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Après circulation extra-corporelle, la mesure continue de la Svo₂ et la co-oxymétrie du sang mêlé ne sont pas interchangeables

[Continuous Svo₂ measurements and co-oximetry are not interchangeable immediately after cardiopulmonary bypass]

De la Division des Soins Intensifs de Chirurgie, Hôpitaux Universitaires de Genève, Genève, Suisse.

Adresser la correspondance à: Dr Karim Bendjelid, Chef de Clinique Scientifique, Division des Soins Intensifs Chirurgicaux, Hôpitaux Universitaires de Genève, CH-1211 Genève 14, Suisse. Téléphone: +41/22 382 74 52; Télécopieur: +41/22 382 74 55; Courriel: Karim.Bendjelid{at}hcuge.ch

Objective: To determine the accuracy of continuous (in vivo) measurement of mixed venous oxygen saturation (SvO₂), using a fibreoptic catheter, in patients having had cardiopulmonary bypass (CPB).

Methods: Using a pulmonary arterial catheter, we prospectively studied 14 patients (age 64 ± 8) having had cardiopulmonary bypass. Mean hematocrit was 30 ± 4%. The catheter was calibrated in vitro and in vivo, according to the manufacturer’s instructions. Fifty-six simultaneous measurements of continuous SvO₂ (CSvO₂) and measured SvO₂ (MSvO₂) were taken with a co-oxymeter and the paired values were analyzed by the linear regression method. To make the two sets of measurements interchangeable, we established, a priori, a maximum limit of 3% (approximately 5% of the measurement), as being an acceptable difference between the two types of measurements.

Results: All the measurements were obtained within four hours of the placement of the catheter. CSvO₂ was weakly correlated with MSvO₂, with a correlation coefficient of r² = 0.49 (P < 0.001). The Bland-Altman analysis demonstrates an objective mean bias of 0.8 ± 3%, with 36% of the values measured falling outside clinically acceptable limits. For values of CSvO₂ 65%, the measurements were not correlated (P = NS). The bias was –2 ± 7%, and 56% of the values measured fell outside clinically acceptable limits.

Conclusions: Our results demonstrate that, in the immediate aftermath of cardiopulmonary bypass, the continuous measurement of SvO₂ and the co-oxymetry determination of mixed blood is not inter-changeable. Other studies will be necessary to confirm these results and enable us to understand the pathophysiological phenomena accounting for the lack of agreement between the two methods.

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