Serum K+3.5-5.5 mmol/L in normal human body. Clinically, hypokalemia is defined as serum K+ <3.5 mmol/L due to insufficient intake or certain diseases or taking certain drugs.
However, a decrease in serum potassium does not necessarily indicate a lack of potassium in the body, but only a decrease in the concentration of potassium in the extracellular fluid, whereas serum potassium does not necessarily decrease when there is a systemic potassium deficiency. Therefore, clinical analysis should take into account the medical history and clinical manifestations. The clinical manifestations of hypokalemia are closely related to the serum potassium concentration and the rate of decrease of potassium concentration as well as the state of the organism.
I. Physiological functions of potassium ion
1, potassium is the material basis for maintaining the resting potential of the cell membrane, which is mainly determined by the permeability of the cell membrane to potassium and the difference in potassium concentration inside and outside the membrane. This potential is an important factor affecting the excitability of neuromuscular tissues.
2. Potassium is involved in a variety of metabolic processes and is closely related to glycogen and protein synthesis. Some intracellular enzymes related to sugar metabolism, such as phosphorylase and sulfhydrylase, must have a high concentration of potassium to be active.
3.Regulate the osmotic pressure and acid-base balance of the intracellular and extracellular fluid. Since a large amount of potassium ions exist inside the cell (about 20 times more than that outside the cell), it not only maintains the osmotic pressure and acid-base balance of the intracellular fluid, but also affects the osmotic pressure and acid-base balance of the extracellular fluid.
II. Several manifestations of hypokalemia
1. Neuromuscular system
Potassium has the function of maintaining neuromuscular stress, and the neuromuscular system can only normalize its stress when the blood potassium is maintained at a certain concentration. When the serum potassium decreases, the excitability and conductivity of the neuromuscular system decreases and muscle weakness occurs.
(1) Skeletal muscle weakness and paralysis
Muscle weakness usually starts in the lower extremities, especially in the quadriceps, and manifests as difficulty walking and unsteadiness in standing; as hypokalemia increases, muscle weakness worsens and involves the trunk and upper extremity muscles until it affects the respiratory muscles and respiratory failure occurs. In general, muscle weakness can occur when the serum potassium concentration is below 3 mmol/L, and paralysis can occur when it is below 2.5 mmol/L. It is also easy to complicate respiratory failure. In patients with pulmonary insufficiency, hypokalemia leading to respiratory failure or exacerbation of respiratory failure is more common, but is easily overlooked clinically.
(2) Smooth muscle weakness and paralysis
It manifests as abdominal distension, constipation, paralytic intestinal obstruction in severe cases, and urinary retention may also occur.
2.Central nervous symptoms
Patients are irritable, mood swings, weakness, and in severe cases, mental depression, drowsiness, confusion and coma.
3.Circulatory system symptoms
Potassium can maintain the function of the heart. During systole, potassium escapes from the cell before actin combines with myosin and ATP, while during diastole, potassium is transferred to the cell, so the concentration of potassium inside and outside the myocardium has an effect on the autoregulation, conduction and endurance of the myocardium.
(1) Arrhythmia: It is related to the abnormalities of excitability of autonomic heart cells and conduction of conduction tissues, mainly manifested by the decrease of excitability of sinus node, the slowdown of conduction of atrioventricular junctional zone, and the increase of excitability of ectopic rhythm cells, so a variety of arrhythmias can occur, including sinus bradycardia, premature atrial or ventricular beats, supraventricular tachycardia and atrial fibrillation, atrioventricular block, and even ventricular tachycardia and ventricular fibrillation. There is also a tendency to digitalis toxicity.
The electrocardiographic manifestations are valuable for the diagnosis of hypokalemia. As hypokalemia worsens, P-wave widening, QRS wave widening and the above-mentioned arrhythmias may appear.
(2) Cardiac insufficiency: The changes in myocardial function and structure caused by severe hypokalemia may directly induce or aggravate cardiac insufficiency, especially in patients with poor underlying cardiac function.
(3) Hypotension: It may be related to vasodilatation caused by phytonadic dysfunction.
4.Rhabdomyolysis
When normal muscle is closed, potassium can be released from the transverse muscle, causing local vasodilation to adapt to energy needs. In severe potassium deficiency, the above role is impaired, and the muscle tissue is relatively ischemic during muscle contraction, and in severe cases, rhabdomyolysis can occur, with a large amount of myosin excreted from the kidney, which can sometimes induce acute renal failure.
5. Acid-base and other electrolyte disorders
In potassium deficiency, the kidney’s ability to reabsorb water decreases, and polyuria and nocturia occur. Thirst occurs due to urination, increased renal NH production, increased acid excretion and increased bicarbonate reabsorption, resulting in metabolic alkalosis. Metabolic alkalosis will result in a corresponding decrease in chlorine retention capacity and a decrease in blood chlorine.
6.Renal function damage
The main pathological changes are renal tubular hypofunction, epithelial cell degeneration, renal interstitial lymphocyte infiltration, and in severe cases, fibrous changes. Clinical manifestations are.
(1) reduced sodium pump activity of renal tubular epithelial cells, reduced intracellular K+, increased hydrogen-sodium exchange, acidic urine, and metabolic alkalosis; increased intracellular Na+, reduced tubular fluid Na+ reabsorption, and hyponatremia.
(2) Decreased concentration function: polyuria, increased nocturia, low specific gravity urine, hypotonic urine, poor response to antidiuretic hormone.
(3) Increased ammonia production capacity, increased acid excretion, increased HCO3- reabsorption, and metabolic alkalosis.
(4) Chronic hyperalgesia. It is more common in patients with chronic, long-term hypokalemia or hypomagnesemia.