dr. Husnil Kadri, M.Kes

Biochemistry Departement Medical Faculty Of Andalas University

Padang

Arterial Blood Gases

• Aids in establishing a diagnosis • Helps guide treatment plan• Aids in ventilator management• Improvement in acid/base

management allows for optimal function of medications

• Acid/base status may alter electrolyte levels critical to patient status/care

Logistics

• When to order an arterial line --–Need for continuous BP monitoring–Need for multiple ABGs

• Where to place – (with antikoagulant)–A. Radial–A. Femoral –A. Brachial–A. Dorsalis Pedis–A. Axillary

The Components

Desired Ranges:–pH ; 7.35 - 7.45–PaCO2 ; 35-45 mmHg–PaO2 ; 80-100 mmHg–HCO3 ; 21-27–O2sat ; 95-100%–Base Excess ; +/-2 mEq/L

Arterial Blood Gases

• Reflect oxygenation, gas exchange, and acid-base balance

• PaO2 is the partial pressure of oxygen dissolved in arterial blood

• SaO2 is the amount of oxygen bound to hemoglobin

Base Excess

Definition: The amount of a strong acid (like HCl) needed to bring blood to 7.40.

• Assumes 100% oxygenation, 37oC, and pCO2 of 40.

Normal = 0

Used to calculate the metabolic component of an acid-base disturbance.

Base Excess calculations

Calculated the same way, in practice, as SID:Buffer Base (SID) = HCO3

- + A-

HCO3 calculated by pH & pCO2 (blood gas machine)

BE = Buffer Base – “expected buffer base” (expected if pH = 7.4 and pCO2 = 40)

A- calculated using pH & hemoglobin (whole blood)OR A- calculated using albumin & phos (plasma)

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Indicators of hypoxaemia and hypoxiaArterial blood gases Lab Findings

PO2

80-100 mm Hg (normal)

60-80 mm Hg (mild hypoxemia)

40-60 mm Hg (moderate hypoxemia)

<40 mm Hg (severe hypoxemia)

SO2

95%-97% (normal)

<90% (may indicate hypoxemia)

pH7.35-7.45 (normal)

<7.35 (acidemia)

>7.45 (alkalemia)

PCO2

35-45 mm Hg (normal)

>45 mm Hg (hypoventilation)

<35 mm Hg (hyperventilation)

Is it Respiratory or Metabolic?

1. Respiratory Acidosis

2. Respiratory Alkalosis

3. Metabolic Acidosis

4. Metabolic Alkalosis

• Increased pCO2 >50

• Decreased pCO2<30

• Decreased HCO3 <18• Increased HCO3 >30

Compensated or Uncompensated—what does this mean?

1. Evaluate pH—is it normal? Yes

2. Next evaluate pCO2 & HCO3

• pH normal + increased pCO2 + increased HCO3 = compensated respiratory acidosis

• pH normal + decreased HCO3 + decreased pCO2 = compensated metabolic acidosis

Compensated vs. Uncompensated

1. Is pH normal? No2. Acidotic vs. Alkalotic3. Respiratory vs. Metabolic

• pH<7.30 + pCO2>50 + normal HCO3 = uncompensated respiratory acidosis

• pH<7.30 + HCO3<18 + normal pCO2 = uncompensated metabolic acidosis

• pH>7.50 + pCO2<30 + normal HCO3 = uncompensated respiratory alkalosis

• pH>7.50 + HCO3>30 + normal pCO2 = uncompensated metabolic alkalosis

Causes of Acidosis• Respiratory

– Hypoventilation– Impaired gas

exchange

• Metabolic– Ketoacidosis

• Diabetes– Renal Tubular Acidosis

• Renal Failure– Lactic Acidosis

• Decreased perfusion

• Severe hypoxemia

Causes of Alkalosis

• Respiratory– Hyperventilation due

to:• Hypoxemia• Metabolic acidosis• Neurologic

–Lesions–Trauma– Infection

• Metabolic– Hypokalemia

– Gastric suction or vomiting

– Hypochloremia

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Mixed Metabolic Acidosis and Chronic Respiratory Alkalosis

Examples:• Sepsis• Addition of respiratory alkalosis to metabolic

acidosis further decreases HCO3- but pH may

remain normal• Lactic acidosis plus respiratory alkalosis due

to severe liver disease, pulmonary emboli, or sepsis

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Mixed Metabolic Alkalosis and Chronic Respiratory Acidosis

Examples:• Patient with COPD receiving glucocorticoids or

diuretics• pCO2 and HCO3

- are increased by both conditions, but pH is neutralized

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Mixed Alkalosis, Severe

Example:• Postoperative patient with severe hemorrhage

stimulating hyperventilation [respiratory alkalosis] plus massive transfusion and nasogastric drainage [metabolic alkalosis]

17

Mixed Chronic Respiratory Acidosis and Acute Metabolic Acidosis

Examples:• COPD [chronic respiratory acidosis] with

severe diarrhoea [metabolic acidosis]. pH is too low for pCO2 of 55 mmHg in chronic respiratory acidosis, indicating low pH due to mixed acidosis, but HCO3

- effect is offset

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Mixed Metabolic Acidosis and Metabolic Alkalosis

Examples:• Gastroenteritis with vomiting [metabolic

alkalosis] and diarrhoea [metabolic acidosis due to loss of HCO3

-]; surprisingly normal findings with marked volume depletion

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Serum Values in Acid-Base DisturbancesCondition Na+

mmol/LCl-

mmol/LHCO3

-

mmol/L

pCO2 mmHg

pH

Normal 140 105 25 40 7.40

Metabolic acidosis 140 115 15 31 7.30

Chronic respiratory alkalosis 136 102 25 40 7.44

Mixed metabolic acidosis and chronic respiratory alkalosis

136 108 14 24 7.39

Metabolic alkalosis 140 92 36 48 7.49

Chronic respiratory acidosis 140 100-102 28 50 7.37

Mixed metabolic alkalosis and chronic respiratory acidosis

140 90 40 67 7.40

Serum Values in Acid-Base Disturbances

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Condition Na+

mmol/LCl-

mmol/LHCO3

-

mmol/L

pCO2 mmHg

pH

Normal 136-145 100-106 24-26 35-45 7.35-7.45

Metabolic alkalosis 139 89 35 47 7.49

Respiratory alkalosis 136 102 20 30 7.44

Mixed alkalosis, mild 139 92 32 39 7.53

Mixed alkalosis, severe 139 92 32 30 7.63

Mixed chronic respiratory acidosis and acute metabolic acidosis

136 102 22 55 7.22

Mixed metabolic acidosis and metabolic alkalosis

140 103 25 40 7.40

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Summary of Pure and Mixed Acid-Base Disorders

Decreased pH

Normal pH Increased pH

pCO2Respiratory acidosis with or without incompletely compensated metabolic alkalosis or coexisting metabolic acidosis

Respiratory acidosis and compensated metabolic alkalosis

Metabolic alkalosis with incompletely compensated respiratory acidosis or coexisting respiratory acidosis

Normal pCO2

Metabolic acidosis Normal Metabolic alkalosis

Source: Adapted from Friedman HH. Problem-oriented medical diagnosis, 3rd ed. Boston: Little, Brown. 1983

References

• Anisman, S. Base Excess & Strong Ion Theories. ppt. 2003.

• Klee, V. Arterial Blood Gas Analysis.ppt. 2012.• Perkins, J. ABG Interpretation. ppt. 2012.• Rashid, FA. Respiratory Mechanisms in Acid-Base

Homeostasis.ppt. 2005.