Inability to Maintain Airway Patency and/or Reflexes

Upper Airway Obstruction

• Altered Mental Status (see Obtundation-Coma)
  • Intoxication
    • Central Nervous System Depressants (Such as Ethanol, Benzodiazepines, etc)
  • Procedural Deep Sedation (with/without Pharmacologic Paralysis) (see Sedation)
    • Bronchoscopy (see Bronchoscopy)
    • Cerebral Angiogram (see Cerebral Angiogram)
    • Coronary Angiogram (see Cardiac Catheterization)
    • Colonoscopy (see Colonoscopy)
    • Esophagogastroduodenoscopy (EGD) (see Esophagogastroduodenoscopy)
    • Other Procedures Which Require Prolonged Immobility
  • Procedural General Anesthesia (with/without Pharmacologic Paralysis) (see General Anesthesia)
    • Surgery
• Bilateral Vocal Fold Immobility (BVFI) (see Bilateral Vocal Fold Immobility)
  • Example: Bilateral Vocal Cord Paralysis (see Vocal Cord Paralysis)
  • Example: Laryngospasm (see Laryngospasm)
• Other Etiologies of Upper Airway Obstruction
  • Example: Anaphylaxis (see Anaphylaxis)
  • Example: Angioedema (see Angioedema)

Lower Airway Obstruction

• Airway Mucous Plugging with Inability to Clear Secretions
• Massive Hemoptysis (see Hemoptysis)

Respiratory Failure (see Respiratory Failure)

Type I Hypoxemic Respiratory Failure

• Acute or Chronic Hypoxemic Respiratory Failure
  • Decreased Mixed Venous Oxygen Saturation (Decreased Mixed Venous pO2)
  • Intrapulmonary Right-to-Left Shunt (see Intracardiac and Extracardiac Shunt)
  • Intracardiac Right-to-Left Shunt (see Intracardiac and Extracardiac Shunt)
  • Worsened V/Q Mismatch (Above Levels Observed as Part of Normal Physiology)
  • Diffusion Limitation

Type II Hypoxemic, Hypercapnic Respiratory Failure

• Acute Hypoxemic, Hypercapnic Respiratory Failure (Acute Hypoventilation, Acute Ventilatory Failure)
  • Decreased Ventilatory Drive
  • Decreased Ventilatory Output Due to Neuromuscular Disease
  • Decreased Ventilatory Output Due to Excessive Ventilatory Demand
• Chronic Hypoxemic, Hypercapnic Respiratory Failure (Chronic Hypoventilation, Chronic Ventilatory Failure)
  • Decreased Ventilatory Drive
  • Decreased Ventilatory Output Due to Neuromuscular Disease
  • Decreased Ventilatory Output Due to Excessive Ventilatory Demand

Physiologic/Clinical Benefits of Positive-Pressure Mechanical Ventilation

Positive-Pressure Mechanical Ventilation Decreases the Work of Breathing

• Increased Work of Breathing (with the Development of Respiratory Muscle Fatigue) is a Common Feature of Various Types of Respiratory Failure
• Depending on the Mode/Settings Utilized, Mechanical Ventilation Can Assume Part or All of the Work of Breathing, Allowing the Respiratory Muscles Time to Recover from Fatigue (Am J Med, 1982) [MEDLINE] (Intensive Care Med, 1998) [MEDLINE]

Positive-Pressure Mechanical Ventilation Improves V/Q Mismatch

• Improvement in V/Q Mismatch on Positive-Pressure Mechanical Ventilation is a Summation of the Following Two Competing Mechanisms
  • Positive-Pressure Mechanical Ventilation Worsens Physiologic (Alveolar) Dead Space (by Distending Alveoli Which May Be Poorly Perfused, Resulting in High V/Q Areas of the Lung)
    • Dead Space = Anatomic Dead Space + Physiologic (Alveolar) Dead Space
    • Note that Positive-Pressure Mechanical Ventilation Does Not Alter the Anatomic Dead Space
  • Positive-Pressure Mechanical Ventilation (Especially with the Application of PEEP) Decreases Atelectasis, Resulting in Decreased Physiologic Shunt
    • Shunt = areas of the lung which are underventilated, relative to perfusion (i.e. areas with low V/Q ratios)

Positive-Pressure Mechanical Ventilation Improves Left Ventricular Failure (see Congestive Heart Failure)

• Positive-Pressure Mechanical Ventilation Decreases Venous Return to the Right Side of the Heart (Preload) and Decreases Left Ventricular Afterload (NEJM, 1991) [MEDLINE]
  • Hemodynamic Effects of Positive-Pressure Mechanical Ventilation are Due to Transmission of the Airway Pressure to the Adjacent Thoracic Structures
    • Transmission is Greatest When There is Low Chest Wall Compliance (Due to Fibrothorax, etc) or High Chest Wall Compliance (Due to COPD, etc)
    • Transmission is Least When There is High Chest Wall Compliance (Due to Sternotomy, etc) or Low Lung Compliance (Due to ARDS, Pulmonary Edema, etc)

Heterogeneity of Ventilation While on Positive-Pressure Mechanical Ventilation

• Distribution of Ventilation (While on Positive-Pressure Mechanical Ventilation) is Heterogenous Due to Regional Differences in Alveolar Compliance, Airway Resistance, and Dependency (Upper Lung Zone vs Lower Lung Zone)
  • More Compliant Lung Zones with Low Airway Resistance Will Be the Most Ventilated (and Most Distended), While Less Compliant Lung Zones with High Airway Resistance Will Be the Least Ventilated (and Least Distended)