Time-course of impairment of respiratory mechanics after cardiac surgery and cardiopulmonary bypass

Objective: Cardiopulmonary bypass (CPB) is associated with abnormalities of lung function characterized by an increase in static elastance of the respiratory system. We examined the following: a) the effects of CPB on the total inspiratory volume-pressure (V-P) relationship of the respiratory system; b) the relative contribution of the chest wall and lung to the impairment of respiratory system mechanics; and c) the time-course of such impairment. Design: Prospective, interventional study. Setting: Surgical and medical intensive care units in a teaching hospital. Patients: Eight adult patients scheduled for elective open heart surgery to correct valvular dysfunction. Interventations: V-P curves (interrupter technique) of the respiratory system were partitioned between the chest wall and lung by measurements of esophageal pressure. Measurements were obtained before sternotomy (control), immediately after, 4 hrs after, and 7 hrs after separation from CPB. Measurements and Main Results: Control V-P relationships of the respiratory system and lung showed lower inflection points (P-flex) at pressure values of 5.9 +/- 2.3 and 4.3 +/- 2.5 cm H2O, respectively. Immediately after and 4 hrs after separation from CPB, both curves had sigmoid shapes because of lower P-flex and formation of upper inflection (UIP) points. The pressures corresponding to the P-flex increased significantly (p < .001) by 56% +/- 3% and 78% +/- 4%, whereas the UIP corresponded to a pressure value of 42.34 +/- 8.5 and 35.6 +/- 7.8 cm H2O in the respiratory system and lung, respectively. A linear V-P relationship of the chest wall was observed during the control condition and after separation from CPB. Four hours later, no further increases in the pressure values corresponding to P-flex were observed on the inspiratory V-P curves of the respiratory system and lung, whereas the UIP occurred at a pressure of 35.6 +/- 9.1 and 29.7 +/- 8.4 cm H2O, respectively. A UIP was present on the V-P curve of the chest wall at a volume of 0.77 +/- 0.02 L. Seven hours after separation from CPB, the inspiratory V-P curves of the respiratory system, chest wall, and lung returned to normal. Conclusions: Sternotomy and CPB produced immediate changes in lung mechanics. Chest wall mechanics were affected only 4 hrs after sternotomy, Seven hours after disconnection from CPB, all mechanics had returned to normal.