Effects of the knee-chest position on cardiac index and propofol requirements during bispectral index (BIS)-guided spine surgery.

INTRODUCTION

The knee-chest (KC) position is often used for spine surgery. It is considered to promote significant changes in venous return and cardiac output. However, the magnitude of these changes and their consequences on intraoperative haemodynamics and anaesthetic requirements remain to be determined. The goal of the present study was to determine the changes in cardiac index and propofol requirements of patients undergoing spine surgery in the KC position.

METHODS

Twenty ASA 1-3 patients scheduled for elective spine surgery were included in the study. A radial artery catheter and an oesophageal Doppler probe were properly positioned after induction of anaesthesia. Anaesthesia consisted of bispectral index (BIS)-guided, plasma target-controlled, propofol-remifentanil anaesthesia. After positioning the patient KC, remifentanil target concentration was maintained throughout the case as in the supine position whilst propofol target concentration was adjusted to maintain BIS values between 40 and 50. Cardiac index, stroke volume, heart rate, end-tidal CO(2) (ETCO(2)), mean arterial pressure, peak and plateau airway pressures, BIS values and plasma target concentrations of propofol and remifentanil were compared 15 min after induction of anaesthesia (in the supine position) and 15 min after placing the patients KC. Data are expressed as mean+/-S.D. except for DeltaPP expressed as a number of patients with DeltaPP greater than 13%.

RESULTS

Cardiac index, stroke volume, mean arterial pressure and propofol target concentration were significantly decreased from supine to KC position: 2.6+/-0.03 to 1.7+/-0.04 l/min/m(2), p<0.0001; 68+/-1.2 to 45+/-1 ml, p<0.0001; 83+/-1.2 to 76+/-1.4 mmHg, p<0.0001 and 3+/-0.06 to 2+/-0.05 microg/ml, p<0.0001, respectively. The number of patients with DeltaPP greater than 13% was zero in the supine position and 18 (90%) in the KC position (p<0.0001).

CONCLUSION

Placing surgical patients in the KC position during BIS guided anaesthesia was associated with marked decrease in cardiac index and propofol requirements. These results suggest that monitoring intraoperative cardiac index via an oesophageal Doppler and depth of anaesthesia with the BIS may be useful in patients undergoing spine surgery in the KC position.