Can high intrinsic Positive End-Expiratory Pressure (PEEP) increase respiratory inspiratory resistance (R inspir) on ventilators (vents)?

Impact of Intrinsic PEEP on Inspiratory Resistance During Mechanical Ventilation

Yes, high intrinsic PEEP can increase inspiratory resistance (R insp) on mechanical ventilators by creating an inspiratory threshold load that must be overcome before airflow can begin.

Mechanism of Intrinsic PEEP and Its Effect on Inspiratory Resistance

Intrinsic PEEP (PEEPi or auto-PEEP) develops when:

• End-expiratory alveolar pressure remains positive despite no external PEEP being applied
• Insufficient expiratory time prevents complete lung emptying before the next breath
• Air trapping occurs due to expiratory flow limitation

Physiological Basis

• PEEPi creates a pressure threshold that must be fully counterbalanced by inspiratory muscles before ventilator triggering can occur 1
• This threshold functions as an additional inspiratory load, effectively increasing inspiratory resistance
• The equation that describes this relationship is:

  Pappl = PEEPi + V̇/C + Rt + (k1V̇ + k2V²)

  where Pappl is applied pressure, PEEPi is intrinsic PEEP, V̇/C represents elastic load, Rt is resistance, and the final terms represent additional resistive components 1

Clinical Implications

Patient-Ventilator Asynchrony

• High PEEPi increases the effort needed to trigger the ventilator beyond the set sensitivity (typically 1-2 cmH2O) 1
• This can lead to:
  • Ineffective triggering attempts
  • Wasted respiratory efforts
  • Increased work of breathing
  • Patient-ventilator asynchrony 2

Respiratory Muscle Function

• Increased inspiratory resistance due to PEEPi can:
  • Impair inspiratory muscle function and coordination 1
  • Contribute to respiratory muscle dysfunction 1
  • Increase energy consumption of respiratory muscles 1

Measurement of Intrinsic PEEP

Two validated methods to measure PEEPi:

1. End-expiratory airway occlusion technique:

   • Performed manually at the expiratory port during the last 0.5 seconds of expiration
   • Can be automated in some ventilators with end-expiratory occlusion hold options 1, 3
   • Measures static PEEPi

2. Simultaneous recording of flow and esophageal pressure (Ppl):

   • Measures dynamic PEEPi as the decrease in Ppl preceding inspiratory flow
   • Requires esophageal balloon placement
   • More accurate in spontaneously breathing patients 1, 3

Management Strategies

To reduce the inspiratory resistance caused by intrinsic PEEP:

1. Apply external PEEP:

   • Set external PEEP at approximately 80-85% of measured intrinsic PEEP 3
   • This counterbalances the inspiratory threshold load without increasing hyperinflation
   • Studies show this reduces work of breathing and improves patient-ventilator synchrony 4, 5

2. Optimize ventilator settings:

   • Reduce respiratory rate to allow longer expiratory time
   • Decrease tidal volume (6-8 mL/kg ideal body weight)
   • Consider permissive hypercapnia if clinically appropriate 3

3. Optimize bronchodilator therapy in patients with obstructive diseases 3

Common Pitfalls

• Setting external PEEP too high can worsen hyperinflation and hemodynamics
• Failing to recognize intrinsic PEEP can lead to increased work of breathing
• Ignoring expiratory muscle activity can confound PEEPi measurements
• Overlooking cardiac effects of high PEEPi (can decrease cardiac output) 1, 3
• Using automatic calculations without verification (automated measurements can be unreliable) 1

Clinical Evidence

Studies have demonstrated that in patients with obstructive airway disease:

• PEEPi acts as an inspiratory threshold load that increases inspiratory resistance 5
• Application of external PEEP reduces this imposed triggering load 5, 6
• This reduction in inspiratory resistance decreases patient effort without changing tidal volume 5
• Proper PEEP application can reduce the metabolic work of the diaphragm by up to 59% 6

In summary, intrinsic PEEP significantly increases inspiratory resistance by creating a threshold load that must be overcome before inspiratory flow can begin, and appropriate management strategies can effectively reduce this additional work of breathing.