EMmedonline: How to select optimal PEEP ?

Optimal PEEP can be estimated from various techniques -

1)Pressure Volume Loop

PV Loops represent the dynamic interaction of changes in pressure and volume during the inspiratory and expiratory portions of a breath. It helps us to monitor lung compliance and airway resistance.
When lung compliance decreases PV loop rotates closer to the X-axis, lying more horizontally whereas when compliance increases, the PV loop rotates towards the Y- axis, lying more vertically.

There two important inflection point on the static PV loop.
The lower inflection point (LIP) is located on the inspiratory limb of PV loop. This represents the point at which compliance increases significantly, likely due to recruitment and opening of alveoli.
The upper inflection point (UIP) on the inspiratory limb is suggested to be a point at which compliance increases significantly, likely due to the recruitment and opening of alveoli. The decrease in compliance and overdistension of alveoli creates a classic beaking apperance to the PV loop.

PV loop can be used to set PEEP by using LIP. Matamis et al suggested setting PEEP above LIP on the inspiratory limb of the PV loop to prevent distal airway collapse and to maximise alveolar recuritment.PEEP is set at 2cm H20 above LIP.

2) Driving Pressure (DP)

DP is calculated as the difference between inspiratory plateau pressure and PEEP or ratio of TV to compliance.

In the absence of respiratory effort by the patient, DP represent the pressure above PEEP which is applied to achieve ventilation.It reflects the size of TV relative to aerated lung volume and therefore correlates with overall lung strain and pulmonary compliance.

ARDS patients with Driving pressure (DP) > 7 have been shown increased risk for mortality, recently studies have shown DP> 14cmH20 on day 1 is associated with worse outcome.
Decrease in DP have been shown to be more strongly associated with lower mortality compared to increase in the Pa02/FiO2 ratio.

3) Stress Index (SI)

SI is measured by determining the slope of the airway pressure time curve during inspiration, based on two timepoints on a dynamic airway pressure scaler.
Measurement of SI requires volume control ventilation and constant flow pattern, which keeps alveolar volume and pressure constant. Under these condition, the slope of the airway pressure rise will represent changes in compliance.

Using SI to determine optimal PEEP involves setting PEEP to a pressure at which SI =1, when it is thought that hyperinflation or recruitment is occuring.
Clinical utility of SI is limited because of the need for quantitative analysis of the shape of the pressure time curve with dedicated instruments or specific ventilators.

4) Transpulmonary Pressure

Transpulmonary pressure (Ptp) is defined as the difference between the airway pressure and pleural space pressure and it represents the pressure required to move air through airways and to overcome elastic recoil.

Pleural pressure is most commonly estimated by measuring esophgeal pressure using esophgeal manometry.
Transpulmonary pressure guided PEEP approaches have shown to improve oxygenation, increase compliance and decrease DP.

5) Electric Impedance

Electrical Impedance Tomography is a non invasive bedside technique which allows real time visualisation of changes in the distribution of ventilation and perfusion.

It involves placing several electrodes around the patients chest, which measures thoracic impedance to small alternating electrical currents that are applied through electrodes. Software analyses this data and creates a image of the lung depicting ventilation and perfusion.

Following a recruitment manuver, lung compliance is estimated for decremental PEEP. The PEEP should then be set at the point of intersection between collapse and over distension percentage curve assessed by EIT.

Reference

Zersen KM. Setting the optimal positive end-expiratory pressure: a narrative review. Front Vet Sci. 2023 Jul 19;10:1083290. doi: 10.3389/fvets.2023.1083290. PMID: 37538169; PMCID: PMC10395088.

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