Arterial blood gas
Encyclopedia
An arterial blood gas is a blood test
that is performed using blood
from an artery
. It involves puncturing an artery with a thin needle and syringe and drawing a small volume of blood. The most common puncture site is the radial artery
at the wrist
, but sometimes the femoral artery
in the groin
or other sites are used. The blood can also be drawn from an arterial catheter. Pulse oximetry
plus transcutaneous carbon dioxide measurement is an alternative method of obtaining similar information as well. An ABG is a test that measures the arterial oxygen tension (PaO2), carbon dioxide tension (PaCO2), and acidity (pH
). In addition, arterial oxyhemoglobin saturation (SaO2) can be determined. Such information is vital when caring for patients with critical illness or respiratory disease. As a result, the ABG is one of the most common tests performed on patients in intensive care units (ICUs).
The test is used to determine the pH
of the blood, the partial pressure
of carbon dioxide
and oxygen
, and the bicarbonate
level. Many blood gas analyzers will also report concentrations of lactate
, hemoglobin
, several electrolyte
s, oxyhemoglobin, carboxyhemoglobin
and methemoglobin. ABG testing is mainly used in pulmonology
and critical care medicine to determine gas exchange
which reflect gas exchange across the alveolar-capillary membrane. ABG testing also has a variety of applications in other areas of medicine. Combinations of disorders can be complex and difficult to interpret, so calculators, nomograms, and rules of thumb are commonly used.
or nurse. 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 selection of which radial artery to draw from is based on the outcome of an Allen's test
. 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 analyzed within 10-15 minutes, it must be placed on ice for valid results. Even when placed on ice, samples should still be analyzed within 1 hour.
Standard blood tests can also be performed on arterial blood, such as measuring glucose
, lactate
, hemoglobins, dys-haemoglobins, bilirubin
and electrolyte
s.
and the partial pressure
s of oxygen and carbon dioxide. The bicarbonate concentration is also calculated. These results are usually available for interpretation within five minutes.
Two methods have been used in medicine in the management of blood gases of patients in hypothermia: pH-stat method and alpha-stat method. Recent studies suggest that the α-stat method is superior.
Both the pH-stat and alpha-stat strategies have theoretical disadvantages. α-stat method is the method of choice for optimal myocardial function. The pH-stat method may result in loss of autoregulation in the brain (coupling of the cerebral blood flow with the metabolic rate in the brain). By increasing the cerebral blood flow beyond the metabolic requirements, the pH-stat method may lead to cerebral microembolisation and intracranial hypertension.
Contamination of the sample with room air will result in abnormally low carbon dioxide and possibly elevated oxygen levels, and a concurrent elevation in pH. Delaying analysis (without chilling the sample) may result in inaccurately low oxygen and high carbon dioxide levels as a result of ongoing cellular respiration.
Blood test
A blood test is a laboratory analysis performed on a blood sample that is usually extracted from a vein in the arm using a needle, or via fingerprick....
that is performed using blood
Blood
Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....
from an artery
Artery
Arteries are blood vessels that carry blood away from the heart. This blood is normally oxygenated, exceptions made for the pulmonary and umbilical arteries....
. It involves puncturing an artery with a thin needle and syringe and drawing a small volume of blood. The most common puncture site is the radial artery
Radial artery
In human anatomy, the radial artery is the main blood vessel, with oxygenated blood, of the lateral aspect of the forearm.-Course:The radial artery arises from the bifurcation of the brachial artery in the cubital fossa. It runs distally on the anterior part of the forearm...
at the wrist
Wrist
In human anatomy, the wrist is variously defined as 1) the carpus or carpal bones, the complex of eight bones forming the proximal skeletal segment of the hand;...
, but sometimes the femoral artery
Femoral artery
The femoral artery is a general term comprising a few large arteries in the thigh. They begin at the inguinal ligament and end just above the knee at adductor canal or Hunter's canal traversing the extent of the femur bone....
in the groin
Groin
In human anatomy, the groin areas are the two creases at the junction of the torso with the legs, on either side of the pubic area. This is also known as the medial compartment of the thigh. A pulled groin muscle usually refers to a painful injury sustained by straining the hip adductor muscles...
or other sites are used. The blood can also be drawn from an arterial catheter. Pulse oximetry
Pulse oximetry
Pulse oximetry is a non-invasive method allowing the monitoring of the oxygenation of a patient's hemoglobin.A sensor is placed on a thin part of the patient's body, usually a fingertip or earlobe, or in the case of an infant, across a foot....
plus transcutaneous carbon dioxide measurement is an alternative method of obtaining similar information as well. An ABG is a test that measures the arterial oxygen tension (PaO2), carbon dioxide tension (PaCO2), and acidity (pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
). In addition, arterial oxyhemoglobin saturation (SaO2) can be determined. Such information is vital when caring for patients with critical illness or respiratory disease. As a result, the ABG is one of the most common tests performed on patients in intensive care units (ICUs).
The test is used to determine the pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
of the blood, the partial pressure
Partial pressure
In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture....
of carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
and oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
, and the bicarbonate
Bicarbonate
In inorganic chemistry, bicarbonate is an intermediate form in the deprotonation of carbonic acid...
level. Many blood gas analyzers will also report concentrations of lactate
Lactic acid
Lactic acid, also known as milk acid, is a chemical compound that plays a role in various biochemical processes and was first isolated in 1780 by the Swedish chemist Carl Wilhelm Scheele. Lactic acid is a carboxylic acid with the chemical formula C3H6O3...
, hemoglobin
Hemoglobin
Hemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of all vertebrates, with the exception of the fish family Channichthyidae, as well as the tissues of some invertebrates...
, several electrolyte
Electrolyte
In chemistry, an electrolyte is any substance containing free ions that make the substance electrically conductive. The most typical electrolyte is an ionic solution, but molten electrolytes and solid electrolytes are also possible....
s, oxyhemoglobin, carboxyhemoglobin
Carboxyhemoglobin
Carboxyhemoglobin is a stable complex of carbon monoxide and hemoglobin that forms in red blood cells when carbon monoxide is inhaled or produced in normal metabolism. Large quantities of it hinder delivery of oxygen to the body...
and methemoglobin. ABG testing is mainly used in pulmonology
Pulmonology
In medicine, pulmonology is the specialty that deals with diseases of the respiratory tract and respiratory disease. It is called chest medicine and respiratory medicine in some countries and areas...
and critical care medicine to determine gas exchange
Gas exchange
Gas exchange is a process in biology where gases contained in an organism and atmosphere transfer or exchange. In human gas-exchange, gases contained in the blood of human bodies exchange with gases contained in the atmosphere. Human gas-exchange occurs in the lungs...
which reflect gas exchange across the alveolar-capillary membrane. ABG testing also has a variety of applications in other areas of medicine. Combinations of disorders can be complex and difficult to interpret, so calculators, nomograms, and rules of thumb are commonly used.
Sampling and analysis
Arterial blood for blood gas analysis is usually drawn by a respiratory therapist and sometimes a phlebotomistPhlebotomist
Phlebotomists are individuals trained to draw blood from a live person or animal.Phlebotomists collect blood primarily by performing venipuncture and, for collection of minute quantities of blood, fingersticks. Blood may be collected from infants by means of a heel stick...
or nurse. Blood is most commonly drawn from the radial artery
Radial artery
In human anatomy, the radial artery is the main blood vessel, with oxygenated blood, of the lateral aspect of the forearm.-Course:The radial artery arises from the bifurcation of the brachial artery in the cubital fossa. It runs distally on the anterior part of the forearm...
because it is easily accessible, can be compressed to control bleeding, and has less risk for occlusion, the selection of which radial artery to draw from is based on the outcome of an Allen's test
Allen's test
In medicine, Allen's test, also Allen test, is used to test blood supply to the hand. It is performed prior to radial arterial blood sampling or cannulation.It is named for Edgar Van Nuys Allen....
. The femoral artery
Femoral artery
The femoral artery is a general term comprising a few large arteries in the thigh. They begin at the inguinal ligament and end just above the knee at adductor canal or Hunter's canal traversing the extent of the femur bone....
(or less often, the brachial artery
Brachial artery
The brachial artery is the major blood vessel of the arm.It is the continuation of the axillary artery beyond the lower margin of teres major muscle. It continues down the ventral surface of the arm until it reaches the cubital fossa at the elbow. It then divides into the radial and ulnar arteries...
) 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
Heparin
Heparin , also known as unfractionated heparin, a highly sulfated glycosaminoglycan, is widely used as an injectable anticoagulant, and has the highest negative charge density of any known biological molecule...
, to prevent coagulation
Coagulation
Coagulation is a complex process by which blood forms clots. It is an important part of hemostasis, the cessation of blood loss from a damaged vessel, wherein a damaged blood vessel wall is covered by a platelet and fibrin-containing clot to stop bleeding and begin repair of the damaged vessel...
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 analyzed within 10-15 minutes, it must be placed on ice for valid results. Even when placed on ice, samples should still be analyzed within 1 hour.
Standard blood tests can also be performed on arterial blood, such as measuring glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
, lactate
Lactic acid
Lactic acid, also known as milk acid, is a chemical compound that plays a role in various biochemical processes and was first isolated in 1780 by the Swedish chemist Carl Wilhelm Scheele. Lactic acid is a carboxylic acid with the chemical formula C3H6O3...
, hemoglobins, dys-haemoglobins, bilirubin
Bilirubin
Bilirubin is the yellow breakdown product of normal heme catabolism. Heme is found in hemoglobin, a principal component of red blood cells. Bilirubin is excreted in bile and urine, and elevated levels may indicate certain diseases...
and electrolyte
Electrolyte
In chemistry, an electrolyte is any substance containing free ions that make the substance electrically conductive. The most typical electrolyte is an ionic solution, but molten electrolytes and solid electrolytes are also possible....
s.
Calculations
The machine used for analysis aspirates this blood from the syringe and measures the pHPH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
and the partial pressure
Partial pressure
In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture....
s of oxygen and carbon dioxide. The bicarbonate concentration is also calculated. These results are usually available for interpretation within five minutes.
Two methods have been used in medicine in the management of blood gases of patients in hypothermia: pH-stat method and alpha-stat method. Recent studies suggest that the α-stat method is superior.
- pH-stat: the arterial carbon dioxide tension (paCO2) is maintained at 5.3 kPa (40 mmHg) and the pH is maintained at 7.40 when measured at the actual patient temperature. It is then necessary to add CO2 to the sample to calculate results.
- α-stat (alpha-stat): the arterial carbon dioxide tension and the pH are maintained at 5.3 kPa (40mmHg) and 7.40 when measured at +37°C. When a patient is cooled down, the pH-value will increase and the pCO2-value and the pO2-value will decrease with lowering of the temperature if measured at the patients temperature.
Both the pH-stat and alpha-stat strategies have theoretical disadvantages. α-stat method is the method of choice for optimal myocardial function. The pH-stat method may result in loss of autoregulation in the brain (coupling of the cerebral blood flow with the metabolic rate in the brain). By increasing the cerebral blood flow beyond the metabolic requirements, the pH-stat method may lead to cerebral microembolisation and intracranial hypertension.
Helpful Guidelines
- A 1mmHg change in PaCO2 above or below 40 mmHg results in 0.008 unit change in pH in the opposite direction.
- The PaCO2 will decrease by about 1 mmHg for every 1 mEq/L reduction in [HCO3-] below 24 mEq/L
- A change in [HCO3-] of 10 mEq/L will result in a change in pH of approximately 0.15 pH units in the same direction.
Reference ranges
These are typical reference ranges, although various analysers and laboratories may employ different ranges.| Analyte | Range | Interpretation |
|---|---|---|
| pH | 7.34-7.44 | The pH PH In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline... or H+ indicates if a patient is acidemic (pH < 7.35; H+ >45) or alkalemic (pH > 7.45; H+ < 35). |
| H+ | 35–45 nmol Mole (unit) The mole is a unit of measurement used in chemistry to express amounts of a chemical substance, defined as an amount of a substance that contains as many elementary entities as there are atoms in 12 grams of pure carbon-12 , the isotope of carbon with atomic weight 12. This corresponds to a value... /L Litre pic|200px|right|thumb|One litre is equivalent to this cubeEach side is 10 cm1 litre water = 1 kilogram water The litre is a metric system unit of volume equal to 1 cubic decimetre , to 1,000 cubic centimetres , and to 1/1,000 cubic metre... (nM) |
See above. |
| Arterial oxygen partial pressure (PaO2) | 11-13 kPa KPA KPA may refer to:* Kenya Ports Authority* Kiln phosphoric acid, a dry process to produce phosphoric acid at high temperature in a kiln* Kilopascal , a unit of pressure* Known-plaintext attack, a method of cryptanalysis* Korean People's Army... or 75-100 mmHg |
A low PaO2 indicates that the patient is not oxygenating properly, and is hypoxemic. (Note that a low PaO2 is not required for the patient to have hypoxemia Hypoxemia Hypoxemia is generally defined as decreased partial pressure of oxygen in blood, sometimes specifically as less than or causing hemoglobin oxygen saturation of less than 90%.-Distinction from anemia and hypoxia:... .) At a PaO2 of less than 60 mm Hg, supplemental oxygen should be administered. At a PaO2 of less than 26 mmHg, the patient is at risk of death and must be oxygenated immediately. |
| Arterial carbon dioxide partial pressure (PaCO2) | 4.7-6.0 kPa or 35-45 mmHg | The carbon dioxide partial pressure (PaCO2 Carbon dioxide Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom... ) is an indicator of CO2 production and elimination: for a constant metabolic rate, the PaCO2 Carbon dioxide Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom... is determined entirely by its elimination through ventilation Ventilation (physiology) In respiratory physiology, ventilation is the rate at which gas enters or leaves the lung. It is categorized under the following definitions:-Sample values:... . A high PaCO2 Carbon dioxide Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom... (respiratory acidosis Respiratory acidosis Respiratory acidosis is a medical condition in which decreased ventilation causes increased blood carbon dioxide concentration and decreased pH .... , alternatively hypercapnia Hypercapnia Hypercapnia or hypercapnea , also known as hypercarbia, is a condition where there is too much carbon dioxide in the blood... ) indicates underventilation (or, more rarely, a hypermetabolic disorder Malignant hyperthermia Malignant hyperthermia or malignant hyperpyrexia is a rare life-threatening condition that is usually triggered by exposure to certain drugs used for general anesthesia; specifically, the volatile anesthetic agents and the neuromuscular blocking agent, succinylcholine... ), a low PaCO2 Carbon dioxide Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom... (respiratory alkalosis Respiratory alkalosis Respiratory alkalosis is a medical condition in which increased respiration elevates the blood pH... , alternatively hypocapnia Hypocapnia Hypocapnia or hypocapnea also known as hypocarbia, sometimes incorrectly called acapnia, is a state of reduced carbon dioxide in the blood. Hypocapnia usually results from deep or rapid breathing, known as hyperventilation.... ) hyper- or overventilation. |
| HCO3− | 22–26 mmol/L | The HCO3− ion indicates whether a metabolic Metabolism Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories... problem is present (such as ketoacidosis Ketoacidosis Ketoacidosis is a metabolic state associated with high concentrations of ketone bodies, formed by the breakdown of fatty acids and the deamination of amino acids. The two common ketones produced in humans are acetoacetic acid and β-hydroxybutyrate.... ). A low HCO3− indicates metabolic acidosis Metabolic acidosis In medicine, metabolic acidosis is a condition that occurs when the body produces too much acid or when the kidneys are not removing enough acid from the body. If unchecked, metabolic acidosis leads to acidemia, i.e., blood pH is low due to increased production of hydrogen by the body or the... , a high HCO3− indicates metabolic alkalosis Metabolic alkalosis Metabolic alkalosis is a metabolic condition in which the pH of tissue is elevated beyond the normal range . This is the result of decreased hydrogen ion concentration, leading to increased bicarbonate, or alternatively a direct result of increased bicarbonate concentrations.-Terminology:*Alkalosis... . As this value when given with blood gas results is often calculated by the analyzer, correlation should be checked with total CO2 levels as directly measured (see below). |
| SBCe | 21 to 27 mmol/L | the bicarbonate concentration in the blood at a CO2 Carbon dioxide Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom... of 5.33 kPa, full oxygen saturation and 37 Celsius. |
| Base excess Base excess In human physiology, base excess and base deficit refer to an excess or deficit, respectively, in the amount of base present in the blood. The value is usually reported as a concentration in units of mEq/L, with positive numbers indicating an excess of base and negative a deficit... |
−2 to +2 mmol/L | >- |
Contamination of the sample with room air will result in abnormally low carbon dioxide and possibly elevated oxygen levels, and a concurrent elevation in pH. Delaying analysis (without chilling the sample) may result in inaccurately low oxygen and high carbon dioxide levels as a result of ongoing cellular respiration.
See also
- Acid-base homeostasisAcid-base homeostasisAcid–base homeostasis is the part of human homeostasis concerning the proper balance between acids and bases, in other words, the pH. The body is very sensitive to its pH level, so strong mechanisms exist to maintain it...
- Anion gapAnion gapThe anion gap is the difference in the measured cations and the measured anions in serum, plasma, or urine. The magnitude of this difference in the serum is often calculated in medicine when attempting to identify the cause of metabolic acidosis...
- Mechanical ventilationMechanical ventilationIn medicine, mechanical ventilation is a method to mechanically assist or replace spontaneous breathing. This may involve a machine called a ventilator or the breathing may be assisted by a physician, respiratory therapist or other suitable person compressing a bag or set of bellows...