MECHANICS OF BREATHING AND PEEP

Key Points

1.      The resting point of outward chest spring and inward lung collapse is the Functional Residual Capacity (FRC): this is a reservoir for gas exchange .The FRC is the lung’s physiologic reserve, it is a reservoir.

2.      Loss of chest wall or lung compliance causes reduced FRC.

3.      Exhalation below FRC is active causing dynamic airway collapse, trapping air in the alveoli (auto PEEP)

4.      At residual volume it is not possible to empty alveoli of air further, due to dynamic airway collapse (airway closure).

5.      The closing volume (CV) is the point at which dynamic compression of the airways begins.

6.      Such airway closure occurs normally within FRC, and it is known as the closing volume (CV). With age and disease the CV moves into the tidal breathing range.

7.      The CV increases with age, smoking, lung disease, and body position (supine > erect).

8.      Airway collapse increases the work of breathing and leads to ventilation-perfusion mismatch

9.      In mechanically ventilated patients airway collapse is prevented by applying positive pressure to the airway throughout the respiratory cycle – CPAP/PEEP

10.  PEEP/CPAP works by increasing FRC, maintaining alveolar recruitment facilitating gas exchange (and removal of CO2 and replenishment of O2), and reducing the workload of breathing.

11.  The patient requires sufficient PEEP to prevent alveolar derecruitment, but not so much PEEP that alveolar overdistension, dead space ventilation and hypotension occurs.

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

13.  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.

14.  Auto-PEEP is gas trapped in alveoli at end expiration, due to inadequate time for expiration, bronchoconstriction or mucus plugging. It increased the work of breathing.

15.  The increased work of breathing associated with auto-PEEP can be offloaded by applying CPAP to the trachea/mouth, and splinting open the connecting airways. The objective is to set the CPAP level above the auto-PEEP level.

 References

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