Mechanical Ventilation: Complete Clinical Guide

1. WHAT IS MECHANICAL VENTILATION?

2. BASIC PHYSIOLOGY

Minute Volume and Alveolar Ventilation

• Minute volume (VE) = Tidal volume (VT) x Respiratory rate (f). Normal = 7-10 L/min.
• Alveolar ventilation (VA) = (VT - VDS) x f, where VDS = dead space volume (~2.2 mL/kg lean body weight in healthy adults).
• Alveolar ventilation and CO2 production together determine PaCO2. Increasing minute ventilation lowers PaCO2; decreasing it raises PaCO2.

Volume-Pressure Relationship

• Decreasing compliance (stiffer lung) → higher pressure for same volume
• Increasing volume → higher pressure
• This is the basis for all ventilator pressure management

Key Airway Pressures

PEEP (Positive End-Expiratory Pressure)

• Extrinsic PEEP (PEEPe): Set on the ventilator; prevents alveolar collapse.
• Intrinsic PEEP (auto-PEEP, iPEEP): Air trapping due to inadequate expiration time (common in asthma/COPD). Worsens work of breathing. Managed by increasing expiratory time, decreasing respiratory rate.

3. INDICATIONS FOR MECHANICAL VENTILATION

Indications for Invasive Mechanical Ventilation (IMV)

• Respiratory or cardiac arrest
• Respiratory failure (hypoxemic or hypercapnic) not corrected by less invasive means
• NIV failure or inability to tolerate NIV
• Persistent diminished consciousness / coma
• Hemodynamic instability unresponsive to fluids and vasopressors
• Persistent inability to clear secretions
• Massive aspiration
• Life-threatening hypoxemia not corrected by supplemental oxygen
• Decreased consciousness or increased agitation compromising airway

Indications for NIV (BiPAP) in COPD Exacerbation

• Respiratory acidosis: PaCO2 ≥45 mmHg and pH ≤7.35
• Severe dyspnea with signs of respiratory muscle fatigue and accessory muscle use
• Persistent hypoxemia despite supplemental oxygen

Contraindications to NIV

• Active vomiting / high aspiration risk
• Respiratory arrest
• Facial trauma preventing mask seal
• Depressed mental status NOT related to hypercapnia
• Uncooperative patient

4. VENTILATOR MODES

Assist-Control (AC) / Continuous Mandatory Ventilation (CMV)

• Best for: patients who are heavily sedated, paralyzed, or in acute distress
• Risk: breath stacking, respiratory alkalosis if patient over-breathes the set rate

Synchronized Intermittent Mandatory Ventilation (SIMV)

• Best for: weaning from ventilator
• Downside: SIMV has the poorest weaning outcomes of all techniques

Pressure Support Ventilation (PSV)

• Best for: spontaneously breathing patients, weaning
• Advantage: patient controls rate and timing, reduces ventilator dyssynchrony

Volume-Cycled Ventilation (VCV / Volume Control)

• Advantage: consistent, reliable VT delivery
• Disadvantage: if compliance worsens (e.g., mucous plug, worsening ARDS), airway pressure climbs; cannot take dynamic changes into account automatically

Pressure-Cycled Ventilation (PCV / Pressure Control)

• Advantage: limits peak pressures, reduces barotrauma risk; patient can draw desired flow
• Disadvantage: VT varies; requires vigilance for falling volumes if compliance changes

5. INITIAL VENTILATOR SETTINGS (Standard Protocol)

Lung-Protective Ventilation (ARDSNet Protocol)

• VT: 6 mL/kg IBW (can go down to 4 mL/kg if needed)
• Plateau pressure: <30 cm H2O
• PEEP: titrated up (FiO2/PEEP table below) to maintain oxygenation
• Permissive hypercapnia is acceptable to keep pressures low

Strategies to Improve Oxygenation

1. Increase FiO2 - effective but FiO2 >50% risks oxygen toxicity and absorptive atelectasis
2. Increase PEEP - recruits collapsed alveoli, improves V/Q matching
3. Recruitment maneuvers (RM) - brief high-pressure inflations to open collapsed alveoli; follow with increased PEEP to keep them open
4. Prone positioning - redistributes perfusion to better-ventilated lung; the PROSEVA trial showed 90-day mortality of 23.6% (prone) vs 41% (supine) in moderate-severe ARDS; recommended ≥16 hours/day

Strategies to Improve Ventilation (CO2 Elimination)

• Increase RR or VT to increase minute ventilation
• Reduce dead space
• In status asthmaticus: low RR (<10/min), high flow rates (>60 L/min), permissive hypercapnia to avoid air trapping

6. VENTILATOR-INDUCED LUNG INJURY (VILI)

7. NONINVASIVE VENTILATION (NIV): CPAP AND BiPAP

History

CPAP (Continuous Positive Airway Pressure)

• Increases FRC (functional residual capacity)
• Recruits collapsed/partially collapsed alveoli (expansion above the inflection point on pressure-volume curve)
• Improves V/Q matching, reduces intrapulmonary shunting
• Redistributes extravascular lung water from alveolar-endothelial space to peribronchial/perihilar areas
• In OSA: acts as pneumatic splint to keep the upper airway patent

• High-flow gas source (60-90 L/min) to prevent pressure from dropping during inspiration
• Tight-fitting mask (maintains level without leaks)
• CPAP via mask should only be used if patient has intact airway reflexes
• Mask CPAP limited to <15 cm H2O (above this = risk of exceeding lower esophageal sphincter pressure → gastric distention, regurgitation)
• Pressures >15 cm H2O require tracheal or tracheostomy tube

• Start at 5-8 cm H2O
• Titrate up for OSA (polysomnography-guided) or hypoxemia

• Obstructive sleep apnea (primary indication)
• Acute cardiogenic pulmonary edema (CPAP shown to reduce intubation rates and trend toward mortality reduction)
• Post-operative hypoxemia and atelectasis
• During weaning as a spontaneous breathing trial mode

BiPAP (Bilevel Positive Airway Pressure)

• EPAP = CPAP = PEEP (functionally equivalent in terms of alveolar end-expiratory pressure)
• IPAP - EPAP = Pressure support (the inspiratory augmentation)
• BiPAP, NPPV (noninvasive positive pressure ventilation), and NIPPV (noninvasive intermittent PPV) are often used interchangeably

• EPAP: same as CPAP - recruits alveoli, increases FRC, improves oxygenation
• IPAP minus EPAP (effective PS): offloads respiratory muscles, augments tidal volume, improves alveolar ventilation and CO2 elimination
• In COPD: IPAP helps overcome iPEEP (intrinsic PEEP), reduces work of breathing
• Decreases respiratory rate, allows more effective lung emptying, larger tidal volumes

• S (Spontaneous): All breaths patient-triggered; machine switches between IPAP/EPAP per patient effort
• T (Timed): Breaths delivered at set rate regardless of patient effort
• S/T: Patient can trigger, but machine provides backup timed breaths if patient fails to trigger within set time - most common clinical mode

NIV Indications by Disease

CPAP vs PEEP vs BiPAP - Key Distinctions

8. CONTRAINDICATIONS AND MONITORING

Ongoing Ventilator Monitoring

• SpO2 continuously; target ≥92-95%
• ABG (baseline, after initiation, and with any changes): pH, PaO2, PaCO2
• Peak and plateau pressures every breath cycle; alert if Pplat >30
• Peak-plateau gradient (>4 cm H2O = bronchospasm/obstruction)
• Capnography (continuous EtCO2 during intubation and ventilation)
• Tidal volumes and minute ventilation
• Auto-PEEP measurement by expiratory hold maneuver

Common Complications

• Ventilator-associated pneumonia (VAP)
• Barotrauma (pneumothorax, pneumomediastinum)
• Oxygen toxicity (FiO2 >50% prolonged)
• Hemodynamic compromise (increased intrathoracic pressure → reduced venous return → decreased cardiac output)
• Diaphragmatic atrophy (prolonged CMV without spontaneous breathing)

9. SPECIAL PROTOCOLS

Asthma / Severe Obstruction Protocol

• Large ET tube (≥8.0 mm adults)
• Ketamine for induction (1-2 mg/kg IV) - bronchodilatory
• Low RR (<10 breaths/min) + high inspiratory flow (>60 L/min) = prolonged expiratory time
• Low VT (6-8 mL/kg) - reduces auto-PEEP risk
• Permissive hypercapnia - accepted; target pH >7.20
• Avoid breath stacking; do not hyperventilate after intubation

ARDS Protocol (ARDSNet)

• VT 6 mL/kg IBW (can reduce to 4 mL/kg)
• Pplat <30 cm H2O
• FiO2/PEEP table titration
• Consider prone positioning ≥16 hrs/day for P/F ratio <150
• Conservative fluid strategy
• Neuromuscular blockade for P/F ≤150 (<36 hrs) - reduces dyssynchrony (ACURASYS; ROSE trial was neutral for mortality)

10. WEANING AND LIBERATION FROM MECHANICAL VENTILATION

Readiness Criteria (Daily Assessment Once Disease Resolving)

Spontaneous Breathing Trials (SBT) - Key Weaning Strategies

1. T-piece trial: Patient disconnected from ventilator, remains intubated, breathes spontaneously through a heated humidified circuit. True test of independent breathing.
2. PSV trial (Pressure Support Ventilation): Patient remains on ventilator with low PS (5-10 cm H2O) and PEEP 5. No mandatory breaths. Most commonly used SBT method.
3. SIMV (decreasing rate): Set RR gradually decreased over hours to days. Poorest weaning outcomes of all techniques; used mainly in surgical/neurosurgical patients.

• Daily SBT (30-120 minutes) is the most important predictor of timely liberation from MV.
• Extubation failure rate: up to 23.5% even when all criteria met.

Pre-Extubation Cuff Leak Test

• Required when laryngeal edema is suspected (angioedema, traumatic intubation)
• Absent cuff leak = preclude extubation; treat with IV corticosteroids x 12-24 hours, then reassess

Post-Extubation

• Patients with COPD: extubation to NPPV (BiPAP) is associated with reduced mortality
• Close monitoring for reintubation need

Summary Comparison: CPAP vs BiPAP vs IMV