Acid-base Balance – Part 2 – Respiratory Acidosis and Respiratory Alkalosis

September 12, 2020Chemical pathologyLab Tests

Sample

1. The better choice is the Radial artery.
   1. The sample may be taken from the femoral artery or brachial.
   2. Blood can be drawn 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 venous blood syringe or tubes are completely filled and apply a tourniquet for a few seconds.
5. Arterial blood is risky and it should be done by the trained person.
   1. Never apply a tourniquet.
   2. Don’t apply the pull to the plunger of the syringe.

Arterial VS Venous blood

1. Arterial blood (ABG)  gives a good mixture of blood from various areas of the body.
2. Arterial blood color is bright red.
3. Arterial blood measurement gives a better status of the lung oxygenation.
   1. If arterial O2 concentration is normal, indicate lung function is normal.
   2. If mixed venous O2 concentration is low, indicating heart and circulation are failing.
4. Arterial blood gives information about the ability of the lung to regulate the acid-base balance through retention or release of CO2.
   1. The effectiveness of the kidneys in maintaining the appropriate bicarbonate level can also be checked.
5. Venous blood  (VBG) gives information about the local area from where the blood sample is taken.
   1. Venous blood color is dark red.
   2. Metabolism of the extremity varies from area to area.
   3. In shock,  the extremities are cold and less blood perfusion.
   4. During the local exercise of the extremities, as opening and closing the fist with power.
   5. In case if there is an infection of the sample area.
6. A blood sample from the central venous catheter is not a good mix of the blood from various parts of the body.  For well-mixed blood sample should be taken from the right ventricle or the pulmonary artery which is not an easy procedure.
7. A blood sample from the central venous catheter:
   1. Shows low O2 concentration, it means that:
      1. Either the lungs have not oxygenated the arterial blood well.
      2. Or the Heart is not circulating the blood effectively.

Difference between arterial and venous blood:

Precautions for the collection of blood

1. Avoid pain and anxiety to 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 finger or fist. This will leads to lower CO₂ and increased acid metabolites.
4. pCO₂ values are lower in the sitting or standing position in comparison with 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 pCO₂  maybe 40% less.
8. Not proper mixing of the blood before running the test may give a false result.
9. A prolonged tourniquet or muscular activity decreases venous pO₂ and pH.
10. Best way to collect arterial or venous blood anaerobically.
11. Arterial blood precautions:
    1. Only syringe and needle, no tourniquet, no pull on the plunger.
12. Venous blood precautions:
    1. The needle and syringe of the heparinized evacuated tube completely 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 an increase in CO2 and a decrease in pH.
       3. This will leads to a dilutional error.
13. Glass collection devices are better than plastic.

Definition of acid-base disturbance and control:

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 is declining 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.
6. The blood pH is normally maintained at 7.38 to 7.42. Any deviation from this range indicates a change in the H⁺ ions concentration.
   1. Blood pH is a negative logarithm of [H⁺] as shown in the following equation:
      1. 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.
7. The following diagram can give you the concept of how pH is maintained by the arterial carbon dioxide tension  (pCO2)  and plasma bicarbonate (HCO3^–).

Acid-base balance mechanism control

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

   The various systems in Acid-base balance

Indications

1. In the case of chronic lung disease.
2. Cardiopulmonary arrest.
3. Sleep apnea.
4. Myasthenia gravis.
5. Laryngospasm.
6. Chronic obstructive pulmonary disease.

Respiratory acidosis

Pathophysiology:

1. Alveolar ventilation provides the necessary oxygen for oxidative metabolism and eliminates the CO2 produced by these metabolic processes.
2. A decrease in alveolar ventilation in relation to the metabolic production of CO2 produces respiratory acidosis by an increase in H2CO3 acid.

The arterial CO2 tension (or pressure) PaCO2 is >45 mm Hg.

Respiratory acidosis mechanism

1. This is seen in respiratory failure where CO2 accumulates, called hypercapnia.
   1. This condition will raise the pCO2 and causes the pH to drop.
   2. To compensate the HCO3^– will increase, but this is not sufficient to restore the pH to a normal level.
   3. Total CO2 may rise to a very high level of chronic respiratory acidosis.
      

      Respiratory acidosis mechanism

Causes of respiratory acidosis:

1. Acute respiratory acidosis:
   1. This occurs with sudden obstruction to:
      1. The airway.
      2. Chest trauma that damages the respiratory muscles.
      3. Acute paralysis or depression of CNS respiratory center.
   2. HCO3^– rises 1 meq/L for each 10 mmHg rise in pCO₂.
2. Chronic respiratory acidosis:
   1. This chronic respiratory acidosis is difficult to treat as compared to acute respiratory acidosis.
   2. This will take place by:
      1. Chronic obstructive pulmonary diseases like bronchitis, emphysema, pulmonary fibrosis, or scarring.
      2. Accumulation of the CO2 lasting days, weeks, or months, will provoke a sustained increase in HCO3^– generation and leads to enhanced renal excretion of the H+ ions with chronic CO 2 retention.
      3. HCO3^– rises 3.5 meq/L for each 10 mm Hg rise in pCO 2.
      4. The serum level of Na⁺ and K⁺ may be normal or mildly raised.
3. Suppression of the medullary respiratory center:
   1. Sleep apnea.
   2. Sedation medicines.
   3. Cardiopulmonary arrest.
4. Upper respiratory obstruction:
   1. Laryngospasm.
   2. Aspiration of the foreign body or vomitus.
   3. Obstruction in the sleep apnea.
5. Defective respiratory muscle function:
   1. Myasthenia gravis.
   2. Guillain-barre syndrome.
   3. Botulism.
   4. Hypokalemia (severe).
   5. Poliomyelitis.
   6. Myxedema.
   7. Amyotrophic lateral sclerosis.
6. Defect in the pulmonary gas exchange:
   1. Acute respiratory distress syndrome.
   2. Pneumothorax.
   3. Hemothorax.
   4. Severe asthma.
   5. Severe pneumonia.
   6. Chronic obstructive pulmonary disease.

Signs and symptoms:

1. There is often breathlessness.
2. The patient is restless.
3. There is apprehension followed by lethargy.
4. The patient will have disorientation.
5. There are muscle twitching and tremors.
6. Skin will be warm and flushed due to raised CO2 causes vasodilatation.
7. The patient will have convulsions and ultimately goes into a coma.

Respiratory Alkalosis

1. Overbreathing causes excessive CO2 exhaled out and causing the blood pH to rise.
   1. Acute respiratory alkalosis interacts with intracellular and protein buffers before affecting the HCO3^– system.
   2. After the adjustment blood HCO3^– drops 5 meq/L for every 10 mmHg decline in pCO2.
   3. Alkalosis causes plasma proteins to have a more negative charge that in turn binds more ionized Ca^++.
   4. This hypocalcemia increases neuromuscular excitability and leads to tetany.
2. Respiratory alkalosis occurs when there are alveolar hyperventilation and excessive reduction in plasma CO2 levels. This is called hypocapnia.
3. In the case of initial hypoxemia, there is increased ventilation which is mostly mediated by the chemoreceptors in the carotid body, these are located near the bifurcation of the carotid artery.
4. Kidneys compensate by decreasing H+ excretion and HCO3¯ reabsorption.
5. The PaCO2 is <35 mm Hg.
   

   Respiratory alkalosis changes

Causes of respiratory alkalosis:

1. Pulmonary diseases due to hypoxemia:
   1. Pneumonia.
   2. Pulmonary embolism.
   3. Pulmonary edema.
   4. High-altitude syndrome.
   5. Severe anemia.
   6. Congestive heart failure.
2. Stimulation of the medullary (respiratory) center:
   1. Hepatic encephalopathy.
   2. Sepsis with fever.
   3. Salicylates toxication.
   4. Hyperventilation syndrome.
   5. Pregnancy when there is increased progesterone.
   6. Cerebrovascular accidents.
   7. Pontine tumors.
3. Hypermetabolic conditions:
   1. Fever.
   2. Anemia.
   3. Thyrotoxicosis.
   4. Hysteria.
   5. Cirrhosis.
   6. Gram-negative sepsis.

Signs and symptoms:

1. The central and peripheral nervous system is stimulated leading to:
   1. Dizziness.
   2. Confusion.
   3. Tingling of the extremities appears first around the mouth and in the fingers and toes, called circumoral and peripheral paresthesia.
   4. Light-headedness and weakness may occur and progress to unconsciousness.
   5. Convulsions.
   6. Ultimately the patient goes into a coma.
   7. Deep and rapid respirations are the primary symptoms that cause respiratory alkalosis.

Diagnosis:

1. The blood pH is >7.42.
2. Decreased pCO2.
3. HCO3: H2CO3  = 20:0.5
4. Decreased H2CO3 level.

Treatment:

1. Ventilators are needed.
2. Treatment is mostly not needed.
3. It important is to diagnose the cause and treat the underlying disease.

Table showing characteristic features of acidosis and alkalosis: