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
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)
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]
