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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 [Na+] – {[Cl−]+[HCO3−]}		<8	Low
		8 to 16	Normal
		>16	High

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
Respiratory alkalosis	↑	↓	↓	Renal

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

Management of Acidosis

pH < 7.35

Acidosis

Determine the primary disorder Metabolic or respiratory?

Check [HCO3-] and PaCO2

Low [HCO3-] and Low to normal PaCO2

High PaCO2 and High to normal [HCO3-]

Metabolic acidosis

Respiratory acidosis

Check for respiratory compensation Calculate expected PCO2

Check for renal compensation Calculate expected [HCO3-]

Decrease in PaCO2=1.25 x (24- measured HCO3-)?

Acute acidosis? Increase [HCO3-]=0.1 x (measure PaCO2-40)?

Chronic acidosis? Increase [HCO3-]=0.1 x (measure PaCO2-40)?

PaCO2 too low? Mixed metabolic acidosis with respiratory alkalosis

PaCO2 too high? Mixed metabolic acidosis with respiratory acidosis

[HCO3-] too low? Mixed respiratory acidosis with metabolic acidosis

[HCO3-] too high? Mixed respiratory acidosis with metabolic alkalosis

[HCO3-] too low? Mixed respiratory acidosis with metabolic acidosis

E04=[HCO3-] too high? Mixed respiratory acidosis with metabolic alkalosis

Measured PaCO2 is equal to expected value? Compensated metabolic acidosis

Measured [HCO3-] is equal to expected value? Compensated respiratory acidosis

Measured [HCO3-] is equal to expected value? Compensated respiratory acidosis

Click here for the management of metabolic acidosis

Click here for the management of respiratory acidosis

Metabolic Acidosis

Category	Disease		Mechanism
			↑ acid production	Loss of bicarbonate	↓ renal acid excretion	pH	Serum AG	Urine AG	Serum Cl	Volume depletion	HTN
Toxin/Medication	Alcohol Methanol Ethylene glycol Ethanol Isopropyl alcohol Propylene glycol		+			↓	↑
	Paraldehyde					↓	↑
	Toluene		+			↓	Nl or ↑
	Pyroglutamic acid (5-oxoproline)		+			↓	↑
	Salicylates					↓↑	↑
	Metformin					↓	↑
	Isoniazid					↓	↑
	Acetazolamide			+		↓	Nl
	Ammonium chloride					↓	Nl
	Amphotericin B					↓	Nl
	Cholestyramine					↓	Nl
	Lactic acidosis		+			↓	↑
Ketoacidosis	Diabetic		+			↓	↑
	Starvation		+			↓	↑
	Alcoholic		+			↓	↑
Systemic	Infection					↓	↑
	Ischemia					↓	↑
Renal	Uremia					↓	↑
	Ureteral diversion			+		↓	Nl
	Renal failure				+	↓	↑
	Renal tubular acidosis	Type I			+	↓	Nl	↑
		Type II		+		↓	Nl	↓
		Type IV			+	↓	Nl	↑
GI	Diarrhea			+		↓	Nl
	Hyperalimentation					↓	Nl
Endocrine	Addison's disease					↓	Nl
	Post hypocapnea					↓	Nl
Category	Disease					pH	Serum AG	Urine AG		Volume depletion	HTN

Metabolic Alkalosis

Category	Disease	pH	Renin	Urine chloride	Volume depletion	HTN
Exogenous HCO3− loads	Acute alkali administration	↑
	Milk-alkali syndrome	↑
Gastrointestinal origin	Vomiting	↑		↓	+
	Nasogastric tube suction	↑		↓	+
	Gastric aspiration
	Congenital chloridorrhea
	Villous adenoma
Renal origin	Diuretics	↑		↓	+
	Posthypercapnic state
	Hypercalcemia/hypoparathyroidism
	Recovery from lactic acidosis or ketoacidosis
	Nonreabsorbable anions including penicillin, carbenicillin
	Hypomagnesemia	↑		Nl		-
	Hypokalemia	↑		Nl		-
	Bartter's syndrome	↑		Nl		-
	Gitelman’s syndrome
	Renal artery stenosis	↑	↑	Nl		+
	Cushing's syndrome	↑	↓	Nl		+
	Hyperaldosteronism	↑	↓	Nl		+
	Licorice ingestion	↑	↓	Nl		-
Category	Disease	pH	Renin	Urine chloride	Volume depletion	HTN

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
• Arterial blood gas
• Metabolic acidosis
• Metabolic alkalosis
• Respiratory acidosis
• Respiratory alkalosis
• Anion gap