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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. 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 (PaO₂), carbon dioxide tension (PaCO₂), and acidity (pH). In addition, arterial oxyhemoglobin saturation (SaO₂) 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 (ICUs).

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,^[1] nomograms, and rules of thumb^[2] are commonly used.

Sampling and analysis[link]

Arterial blood for blood gas analysis is usually drawn by a respiratory therapist and sometimes a phlebotomist, nurse or doctor.^[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 selection of which radial artery to draw from is based on the outcome of an Allen's test. 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 analyzed within 10–15 minutes, it must be placed on ice for valid results. Even when placed on ice, samples should still be analyzed within 1 hour.^[4]

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

Calculations[link]

Blood gas analyzer

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 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 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[link]

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.

Parameters and reference ranges[link]

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 (without chilling the sample) may result in inaccurately low oxygen and high carbon dioxide levels as a result of ongoing cellular respiration.

pH[link]

The normal range for pH is 7.35-7.45. As the pH decreases (<7.35), it infers acidosis, while if the pH increases (>7.45) it infers 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 (Pa CO2 > 45 mmHg), 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 lowering 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 alkolosis (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 taken to normalise the pH. The respiratory pathway tries to compensate for the change in pH in a matter of 2–4 hours. However 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, one must immediately comprehend that the problem has been present over a couple of days, and metabolic compensation took place over a blood acedemia problem.

Most often than not it is much more easier to correct acute pH derangements by adjusting respirations. 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 abrubtly, means that the bicarbonate will be in excess, and thus will cause a metabolic alkalosis. In such a case, Carbon Dioxide levels should be slowly diminished.

See also[link]

• Acid-base homeostasis
• Anion gap
• Mechanical ventilation
• Radial artery puncture

References[link]

External links[link]

• 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

Blood gas is a term used to describe a laboratory test of blood where the purpose is primarily to measure ventilation and oxygenation. The source is generally noted by an added word to the beginning; arterial blood gases come from arteries, venous blood gases come from veins and capillary blood gases come from capillaries^[1].

Values measured[link]

• pH — The acidity or basicity of the blood.
• PaCO2 — The partial pressure of carbon dioxide in the blood.
• PaO2 — The partial pressure of oxygen in the blood.
• HCO3 — The level of bicarbonate in the blood.
• BE — The base-excess of bicarbonate in the blood.

Purposes for testing[link]

• Acidosis^[1]
• Diabetic ketoacidosis^[1]
• Lactic acidosis^[1]
• Metabolic acidosis^[1]
• Respiratory acidosis^[1]
• Respiratory alkalosis^[1]

Abnormal results[link]

Abnormal results may be due to lung, kidney, or metabolic diseases. Head or neck injuries or other injuries that affect breathing can also lead to abnormal results^[1].

References[link]

1. ^ ^{a b c d e f g h} Seifter JL. Acid-base disorders. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 119

Arterial blood is the oxygenated blood in the circulatory system found in the lungs, the left chambers of the heart, and in the arteries.^[1] It is bright red in color, while venous blood is dark red in color (but looks purple through the opaque skin). It is the contralateral term to venous blood.

Framed in the cardiac cycle, often historically accredited to the Wiggers diagram, arterial blood has just passed through the lungs and is ready to boost oxygen to sustain the peripheral organs. The essential difference between venous and arterial blood is the curve of the oxygen saturation of hemoglobin. The difference in the oxygen content of the blood between the arterial blood and the venous blood is known as the arteriovenous oxygen difference.^[2]

References[link]