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Arterial blood gas

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An arterial blood gas 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.

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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.

Extraction and Analysis

Arterial blood for blood gasses is usually extracted by a phlebotomist or nurse, and occasionally a respiratory therapist.^[1] 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 be analyzed within 30 minutes.^[2]

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, haemoglobins, 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 70-100 mmHg	-Reference: Mahoney JJ et al. Chapter 9 - Arerial Blood Gas Analysis In: Respiratory Care - A Guide to Clinical Practice. 1997 Fourth edition. Lippincott.
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.^[3] A high PCO₂ (respiratory acidosis) indicates underventilation, a low PCO₂ (respiratory alkalosis) hyper- or overventilation.
HCO₃^-	22 - 30 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.
SBC_e	21 to 27 mM	the bicarbonate concentration in the blood at a CO₂ of 5.33kPa, full oxygen saturation and 37 degrees Celcius.^[4]
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 ^[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 ^[6]
total O₂ (tO_2e)		This is the sum of oxygen solved in plasma and chemically bound to hemoglobin. ^[7]

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.

References

^ Aaron SD, Vandemheen KL, Naftel SA, Lewis MJ, Rodger MA (2003). "Topical tetracaine prior to arterial puncture: a randomized, placebo-controlled clinical trial". Respir Med. 97 (11): 1195-1199. PMID 14635973.
^ Mahoney JJ, Harvey JA, Wong RL, Van Kessel AL (1991). "Changes in oxygen measurements when whole blood is stored in iced plastic or glass syringes". Clin Chem. 37 (7): 1244-1248. PMID 1823532.
^ Baillie K, Simpson A. Altitude oxygen calculator. Apex (Altitude Physiology Expeditions). Retrieved on 2006-08-10. - Online interactive oxygen delivery calculator
^ Acid Base Balance (page 3)
^ Walter F., PhD. Boron. Medical Physiology: A Cellular And Molecular Approaoch. Elsevier/Saunders. ISBN 1-4160-2328-3. Page 849
^ CO2: The Test
^ Hemoglobin and Oxygen Transport. Charles L. Webber, Jr., Ph.D.

v • d • e Respiratory system, physiology: respiratory physiology
Volumes	lung volumes - vital capacity - functional residual capacity - respiratory minute volume - closing capacity - dead space - spirometry - body plethysmography - peak flow meter - thoracic independent volume - bronchial challenge test
Airways	ventilation (V) (positive pressure) - breath (inhalation, exhalation) -respiratory rate - respirometer - pulmonary surfactant - compliance - hysteresivity - airway resistance
Blood	pulmonary circulation - perfusion (Q) - hypoxic pulmonary vasoconstriction - pulmonary shunt
Interactions	ventilation/perfusion ratio (V/Q) and scan - zones of the lung - gas exchange - pulmonary gas pressures - alveolar gas equation - hemoglobin - oxygen-haemoglobin dissociation curve (2,3-DPG, Bohr effect, Haldane effect) - carbonic anhydrase (chloride shift) - oxyhemoglobin - respiratory quotient - arterial blood gas - diffusion capacity - Dlco
Control of respiration	pons (pneumotaxic center, apneustic center) - medulla (dorsal respiratory group, ventral respiratory group) - chemoreceptors (central, peripheral) - pulmonary stretch receptors (Hering-Breuer reflex)
Insufficiency	high altitude - oxygen toxicity - hypoxia

v • d • e Urinary system, physiology: renal physiology and acid base physiology
Filtration	Renal blood flow - Ultrafiltration - Countercurrent exchange
Hormones affecting filtration	Antidiuretic hormone (ADH) - Aldosterone - Atrial natriuretic peptide
Secretion/clearance	Pharmacokinetics - Clearance of medications
Reabsorption	Solvent drag -Na⁺ - Cl^- -urea -glucose -oligopeptides -protein
Endocrine	Renin - Erythropoietin (EPO) - Calcitriol (Active vitamin D) - Prostaglandins
Assessing Renal function / Measures of dialysis	Glomerular filtration rate - Creatinine clearance - Renal clearance ratio - Urea reduction ratio - Kt/V - Standardized Kt/V - Hemodialysis product - PAH clearance (Effective renal plasma flow - Extraction ratio)
Acid base physiology	Fluid balance - Darrow Yannet diagram - Body water - Interstitial fluid - Extracellular fluid - Intracellular fluid/Cytosol - Plasma - Transcellular fluid - Base excess - Davenport diagram - Anion gap - Arterial blood gas
Buffering/compensation	Bicarbonate buffering system - Respiratory compensation - Renal compensation

It has been suggested that Arterial blood gas sampling be merged into this article or section. (Discuss)

Categories: Respiratory therapy | Pulmonology | Metabolism | Chemical pathology | Medical tests

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Arterial_blood_gas". A list of authors is available in Wikipedia.