Should PEEP Be Used in Airflow Obstruction?
Highlights
- (3) Dynamic hyperinflation with flow limitation. (Page 1)
- It is important to emphasize that there are three distinct forms of auto-PEEP: (1) Auto-PEEP without lung disten tion. (Page 1)
- (2) Dynamic hyperinflation without flow limitation (Page 1)
- When PEEP is added to the central air way of a passively ventilated patient with auto-PEEP, its effect on expiratory air flow and lung volume may depend on the presence and extent of flow limitation. (Page 1)
- raising the set level of end-expiratory pres sure might ease the inspiratory workload by improving lung mechanics or by facil itating the off-loading of inspiratory work to the expiratory side. The disten tion expected from PEEP (a function of chest compliance) will not occur if ac tive contraction of the expiratory mus cles drives the chest below the appropri ate equilibrium volume. Although expi ratory work is performed in opposing PEEP, inspiration is aided by the boost received as the expiratory muscles relax, allowing the chest to recoil outward to its equilibrium position. Such "work shar ing" represents one potential mechanism by which the application of a low level of PEEP or CPAP could make a breath less patient with (or without) moderate airflow obstruction more comfortable (7). Unfortunately, dynamic airway col lapse frustrates this work-sharing mech anism by rendering expiratory muscle contraction ineffective in speeding air flow. Yet, in those with tidal flow limita tion, the addition of PEEP might still be helpful, not only by retarding flow but also by narrowing the end-expiratory gra dient between alveolar and central air way pressure. (Page 2)
- in patients with extensive dynamic small airway collapse, the addi tion of PEEP (or CPAP) less than the original levelof auto-PEEP will increase averageend-expiratory Palv and lung vol ume by only a small amount, but narrow the Palv - Paw considerably. In such pa tients, low level PEEP can make breath ing more comfortable without causing significant distention. In others - those without extensive flow limitation during tidal breathing or those in whom end expiratory airway pressure exceeds Pem - additional PEEP will cause propor tional increases in Palv, hyperinflation, and the attendant risks of barotrauma, hemodynamic compromise, and impaired inspiratory muscle activity. (Page 2)
- Because the detrimental hemodynamic effects of PEEP are associated with lung disten tion, the addition of PEEP should not cause hemodynamic deterioration until this critical value is exceeded. (Page 2)
- The primary objective of PEEP in air flow obstruction is to improve the work load and relieve dyspnea (Page 2)
- PEEP not only sets the end-expiratory system pressure but also adds flow resistance (Page 2)
- If add ing PEEP narrows the end-expiratory difference between alveolar and central airway pressures [d(Palv - Paw)], the effective triggering sensitivity of the ven tilator improves. Note that improved sen sitivity and decreased work of breathing require that the added PEEP cause little increase in end-expiratory alveolar pres sure and lung volume; for these patients, PEEP and auto-PEEP are not algebrai cally additive. (Page 2)
- there is a subset of patients with auto-PEEP whose problems relate pri marily to flow limitation, dynamic air way compression, and dyspnea. In these, the application of low levelsof PEEP (as a rule less than the original level of auto PEEP) may relieve dyspnea by making the machine easier to trigger or the spon taneous breath easier to draw. It is also possible that some patients who have high levels ofVE and modest airflow ob struction (but no flow limitation) would benefit by work sharing. (Page 3)
Should PEEP Be Used in Airflow Obstruction?
Highlights
- (3) Dynamic hyperinflation with flow limitation. (Page 1)
- It is important to emphasize that there are three distinct forms of auto-PEEP: (1) Auto-PEEP without lung disten tion. (Page 1)
- (2) Dynamic hyperinflation without flow limitation (Page 1)
- When PEEP is added to the central air way of a passively ventilated patient with auto-PEEP, its effect on expiratory air flow and lung volume may depend on the presence and extent of flow limitation. (Page 1)
- raising the set level of end-expiratory pres sure might ease the inspiratory workload by improving lung mechanics or by facil itating the off-loading of inspiratory work to the expiratory side. The disten tion expected from PEEP (a function of chest compliance) will not occur if ac tive contraction of the expiratory mus cles drives the chest below the appropri ate equilibrium volume. Although expi ratory work is performed in opposing PEEP, inspiration is aided by the boost received as the expiratory muscles relax, allowing the chest to recoil outward to its equilibrium position. Such "work shar ing" represents one potential mechanism by which the application of a low level of PEEP or CPAP could make a breath less patient with (or without) moderate airflow obstruction more comfortable (7). Unfortunately, dynamic airway col lapse frustrates this work-sharing mech anism by rendering expiratory muscle contraction ineffective in speeding air flow. Yet, in those with tidal flow limita tion, the addition of PEEP might still be helpful, not only by retarding flow but also by narrowing the end-expiratory gra dient between alveolar and central air way pressure. (Page 2)
- in patients with extensive dynamic small airway collapse, the addi tion of PEEP (or CPAP) less than the original levelof auto-PEEP will increase averageend-expiratory Palv and lung vol ume by only a small amount, but narrow the Palv - Paw considerably. In such pa tients, low level PEEP can make breath ing more comfortable without causing significant distention. In others - those without extensive flow limitation during tidal breathing or those in whom end expiratory airway pressure exceeds Pem - additional PEEP will cause propor tional increases in Palv, hyperinflation, and the attendant risks of barotrauma, hemodynamic compromise, and impaired inspiratory muscle activity. (Page 2)
- Because the detrimental hemodynamic effects of PEEP are associated with lung disten tion, the addition of PEEP should not cause hemodynamic deterioration until this critical value is exceeded. (Page 2)
- The primary objective of PEEP in air flow obstruction is to improve the work load and relieve dyspnea (Page 2)
- PEEP not only sets the end-expiratory system pressure but also adds flow resistance (Page 2)
- If add ing PEEP narrows the end-expiratory difference between alveolar and central airway pressures [d(Palv - Paw)], the effective triggering sensitivity of the ven tilator improves. Note that improved sen sitivity and decreased work of breathing require that the added PEEP cause little increase in end-expiratory alveolar pres sure and lung volume; for these patients, PEEP and auto-PEEP are not algebrai cally additive. (Page 2)
- there is a subset of patients with auto-PEEP whose problems relate pri marily to flow limitation, dynamic air way compression, and dyspnea. In these, the application of low levelsof PEEP (as a rule less than the original level of auto PEEP) may relieve dyspnea by making the machine easier to trigger or the spon taneous breath easier to draw. It is also possible that some patients who have high levels ofVE and modest airflow ob struction (but no flow limitation) would benefit by work sharing. (Page 3)