Effects of open heart surgery on respiratory mechanics

Abstract

Objective: To investigate the changes in respiratory mechanics in patients undergoing cardiac surgery before and after the operation. Design: Prospective physiological study. Setting: Operating theatre of the Institute of Cardiac Surgery, Verona, Italy. Patients: 8 patients needing heart surgery because of a coronary bypass or mitral valve replacement. Measurements and results: We measured respiratory mechanics before and immediately after the surgical procedure with two techniques: (1) the rapid airway occlusion technique during constant flow inflation at different lung volumes, and (2) the negative expiratory pressure (NEP) technique. We found that static and dynamic elastance (15.3 ± 3.3 and 19.0 ± 5.5 cmH2O/l, respectively) and respiratory resistance, both airway and total flow resistance (5.8 ± 2.5 and 10.3 ± 4 cmH2O·l-1·s, respectively) before surgery were slightly higher than in normal anaesthetised subjects. In all patients, the static inflation V-P curves fitted the power function and exhibited a slight upward concavity towards the volume axis (a = 16.9 ± 3.5, b = 0.74 ± 0.07), indicating that elastance decreased with inflating volume. Whereas elastance increased by 30%, neither intrinsic positive end-expiratory pressure, which was small, averaging 1.5 ± 1.2 cmH2O, nor flow resistance changed after surgery. With the NEP technique, four patients exhibited expiratory flow limitation during the tidal expiration, for about 67% of the exhaled volume, without changes after surgery. Arterial carbon dioxide tension (32 ± 4 mmHg) and pH (7.46 ± 0.07) did not change, whereas arterial oxygen tension (PaO2) (242 ± 34 mmHg) decreased significantly by about 70 mmHg, on average, with a constant fractional inspired oxygen (0.50). Conclusions: This study shows that (1) respiratory mechanics can be abnormal in patients undergoing cardiac surgery, including expiratory flow limitation; (2) elastance increases and PaO2 decreases after surgery; (3) simple, noninvasive techniques are available to measure respiratory mechanics in the operating theatre.