What are the key principles and parameters to focus on for a general adult population requiring mechanical ventilation in Egan's book of Respiratory Therapy (RT)?

Understanding Mechanical Ventilation Studies in Egan's Respiratory Therapy

Focus on lung-protective ventilation as the foundational principle: use tidal volumes of 6-8 ml/kg predicted body weight (not actual weight), maintain plateau pressures strictly below 30 cmH₂O, and start with Assist/Control mode for complete initial ventilatory support. 1, 2, 3

Core Ventilator Parameters to Master

Tidal Volume Calculations

• Always calculate using predicted body weight (PBW), never actual body weight 2, 3
  • Males: PBW = 50 + 0.91 × [height (cm) - 152.4] kg 1
  • Females: PBW = 45.5 + 0.91 × [height (cm) - 152.4] kg 1
• Start at 6 ml/kg PBW and increase only to 8 ml/kg if the lower volume is not tolerated 1, 2
• This lung-protective strategy reduces mortality in ARDS and sepsis-induced respiratory failure 3

Pressure Limits

• Maintain plateau pressure ≤30 cmH₂O at all times to prevent ventilator-induced lung injury and barotrauma 1, 2, 3
• In children, target even lower at <28 cmH₂O 1
• Driving pressure (plateau pressure minus PEEP) is emerging as a better predictor of outcomes than tidal volume or plateau pressure alone 1

PEEP Settings

• Set initial PEEP at minimum 5 cmH₂O—zero PEEP is explicitly not recommended 2
• For moderate-to-severe ARDS (PaO₂/FiO₂ <200), use higher PEEP levels titrated to FiO₂ requirements 1
• In COPD patients, use PEEP of 4-8 cmH₂O to offset intrinsic PEEP and improve triggering, but never set external PEEP higher than measured intrinsic PEEP 2, 4

Understanding Ventilator Modes

Volume-Controlled vs. Pressure-Controlled Ventilation

• In volume-targeted ventilation, you set the tidal volume and inspiratory time; the ventilator generates whatever pressure is necessary to deliver that volume 3, 5
• In pressure-targeted ventilation, you set the inspiratory pressure level; the delivered tidal volume varies based on lung compliance, airway resistance, and inspiratory time 1, 3

Primary Modes for Initial Management

Assist/Control (AC) Ventilation:

• Start with AC mode when initiating mechanical ventilation, as it provides complete ventilatory support immediately after intubation and prevents central apneas 3
• Guarantees a preset number of mandatory breaths per minute while allowing patient-triggered breaths, with all breaths delivering identical preset parameters 3

Controlled Mechanical Ventilation (CMV):

• Provides full ventilatory support with no patient effort required, delivering preset breaths at fixed intervals regardless of patient respiratory drive 1, 3

Synchronized Intermittent Mandatory Ventilation (SIMV):

• Synchronizes patient-triggered breaths with machine-delivered breaths, delaying the next mandatory breath when a patient triggers 3

Pressure Support Ventilation (PSV):

• The patient's respiratory effort triggers the ventilator both on and off, with the patient determining respiratory frequency and timing of each breath 1, 3

Disease-Specific Ventilation Strategies

ARDS Management

• Use AC with low tidal volumes (6 ml/kg PBW) and plateau pressure ≤30 cmH₂O 1, 3
• For severe ARDS (PaO₂/FiO₂ <100), implement prone positioning for more than 12 hours per day—this is a strong recommendation with moderate confidence in mortality reduction 1, 3
• Consider recruitment maneuvers in moderate-to-severe ARDS, though evidence quality is lower 1
• Strongly avoid routine use of high-frequency oscillatory ventilation in moderate or severe ARDS 1

COPD-Specific Modifications

• Set respiratory rate at 10-15 breaths/min, preferring the lower end to allow adequate expiratory time 2
• Use prolonged expiratory time with I:E ratio of 1:2 to 1:4 to prevent breath stacking and auto-PEEP 2
• Titrate FiO₂ to SpO₂ 88-92% (not higher) to avoid worsening hypercapnia from excessive oxygen 2
• Accept permissive hypercapnia with pH >7.2 to reduce barotrauma risk 1, 2

Post-Cardiac Arrest

• Avoid hyperventilation and target normocapnia with PaCO₂ 40-45 mmHg 3
• Hyperventilation causes cerebral vasoconstriction, hemodynamic instability, and increased mortality 3

Critical Monitoring Parameters

Initial Assessment

• Obtain arterial blood gas before initiating ventilation and recheck 30-60 minutes after any ventilator change 2
• Position patient with head of bed elevated 30 degrees before induction if not contraindicated 2

Oxygenation Targets

• For COPD patients: SpO₂ 88-92% 2
• For other patients: SpO₂ 88-95% 2
• Use the lowest FiO₂ possible to achieve target saturation 2

Common Pitfalls to Avoid

Critical Errors:

• Never use actual body weight for tidal volume calculations—this is the most common error leading to ventilator-induced lung injury 3
• Never use excessive FiO₂ in COPD, as oxygen administration corrects hypoxemia but worsens V/Q mismatch and contributes to increased PaCO₂ 2
• Avoid hyperventilation, which causes auto-PEEP and hemodynamic compromise in obstructive lung disease 2, 3
• Never use high respiratory rates that prevent adequate expiratory time, as this causes dangerous auto-PEEP accumulation 2

Terminology Confusion:

• Be aware that terminology for ventilation modes varies between ventilator manufacturers, potentially causing confusion 3
• The same mode may be called by different names on different ventilators 1

Alternative Modes for Refractory Cases

When conventional AC or SIMV with lung-protective ventilation fails:

• Consider Airway Pressure Release Ventilation (APRV) for refractory hypoxemia with ARDS and ventilator asynchrony 4
• Consider Neurally Adjusted Ventilatory Assist (NAVA) for significant patient-ventilator asynchrony with intact respiratory drive 4
• For COPD exacerbations with acute hypercapnic respiratory failure, use CPAP 4-8 cmH₂O plus PSV 10-15 cmH₂O as the most effective non-invasive approach 4

Adjunctive Medical Management

Bronchodilators:

• Administer nebulized salbutamol 2.5-5 mg or ipratropium 0.25-0.5 mg every 4-6 hours via ventilator circuit 2

Corticosteroids:

• Administer prednisolone 30 mg/day orally or hydrocortisone 100 mg IV for 7-14 days 2

Antibiotics:

• Use amoxicillin or tetracycline as first-line unless previously ineffective 2

Non-Invasive Ventilation Considerations

• Avoid non-invasive positive pressure ventilation (NIPPV) in sepsis-related ALI/ARDS patients, as delays in intubation may result in worse complications 1
• NIPPV is most effective in selected patients with normal or near-normal mental status without significant respiratory secretions and expected resolution within 72 hours 1
• Early reports did not favor NIV in COVID-19 due to concerns about large tidal volumes, high transpulmonary pressures, and aerosol generation 1