MECHANICS OF BREATHING AND PEEP

      
   

 

      
     

What is a recruitment maneuver?

  • ·Recruitment maneuvers are used to reinflate collapsed alveoli, a sustained pressure above the tidal ventilation range is applied, and PEEP is used to prevent derecruitment.

There is strong evidence to support the hypothesis that PEEP has  a protective effect in acute lung injury, and probably in the prevention of ventilator induced lung injury. The reasons proposed for this effect are: 1. Curtailment of quantitative and qualitative surfactant depletion 2. Reduced shearing stresses within junctional tissues, reduced parenchymal injury and cytokine release. We also know that to optimize lung compliance with modern low stretch (low tidal volume) ventilation strategies, the lung needs to be recruited and compliant.
PEEP only prevents derecruitment. In the past when large tidal volume were used to ventilate patients,, most lung units were probably opened the repetitive high pressure expansion. Many “sticky” lung units will not reopen in the normal tidal volume and protective pressure range.
If you have ever witnessed re-expansion of the lung (or performed it) during thoracic surgery, you will have noticed that the most effective way of  performing this maneuver is to apply long, infrequent sustained inflations (rapid “bagging” with large tidal volumes is quite ineffective): the lung segments re-emerge slowly. This method should be applied to lung recruitment in intensive care. Every time a patient is disconnected from a ventilator, the lungs derecruit – whole segments of the lung will collapse, and will not inflate with re-instatement of previous ventilator settings. A recruitment maneuver is required.

In the absence of PEEP many alveoli expand and collapse during the respiratory cycle. Others, "sticky" alveoli do not participate in gas exchange.

Using PEEP, the alveoli that open in inspiration remain recruited. However some "sticky" alveoli remain collapsed.

Application of sustained pressure to the lung pops open the residual alveoli, and PEEP maintains recruitment

Recruitment Maneuvers

Anesthesiologists routinely perform such maneuvers in the operating room, albeit in a slightly uncontrolled manner (a tidal volume is delivered through a manual breathing system and held, the airway pressure is not measured). There are many methods of performing sustained inflation maneuvers in intensive care while being in complete control of the airway pressure.
Two factors influence whether or not recruitment maneuvers are successful: the pressure applied must be in excess of the current plateau pressure, and the pressure must be sustained, in order to inflate lung units with long time constants. The most effective method of doing this is to apply CPAP to the airway. Amato (1998) applied CPAP at 10cmH2O above the plateau pressure for 30 to 60 seconds to restore alveolar recruitment. Following the period of sustained inflation, the ventilation settings are returned to previous levels. It is not necessary to increase the PEEP, as the lung volumes from the same amount of PEEP as before should be higher. A successful procedure will result in improved oxygenation, reduced end-tidal CO2 and improved compliance.

On occasion, it may be necessary to increase the CPAP further to guarantee recruitment – up to 20cmH2O above the pressure limit. Indeed sometimes very high pressures are required to recruit collapsed lung tissue: Medoff (2000) and colleagues have reported a case where 40cmH2O of PEEP and 20cmH2O of pressure control was required to reinflate the lungs of a young woman with Streptococcal sepsis. Interestingly, the PEEP required to prevent derecruitment in this patient was significantly higher than that predicted by Pflex.

Undoubtedly recruitment maneuvers will become an integral part of the management of ARDS (using low stretch protocols). However the ideal level of PEEP required to prevent derecruitment remains to be established.

The following is a suggested batting order for recruitment maneuvers, adapted from Kacmarek and Schwartz (2000):

Performance of Recruitment Maneuvers:

1.      Ensure hemodynamic stability.

2.      Set the FiO2 to 1.0

3.      Wait 10 minutes.

4.      Recruit with 30* cmH2O CPAP for 30-40 seconds (* or 10 cmH2O above the plateau pressure level).

5.      If unresponsive, wait 15 minutes and then recruit with 35 cmH2O  CPAP for 30 to 40 seconds.

6.      If unresponsive, wait 15 minutes and then recruit with 40 cmH2O  CPAP for 30 to 40 seconds.

7.      If unresponsive, wait 15 minutes and then recruit with 20 cmH2O  pressure control and  40 cmH2O CPAP for 2 minutes.

8.      If the patient remains unresponsive, consider prone positioning.

 

        
   

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