Arterial blood gas test

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Arterial-blood gas test
MeSHD001784
MedlinePlus003855
LOINC24336-0

An arterial blood gas (ABG) test, or arterial blood gas analysis (ABGA) measures the amounts of arterial gases, such as

arterial catheter
.

An ABG test measures the

intensive-care units. In other levels of care, pulse oximetry plus transcutaneous carbon-dioxide measurement is a less invasive, alternative method of obtaining similar information.[citation needed
]

An ABG test can also measure the level of

nomograms, and rules of thumb[3]
are commonly used.

ABG samples originally were sent from the clinic to the medical laboratory for analysis. Newer equipment lets the analysis be done also as point-of-care testing, depending on the equipment available in each clinic.

Sampling and analysis

Bench top analyzer ABL800 FLEX - Radiometer Medical
Roche Diagnostics
).

Arterial blood for

nurse, a paramedic or a doctor.[4] 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 vascular occlusion. The selection of which radial artery to draw from is based on the outcome of an Allen's test. The brachial artery (or less often, the femoral 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.[5]

There are plastic and glass syringes used for blood gas samples.

hemoglobins, dyshemoglobins, bilirubin and electrolytes.[citation needed
]

Derived parameters include bicarbonate concentration, SaO2, and base excess. Bicarbonate concentration is calculated from the measured pH and PCO2 using the Henderson-Hasselbalch equation. SaO2 is derived from the measured PO2 and calculated based on the assumption that all measured hemoglobin is normal (oxy- or deoxy-) hemoglobin.[10]

Calculations

Detail of measurement chamber of a modern blood gas analyzer showing the measurement electrodes. (Cobas b 121 - Roche Diagnostics)

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.[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.[citation needed]

  • pH-stat: The pH and other ABG results are measured at the patient's actual temperature. The goal is to maintain a pH of 7.40 and the arterial carbon dioxide tension (paCO2) at 5.3 kPa (40 mmHg) at the actual patient temperature. It is necessary to add CO2 to the oxygenator to accomplish this goal.
  • α-stat (alpha-stat): The pH and other ABG results are measured at 37 °C, despite the patient's actual temperature. The goal is to maintain the arterial carbon dioxide tension at 5.3 kPa (40mmHg) and the pH at 7.40 when measured at +37 °C.

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.[10]

Guidelines

  1. A 1 mmHg change in PaCO2 above or below 40 mmHg results in 0.008 unit change in pH in the opposite direction.[11]
  2. The PaCO2 will decrease by about 1 mmHg for every 1 mEq/L reduction in [HCO−
    3    ] below 24 mEq/L
  3. A change in [HCO−
    3    ] of 10 mEq/L will result in a change in pH of approximately 0.15 pH units in the same direction.
  4. Assess relation of pCO2 with pH: If pCO2 & pH are moving in opposite directions i.e., pCO2 ↑ when pH is <7.4 or pCO2 ↓ when pH > 7.4, it is a primary respiratory disorder. If pCO2 & pH are moving in same direction i.e., pCO2 ↑when pH is >7.4 or pCO2 ↓ when pH < 7.4, it is a primary metabolic disorder.[12]

Parameters and reference ranges

See also: Reference ranges for blood tests § Acid–base and blood gases

These are typical

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

Analyte Range Interpretation
pH 7.34–7.44[13] The pH or H+ indicates if a person is acidemic (pH < 7.35; H+ >45) or alkalemic (pH > 7.45; H+ < 35).
H+ 35–45 nmol/L (nM)
Arterial
oxygen partial pressure
(PaO2) 		10–13 kPa
75–100 mmHg[13] 		A low PaO2 indicates abnormal oxygenation of blood and a person is known as having hypoxemia. (Note that a low PaO2 is not required for the person to have
supplemental oxygen
should be administered.
Arterial
carbon dioxide partial pressure
(PaCO2) 		4.7–6.0 kPa
35–45 mmHg[13] 		The carbon dioxide partial pressure (Pa
ventilation.[14] A high PaCO2 (respiratory acidosis, alternatively hypercapnia) indicates underventilation (or, more rarely, a hypermetabolic disorder), a low PaCO2 (respiratory alkalosis, alternatively hypocapnia
) hyper- or overventilation.
HCO−3 22–26 mEq/L The HCO−3 ion indicates whether a metabolic problem is present (such as ketoacidosis). A low HCO−3 indicates metabolic acidosis, a high HCO−3 indicates metabolic 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 of 5.33 kPa, full oxygen saturation and 37 Celsius.[15]
Base excess −2 to +2 mmol/L The base excess is used for the assessment of the metabolic component of
acid-base disorders, and indicates whether the person has metabolic acidosis or metabolic alkalosis. Contrasted with the bicarbonate levels, the base excess is a calculated value intended to completely isolate the non-respiratory portion of the pH change.[16]

There are two calculations for base excess (extra cellular fluid - BE(ecf); blood - BE(b)). The calculation used for the BE(ecf) = [HCO−3]− 24.8 + 16.2 × (pH − 7.4). The calculation used for BE(b) = (1 − 0.014 × Hgb) × ([HCO−3]− 24.8 + (1.43 × Hgb + 7.7) × (pH − 7.4).

total CO2 (tCO2 (P)c) 23–30 mmol/L[17]
100–132 mg/dL[18] 		This is the total amount of CO2, and is the sum of HCO−3 and PCO2 by the formula: tCO2 = [HCO−3] + α×PCO2, where α=0.226 mM/kPa, HCO−3 is expressed in millimolar concentration (mM) (mmol/L) and PCO2 is expressed in kPa
O2 Content (CaO2, CvO2, CcO2) 94-100%[19]
(mL O2/dL blood) 		This is the sum of oxygen dissolved in plasma and chemically bound to hemoglobin as determined by the calculation: CaO2 = (PaO2 × 0.003) + (SaO2 × 1.34 × Hgb) where hemoglobin concentration is expressed in g/dL.[20]

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.

pH

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 = 0.6

The normal range for pH is 7.35–7.45. As the pH decreases (< 7.35), it implies

acid-base homeostasis. As carbon dioxide concentrations continue to increase (PaCO2 > 45 mmHg), a condition known as respiratory acidosis occurs. The body tries to maintain homeostasis by increasing the respiratory rate, a condition known as tachypnea. This allows much more carbon dioxide to escape the body through the lungs, thus increasing the pH by having less carbonic acid. If a person is in a critical setting and intubated, one must increase the number of breaths mechanically.[citation needed
]

mechanical ventilator in a critical care setting. The action to be taken is to calm the person and try to reduce the number of breaths being taken to normalize the pH. The respiratory pathway tries to compensate for the change in pH in a matter of 2–4 hours. If this is not enough, the metabolic pathway takes place.[citation needed
]

Under normal conditions, the Henderson–Hasselbalch equation will give the blood pH

where:

The kidney and the liver are two main organs responsible for the metabolic homeostasis of pH. Bicarbonate is a base that helps to accept excess hydrogen ions whenever there is acidaemia. However, this mechanism is slower than the respiratory pathway and may take from a few hours to 3 days to take effect. In acidaemia, the bicarbonate levels rise, so that they can neutralize the excess acid, while the contrary happens when there is alkalaemia. Thus when an arterial blood gas test reveals, for example, an elevated bicarbonate, the problem has been present for a couple of days, and metabolic compensation took place over a blood acidaemia problem.[citation needed]

In general, it is much easier to correct acute pH derangement by adjusting respiration. Metabolic compensations take place at a much later stage. However, in a critical setting, a person with a normal pH, a high CO2, and a high bicarbonate means that, although there is a high carbon dioxide level, there is metabolic compensation. As a result, one must be careful as to not artificially adjust breaths to lower the carbon dioxide. In such case, lowering the carbon dioxide abruptly means that the bicarbonate will be in excess and will cause a metabolic alkalosis. In such a case, carbon dioxide levels should be slowly diminished.[citation needed]

See also

References

  1. ^ Dr Colin Tidy (26 Jan 2015). "Arterial Blood Gases - Indications and Interpretation". Patient. Reviewed by Dr Adrian Bonsall. Retrieved 2017-01-02.
  2. ^ Baillie K. "Arterial Blood Gas Interpreter". prognosis.org. Archived from the original on 2013-03-12. Retrieved 2007-07-05. - Online arterial blood gas analysis
  3. PMID 18308967
    .
  4. PMID 14635973
    .
  5. PMID 29489243. Retrieved August 13, 2020. {{cite journal}}: Cite journal requires |journal= (help
    )
  6. PMID 17722540
    .
  7. ^ Potter, Lewis (7 January 2014). "How to take an Arterial Blood Gas (ABG) - OSCE Guide". Geeky Medics. Retrieved 24 February 2023.
  8. PMID 11568098
    .
  9. ISBN 978-1-56238-545-3. Archived from the original
    on 2015-05-11. Retrieved 2015-04-27.
  10. ^
    PMID 8865418
    .
  11. ^ Stoelting: Basics of Anesthesia, 5th ed. p 321.
  12. ^ "Arterial Blood Gas (ABG) In 4 Steps". www.edulanche.com/. EduLanche. Retrieved 2016-05-13.
  13. ^ a b c Normal Reference Range Table from The University of Texas Southwestern Medical Center at Dallas. Used in Interactive Case Study Companion to Pathologic basis of disease.
  14. ^ Baillie K, Simpson A. "Altitude oxygen calculator". Apex (Altitude Physiology Expeditions). Archived from the original on 2017-06-11. Retrieved 2006-08-10. - Online interactive oxygen delivery calculator
  15. ^ "Acid Base Balance (page 3)". June 13, 2002. Archived from the original on 2002-06-13.
  16. ^ "RCPA Manual: Base Excess (arterial blood)".
  17. ^ "ABG (Arterial Blood Gas)". Brookside Associates. Retrieved 2017-01-02.
  18. ^ Derived from molar values using molar mass of 44.010 g/mol
  19. ^ "Blood Gases". Retrieved 2023-04-18.
  20. ^ "Hemoglobin and Oxygen Transport Charles L". www.meddean.luc.edu.

External links

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