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, carbon dioxide tension, and acidity. In addition,
arterial oxyhemoglobin saturation 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.
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 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.
Sampling and analysis
Arterial blood for blood gas analysis is usually drawn by a respiratory therapist and sometimes
a phlebotomist, nurse or doctor. 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 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. There are plastic and glass syringes used
for blood gas samples. Most syringes come pre-packaged and contain a small amount of
heparin, to prevent coagulation or 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
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 a plastic blood gas syringe is used, the sample should be transported and
kept at room temperature and analyzed within 30 min. If prolonged time delays are expected
prior to analysis, the sample should be drawn in a glass syringe and immediately placed
on ice . 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.
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 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 at 5.3 kPa at the actual patient temperature. It is necessary
to add CO2 to the oxygenator to accomplish this goal.
α-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 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. 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 1 mmHg 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.
Parameters and reference ranges These are typical reference ranges, although
various analysers and laboratories may employ different ranges.
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
may result in inaccurately low oxygen and high carbon dioxide levels as a result of
ongoing cellular respiration. pH
The normal range for pH is 7.35–7.45. As the pH decreases, it implies acidosis, while
if the pH increases it implies alkalosis. In the context of arterial blood gases, the
most common occurrence will be that of respiratory acidosis. Carbon dioxide is dissolved in the
blood as carbonic acid, a weak acid; however, in large concentrations, it can affect the
pH drastically. 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, a condition known as respiratory acidosis occurs. The body tries to maintain
homeostasis by increasing the respiratory rate, a condition known as tachypneoa. This
allows much more carbon dioxide to escape the body through the lungs, thus increasing
the pH by having less carbonic acid. If a patient is in a critical setting and intubated,
one must increase the number of breaths mechanically. On the other hand, respiratory alkalosis 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
taken to normalise 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. 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 neutralise 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 acedemia problem.
In general, it is much easier to correct acute pH derangements by adjusting respiration.
Metabolic compensations take place at a much later stage. However, in a critical setting,
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 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.
See also Acid-base homeostasis
Anion gap Mechanical ventilation
Radial artery puncture References
External links Blood Gas Interpretation - Simplified process
for blood gas interpretation. Video tutorial on arterial blood gas
Online arterial blood gas interpreter An online model of arterial blood gas changes
with respiration Interactive ABG quiz
Practice interpreting sample arterial blood gas presentations