PEEP + Auto-PEEP

Definitions

  • Extrinsic PEEP: PEEP applied by the clinician
  • Auto-PEEP (Intrinsic PEEP): PEEP which develops due to intrinsic properties of the respiratory system
  • Total PEEP = Extrinsic PEEP + Auto-PEEP
  • Dynamic Hyperinflation:

Positive End-Expiratory Pressure (PEEP)

Physiologic Effects of PEEP

  • Decreases Intrapulmonary Shunting
  • Prevents Alveolar De-Recruitment (Collapse) at End-Expiration: rather than increasing alveolar recruitment
  • May Potentially Increase Physiologic Dead Space in Some Lung Regions: by increasing V/Q ratio in those areas
  • Decreases Dynamic Airway Compression

Adverse Effects of PEEP

  • Increase in Intrathoracic Pressure: potentially decreasing venous return -> decreasing CO -> decreasing mixed venous pO2 -> decreasing pO2
  • Exacerbation of R->L Shunting Through Patent Foramen Ovale (PFO) (see Patent Foramen Ovale, [[Patent Foramen Ovale]]) [MEDLINE]

Potential Techniques to Set the Optimal Amount of PEEP

  • Maintain PEEP to Keep Tidal Ventilation Above the Lower Inflection Point on the Static Pressure-Volume Curve of the Lung [MEDLINE]
    • Note: this is the static curve, not the dynamic curve obtained during mechanical ventilation
    • This may reduce the potential for sheer forces exacerbating lung injury in ALI/ARDS
    • However, this technique is cumbersome and trials do not indicate a mortality benefit

Trials Examining Intra-Operative PEEP

  • PROVHILO Trial Examining Intra-Operative PEEP (Lancet, 2014) [MEDLINE]
    • Study: multi-center RCT in Europe/North America/South America (n = 900) comparing high level of positive end-expiratory pressure (12 cm H2O) with recruitment maneuvers (higher PEEP group) or a low level of pressure (≤2 cm H2O) without recruitment maneuvers (lower PEEP group)
    • Main Findings
      • High PEEP during open abdominal surgery does not protect against post-operative pulmonary complications
      • High PEEP group had higher rates of intraoperative hypotension and vasoactive drug requirement, as compared to low PEEP group

Auto Positive End-Expiratory Pressure (Auto-PEEP)

Etiology of Auto-PEEP

  • Patient Factors
  • Ventilator Factors
    • Large Tidal Volume (VT)
      • This may also occur during cardiopulmonary resuscitation with aggressive bag ventilation (NEJM, 1996) [MEDLINE]
    • High Inspiratory:Expiratory (I:E) Ratio
      • This may also occur during cardiopulmonary resuscitation with aggressive bag ventilation (NEJM, 1996) [MEDLINE]

Adverse Effects of Auto-PEEP

  • Barotrauma: related mainly to increased lung volumes
  • Dyspnea (see Dyspnea, [[Dyspnea]])
  • Hemodynamic Compromise
    • Hypotension (see Hypotension, [[Hypotension]]): due to elevation of pleural and right atrial pressures, resulting in decreased venous return to the right side of the heart
      • Risk is Especially High in the Setting of Dehydration
  • Increased Work of Breathing: related mainly to increased lung/chest wall volumes (lung and chest wall are less compliant at high lung volumes) and the requirement to generate a negative inspiratory force to overcome the positive auto-PEEP
  • Interference with Hemodynamic Measurements
    • Impaired Measurement of Pulmonary Capillary Wedge Pressure (PCWP)
    • Dynamic Hyperinflation Amplifies the Respiratory Variation of Arterial Pulse Pressure and Contributes to Pulsus Paradoxus in Mechanically Ventilated Patients [MEDLINE]
  • Interference with Measurement of Respiratory System Mechanics: unmeasured total PEEP will lead to an inaccurate assessment of the calculated driving pressure (with inaccurate assessment of the lung compliance)
  • Interference with Pressure-Regulated Ventilation
  • Patient-Ventilator Dyssynchrony

Management of Auto-PEEP

  • Treat Airway Obstruction
  • Treat Agitation: wtih sedation/paralysis, as required
  • Treat Acidosis: treat underlying cause, use bicarbonate, etc
  • Treat Fever (see Fever, [[Fever]]): with antipyretics
  • Synchronized Intermittent Mandatory Ventilation (SIMV) Mode (Instead of Assist Control): prevents breath stacking
  • Increase Inspiratory Flow Rates: decreases inspiratory time and increases expiratory time
  • Decrease Respiratory Rate
    • Permissive Hypercapnia: intentional hypoventilation to lower pH (usually with sedation and paralysis)
      • Contraindicated in presence of increased intracranial pressure
  • Application of Extrinsic PEEP [MEDLINE]
    • Mechanism
      • Decreases Dynamic Airway Compression
      • Counteracts the Critical Closing Pressure that Causes Small Airway Collapse in Asthma/COPD
    • Technique
      • Add PEEP to point just below where PIP and plateau pressures start to increase
      • Use extrinsic PEEP in an amount less than approximately 80% of the amount of auto-PEEP
    • Efficacy: the efficacy of applied PEEP in decreasing auto-PEEP depends on the level of PEEP used and whether flow limitation is present -> applying PEEP to lungs without flow limitation simply distends them further and is ineffective

References

  • Occult positive end-expiratory pressure in mechanically ventilated patients with airflow obstruction: The auto-PEEP effect. Am Rev Respir Dis 1982; 126:166-170 [MEDLINE]
  • Determination of auto-PEEP during spontaneous and controlled ventilation by monitoring changes in end-expiratory thoracic gas volume. Chest 1989; 96:613-616 [MEDLINE]
  • Should PEEP be used in airflow obstruction? Am Rev Respir Dis 1989; 140:1-3 [MEDLINE]
  • Auto-PEEP during CPR: an “occult” cause of electromechanical dissociation? Chest 1991;99:492–493 [MEDLINE]
  • Physiologic effects of positive end-expiratory pressure in chronic obstructive pulmonary disease during acute ventilatory failure and controlled mechanical ventilation. Am Rev Respir Dis. 1993;147:5–13 [MEDLINE]
  • Positive end-expiratory pressure increases the right to-left shunt in mechanically ventilated patients with patent foramen ovale. Ann Intern Med 1993; 119:887-894 [MEDLINE]
  • Interaction between intrinsic positive end-expiratory pressure and externally applied positive end-expiratory pressure during controlled mechanical ventilation. Crit Care Med 1993; 21:348-356 [MEDLINE]
  • Auto-PEEP and electromechanical dissociation. N Engl J Med 1996;335:674–675 [MEDLINE]
  • Pressure-volume curves and compliance in acute lung injury: evidence of recruitment above the lower inflection point. Am J Respir Crit Care Med. 1999 Apr;159(4 Pt 1):1172-8 [MEDLINE]
  • Use of pulse oximetry to recognize severity of airflow obstruction in obstructive airway disease: correlation with pulsus paradoxus. Chest 1999;115:475–481 [MEDLINE]
  • Intrinsic (or auto-) positive end-expiratory pressure during spontaneous or assisted ventilation. Intensive Care Med 2002;28:1552 [MEDLINE]
  • The National Heart, Lung, and Blood Institute ARDS Clinical Trials Network. Higher versus lower positive end-expiratory pressures in patients with the acute respiratory distress syndrome. N Engl J Med 2004;351:327-36 [MEDLINE]
  • Positive-end expiratory pressure setting in adult acute lung injury and acute respiratory distress syndrome: a randomized, controlled trial. JAMA 2008;299:646 [MEDLINE]
  • Dynamic hyperinflation and auto-positive end-expiratory pressure: lessons learned over 30 years. Am J Respir Crit Care Med. 2011;184:756–762 [MEDLINE]
  • Patient-ventilator interactions. Implications for clinical management. Am J Respir Crit Care Med. 2013;188:1058–1068 [MEDLINE]
  • High versus low positive end-expiratory pressure during general anaesthesia for open abdominal surgery (PROVHILO trial): a multicentre randomised controlled trial. Lancet. 2014 Aug 9;384(9942):495-503. doi: 10.1016/S0140-6736(14)60416-5. Epub 2014 Jun 2 [MEDLINE]
  • Asynchronies during mechanical ventilation are associated with mortality. Intensive Care Med. 2015;41(4): 633–641; published online Feb 2015 [MEDLINE]