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Acid-Base Balance:- Part 4 – Arterial Blood gases (Blood Gases), Acid-Base balance Mechanism

May 17, 2022Chemical pathologyLab Tests2

Arterial Blood Gases

Sample

  1. The better choice is the Radial artery.
    1. Can draw the blood 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 tube is 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.

Arterial vs. Venous blood

Arterial blood gases (ABG):

  1. It 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 lung oxygenation.
    1. If arterial O2 concentration is normal, indicate lung function is normal.
    2. If mixed venous O2 concentration is low, indicating the heart and circulation are failing.
  4. Arterial blood gives information about the lung’s ability to regulate the acid-base balance through the
    retention or release of CO2.

    1. Can also be checked the effectiveness of the kidneys in maintaining the appropriate bicarbonate
      level.

Venous blood  gases (VBG)

  1. It 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, such as opening and closing the fist with power.
    5. In case if there is an infection of the sample area.
  2. A blood sample from the central venous catheter is not a good mix of 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.
  3. A blood sample from the central venous catheter:
    1. Shows low O2 concentration, which means that:
      1. Either the lungs have not oxygenated the arterial blood well.
      2. Or the Heart is not circulating the blood effectively.
    2. 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 to 7.45 (7.40) 7.32 to 7.43  (7.37)
      pCO2 mmHg 35  to 45 (40) 41  to  51 (45)
      CO2 contents  meq/L 22 to 28 (25) 24 to 30 (27)
      Bicarbonate (HCO3–)
      1. 22 to 28 mmol/L
      2. 20 to 28 meq/L
      1. 23 to 29 mmol/L
      2. 22 to 30 meq/L
      pO2 mmHg 80 to 100 37 to 43 (40)
      O saturation 95% 70 to 75%

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 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 by 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. 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 an increase in CO2 and a decrease in pH.
      3. This will leads to a dilutional error.
    3. Glass collection devices are better than plastic.

Purpose of the test (Indications)

  1. This test is done on mostly hospitalized patients.
  2. Mostly the patients are on a ventilator or unconscious.
    1. It monitors critically ill nonventilator patients.
  3. For patients in pulmonary distress.
  4. To assess the respiratory (ventilation), metabolic (renal) acid/base, and electrolyte imbalance.
  5. Its primary use is to monitor arterial blood gases and the pH of the blood.
  6. Also used to monitor oxygenation.
  7. Used to qualify a patient for the use of oxygen at home.
  8. This is used as preoperative baseline parameters.

Precautions

  1. Avoid in patients with coagulopathy.
  2. Avoid in a patient with AV fistula.

Definition of the arterial blood gases

  1. Arterial blood gases are a common test that provides useful and potentially life-saving treatment.
  2. The following parameters are measured, and the rest are calculated:
    1. pH will give us information about the acid-base balance.
    2. pO2 will tell us oxygenation (ventilation).
    3. pCO2 will also tell us about the acid-base balance.
        1. pH = 7.35 to 7.45.
        2. pO2 = >80 mm Hg.
        3. pCO2 = 35 to 45 mm Hg.
      1. The above three parameters are really needed to help the patients with their management.

Acid-base system:

The H+ ions concentration is commonly expressed as the pH.

    1. The buffering system becomes active in response to a change in the acid-base status.
    2. Buffers can absorb excessive H+ (acid) or  OH‾ (base) without any
      change in the pH.
  1. The body normally maintains the arterial blood pH within a definite range of 7.35 to 7.45.
  2. The H+ ions concentration must be regulated within the narrow range for the body to maintain the body’s
    normal functionality.

    1. The slight change in the H+ concentration will change the body pH.
    2. H+ is needed for the :
      1. For the maintenance of cell integrity.
      2. It helps in the speed of the enzymatic reaction.

pH maintained by the interaction of three buffer systems in our body are:

The buffer system

  1. It works through the retention or excretion of the H+.
    1. Other minor buffer systems are phosphate and proteins.
      Acid-base buffer system

      Acid-base buffer system

The respiratory buffer system:

  1. It works through Carbonic acid-bicarbonate (H2CO3 – HCO3¯).
    This will cover 80% of the pH control, and the renal system does the rest.
  2. There is a ratio of one part of Carbonic acid (H2CO3) and twenty parts of bicarbonate.
    (HCO3−).

    Buffer system, carbonic acid, and bicarbonate

    Buffer system, carbonic acid, and bicarbonate

  3. The carbonic acid level can be measured indirectly by measuring the pCO2 level.
  4. The lung controls the pCO2.
    1. More CO2 retained and more H2CO3 will lead to acidosis.
    2. Less CO2 and there will be fewer H2CO3 will lead to alkalosis.

Bicarbonate (20 parts) / carbonic acid (one part):

  1. =  (HCO3 / H2CO3) is the most important buffering system.
  2. This buffer system works in the lungs and as well as in the kidneys.
  3. The greater the CO2 partial pressure pCO2, the more carbonic acid (H2CO3) forms.
    Can express this relationship as :
    H2CO3 = 0.03 x pCO2  (mm Hg)
    0.03 represents the solubility coefficient for the CO2 in the water.
    The pCO2 of the arterial blood is around 44 mm Hg. Therefore the amount of H2CO3 is
    equal to 1.2 mmol/L = 0.03 x 40 = 1.2 mmol/L

Acids or chemical substances can donate H+ ions.

  1. Bases or substances that can accept H+ ions.
    1. Strong acids readily give up H+, whereas strong base readily accepts H+.
  2. Respiratory and metabolic disorder depends on the correct measurement of:
    1. O2
    2. CO2
  3. Acid/base assessed by:
      1. Total CO2
      2. Plasma pH
  4. pCO2CO2 and H2O reaction
    1. The lungs can decrease the amount of H2CO3 by blowing off the CO2 and leaving H2O.
      H2CO3    →  CO2 + H2O
    2. The kidneys can reabsorb HCO3¯or regenerate new HCO3¯ from the CO2 and water.
      H2O  + CO2  →  HCO3¯  + H+
      Renal regulation is slow, / Pulmonary regulation is fast.

 

The renal system:

  1. It works through excretion or retention of H+, HCO3–, Na+, K+, and Cl¯.
  2. The distle tubule of the kidney regulates the acid-base balance.
  3. It secretes H+ ions into the urine and reabsorbs the HCO3¯.
  4. H2PO4¯ and NH4+ are also secreted into the urine.
    Kidneys role in the acid-base balance

    Kidneys role in
    the acid-base balance

The body acids exist in two forms:

  1. Volatile can be eliminated as CO2.
  2. Nonvolatile. are sulphuric acid, phosphoric acid, and other organic acids. These are produced by the metabolism of protein, carbohydrates, and fats.
  3. The volatile acid is carbonic acid (H2CO3) which will form by the hydration of the CO2.
    Respiratory acidosis

    Respiratory acidosis and compensation mechanism

    Role of kidneys and lungs in acid-base balance

    Role of kidneys and lungs in acid-base balance

In short, lungs and kidneys, with the help of buffer systems, are the prime regulator of acid-base
balance.

  1. The respiration process supplies oxygen to the tissues and removes the carbon dioxide produced by cellular
    metabolic activity.
  2. External respiration:
    1. Where oxygen in the air is exchanged at the alveolar level with carbon dioxide in the blood.
  3. Internal respiration:
    1. Take place at the tissue level, where oxygen in the blood is delivered to the cells, and
      carbon dioxide is transferred from the cells to the blood for disposal.
Role of lungs in case of Respiratory acidosis

Role of lungs in case of Respiratory acidosis

  1. The medullary center,  the brain stem, controlled respiration by increasing CO2 and decreasing O2.
    Role of brain stem in acid-base balance

    Role of the brain stem in acid-base balance

Hydrogen (H+) ions and pH:

  1. The H+ ions concentration is commonly expressed as the pH, The negative logarithm of H+ ions in the
    solution.
  2. The logarithm value means that if the pH changes from 7 to 6, then the H+ ions change tenfold.
  3. As the H+ ions increase, the pH decreases; likewise, if the H+ions decrease, then pH increases.
  4. The more H+ ions =, the more acidic solution, and the lower the pH.
  5. The lower the H+ ions =, the more basic solution and higher pH.
  6. pH <7.4 = acidic.
  7. pH >7.4 = basic.
    Different body fluids pH value Reason for the pH
    Arterial blood 7.38 to 7.42 There is less H2CO3
    Venous blood 7.37 There is more H2CO3
    Cerebrospinal fluid 7.32 Decrease HCO3¯ and increased CO2
    Urine 5.0 to 6.0 Increased H+ ions excretion
    Pancreatic fluid 7.8 to 8.0 There are HCO3¯
    Gastric contents 1.0 to 3.0 HCL production
  8. Henderson- Hasselbalch equation give the idea about the blood gas measurement.
    1. Henderson-Hasselbalch equation = pH = pKa + log (base/acid)

pH calculation formula

  1. pH calculation formula:
    1. Where pH = 7.4
    2. pKa = 6.1
    3. Base (bicarbonate) = 24
    4. Acid = dissolved PaCO2 = 0.03 x PaCO2 = 0.03 x 40 = 1.2
    5. pH = 7.4 = 6.1 + log(24/1.2) = 6.1 + 1.3 = 7.4
  2. The body makes every effort to maintain the validity of the above equation.
  3. Our body will maintain a pH of  7.4, and pK is constant to alter the bicarbonate and paCO2
    (H2CO3)
    .
  4. The pH is dependent upon the total concentration of:
    1. CO2.
    2. HCO–3.
    3. Dissolved CO2.
    4. H+ ions.
    5. All these are interrelated.
  5. The body normally maintains the arterial pH between 7.35 to 7.45. This takes place through the buffer system of bicarbonate.
    acid-base balance: Role of H+ ions for acidemia and alkalemia

    acid-base balance: Role of H+ ions for acidemia and alkalemia

Buffer pairs Buffer system pK value Chemical reaction
HCO3¯ / H2CO3 Bicarbonate/carbonic acid 6.1 H+ + HCO3 >< H2O + CO 2
HPO4- /H2PO4- Phosphate 6.8 H2PO4 ↔ H+  + HPO4-
Hb / HHb Hemoglobin 7.3 HHb ↔H+ + Hb
Pr- /HPr Protein in blood 6.7 HPr ↔ H+ + Pr-

Common acid/base disorders  examples are:

  1. Lactic acidosis and diabetic ketoacidosis.
      1. Intermediate organic acids are lactic acid and β-hydroxybutyric acid.
      2. The above acids are metabolized to CO2 and water.
    1. These may accumulate and cause acidemia.
  2. Body acids are formed as end products of cellular metabolism. The average person generates acid 50 to 100 meq/day from the metabolism of protein, carbohydrates, and fats and stool loss.
  3. To neutralize this acid formation, the body needs to maintain the pH in the range; in that case, an equal amount of acid needs to be neutralized or excreted.

Now our various buffer systems come into play like:

  1. Retention or excretion of H+ ions.
  2. Respiratory system.
  3. Renal system.
  4. These systems are interrelated and work together.

Blood arterial gases

pH

  1. The pH of a solution is the negative logarithm of the hydrogen ion activity.
        1. pH = -logaH+.
  2. The acid-base status of the body is assessed by:
        1. pH.
        2. pCO2.
  3. While blood passes through the lung, O2 moves to the blood, and CO2 goes into the lung.
Role of the lungs in Acid-base balance

Role of the lungs in Acid-base balance

  1. As the blood hydrogen concentration increases, the pH decreases, and if hydrogen ions decrease, the pH increases.
    1. The decrease of one unit of pH represents a 10 times increase in H+ activity.
  2. The average pH of the blood 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.
Acid-base metabolism control

Acid-base metabolism control

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

pCO2 (Partial pressure of the carbon dioxide CO2)

  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 pCO2 level drops will depend upon the breathing rate.
  4. The faster and more deeply one breaths, the more CO2 is blown off, and the pCO2 level drop.
    Acid-base balance: Role of breathing and pCO2

    Acid-base balance: Role of breathing and pCO2

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

HCO3 or CO2 content

  1. Method to measure CO2 contents:
  2. 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 plasma is separated.
    3. Next, plasma is analyzed for CO2 by converting HCO3– and H2CO3 to the gas form.
  3. Most of the CO2 contents are HCO3¯ in the blood because the dissolved amount of the CO2 and H2CO3 contents are very small.
    1. The HCO3– ions can be measured directly as HCO3– or indirectly by CO 2 contents.
  4. Total CO2 =  HCO3¯ + Dissolved CO2.
  5. The most important buffer system of the plasma is HCO3¯ / H2CO3.
  6. It is also present in the RBC but at a lower concentration.
  7. The ratio of base: acid = 20: 1 in plasma.
  8. The kidney regulates HCO3¯ ions, and it is the measure of the metabolic (Renal) component of the acid-base balance.
  9. CO2 contents should not be confused with pCO2.
  10. 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 increase and the pH also increases.
  11. 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 an increased amount of HCO3¯ to lower the pH.
 Clinical conditions pH Bicarbonate (HCO3) level
Metabolic acidosis decreased decreased
Metabolic alkalosis increased increased
Respiratory alkalosis increased decreased
Respiratory acidosis decreased increased

pO2

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

O2 saturation

  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. pO2.
    3. Hemoglobin.
    4. This indicates the amount of O2 available to the tissues for oxygenation.
  3. When hemoglobin 92 to 100% carries O2, then perfusion or oxygen supply to the tissue is normal.
  4. With the decrease of the pO2 level, the saturation of hemoglobin also decreases.
  5. When the O2 saturation is 70% or low, the tissues cannot get adequate oxygen.
  6. Precaution:
    1. Please avoid smoking or exposure to close second-hand smoke or CO (carbon monoxide). In such cases, the COHb level is increased.
    2. Avoid the use of paint or varnish.

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

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

 Chemicals Arterial Venous
pH 7.35 to 7.45 7.31 to 7.41
pCO2 35 to 45 mm Hg 40 to 50 mm Hg
pO2 75 to 100 mm Hg 40 to 50 mm Hg
O content 15 to 22 % 11 to 16 %
HCO3 21 to 28 meq/L
Anion Gap 3 to 10

Source 3

Normal Values of Analytes 

Blood Venous Arterial
pH 7.32 to 7.43 7.35 to 7.45
Bicarbonate (HCO3) 22 to 26 mmol/L
Albumin 3.5 to 5.0 G/dL
pCO2 Male = 35 to 48 mm HgFemale = 32 to 45 mm Hg
Anion Gap 3 to 10
Oxygen saturation O2 94 to 98% (decrease with age)
O2 content 11 to 16% 15 to 22%
pO2 80 to 108 mm Hg (depends on altitude)
Source 4

Arterial blood gases ordered in routine:

Biochemical parameter Adult Pediatric group
pH 7.35 to 7.45 7.32 to 7.42
pCO2 35 to 45 mm Hg 30 to 40 mm Hg
pO2 >80  mm Hg 80 to 100 mm Hg
O2 saturation >94%
CO2 content 45 to 51 vol% (19.3 to 22.4 mmol/L)
O2 content 15 to 22 vol% (6.6 to 9.7 mmol/L)
Base Excess >2 meq/L (>2 mmol/L)
Base deficit < – 2 meq/L  (< – 2 mmol/L)
HCO3 22 to 26 meq/L (22 to 26 mmol/L)

Interpretations 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.

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.
      Acid-base balance: Respiratory acidosis and compensatory mechanism

      Acid-base balance: Respiratory acidosis and compensatory mechanism

Metabolic acidosis

  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.
    Metabolic acidosis changes and findings

    Arterial Blood gases: Metabolic acidosis changes and findings

    Acid-base balance: Metabolic acidosis and compensatory mechanism

    Acid-base balance: Metabolic acidosis and compensatory mechanism

Respiratory alkalosis

  1. pH and HCO3– go in the opposite direction.
  2. pH high.
  3. pCO2 low.
  4. HCO3– is normal or slightly decreased.
    1. Seen in hyperventilation.
    2. Excessive loss of CO2.
      Arterial Blood gases: Respiratory alkalosis and compensatory mechanism

      Arterial Blood gases: Respiratory alkalosis and compensatory mechanism

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.
    Acid-base balance: Metabolic alkalosis and compensatory mechanism

    Acid-base balance: Metabolic alkalosis and compensatory mechanism

Urine pH:

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

Interpretation of the various parameters:

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.

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:

Clinical condition pH HCO3 pCO2 Etiology
Metabolic acidosis <7.4 low low Diabetic ketoacidosis. lactic acidosis.
Metabolic alkalosis >7.4 high high Vomiting
Respiratory acidosis <7.4 high high COPD, weakness of respiratory muscles
Respiratory alkalosis >7.4 low low Anxiety and pain

Critical values

Biochemical parameter Less than More than
pH 7.25 7.55
pCO2 20 mm Hg 60 mm Hg
HCO3 15 meq/L 40 meq/L
pO2 40 mm Hg
O2 saturation 75% or lower
Base Excess ± 3meq/L

Signs and symptoms summary of the acid-base system changes:

  1. Acidosis leads to coma and death due to depression of 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.

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

Lab test Importance
pH This will tell:

  1. Increased pH value indicates alkalosis
  2. Decreased value of pH indicates acidosis
pCO2 This is the partial pressure of CO2, and it will tell:

  1. The respiration modulates this pCO2
  2. This is the index of ventilation
pO2 This is the partial pressure of the O2 in the arterial blood and tells:

  1. Low values indicate hypoxia
  2. pO2 is the indirect measure of O2 contents of arterial blood.

 


Possible References Used
Go Back to Chemical pathology

Comments

Angelique Breske Reply
September 29, 2020

I agree with you

Charlsie Burruel Reply
October 12, 2020

I agree with you

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