Acute Respiratory Failure

Introduction

• Third session in the ICU curriculum
• Focus: acute respiratory failure
• Topics covered:
  • Defining hypoxemic and hypercapnic respiratory failure
  • Examples of respiratory distress
  • Patients benefiting from non-invasive positive pressure ventilation (NIPPV)
  • Common reasons for intubation and mechanical ventilation
  • Four major ventilator variables and their effects on oxygenation and ventilation

Case Study

• 55-year-old male with COPD
• Presented with dyspnea, oxygen saturation 72% on room air
• Required 3 liters nasal cannula to maintain oxygen saturation >90%
• Chest X-ray: no focal consolidation or opacities
• Respiratory viral panel: positive for rhinovirus
• Developed worsening symptoms: more tired, increased breathing effort
• Required 10 liters nasal cannula to maintain oxygen saturation >90%
• ABG on 10 liters nasal cannula: pH 7.15, pCO2 65, pO2 52

Major Types of Respiratory Failure

Hypoxemic Respiratory Failure (Type 1)

• Definition: Oxygen saturation <90% or PaO2 <60 mmHg on room air
• Causes: Low inspired oxygen, hypoventilation, diffusion restriction, shunt, VQ mismatch
• Main Cause in ICU: VQ mismatch
• Examples of Alveolar Filling: Blood (hemorrhage), pus (pneumonia), water (pulmonary edema), cells (cancer), fat, calcium
• Treatment: Nasal cannula, non-rebreather, heated high flow nasal cannula, invasive mechanical ventilation

Hypercapnic Respiratory Failure (Type 2)

• Definition: PaCO2 >45 mmHg + respiratory acidosis (pH <7.3)
• Causes: Hypoventilation (decreased minute ventilation)
• Minute Ventilation Equation: Tidal volume × Respiratory rate
• Classification of Causes: "Won't breathe", "Can't breathe", "Can't breathe enough"
  • Won't breathe: CNS causes (sedative/narcotic overdose)
  • Can't breathe: Neuromuscular dysfunction (Guillain-Barre, Myasthenia Gravis)
  • Can't breathe enough: Pulmonary causes (COPD, asthma exacerbations)
• Treatment: Non-invasive positive pressure ventilation (NIPPV) using CPAP or BiPAP

Non-Invasive Positive Pressure Ventilation (NIPPV)

• Use in COPD/Asthma Exacerbations:
  • Breaks the vicious cycle of dynamic hyperinflation, air trapping, and increased effort
  • Stents open obstructed airways, reduces respiratory effort, decreases PCO2, and increases pH
• Data Supporting Use: Landmark study by Brochard et al. (1995), Cochrane review (2017)
• Use in Cardiogenic Pulmonary Edema:
  • Benefits from increased intrathoracic pressure: decreased preload, decreased afterload, improved forward flow
  • Pushes fluid from alveoli into interstitial space
• Contraindications: Copious secretions, altered mental status, inability to protect the airway, facial trauma, hemodynamic instability

Indications for Intubation and Mechanical Ventilation

• Head/CNS: Altered mental status, unprotected airway, facial trauma, airway edema
• Cardiac: Cardiac arrest, cardiogenic pulmonary edema (failed NIPPV)
• Pulmonary: Failure of NIPPV or heated high flow nasal cannula, ARDS, massive hemoptysis
• GI: Massive hematemesis, facilitation of procedures (e.g., EGD)
• Total Body: Shock (type 4 respiratory failure)

Ventilator Variables

Oxygenation Variables

• FiO2: Fraction of inspired oxygen
• PEEP: Positive end-expiratory pressure

Ventilation Variables

• Tidal Volume
• Respiratory Rate

Session Summary

• Defined hypoxemic and hypercapnic respiratory failure
• Differential diagnosis for each form
• Identified patients for NIPPV
• Discussed indications for intubation/mechanical ventilation
• Reviewed ventilator variables affecting oxygenation and ventilation

Thank you for your participation!