An experimental study was conducted to examine the effect of backrest position on mixed venous oxygen saturation (SvO$sb2$); to determine the relationship between SvO$sb2$ and cardiac output/index; and to determine the relationship between intraoperative factors and postoperative SvO$sb2$.

Thirty subjects were studied 6.0-9.5 hours after coronary artery bypass surgery; all were mechanically ventilated. Subjects were randomly assigned to one of three groups and baseline data were recorded on all subjects in a flat position. The backrest was adjusted according to group assignment: Group 1--20$spcirc$; Group 2--40$spcirc$; Group 3--flat (control). Measurements were taken at 0, 5, 15, and 30 minutes after change in position. After 30 minutes, patients were returned to the flat position and measurements were repeated.

There were no significant differences in SvO$sb2$ from baseline among the three groups while the backrest was elevated (p =.081) or when flat (p =.405). There were no significant differences in SvO$sb2$ from baseline among the times that SvO$sb2$ was measured in either the elevated (p =.344) or flat positions (p =.118). There were no significant interaction effects of position and time on difference in SvO$sb2$ from baseline in either the elevated (p =.134) or flat positions (p =.218).

There were no significant correlations between baseline SvO$sb2$ and cardiac output (r =.266; p =.156) or cardiac index (r =.261; p =.163). There were no significant correlations between time on cardiopulmonary bypass and baseline SvO$sb2$ (r = $-.134$; p =.481). There were no significant differences between the two surgical techniques (saphenous vein graft and internal mammary) and baseline SvO$sb2$ (p =.457).

The findings support elevating the head of the bed 20$spcirc$ to 40$spcirc$ after cardiac surgery once hemodynamic stability has been established. However, nurses should continue to observe for effects of position change on SvO$sb2$. Additional findings of the study support case studies which noted a decrease in SvO$sb2$ with activity and suctioning. When SvO$sb2$ does change, the patient should be assessed for changes in cardiac output, hemoglobin, arterial oxygen saturation, and/or oxygen consumption.