MECHANICS OF BREATHING AND PEEP

     
   

 

     
     

How do you set the ideal level of PEEP?

  • ·The ideal level of PEEP is that which prevents derecruitment of the majority of alveoli, while causing minimal overdistension.

The ideal level of PEEP is that which puts the majority of lung units on the favorable part of the pressure-volume curve (remember each lung unit has a different curve), maximizes gas exchange and minimizes over-distention. This is easier said than done. In patients who are on mechanical ventilators one can adjust the PEEP and look for the biggest tidal volume that will arise using the same peak airway pressure. Alternatively, one can plot a pressure volume curve (see figure 4), although this is very hard work: usually the level of PEEP chosen is a “guesstimate”. Some authorities suggest that you should start with a high level of PEEP (20cmH2O for example), reduce the FiO2 to 0.45 or less, and progressively reduce the PEEP every 20mins or so until the tidal volumes fall or the patient begins to desaturate. At this point a recruitment maneuver should be performed (see below) and the PEEP set 2-5 cmH2O above this level.

 Volume-Pressure relationship in idealized lung. The optimal volume-pressure relationship is between the lower inflection point (Pflex) and the upper inflection point (Pmax).

 

Many authors have advocated the construction of quasi-static pressure volume curves with a large air filled “super-syringe” in order to plot volume pressure curves for the lung. The objective is to determine the critical opening pressure for the majority of alveoli – the lower inflection point (Pflex) of the pressure volume curve. At this point, the majority of heretofore collapsed alveoli, open and lung becomes more compliant. Likewise, there is a point where the pressure volume curve flattens – the Pmax – the upper inflection point, there the lungs lose their compliance. Amato (1998) and colleagues have suggested that the optimal PEEP one or two cmH2O above the lower inflection point, as this provides a springboard for lung inflation. Moreover, it has been postulated that tidal ventilation across Pflex is possible harmful, as the phasic opening and closing of injured lung units is thought to contribute to ventilator induced lung injury.

Figure1

Undoubtedly, the combination of PEEP and low tidal volumes prevents volutrauma, but the exact amount of PEEP applied is very controversial. The reason for this is hysteresis – the tendency of the lungs, due to surfactant, to exist at higher volumes in expiration than in inspiration. Figure 1 demonstrates a tidal volume plus inspiratory reserve volume breath for a normal lung. As you can see, the breath begins and ends at FRC, but the volumes in the lungs differ considerably in expiration as compared to inspiration.

Figure 2

Figure 2 describes the situation in an injured lung: the lung becomes more compliant in inspiration at the point Pflex, and is inflated to Pmax. As you can see, the lung volumes are higher in expiration than in inspiration. Thus, if the lung is inflated an PEEP is applied at the same point as Pflex, then, as the lung deflates to this point, it arrives at a higher volume in expiration than in inspiration.

Figure 3

The next breath (figure 3), which starts at the new FRC, is at a higher volume than expected,    and the lung is less compliant at this stage. This is not necessarily a bad thing, as the objective is to prevent derecruitment. Nevertheless, it appears that Pflex overestimates the PEEP required to prevent alveolar derecruitment.

       
   

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