The oxygen affinity of haemoglobin in chronic renal failure

Cerebral blood flow regulation in end-stage kidney disease

Patients with chronic kidney disease (CKD) and end-stage kidney disease (ESKD) experience an increased risk of cerebrovascular disease and cognitive dysfunction. Hemodialysis (HD), a major modality of renal replacement therapy in ESKD, can cause rapid changes in blood pressure, osmolality, and acid-base balance that collectively present a unique stress to the cerebral vasculature. This review presents an update regarding cerebral blood flow (CBF) regulation in CKD and ESKD and how the maintenance of cerebral oxygenation may be compromised during HD. Patients with ESKD exhibit decreased cerebral oxygen delivery due to anemia, despite cerebral hyperperfusion at rest. Cerebral oxygenation further declines during HD due to reductions in CBF, and this may induce cerebral ischemia or "stunning." Intradialytic reductions in CBF are driven by decreases in cerebral perfusion pressure that may be partially opposed by bicarbonate shifts during dialysis. Intradialytic reductions in CBF have been related to several variables that are routinely measured in clinical practice including ultrafiltration rate and blood pressure. However, the role of compensatory cerebrovascular regulatory mechanisms during HD remains relatively unexplored. In particular, cerebral autoregulation can oppose reductions in CBF driven by reductions in systemic blood pressure, while cerebrovascular reactivity to CO₂ may attenuate intradialytic reductions in CBF through promoting cerebral vasodilation. However, whether these mechanisms are effective in ESKD and during HD remain relatively unexplored. Important areas for future work include investigating potential alterations in cerebrovascular regulation in CKD and ESKD and how key regulatory mechanisms are engaged and integrated during HD to modulate intradialytic declines in CBF.

Changes of Arterio-venous Differences in pH and pCO2 by Hemodialysis

Normally the differences in arterial-venous pH (A-VpH) and veno-arterial pCO2 (V-ApCO2) are small and constant. This study deals with A-VpH and V-ApCO2 and their effect on arterial-venous saturation hemoglobin percentage (A-VSHb%) in uremic patients under hemodialysis (HD). In 17 uremic patients under HD with acetate, blood samples were collected anaerobically in heparinized syringes from artery (fistula) and vein (forearm without fistula) pre- and post-HD. In these samples pH, pCO2 and SHb% were determined and A-VpH, V-ApCO2 and A-VSHb% were estimated. Comparison between the values pre- and post-HD of A-VpH, V-ApCO2 and A-VSHb% shows that these three values were decreased significantly post-HD (p<0.001). The correlation of all values (pre- and post-HD) of A-VpH and V-ApCO2 with that of A-VSHb% was significant and positive (r=0.514 p<0.01, r=0.505 p<0.01, respectively).

Control-relevant erythropoiesis modeling in end-stage renal disease

Anemia is prevalent in end-stage renal disease (ESRD). The discovery of recombinant human erythropoietin (rHuEPO) over 30 years ago has shifted the treatment of anemia for patients on dialysis from blood transfusions to rHuEPO therapy. Many anemia management protocols (AMPs) used by clinicians comprise a set of experience-based rules for weekly-to-monthly titration of rHuEPO doses based on hemoglobin (Hb) measurements. In order to facilitate the design of an AMP using model-based feedback control theory, we present a physiologically relevant erythropoiesis model and demonstrate its applicability using clinical data.

Opposite effects of urea on hemoglobin-oxygen affinity in anemia of chronic renal failure

We studied the action of urea on the spin-spin relaxation rate of 2,3-diphosphoglycerate (2,3-DPG) phosphorus atoms in normal and uremic erythrocytes. At concentrations from 10 to 60 mM, urea increased the relaxation rates of 2,3-DPG P-3 phosphorus atoms. This evidenced a stronger binding of 2,3-DPG to hemoglobin (Hb), suggesting that the deoxyform of Hb was stabilized. This hypothesis was confirmed by measurements of the association constant of oxygen to hemoglobin (K) in normal erythrocytes in presence of urea concentrations in the range of those observed in uremic patients (30 mM). Indeed, the observed decrease in K suggests that the T structure of hemoglobin is stabilized. By contrast, with higher urea concentrations (120 mM), measurements of P50 showed an increase in the hemoglobin affinity for oxygen (decrease in P50). Moreover, the relaxation rates of 2,3-DPG P-3 phosphorus atoms were not modified, which is consistent with the simultaneous increase of K. This may be attributed to the formation of carbamylated hemoglobin in presence of urea. These results suggest two opposite effects of urea on Hb-O2 affinity: the first reinforces 2,3-DPG-Hb binding and leads to a decrease in O2 affinity; the second, mediated by carbamylation of Hb, hinders the binding of 2,3-DPG and increases the O2 affinity. These findings are consistent with the fact that, despite the presence of carbamylated hemoglobin, uremic patients do not present increased Hb-O2 affinity.

End-stage kidneys are capable of increased erythropoietin production

A relative deficiency of erythropoietin (EPO) is the most important factor responsible for the anaemia of end-stage renal failure. Patients on continuous ambulatory peritoneal dialysis usually maintain a higher haemoglobin concentration than patients on other forms of dialysis. The precise mechanism is uncertain, and there is disagreement over the role of increased EPO production. An 11-year-old boy with end-stage renal failure maintained on overnight cycling peritoneal dialysis developed a reticulocytosis, followed by a marked increase in haemoglobin concentration, shortly after his dialysis schedule was altered to include a full peritoneal cavity during the daytime. This improvement in erythropoiesis was closely associated with an increase in serum EPO concentration. We suggest that the alteration in dialysis may have resulted in enhanced clearance of an inhibitor of EPO production and discuss the possible mechanisms involved.

Pheochromocytoma, polycythemia, and venous thrombosis

Polycythemia is rarely associated with pheochromocytoma. A patient with a 22-year history of malignant pheochromocytoma is presented in whom major complications developed as a result of long-standing polycythemia, apparently due to secretion of erythropoietin by the tumors. Despite attempts to reduce tumor burden by surgery, chemotherapy, and large doses of I-131-metaiodobenzylguanidine, polycythemia persisted. Extensive venous thrombosis developed requiring hospitalization and anticoagulation. Thus, polycythemia itself may be a cause of major morbidity in patients with pheochromocytoma, and prophylactic measures may be warranted. Review of the 130 patients with benign and malignant pheochromocytoma studied since the introduction of I-131-metaiodobenzylguanidine in 1980 revealed another six patients with hematocrits over 50 but only one had a hematocrit greater than 55 and required regular phlebotomy. In contrast, anemia (hematocrit less than 35) due to variety of causes was present in 18 cases.

Androgen therapy in hemodialysis patients: I. Effects on red cell oxygen transport

Measures related to red cell oxygen transport were evaluated in 17 hemodialysis patients receiving androgen therapy, 15 untreated hemodialysis patients and 15 normal subjects. Hemoglobin levels were higher in androgen-treated patients than in the untreated population and were directly related to the reticulocyte index. Hill coefficients were normal, and in vivo P5O values were increased to the same degree in both dialysis groups. However, DPG and serum phosphate explained 70% and 12%, respectively, of the variance in P5O in untreated patients but only 2% and 5% in androgen-treated subjects. In contrast, sample pH explained 34% of the variance in P5O in the androgen-treated group and less than 1% in the untreated dialysis population. Despite the relative importance of pH as a determinant of P5O in patients on androgen therapy, the Bohr coefficient in this group was only about half of that in untreated dialysis subjects. Androgen-treated patients also had lower red cell ATP levels. Finally, the expected correlation of MCHC with pH was noted in untreated dialysis subjects but not in patients receiving androgens. We conclude that androgen therapy in hemodialysis patients in addition to increasing red cell production, directly alters red cell metabolism. Moreover, although the androgen regimens used did not change the net oxygen transport characteristics of hemoglobin, they decreased the responsiveness of hemoglobin-oxygen affinity to changes in pH, DPG and phosphate. Thus, red cell adaptation to changes in oxygen supply and/or demand may be limited in androgen treated patients, and the improvement in clinical performance expected from androgen-stimulated erythropoiesis may not be realized.

Anaesthesia for the patient with impaired renal function

Patients with renal disease are at risk of further deterioration of renal function and acute tubular necrosis when subjected to anaesthesia and surgery. Optimal fluid loading and careful selection of anaesthetic techniques and agents, appropriate monitoring and the use of mannitol and dopamine assist in the maintenance of renal blood flow and help preserve renal function in these patients. In association with renal failure, physiological changes in other systems result in reduced oxygen supply to the tissues, metabolic disturbances, impairment of the coagulation and immune defence mechanisms and an increased risk of cardiac and cerebrovascular catastrophe. Although many anaesthetic techniques including regional analgesia may be used successfully in these patients caution with most drugs, especially pethidine, phenoperidine, suxamethonium and all non-depolarising neuromuscular relaxants is recommended. Of the volatile anaesthetics currently available, halothane is the agent of choice. Oxygen therapy and close monitoring of cardiorespiratory function are necessary postoperatively.

Haemoglobin concentration and serum erythropoietin in renal dialysis and transplant patients

In patients with chronic renal failure the use of the relatively new dialysis technique of continuous ambulatory peritoneal dialysis (CAPD), unlike other forms of dialysis, is consistently associated with an increase in Hb concentration, but the mechanism remains obscure. We measured Hb, haematocrit, S-erythropoietin and Hb-oxygen affinity in 3 groups of patients with chronic renal failure. (1) Untreated patients starting on haemodialysis. (2) Patients on intermittent peritoneal dialysis changing to CAPD. (3) Patients from the above 2 groups receiving renal transplants. In addition, red cell mass, plasma volume and red cell survival were measured in (2), before starting CAPD and at 6 months. There were distinctly different patterns of change in Hb concentration, Hb-oxygen affinity and S-erythropoietin in the 3 groups of patients. The increase in Hb concentration with CAPD is due to both a fall in plasma volume and an increase in red cell mass, with an increase in red cell survival. There was no change in Hb-oxygen affinity or serum erythropoietin concentration. The improvement in red cell mass and survival may be related to increased clearances of substances with mol. wt.s between 500 and 5000 daltons which accumulate in renal failure (uraemic middle molecules).

The effect of hemodialysis on oxygen transport in chronic uremics

Previous studies from our laboratory suggested a decrease in tissue oxygen delivered during hemodialysis of chronic uremic patients due to an increase in hemoglobin-oxygen affinity, i.e. decrease in P50. This current study was designed to determine whether the changes in cardiac index and/or tissue oxygen extraction could compensate for increases in hemoglobin-oxygen affinity previously observed, so that total tissue delivery was unchanged during hemodialysis. This study demonstrated in patients during a 6-hour hemodialysis that: 1) no change occurs in hemoglobin-oxygen affinity expressed as P50, with unchanging plasma inorganic phosphate and red blood cell pH; 2) increased extraction of oxygen from tissue, measured via (A-V) O2 difference compensates for decreased oxygen delivery due to decreased cardiac output, and although there was no demonstrable change in hemoglobin-oxygen affinity, it would appear that increased tissue extraction of oxygen via other mechanisms should be easily able to compensate for previously reported changes in P50.

Hematologic complications of chronic renal failure

Uremia interferes with erythropoiesis, granulocyte, platelet, and immune functions. As a result, uremic patients are almost invariably anemic, and have a high incidence of infections and hemorrhagic complications. The anemia of renal failure, which is caused primarily by damage to the site of erythropoietin production is often complex, and complicated by hemolysis from a variety of mechanisms, iron deficiency, and so forth. Although hemodialysis ameliorates some of the hematologic complications to a variable degree, they remain a serious hinderance to the well being of this group of patients. Progress in understanding the mechanism of these problems and their therapy has been reviewed here.

Erythrocyte organic phosphates in the anemia of renal failure in childhood

Erythrocyte 2,3-diphosphoglycerate (2,3-DPG ) and adenosinetriphosphate (ATP) levels were determined in 43 children with chronic renal failure on conservative treatment (CT), and 12 children on regular hemodialysis (HD) immediately before and after a HD session. The results were compared to non-anemic and anemic controls. In spite of anemia, erythrocyte 2,3-DPG in renal failure was similar to non-anemic controls at normal blood pH, but rose during dialysis as a result of alkalosis. In contrast, ATP levels were high already at a normal blood pH. 2,3-DPG correlated with packed cell volume (PCV) in children with renal failure but at lower concentrations compared to controls. Both organic phosphates in the erythrocytes showed a significant correlation with blood pH. The poor increase of 2,3-DPG, in combination with elevated ATP levels, suggests uremia-induced inhibition of 2,3-DPG synthesis.

Red cell 2,3-diphosphoglycerate and oxygen affinity

The ease with which haemoglobin releases oxygen to the tissues is controlled by erythrocytic 2,3-diphosphoglycerate (2,3-DPG) such that an increase in the concentration of 2,3-DPG decreases oxygen affinity and vice versa. This review article describes the synthesis and breakdown of 2,3-DPG in the Embden-Meyerof pathway in red cells and briefly explains the molecular basis for its effect on oxygen affinity. Interaction of the effects of pH, Pco2, temperature and 2,3-DPG on the oxyhaemoglobin dissociation curve are discussed. The role of 2,3-DPG in the intraerythrocytic adaptation to various types of hypoxaemia is described. The increased oxygen affinity of blood stored in acid-citrate-dextrose (ACD) solution has been shown to be due to the decrease in the concentration of 2,3-DPG which occurs during storage. Methods of maintaining the concentration of 2,3-DPG in stored blood are described. The clinical implication of transfusion of elderly people, anaemic or pregnant patients with ACD stored blood to anaesthetically and surgically acceptable haemoglobin concentrations are discussed. Hypophosphataemia in association with parenteral feeding reduces 2,3-DPG concentration and so increases oxygen affinity. Since post-operative use of intravenous fluids such as dextrose or dextrose/saline also lead to hypophosphataemia, the addition of inorganic phosphorus to routine post-operative intravenous fluid may be advisable. Disorders of acid-base balance effect oxygen affinity not only by the direct effect of pH on the oxyhaemoglobin dissociation curve but by its control of 2,3-DPG metabolism. Management of acid-base disorders and pre-operative aklalinization of patients with sickle cell disease whould take account of this. It is known that anaesthesia alters the position of the oxyhaemoglobin dissociation curve, but it is thought that this is independent of any effects which anaesthetic agents may have on 2,3-DPG concentration. In vitro manipulation of 2,3-DPG concentration with steroids has already been carried out. Elucidation of the role of 2,3-DPG in the control of oxygen affinity may ultimately lead to iatrogenic manipulation of oxygen affinity in vivo.