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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			Clinical									Paraclinical																			Gold standard diagnosis	Other findings
						Symptoms					Signs				Lab data																	Imaging
															ABG				CBC		Electrolyte			Renal		LFT		Enzyme		U/A
			↑ acid production	Loss of bicarbonate	↓ renal acid excretion	Fever	N/V	Diarrhea	Dyspnea	Edema	Toxic/ill/Shock	BP	Dehydration	LOC	pH	HCO3-	O2	Serum AG	WBC	Hb	Cl-	K+	Na+	Bun	Cr	Bili	Aminase	CK	Renin	Urine pH	Urine AG	US	CT scan
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

Metabolic Alkalosis

Category	Disease	Mechanism			Clinical						Paraclinical									Gold standard diagnosis	Other findings
					Symptoms			Signs			Lab data							Imaging
											ABG		U/A	Electrolytes			Renin
		↑ acid production	Loss of bicarbonate	↓ renal acid excretion	Fever	Dyspnea	Edema	Toxic/ill	BP	Dehydration	pH	Serum AG	Urine Cl-	Cl-	K+	Na+		US	CT scan
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				↓

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