Metabolic acidosis

Introduction The pH of the human arterial blood is the negative logarithm of the H concentration in the blood, which is normally 7.35 to 7.45, and the equilibrium value is 7.40. There is very little H in body fluids, mainly due to endogenous metabolism. The body has a fairly complete adjustment mechanism for acid-base load, including buffering, compensation and corrective action. Metabolic acidosis is the most common acid-base balance disorder characterized by a primary HCO3-reduction (<21 mmol/L) and a reduced pH (<7.35). Metabolic acidosis, the symptoms of acidosis, is initially inconspicuous and is often masked by its primary disease.

Cause

Cause:

1. Tissue hypoxia:

In the absence of oxygen, the glycolysis process is strengthened, lactic acid production is increased, and accumulation due to insufficient oxidation process leads to an increase in blood lactate levels, which leads to acidosis. This acidosis is very common, and tissue hypoxia caused by various causes can be caused.

2. Diabetes factors:

Ketoacidosis is a condition in which a large amount of body fat is used, such as diabetes, hunger, vomiting symptoms in a long-term pregnancy reaction, alcoholism and vomiting, and food intake in the liver for several days. Ketone formation increases and exceeds the amount of extrahepatic utilization, resulting in ketosis. Ketone bodies include acetone, beta-hydroxybutyrate, acetoacetate, the latter two being organic acids, leading to metabolic acidosis. This acidosis is also an increase in normal blood chlorine metabolic acidosis.

3. Severe diarrhea:

As diarrhea loses a lot of alkaline substances, it causes water, electrolyte and acid-base balance disorders.

mechanism:

1. The buffering effect of blood and the compensation of intracellular buffer:

In metabolic acidosis, the increased H+ in the blood can be immediately buffered by the plasma buffer system. Through the above reaction, plasma HCO3- and buffer base are consumed, and the generated H2CO3 can be excreted by the lungs. The intracellular buffer is mostly acidosis for 2 to 4 hours. After the occurrence, the increased H+ in the extracellular fluid is transferred to the cells, which is buffered by the intracellular buffer base, and the intracellular K+ is transferred to the outside of the cell to maintain the internal and external electrical balance of the cells, so acidosis easily causes hyperkalemia.

2, lung regulation:

Increased blood H+ concentration stimulates carotid body and aortic chemoreceptors, reflexively causes respiratory center excitement, significantly increases lung ventilation, causes PaCO2 to decrease secondaryly, maintains HCO3-/H2CO3 ratio close to normal, makes blood The pH value tends to be normal, and the compensatory response of the breath is very rapid. Generally, the respiratory enhancement occurs after 10 minutes of acidosis, and the compensation is reached after 30 minutes, and the compensation peak is reached after 12 to 24 hours. The maximum limit of compensation is PaCO2 reduced to 10 mmHg. (1.33kPa).

3, kidney regulation:

In addition to metabolic acidosis caused by abnormal renal function, metabolic acidosis caused by other causes, the kidney exerts an important compensatory function through acid and alkali retention, and carbonic anhydrase activity in renal tubular epithelial cells increases during acidosis, promoting kidney Small tube secretion of H+ and reabsorption of HCO3- increased, phosphate acidification increased, but renal tubular secretion of NH4+ is the most important compensation mechanism, renal compensation generally exerts the maximum effect within 3 to 5 days, and potassium balance often occurs during acidosis. Great influence, acidosis, early kidneys reduce K+ excretion, but then increase significantly, this is due to acidosis, blood potassium rise, stimulate aldosterone secretion and inhibit the reabsorption of HCO3- in the proximal tubules, so that the distal tubules are reached. Increase, so that the urinary potassium excretion increases.

an examination

Related inspection

Serum Selenium (Se) Carbon Dioxide Partial Pressure (PCO2, PCO2) Total Carbon Dioxide (TCO2) Standard Bicarbonate (SB) Buffer Base (BB)

First, medical history

There are many reasons for metabolic acidosis. Diseases such as diabetes and renal failure are combined with metabolism to overcome acidosis, marking the disease has reached a serious stage. Severe diarrhea or intestinal thinness leads to a large number of digestive fluids; excessive intake of acid or acid-forming substances, enhanced anaerobic glycolysis, and increased catabolism and reduced anabolism should also be noted.

Metabolic acidosis generally has no specific symptoms, respiratory activity and facial flushing are often overlooked, followed by carving, nausea, weakness and burnout. Cardiovascular changes can occur when severe acidemia occurs, such as ventricular arrhythmias, decreased ventricular function, decreased blood pressure or even shock, and symptoms of central nervous system disorders such as lethargy or coma.

Second, physical examination

Respiratory activity is a compensatory response to the body during acidemia. Examination can reveal that the breathing is deep and fast, and the ventilation per minute can be increased by 4-8 times, even if the patient feels difficulty breathing or respiratory distress.

In the case of acidemia, the peripheral blood vessels dilate, so the facial skin is flushed. As cardiovascular susceptibility to catecholamines is reduced, ventricular function is reduced, myocardial contractility is reduced, blood pressure is reduced, and even shock occurs. If a ventricular arrhythmia occurs, the pulse and electrocardiogram will respond. Generally, it is considered that the occurrence of ventricular arrhythmia in acidemia is related to hyperkalemia.

In acidemia, anorexia and nausea can cause weight loss, and the nervous system is mild and weak, and severe cases can cause drowsiness and coma.

Long-term or recurrent acidosis can cause various bone diseases. In children, it can affect the growth and development of the body, and even fibrosis and rickets, and osteomalacia or osteoporosis occurs in adulthood. Henderson's test: For patients with deep, fast, and suspected metabolic acidosis, stop breathing and stop breathing as a breath holding. If the breath holding time is less than 20 seconds, it is a virtue. The pine test was positive. This positive generally indicates that the patient is in a severe or severe metabolic acidosis state because the HCO3- is reduced, the pH is lowered, the respiratory center excitability is enhanced, respiratory activity is enhanced, and H2CO3 compensatory discharge is increased.

Third, laboratory inspection

(1) Blood gas analysis:

Blood gas, PCO2, SB, AB, BB, BE and other values can be obtained by blood gas analysis, and various simple and mixed acid-base balance disorders can be basically clarified. Metabolic acidosis is characterized by a decrease in pH. SB, AB, BB, and BE have fewer primary bases and AB equals SB, and PCO2 compensatory decreases.

(2) Determination of anion gap (AG):

The AG value of metabolic acidosis can be normal or increased, but will not decrease. AG is hyperchloremic metabolic acidosis when normal, and normal blood chloride metabolic acidosis when AG is increased. The determination of AG contributes to the analysis of the causes of metabolic acidosis.

(3) Other:

If the AG value is high, there are diseases or factors causing lactic acidosis. Exogenous supplementation of HCO3- can not increase the plasma HCO3- concentration steadily, suggesting that the acid is continuously produced in the body. Highly suspected lactic acidosis, plasma lactic acid is determined. Level, more than 4mmol / L can diagnose lactic acidosis.

The diagnosis of ketoacidosis needs to prove the presence of ketosis. The blood ketone body can be tested with a nitroprusside tablet. This reagent reacts with a 1:1 diluted serum. If it is "4+", it strongly indicates ketoacidosis. Since this reagent reacts only with acetic acid and acetone, it does not react with -hydroxybutyric acid. When diabetic ketoacidosis occurs, the hydroxy acid accounts for about 75% of the circulating ketone body, and the detection of -hydroxybutyric acid is difficult. An indirect method for reflecting all ketone bodies is to add a few drops of hydrogen peroxide to the urine sample, convert the -hydroxybutyric acid to acetic acid by a non-enzymatic reaction, and then detect it with a nitroprusside tablet. Diabetic acidosis is the most common ketoacidosis in the clinic.

Fourth, equipment inspection

At present, the domestic blood gas analyzer has been quite popular, and it is convenient, fast and accurate to detect with this instrument. A small amount of blood can be used to obtain all the data in a short time. Since the price of the instrument is still high, some hospitals fail to purchase, and blood pH, HCO3- and PCO2 are still measured separately. Whether the measurement results are accurate should be verified.

1. Calculation and verification method:

Two of the three values of pH, HCO3- and PCO2 are substituted into the equation: [H+]=24X PCO2HCO3 or the unit of [H+] is nmol/L, and the unit of PCO2 is mmHg (calculated as kPa in mmHg, kPa = 0.1333 × mmHg), and the unit of HCO3- is mmol/L. The calculated third value is consistent with the actual measured value, indicating that the measurement result is reliable, otherwise the measurement result is inaccurate, and at least one of the measurement results is incorrect. In the detection of the blood gas analyzer, it actually measured [H] and PCO2, and HCO3- was obtained by calculation.

2. Search verification method:

Use the Siggaand-Andersen acid-base equilibrium nomogram to verify that if the three values measured above are connected in a straight line or an approximate line on the graph, the measured values are credible, otherwise at least one of them is wrong.

Differential diagnosis

Differential diagnosis:

First, respiratory alkalosis

Respiratory alkalosis can cause a range of symptoms, including dizziness, due to increased stress in the central and peripheral nervous systems. The limbs and the area around the mouth are abnormal, muscle spasms, hands and feet, etc., may have swelling or pain in the chest. In addition, various supraventricular and ventricular arrhythmias can occur.

Respiratory alkalosis can cause cerebral blood flow to decrease, and cerebral blood flow is also one of the causes of abnormal nervous system function. The experiment reported that when the PCO2 decreased by 2.6 kPa (20 mmHg), the cerebral blood flow could be reduced by 35%-40%.

Abnormalities in nervous system function mainly occur in acute respiratory alkalosis, but rarely occur in chronic respiratory alkalosis.

Second, respiratory acidosis

In addition to the symptoms of respiratory diseases and respiratory function compensation, patients with severe respiratory acidosis often have a variety of neurological symptoms, such as headache, blurred vision, restlessness, irritability, further development to tremors, paralysis and lethargy, coma (The so-called carbon dioxide anesthesia). As acidemia causes cerebral vasodilation and increased cerebral blood flow, it can cause elevated cerebrospinal fluid pressure and papilledema. HCO3- in the blood is slow through the blood-brain barrier, and carbon dioxide can quickly cross the blood-brain barrier and reach equilibrium. Therefore, when acute hypercapnia, ie acute respiratory acidosis, the pH in the brain and cerebrospinal fluid is lower than that of chronic respiration. Acidosis is obvious, and the decrease of PH in brain and cerebrospinal fluid is greater than that of arterial blood. Therefore, neurological dysfunction in acute respiratory acidosis is more obvious than chronic respiratory acidosis and metabolic acidosis.

Respiratory acidosis often involves metabolic acidosis, and the combined arterial blood pH can be significantly reduced. For example, lowering to 7.10 or less often causes severe hypotension due to arrhythmia and peripheral vasodilation. At this time, the decrease in pH, that is, acidemia, weakens the contractile reactivity of blood vessels to catecholamines, so it is difficult to increase blood pressure by using a vasculature before the pH is restored to 7.15-7.20.

Chronic respiratory acidosis often associated with pulmonary heart disease and systemic edema. However, cardiac output and glomerular filtration rate are often normal or near normal.

Third, metabolic alkalosis

Severe metabolic alkalosis can cause functional abnormalities in the peripheral nerves of the central nervous system, such as paresthesia, muscle spasms, headaches, and irritability. Ankle and disturbance of consciousness, the above-mentioned abnormalities of central nervous function are obvious in patients with alkalosis after hypercapnia, because patients with chronic hypercapnia, increased PaCO2, and increased compensatory HCO3- in plasma and cerebrospinal fluid, when mechanical ventilation is applied After treatment, PCO2 decreased rapidly, and HCO3- could not quickly pass through the blood-brain barrier, resulting in a sudden increase in the pH of the cerebrospinal fluid. Therefore, the speed of correcting chronic respiratory acidosis should not be too fast.

Fourth, metabolic acidosis

Respiratory activity is a compensatory response to the body during acidemia. Examination can reveal that the breathing is deep and fast, and the ventilation per minute can be increased by 4-8 times, even if the patient feels difficulty breathing or respiratory distress.

In the case of acidemia, the peripheral blood vessels dilate, so the facial skin is flushed. As cardiovascular susceptibility to catecholamines is reduced, ventricular function is reduced, myocardial contractility is reduced, blood pressure is reduced, and even shock occurs. If a ventricular arrhythmia occurs, the pulse and electrocardiogram will respond. Generally, it is considered that the occurrence of ventricular arrhythmia in acidemia is related to hyperkalemia.

In acidemia, anorexia and nausea can cause weight loss, and the nervous system is mild and weak, and severe cases can cause drowsiness and coma. Long-term or recurrent acidosis can cause various bone diseases. In children, it can affect the growth and development of the body, and even fibrosis and rickets, and osteomalacia or osteoporosis occurs in adulthood. Henderson's test: For patients with deep, fast, and suspected metabolic acidosis, stop breathing and stop breathing as a breath holding. If the breath holding time is less than 20 seconds, it is a virtue. The pine test was positive. This positive generally indicates that the patient is in a severe or severe metabolic acidosis state because the HCO3- is reduced, the pH is lowered, the respiratory center excitability is enhanced, respiratory activity is enhanced, and H2CO3 compensatory discharge is increased.

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