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

May 17, 2022Chemical pathology Lab Tests 2

Arterial Blood Gases

Sample

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.
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.
Arterial blood is better than venous blood.
For venous blood syringe or tube is filled, and apply a tourniquet for a few seconds.
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):

It 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 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.
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)

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.
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.

A blood sample from the central venous catheter:

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.

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^–)	22 to 28 mmol/L 20 to 28 meq/L	23 to 29 mmol/L 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:

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.
Keep blood cool during transit.
Don’t clench your 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 pCO_{2 by}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.
The best way to collect arterial or venous blood is anaerobic.
Arterial blood precautions:
1. Only syringe and needle, no tourniquet, no pull on the plunger.
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)

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

Precautions

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

Definition of the arterial blood gases

Arterial blood gases are a common test that provides useful and potentially life-saving treatment.
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. 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.
The body normally maintains the arterial blood pH within a definite range of 7.35 to 7.45.
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

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

The respiratory buffer system:

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

Buffer system, carbonic acid, and bicarbonate
The carbonic acid level can be measured indirectly by measuring the pCO2 level.
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):

= (HCO³ / H₂CO₃) is the most important buffering system.
This buffer system works in the lungs and as well as in the kidneys.
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.

Bases or substances that can accept H⁺ ions.
1. Strong acids readily give up H⁺, whereas strong base readily accepts H⁺.
Respiratory and metabolic disorder depends on the correct measurement of:
1. O₂
2. CO₂
Acid/base assessed by:
1. 1. Total CO₂
  2. Plasma pH
pCO₂
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:

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

Kidneys role in
the acid-base balance

The body acids exist in two forms:

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

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.

The respiration process supplies oxygen to the tissues and removes the carbon dioxide produced by cellular
metabolic activity.
External respiration:
1. Where oxygen in the air is exchanged at the alveolar level with carbon dioxide in the blood.
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

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

Hydrogen (H⁺) ions and pH:

The H⁺ ions concentration is commonly expressed as the pH, The negative logarithm of H+ ions in the
solution.
The logarithm value means that if the pH changes from 7 to 6, then the H+ ions change tenfold.
As the H⁺ ions increase, the pH decreases; likewise, if the H+ions decrease, then pH increases.
The more H⁺ ions =, the more acidic solution, and the lower the pH.
The lower the H⁺ ions =, the more basic solution and higher pH.
pH <7.4 = acidic.

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

Henderson- Hasselbalch equation give the idea about the blood gas measurement.
1. Henderson-Hasselbalch equation = pH = pKa + log (base/acid)

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
The body makes every effort to maintain the validity of the above equation.
Our body will maintain a pH of 7.4, and pK is constant to alter the bicarbonate and paCO_2

(H2CO3).
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.
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

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:

Lactic acidosis and diabetic ketoacidosis.
1. 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.
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.
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:

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

Blood arterial gases

pH

The pH of a solution is the negative logarithm of the hydrogen ion activity.
1. 1. 1. pH = -logaH⁺.
The acid-base status of the body is assessed by:
1. 1. 1. pH.
    2. pCO2.
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

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.
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

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

pCO2 (Partial pressure of the carbon dioxide CO2)

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.
pCO₂ in the blood is 10% in the plasma and 90% carried by the red blood cells.
With respiration, CO₂ is breathed out, and pCO₂ level drops will depend upon the breathing rate.
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

pCO2 is referred to as the respiratory component in acid-base determination because the lungs control this value.
As the CO₂ level increases, the pH level will decrease.
The pCO₂ level in blood and CSF is a major stimulant to the breathing center in the brain.
As the pCO2 level rises, breathing is stimulated.
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.

In metabolic acidosis, the lungs try to compensate by more blowing of CO₂ to raise pH.
In metabolic alkalosis, the lungs try to compensate by retaining the CO₂ to lower pH.

HCO₃ or CO₂ content

Method to measure CO2 contents:
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.
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.
Total CO₂ = HCO₃¯ + Dissolved CO₂.
The most important buffer system of the plasma is HCO₃¯ / H2CO₃.
It is also present in the RBC but at a lower concentration.
The ratio of base: acid = 20: 1 in plasma.
The kidney regulates HCO3¯ ions, and it is the measure of the metabolic (Renal) component of the acid-base balance.
CO2 contents should not be confused with pCO₂.
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.
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

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.
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.
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

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 O₂and hemoglobin.
3. This is not the O₂ content.
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.
When hemoglobin 92 to 100% carries O2, then perfusion or oxygen supply to the tissue is normal.
With the decrease of the pO₂ level, the saturation of hemoglobin also decreases.
When the O2 saturation is 70% or low, the tissues cannot get adequate oxygen.
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

The actual amount of O₂in the blood is termed the O₂ content.
1. Normally all O₂ is bound to hemoglobin.
About 98% of all O2 delivered to the tissue is transported in combination with the hemoglobin.
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

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

pH and HCO₃^– go in the opposite direction.
pH lower, pCO₂ high, HCO₃^– high.
Seen in respiratory depression due to any cause.
1. Hypoventilation.
2. Excessive retention of CO₂.
  Acid-base balance: Respiratory acidosis and compensatory mechanism

Metabolic acidosis

pH and HCO₃^– go in the same direction.
pH low, pCO₂low, HCO₃^– low.
Seen in diabetes, shock, renal failure, and an intestinal fistula.
Arterial Blood gases: Metabolic acidosis changes and findings

Acid-base balance: Metabolic acidosis and compensatory mechanism

Respiratory alkalosis

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

Metabolic Alkalosis

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