Comparison of central venous pressure and venous oxygen saturation from venous catheters placed in the superior vena cava or via a femoral vein: the numbers are not interchangeable.

OBJECTIVE

To compare venous pressure and haemoglobin oxygen saturation measured from a catheter in the superior vena cava (SVC) with a catheter inserted via the femoral vein, and to assess the agreement of these measurements. To assess the effect of intra-abdominal pressure and intrathoracic pressures on these measurements.

DESIGN, SETTING AND PARTICIPANTS: Prospective study of patients in an adult intensive care unit, Alfred Hospital, Melbourne, Australia.

MAIN OUTCOME MEASURES

Central venous pressure (CVP), femoral venous pressure (FVP), venous haemoglobin oxygen saturation in the SVC (SO₂C) and via the femoral vein (SO₂F), agreement between these measures using the Bland-Altman method, and the effect of intra-abdominal pressure and intrathoracic pressure.

RESULTS

43 patients were included; the mean bias for FVP -CVP was 1.05 mmHg (95% CI, 0.30-1.79 mmHg), with limits of agreement of -3.79 to 5.89 mmHg (95% CI, -5.08 to 7.18 mmHg). The bias for SO₂F -SO₂C was -3.21 (95% CI, -6.33 to -0.10), with limits of agreement of -22.43 to 16.01 (95% CI, -27.81 to 21.39). Intra-abdominal pressure had a significant (P < 0.01) effect on both the FVP and on the difference (FVP -CVP).

CONCLUSIONS

This study demonstrates poor agreement between CVP and FVP and between SO₂C and SO₂F and that the measurements taken from these two sites are not interchangeable clinically.