Arterial blood gas analizer

1. Arterial blood gas
An arterial blood gas (ABG) test is a blood gas test of
blood from an artery; it is thus a blood test that mea-
sures the amounts of certain gases (such as oxygen and
carbon dioxide) dissolved in arterial blood. An ABG
test 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,[1]
but sometimes the femoral artery in the groin or other
sites are used. The blood can also be drawn from an
arterial catheter. An ABG test measures the blood gas
tension values of arterial oxygen tension (PaO2), arte-
rial carbon dioxide tension (PaCO2), and acidity (pH). In
addition, arterial oxygen saturation (SaO2) can be deter-
mined. Such information is vital when caring for patients
with critical illness or respiratory disease. Therefore, the
ABG test is one of the most common tests performed on
patients in intensive care units (ICUs). In other levels of
care, pulse oximetry plus transcutaneous carbon dioxide
measurement is an alternative method of obtaining simi-
lar information less invasively.
Modern blood gas analyzer. This device is capable of reporting
pH, pCO2, pO2, SatO2, Na+
, K+
, Cl−
, Ca2+
, Hemoglobin (to-
tal and derivatives: O2Hb, MetHb, COHb, HHb, CNHb, SHb ),
Hematocrit, Total bilirubin, Glucose, Lactate and Urea. (Cobas
b 221 - Roche Diagnostics).
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, sev-
eral electrolytes, 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,[2]
nomograms, and rules of thumb[3]
are com-
monly used.
1 Sampling and analysis
Blood gas analyzer
Arterial blood for blood gas analysis is usually drawn
by a respiratory therapist and sometimes a phlebotomist,
nurse, paramedic or doctor.[4]
Blood is most commonly
drawn from the radial artery because it is easily accessi-
ble, 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
brachial artery (or less often, the femoral artery) is also
used, especially during emergency situations or with chil-
dren. Blood can also be taken from an arterial catheter
already placed in one of these arteries.
There are plastic and glass syringes used for blood gas
samples. Most syringes come pre-packaged and con-
tain a small amount of heparin, to prevent coagulation.
Other syringes may need to be heparinised, by drawing
up a small amount of liquid heparin and squirting it out
again to remove air bubbles. Once the sample is ob-
tained, care is taken to eliminate visible gas bubbles, as
these bubbles can dissolve into the sample and cause in-
accurate results. The sealed syringe is taken to a blood
gas analyzer. If a plastic blood gas syringe is used, the
sample should be transported and kept at room temper-
ature and analyzed within 30 min. If prolonged time
1

2. 2 2 PARAMETERS AND REFERENCE RANGES
delays are expected (i.e., greater than 30 min) prior to
analysis, the sample should be drawn in a glass syringe
and immediately placed on ice.[5]
Standard blood tests
can also be performed on arterial blood, such as mea-
suring glucose, lactate, hemoglobins, dys-haemoglobins,
bilirubin and electrolytes.
1.1 Calculations
Detail of measurement chamber of a modern blood gas analyzer
showing the measurement electrodes. (Cobas b 121 - Roche Di-
agnostics)
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 concen-
tration is also calculated. These results are usually avail-
able for interpretation within five minutes.
Two methods have been used in medicine in the manage-
ment 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 pH and other ABG results are mea-
sured 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 car-
bon 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 theoreti-
cal 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 (cou-
pling of the cerebral blood flow with the metabolic rate
in the brain). By increasing the cerebral blood flow be-
yond the metabolic requirements, the pH-stat method
may lead to cerebral microembolisation and intracranial
hypertension.[6]
1.2 Helpful guidelines
1. A 1 mmHg change in PaCO2 above or below 40
mmHg results in 0.008 unit change in pH in the op-
posite direction.[7]
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.
2 Parameters and reference ranges
These are typical reference ranges, although various anal-
ysers and laboratories may employ different ranges.
Contamination of the sample with room air will result in
abnormally low carbon dioxide and possibly elevated oxy-
gen levels, and a concurrent elevation in pH. Delaying
analysis (without chilling the sample) may result in in-
accurately low oxygen and high carbon dioxide levels as
a result of ongoing cellular respiration. A calculator for
predicted reference normal values of arterial blood gas
parameters is available online.
2.1 pH
The normal range for pH is 7.35–7.45. As the pH de-
creases (<7.35), it implies acidosis, while if the pH in-
creases (>7.45) it implies alkalosis. In the context of
arterial blood gases, the most common occurrence will
be that of respiratory acidosis. Carbon dioxide is dis-
solved in the blood as carbonic acid, a weak acid; how-
ever, in large concentrations, it can affect the pH dras-
tically. Whenever there is poor pulmonary ventilation,
the carbon dioxide levels in the blood are expected to
rise. This leads to a rise of carbonic acid, leading to a
decrease in pH. The first buffer of pH will be the plasma
proteins, since these can accept some H+ ions to try and
maintain 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 pa-
tient is in a critical setting and intubated, one must in-
crease the number of breaths mechanically.
On the other hand, respiratory alkalosis (Pa CO2 <
35mmHg) occurs when there is too little carbon dioxide
in the blood. This may be due to hyperventilation or else
excessive breaths given via a mechanical ventilator in a
critical care setting. The action to be taken is to calm
the patient and try to reduce the number of breaths being

3. 3
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.
Under normal conditions, the Henderson–Hasselbalch
equation will give the blood pH
pH = 6.1 + log10
(
[HCO−
3 ]
0.03 × PaCO2
)
, where:
• 6.1 is the acid dissociation constant of carbonic acid
(H2CO3) at normal body temperature
• [HCO3
−
] is the concentration of bicarbonate in the
blood in mEq/L
• PaCO2 is the partial pressure of carbon dioxide in
the arterial blood in torr
The kidney and the liver are two main organs responsi-
ble for the metabolic homeostasis of pH. Bicarbonate is
a base that helps to accept excess hydrogen ions when-
ever 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 ex-
cess acid, while the contrary happens when there is alka-
laemia. 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 acedemia problem.
In general, it is much easier to correct acute pH derange-
ment by adjusting respiration. Metabolic compensations
take place at a much later stage. However, in a critical set-
ting, a patient 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 re-
sult 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.
3 See also
• Acid-base homeostasis
• Anion gap
• Mechanical ventilation
• Radial artery puncture
• Acidosis
• Alkalosis
• Chemical equilibrium
• pCO2
• pH
• pKa
4 References
[1] “Arterial Blood Gases - Indications and Interpretation”.
patient.info/doctor. 20 December 2010. Retrieved 10
February 2013.
[2] Baillie K. “Arterial Blood Gas Interpreter”. prognosis.org.
Retrieved 2007-07-05. - Online arterial blood gas analysis
[3] Baillie, JK (2008). “Simple, easily memorised “rules of
thumb” for the rapid assessment of physiological compen-
sation for acid-base disorders”. Thorax 63 (3): 289–90.
doi:10.1136/thx.2007.091223. PMID 18308967.
[4] 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.
[5] Procedures for the Collection of Arterial Blood Speci-
mens; Approved Standard—Fourth Edition (Procedures
for the Collection of Arterial Blood Specimens; Approved
Standard—Fourth Edition ). Clinical and Laboratory
Standards Institute. 2004. ISBN 1-56238-545-3.
[6] Kofstad J (1996). “Blood Gases and Hypothermia: Some
Theoretical and Practical Considerations”. Scand J Clin
Lab Invest. (Suppl) 224: 21–26. PMID 8865418.
[7] Stoelting: Basics of Anesthesia, 5th ed. p 321.
[8] 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.
[9] Derived from mmHg values using 0.133322 kPa/mmHg
[10] Baillie K, Simpson A. “Altitude oxygen calculator”. Apex
(Altitude Physiology Expeditions). Retrieved 2006-08-
10. - Online interactive oxygen delivery calculator
[11] Acid Base Balance (page 3)
[12] RCPA Manual: Base Excess (arterial blood)
[13] The Medical Education Division of the Brookside
Associates--> ABG (Arterial Blood Gas) Retrieved on
Dec 6, 2009
[14] Derived from molar values using molar mass of 44.010
g/mol
[15] CO2: The Test
[16] Hemoglobin and Oxygen Transport. Charles L. Webber,
Jr., Ph.D.

4. 4 5 EXTERNAL LINKS
5 External links
• Online arterial blood gas calculator
• An online model of arterial blood gas changes with
respiration
• Interactive ABG quiz
• Practice interpreting sample arterial blood gas pre-
sentations

5. 5
6 Text and image sources, contributors, and licenses
6.1 Text
• Arterial blood gas Source: https://en.wikipedia.org/wiki/Arterial_blood_gas?oldid=719958955 Contributors: Bryan Derksen, Alex.tan,
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Izkala and Anonymous: 133
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