PEEP, Auto-Peep, and Waterfalls
Highlights
- 2 This is termed dynamic hyperinflation, and the factors that determine its development include increased tidal volume , prolonged time constant (resistance x compliance) for emptying the respiratory system, and a shortened expiratory time.1 Such hyperinflation has a number of adverse effects:3 the respiratory muscles operate at an unfavorable position on their length-tension curve; elastic recoil of the chest wall is directed inwards, thereby causing an extra elastic load; and breathing takes place at the upper, less compliant portion of the pressure-volume curve of the lung. These factors cause a decrease in the efficiency of force generation by the respiratory muscles and an increase in the work of breathing (Page 1)
- In the presence of hyperinflation, however, to achieve a decrease in alveolar pressure below ambient pressure, a much greater decrease in pleural pressure is required; this greater decrease in pleural pressure is analogous to the further stretching of an already hyperextended spring. In this setting, if ambient pressure is elevated by the application of external PEEP, inspiration is more easily accomplished because alveolar pressure needs to be decreased only below the level of external PEEP (rather than below zero). Thus, we have the paradox whereby external PEEP, which is most commonly employed to induce hyperinflation in patients with diffuse micro-atelectasis, as in the adult respiratory distress syndrome, is being used to decrease the work of breathing induced by hyperinflation consequent to auto-PEEP! (Page 1)
- in the presence of auto-PEEP, external PEEP should not impede expiratory airflow or cause hyperinflation as long as its value is no higher than the critical closing pressure (Page 2)
- taking the waterfall perspective in patients with airflow limitation presents a certain irony: the external PEEP appears to function much like pressure-support ventilation, in that it augments inspiration but is functionally absent in expiration. (Page 2)
PEEP, Auto-Peep, and Waterfalls
Highlights
- 2 This is termed dynamic hyperinflation, and the factors that determine its development include increased tidal volume , prolonged time constant (resistance x compliance) for emptying the respiratory system, and a shortened expiratory time.1 Such hyperinflation has a number of adverse effects:3 the respiratory muscles operate at an unfavorable position on their length-tension curve; elastic recoil of the chest wall is directed inwards, thereby causing an extra elastic load; and breathing takes place at the upper, less compliant portion of the pressure-volume curve of the lung. These factors cause a decrease in the efficiency of force generation by the respiratory muscles and an increase in the work of breathing (Page 1)
- In the presence of hyperinflation, however, to achieve a decrease in alveolar pressure below ambient pressure, a much greater decrease in pleural pressure is required; this greater decrease in pleural pressure is analogous to the further stretching of an already hyperextended spring. In this setting, if ambient pressure is elevated by the application of external PEEP, inspiration is more easily accomplished because alveolar pressure needs to be decreased only below the level of external PEEP (rather than below zero). Thus, we have the paradox whereby external PEEP, which is most commonly employed to induce hyperinflation in patients with diffuse micro-atelectasis, as in the adult respiratory distress syndrome, is being used to decrease the work of breathing induced by hyperinflation consequent to auto-PEEP! (Page 1)
- in the presence of auto-PEEP, external PEEP should not impede expiratory airflow or cause hyperinflation as long as its value is no higher than the critical closing pressure (Page 2)
- taking the waterfall perspective in patients with airflow limitation presents a certain irony: the external PEEP appears to function much like pressure-support ventilation, in that it augments inspiration but is functionally absent in expiration. (Page 2)