Blood Gas Analysis

Blood Gas analysis

DR. MANSOOR AQIL

ASSOCIATE PROFESSOR,KING SAUD UNIVERSITY HOSPITALS

RIYADH.

Clinical case

pH 7.62

pCO2 30mmHg

PO2 80 mmHg

HCO3 30 mmol/L

Diagnosis

Respiratory Alklosis

• With Metabolic Alklosis

pH = Alkalemia (Alkalosis)

pH = Acidemia (Acidosis)

Maintained within narrow limits

pH 7.36 to 7.44

BLOOD pH

NORMAL

RESPIRATORY COMPONENT METABOLIC COMPONENT

ACID(CO2)

BASE (HCO3)

7.4

ACIDOSIS ALKALOSIS

7.87.0

BLOOD pH

The challenge

Volatile ACID (CO2) &

Fixed acids

7.4

ACIDOSIS

7.87.0

Defense of normal alkalinity

Types of Acids

Volatile acids

• Easily move from liquid to gas state within

the body– Lung can remove

– H2CO3 + renal enzyme H2O + CO2 (both of which are

exhaled)

– Carbon dioxide is therefore considered an acid

Types of Acids

Nonvolatile acids (Fixed acids)

• Cannot be changed to gas state within the

body– Examples

– Keto acids

– Lactic acids

The challenge

Sources of acids:

Volatile acidCO2 + H2O H2CO3 H+ + HCO3

Fixed acids• Organic and inorganic source

– Lactic acid, ketones, Sulfuric and phosphoric acid

• Kidney plays an important role handling fixed acids.

CO2 15000 mmol/day

CO2 + H2O H2CO3 H+ + HCO3-

Noncarbonic acids 70 mmol/day

HYDROGEN ION SOURCES

Acute (minutes to hours)

Ventilation

Buffering

DEFENCE AGAINST pH CHANGE

Acute (minutes to hours)

Long term

Renal excretion

Hepatic metabolism

DEFENCE AGAINST pH CHANGE

Chemical Buffers• The body uses pH buffers in the blood to guard

against sudden changes in acidity

• A pH buffer works chemically to minimize changes in the pH of a solution

Buffer

1. Intracellular Buffers1. Proteins

2. Haemoglobin

3. Phosphate

2. Extracellular Buffers1. Proteins

2. Bicarbonate

BUFFERS

Buffer systems

Intracellular

Phosphate buffers

Protein buffers

Haemoglobin

Amino acidAll proteins

Plasma proteins

Exracellular

Carbonic acid bicarb buffers

Biological systems and Buffering:

The power of a buffer depends on:

1.Concentration of the buffer.

2.Whether the pK is close to the pH of the system.

Bicarbonate buffer systems:


Maintains a ratio of 20 parts bicarbonate to 1 part carbonic acid

Bicarbonate Buffer System

If strong acid is added:

HCl + NaHCO3 = H2CO3 + NaCl

Hydrogen ions released combine with the bicarbonate ions and form carbonic acid (a weak acid)

The pH of the solution decreases only slightly

20

BICARBONATE BUFFER SYSTEM H2CO3 H+ + HCO3

-

• Hydrogen ions generated by metabolism or by ingestion react with bicarbonate base to form more carbonic acid

HCO3-H2CO3

BICARBONATE BUFFER SYSTEM Equilibrium shifts toward the formation of acid

• Hydrogen ions that are lost (vomiting) causes carbonic acid to dissociate yielding replacement H+ and bicarbonate

H+ HCO3-

H2CO3

Bicarbonate Buffer System

If strong base is added:

NaOH + H2CO3 = NaHCO3 + H2O

It reacts with the carbonic acid to form sodium

bicarbonate (a weak base)

The pH of the solution rises only slightly

This system is the only important ECF buffer



pK = 6.1 [HCO3-] = 24 mmol/L


Can eliminate CO2 & H+ by VA

pK is not very close to blood pH of 7.4

Phosphate buffer systems

Phosphate buffer H2PO4- / HPO4

pK = 6.8 and has a low concentration.

Role as intracellular and urinary buffer.

Phosphate buffer systems

H2PO4- / HPO4

-2

Protein buffers:

A. Amino acid residues of proteins take up H+

(pK=7.0) are most important

NH2 NH3-

B. Hemoglobin is important due to high concentration and its increased buffering capacity when deoxygenated.

Relative Buffering power:

Phosphate 0.3

HCO3-

1

Plasma proteins 1.4

Hemoglobin6.5

Relative Buffering power:

Most important buffer is protein. 75% of all buffering power of the body is within cells as protein

Compensation

A. Compensatory response involves the system opposite to the one that caused the primary disturbance.

B. Compensation moves pH towards the original normal value but not completely.

Renal buffering mechanisms

Renal - kidney excretes H+ and

replenishes [HCO3-] .

But, this is a slow process taking hours

to days.



METABOLIC DISORDERS

RESPIRATORY ACIDOSIS

ACID(CO2)

BASE (HCO3)


7.8

7.4

7.0

RESPIRATORY ACIDOSIS

H2O + CO2 H2CO3 H+ + HCO3

-

Cause - hypoventilation

Retention of CO2

Drives equation rightward

Increases both [H+] and [HCO3-]

RESPIRATORY ALKALOSIS

ACID(CO2)

BASE (HCO3)

7.0

7.4

7.8


RESPIRATORY ALKALOSIS

H2O + CO2 H2CO3 H+ + HCO3-

2. Respiratory Alkalosis

cause - hyperventilation

Blows off CO2

Drives equation leftward decreasing both [H+] and [HCO3-]

METABOLIC ACIDOSIS

ACID(CO2)

BASE (HCO3)


7.8

7.4

7.0

Metabolic Acidosis

Deficit in HCO3- and decreased pH

Causes:

1. Increased production of nonvolatile acids.

2. Decreased H+ secretion in the kidney

3. Increased HCO3- loss in kidney

4. Increased Cl- reabsorption by the kidney.

Metabolic Acidosis

Body response is increased

ventilation to blow off excess

CO2

METABOLIC ALKALOSIS

ACID(CO2)

BASE (HCO3)


7.0

7.4

7.8

Metabolic Alkalosis

Primarily due to Increased HCO3- , increased pH

Causes

• Administration of excess HCO3-

• Increased secretion of H+ by kidney and gut• Sudden volume contraction which leads to increased Na+ retention.This

leads to water and HCO3- to follow the Na+

PARTIALLY COMPENSATED RESPIRATORY ACIDOSIS

ACID(CO2)

BASE (HCO3)


7.4

7.87.0

PARTIALLY COMPENSATED RESPIRATORY ALKALOSIS

METABOLIC COMPONENT

ACID(CO2)

BASE (HCO3)

RESPIRATORY COMPONENT

7.4

7.0 7.8

ACID(CO2)

BASE (HCO3

)RESPIRATORY COMPONENT METABOLIC COMPONENT

7.4

7.87.0

PARTIALLY COMPENSATED METABOLIC ACIDOSIS

PARTIALLY COMPENSATED METABOLIC ALKALOSIS

ACID(CO2)

BASE (HCO3)


7.4

7.0 7.8

MIXED ACIDOSIS

ACID(CO2)

BASE (HCO3)


7.8

7.4

7.0

COMPENSATED STATE

ACID(CO2)

BASE (HCO3)


7.4ACIDOSIS ALKALOSIS

7.87.0

pH PaCO2 HCO3-

Acute ventilatory failure (acute respiratory acidosis)

N

Chronic ventilatory failure(compensated respiratory acidosis)

pH PaCO2 HCO3-

Normal

pH PaCO2 HCO3-

N

Acute alveolar hyperventilation(acute respiratory alkalosis)

pH PaCO2 HCO3-

Normal

Chronic alveolar hyperventilation(compensated respiratory alkalosis)

pH PaCO2 HCO3-

Normal

pH PaCO2 HCO3-

Normal

Acute metabolic acidosis

pH PaCO2 HCO3-

Normal

Chronic metabolic acidosis

pH PaCO2 HCO3-

Normal

pH PaCO2 HCO3-

Normal

Acute Metabolic Alkalosis

pH PaCO2 HCO3-

Normal

Chronic metabolic alkalosis

pH PaCO2 HCO3-

Normal

pH PaCO2 HCO3-

Normal

Anion Gap

AG = [Na + ] - [Cl ‾ + HCO3‾ ]

• AG represents unaccounted for anions (R ‾ )

• Normal anion gap = 10

Anion Gap

Cations are Na + & K +

Major Anions are Cl ‾ & HCO3 ‾

Anion gap

Unmeasured Anions vs

Unmeasured Cations

Proteins, mostly albumin 15 mEq/L

Calcium 5 mEq/L

Organic acids 5 mEq/L Potassium 4.5 mEq/L

Phosphates 2 mEq/L Magnesium 1.5 mEq/L

Sulfates 1 mEq/L Totals: 23 mEq/L 11 mEq/L

Rules For Analyzing The ABG’s

• Look at the anion gap.

If elevated and metabolic acidosis is present, it’s anion gap metabolic acidosis.

If not elevated, it’s non-anion gap metabolic acidosis.

If more than 20, there is definitely anion gap acidosis even if the PH and the HCO3 are normal.

Differential diagnosis of metabolic acidosis

Normal anion gap

Renal tubular acidosis

Dirrhoea

Carbonic anhydrase inhibition

Ureteral diversion

Early renal failure

Hydronephrosis

HCL administration

Saline administration

Elevated anion gap

Uremia

Ketoacidosis

Lactic acidosis

Methanol toxicity

Ethylene glycol toxicity

Salicylate

Paraldehyde

DIAGNOSIS OF ACID BASE DISTURBANCE

Determining the predicted “Respiratory pH”

Acute 10 mmHg increase in PCO2 results in pH

decrease of approximately 0.05 units

Acute 10 mmHg decrease in PCO2 results in pH

increase of approximately 0.10 units


First determine the difference between the measured

PaCO2 and 40 mmHg and move the decimal point two

places left.

60 - 40 = 20 X 1/2 0.10

40 – 30 = 10 0.10


1. If the PaCO2 is greater than 40 subtract half of the difference from 7.40

2. ? If this Pt has pH = 7.2

3. ? If this Pt has pH = 7.33

60 - 40 = 20 X ½ =10 = 0.10

pH = 7.40 – 0.10 = 7.30


If the PaCO2 is less than 40 add the difference to 7.40

40 - 30 = 10 0.10

pH = 7.40 + 0.10 = 7.50


pH 7.04

PCO2 76

76 - 40 = 36 X ½ = 18 0.18

7.40 - 0.18 = 7.22


pH 7.21

PCO2 90

90 - 40 = 50 X ½ = 25 0.25

7.40 – 0.25 = 7.15


pH 7.47

PCO2 18

40 – 18 = 22 0.22

7.40 + 0.22 = 7.62

Determining the Metabolic component

RULE

10 mmol/L variance from the normal buffer base represents a pH change of approximately 0.15 units.

pH 7.21

PCO2 90

90 - 40 = 50 X ½ = 0.25

7.40 – 0.25 = 7.15


7.21 -7.15 = 0.06 X 2/3 = 0.04 = 4 mmol/L base excess

pH 7.04

PCO2 76

76 - 40 = 36 X ½ = 0.18

7.40 - 0.18 =7.22


7.22 -7.04 = 0.18 X 2/3 =12 mmol/L base deficit

pH 7.47

PCO2 18

40 – 18 = 22 = 0.22

7.40 + 0.22 = 7.62


7.62-7.47 = 0.15 X 2/3 =10 mmol/L base deficit


Diagnosis of acid base disturbance

1. Examine arterial pH: Is acidemia or alkalemia present?

2. Examine PaCO2: Is the change in PaCO2 consistent with a

respiratory component?

3. If the change in PaCO2 does not explain the change in

arterial pH, does the change in [HCO3–] indicate a

metabolic component?

4. Make a tentative diagnosis (see Table).

Diagnosis of acid base disturbance

5. Compare the change in [HCO3–] with the change in PaCO2. Does a

compensatory response exist (Table)?

6. If the compensatory response is more or less than expected, by definition

a mixed acid–base disorder exists.

7. Calculate the plasma anion gap in the case of metabolic acidosis.

8. Measure urinary chloride concentration in the case of metabolic

alkalosis.

Table . Normal Compensatory Responses in Acid–Base Disturbances.

Disturbance Response Expected Change

Respiratory acidosis

Acute [HCO3–]

1 mEq/L/10 mm Hg increase in PaCO2

Chronic [HCO3–]

4 mEq/L/10 mm Hg increase in PaCO2

Respiratory alkalosis

Acute [HCO3–]

2 mEq/L/10 mm Hg decrease in PaCO2–

Chronic [HCO3–]

4 mEq/L/10 mm Hg decrease in PaCO2

Metabolic acidosis PaCO2

1.2 x the decrease in [HCO3

–]

Metabolic alkalosis PaCO2

0.7 x the increase in [HCO3

–]

Clinical case

pH 7.62

pCO2 30mmHg

PO2 80 mmHg

HCO3 30 mmol/L

Diagnosis

Respiratory Alklosis

• With Metabolic Alklosis

Clinical case

pH 7.15pCO2 22 mm HgPO2 90 mmHgHCO3 9 mmol/L

•Partially compensated Metabolic acidosis

Diagnosis

Clinical case


•Partially compensated Metabolic acidosis

Diagnosis

Clinical case


• Acute respiratory Alklosis

Diagnosis

Clinical case


•Partially compensated Respiratory acidosis

Diagnosis

Thank U

Base Excess/ Deficit

The degree of deviation from normal total body buffer base can be calculated independent of compensatory PCO2 changes

The amount of acid of base that must be added to return the blood pH to 7.4 and PCO2 to 40 at full O2 saturation and 370 C

Documents

Blood Gas Analysis