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Approach

Normal range

Blood gas analysis	Vessel	Range	Interpretation
Oxygen Partial Pressure (pO2)	Arterial	80 to 100 mmHg (10.6 to 13.3 kPa)	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 ([H+] 35 to 45 nmol/L)	Normal
		>7.45	Alkalemia
	Venous	7.26 to 7.46	Normal
Carbon Dioxide Partial Pressure (pCO2)	Arterial	<35 mmHg	Low
		35 to 45 mmHg (4.7 to 6.0 kPa)	Normal
		>45 mmHg	High
	Venous	40 to 45 mmHg	Normal
[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 formula

Primary disorder	pH	PaCO2	[HCO3−]	Compensation
Metabolic acidosis	↓	↓	↓	Respiratory
Metabolic alkalosis	↑	↑	↑	Respiratory
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

Approach to Metabolic Acidosis

pH<7.35 And [HCO3-]<24 meq/L

Metabolic acidosis

Calculate the anion gap (AG) Na+ - Cl- - HCO3-

Low AG AG<8

Normal AG 8<AG<16

High AG AG>16

Check albumin Correct the AG if albumin is low For every decrease of 1 g/dl of albumin, AG is decreased by 2.5 meg/L

Check Ca2+, Mg2+, K+, immunoglobulins High levels of these unmeasured cations decrease the AG

Check urine AG Na+ + K+ - Cl-

Check ΔAG/ΔHCO3-

R/O low Ca2+, Mg2+, K+

Negative urine AG GI causes RTA type II

Positive urine AG Renal failure RTA type I RTA type IV

ΔAG/ΔHCO3-<1 High AG metabolic acidosis combined with normal AG metabolic acidosis

1<ΔAG/ΔHCO3-<2 Pure high AG metabolic acidosis

ΔAG/ΔHCO3->2 High AG metabolic acidosis combined with metabolic alkalosis

Metabolic Acidosis

Category	Disease		pH	Serum AG	Urine AG	Urine chloride	Volume depletion	HTN	Loss of consciousness	Fever
Toxin/Medication	Methanol		↓	↑
	Paraldehyde		↓	↑
	Propylene glycol		↓	↑
	Ethylene glycol		↓	↑
	Ethanol		↓	↑
	Isopropyl alcohol		↓	↑
	Toluene		↓	Nl
	Pyroglutamic acid (5-oxoproline)		↓	↑
	Salicylates		↓↑	↑
	Metformin		↓	↑
	Isoniazid		↓	↑
	Acetazolamide		↓	Nl
	Ammonium chloride		↓	Nl
	Amphotericin B		↓	Nl
	Cholestyramine		↓	Nl
	Lactic acidosis		↓	↑
	Uremia		↓	↑
Ketoacidosis	Diabetic		↓	↑
	Starvation		↓	↑
	Alcoholic		↓	↑
	Infection		↓	↑
	Ischemia		↓	↑
	Diarrhea		↓	Nl
	Ureteral diversion		↓	Nl
	Renal failure		↓	↑
	Renal tubular acidosis	Type I	↓	Nl	↑
		Type II	↓	Nl	↓
		Type IV	↓	Nl	↑
	Hyperalimentation		↓	Nl
	Addison's disease		↓	Nl
	Post hypocapnea		↓	Nl
	Vomiting		↑			↓	+
	Nasogastric tube suction		↑			↓	+
	Over-diuresis		↑			↓	+
	Cushing's syndrome		↑			Nl		+
	Hyperaldosteronism		↑			Nl		+
	Renal artery stenosis		↑			Nl		+
	Hypomagnesemia		↑			Nl		-
	Hypokalemia		↑			Nl		-
	Bartter's syndrome		↑			Nl		-
	Licorice ingestion		↑			Nl		-
Category	Disease		pH	Serum AG	Urine AG	Urine chloride	Volume depletion	HTN	Loss of consciousness	Fever

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-diarrhea and lactic acidosis, toluene toxicity, treatment of diabetic ketoacidosis	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