Oxygen extraction ratio (OER): a marker of hemodynamic stress in hemodialysis (HD)-L-


Hemodialysis treatment exposes patients to cardiovascular stress and to the risk of intradialytic complications.

Control systems (e.g: blood pressure, blood volume monitoring) reduce events but do not measure the entity of the hemodynamic stress.

Oxygen extraction ratio (n.v 20%-25% ) is an important hemodynamic parameter measured in Intensive Care Units to assess the tissue oxygenation (Figura 1).

Pathologic increments in OER are indicative of parenchymal hypoperfusion and of inadequate adaptation of tissues to hypoxia (Figura 1).

We hypothesized that OER changes, if detectable during HD sessions, might be a marker of hemodynamic stress.


OER can be measured as the ratio between peripheral O2 saturation (SO2) and central venous O2 saturation (ScvO2).

We sampled OER basally, 15’, 30’, 60’, 120’ and at the end of HD sessions in ten chronic hemodialysis patients. Blood saturations were measured by a digital pulse oximeter (Max Puls two) and a blood gas analysis (GEM 4000; IL) of a central venous sample. Accordingly, the study was limited to patients with central venous catheter (CVC).

OER changes were evaluated on three consecutive HD sessions (1 long and 2 short intervals). In these sessions, to reduce hemodynamic stress, ultrafiltration rate was kept at <10ml/kg/h.

Blood pressure (BP), heart rate (HR), blood volume (BV), and symptoms were also recorded.

Exclusion criteria: acute illness and/or dialysis vintage <6 months.


We enrolled ten patients: age 72±14 y.o., M/F = 2/8, HD since 38±41 months.

Changes in OER were invariably detectable in all of the studied sessions. Moreover, there was a substantial repeatability of values in each patients in the three consecutive HD sessions. Therefore, for further statistics, we decided to calculate for each patient, the average of 3 values.

Average OER values were increased even before dialysis (35±7% vs normal reference range 20-25% ). We recorder a further increment of OER up to a final value of 47±8% (end of HD) (p<.0002) (Figura 2).

BV decreased up to -8±4% (p<.0001), with non significant changes in BP and HR (Figura 3).

All sessions were free of symptoms.

Discussion and conclusion

Surprisingly OER values were increased even in basal condition. This may be related to a reduced O2 parenchymal perfusion due to fluid overload.

OER showed a progressive increase during HD in front of stable BP and HR values and in the absence of symptoms.

The progressive OER increment can be considered to represent a measurement of the ongoing parenchymal adaptation to hemodynamic stress.

Accordingly we could regard it as a simple and cheap tool to measure the entity of tissue stress resulting from hemodialysis.

OER may be useful to monitor patients at highest hemodynamic risk.