Acid-base Balance:- Part 2 – Metabolic acidosis, and Metabolic Alkalosis, Anion Gap

Acid-base Balance

Sample for acid-base balance

1. The better choice is the Radial artery.
   1. May take the sample from the femoral artery or brachial.
   2. Can draw the blood from the indwelling arterial line.
2. The tests are done immediately because oxygen and carbon dioxide are unstable.
   1. Place the sample on ice and immediately transfer it to the lab.
3. Arterial blood is better than venous blood.
4. For a venous blood syringe or tube, be filled, and apply a tourniquet for a few seconds.
5. Arterial blood is risky, and a trained person should do it.
   1. Never apply a tourniquet.
   2. Don’t apply the pull to the plunger of the syringe.

Indications for Acid-base Balance

It is advised in:

1. Diabetes mellitus.
2. Starvation.
3. Lactic acidosis.
4. Ingestion of NH4CL, ethylene glycol, methanol, salicylates, and paraldehyde.
5. In the case of diarrhea.
6. In the case of renal failure.
7. In the case of proximal tubular acidosis.

Precautions for the collection of blood

1. Avoid pain and anxiety in the patient, which will lead to hyperventilation.
   1. Hyperventilation due to any cause leads to decreased CO2 and increased pH.
2. Keep blood cool during transit.
3. Don’t clench your finger or fist. This will lead to lower CO₂ and increased acid metabolites.
4. pCO₂ values are lower in the sitting or standing position than the supine position.
5. Don’t delay the performance of the test.
6. Avoid air bubbles in the syringe.
7. Excess of heparin decreases the pCO_{2 by} maybe 40% less.
8. Not properly mixing the blood before running the test may give a false result.
9. A prolonged tourniquet or muscular activity decreases venous pO₂ and pH.
10. The best way to collect arterial or venous blood is anaerobic.
11. Arterial blood precautions:
    1. Only syringe and needle, no tourniquet, no pull on the plunger.
12. Venous blood precautions:
    1. Needle and syringe of the heparinized evacuated tube filled, drawn a few seconds after the tourniquet.
    2. Liquid heparin is the only suitable anticoagulant with the proper amount.
       1. Less amount will lead to clot formation.
       2. The increased amount will lead to increased CO2 and a decrease in pH.
       3. This will lead to a dilutional error.
    3. Glass collection devices are better than plastic.

Pathophysiology

Definition of acid-base balance:

1. This regulation of the extracellular fluid environment involves the ratio of acid to base, measured clinically as pH.
   1. Physiologically all positively charged ions are called acids, and all negatively charged ions are bases.
2. Physiological changes in the concentration of H⁺ ions in the blood lead to acid-base balance.
3. A systemic increase in the H⁺ ions concentration is called acidosis.
4. A systemic decrease in the H⁺ ions is called alkalosis.
5. The acid-base must be regulated within a narrow range for the body to function normally.
   1. A very slight change in the pH will affect the body.
   2. A slight change in the H⁺ ions can bring changes in the cell and tissue.

H⁺ ions are needed for the following:

1. To maintain the integrity of the membrane.
2. Speed of the metabolic reactions.
3. Any change in the pH will lead to more harmful effects than other diseases.
4. The symbol pH represents the power of H⁺.
5. When pH changes, one unit like 7.0 to 6.0 = [H⁺] [H⁺] = H⁺ ions concentration changes 10 folds.

Body acids are formed from end products of:

1. Metabolism of proteins.
2. Metabolism of Carbohydrates.
3. Metabolism of fats.
4. This must be balanced by the number of basic substances in the body to maintain the normal pH.
   1. Lungs, kidneys, and bones are the major organs involved in regulating acid-base balance.

Body acids are of two types:

1. Volatile acids:
   1. Carbonic acid (H2CO₃) is a weak acid, and it does not easily release the H⁺ ions.
   2. The presence of carbonic anhydrase enzyme can eliminate CO2 gas and water H2O.
   3. CO2 is eliminated through the lungs.
2. Nonvolatile acids:
   1. These are sulfuric acid, phosphoric acid, and other organic acids eliminated through the kidneys.
   2. These are strong acids and readily give up their H⁺ ions.
   3. Nonvolatile acids are secreted into the urine by the renal tubules.
   4. These acids are about 150 meq/L of H+ ions per day or about 1 meq/kg body weight.

Various Buffer Systems and their role in the acid-base system:

• Acid-base control by the various organs of the body.

The pH of different body fluids:

pH significance in our life:

• For normal body functions, the pH range is very narrow and needs to be maintained within these limits.

Acid-base balance types:

1. H⁺ ions and electrolytes disturbances may be:
   1. Acute.
   2. Chronic.
   3. Modest or severe.
   4. Simple or mixed.
2. When there is an accumulation of H+ ions is called acidosis.
   1. When blood pH declines below 7.3, this process is called acidemia.
3. When there is a deficiency of H+ ions is called alkalosis.
   1. Blood pH rises above 7.45 is called alkalemia.
4. There are conditions related to the respiratory system that leads to respiratory acidosis or alkalosis.
5. There are metabolic conditions related to kidneys, and abnormality of intake/output leads to metabolic acidosis/alkalosis.

The blood pH:

1. It is normally maintained at 7.38 to 7.42. Any deviation from this range indicates a change in the H⁺ ions concentration.
2. Blood pH is a negative logarithm of [H⁺], as shown in the following equation:
   1. pH = log₁₀ [H⁺]
   2. This equation shows that an increase in the H⁺ ions will lead to a fall in the blood pH is called acidemia.
   3. So a decrease in the H⁺ ions will lead to an increase in the pH of the blood, called alkalemia.
   4. The conditions which cause the change in the pH are called acidosis and alkalosis.
3. H⁺ ion changes in the blood lead to acid-base imbalance.
   1. A systemic increase in the H+ ions is called acidosis.
   2. In the case of acidemia pH of the arterial blood is <7.4.
4. While in alkalemia, the pH of the arterial blood is >7.4.
   1. A systemic decrease in the H⁺ ions in the systemic blood is called alkalosis.
5. The following diagram can explain how pH is maintained by the arterial carbon dioxide tension  (pCO2)  and plasma bicarbonate (HCO3^–).

Acid-base balance and diet

6. Plasma HCO3^– decrease in the plasma caused by gastrointestinal or renal losses will increase H⁺ ions and lowers the pH.

Acid-base buffer system

Acid-base balance, the role of lungs and kidneys

Anion gap:

Definition of the anion gap:

1. Anion gap refers to anions usually not measured in the laboratory, like sulfate, phosphate, and lactate.
2. The anions usually measured are Chloride (Cl^–) and bicarbonate (HCO3^–).
3. The sum of the anions is subtracted from the sum of cations (Na⁺ ); there is a gap of around 10 to 12 meq/L, which is called an anion gap. An elevated anion gap gives clues for acidosis.
4. The anion gap is measured in meq/L.
5. This is the difference between the plasma concentration of major cation sodium (Na⁺) and other anions are HCO3^– and Cl^–.
   1. Anion gap = [Na⁺] – ([HCO3^–] + [Cl^–])
6. The normal anion gap is 3 to 13 meq/L, and the mean is 10 meq/L.
7. This is dependent mainly on the plasma protein, primarily albumin.
8. 2.5 meq/L falls for every 1 gram/dl of albumin concentration in the blood.
9. The importance of the anion gap is to identify the etiology of metabolic acidosis.

Causes of a high anion gap (>12 meq/L):

1. Methanol toxicity.
2. Uremia due to renal failure.
3. Starvation.
4. Diabetes mellitus (ketoacidosis).
5. Lactic acidosis.
6. Salicylates toxicity.
7. Ethyl alcohol toxicity.
8. Isoniazid toxicity.
9. Iron toxicity.

Causes of decreased anion gap (<6 meq/L):

1. Hypoalbuminemia.
2. Plasma cell disorders.
3. Bromide intoxication.

Causes of normal anion gap (6 to 12 meq/L):

1. Intestinal fistula.
2. Pancreatitis.
3. Renal tubular acidossis.
4. Acid and chloride administration:
   1. NH4Cl, and HCL for the treatment of severe metabolic acidosis.
   2. Hyperalimentataion.
5. Bicarbonate or other alkali losses:
   1. Diarrhea.
   2. Recovery from ketoacidosis.

Metabolic acidosis

Definition of metabolic acidosis:

1. Metabolic acidosis occurs whenever there is a primary decrease in the HCO3¯ in the blood.
2. This may occur due to:
   1. Exogenous acid administration.
   2. Endogenous acid production.
   3. Impaired renal H⁺ secretion.
   4. HCO3^– losses from the kidney or in the gastrointestinal secretions.

Mechanism of metabolic acidosis

Causes of metabolic acidosis:

1. In metabolic acidosis, noncarbonic acid increases, or HCO3¯ is lost from the extracellular space.
   1. The buffering system becomes active and maintains the pH.
   2. In case of the buffering system’s failure, the anion gap  HCO3¯: H2CO3 = 20:1 changes.
2. Increased noncarbonic acid with an elevated anion gap and Increased H⁺ load:
   1. Diabetes mellitus with ketoacidosis. There is a production of acetoacetic acid and β-hydroxybutyric acid in diabetic acidosis.
   2. In the case of starvation.
   3. Lactic acidosis in shock and hypoxemia. There is the production of lactic acid.
   4. Ingestion of drugs like NH4CL, salicylates, methanol, ethylene glycol, and paraldehyde.
3. Decreased H⁺ ions excretion was seen in:
   1. Uremia.
   2. Distal renal tubular acidosis (decreased renal H⁺ secretion).
      1. There is an accumulation of the acid that consumes the bicarbonate (HCO3¯).
4. Bicarbonate (HCO3¯) loss from the extracellular space and normal anion gap:
   1. Renal failure.
   2. Diarrhea.
   3. Proximal tubular acidosis (there is renal HCO3¯ loss).
      1. Plasma HCO3¯ falls, associated with a rise in the concentration of the inorganic anions, mostly CL¯ or a fall in Na⁺ concentration.

Biochemical changes in metabolic acidosis:

Compensation for metabolic acidosis:

1. Hyperventilation by rapid breathing from the lungs will blow off CO2.
2. Kidneys will conserve HCO3¯ and eliminate H⁺ ions in the urine, where urine will be acidic.

Acid-base balance: Compensatory metabolic acidosis

Signs and symptoms of metabolic acidosis:

1. Kussmaul respiration suggests metabolic acidosis.
2. The early symptom is a headache and lethargy.
3. There is anorexia, nausea, vomiting, diarrhea, and abdominal discomfort.
4. If acidosis progresses, then ultimately, the end is death.
5. The patient can have neurological, respiratory, gastrointestinal, and cardiovascular signs and symptoms.
6. Deep rapid respiration indicates respiratory compensation.
   1. There is increased tidal volume rather than respiratory rate; this characterizes these ventilatory changes resulting from stimulating the brain stem respiratory center by the low pH.
   2. Decreased blood pH leads to:
      1. Decreased myocardial contraction, causing decreased blood pressure.
      2. Arterial vasodilatation.
      3. pH below 7.15 to 7.20, the effect of acidemia is prominent.
7. Ketoacidosis is associated with increased thirst and polyuria.
8. There is secondary hypotension in severe acidotic patients.
9. Severe acidosis produces life-threatening dysrhythmias, like ventricular fibrillation.
10. Ultimately the patient will go into a coma.

Diagnosis of metabolic acidosis:

1. Take the history of the patient.
2. There are clinical signs and symptoms.
3. Lab. findings are:
   1. pH = <7.35. (low pH).
   2. HCO3¯ = <24 meq/L (low plasma bicarbonate).
   3. Anion gap = >14 meq/L  seen in:
      1. High-anion metabolic acidosis.
      2. Lactic acidosis.
      3. Ketoacidosis.
      4. Asprin over-dose.
      5. Renal failure.
      6. Overuse of alcohol.
   4. Anion gap = 12 meq/L or less is seen in:
      1. Increased acid load.
      2. Rapid I/V saline administration.
      3. Other diseases are characterized by HCO3^– loss.
4. Urine pH = <4.5 in the absence of renal disease.
5. Lactic acid = Increased (in lactic acidosis).
6. There may be concomitant hypokalemia or hyperkalemia, which helps in the diagnosis.

Treatment of metabolic acidosis:

1. Until arterial pH falls below 7.15 to 7.20, acidemia’s adverse effect is usually compensated for by elevated plasma catecholamines.
2. In case of severe acidosis, give NaHCO3 to elevate the pH.
3. Correct the sodium and water deficiency.
4. Give lactate ringer solution.
5. Correct the electrolyte imbalance.
6. Try to treat the underlying cause of the acidosis.
7. In case mechanical ventilation may be needed.
8. May need dialysis for patients with renal failure.
9. Needs antibiotics to treat the infection.

Metabolic Alkalosis

Definition of metabolic alkalosis:

1. There is excessive loss of metabolic acids.
2. An increase in the plasma HCO3‾.
3. An arterial pH >7.4 leads to metabolic alkalosis.

Causes of metabolic alkalosis:

1. This common condition is often induced by diuretic therapy or loss of gastric secretions (in vomiting or nasogastric suction).
2. This condition is caused by:
   1. There must be an initial increase in the HCO3^– level caused by the H⁺ ions loss in the gastrointestinal secretions or the urine.
   2. H⁺ ions move into the cell.
   3. Akali administration.
      1. Volume contraction around a relatively constant amount of extracellular HCO3^-.
3. One of the following factors in case of absence of renal failure to maintain high HCO3^–:
   1. Chloride (Cl^–) depletion.
   2. Hypochloremia or hypokalemia.
   3. Effective circulating volume depletion.
4. This occurs in the acid loss by vomiting or nasogastric suction.
   1. Pyloric or upper duodenal obstruction.
   2. In the case of villous adenoma.
5. Prolonged diuretic therapy.
6. Cystic fibrosis.
7. Primary Hyperaldosteronism leads to retention of the NaHCO3 and loss of H+ and K+.
8. Secondary hyperaldosteronism.
9. Bilateral adrenal hyperplasia.
10. Congenital adrenal hyperplasia.
11. Cushing’s syndrome.
12. Pituitary adenoma secreting ACTH (Cushing’s syndrome).
13. Exogenous cortical therapy.
14. Excessive licorice ingestion.
15. Diuretics also produce mild alkalosis because they produce excretion of Na⁺, K+, and CL¯ than HCO3¯.
16. Milk-alkali syndrome.
17. Massive blood transfusion.
18. High doses of carbenicillin or penicillin.

Changes in metabolic alkalosis:

Summary of metabolic alkalosis:

1. Before alkalosis HCO3: H2CO3 ratio is 20:1.
2. Then pH increases, PCO2 no change, and HCO3¯ also increases.
   1. HCO3: H2CO3 = 40 :1
3. HCO3¯ increases because of the loss of CL¯ ions or excess ingestion of NaHCO3.

Compensatory mechanism of metabolic alkalosis:

1. Breathing will be suppressed to hold the CO2.
   1. The increase in the pH depresses the respiratory center causing retention of CO2, which will increase H2CO3 and CO2.
2. The kidney will conserve H⁺ ions and excrete more HCO3¯ in the urine, which will be alkaline.

Metabolic alkalosis

Acid-base balance: Metabolic alkalosis and compensatory mechanism

Signs and symptoms of metabolic alkalosis:

1. The patients are irritated, twitching, and confused.
2. There are nausea, vomiting, and diarrhea.
3. Some patients may have severe cramping, paresthesia, or even tetany, but in others with similar electrolytes, data have no such S/S; the reason is unknown.
4. Ask about the history of vomiting or diuretic therapy.
5. There is a weakness.
6. There are muscle cramps.
7. There are hyperactive reflexes.
8. There is shallow and slow respiration. There are cyanosis and apnoea.
9. There will be tetany.
10. The patient will have confusion and convulsions.
11. There are cardiovascular abnormalities due to hypokalemia.
    1. Ultimately patient will have atrial tachycardia.

Lab diagnosis of metabolic alkalosis:

1. The arterial blood shows increased pH and HCO3¯.
2. pH = >7.45.
3. HCO3^– = >29 meq/L.
4. K⁺ = <3.5 meq/L (low).
5. Calcium  (Ca⁺⁺)= <8.9 mg/dl.
6. Chloride (Cl^–) = <98 meq/L..
7. There may be an increased anion gap.
8. Measurement of the Na⁺ in a random urine sample differentiates urinary volume depletion Na⁺ <20 meq/L and euvolemic Na⁺ >40 meq/L.
   1. Metabolic alkalosis is the condition in which volume depletion may not lead to a low urinary Na⁺.
   2. The capacity to retain the Na⁺ in this situation is maybe antagonized by the need to excrete HCO3^– (as Na+ salt) to correct the alkalosis.
   3. In such cases, a random urinary Cl^– determination is more useful.

Summary of metabolic alkalosis:

Treatment of metabolic alkalosis:

1. In the case of mild alkalosis, the patient can tolerate it.
2. In the case of severe cases of pH >7.6, urgent treatment is needed.
3. Can give KCl and normal saline.
4. Discontinue diuretics and supplementary KCl.

Table showing characteristic features of acidosis and alkalosis:

Panic values:

Summary of the parameters needed for the acid-base balance:

Summary of metabolic and respiratory acidosis/alkalosis changes in the blood:

Acid-base balance summary

Questions and answers: