Arterial blood gas

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-In-Chief: Priyamvada Singh, M.B.B.S. [2]

Overview

An arterial blood gas (also called "ABG'S") is a blood test that is performed specifically on arterial blood, to determine the concentrations of carbon dioxide, oxygen and bicarbonate, as well as the pH of the blood. Its main use is in pulmonology, to determine gas exchange levels in the blood related to lung function, but it is also used in nephrology, and used to evaluate metabolic disorders such as acidosis and alkalosis. As its name implies, the sample is taken from an artery, which is more uncomfortable and difficult than venipuncture.

Indications

• Identification of acid-base disturbances
• Measurement of the partial pressures of oxygen and carbon dioxide
• Assessment of the response to therapeutic interventions
• Collection of a blood sample when venous sampling is not feasible

Contraindications

• Radial samples are contraindicated in abnormal modified Allen's test
• Abnormal anatomy at the puncture site such as:
• Congenital malformations
• Burns
• Aneurysm
• Arteriovenous fistula
• Active Raynaud's syndrome

Extraction and Analysis

• Arterial blood for blood gas analysis is usually extracted by a phlebotomist, nurse, or respiratory therapist.
• Blood may be taken from an easily accessible artery (typically the radial artery, but during unusual or emergency situations the brachial or femoral artery may be used), or out of an arterial line.
• The syringe is pre-packaged and contains a small amount of heparin, to prevent coagulation or needs to be heparinised, by drawing up a small amount of heparin and squirting it out again.
• Once the sample is obtained, care is taken to eliminate visible gas bubbles, as these bubbles can dissolve into the sample and cause inaccurate results.
• The sealed syringe is taken to a blood gas analyzer.
• If the sample cannot be immediately analyzed, it is chilled in an ice bath in a glass syringe to slow metabolic processes which can cause inaccuracy.
• Samples drawn in plastic syringes should not be iced and should always be analyzed within 30 minutes.
• The machine used for analysis aspirates this blood from the syringe and measures the pH and the partial pressures of oxygen and carbon dioxide. The bicarbonate concentration is also calculated. These results are usually available for interpretion within five minutes.
• Standard blood tests can also be performed on arterial blood, such as measuring glucose, lactate, hemoglobins, dys-haemoglobins, bilirubin and electrolytes.
• Contamination with room air will result in abnormally low carbon dioxide and (generally) normal oxygen levels. Delays in analysis (without chilling) may result in inaccurately low oxygen and high carbon dioxide levels as a result of ongoing cellular respiration.
• Lactate level analysis is often featured on blood gas machines in neonatal wards, as infants often have elevated lactic acid.

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Reference Ranges and Interpretation

Interpretation

• Arterial blood gas is interpreted in the following sequence for alkalosis and acidosis:

Step 1

• Normal pH is 7.35 - 7.45.
• pH < 7.35 is acidosis and > 7.45 alkalosis

Step 2

• Normal CO2 is 4.7 to 6.0 kPa or 35 -45 mm Hg.
• Check for CO2 whether acidosis (> 45) or alkalosis (< 35)

Step 3

• Normal HCO3 (bicarbonates) 22 - 28 mmoL/liter.
• HCO3 < 22 acidosis, Hco3 > 28 alkalosis

Step 4

• Match whether pH is matching with carbondioxide or bicarbonate to determine the primary defect.
• If pH matches CO2 the primary defect is respiratory, whereas if pH matches HCO3 the primary defect is metabolic

Step 5

• After determining the primary defect check the opposite factor to see whether the defect is uncompensated, partially or fully compensated. For instance, the primary defect is respiratory acidosis then check the opposite factor i.e. HCO3 for compensation.

Step 6

• Check for oxygen saturation to see if hypoxemia is present or not

Examples

Example 1

• pH = 7.01, CO2 = 28 mm Hg, HCO3 = 10 mmol/L, Oxygen saturation = 95%, pO2 = 95
• Step 1 - pH = 7.01, acidosis
• Step 2 - CO2 = 28 mm Hg, alkalosis
• Step 3 - HCO3 = 10 mmoL/L, acidosis
• Step 4 - Match the pH - pH is acidosis and HCO3 is acidosis so the primary defect is metabolic acidosis
• Step 5 - Since the primary defect is metabolic, check CO2 for compensation. Since CO2 is opposite to the pH it is trying to compensate. However, the pH is still acidosis and not normal so the compensation is only partial.
• Step 6 - Oxygen saturation is normal so no hypoxemia.
• Conclusion - Partially compensated metabolic acidosis without hypoxemia

Example 2

• pH = 7.50, CO2 = 40 mm Hg, HCO3 = 32 mmol/L, Oxygen saturation = 95%, pO2 = 90
• Step 1 - pH = 7.50, alkalosis
• Step 2 - CO2 = 40 mm Hg, normal
• Step 3 - HCO3 = 32 mmoL/L, alkalosis
• Step 4 - Match the pH - pH is alkalosis and HCO3 is alkalosis so the primary defect is metabolic alkalosis
• Step 5 - Since the primary defect is metabolic, check CO2 for compensation. Since CO2 is normal so it is uncompensated as CO2 is not trying to compensate.
• Step 6 - Oxygen saturation is normal but pO2 is low so hypoxemia.
• Conclusion - Uncompensated metabolic alkalosis with hypoxemia

Example 3

• pH = 7.44, CO2 = 20 mm Hg, HCO3 = 10 mmol/L, Oxygen saturation = 95%, pO2 = 95%
• Step 1 - pH = 7.44, normal
• Step 2 - CO2 = 20 mm Hg, alkalosis
• Step 3 - HCO3 = 10 mmoL/L, acidosis
• Step 4 - Match the pH - pH is normal but a pH of 7.44 is more inclined towards CO2 (alkalosis) so the primary defect is respiratory alkalosis
• Step 5 - Since the primary defect is respiratory, check HCO3 for compensation. Since pH is normal so it is fully compensated.
• Step 6 - Oxygen saturation is normal but pO2 is low so hypoxemia.
• Conclusion - Fully compensated respiratory alkalosis without hypoxemia.

Reference Ranges

Oxygen Partial Pressure (pO2)
Arterial pO2	70-100 mm Hg
Venous pO2	35-40 mmHg
Oxygen Saturation (SO2)
Arterial SO2	< 95%
Venous SO2	70-75%
Carbon Dioxide Partial Pressure (pCO2)
Arterial pCO2	35-45 mmHg
Venous pCO2	40-50 mmHg
Serum Bicarbonate (HCO3)
Arterial HCO3	20-27 mmol/l
Venous HCO3	19-28 mmol/l
pH
Arterial pH	7.35-7.45
Venous pH	7.26-7.46
Base Excess (BE)
Arterial BE	-3.4 - +2.3 mmol/l
Venous BE	-2 - -5 mmol/l

These are typical reference ranges, although various analysers and laboratories may employ different ranges.

Analyte	Range	Interpretation
pH	7.35 - 7.45	The pH or H+ indicates if a patient is acidemic (pH < 7.35; H+ >45) or alkalemic (pH > 7.45; H+ < 35).
H+	35 - 45 nmol/l (nM)	See above.
pO2	9.3-13.3 kPa or 80-100 mmHg	Normal pO2 is 80-100 mmHg (age-dependent).
pCO2	4.7-6.0 kPa or 35-45 mmHg	The carbon dioxide and partial pressure (PCO2) indicates a respiratory problem: for a constant metabolic rate, the PCO2 is determined entirely by ventilation. A high PCO2 (respiratory acidosis) indicates underventilation, a low PCO2 (respiratory alkalosis) hyper- or overventilation.
HCO3-	22 - 26 mmol/l	The HCO3- ion indicates whether a metabolic problem is present (such as ketoacidosis). A low HCO3- indicates metabolic acidosis, a high HCO3- indicates metabolic alkalosis.
SBCe	21 to 27 mM	the bicarbonate concentration in the blood at a CO2 of 5.33kPa, full oxygen saturation and 37 degrees Celcius.
Base excess	-2 to +2 mmol/l	The base excess indicates whether the patient is acidotic or alkalotic. A negative base excess indicates that the patient is acidotic. A high positive base excess indicates that the patient is alkalotic.
HPO42−	0.8 to 1.5 mM
total CO2 (tCO2 (P)c)	25 to 30 mM	This is the total amount of CO2, and is the sum of HCO3- and pCO2 by the formula: tCO2 = [HCO3-] + α*pCO2, where α=0.226 mM/kPa, HCO3- is expressed in molars (M) and pCO2 is expressed in kPa
total O2 (tO2e)		This is the sum of oxygen solved in plasma and chemically bound to hemoglobin.

Sources of error

• When the sample is left for prolonged periods at room temperature, consumption of oxygen may result in a falsely low PaO2. The sample should be analyzed within 15 minutes. [16-19]
• On the other side, air bubbles exist in the sample can cause a falsely high PaO2 and a falsely low PaCO2. Remove the bubbles after the sample has been withdrawn can minimize this effect. [17,21]. [10]
• If acidic heparin is used, heparin can decrease the pH. Heparin amount should be minimized.
• If compared to pulmnoary artery catheter, arterial pH was higher and PaCO2 was lower.

Related Chapters

• Metabolic acidosis
• Metabolic alkalosis
• Respiratory acidosis
• Respiratory alkalosis

References

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