Oxygen extraction ratio to identify patients at increased risk of intradialytic hypotension

Abstract

Intradialytic hypotension (IDH) is a hemodynamic phenomenon recently associated with decreased blood oxygen saturation (SO₂). The ratio between peripheral oxygen saturation (SpO₂) and central venous SO₂ (ScvO₂) or Oxygen Extraction Ratio (OER), which represents a roughly estimate of the amount of oxygen claimed by peripheral tissues, might be used to estimate haemodialysis (HD) related hypoxic stress. Aim of this pilot study was to evaluate the relationship between OER increments during dialysis sessions (ΔOER) and episodes of IDH. We enrolled chronic HD patients with permanent central venous catheter (CVC) and no fistula, in whom ScvO₂ measurement is at hand. OER ([(SpO₂ - ScvO₂)/SpO₂] × 100) was measured in three consecutive HD sessions (HD OER sessions) before HD, after 15', 30' and 60' min and at the end of HD. Then, a one-year follow-up was planned to record the number of IDH episodes. In the 28 enrolled patients (age 74 ± 2.6 years), during 12 ± 1.2 months of follow up, incidence of IDH was 3.6%. We divided patients into two groups, above or below the median value of ΔOER at the end of HD, which was 36%. In these groups, the average incidence of IDH was 7% and 2% respectively (p < 0.01), while OER values before HD were not different. Notably, in the high ΔOER group the OER increment was evident since after 15' and was significantly higher than in the low ∆OER group (∆OER-15' = 19 ± 3.0% vs. 9.0 ± 3.0%; p < 0.05). By comparison, blood volume changes overlapped in the two groups (average change - 9 ± 0.8%). Values of ∆OER > 19% after only 15' of HD treatment or > 36% at the end of the session characterize patients with higher rates of hypotension. Intradialytic ∆OER, a parameter of tissue hypoxic stress, identifies more fragile patients at greater risk of IDH.

Figure 1

Oxygen saturation trends during dialysis. In the population as a whole, SpO₂ was stable, while ScvO₂ decreased after 30 min and OER increased after 15 min of HD initiation (repeated measures ANOVA: p < 0.001 for both variables). Bonferroni post-hoc test vs. basal values: *p < 0.01; °p < 0.001. SpO₂: peripheral oxygen saturation; ScvO₂: central venous oxygen saturation; OER: oxygen extraction rate; HD: hemodialysis.

Figure 2

Comparison of ScvO₂ trends during HD between the two groups ΔOER > 36% and ΔOER ≤ 36%. ScvO₂ decreased significantly (ANOVA, p < 0.001) in both groups but without differences between them, in particular after 30 min. ScvO₂: central venous oxygen saturation; ∆OER: percent change in OER; HD: haemodialysis.

Figure 3

During HD ∆OER increased in both groups with either low or high ΔOER (repeated measures ANOVA, p < 0.001), but in the ΔOER > 36% group the increment was significantly higher than in the ΔOER ≤ 36% group after 15 min, 30 min and 60 min. Bonferroni post-hoc test, ΔOER > 36% vs. ΔOER ≤ 36% *p < 0.05; °p < 0.001. ∆OER: percent change in OER; HD: haemodialysis;

Figure 4

Overlapping trends of hemodialysis induced changes in blood volume in ΔOER > 36% and ΔOER ≤ 36% groups.

Figure 5

The ΔOER > 36% group had higher IDH incidence than ΔOER ≤ 36% group. Bonferroni test p  0.01. ∆OER: percent change in OER; IDH: intradialytic hypotension.