What is the role of kidney in acid base balance? What are the anaesthetic considerations in a dialysis patient?

The role of kidney in acid base homeostasis

Renal compensation in Metabolic Acidosis

1) Increased Reabsorption of HCO₃

2) Increased excretion of titrable Acids

3) Increased production of ammonia

Increased Reabsorption of HCO₃

almost 70-80% of all HCO₃ filtered by the kidneys is reabsorbed in proximal convoluted tubules

H⁺ is titrated into the tubular fluid and carbonic anhydrase catalyses the production of H₂O and CO₂which is rapidly reabsorbed and carbonic anhydrase within the cell converts CO₂ back into HCO₃which exits into the peritubular blood

Increased Excretion of Titratable Acids

The H⁺ that was secreted into the tubular fluid to recover the HCO₃⁻ combines with urinary buffers as HPO₄²⁻ to form H² PO⁴ which due to its charge can't be absorbed back and is excreted from body 

Increased production of Ammonia

The Deamination of Glutamine in the proximal convoluted tubules is the principle source of NH₃ production in the kidneys which is secreted into the tubular fluid as a buffer and combines with H⁺ to form NH₄⁺ which is readily not absorbed back and excretion of NH₄ in urine effectively eliminates H⁺

Renal compensation in Metabolic Alkalosis: The kidneys have good capacities of reabsorption or excretion of HCO₃which protects against metabolic alkalosis per se hence metabolic alkalosis  generally occurs in association with sodium depletion or mineralocorticoid excess. Sodium depletion enhances the Na⁺ reabsorption into the tubular blood  which is coupled with Cl⁻ to maintain neutrality which again causes H⁺ secretion and HCO³reabsorption causing metabolic alkalosis

The anaesthetic concerns in dialysis patients

1. Hypervolumia
2. Acidemia
3. hyperkalemia
4. Cardiac : Hypertension is most common
5. Pulmonary congestion and edema 
6. Hematological : Anemia due to insufficiente production of erythropoietin,platelet dysfunction leading to prolonged bleeding and clotting time
7. Hypoalbuminemia; due to hemodilution, impaired synthesis and increased loss leading to alterations in drug effect of high protein binding drugs like diazepam, warfarin and phenytoin but anaesthetic drugs are less albumin binding and free fraction of drug increases modulation needful in drug dosage
8. Patients weight need to be recorded so dry weight can guide to dialysis
9. Last dialysis time as immediately patients are hypovolumic with increased risk of hypotension
10. Use of Heparin in dialysis can potentiate blood loss during surgery so titres need to be done and antagonist used if more
11. Care of AV fistula 
12. Fluid therapy should be guarded on intra and post operative periods