The Effect of External PEEP on Work of Breathing in Patients with Auto-PEEP

Chin, Jae Yong; Lim, Chae Man; Koh, Younsuck; Park, Pyung Whan; Choi, Jong Moo; Lee, Sang Do; Kim, Woo Sung; Kim, Dong Soon; Kim, Won Dong

Abstract

BACKGROUND
Auto-PEEP which develops when expiratory lung emptying is not finished until the beginning of next inspiration is frequently found in patients on mechanical ventilation. Its presence imposes increased risk of barotrauma and hypotension, as well as increased work of breathing (WOB) by adding inspiratory threshold load and/or adversely affecting to inspiratory trigger sensitivity. The aim of this study is to evaluate the relationship of auto-PEEP with WOB and to evaluate the effect of PEEP applied by ventilator (external PEEP) on WOB in patients with auto-PEEP. METHOD: 15 patients, who required mechanical ventilation for management of acute respiratory failure, were studied. First, the differences in WOB and other indices of respiratory mechanics were examined between 7 patients with auto-PEEP and 8 patients without auto-PEEP. Then, we applied the 3 cm H2O of external PEEP to patients with auto-PEEP and evaluated its effects on lung mechanics as well as WOB. Indices of respiratory mechanics including tidal volume (V(T)), repiratory rate, minute ventilation (V(E)), peak inspiratory flow rate (PIFR), peak expiratory flow rate (PEFR), peak inspiratory pressure (PIP), T(I)/T(TOT), auto-PEEP, dynamic compliance of lung (Cdyn), expiratory airway resistance (RAWe), mean airway resistance (RAWm), P(0.1), work of breathing performed by patient (WOB), and pressure-time product (PTP) were obtained by CP-100 Pulmonary Monitor (Bicore,USA). The values were expressed as meanSEM (standard error of mean).
RESULTS
1) Comparison of WOB and other indices of respiratory mechanics in patients with and without auto-PEEP: There was significant increase in WOB (1.71 +/-0.24 vs 0.500.19 J/L, p=0.007), PTP (317+/-70 vs 98+/-36 cm H2O * sec/min, p=0.023), RAWe (35.6+/-5.7 vs 18.2+/-2.3 cm H20/L/sec, p=0.023), RAWm (28.8+/-2.5 vs 11.9+/-2.0 cm H2O/L/sec, p=0.001) and P0.1 (6.21.0 vs 2.9+0.6 cm H2O, p=0.021) in patients with auto-PEEP compared to patients without auto-PEEP. The differences of other indices including V(T), PEFR, V(E) and T(I)/T(TOT) showed no significance. 2) Effect of 3 cm H2O external PEEP on respiratory mechanics in patients with auto-PEEP: When 3 cm H2O of external PEEP was applied, there were significant decrease in WOB (1.71+/-0.24 vs 1.20+/-0.21 J/L, p=0.021) and PTP (317+/-70 vs 231+/-55 cm H2O * sec/min, p=0.038). RAWm showed a tendency to decrease (28.8+/-2.5 vs 23.9+/-2.1 cm H2O, p=0.051). But PIP was increased with application of 3 cm H2O of external PEEP (16+/-2 vs 22+/-3 cm H2O, p=0.008). V(T), V(E), PEFR, T(I)/T(TOT) and Cdyn did not change significantly.
CONCLUSION
The presence of auto-PEEP in mechanically ventilated patients was accompanied with increased WOB performed by patient, and this WOB was decreased by 3 cm H2O of externally applied PEEP. But, with 3 cm H2O of external PEEP, increased PIP was noted, implying the importance of close monitoring of the airway pressure during application of external PEEP.