Arterial blood gases

How will you define arterial blood gases?

1. Blood arterial gases measure the balance between Oxygen (O2) and carbon dioxide (CO2), giving information about the function of the lungs.
2. It also measures the acid-base balance in the body.
3. Lunga and kidneys work together to keep the acid-base balance.
4. Blood arterial gases measure various gases in the arterial blood. It will tell the metabolic and respiratory status.
5. Arterial blood gases include:
   1. The partial pressure of oxygen (paO2).
   2. Oxygen saturation.
   3. The partial pressure of carbon dioxide (paCO2).
   4. Bicarbonate level (HCO3^–).
   5. pH level.

What are the blood arterial gases parameters?

The pH of the blood

1. The pH of a solution is the negative logarithm of the hydrogen ion activity.
   1. 1. 1. pH = ^–logaH⁺.
2. The acid-base status of the body is assessed by:
   1. 1. 1. pH.
         2. pCO2.
3. While blood passes through the lung, O2 moves to the blood, and CO₂ enters the lung.

4. As the blood hydrogen concentration increases, the pH decreases; if hydrogen ions decrease, the pH increases.
   1. The decrease of one pH unit represents a 10 times increase in H⁺ activity.
5. The average blood pH of 7.40 is equal to the H⁺ ions concentration of 40 nmol/L.
   1. The lungs and the kidneys regulate the pH of the plasma.

6. The acids found in the blood are carbonic acid (H₂CO₃), dietary acids, keto acid, and lactic acid.
7. pH indicates acidity and alkalinity.
8. In respiratory or metabolic alkalosis, the pH will be high.
9. Respiratory acidosis or metabolic acidosis will decrease the pH value.
   1. pH alkaline when it is >7.4.
   2. pH is acidic when it is <7.35.
   3. Acidemia = pH <7.35
   4. Alkalemia = pH >7.45

How will you discuss the pCO2 (Partial pressure of the carbon dioxide)?

1. pCO₂ measures the partial pressure of CO₂ gas in the blood (arterial blood, plasma, or serum) measured in mm Hg.
   1. This is proportional to the amount of dissolved CO2 or the concentration of the CO2.
   2. pCO2 is a measurement of ventilation capability.
   3. The units are the same as pO2.
2. pCO₂ in the blood is 10% in the plasma and 90% carried by the red blood cells.
3. With respiration, CO₂ is breathed out, and the pCO₂ level drops will depend on the breathing rate.
4. The faster and more deeply one breaths, the more CO₂ is blown off, and the pCO2 level drops.

5. pCO2 is the respiratory component in acid-base determination because the lungs control this value.
6. As the CO₂ level increases, the pH level will decrease.
7. The pCO₂ level in blood and CSF is a major stimulant to the breathing center in the brain.
8. As the pCO2 level rises, breathing is stimulated.
9. When the brain can not cope with increased pCO₂ and cannot blow off excess CO_2, the brain is depressed, ventilation decreases, and the patient goes into a coma.
10. In metabolic acidosis, the lungs try to compensate by blowing more CO₂ to raise pH.
11. In metabolic alkalosis, the lungs try to compensate by retaining the CO₂ to lower pH.

What is the HCO₃ or CO₂ content?

What are the methods to measure CO2 contents?

1. The total CO2 contents are determined from the heparinized arterial or venous blood drawn anaerobically.
   1. This can be done in a vacuum tube or a syringe, which is quickly capped.
   2. The blood is centrifuged, and the plasma is separated.
   3. Next, plasma is analyzed for CO2 by converting HCO3^– and H2CO3 to the gas form.

Write briefly HCO3^– and CO2?

1. Most of the CO₂ contents are HCO₃¯ in the blood because the dissolved amount of the CO2 and H2CO3 contents are very small.
2. The HCO3^– ions can be measured directly as HCO3^– or indirectly by CO 2 contents.
3. Total CO₂ =  HCO₃¯ + Dissolved CO₂.
4. The most important buffer system of the plasma is HCO₃¯ / H2CO₃.
5. It is also present in the RBC but at a lower concentration.
6. The ratio of base: acid = 20: 1 in plasma.
7. The kidney regulates HCO3¯ ions, and it is the measure of the metabolic (Renal) component of the acid-base balance.
8. CO2 contents should not be confused with pCO₂.
9. CO2 contents are indirectly measured by HCO3¯.
   1. HCO₃¯  : Dissolved CO₂ =  25 : 1
   2. Any change in the above equation leads to a change in the pH.
   3. As the HCO3¯ level increases, the pH also increases.

Write briefly about the HCO3¯ level?

1. In metabolic alkalosis, the HCO₃^– level is elevated.
   1. In metabolic acidosis, the HCO₃^– level is decreased.
2. In respiratory acidosis, kidneys attempt to compensate for increased reabsorption of HCO3¯.
   1. In respiratory alkalosis, kidneys excrete more HCO3¯ to lower the pH.

What is the role of pH and Bicarbonate in the acid-base system?

Write briefly about pO_2?

1. Oxygen in the blood is carried in two forms:
   1. Dissolved in plasma = <2%.
   2. Combined with hemoglobin = 98%.
2. This partial pressure of the oxygen gas determines the force it exerts in attempting to diffuse through the pulmonary membrane.
   1. The pO2 reflects the amount of oxygen passing from the pulmonary alveoli to the blood.
3. pO₂ is the measure of the pressure of O₂ present in the plasma.
   1. pO₂ is the indirect measure of O₂ contents of arterial blood.
4. The pO₂ level is decreased in the following ways:
   1. Pneumonia.
   2. Shock lung.
   3. Congestive heart failure.
   4. Congenital heart diseases.
   5. Patient under-ventilated.

Write O2 saturation briefly?

1. O2 saturation indicates % of hemoglobin saturated with oxygen. OR:
   1. This measurement is the ratio between the actual O₂ content of the hemoglobin and the potential maximum carrying capacity of the hemoglobin.
   2. O₂% saturation is the percentage indicating the relationship between O₂ and hemoglobin.
   3. This is not the O₂ content.
2. The combined measurement of:
   1. O₂ saturation.
   2. pO_2.
   3. Hemoglobin.
   4. This indicates the amount of O₂ available to the tissues for oxygenation.
3. When 92% to 100% of hemoglobin carries O2, perfusion or oxygen supply to the tissue is normal.
4. With the decrease of the pO₂ level, the saturation of hemoglobin also decreases.
5. The tissues cannot get adequate oxygen when the O2 saturation is 70% or low.
6. What are the Precautions for O2?
   1. Please avoid smoking or exposure to secondhand smoke or CO (carbon monoxide). In such cases, the COHb level increases.
   2. Avoid the use of paint or varnish.

 What is the O₂ content?

1. The actual amount of O₂  in the blood is termed the O₂ content.
   1. Normally, all O₂ is bound to hemoglobin.
2. About 98% of all O2 delivered to the tissue is transported in combination with the hemoglobin.
3. The following formula calculates O2 contents:
   • • O₂ content = O₂ saturation x Hb x 1.34 + pO2 × 0.003

What are the normal electrolytes and blood gases?

Source 1

pH

• Adult / child = 7.35 to 7.45
• Newborn = 7.32 to 7.49
• 2 months  to 2 years = 7.34 to 7.46
• pH venous blood = 7.31 to 7.41
  

  Body fluids	pH
  Arterial blood	7.38 to 7.42
  Venous blood	7.37
  Cerebrospinal fluid	7.32
  Pancreatic fluid	7.8 to 8.0
  Gastric juice	1.0 to 3.0
  Urine	5 .0 to 6.0

pCO₂

• Adult /child = 35 to 45 mm Hg
• Child <2 years = 26 to 41 mm Hg
  • Venous blood = 40 to 50 mm Hg

HCO₃

• Adult / child = 21 to 28 mEq/L
• Newborn / infants =16 to 24 mEq/L

pO₂

• Adult/child = 80 to 100 mm Hg
• Newborn = 60 to 70 mm Hg
  • Venous blood = 40 to 50 mm Hg

O₂ saturation

• Adult/child = 95 to 100%
• Old people = 95%
• Newborn = 40 to 90%

O₂ content

• Arterial blood = 15 to 22 vol%
• Venous blood = 11 to 16 vol%

Source 2

Source 3

Normal Values of Analytes 

What are the Arterial blood gases?

What are the interpretations of and role of arterial gases in the acid-base balance?

Normal picture =  pH normal,  PCO₂ normal, HCO₃ normal.

• Acidemia means arterial blood pH <7.4.
  • Acidosis means a systemic increase in H⁺ ions.
• Alkalemia means arterial blood pH >7.4.
  • Alkalosis means a systemic decrease in H⁺ ions.

What are the findings of Respiratory acidosis?

1. pH and HCO₃^– go in the opposite direction.
2. pH lower, pCO₂ high, HCO₃^– high.
3. Seen in respiratory depression due to any cause.
   1. Hypoventilation.
   2. Excessive retention of CO₂.

1. pH and HCO₃^– go in the same direction.
2. pH low, pCO₂ low,  HCO₃^– low.
3. Seen in diabetes, shock, renal failure, and an intestinal fistula.

What are the findings of Respiratory alkalosis?

1. pH and HCO₃^– go in the opposite direction.
2. pH is high.
3. pCO₂ low.
4. HCO₃^– is normal or slightly decreased.
   1. Seen in hyperventilation.
   2. Excessive loss of CO2.

What are the findings of Metabolic Alkalosis?

1. pH and HCO₃^– go in the same direction.
2. HCO3 is >30 meq/L.
3. pH high,  pCO₂ high,  HCO₃^– high.
4. Urine pH >7.0 (Unless there is severe hypokalemia).
5. Serum K is usually low.
6. Seen in sodium bicarbonate overdose, prolonged vomiting, and nasogastric drainage.

Interpretation of the various parameters:

Urine pH:

• pH = < than 7.4 = acidosis.
• pH = > than 7.4 = alkalosis.

pCO₂

1. pCO2 is high = It is respiratory acidosis.
2. pCO2 is low = It is metabolic acidosis.
3. pCO2 is low = It is respiratory alkalosis.
4. pCO2 is high = It is metabolic alkalosis.

HCO₃^–

1. HCO₃^– high = It is in respiratory acidosis.
2. HCO₃^–  low = It is in metabolic acidosis.
3. HCO₃^–  low = It is in respiratory alkalosis.
4. HCO₃^–  high = It is in metabolic alkalosis.

Anion gap

Calculation of Anion gap = Na (140 ) + K (4) — Cl (110 ) + HCO3(24) = 10 meq/L

• • Normal anion gap =   10 to 12 meq/L   = < 12

Table showing the values of pH, HCO3, pCO2, and etiology:

What are the critical values?

What is the result of acid-base system changes?

1. Acidosis leads to coma and death due to depression in the CNS.
2. Alkalosis leads to irritability, tetany, and possible death due to the stimulation of the CNS.
3. The acidosis state is more threatening than alkalosis.

How will you summarize the parameters needed for the acid-base balance?