Effects of open heart surgery on respiratory mechanics.

Abstract

To investigate the changes in respiratory mechanics in patients undergoing cardiac surgery before and after the operation. Prospective physiological study. Operating theatre of the Institute of Cardiac Surgery, Verona, Italy. 8 patients needing heart surgery because of a coronary bypass or mitral valve replacement. 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 cmH(2)O/l, respectively) and respiratory resistance, both airway and total flow resistance (5.8 +/- 2.5 and 10.3 +/- 4 cmH(2)O. 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 cmH(2)O, 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 mm Hg) and pH (7.46 +/- 0.07) did not change, whereas arterial oxygen tension (PaO(2)) (242 +/- 34 mm Hg) decreased significantly by about 70 mm Hg, on average, with a constant fractional inspired oxygen (0. 50). This study shows that (1) respiratory mechanics can be abnormal in patients undergoing cardiac surgery, including expiratory flow limitation; (2) elastance increases and PaO(2) decreases after surgery; (3) simple, noninvasive techniques are available to measure respiratory mechanics in the operating theatre.