Post-operative myocardial dysfunction does not affect the physiological response to early mobilization after coronary artery bypass grafting.

BACKGROUND: An acute increase in oxygen demand can be compensated for either by increased cardiac index (CI) or increased oxygen extraction, resulting in reduced mixed venous oxygen saturation (SvO2). We tested the hypothesis that post-operative cardiac dysfunction may explain why oxygen extraction alone is increased during early mobilization after cardiac surgery. METHODS: Twenty patients with a pre-operative ejection fraction > 50% were included in an open prospective observational study comparing the changes in SvO2 and hemodynamics during mobilizations immediately prior to surgery and on the first post-operative morning. RESULTS: Mobilization induced an absolute reduction in SvO2 of 17.7 +/- 7.4% pre- and 19.0 +/- 5.5% post-operatively (NS). ANOVA for a series of measurements throughout the mobilization sequence identified no different effect on SvO2 between pre- and post-operative mobilizations (P = 0.567). The SvO2 level was reduced post-operatively resulting in a SvO2 during standing exercise of 55% before and 49% after the surgery (P < 0.01). Mobilization increased the heart rate (HR) and decreased the stroke volume index (SVI), leaving CI unchanged. This response was similar pre- and post-operatively (NS). Compared with pre-operative measurements, CI and HR increased post-operatively while SVI remained unchanged despite elevated cardiac filling pressures and reduced systemic vascular resistance. The left ventricular stroke work index was reduced, indicating reduced myocardial performance. CONCLUSION: Myocardial function was reduced on the first morning after coronary artery bypass grafting (CABG), but during post-operative mobilization this reduction did not significantly influence the changes in CI or SvO2.

Mixed venous oxygen desaturation during early mobilization after coronary artery bypass surgery.

BACKGROUND: Early postoperative mobilization induces a marked reduction in mixed venous oxygen saturation (S(v)O(2)) after aortic valve replacement. We investigated whether a similar desaturation occurs among coronary artery bypass grafting (CABG) patients, and if the desaturation was related to the preoperative ejection fraction (EF). METHODS: Thirty-one CABG patients with a wide range in EF were included in an open observational study. We recorded hemodynamic and oxygenation variables during mobilization on postoperative day 1 and day 2 using a pulmonary artery catheter. RESULTS: Patients with an EF ranging from 24 to 87% were mobilized without clinical problems. S(v)O(2) at rest was 65.4 +/- 4.9% (mean +/- SD) on day 1 and 64.3 +/- 5.8% on day 2 (NS). During mobilization, cardiac index and oxygen delivery were reduced while oxygen consumption was increased (P-values: 0.000, 0.007 and 0.000, respectively). Consequently, oxygen extraction increased, resulting in a marked reduction in S(v)O(2)-42.9 +/- 8.3% on day 1 and 47.4 +/- 8.5% on day 2 (P = 0.025 between days). Several pre-, intra- and postoperative factors were tested as possible predictors for S(v)O(2) during mobilization. No factor contributed substantially. CONCLUSION: Patients with CABG exhibit a marked desaturation during early postoperative mobilization. Preoperative ejection fraction did not affect S(v)O(2) during exercise. The clinical consequences and underlying mechanism require further investigation.