Arterial blood gas

Arterial blood gas
Diagnostics
MeSH	D001784
MedlinePlus	003855

An arterial blood gas (ABG) test is a blood gas test of blood from an artery; it is thus a blood test that measures 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, 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), arterial carbon dioxide tension (PaCO₂), and acidity (pH). In addition, arterial oxygen saturation (SaO₂) can be determined. 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 similar information less invasively.

Modern blood gas analyzer. This device is capable of reporting pH, pCO₂, pO₂, SatO₂, Na⁺, K⁺, Cl⁻, Ca²⁺, Hemoglobin (total 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, 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

File:BGA.jpg

Blood gas analyzer

Arterial blood for blood gas analysis is usually drawn by a respiratory therapist and sometimes a phlebotomist, nurse, paramedic 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 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.

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. 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 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 (i.e., greater than 30 min) 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

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.

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 (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.

Helpful 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.
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

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

Analyte	Range	Interpretation
pH	7.34-7.44	The pH or H+ indicates if a patient is acidemic (pH < 7.35; H+ >45) or alkalemic (pH > 7.45; H+ < 35).
H+	35–45 nmol/L (nM)	See above.
Arterial oxygen partial pressure (PaO2)	11-13 kPa or 75-100 mmHg	A low PaO2 indicates that the patient is not oxygenating properly, and is hypoxemic. (Note that a low PaO2 is not required for the patient to have hypoxia.) 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.
Arterial carbon dioxide partial pressure (PaCO2)	4.7-6.0 kPa or 35-45 mmHg	The carbon dioxide partial pressure (PaCO2) is an indicator of CO2 production and elimination: for a constant metabolic rate, the PaCO2 is determined entirely by its elimination through ventilation. 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.
HCO3−	22–26 mEq/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. 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.
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 patient 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. There are two calculations for base excess (extra cellular fluid - BE(ecf); blood - BE(b)). The calculation used for the BE(ecf) = cHCO3 - 24.8 +16.2 X (pH-7.4). The calculation used for BE(b) = (1-0.014 x hgb) x (cHCO3 - 24.8 + (1.43 x hgb + 7.7) x (pH -7.4).
total CO2 (tCO2 (P)c)	23-30 mmol/L or 100-132 mg/dL	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
O2 Content (CaO2, CvO2, CcO2)	vol% (mL oxygen/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.

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. A calculator for predicted reference normal values of arterial blood gas parameters is available online.

pH

The normal range for pH is 7.35–7.45. As the pH decreases (<7.35), it implies acidosis, while if the pH increases (>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 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 (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 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 (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. If this is not enough, the metabolic pathway takes place.

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

$pH = 6.1 + \log_{10} \left ( \frac{[HCO_3^-]}{0.03 \times PaCO_2} \right )$

, 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 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

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

• Acid-base homeostasis
• Anion gap
• Mechanical ventilation
• Radial artery puncture
• Acidosis
• Alkalosis
• Chemical equilibrium
• pCO2
• pH
• pKa

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 presentations