- Arterial blood gas sampling
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gassampling is a medical technique used to check gas levels in the blood. It typically involves using a thin needle and syringeto puncture an artery, usually in the wrist, and withdraw a small amount of blood. This technique is useful for making sure that certain parts of the blood's chemistryare normal. This technique is commonly used on patients whose breathing is controlled by a mechanical respiratoror who are having serious difficulties with breathing. When this procedure is performed on a small artery in the wrist, it is unlikely to cause serious complications, and the information gained may save the patient's life.
The purpose of arterial blood gas sampling is to assess patients'
respiratorystatus as well as acid-base balanceor for laboratory testing when venous blood is unavailable, and is frequently requested for seriously ill patients. An arterial blood gas (ABG) will help in the assessment of oxygenation, ventilation, and acid-base homeostasis. It can also aid in the determination of poisonings ( carboxyhemaglobinemiaor methemaoglobinemia) and in the measurement of lactate concentration.
It is sometimes called "arterial blood gas analysis" or "ABG" sampling."
Proper collection of arterial blood samples.
Arterial Blood Gas Test Definition
Blood is drawn anaerobically from a peripheral
artery( Radial, Brachial, Ulnar, Femoral, Axillary, Posterior Tibialor Dorsalis pedis) via a single percutaneous needle puncture, or from an indwelling arterial cannulaor catheterfor multiple sample.
Each method provides a blood
specimenfor direct measurement of:
*partial pressures of
hydrogen ionactivity (pH);
Purpose and indications
The purpose of arterial blood gas sampling is to assess patients respiratory status as well as acid base balance or for laboratory testing when venous blood is unavailable, and is frequently requested for seriously ill patients. So, an arterial blood gas (ABG) will help in the assessment of oxygenation, ventilation, and acid-base homeostasis. It can also aid in the determination of poisonings (carboxyhemaglobinemia or methemaoglobinemia) and in the measurement of
Pulse oximetry will give a reasonable estimate of the adequacy of oxygenation in many circumstances but does not assess acid-base status or ventilation and should not be used alone in cases where these measurements are important.
Basic conditions diagnosed by ABG's:
*Anything which prevents the body from getting rid of excess CO2, increases acid which decreases pH
*Anything which makes the body lose CO2, decreases acid, which increases pH
*Anything which increases HCO3 increases base which increases
*Anything which decreases HCO3 decreases base which decreases pH
Apart from helping to establish a diagnosis, blood gases may also help to as certain the severity of a particular condition (e.g. metabolic acidosis in sepsis). This information can help to establish diagnosis, monitor severity, progression, and prognosis as well as guide therapy of:
Table 1. Normal Values in an ABG report at sea level:
The pH of the blood is maintained within a normal range by a number of compensatory mechanisms, the most important being the body buffer mechanisms and the renal and respiratory systems. The degree of compensation varies between individuals and depends on the severity and duration of the primary problem and associated medical comorbidities. Respiratory compensation for metabolic problems is usually rapid and almost complete. The lungs respond quickly by increasing ventilation to blow off excessive carbon-dioxide (in
metabolic acidosis) or decreasing ventilation to retain carbon-dioxide (in metabolic alkalosis). The latter compensation is less complete than the former for obvious reasons. The renal compensation for respiratory imbalances is slow and incomplete. The kidneys regulate extracellular fluid H+ ion concentration by secretion of H+ ions, reabsorption of filtered HCO3− ions, and the production of new HCO3− ions. Excess HCO3− is filtered into the renal tubules and eliminated in the urine. Depending on the need to excrete either an acid or a base load, the kidneys can excrete urine with a pH ranging from 4.5 to 8.0. A rough guide to the degree of compensation to primary changes in CO2 and HCO3 as a result of respiratory and metabolic imbalances respectively is shown in table 3.
Table 3. Metabolic Alkalosis:
Metabolic alkalosis can result from the loss of acid, addition of alkali or both in the kidneys or elsewhere. Extrarenal sites include stomach (loss of acid), redistribution of alkali from the intracellular stores to the ECF (as in potassium or chloride depletion), oral administration (antacids, ion-exchange resins, milk alkali syndrome, oral HCO3−) and parenteral administration of alkali (citrate in blood transfusions, bicarbonate in severe metabolic acidosis). Renal causes of alkali excess include mineralocorticoid excess, response to long-standing hypercapnia (persists even after correction of respiratory acidosis), hypokalemia (promotes H+ secretion in the distal nephron) and ECF volume depletion (impaired HCO3− excretion). Certain conditions can cause metabolic alkalosis by a number of mechanisms (e.g. diuretic use causes both ECF depletion and hypokalemia).
The principal cause of respiratory alkalosis (hypocapnia) is hypoxia and its causes (type I respiratory failure), further treatment of which has been detailed before. Other causes of acute respiratory alkalosis include anxiety, fever, pain, sepsis, hepatic failure, CNS disorders (stroke, infections), pulmonary disorders without hypoxia (infections and interstitial lung disease), delirium tremens and drugs (salicylate intoxication). Chronic causes include high altitude hypoxia, chronic hepatic failure, chronic pulmonary disease, CNS trauma, anaemia, hyperthyroidism, beriberi and pregnancy. Treatment should be directed towards the cause.
Contraindications/concerns for arterial puncture
Approved puncture sites include radial, dorsalis pedis, and brachial arteries. The brachial artery will not be used on patients in Children’s Hospital. In the Emergency Department, femoral artery is an approved puncture site.Brachial and femoral arteries should be reserved as a last option. The radial artery on non dominant hand is the ideal site for an arterial puncture for the following reasons:
*It is small, but superficial and easily accessible, and stabilized.
*It is easily compressible with better control of bleeding
*There is no nerve near by to worry about.
*The collateral arch with ulnar artery minimizes the risk of occlusion.
The syringe has to be heparinized to prevent clotting. It is important to have the right amount of heparin in the syringe. “Too much” or “too little heparin can alter the results.”
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