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.