Chapter 313: Mechanical Ventilatory Support¶
Part 8: Critical Care Medicine
KEY CLINICAL POINTS¶
- Mechanical ventilation is critical for patients with hypoxemic or hypercapnic respiratory failure, ARDS, and airway obstruction.
- Optimal ventilation requires balancing tidal volume (6-8 mL/kg IBW), PEEP, and respiratory rate to prevent barotrauma and volume trauma.
- Noninvasive ventilation (NIV) is beneficial for acute respiratory failure with reversible causes, reducing ICU admission and intubation.
- Ventilator-associated pneumonia (VAP) is a major complication, with prevention strategies including head-of-bed elevation and daily sedation interruption.
- Successful weaning from mechanical ventilation depends on resolving the underlying disease, adequate respiratory drive, and minimal secretions.
1. DEFINITION & OVERVIEW¶
Mechanical ventilation delivers positive pressure gas to patients with acute/chronic respiratory failure. Hypoxemic failure is due to ventilation-perfusion mismatch (e.g., ARDS, pneumonia), while hypercapnic failure is caused by obstructive lung disease (COPD, asthma) or neuromuscular disorders. Noninvasive ventilation (NIV) is used for patients with intact airway protection.
Table 313-1: Key Features of Commonly Used Mechanical Ventilation Modes¶
| MODE | VARIABLES SET BY CLINICIAN (INDEPENDENT) | MONITORED VARIABLES (DEPENDENT) | ADVANTAGES | DISADVANTAGES |
|---|---|---|---|---|
| Assist control–volume control | VT, Respiratory rate, PEEP, FIO2, Inspiratory flow rate | Peak inspiratory pressure, End-inhalation pressure, VE | Guarantees minimum VT and VE | Barotrauma from high plateau pressure, Patient-ventilator dyssynchrony |
| Assist control–pressure control | Inspiratory driving pressure, Respiratory rate, PEEP, FIO2 | VT, VE | Limits barotrauma if patient effort minimal | Variable VT due to changing compliance |
| Pressure-regulated volume control | VT, Respiratory rate, PEEP, FIO2 | Peak inspiratory pressure, End-inhal, VE | Patient effort varies inspiratory flow | VT may exceed set values |
| Pressure support | FIO2, PEEP, Maximum inspiratory pressure | VT, VE, Respiratory rate | Preserves patient effort | Apnea/hypoventilatio n possible |
Table 313-2: Common Contraindications to Noninvasive Ventilation¶
| CONTRAINDICATION |
|---|
| Inability to protect the airway (e.g., severe encephalopathy) |
| High risk for aspiration (e.g., vomiting, upper GI bleeding) |
| Difficulty clearing respiratory secretions |
| Facial trauma/surgery, upper airway obstruction |
| Significant hemodynamic instability |
1.1 ARDS and Lung Recovery¶
ARDS survivors often regain near-normal lung function within 6 months, but long-term sequelae include exercise limitation and reduced quality of life. Recovery correlates with initial lung injury severity (e.g., static compliance, PEEP requirements). Psychological comorbidities (depression, PTSD) are common in survivors.
1.2 Ventilator Mechanics¶
Positive pressure ventilation inflates lungs via transpulmonary pressure. In ARDS, stiff lungs require higher pressures for tidal volumes, risking respiratory muscle fatigue. Ventilator settings must balance oxygenation, CO2 removal, and lung protection.
2. PRINCIPLES OF MECHANICAL VENTILATION¶
Optimal ventilation requires balancing tidal volume (6-8 mL/kg IBW), PEEP, and respiratory rate to prevent barotrauma and volume trauma. Pressure-regulated volume control (PRVC) adapts to patient effort while maintaining target VT. PEEP should be set at the lower inflection point of the pressure-volume curve to prevent alveolar collapse.
Table 313-3: Adverse Effects of Hypercapnia¶
| ADVERSE EFFECT |
|---|
| Pulmonary arterial vasoconstriction (right heart failure) |
| Rightward shift of oxyhemoglobin curve |
| Cerebral vasodilation and increased ICP |
| Sympathetic-adrenal stimulation |
| Reduced cardiac contractility (especially with b-blockers) |
2.1 Tidal Volume and PEEP Optimization¶
Low tidal volume (6 mL/kg IBW) and moderate PEEP (5-10 cmH2O) reduce lung injury. Higher PEEP (10-20 cmH2O) may be needed for ARDS to prevent atelectasis. End-inspiratory pressure should remain <30 cmH2O to avoid overdistension.
2.2 Ventilation Modes¶
Volume control (AC) guarantees VT but risks barotrauma. Pressure control (PCV) limits peak pressure but allows variable VT. Pressure support (PSV) preserves patient effort but may lead to hypoventilation.
3. MECHANICAL VENTILATION MODES¶
Assist control (AC), pressure control (PCV), pressure support (PSV), and volume support modes are used based on patient needs. NIV (CPAP/BiPAP) is preferred for reversible respiratory failure. PRVC adapts to patient effort while maintaining target VT.
3.1 Assist Control (AC)¶
Clinician controls VT, respiratory rate, PEEP, and FIO2. Patients can trigger additional breaths. Risks include barotrauma and dyssynchrony.
3.2 Pressure Control (PCV)¶
Inspiratory pressure is set, with VT dependent on lung compliance. Limits peak pressure but may result in variable VT. Used in ARDS to prevent overdistension.
3.3 Pressure Support (PSV)¶
Patient-initiated breaths are supported by preset pressure. Preserves spontaneous breathing but may lead to hypoventilation if patient effort is inadequate.
4. NONINVASIVE POSITIVE PRESSURE VENTILATION¶
NIV (CPAP/BiPAP) is used for acute respiratory failure with reversible causes (e.g., COPD exacerbations). It improves gas exchange and reduces ICU admission. Contraindications include inability to protect the airway or hemodynamic instability.
4.1 Indications and Outcomes¶
NIV reduces intubation rates in COPD exacerbations and acute cardiogenic pulmonary edema. Nocturnal NIV improves outcomes in hypercapnic patients with chronic respiratory failure.
4.2 Risks and Monitoring¶
PES (post-extubation stridor) risk factors include prolonged intubation, trauma, or previous PES. Cuff leak tests help identify PES risk. Monitor for hypercapnia and ventilatory effort.
5. COMPLICATIONS OF MECHANICAL VENTILATION¶
Common complications include ventilator-associated pneumonia (VAP), barotrauma (pneumothorax, pneumomediastinum), and hypercapnia. Prolonged ventilation increases risk of ICU-acquired weakness and delirium.
5.1 Airway Complications¶
Endotracheal tube placement can cause vocal cord injury, tracheal stenosis, or stricture. PES risk factors include prolonged intubation, trauma, or previous PES.
5.2 Hypercapnia and Acidosis¶
Hypercapnia causes pulmonary vasoconstriction, increased ICP, and reduced cardiac contractility. Permissive hypercapnia may be tolerated down to pH 7.2 but requires monitoring.
5.3 Ventilator-Associated Pneumonia¶
VAP occurs in 15% of ventilated patients, with mortality ~50%. Prevention includes head-of-bed elevation, daily sedation interruption, and strict hand hygiene.
6. LIBERATION FROM MECHANICAL VENTILATION¶
Weaning involves assessing readiness for spontaneous breathing, often via spontaneous breathing trial (SBT). Criteria include FIO2 <0.5, PEEP <8 cmH2O, SaO2 >90%, and stable hemodynamics.
Table 313-4: Algorithm for Discontinuing Mechanical Ventilation¶
| STEP | CRITERIA |
|---|---|
| 1 | Underlying process improved, patient awake, minimal sedation |
| 2 | FIO2 £0.5, PEEP <8 cmH2O, SaO2 >88% |
| 3 | Stable hemodynamics, minimal secretions, adequate cough |
| 4 | Spontaneous breathing trial (SBT) with pressure support 5-7 cmH2O |
| 5 | SBT success: RR <35, SaO2 >90%, no anxiety/diaphoresis |
| 6 | Extubation with NIV or high-flow O2 for high-risk patients |
6.1 Spontaneous Breathing Trial (SBT)¶
SBT uses minimal pressure support (5-7 cmH2O) to assess respiratory effort. Success criteria: comfortable breathing, RR <35, SaO2 >90%, and minimal secretions.
6.2 Weaning Algorithms¶
Algorithm includes daily readiness assessment, SBT, and transition to NIV or high-flow oxygen. High-risk patients (age >65, COPD, APACHE-II >12) require closer monitoring.
7. KEY POINTS & CLINICAL PEARLS¶
- Use low tidal volumes (6 mL/kg IBW) and moderate PEEP to prevent lung injury.
- Monitor for hypercapnia and adjust ventilator settings to avoid barotrauma.
- NIV is effective for acute respiratory failure with reversible causes but has contraindications.
- Weaning success depends on resolving the underlying disease and adequate respiratory drive.
- Early mobilization and daily sedation interruption reduce ICU-acquired weakness.