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

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Abstract
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. Prospective, interventional study. Surgical and medical intensive care units in a teaching hospital. Eight adult patients scheduled for elective open heart surgery to correct valvular dysfunction. 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. Control V-P relationships of the respiratory system and lung showed lower inflection points (Pflex) at pressure values of 5.9 +/- 2.3 and 4.3 +/- 2.5 cm H2 O, respectively. Immediately after and 4 hrs after separation from CPB, both curves had sigmoid shapes because of lower Pflex and formation of upper inflection (UIP) points. The pressures corresponding to the Pflex 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 H2 O 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 Pflex 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 H2 O, 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. 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. (Crit Care Med 1999; 27:1454-1460)

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