---

Mechanical Ventilation: Complete Explanation

---

1. Definition and Goals

• Maintain adequate gas exchange (oxygenation and CO2 elimination)
• Rest the respiratory muscles and decrease the oxygen cost of breathing
• Minimize iatrogenic consequences - including hyperinflation, infection, oxygen toxicity, respiratory muscle injury, and ventilator-induced lung injury (VILI)

• Goldman-Cecil Medicine, p. 1061

---

2. Physiological Basis: How Ventilation Works

Oxygenation

1. Increasing FiO2 - raises the partial pressure of oxygen available for hemoglobin transport. FiO2 >50% is generally tolerated but risks oxygen toxicity and absorptive atelectasis.
2. Increasing mean airway pressure - via increasing PEEP, driving pressure, or adjusting the I:E ratio to raise inspiratory time. Plateau pressure must be limited to minimize barotrauma and VILI.
3. Recruitment maneuvers (RM) - transient increases in airway pressure to re-open collapsed alveoli, followed by increased PEEP to prevent recollapse. Risk: barotrauma and transient decreased cardiac output.

Ventilation (CO2 Elimination)

• Current Surgical Therapy 14e, p. 1575

---

3. Types of Mechanical Ventilation

3a. Invasive vs. Noninvasive

---

4. Modes of Mechanical Ventilation

4a. Controlled Ventilation (CMV)

• A preset tidal volume (volume-controlled) or preset inspiratory pressure (pressure-controlled) is delivered at a predetermined rate and insufflation time, regardless of patient effort.
• Used in heavily sedated or paralyzed patients and in severe ARDS or shock requiring full support.
• Disadvantage: Rapid respiratory muscle atrophy from disuse; altered gas distribution from supine position and lack of diaphragm activity.

4b. Assist-Control Ventilation (AC)

• Most commonly used mode in ICUs.
• The ventilator delivers full preset breaths whenever triggered - either by the patient's inspiratory effort or by the ventilator timer if no patient effort occurs in the set interval.
• Every breath (patient-triggered or machine-triggered) receives the full preset support.
• Advantage: Low work of breathing; tidal volume is guaranteed.
• Disadvantage: Risk of breath stacking and respiratory alkalosis if patient over-breathes.

4c. Synchronized Intermittent Mandatory Ventilation (SIMV)

• The ventilator delivers a mandatory breath rate and also permits the patient to take additional spontaneous variable breaths above the set rate.
• Mandatory breaths are synchronized with the patient's inspiratory effort to minimize breath stacking.
• If the patient does not trigger, the ventilator delivers mandatory breaths at the set rate.
• Spontaneous breaths between mandatory breaths are unsupported (or can be supported with pressure support).

4d. Spontaneous Modes (Pressure Support Ventilation - PSV)

• No mandatory breaths - respiratory rate is entirely patient-determined.
• The ventilator provides a set pressure boost to augment each patient-initiated breath.
• Requires intact respiratory drive; apnea alarms and back-up rates are mandatory.
• Used for patient comfort and to assist with ventilator liberation (weaning).

4e. Bilevel Ventilation (APRV / BiLevel)

---

5. Volume vs. Pressure Breaths

---

6. Key Ventilator Settings

• Murray & Nadel's Textbook of Respiratory Medicine; Current Surgical Therapy 14e

---

7. Pressure-Volume Curve and the "Safe Window"

• Zone of overdistension (barotrauma/volutrauma): Too high a pressure or volume stretches and ruptures alveoli.
• Zone of derecruitment and atelectasis: Too little pressure allows alveoli to collapse.
• Safe window (green): The clinical goal - use sufficient PEEP to keep alveoli open while using small tidal volumes to stay below the overdistension threshold.
• Current Surgical Therapy 14e, Fig. 1

---

8. Ventilator-Induced Lung Injury (VILI)

---

9. Auto-PEEP and Dynamic Hyperinflation

• High minute ventilation
• Increased expiratory airway resistance (e.g., COPD, asthma)
• Increased respiratory system compliance
• Short expiratory time

• Goldman-Cecil Medicine, p. 1063

---

10. Hemodynamic Effects of Positive-Pressure Ventilation

• Increases intrathoracic pressure → decreases venous return → reduced preload → may drop cardiac output
• At high lung volumes: Compresses alveolar vessels → increased pulmonary vascular resistance → right ventricular strain → acute cor pulmonale
• ARDS patients tolerate relatively higher PEEP because low lung compliance limits the transmission of airway pressure to pleural pressure
• COPD patients tolerate lower PEEP because high compliance transmits more pressure to pleura
• Obese patients tolerate PEEP ~8 cmH2O higher than non-obese patients due to higher baseline intrathoracic pressures from abdominal/chest wall load
• Goldman-Cecil Medicine, p. 1063

---

11. Disease-Specific Ventilation Strategies

11a. ARDS (Acute Respiratory Distress Syndrome)

• Lung-protective ventilation: VT 6 mL/kg IBW, plateau pressure <30 cmH2O
• Higher PEEP to prevent derecruitment; titrate using PEEP-FiO2 tables
• Prone positioning for moderate-severe ARDS (PaO2/FiO2 <150): improves dorsal recruitment
• Permissive hypercapnia acceptable to avoid volutrauma
• Neuromuscular blockade in severe early ARDS to reduce patient-ventilator dyssynchrony and VILI

11b. COPD / Status Asthmaticus

• Avoid dynamic hyperinflation - use low respiratory rates (10-12/min), smaller tidal volumes, long expiratory times
• Accept permissive hypercapnia (controlled hypoventilation)
• PEEP should be set at 80-85% of auto-PEEP to reduce inspiratory trigger work without worsening hyperinflation
• NIV preferred initially to avoid intubation

11c. Neuromuscular Disease

• VT 6-8 mL/kg, rates slightly below spontaneous, 5-10 cmH2O PEEP
• Pressure-limited ventilation acceptable if pressure difference (Pi - Pe) is at least 8-10 cmH2O
• NIV preferred when possible; failed NIV (especially with bulbar involvement) requires intubation

---

12. Noninvasive Ventilation (NIV)

---

13. Monitoring During Mechanical Ventilation

• Continuous: ECG, pulse oximetry, capnometry (EtCO2)
• Airway pressures: Peak, plateau, mean, and PEEP - measured continuously
• Tidal volumes: Inhaled and exhaled (mechanical + spontaneous)
• FiO2: Should match set values
• Chest radiograph: To confirm ETT position, detect pneumothorax, pulmonary edema
• Arterial blood gas (ABG): Via arterial line for accurate pH, PaO2, PaCO2
• Fluid balance: Urine output monitoring

• Rising plateau pressure = worsening compliance
• Rising peak + hypotension = suspect pneumothorax
• Rising peak only (plateau stable) = increased airway resistance (secretions, bronchospasm, ETT obstruction)
• Dynamic hyperinflation = drop in BP + rising plateau → disconnect from ventilator to release trapped gas
• Morgan & Mikhail's Clinical Anesthesiology, p. 2538

---

14. Complications of Mechanical Ventilation

Pulmonary

Infectious

• Defined as pneumonia ≥48 hours after initiating mechanical ventilation
• Occurs in up to 15% of ventilated patients; ~50% mortality when it occurs
• Pathogens: S. aureus, Pseudomonas aeruginosa, gram-negative enteric rods (MRSA in select patients)
• Empiric treatment: IV beta-lactam (piperacillin-tazobactam, cefepime) ± MRSA coverage (vancomycin or linezolid)
• Treatment duration: 7 days when possible

• Head-of-bed elevation 30-45° (70% VAP reduction vs. supine)
• Specialized ETT with suction port above cuff (50% VAP reduction)
• Minimize ventilator circuit tubing changes
• Hand hygiene before handling circuits
• Daily sedation interruption and readiness-for-extubation assessment
• Oral/dental care

Cardiovascular

• Hemodynamic compromise in up to 40% of emergency intubations
• Decreased venous return, cardiac output, and BP from high intrathoracic pressures

Extrathoracic

• GI stress ulcers and bleeding (requires prophylaxis with H2 blockers or PPIs)
• Deep venous thrombosis / pulmonary embolism (requires pharmacologic or mechanical DVT prophylaxis)
• Delirium and sleep disruption
• ICU-acquired weakness (ICUAW) from prolonged sedation, paralysis, disuse atrophy
• Harrison's Principles of Internal Medicine, 22e, p. 2351

---

15. Liberation (Weaning) from Mechanical Ventilation

15a. Readiness Criteria (Daily Screening)

• Underlying cause of respiratory failure is improving or resolved
• Patient is awake, alert, off or on minimal sedation
• FiO2 ≤ 0.5, PEEP < 8 cmH2O, SpO2 > 88%
• Hemodynamically stable (no vasopressors or minimal doses)
• Manageable secretions with adequate cough
• pH > 7.25

15b. Spontaneous Breathing Trial (SBT)

• Set pressure support to 5-7 cmH2O (to compensate for ETT resistance)
• Patient breathes spontaneously for 30-120 minutes
• Passes SBT if: Comfortable, SpO2 stable, RR <35/min, HR and BP stable, no distress or diaphoresis

15c. Rapid Shallow Breathing Index (RSBI)

• RSBI < 105: Predicts successful extubation (sensitivity ~97%)
• RSBI > 105: Predicts weaning failure

15d. Additional Weaning Parameters

15e. Methods of Weaning (when gradual withdrawal needed)

1. Daily SBT (T-piece or low PSV): Most evidence-based. Pass → extubate.
2. Gradual PSV reduction: Decrease pressure support by 2-4 cmH2O increments until 5-7 cmH2O tolerated.
3. SIMV rate reduction: Progressively lower mandatory rate. Now considered least preferred (slower weaning).

15f. Tracheostomy

• Consider when a patient has required intubation for ≥ 2 weeks or is predicted to require prolonged ventilation.
• Early tracheostomy (<1 week) is generally no better than waiting 10-15 days.
• Allows weaning while maintaining airway access, reducing sedation requirements, and enabling communication.
• Harrison's Principles of Internal Medicine, 22e; Morgan & Mikhail's Clinical Anesthesiology, p. 2539

---

16. Recent Evidence (2024-2026)

1. Intraoperative ventilation (PMID: 40133080, Anaesthesia, 2025): Network meta-analysis of RCTs found that lung-protective intraoperative ventilation strategies remain superior, though optimal combinations of VT, PEEP, and recruitment maneuvers are still debated.
2. Mechanical power and VILI (PMID: 41270941, Respir Med, 2025): Mechanical power (MP) - the total energy delivered to the lung per minute (encompassing pressure, tidal volume, flow, and rate) - is emerging as a more unified index of VILI risk than individual parameters. Limiting MP may reduce VILI beyond traditional lung-protective settings alone.

---

Summary Table: Quick Reference

---