Sandbox: sadaf

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	CO2	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 Primary cause: hypoventilation

PaCO2 Normal or < 35mm Hg = Metabolic acidosis

Check PaCO2

PaCO2 > 45mm Hg = Metabolic alkalosis

PaCO2 < 35mm Hg = Respiratory alkalosis Primary cause: hyperventilation

HCO3- > 29 eg. vomiting

Check HCO3-

Normal or slight decrease = Acute respiratory alkalosis eg. fever, panic attack

Decreased < 24 = Chronic respiratory alkalosis eg. Anemia, CNS causes

Management of Acidosis

pH < 7.35

Acidosis (High [H+])

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

Disease	PH	Poisoning	AG	Loss of consciousness	Fever
Methanol	Low	Yes	High
Metformin	Low		High
Uremia	Low		High
Diabetic ketoacidosis	Low		High
Paraldehyde	Low	Yes	High
Propylene glycol	Low	Yes	High
Infection	Low		High
Ischemia	Low		High
Isoniazid	Low		High
Ethylene glycol	Low	Yes	High
Ethanol	Low	Yes	High
Salicylates	Low/high	Yes	High
Starvation	Low		High
Diarrhea	Low		Nl
Ureteral diversion	Low		Nl
Renal tubular acidosis	Low		Nl
Hyperalimentation	Low		Nl
Addison's disease	Low		Nl
Acetazolamide	Low		Nl
Ammonium chloride	Low		Nl
Congenital chloride diarrhea	Low		Nl
Amphotericin B	Low		Nl
Toluene	Low		Nl
Cholestyramine	Low		Nl
post hypocapnea	Low		Nl

Related Chapters

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