Quick interpretation of Blood Gases

Dr. P K Maharana

Normal Values

• pH 7.4 ( range 7.34 – 7.44)• H+ 35 – 45 mmol / L

• PaCO2 40mmhg (35-45)

• PaO2 100mmHg( 75 – 100)

• HCO3 24 mEq/L(22- 26)

• Base Excess -2 to +2 mmol /L

pH

• Acidosis pH < 7.35

• Alkalosis pH > 7.45

Changes in pH, PCO2 & HCO3Respiratory Acidosis High PaCO2 ( > 45)

Low pH ( < 7.35)

Normal Bicarbonate

Respiratory Alkalosis Low PaCO2

High pH

Normal Bicarbonate

Respiratory Acidosis with Compensatory Metabolic Alkalosis

High PaCO2

Normal pH

High Bicarbonate

Metabolic Acidosis Normal PaCO2

Bicarbonate Low

Low pH

Metabolic Alkalosis Normal PaCO2

Bicarbonate High

pH high

Metabolic Acidosis with

compensatory Alkalosis PCO2 High

Bicarbonate High

pH Low

Anion Gap

• This is useful in any cause of metabolic acidosis. In plasma, the sum of the cations (sodium plus potassium) is normally greater than that of the anions (chloride plus bicarbonate) by approximately 14 mmol/L (normal range 10-18 mmol/L).

• This is known as the anion gap. In some disorders, either the positive or negative ions may increase, leading to a change in the anion gap

Raised Anion gap Metabolic Acidosis

• Accumulation of acids, for example: 

• Ketoacids in diabetic ketoacidosis (DKA).• Lactic acid, eg shock, infection.• Drugs/toxins, eg salicylates, ethylene glycol,

methanol.

MUDPILES' mnemonic (methanol, uraemia, DKA, paraldehyde, infection/ ischaemia / isoniazid, lactic acidosis, ethylene

glycol/ethanol, salicylates/starvation).

Normal Anion Gap Metabolic Acidosis

• Loss of bicarbonate or ingestion of acid, for example:

• Gastrointestinal tract (GIT) causes, eg diarrhoea, pancreatic fistula.

• Renal tubular acidosis.

• Addison's disease.

• Drugs, eg carbonic anhydrase inhibitors

Arterial Blood Samples

• Contraindications for Radial Artery Puncture :-

• Infection• Unable palpate the

pulse• Coagulation Disorder• Positive Allen Test

• Samples can be drawn from Radial, Femoral, Brachial or from an Arterial Line

Allen Test

Respiratory Acidosis

Causes :- • CNS depression due to Head Injury• CNS depression due to drugs ( sedatives,

narcotics, anesthesia).

• Impaired respiratory muscle function( spinal cord injury, neuromuscular diseases, muscle relaxants)

• Pulmonary Diseases ( pneumonia, atelectasis, pneumothorax, bronchial obstruction, pulmonary oedema,

pulmonary embolism) • Hypoventilation (due to Pain, Abdominal distention,

chest wall injury or deformity)

Treatment of Respiratory Acidosis

• Treatment of Cause.

• O2 therapy

• Respiratory support ( Mechanical Ventilation)

Respiratory Alkalosis

(pH > 7.45, PaCO2 < 35 mmHg)

• Causes :-

• Physiological Hyperventilation( fear, pain, anxiety)

• Medications.( Respiratory Stimulants)

• Increased Metabolic states (fever, sepsis, pregnancy, thyrotoxicosis)

• Central Nervous System Lesions

Treatment

• Treatment of underlying cause.

• Patients should be monitored for exhaustion & respiratory muscle fatigue leading to respiratory failure.

Metabolic Acidosis

(pH <7.35, Bicarbonate < 22 mmol/L)

Causes:-Renal Failure.Diabetic ketoacidosis.Starvation.Anaerobic Metabolism.Ingestion of Salicylates.

Signs & Symptoms

• Nausea, Vomiting

• Kussmaul’s Breathing

• Headache, confusion, lethargy, restlessness, stupor, coma.

• Hypotension, warm flushed skin, Dysrhythmias

Treatment

• Treatment of the cause.

• Presence of metabolic acidosis should spur a search for hypoxia somewhere in the body.

Hypoxia Anaerobic Metabolism Acidosis

The only way is to improve tissue perfusion.

Once tissue perfusion is restored look for other causes.

Treatment Acidosis (cont-)

• Routine use of Sodium Bicarbonate should be avoided as it leads to alkalosis & Hypernatremia.

• It only should be used in bicarbonate-responsive acidosis, like renal failure.

Metabolic Alkalosis

• pH > 7.45, Bicarbonate > 29 mEq /L Causes:-A) Excess of Base Ingestion of Antacids,

Excess use of bicarbonates, Use of Lactate in dialysis.

B) Loss of Acids Protracted vomiting, Gastric suction, hypochloremia, excess use of diuretics, high level of Aldosterone.

Treatment of metabolic Alkalosis

• It is one of the most difficult situation to treat. In a hospitalized patient mostly iatrogenic.

• Acetazolamide ( diamox) is the drug commonly used to treat but the effects are slow.

Case Study

1. pH 7.21, pCO2 60 HCO3 24

a. Normal

b. Respiratory acidosis without compensation

c. Metabolic acidosis with partial compensation

d. Respiratory acidosis with complete compensation

2.pH 7.50 pCO2 29 HCO3 24

A. Normal

B. Respiratory acidosis with compensation

C. Respiratory alkalosis without compensation

D. Metabolic alkalosis with partial compensation

3. pH 7.28 pCO2 40 HCO3 18

A. Respiratory acidosis without compensation

B. Respiratory alkalosis with partial compensation

C. Metabolic alkalosis with partial compensation

D. Metabolic acidosis without compensation

Case Study4.pH 7.45 pCO2 26 HCO3 16

A. Normal

B. Respiratory acidosis fully compensated

C. Respiratory alkalosis fully compensated

D. Metabolic alkalosis fully compensated

5. pH 7.33 pCO2 60 HCO3 34

A. Normal ABG values

B. Respiratory acidosis without compensation

C. Respiratory acidosis with partial compensation

D. Respiratory acidosis with full compensation

6. pH 7.48 pCO2 42 HCO3 30

A. Metabolic acidosis without compensation

B. Respiratory alkalosis with partial compensation

C. Respiratory alkalosis with full compensation

D. Metabolic alkalosis without compensation

Case study7.pH 7.38 pCO2 38 HCO3 24

A. Respiratory alkalosis

B. Normal

C. Metabolic Alkalosis

D. None of the above

8.. pH 7.21 pCO2 60 HCO3 24

A. Normal

B. Respiratory acidosis without compensation

C. Metabolic acidosis with partial compensation

D. Respiratory acidosis with complete compensation

9. pH 7.48 pCO2 28 HCO3 20

A. Respiratory alkalosis with partial compensation

B. Respiratory alkalosis with complete compensation

C. Metabolic alkalosis without compensation

D. Metabolic alkalosis with complete compensation

Question Answer.1.The solution that would be most alkalotic

would be the one with a pH of A. Four

B. Seven

C. Nine

D. Fourteen

2.The normal pH range for blood is

A. 7.00 – 7.25

B. 7.30 – 7.40

C. 7.35 – 7.45

D. 7.45 – 7.55

3.The respiratory system compensates for changes in the pH level by responding to changes

in the levels of:

A. CO2

B. H2O

C. H2CO3

D. HCO3

4. The kidneys compensate for acid-base imbalances by excreting or retaining:

A. Hydrogen ions

B. Carbonic acid

C. Sodium Bicarbonate

D. Water

Question & AnswerAll of the following might be a cause of respiratory acidosis except the followings

A. Sedation

B. Head trauma

C. COPD

D. Hyperventilation

A patient with a prolonged episode of nausea, vomiting and diarrhea has an ABG ordered on admission. You might

expect the results to show

A. Metabolic acidosis

B. Metabolic alkalosis

C. Respiratory acidosis

D. Respiratory alkalosis

A calculated ABG value that indicates excess or insufficiency of sodium bicarbonate in the system is:

A. HCO3

B. Base excess

C. PaO2

D. pH

You are reviewing the results of an ABG. Both the pH and the CO2 values are abnormal. The primary problem is:

A. Respiratory

B. Renal

C. Metabolic

D. Compensation