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

Sample
- The better choice is the Radial artery.
- The sample may be taken from the femoral artery or brachial.
- Blood can be drawn from the indwelling arterial line.
- The tests are done immediately because oxygen and carbon dioxide are unstable.
- Place the sample on ice and immediately transfer it to the lab.
- Arterial blood is better than venous blood.
- For venous blood syringe or tubes are completely filled and apply a tourniquet for a few seconds.
- Arterial blood is risky and it should be done by the trained person.
- Never apply a tourniquet.
- Don’t apply the pull to the plunger of the syringe.
Arterial VS Venous blood
- Arterial blood (ABG) gives a good mixture of blood from various areas of the body.
- Arterial blood color is bright red.
- Arterial blood measurement gives a better status of the lung oxygenation.
- If arterial O2 concentration is normal, indicate lung function is normal.
- If mixed venous O2 concentration is low, indicating heart and circulation are failing.
- Arterial blood gives information about the ability of the lung to regulate the acid-base balance through retention or release of CO2.
- The effectiveness of the kidneys in maintaining the appropriate bicarbonate level can also be checked.
- Venous blood (VBG) gives information about the local area from where the blood sample is taken.
- Venous blood color is dark red.
- Metabolism of the extremity varies from area to area.
- In shock, the extremities are cold and less blood perfusion.
- During the local exercise of the extremities, as opening and closing the fist with power.
- In case if there is an infection of the sample area.
- 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.
- A blood sample from the central venous catheter:
- Shows low O2 concentration, it means that:
- Either the lungs have not oxygenated the arterial blood well.
- Or the Heart is not circulating the blood effectively.
- Shows low O2 concentration, it means that:
Difference between arterial and venous blood:
Biochemical parameters | Arterial blood | Venous blood |
Use | For blood gases | For all routine lab test |
Color | Bright red | Dark red |
pH | 7.35-7.45 | 7.32-7.43 (7.37) |
pCO2 mmHg | 35 -45 | 41 – 51 |
Bicarbonate mmol/L | 22-28 | 23-29 |
pO2 mmHg | 80-100 | 30 -40 |
O saturation | 95% | 70 to 75% |
Precautions for the collection of blood
- Avoid pain and anxiety to the patient which will lead to hyperventilation.
- Hyperventilation due to any cause leads to decreased CO2 and increased pH.
- Keep blood cool during transit.
- Don’t clench finger or fist. This will leads to lower CO2 and increased acid metabolites.
- pCO2 values are lower in the sitting or standing position in comparison with the supine position.
- Don’t delay the performance of the test.
- Avoid air bubbles in the syringe.
- Excess of heparin decreases the pCO2 maybe 40% less.
- Not proper mixing of the blood before running the test may give a false result.
- A prolonged tourniquet or muscular activity decreases venous pO2 and pH.
- Best way to collect arterial or venous blood anaerobically.
- Arterial blood precautions:
- Only syringe and needle, no tourniquet, no pull on the plunger.
- Venous blood precautions:
- The needle and syringe of the heparinized evacuated tube completely filled, drawn a few seconds after the tourniquet.
- Liquid heparin is the only suitable anticoagulant with the proper amount.
- Less amount will lead to clot formation.
- The increased amount will lead to an increase in CO2 and a decrease in pH.
- This will leads to a dilutional error.
- Glass collection devices are better than plastic.
Definition of acid-base disturbance and control:
- H+ ions and electrolytes disturbances may be:
- Acute.
- Chronic.
- Modest or severe.
- Simple or mixed.
- When there is an accumulation of H+ ions is called acidosis.
- When blood pH is declining below 7.3, this process is called acidemia.
- When there is a deficiency of H+ ions is called alkalosis.
- Blood pH rises above 7.45 is called alkalemia.
- There are conditions related to the respiratory system that leads to respiratory acidosis or alkalosis.
- There are metabolic conditions related to kidneys and abnormality of intake/output leads to metabolic acidosis/alkalosis.
- 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.
- Blood pH is a negative logarithm of [H+] as shown in the following equation:
-
- pH = log10 [H+]
- This equation shows that an increase in the H+ ions will lead to a fall in the blood pH is called acidemia.
- So a decrease in the H+ ions will lead to an increase in the pH of the blood called alkalemia.
- The conditions which cause the change in the pH are called acidosis and alkalosis.
-
- Blood pH is a negative logarithm of [H+] as shown in the following equation:
- The following diagram can give you the concept of how pH is maintained by the arterial carbon dioxide tension (pCO2) and plasma bicarbonate (HCO3–).
- Plasma HCO3– decrease in the plasma caused by gastrointestinal or renal losses will increase H+ ions and lowers the pH.
Indications
- In the case of chronic lung disease.
- Cardiopulmonary arrest.
- Sleep apnea.
- Myasthenia gravis.
- Laryngospasm.
- Chronic obstructive pulmonary disease.
Respiratory acidosis
Pathophysiology:
- Alveolar ventilation provides the necessary oxygen for oxidative metabolism and eliminates the CO2 produced by these metabolic processes.
- 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.
- This is seen in respiratory failure where CO2 accumulates, called hypercapnia.
- This condition will raise the pCO2 and causes the pH to drop.
- To compensate the HCO3– will increase, but this is not sufficient to restore the pH to a normal level.
- Total CO2 may rise to a very high level of chronic respiratory acidosis.
Causes of respiratory acidosis:
- Acute respiratory acidosis:
- This occurs with sudden obstruction to:
- The airway.
- Chest trauma that damages the respiratory muscles.
- Acute paralysis or depression of CNS respiratory center.
- HCO3– rises 1 meq/L for each 10 mmHg rise in pCO2.
- This occurs with sudden obstruction to:
- Chronic respiratory acidosis:
- This chronic respiratory acidosis is difficult to treat as compared to acute respiratory acidosis.
- This will take place by:
- Chronic obstructive pulmonary diseases like bronchitis, emphysema, pulmonary fibrosis, or scarring.
- 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.
- HCO3– rises 3.5 meq/L for each 10 mm Hg rise in pCO 2.
- The serum level of Na+ and K+ may be normal or mildly raised.
- Suppression of the medullary respiratory center:
- Sleep apnea.
- Sedation medicines.
- Cardiopulmonary arrest.
- Upper respiratory obstruction:
- Laryngospasm.
- Aspiration of the foreign body or vomitus.
- Obstruction in the sleep apnea.
- Defective respiratory muscle function:
- Myasthenia gravis.
- Guillain-barre syndrome.
- Botulism.
- Hypokalemia (severe).
- Poliomyelitis.
- Myxedema.
- Amyotrophic lateral sclerosis.
- Defect in the pulmonary gas exchange:
- Acute respiratory distress syndrome.
- Pneumothorax.
- Hemothorax.
- Severe asthma.
- Severe pneumonia.
- Chronic obstructive pulmonary disease.
Signs and symptoms:
- There is often breathlessness.
- The patient is restless.
- There is apprehension followed by lethargy.
- The patient will have disorientation.
- There are muscle twitching and tremors.
- Skin will be warm and flushed due to raised CO2 causes vasodilatation.
- The patient will have convulsions and ultimately goes into a coma.
Respiratory Alkalosis
- Overbreathing causes excessive CO2 exhaled out and causing the blood pH to rise.
- Acute respiratory alkalosis interacts with intracellular and protein buffers before affecting the HCO3– system.
- After the adjustment blood HCO3– drops 5 meq/L for every 10 mmHg decline in pCO2.
- Alkalosis causes plasma proteins to have a more negative charge that in turn binds more ionized Ca++.
- This hypocalcemia increases neuromuscular excitability and leads to tetany.
- Respiratory alkalosis occurs when there are alveolar hyperventilation and excessive reduction in plasma CO2 levels. This is called hypocapnia.
- 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.
- Kidneys compensate by decreasing H+ excretion and HCO3¯ reabsorption.
- The PaCO2 is <35 mm Hg.
Causes of respiratory alkalosis:
- Pulmonary diseases due to hypoxemia:
- Pneumonia.
- Pulmonary embolism.
- Pulmonary edema.
- High-altitude syndrome.
- Severe anemia.
- Congestive heart failure.
- Stimulation of the medullary (respiratory) center:
- Hepatic encephalopathy.
- Sepsis with fever.
- Salicylates toxication.
- Hyperventilation syndrome.
- Pregnancy when there is increased progesterone.
- Cerebrovascular accidents.
- Pontine tumors.
- Hypermetabolic conditions:
- Fever.
- Anemia.
- Thyrotoxicosis.
- Hysteria.
- Cirrhosis.
- Gram-negative sepsis.
Signs and symptoms:
- The central and peripheral nervous system is stimulated leading to:
- Dizziness.
- Confusion.
- Tingling of the extremities appears first around the mouth and in the fingers and toes, called circumoral and peripheral paresthesia.
- Light-headedness and weakness may occur and progress to unconsciousness.
- Convulsions.
- Ultimately the patient goes into a coma.
- Deep and rapid respirations are the primary symptoms that cause respiratory alkalosis.
Diagnosis:
- The blood pH is >7.42.
- Decreased pCO2.
- HCO3: H2CO3 = 20:0.5
- Decreased H2CO3 level.
Treatment:
- Ventilators are needed.
- Treatment is mostly not needed.
- It important is to diagnose the cause and treat the underlying disease.
Table showing characteristic features of acidosis and alkalosis:
Clinical condition | Etiology of the condition | pH (7.37 to 7.43) | HCO3– (19 to 25 meq/L) | pCO2 (38 to 42 mmHg) |
Acute respiratory acidosis |
|
<7.35 | >27 meq/L | 50 to 100 mm Hg |
Chronic respiratory acidosis |
|
<7.35 | >35 | 50 to 100 |
Respiratory alkalosis |
|
>7.45 | 14 to 20 | <30 |
Metabolic acidosis |
|
<7.35 | <15, may become zero | <30 |
Metabolic alkalosis |
|
>7.45 | >27 | 45 to 55 |