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Positive End

2023-08-19 11:12| 来源: 网络整理| 查看: 265

OVERVIEWEXTRINSIC, INTRINSIC, AND TOTAL PEEPADVANTAGES OF PEEPDISADVANTAGES OF PEEPVARIATION IN EFFECTS OF PEEPDETERMINING OPTIMAL PEEPEVIDENCEAN APPROACHFIGURESREFERENCES / LINKS OVERVIEW

Positive End-Expiratory Pressure (PEEP) is the maintenance of positive pressure (above atmospheric) at the airway opening at the end of expiration. PEEP acts to distend distal alveoli, assuming there is no airway obstruction.

PEEP is routinely used in mechanical ventilation to prevent collapse of distal alveoli, and to promote recruitment of collapsed alveoliHow to optimise PEEP is a controversial topic, but should involve (1) optimisation of oxygenation and (2) minimisation of ventilator induced lung injury (VILI), and should should be individualised for a given patient“High” PEEP is used as part of an Open Lung Approach To Ventilation for Acute Respiratory Distress Syndrome (ARDS)In spontaneous ventilation using non-invasive ventilation (NIV), CPAP (continuous positive airway pressure) is analogous to PEEP, but the pressure applied is maintained throughout the respiratory cycle (during both inspiration and expiration). EXTRINSIC, INTRINSIC, AND TOTAL PEEP

Extrinsic PEEP (PEEPe) is applied by placing resistance in the expiratory limb of a ventilator circuit

a threshold resistor is preferred, as resistance to flow is minimal once threshold pressure is reacheda solenoid valve is commonly used in ventilators

Intrinsic PEEP (PEEPi) or autoPEEP

elevation in the static recoil pressure of the lungs above the set PEEPe at end expirationdue to insufficient expiratory time (Te), typically in the presence of severe air-flow obstruction (e.g. bronchospasm in asthma)less likely to be uniformly distributed than PEEPealso termed “occult PEEP” by as PEEPi is not apparent on proximal airway pressure recordings (Pepe & Marini, Marino, 2013)see Intrinsic PEEP (PEEPi)

Total PEEP (PEEPtot)

PEEPtot = PEEPi + PEEPe ADVANTAGES OF PEEP

Advantages of PEEP include:

increased airway pressure (improves oxygenation and alveolar recruitment)increased functional residual capacity (FRC) (prevention of airway and alveolar collapse)increase arterial oxygen tension (PaO2)increased capillary-alveoli interface for gas exchangeextra-vascular lung water (EVLW) may be displaced from alveolar interstitium to peribronchial interstitiumminimises denitrogenation atelectasis and oxygen toxicity (by allowing lower FiO2)maximises recruitment of alveoliprevents cyclic de-recruitment on expiration (decreases atelectrauma and VILI) decreases biotrauma from alveolar collapse (e.g. release of inflammatory mediators)improved lung compliance (inspiration begins on a steeper portion of the volume-pressure curve)decreased airway resistancedecreased ventilation/perfusion (V/Q) mismatch and shunt fractionimproved distribution of inspired gasdecreased work of breathing (less effort to trigger inspiration in spontaneous ventilation modes as alveolar pressure only needs to decrease to the level of PEEP for inspiration to occur)prevention of surfactant aggregation reducing alveolar collapsedecreased inflammatory response to mechanical ventilationdecrease in left ventricular (LV) afterload (due to decreased LV transmural pressure)decreased preload and work of breathing also help in acute pulmonary oedema DISADVANTAGES OF PEEP

Disadvantages of PEEP include:

impaired carbon dioxide (CO2) eliminationif decreased tidal volume due to lower driving pressure or poor compliance due to alveolar over-distentionoverinflation of non-dependent alveoli (e.g. in focal ARDS, less likely if Pplat Pa >Pv)exacerbation of right-to-left intracardiac shunt (if present)due to increased PVRdecreased hepatic artery and portal venous flow (due to increased intrathoracic pressure)causes liver congestion and LFT changesincreased intracranial pressure (ICP) (due to increased intrathoracic pressure)likely insignificant < PEEP 15 cmH20 (Huynh et al, 2002; Boone et al, 2017)decreased peribronchial lymphatic flowtheoretically may decrease the clearance of pulmonary oedema and pneumoniadecreased splanchnic blood flowreduced urine output due to:increased antidiuretic hormone (ADH)increased atrial natriuretic peptide (ANP)decreased glomerular filtration rate (GFR) VARIATION IN EFFECTS OF PEEP

The effects of PEEP vary with different disease states

e.g. patients with “recruitable” lung are more likely to be “PEEP responders” and have improved oxygenation and/or alveolar stability with an increase in PEEPe.g. PEEP will cause less increase in intrathoracic pressure if lungs are poorly compliant (e.g. severe ARDS)e.g. regional overdistention is more likely with focal lung diseases (e.g. lobar pneumonia)

Not all ARDS patients are “PEEP responsive” and the degree of lung recruitability in “responders” is variable (Gattinoni et al, 2020)

only ~50% of all ARDS patients respond to higher airway pressure by decreasing the percentage of non-aerated lung tissue DETERMINING OPTIMAL PEEP

Optimal PEEP level ultimately represents a balance between regional areas of overstretching and regional derecruitment to prevent VILI, while achieving optimal oxygenation and minimsing harmful effects such as haemodynamic compromise (Schmidt, 2012; Miller et al, 2012).

no agreed upon ideal method of determinationoptimal PEEP may:change over time in an individual patient (e.g. disease progression, positioning) be independent of oxygenation (e.g. due to hyperinflation injury) as increased oxygenation does not correlate with improved alveolar stability (Nieman et al, 2019; Schmidt, 2012)be independent of respiratory mechanicsCertain patients groups may have different PEEP requirements e.g. in obesity, patients typically need higher PEEP than non-obese patients to maintain alveolar recruitmente.g. in asthma, can use either ZEEP (PEEP = 0) or 2/3 of measured auto-PEEP (no high-level evidence for either, approach however PEEP is preferred in spontaneous ventilation modes to reduce work of breathing)

There are a number of methods suggested to determine the optimum PEEP setting, all of which have pros and cons:

adjust according to a sliding scale of FiO2 requirements (e.g. as per ARDSNet Ventilation Strategy for protective lung ventilation)a simple, pragmatic approach that focus on optimising oxygenation and PlatPEEP settings at low FiO2 typically are lower than those suggested by PEEP optimisation strategies based on lung recruitment (e.g. Open Lung Approach To Ventilation).perform recruitment manoeuvre (RM) (see Recruitment manoeuvres in ARDS) then adjust to either optimal SpO2, static compliance, or other parameter listed below.Staircase recruitment manoeuvres are generally cautioned against due to increased mortality seen in the intervention arm of the ART trial (ART trial investigators, 2017)set PEEP according to pressure-volume loop analysis. Various approaches have been proposed including setting PEEP based on:inflation limb lower inflection point (Pflex),deflation limb upper Pflex,maximal compliance,true inflection point of the deflation limb, the degree of hysteresis, orstress index (a parameter derived from the PV loop, a straight line (0.95 > SI

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