an arterial blood gas

8/8/2019 An Arterial Blood Gas

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An arterial blood gas (ABG) is ablood test that is performed usingblood from an artery. Itinvolves puncturing an artery with a thin needle and syringe and drawing a small volume ofblood. The most common puncture site is theradial artery at the wrist, but sometimes thefemoralartery in the groin or other sites are used. The blood can also be drawn from an arterial catheter.

The test is used to determine thepH of the blood, thepartial pressureofcarbon dioxide and

oxygen, and thebicarbonatelevel. Many blood gas analyzers will also report concentrations oflactate,hemoglobin, several electrolytes, oxyhemoglobin,carboxyhemoglobinandmethemoglobin. ABG testing is mainly used inpulmonology, to determinegas exchange levelsin the blood related to lung function, but 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.

[edit] Extraction and analysisArterial blood for blood gas analysis is usually extracted by aphlebotomist,nurse, orrespiratory

therapist.[3] Blood is most commonly drawn from theradial artery because it is easily accessible,can be compressed to control bleeding, and has less risk for occlusion. The femoral artery (orless often, thebrachial artery) is also used, especially during emergency situations or withchildren. 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 ofheparin, to preventcoagulation orneeds 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 candissolve into the sample and cause inaccurate results. The sealed syringe is taken to ablood gasanalyzer. If the sample cannot be immediately analyzed, it is chilled in an ice bath in a glasssyringe to slow metabolic processes which can cause inaccuracy. Samples drawn in plasticsyringes are not iced and are analyzed within 30 minutes.[4]

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

[edit] Calculations

Arterial blood gas device.
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The machine used for analysis aspirates this blood from the syringe and measures thepHand thepartial pressuresof oxygen and carbon dioxide. The bicarbonate concentration is also calculated.These results are usually available for interpretation within five minutes.

Much controversy exists about optimal blood gas management of hypothermic patients.[citationneeded] 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 -statmethod 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 isthen necessary to add CO2 to the sample to calculate results.

-stat (alpha-stat): the arterial carbon dioxide tension and the pH are maintained at 5.3kPa (40mmHg) and 7.40 when measured at +37C. When a patient is cooled down, thepH-value will increase and the pCO2-value and the pO2-value will decrease withlowering of the temperature if measured at the patients temperature.

Both the pH-stat and alpha-stat strategies have theoretical disadvantages. -stat method is themethod of choice for optimal myocardial function. The pH-stat method may result in loss ofautoregulation in the brain (coupling of the cerebral blood flow with the metabolic rate in thebrain). By increasing the cerebral blood flow beyond the metabolic requirements, the pH-statmethod may lead to cerebral microembolisation and intracranial hypertension.[5]

[edit] Helpful Guidelines

1. A 1mmHg change in PaCO2 above or below 40 mmHg results in 0.008 unit change in pHin 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.

[edit] Reference ranges and interpretationThese are typical reference ranges, although various analysers and laboratories may employdifferent ranges.

Analyte Range Interpretation

pH 7.357.45ThepHor H+ indicates if a patient is acidotic (pH < 7.35; H+ >45) oralkalemic (pH > 7.45; H+ < 35).

H+3545 nmol/L(nM)

See above.

PaO2

9.313.3 kPaor 80100mmHg

A low O2 indicates that the patient is not respiring properly, and is

hypoxemic. At a PaO2 of less than 60 mm Hg, supplemental oxygenshould be administered. At a PaO2 of less than 26 mmHg, the patientis at risk of death and must be oxygenated immediately.

PaCO2 4.76.0 kPa or3545 mmHg

The carbon dioxide partial pressure (PaCO2) indicates a respiratoryproblem: for a constant metabolic rate, the PaCO2 is determinedentirely by ventilation.[7] A high PaCO2(respiratory acidosis)indicates underventilation, a low PaCO2 (respiratory alkalosis) hyper-or overventilation. PaCO2 levels can also become abnormal when the
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respiratory system is working to compensate for a metabolic issue soas to normalize the blood pH. An elevated PaCO2 level is desired insome disorders associated with respiratory failure; this is known aspermissive hypercapnia.

HCO3 2226

mmol/L

The HCO3 ion indicates whether ametabolic problem is present

(such as ketoacidosis). A low HCO3

indicatesmetabolic acidosis, ahigh HCO3 indicates metabolic alkalosis. HCO3

levels can alsobecome abnormal when the kidneys are working to compensate for arespiratory issue so as to normalize the blood pH.

SBCe21 to 27mmol/L

the bicarbonate concentration in the blood at a CO2 of 5.33 kPa, fulloxygen saturation and 37 degrees Celsius.[8]

Base excess3 to +3mmol/L

The base excess is used for the assessment of the metaboliccomponent of acid-base disorders, and indicates whether the patienthas metabolic acidosis or metabolic alkalosis. A negative base excessindicates that the patient has metabolic acidosis (primary orsecondary to respiratory alkalosis). A positive base excess indicatesthat the patient has metabolic alkalosis (primary or secondary torespiratory acidosis).[9]

HPO42 0.8 to 1.5

[10]

mM

total CO2(tCO2 (P)c)

25 to 30mmol/L

This is the total amount of CO2, and is the sum of HCO3 and PCO2

by the formula:tCO2 = [HCO3

] + *PCO2, where =0.226 mM/kPa, HCO3 is

expressed in millimolar concentration (mM) (mmol/l) and PCO2 isexpressed in kPa [11]

O2 Content(CaO2,CvO2, CcO2)

vol% (mLoxygen/dLblood)

This is the sum of oxygen dissolved in plasma and chemically boundto hemoglobin. [12]

Contamination with room air will result in abnormally low carbon dioxide and (generally)normal oxygen levels. Delays in analysis (without chilling) may result in inaccurately lowoxygen and high carbon dioxide levels as a result of ongoing cellular respiration.

Lactate level analysis is often featured on blood gas machines in neonatal wards, as infants oftenhave elevatedlactic acid.
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an arterial blood gas

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