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Blood Gas Analysis

Blood gas analysis	Vessel	Range	Interpretation
Oxygen Partial Pressure (pO2)	Arterial	80 to 100 mmHg	Normal
		<80  mmHg	Hypoxia
	Venous	35 to 40 mmHg	Normal
Oxygen Saturation (SO2)	Arterial	>95%	Normal
		<95%	Hypoxia
	Venous	70 to 75%	Normal
pH	Arterial	<7.35	Acidemia
		7.35 to 7.45	Normal
		>7.45	Alkalemia
	Venous	7.26 to 7.46	Normal
Carbon Dioxide Partial Pressure (pCO2)	Arterial	<35 mmHg	Low
		35 to 45 mmHg	Normal
		>45 mmHg	High
	Venous	40 to 45 mmHg	Normal
Bicarbonate (HCO3−)	Arterial	<22 mmol/L	Low
		22 to 26 mmol/L	Normal
		>26 mmol/L	High
	Venous	19 to 28 mmol/L	Normal
Base Excess (BE)	Arterial	<−3.4	Acidemia
		−3.4 to +2.3 mmol/L	Normal
		>2.3	Alkalemia
	Venous	−2 to −5 mmol/L	Normal
Osmolar gap = Osmolality – Osmolarity		>10	Abnormal
Anion gap = [[[Sodium|Na]]+] – {[[[Chloride|Cl]]−]+[[[Bicarbonate|HCO3]]−]} Corrected AG = (measured serum AG) + (2.5 x [4.5 − Alb])		<8	Low
		8 to 16	Normal
		>16	High

Compensation

• There are compensation mechanisms in the body in order to normalizing the pH inside the blood.
• The amount of compensation depends on proper functioning of renal and respiratory systems. However, it is uncommon to compensate completely. Compensatory mechanisms might correct only 50–75% of pH to normal.
• Acute respiratory compensation usually occurs within first day. However, chronic respiratory compensation takes 1 to 4 days to occur.
• Renal compensation might occur slower than respiratory compensation.

Primary disorder	pH	PaCO2	[HCO3−]	Compensation	Compensation formula
Metabolic acidosis	↓	↓	↓	Respiratory	Expected paCO2 = 1.5 x serum HCO3− + 8 ± 2 (Winters' formula) Expected paCO2 = Serum HCO3− + 15
Metabolic alkalosis	↑	↑	↑	Respiratory	Expected paCO2 = 0.5 − 1 increase/ every 1 unit increase in serum HCO3− from 24
Respiratory acidosis	↓	↑	↑	Renal	Acute: HCO3− increases by 1mEq/L for every 10 mmHg increase in paCO2 above 40 Chronic: HCO3− increases by 3.5mEq/L for every 10 mmHg increase in paCO2 above 40
Respiratory alkalosis	↑	↓	↓	Renal	Acute: HCO3− decreases by 2mEq/L for every 10 mmHg derease in paCO2 above 40 Chronic: HCO3− decreases by 5mEq/L for every 10 mmHg decrease in paCO2 above 40

Approach to acid–base disorders

Check pH on ABG

pH < 7.35= Acidosis

pH > 7.45= Alkalosis

Check PaCO2

PaCO2 > 45mm Hg = Respiratory acidosis

PaCO2 Normal or < 35mm Hg = Metabolic acidosis

Check PaCO2

PaCO2 > 45mm Hg = Metabolic alkalosis

PaCO2 < 35mm Hg = Respiratory alkalosis

[HCO3-] > 29

Check [HCO3-]

Normal or slight decrease = Acute respiratory alkalosis

Decreased < 24 = Chronic respiratory alkalosis

Mixed Acid−Base Disorders

Disorder	Key features	Examples
Metabolic acidosis & respiratory alkalosis	High− or normal−AG metabolic acidosis Prevailing PaCO2 below predicted value	Lactic acidosis Sepsis in ICU
Metabolic acidosis & respiratory acidosis	High− or normal−AG metabolic acidosis Prevailing PaCO2 above predicted value	Severe pneumonia Pulmonary edema
Metabolic alkalosis & respiratory alkalosis	PaCO2 does not increase as predicted pH higher than expected	Liver disease Diuretics
Metabolic alkalosis & respiratory acidosis	PaCO2 higher than predicted pH normal	COPD on diuretics
Metabolic acidosis & metabolic alkalosis	Only detectable with high−AG acidosis ∆AG >> ∆[HCO3−]	Uremia with vomiting
Metabolic acidosis & metabolic acidosis	Mixed high−AG & normal−AG acidosis ∆[HCO3−] accounted for by combined change in ∆AG and ∆Cl−	Diarrhea and lactic acidosis Toluene toxicity Treatment of diabetic ketoacidosis

Related Chapters

• Acid–base homeostasis
• Acid–base imbalance
• Acidosis
• Alkalosis
• Arterial blood gas
• Metabolic acidosis
• Metabolic alkalosis
• Respiratory acidosis
• Respiratory alkalosis
• Anion gap