Since the inception of ARDS Network and the landmark study (ARMA) of low tidal volume ventilation in ARDS, more emphasis has been to quantify the mechanical forces (stress) acting over the lungs causing lung expansion (strain) during mechanical ventilation. The objective of protective lung strategy is to avoid end-expiratory alveolar collapse and atelectrauma and avoid end-inspirstory alveolar distension and volutrauma. Driving pressure and transpulmonary pressure have been more studied recently as targets in protective lung ventilation in addition to low tidal volume, appropriate PEEP, and limited plateau pressure.
The purpose of this blog is not to review the scientific evidence of these targets but to present a practical guidance on how we apply mechanical ventilation to achieve these targets. The above graph shows a patient with ARDS and a single pressure/volume curve (PV Tool) representing the required pressure for the set tidal volume starting from PEEP of 5 cm H2O. The slop of the curve is the compliance of the respiratory system (volume/pressure). The lower inflection point (LIP) is determined when the slop of the inspiratory limb is changed representing alveolar recruitment. The upper inflection point (UIP) is where the slop of the curve is changed again at higher pressure and represents the point where higher pressure may cause alveolar distension and possible barotrauma.
The first target is to determine the appropriate PEEP at which alveolar recruitment is achieved. Traditionally, PEEP level is determined based on the ARDSnet PEEP/FiO2 table with two different set of values for different severity of ARDS. This approach fails to account for the physiological parameters of the respiratory system and the variations among patients. Using the lower inflection point determined on the PV-curve is a better estimate of the appropriate PEEP when it is set at 2 cm H2O higher than the LIP. The reason why we go higher than the value of the LIP is to open the alveoli in most dorsal region of the lungs. These alveoli have a higher LIP compared to the global LIP. I have explained this in a previous blog that you can review here. The most favorite way for me to determine the appropriate PEEP is to use esophageal manometry to measure intrathoracic pressure and keep the end-expiratory transpulmonary pressure (PL EE) at 0-10 cm H2O. This method is preferred in morbidly obese patients and in patients with increased intra-abdominal pressure.
The second target is both the tidal volume not to exceed 6 mL/kg of Ideal Body Weight (IBW) and the plateau pressure not to exceed 30 cm H2O. In volume control mode of ventilation, the tidal volume is set at 6 mL/kg of IBW and decreased gradually (down to 4 mL/kg) to maintain a target plateau pressure of 30 cm H2O or less (preferred less than 28 cm H2O). In pressure control mode of the ventilation, the pressure control level is adjusted to achieve the target tidal volume of 4-6 mL/kg of IBW. The PV curve can be used to set a limit on the plateau pressure 2 cm H2O below the upper inflection point. If an esophageal pressure monitor is inserted, then the volume and the plateau pressure can be adjusted to achieve an end-inspiratory transpulmonary pressure (PL EI) of less than 15-20 cm H2O.
Once the PEEP, plateau pressure, and tidal volume are set at target, we need to ensure that the resultant driving pressure (∆P: plateau pressure-PEEP) is at a target of ≤15-18 cm H2O. This is the most important force as it is the direct stress on the lung of the applied pressure that causes strain on the alveoli. If an esophageal monitor is in place, the difference between end-inspiratory transpulmonary pressure and the end-expiratory transpulmonary pressure (∆PL: PL EI - PL EE) represents the true driving pressure as it eliminates the force needed to expand the chest wall. The volume or the pressure can be adjusted to target ∆PL of less than 12 cm H2O.