The influence of left lateral position on cardiac output changes after head up tilt measured by impedance cardiography.

Background: The value of the impedance cardiography (IC) method for measuring cardiac output (CO) with the subject in the left lateral position has not yet been established. We compared the CO after a 30 degrees head-up tilt with the subjects in the supine and left lateral position.

Methods: Thirty healthy young volunteers were placed in the supine horizontal position. CO, stroke volume (SV) and heart rate (HR) were measured for five minutes using impedance cardiography (NCOMM3, BoMed Medical Manufacturing, Location). Then a 30 degrees head-up tilt was done and the measurements were continued for an additional five minutes. After that the subjects were placed in the left lateral horizontal position and the measuring procedure was repeated.

Results: After the tilt, SV and cardiac index (CI) decreased and HR increased statistically significantly in the supine and in the left lateral position. SV and CI also decreased statistically significantly but HR did not change after the shift from the supine to the left lateral position. Although CI was smaller in the left lateral position, the time course of CI change after the tilt was identical in the supine and in the left lateral position.

Conclusion: When using the IC method of SV measurement, the absolute value of the CI changes when moving from the supine to lateral positions. It is not clear whether this change is physiologic or an artifact of the measurement technique. However, changes in CI in response to a 30 degree head up tilt are the same in either position. We conclude that changes in CI can be measured with the IC method in the lateral position.