Oxygen and 2,3 biphosphoglycerate (2,3-BPG) during haemodialysis

The oxygen cascade in patients treated with hemodialysis and native high-altitude dwellers: lessons from extreme physiology to benefit patients with end-stage renal disease

Patients treated with hemodialysis (HD) repeatedly undergo intradialytic low arterial oxygen saturation and low central venous oxygen saturation, reflecting an imbalance between upper body systemic oxygen supply and demand, which are associated with increased mortality. Abnormalities along the entire oxygen cascade, with impaired diffusive and convective oxygen transport, contribute to the reduced tissue oxygen supply. HD treatment impairs pulmonary gas exchange and reduces ventilatory drive, whereas ultrafiltration can reduce tissue perfusion due to a decline in cardiac output. In addition to these factors, capillary rarefaction and reduced mitochondrial efficacy can further affect the balance between cellular oxygen supply and demand. Whereas it has been convincingly demonstrated that a reduced perfusion of heart and brain during HD contributes to organ damage, the significance of systemic hypoxia remains uncertain, although it may contribute to oxidative stress, systemic inflammation, and accelerated senescence. These abnormalities along the oxygen cascade of patients treated with HD appear to be diametrically opposite to the situation in Tibetan highlanders and Sherpa, whose physiology adapted to the inescapable hypobaric hypoxia of their living environment over many generations. Their adaptation includes pulmonary, vascular, and metabolic alterations with enhanced capillary density, nitric oxide production, and mitochondrial efficacy without oxidative stress. Improving the tissue oxygen supply in patients treated with HD depends primarily on preventing hemodynamic instability by increasing dialysis time/frequency or prescribing cool dialysis. Whether dietary or pharmacological interventions, such as the administration of L-arginine, fermented food, nitrate, nuclear factor erythroid 2-related factor 2 agonists, or prolyl hydroxylase 2 inhibitors, improve clinical outcome in patients treated with HD warrants future research.

Reductions in red blood cell 2,3-diphosphoglycerate concentration during continuous renal replacment therapy

BACKGROUND AND OBJECTIVES

Hypophosphatemia is a frequent complication during continuous renal replacement therapy (CRRT), a dialytic technique used to treat AKI in critically ill patients. This study sought to confirm that phosphate depletion during CRRT may decrease red blood cell (RBC) concentration of 2,3-diphosphoglycerate (2,3-DPG), a crucial allosteric effector of hemoglobin's (Hgb's) affinity for oxygen, thereby leading to impaired oxygen delivery to peripheral tissues.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Phosphate mass balance studies were performed in 20 patients with severe AKI through collection of CRRT effluent. RBC concentrations of 2,3-DPG, venous blood gas pH, and oxygen partial pressure required for 50% hemoglobin saturation (P50) were measured at CRRT initiation and days 2, 4, and 7. Similar measurements were obtained on days 0 and 2 in a reference group of 10 postsurgical patients, most of whom did not have AKI. Associations of 2,3-DPG with laboratory parameters and clinical outcomes were examined using mixed-effects and Cox regression models.

RESULTS

Mean 2,3-DPG levels decreased from a mean (±SD) of 13.4±3.4 µmol/g Hgb to 11.0±3.1 µmol/g Hgb after 2 days of CRRT (P<0.001). Mean hemoglobin saturation P50 levels decreased from 29.7±4.4 mmHg to 26.7±4.0 mmHg (P<0.001). No significant change was seen in the reference group. 2,3-DPG levels after 2 days of CRRT were not significantly lower than those in the reference group on day 2. Among patients receiving CRRT, 2,3-DPG decreased by 0.53 µmol/g Hgb per 1 g phosphate removed (95% confidence interval 0.38 to 0.68 µmol/g Hgb; P<0.001). Greater reductions in 2,3-DPG were associated with higher risk for death (hazard ratio, 1.43; 95% confidence interval, 1.09 to 1.88; P=0.01).

CONCLUSIONS

CRRT-induced phosphate depletion is associated with measurable reductions in RBC 2,3-DPG concentration and a shift in the O2:Hgb affinity curve even in the absence of overt hypophosphatemia. 2,3-DPG reductions may be associated with higher risk for in-hospital death and represent a potentially avoidable complication of CRRT.

Assessment of pH and oxygen status during hemodialysis using the arterial blood line in patients with an arteriovenous fistula

BACKGROUND

In patients with arteriovenous fistulas, assessment of pH and oxygen status during hemodialysis (HD) using the extracorporeal dialysis arterial blood line is widely used both in daily routine and in most studies investigating hypoxia during HD. We designed this study to evaluate whether results of blood gas samples drawn from the extracorporeal arterial line were clinically acceptable in assessing oxygen status.

METHODS

We compared samples drawn from the extracorporeal arterial line with conventionally arterial punctures during 18 routine HD sessions. The samples were drawn simultaneously and analyzed immediately for blood gases, pH and hemoximetry values.

RESULTS

No significant difference was found between the values from the radial artery and the extracorporeal arterial blood line except for FMetHb.

CONCLUSION

Thus, obtaining samples from the extracorporeal dialysis arterial blood line to evaluate the parameters of the oxygen status (pH, pO(2), pCO(2), ctHb, sO(2), FCOHb and ctO(2)) during routine HD is a clinically convenient and accurate sampling approach.