Dynamic Hyperinflation - Complete Notes for MD Examination (10 Marks)

1. Definition

• It is synonymous with intrinsic PEEP (iPEEP / auto-PEEP)
• Auto-PEEP = alveolar pressure minus airway-opening pressure at end-expiration
• DH = the volume manifestation; auto-PEEP = the pressure manifestation of the same phenomenon

"Dynamic hyperinflation is an increase in end-expiratory lung volume above the value that would be obtained if there was complete exhalation to the static functional residual capacity." - Goldman-Cecil Medicine

2. Pathophysiology / Mechanism

2a. Core Mechanism

1. Expiratory flow limitation (EFL) is present - expiratory flow does not increase despite increasing alveolar-to-atmospheric pressure gradient (as in COPD/asthma)
2. Expiratory time is insufficient - the next inhalation begins before complete exhalation
3. Each successive breath adds a "residual volume increment" - leading to a progressively rising EELV

2b. Determinants of Auto-PEEP / DH (4 key factors):

2c. PEEP₁ (intrinsic PEEP)

• The resulting positive end-expiratory alveolar pressure is called PEEP₁ or auto-PEEP
• It acts as an inspiratory threshold load - the patient must first overcome PEEP₁ before generating any inspiratory flow
• The ventilator does not "see" this pressure because the pressure drop is upstream of where airway pressure is measured in the circuit

3. Causes / Clinical Settings

3a. Obstructive Airway Diseases (commonest)

• COPD (emphysema + chronic bronchitis) - loss of elastic recoil + airflow obstruction
• Acute severe asthma (status asthmaticus) - bronchoconstriction + air-trapping
• Bronchiectasis - secretion-related obstruction

3b. Mechanical Ventilation Settings

• High respiratory rate (RR > 20/min) - shortened expiratory time
• Large tidal volumes
• Short I:E ratio (insufficient expiratory phase)
• High minute ventilation

3c. Other

• Tachypnea from any cause (pain, anxiety, hypoxemia) - even without intrinsic lung disease, a very high respiratory rate can produce DH
• Post-exercise in COPD patients

4. Consequences / Physiological Effects

4a. Respiratory Effects

4b. Cardiovascular Effects

• Elevated intrathoracic pressure raises right atrial pressure
• Decreased venous return - reduced driving pressure for systemic venous return
• Right ventricular strain - increased RV afterload
• Hypotension - especially immediately after intubation/sedation when compensatory mechanisms are blunted

4c. Ventilator-Specific Effects

• Ineffective triggering - patient must generate effort to overcome auto-PEEP before the ventilator detects the inspiratory effort; "failed efforts" occur in >10% of breathing attempts in ~25% of patients on pressure support
• Underestimation of respiratory rate - the ventilator's counted rate is lower than the patient's actual rate
• Falsely low compliance measurements - if auto-PEEP is not factored in

5. Measurement / Detection

5a. Clinical Signs

• Flow at end-expiration remains detectable on the flow-time waveform (flow does not return to zero)
• End-expiratory flow velocity > 0 on the ventilator screen
• Patient making inspiratory efforts that fail to trigger the ventilator (ineffective efforts)
• Tachypnea with pursed-lip breathing

5b. Bedside Measurement (Mechanically Ventilated Patient)

• In a patient with no spontaneous breathing effort, occlude the expiratory port at end-expiration
• Alveolar, central airway, and ventilator circuit pressures equilibrate
• Auto-PEEP = measured pressure - set PEEP (read on the ventilator manometer)

• Perform a prolonged expiration (20-30 seconds) to atmosphere
• Volume of gas released = volume of dynamic hyperinflation

5c. Spirometric/Exercise Surrogate

• Inspiratory capacity (IC) progressively falls as EELV rises during exercise
• IC is the most practical marker of dynamic hyperinflation during exercise testing
• IC = TLC - EELV (if TLC is constant, ↓IC = ↑EELV)

6. Clinical Implications in COPD

• Progressive exercise limitation in COPD is driven largely by DH
• DH raises WOB and exacerbates dyspnea during exertion
• Anxiety episodes can cause tachypnea → DH → more dyspnea → a vicious cycle
• Interventions reducing DH improve exercise capacity:
  • Inhaled bronchodilators (LABA/LAMA) - reduce airway resistance
  • Oxygen supplementation - reduces minute ventilation demand
  • Pursed-lip breathing - slows respiratory rate, prolongs expiratory phase
  • Lung volume reduction surgery (LVRS) - improves elastic recoil and reduces DH
  • Pulmonary rehabilitation

7. Management

7a. Non-Ventilated Patient (COPD/Asthma)

• Bronchodilators (SABA + SAMA nebulization) - reduce airway resistance and time constant
• Systemic corticosteroids - reduce airway inflammation and secretions
• Heliox - low-density gas mixture, reduces turbulent flow, improves expiratory flow
• Pursed-lip breathing - self-PEEP stenting of airways, slows RR, prolongs expiration
• NIV/BiPAP - external PEEP offsets auto-PEEP, reduces inspiratory threshold load
• Positioning - upright reduces abdominal load on diaphragm

7b. Mechanically Ventilated Patient

• Mode: Volume assist-control
• RR: 10-14/min
• TV: ≤ 8 mL/kg predicted BW
• Inspiratory time: 0.8-1.0 seconds
• PEEP: 5 cmH₂O
• SpO₂ target: 88-92%

8. Auto-PEEP and Triggering Failure (High-Yield for Exams)

• If auto-PEEP = 10 cmH₂O and trigger sensitivity is set at -2 cmH₂O, the patient must generate -12 cmH₂O effort to trigger the ventilator
• Failed triggers = patient effort does not produce a breath; seen on flow-time waveform as small deflections without delivered breaths
• Solution: apply external PEEP ≈ 75-85% of auto-PEEP to reduce the threshold the patient must overcome

9. Summary - High-Yield Points for Exam

• Goldman-Cecil Medicine International Edition (2 vol.), p. 1064-1065
• Murray & Nadel's Textbook of Respiratory Medicine (2 vol.) - Expiratory Flow Limitation chapter, p. 716
• Fishman's Pulmonary Diseases and Disorders (2 vol.) - Natural History of COPD chapter
• Murray & Nadel's - Oxygen Cost of Breathing chapter
• Rosen's Emergency Medicine - Respiratory Failure & Invasive Ventilation chapter, p. 948