressure, and driving pressure. 4

• Perform recruitment maneuvers before PEEP selection. 4
• Use higher PEEP for moderate-severe ARDS (PaO₂/FiO₂ < 150 mmHg). 2
• PEEP ameliorates changes in closing volume and lung derecruitment, providing dramatic improvements in PaO₂. 4
• Consider esophageal pressure measurement to estimate transpulmonary pressure for individualized PEEP titration. 4

Indications for Invasive Mechanical Ventilation

Proceed to intubation and invasive ventilation when any of the following are present:

• Life-threatening hypoxemia: PaO₂/FiO₂ ≤150 mmHg, particularly if <100 mmHg despite optimized oxygen therapy. 2
• Severe tachypnea: respiratory rate >35 breaths/min despite optimal medical therapy. 4, 2
• Refractory hypoxemia: PaO₂ <60 mmHg despite high-flow oxygen. 4
• Respiratory distress with increased work of breathing and signs of respiratory muscle fatigue despite oxygen and noninvasive support. 2
• Impaired mental status or inability to protect the airway. 4, 2
• Severe acidosis: pH <7.25 with hypercapnia. 2
• Cardiovascular instability complicating respiratory failure (hypotension, arrhythmias, myocardial infarction). 2

Noninvasive Support Considerations

Noninvasive support with close monitoring is reasonable in less severely ill patients, but requires vigilant assessment for failure. 4

• High-flow nasal cannula (HFNC) reduces intubation rates and improves survival in patients with PaO₂/FiO₂ ≤200 mmHg compared to standard oxygen or face-mask NIV. 4
• If noninvasive support fails to improve within 1-2 hours, proceed immediately to invasive ventilation—delayed intubation increases mortality. 4, 2
• Monitored tidal volumes persistently >9.5 mL/kg PBW during NIV suggest the need for intubation. 4
• Rapid shallow breathing index (RSBI) >105 breaths/min/L is associated with need for intubation. 4

Airway Management

Use orotracheal intubation as the preferred route due to increased rates of nosocomial sinusitis with nasotracheal tubes, which contributes to ventilator-associated pneumonia (VAP) and mortality. 4

Adjunctive Therapies for Severe ARDS (PaO₂/FiO₂ <150 mmHg)

When PaO₂/FiO₂ <150 mmHg despite optimized ventilation, implement the following:

• Prone positioning for >12 hours per day—this improves oxygenation in approximately 65% of patients. 4, 2
• Consider neuromuscular blocking agents within the first 48 hours for severe refractory hypoxemia, especially with ventilator-patient dyssynchrony. 4, 2
• Deep sedation and analgesia within the first 48 hours of mechanical ventilation. 2

Rescue Therapies for Refractory ARDS

Consider ECMO when:

• PaO₂/FiO₂ <100 mmHg despite optimized PEEP and neuromuscular blockade 2
• pH <7.15 with excessive compensatory respiratory acidosis 2
• Plateau pressure >30 cmH₂O despite lung-protective ventilation 2
• Mechanical power ≥27 J/min 2

Fluid Management

Advocate judicious fluid resuscitation and/or fluid restriction when possible in ARDS, as improvements in physiology and outcome occur when patients lose weight or microvascular pressures fall. 4

• Fluid restriction strategies do not increase complications such as renal failure or hemodynamic compromise. 4
• In hypo-oncotic patients with established lung injury, albumin combined with furosemide may improve physiology and reduce duration of mechanical ventilation. 4

Critical Pitfalls to Avoid

Never use actual body weight for tidal volume calculations—always use predicted body weight. 2, 3

Never hyperventilate patients—target normocapnia (PaCO₂ 40-45 mmHg) to prevent cerebral vasoconstriction, hemodynamic instability, and increased mortality. 2, 3

Do not delay intubation when noninvasive support fails—delayed intubation is associated with increased mortality. 4

Avoid high-frequency oscillatory ventilation (HFOV) routinely in ARDS—it does not improve outcomes and may be harmful. 2, 3

Monitor for patient-ventilator dyssynchrony closely, as even assisted ventilation can induce VILI through generation of high tidal volumes and transpulmonary pressures. 3

Monitoring Requirements

Continuously monitor the following parameters:

• Plateau pressure, driving pressure, and oxygenation index 2
• Patient-ventilator synchrony 2
• Serial arterial blood gases to guide ventilator adjustments 2
• Pulse oximetry and capnography for adequate oxygenation and ventilation 2

Weaning Considerations

Consider weaning when PaO₂/FiO₂ >200 mmHg and PEEP <10 cmH₂O. 4

• Reduce sedation and transition to partial ventilator support to promote respiratory muscle activity when gas exchange, respiratory mechanics, and hemodynamic status have improved. 4