THE EFFECT OF CHANGING END-EXPIRATORY PRESSURE ON RESPIRATORY SYSTEM MECHANICS IN OPEN-CHEST AND CLOSED-CHEST ANESTHETIZED, PARALYZED PATIENTS

The decrease in functional residual capacity (FRC) with anesthesia may cause lung volume to decrease below closing volume, thereby impairing oxygenation. Increasing end-expiratory pressure (EEP) reexpands atele ctatic areas in anesthetized, ventilated patients, but its effect on p ulmonary mechanics is less well understood. We studied the effect of v arying EEP on the mechanical behavior of the respiratory system in pat ients undergoing either closed (Group 1) or open-chest (Group 2) surgi cal procedures. We measured airway opening pressure (Pao), flow (ir), and esophageal pressure (Pes) (in Group 1 only) at EEPs of 0, 2.5, 5, and 10 cm H2O. Dynamic elastance (E) and resistance (R) for the respir atory system (RS), the lung (L), and the chest wall (CW) were estimate d by fitting the equation P = RV + EV + K to the measured data by mult iple linear regression where P was either Pao, Pes, or Pao-Pes. Group 1 E(L) decreased with increases in EEP to 5 cm H2O and then began to i ncrease with EEP above this level. The same occurred in Group 2 before opening the chest. After opening the chest in Group 2, E(L) increased as EEP increased at all values above 0 cm H2O. The magnitudes of R(RS ) and R(L) were similar in both groups of subjects and in each group t hese quantities decreased with increases in EEP. Dynamic E(L) responde d differently to changes in EEP in subjects with open-chest and closed -chest procedures. We attribute this difference to overdistension of t he remaining ventilable lung tissue at all levels of EEP in open-chest patients.