Acute Respiratory Distress Syndrome (ARDS) with Refractory Hypoxemia (see Acute Respiratory Distress Syndrome, [[Acute Respiratory Distress Syndrome]])

Clinical Efficacy

• Large Randomized Controlled Trial of APRV (Acta Anaesthesiol Scand, 2004) [MEDLINE]: RCT (n = 58) comparing APRV with SIMV with PS (study was terminated early for futility)
  • No Mortality Benefit at 28 Days and 1 Year
  • No Difference in Ventilator-Free Days at 28 Days
  • However, Proning was Used in Both Arms and its Effects May Have Overshadowed the Potential Effects of APRV in this Study
• Randomized Trial of APRV in Adult Trauma Patients with Respiratory Failure (J Trauma, 2010) [MEDLINE]: n= 63
  • For Adult Trauma Patients Requiring Mechanical Ventilation >72 hrs, APRV Had a Similar Safety Profile as Low Tidal Volume Ventilation
  • Trends for APRV Patients to Have Increased Ventilator Days, ICU Length of Stay, and Ventilator-Associated Pneumonia May Be Explained by Initial Higher Acute Physiology and Chronic Health Evaluation II Scores

General Comments

• Rationale: based on open lung approach
• Concept: inverse ratio, pressure controlled, intermittent mandatory ventilation with unrestricted spontaneous breathing -> allows patient to breathe spontaneously while receiving high airway pressure with an intermittent pressure release
  • Historically, APRV Has Been Viewed as “Alternating Levels of CPAP”: this gave rise to the P high, P low, etc terminology for settings

Similarities/Differences Between Airway Pressure Release Ventilation (APRV) and Bi-Level Ventilation

• Confusion Exists in the Literature Regarding Distinction Between APRV and Bi-Level Ventilation
  • Review of 50 published studies noted that 78% of them described APRV, whle 22% described Bi-Level Ventilation (Intensive Care Med, 2008) [MEDLINE]
  • Proprietary Modes Vary by Manufacturer
    • Airway Pressure Release Ventilation (APRV): Drager Evita ventilators
    • BiLevel: Puritan-Bennett 840 ventilator (by Covidien)
    • BiPhasic: CareFusion ventilators
    • Bi-Vent: Maquet Servo-i ventilators
    • DuoPAP: Hamilton C-1 ventilator
• Similarities
  • Both modes allow unrestricted spontaneous breathing during and between mandatory breaths
• Differences
  • APRV uses extreme I:E ratios (>2:1), whle Bi-Level Ventilation usually does not
    • APRV usually keeps the duration of T low at < or = to 1.5 sec, while Bi-Level Ventilation has no restriction on T low
  • APRV supplies higher mean airway pressure, but lower minute ventilation (VE) than Bi-Level Ventilation

Advantages of Airway Pressure Release Ventilation

• Alveolar Recruitment: due to high airway pressure and diaphragmatic contraction during spontaneous breathing
• Improved Oxygenation: spontaneous breaths allow more even distribution of ventilation (decreasing shunt)
• Preservation of Spontaneous Breathing: with spontaneous breathing, APRV is better tolerated than inverse ratio ventilation (without the need for deep sedation/paralysis)
  • However, in the Absence of Spontaneous Breathing (i.e. During Paralysis), APRV is Functionally Equivalent to Inverse Ratio Ventilation (Due to the Relatively Long Times Spent at High Pressure)
• Improved Hemodynamics: spontaneous breaths augment cardiac filling
• Potential Lung-Protective Effects

Disadvantages of Airway Pressure Release Ventilation

• Risk of Volutrauma: due to spontaneous breathing during high pressure (with concomitant generation of large tidal volumes and large negative pleural pressure swings)
• Increased Work of Breathing
• Increased Energy Expenditure Related to Spontaneous Breathing

Determinants of Ventilator-Provided Tidal Volume During Airway Pressure Release Ventilation

• Lung Compliance
• Airway Resistance
• Timing and Duration of Pressure Release

Time Ratio in Airway Pressure Release Ventilation

• Time Ratios Reported in Literature: 1:1 to 9:1
• Significance of Time Ratio
  • The greater the percentage of the total time spent at high pressure (80-95%), the greater the alveolar recruitment
  • The lesser the percentage of the total time spent at low pressure (usually 0.2-0.8 sec in adults), the less alveolar de-recruitment occurs
  • If the Time Spent at Low Pressure is Too Short, Expiration Will Be Incomplete and Auto-PEEP Will Develop
    • However, Some APRV Regimens Use P Low of 0 cm H2O with the Required Development of Auto-PEEP
    • There is a Theoretical Concern About Developing Auto-PEEP, Since (Unlike Applied PEEP Which Distributes Evenly) Auto-PEEP Distributes Predominantly to Lung Units with the Highest Airway Resistance and Lowest Compliance (Chest, 1995) [MEDLINE]
      • Lung Units with Partially Obstructed Airways and Atelectatic Lung Units Will Consequently Have Higher PEEP than the Set P Low

Airway Pressure Release Ventilation Settings and Their Relationship to Gas Exchange

• Clinical Determinants of Oxygenation (pO2)
  • FIO2
  • Amount of P high
  • Time Spent at T high
• Clinical Determinants of Ventilation (pCO2)
  • Delta P (P high – P low): Larger Delta P = More Volume Per Release = More CO2 Excretion Per Release
  • Patient’s Spontaneous Breathing: spontaneous breathing typically only occurs during the P high (due to the short duration of time spent at P low)