In what circumstances have you seen it?

To my understanding, the Veinous O2 saturation cannot be equal to arterial O2 sat except in cases of shunt between systemic and pulmonary circulation. It can be elevated in cases of Low O2 consumtion (anaethesia) or in cases of high CO (cirrhosis - AV shunts - Sepsis - etc). Are you sure about the measures?

I have seen it in 3 cases confirmed by several measures by different machines ,and i didnot find any explanation clincally

This is probably a laboratory error. To have equal SaO2 an SvO2 mean that the patient has no extraction of O2 (dead).

I saw it in hemodialysis patient with partially occlused arteriovenous fistula and omolateral central venous catheter.

note that he was not .... dead

:)

It can happen if you have an anomalous pulmonary vein return to the right atria. Perform TEE , and look at the sample from femoral vein or from left sided central vein

Can it be possible to have 3 patients with this anomaly?

It depends on the interval you have seen it. I work in cardiac center for 25 years I have met with it, but ASD is not an absolutely rare disease. I really cannot explain how often it really occurs. I have tested a left sided central vein in these occasions and they were really venous.:

I have seen anomalous pulmonary veins a few times. But it only affects o2 sat. In right parts of the heart. Where did you take the tested bloodsample?