Arterial blood gas

An arterial blood gas (ABG) 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.

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 electrolytes, oxyhemoglobin, carboxyhemoglobin and methemoglobin. ABG testing is mainly used in pulmonology, to determine gas exchange levels in the blood related to lung function, but has a variety of applications in other areas of medicine. Combinations of disorders can be complex and difficult to interpret, so calculators [1], nomograms, and rules of thumb[2] are commonly used.

Contents

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 are not iced and are analyzed within 30 minutes.[4]

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

Calculations

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.

Much controversy exists about optimal blood gas management of hypothermic patients.[citation needed] 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. 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.[5]

Helpful Guidelines

  1. A 1mmHg change in PaCO2 above or below 40 mmHg results in 0.008 unit change in pH in the opposite direction. [6]
  2. The PaCO2 will decrease by about 1 mmHg for every 1 mEq/L reduction in [HCO3-] below 24 mEq/L
  3. 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 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 acidotic (pH < 7.35; H+ >45) or alkalemic (pH > 7.45; H+ < 35).
H+ 35–45 nmol/L (nM) See above.
PaO2 9.3–13.3 kPa or 80–100 mmHg A low O2 indicates that the patient is not respiring properly, and is hypoxemic. 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.
PaCO2 4.7–6.0 kPa or 35–45 mmHg The carbon dioxide partial pressure (PaCO2) indicates a respiratory problem: for a constant metabolic rate, the PaCO2 is determined entirely by ventilation.[7] A high PaCO2 (respiratory acidosis) indicates underventilation, a low PaCO2 (respiratory alkalosis) hyper- or overventilation. PaCO2 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 PaCO2 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.[8]
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).[9]
HPO42− 0.8 to 1.5 [10] 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 [11]
O2 Content (CaO2, CvO2, CcO2) vol% (mL oxygen/dL blood) This is the sum of oxygen dissolved in plasma and chemically bound to hemoglobin. [12]

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

Pathophysiology sample values
BMP/ELECTROLYTES:
Na+=140 Cl−=100 BUN=20 /
Glu=150
K+=4 CO2=22 PCr=1.0 \
ARTERIAL BLOOD GAS:
HCO3-=24 paCO2=40 paO2=95 pH=7.40
ALVEOLAR GAS:
pACO2=36 pAO2=105 A-a g=10
OTHER:
Ca=9.5 Mg2+=2.0 PO4=1
CK=55 BE=−0.36 AG=16
SERUM OSMOLARITY/RENAL:
PMO = 300 PCO=295 POG=5 BUN:Cr=20
URINALYSIS:
UNa+=80 UCl−=100 UAG=5 FENa=0.95
UK+=25 USG=1.01 UCr=60 UO=800
PROTEIN/GI/LIVER FUNCTION TESTS:
LDH=100 TP=7.6 AST=25 TBIL=0.7
ALP=71 Alb=4.0 ALT=40 BC=0.5
AST/ALT=0.6 BU=0.2
AF alb=3.0 SAAG=1.0 SOG=60
CSF:
CSF alb=30 CSF glu=60 CSF/S alb=7.5 CSF/S glu=0.4

References

  1. ^ Baillie K. "Arterial Blood Gas Interpreter". prognosis.org. http://www.prognosis.org/arterial_blood_gas_calculator.php. Retrieved 2007-07-05.  - Online arterial blood gas analysis
  2. ^ Baillie JK (2008). "Simple, easily memorised "rules of thumb" for the rapid assessment of physiological compensation for acid-base disorders". Thorax 63 (3): 289–90. doi:10.1136/thx.2007.091223. PMID 18308967. 
  3. ^ 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. doi:10.1016/S0954-6111(03)00226-9. PMID 14635973. 
  4. ^ 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. 
  5. ^ Kofstad J (1996). "Blood Gases and Hypothermia: Some Theoretical and Practical Considerations". Scand J Clin Lab Invest. (Suppl) 224: 21-26. PMID 8865418. 
  6. ^ Stoelting: Basics of Anesthesia, 5th ed. p 321.
  7. ^ Baillie K, Simpson A. "Altitude oxygen calculator". Apex (Altitude Physiology Expeditions). http://www.altitude.org/oxygen_levels.php. Retrieved 2006-08-10.  - Online interactive oxygen delivery calculator
  8. ^ Acid Base Balance (page 3)
  9. ^ RCPA Manual: Base Excess (arterial blood)
  10. ^ Walter F., PhD. Boron (2005). Medical Physiology: A Cellular And Molecular Approaoch. Elsevier/Saunders. ISBN 1-4160-2328-3.  Page 849
  11. ^ CO2: The Test
  12. ^ Hemoglobin and Oxygen Transport. Charles L. Webber, Jr., Ph.D.

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