Arterial blood gas

Arterial blood gas

Extraction and analysis

Arterial blood for blood gas analysis is usually extracted by a phlebotomist, nurse, or respiratory therapist.^[3] Blood is most commonly drawn from the radial artery because it is easily accessible, can be compressed to control bleeding, and has less risk for occlusion. The femoral artery (or less often, the brachial artery) is also used, especially during emergency situations or with children. Blood can also be taken from an arterial catheter already placed in one of these arteries.

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.^[4]

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 interpretation within five minutes.

Standard blood tests can also be performed on arterial blood, such as measuring glucose, lactate, hemoglobins, dys-haemoglobins, bilirubin and electrolytes.

Reference ranges and interpretation

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 90-100 mmHg	A low O2 indicates that the patient is not respiring properly, and is hypoxemic. At a PO2 of less than 60 mm Hg, supplemental oxygen should be administered. At a PO2 of less than 26 mm Hg, the patient is at risk of death and must be oxygenated immediately.
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.[5] A high PCO2 (respiratory acidosis) indicates underventilation, a low PCO2 (respiratory alkalosis) hyper- or overventilation. PCO2 levels can also become abnormal when the respiratory system is working to compensate for a metabolic issue so as to normalize the blood pH. An elevated PCO2 level is desired in some disorders associated with respiratory failure; this is known as permissive hypercapnia.
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. HCO3- levels can also become abnormal when the kidneys are working to compensate for a respiratory issue so as to normalize the blood pH.
SBCe	21 to 27 mmol/l	the bicarbonate concentration in the blood at a CO2 of 5.33 kPa, full oxygen saturation and 37 degrees Celsius.[6]
Base excess	-3 to +3 mmol/l	The base excess is used for the assessment of the metabolic component of acid-base disorders, and indicates whether the patient has metabolic acidosis or metabolic alkalosis. A negative base excess indicates that the patient has metabolic acidosis (primary or secondary to respiratory alkalosis). A positive base excess indicates that the patient has metabolic alkalosis (primary or secondary to respiratory acidosis).[7]
HPO42−	0.8 to 1.5 [8] mM
total CO2 (tCO2 (P)c)	25 to 30 mmol/l	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 millimolar concentration (mM) (mmol/l) and PCO2 is expressed in kPa [9]
total O2 (tO2e)		This is the sum of oxygen solved in plasma and chemically bound to hemoglobin. [10]

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.

See also

Sample values
BMP/ELECTROLYTES:
Na+=133	Cl-=107	BUN=27	/
			Glu=164
K+=5.2	CO2=22	Cr=1.1	\
ARTERIAL BLOOD GAS:
HCO3-=24	PCO2=40	PO2=95	pH=7.40
PROTEIN/LIVER FUNCTION TESTS:
Ca=9.5	TP=7.6	AST=26	LDH=100
PO4=1	Alb=4.2	ALT=47	ALP=71
TBIL=0.7	BC=0.5	BU=0.2
OTHER:
CK=55		MO = 290
CALCULATIONS:
BUN:Cr=24.5	BE=-0.4	AG=1
CO=285		OG = 5

• Acid-base homeostasis
• Anion gap
• Mechanical ventilation
• From Wikipedia