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Arterial Blood Gas, ABG

Arterial Blood Gas

An arterial blood gas, it is a test of blood. It measures the acidity (pH), level of oxygen and carbon dioxide from the blood. This test shows the function of the lungs. It shows, how well your lungs are able to move oxygen  and carbon dioxide into the blood.

Measured Parameters

pH-  Measurement of Hydrogen ion concentration in the blood and denotes acidity or alkanity

HCO3– Measurement of Bicarbonate in the blood normal 18-14mml/L

PaCO2– Partial pressure of Carbon Dioxide in blood normal 35-50 mmHg

PaO2- Arterial partial pressure of oxygen in blood normal 60-80 mm Hg

BE- Base excess measurement at metabolic component of acid acid base balance normal-5-+3meq/L

H+ – Hydrogen ion concentration in milli equivalents per litre in blood normal 35-45 meq/L

Terms

Acididemia: Elevated hydrogen ions in blood (pH falls)

Alkalemia: Decreased hydrogen ions in the blood (pH rises)

Metabolic Acidosis: BE LESS THAN -5 i.e. metabolic factors that favor decreased H+ concentration

Metabolic Alkalosis: BE greater than +3 i.e. metabolic factors that favor decreased H+ concentration

Respiratory Acidosis:  PCO2 greater than 50 mmHg.

Respiratory Alkalosis: PCO2 less than 35 mmHg.

Compensation: Occurs in response to primary disturbance in acid-base equilibrium whereby the change in the pH is relieved.

Acid Base Balance

Acid

An Acid is a substance , which gives up hydrogen ions (H+) when in solution. Some substances give up hydrogen ions more readily than others. These are strong acids, such as hydrochloric acid. Carbonic acid does not readily give up hydrogen ions so this is a weak acid.

Base

A base is an alkaline substance, which accepts hydrogen ions. When an acid is added to base in solution, a weaker acid is formed as the base accepts the hydrogen ions given up by the acid.

pH

The acidity of alkalinity of a solution is determined by the hydrogen ion concentration. The hydrogen ion concentration is expressed as a value known as pH   the pH of a solution is inversely proportional to the hydrogen ion concentration. So a solution with a pH of 2.0 has more hydrogen ions than a solution with a pH of 3.0 and therefore is more acidic. A change of 1 unit in pHis equal to a change of 10 units in the hydrogen ion concentration, so a small change in pH is significant.

Buffers

A buffer minimizes changes in the ph of a solution when acid or base is added. This means that a strong acid or base will become weaker. Normally sufficient buffers exist in the body to maintain the pH within narrow limits.

There are three important buffers in the body…..

  1. Bicarbonate
  2. Hemoglobin
  3. Phosphate

Physiological Maintenance of Acid / Base Balance

Normal metabolism results in the production of acids, which must be neutralized before being excreted from the body.

There are main methods of excretion of acids by the body…

The Lungs

This is the main path of excretion for carbonic acid, which is converted to carbon dioxide and water. If the blood pH falls the respirator center in the medulla of the brain reflects the change and triggers an increase in the rate of breathing to blow off the carbon dioxide.

The Kidneys

This is the route of excretion of hydrogen ions. The Kidneys can also conserve bicarbonate.

Hemoglobin

A protein buffer in hemoglobin acts to neutralize acid, particularly carbon acid which it converts to bicarbonate before releasing it as sodium bicarbonate into the plasma

Normal Blood Gas Values

When considering blood gases we usually consider 5 values

  1. pH
  2. partial pressure of oxygen (PaO2)
  3. partial pressure of carbon dioxide (PaCO2)
  4. bicarbonate(HCO3)
  5. base excess(BE)

pH

The pH, is a measure of the hydrogen ion concentration in the blood and denotes acidity or alkalinity.

The normal pH of  arterial  blood is 7.35 to 7.45 .levels below 7.35 show increasing acidity and levels above 7.45 show increasing alkalinity.

PaO2

This  measures the partial pressure of oxygen dissolved in the blood. The normal PaO2 of arterial blood is 60-80mmHg.

PaCO2

This is the partial pressure of carbon dioxide in the blood. The normal PaCO2 level in arterial blood is 35-45mmHg.

HCO3

This measures the amount of bicarbonate in the blood.

The normal limits in arterial blood are 18-24mm/l.

Base Excess

The base excess indicates the amount