Hyperkalemia pathophysiology: Difference between revisions
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===Drugs causing hyperkalemia=== | ===Drugs causing hyperkalemia=== | ||
Revision as of 18:44, 2 July 2018
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-In-Chief: Priyamvada Singh, M.B.B.S. [2]
Overview
Potassium is the most abundant intracellular cation and is critically important for many physiologic processes.The normal range of potassium in blood is 3.5-5.1meq/L .Hyperkalemia develops when the level of potassium exceeds 5.5 meq/L in blood which can be due to an increase in intake of potassium, excessive production as seen in tissue breakdown, ineffective elimination of potassium or some drugs. The potassium levels in the body are highly regulated mainly by renal excretion.The gut excretes a minimal amount of dietary potassium (approximately 10%). Hyperkalemia is very common in patients with chronic kidney disease as potassium is not effectively excreted from the body.Potassium is involved in maintaining transmembrane potentials of cells,so imbalance in potassium levels can lead to disruption of cell membrane potentials and can cause hyperexcitablity leading to fatal cardiac arrhythmias and effecting nervous system.
Pathophysiology
Physiological role of potassium
Potassium is the major intracellular cation and sodium is the major extracellular cation.Almost all cells possess an Na+-K+-ATPase, which pumps Na+ out of the cell and K+ into the cell and leads to a K+ gradient across the cell membrane (K+in>K+out) that is partially responsible for maintaining the potential difference across the membrane.This potential difference called the transmembrane potential is responsible for the excitability of the cells.
Factors affecting transcellular shift of potassium
The distribution of potaasium inside and outside the cells is maintained by various pumps,osmolarity,ph and the hormones insulin, aldosterone, beta 2-catecholamines, alpha-catecholamines, and prostaglandins.
- Insulin regulates potassium uptake into the cells through GLUT receptors on the cell membranes by increasing the activity of Na+-K+-ATPase pump.
- Catecholamines regulate potassium uptake into the cells through β2-Receptor–induced stimulation of Na+-K+-ATPase pump.
- Increased osmolarity as in hyperglycemia causes water efflux from the cells that drags potassium along.
- In acidosis,the decreased extracellular ph decreases the rate of Na+-H+ exchange (NHE1) and inhibit the inward rate of Na+-3HCO3 cotransport,thus decreasing intracellular Na+ levels which in turn decreases the activity of Na+-K+-ATPase pump and decreasing intracellular K+ levels.
- In alkalosis,the increased extracellular ph increases the rate of Na+-H+ exchange (NHE1) and increases the inward rate of Na+-3HCO3 cotransport,thus increasing intracellular Na+ levels which in turn increases the activity of Na+-K+-ATPase pump and increasing intracellular K+ levels.
Role of kidneys
The source of potassium to the body is diet.The potassium levels in the body are dependent on dietary intake,tissue breakdown,gastrointestinal absorption and losses and most important is renal regulation via absorption and secretion.Kidney play a important role in keeping the balance of potassium. At the glomerulus, potassium is freely filtered and then largely reabsorbed in the proximal tubule and thick ascending loop of Henle (>60 % of filtered potassium). The cortical collecting duct receives 10–15% of filtered potassium and constitutes the kidney’s major site of potassium excretion. Potassium excretion at the cortical collecting duct depends on the amount of sodium delivered there and the activity of aldosterone.It does so by the following ways.
- increases intracellular K+ concentration by stimulating the activity of the Na+-K+-ATPase in the basolateral membrane.
- stimulates Na+ reabsorption across the luminal membrane, which increases the electronegativity of the lumen, thereby increasing the electrical gradient favoring K+ secretion.If the rate of delivery of sodium and water is very high in the distal tubules then it will cause more Na+ reabsorption and more K+ secretion.
- has a direct effect on the luminal membrane to increase K+ permeability.
Aleration in the levels of potassium occur due to disruption in the above mentioned mechanisms that regulate potassium homeostasis. Hyperkalemia is the increased levels of potassium (>5.1 meq/L) in blood.It can be caused by increase in transcellular shift of potassium,impaired excretion from the kidneys or excessive intake.
Pathogenesis
Hyperkalemia means excessive potassium in the blood(>5.1 meq/L).It can result from excessive potassium intake,increased tissue breakdown,increased transcellular shift or impaired excretion from the body.
- Increased uptake-
The only source of potassium to our body is by diet.If potassium rich diet is consumed or given parenterally,it can lead to hyperkalemia.However in individuals with normal renal function,potassium levels are regulated and excess potassium is excreted.
- Transcellular shift
- Change in extracellular ph-Decrease in ph as in mineral acidosis leads to increased shift of potassium from intracellular to extracellular space.
- Decrease in insulin as in Diabetes mellitus can lead to increased extracellular accumulation of potassium.
- Increase in osmolarity as in hyperglycemia will cause extracellular shift of potassium.
- Decreased catecholamines or reduced function as with beta blockers use will lead to decreased uptake by cells resulting in extracellular accumulation of potassium.
- Tissue breakdown
- Increased tissue breakdown can occur as in rhabdomylosis or chemotherapy induced tissue breakdown.
- Pseudohyperkalemia referred to as increased potassium levels occurring invitro which usually occurs while collecting a blood sample,thrombocytosis or in myeloproliferative diseases.
- Impaired excretion
Potassium levels in body are regulated by the kidneys.Any impairment in the excretion mechanisms can result in hyperkalemia
- Reduced GFR(<15ml/min) results in decreased urine flow and hence decreased sodium and water delivery to the distal tubules resulting in decreased secretion of potassium.
- Decrease in the levels of Aldosterone as in primary hypoaldosteronism and secondary hypoaldosteronism results in impaired excretion of potassium.
- Pseudohypoaldostrenism-The levels of aldosterone are within normal limits but there is resistance to aldosterone in the kidneys and is not able to exert its function.
- Chronic kidney diseases-The overall renal function is impaired resulting in decreased secretion of potassium as in various nephropathies.
- Renal parenchymal damage-This can occur in obstructive uropathy or AKI in which the renal parenchyma would not be able to effectively excrete potassium.
- Constipatiton-Minor amount of potassium is also excreted by stools,so chronic constipation can lead to hyperkalemia however it is very rare.
| Trans-cellular shifts | Renal secretion impairment | GI cause | Increased tissue breakdown | Increased intake of potassium | |
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Renal Parenchymal disrorder |
Defect in Potassium secretion
Secondary hypoaldostrenosim
Pseudohypoaldosteronism Dehydration Heart failure |
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Drugs causing hyperkalemia
A lot of drugs are responsible for causing hyperkalemia.They do so by multiple mechanisms which are listed below
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