Blood gas interpretation & calculations

Written by miriam breeze
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Blood gas interpretation & calculations
Arterial Blood Gas Readings (brightly coloured science test tubes image by Steve Johnson from Fotolia.com)

The normal pH of human blood is between 7.35 and 7.45. Any readings above or below this range are incompatible with life. Two body systems are responsible for maintaining the narrow range: the renal system via the kidneys and the respiratory systems via the lungs.

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Classifying Arterial Blood Gas PH

An arterial blood gas (ABG) reading with pH above 7.45 is classified as alkaline/alkolotic. A pH below 7.35 is classified as acid/acidic.

Arterial blood gas pH readings are classified as alkalotic, acidic or normal. Abnormal readings are attributed to either a disorder of the renal system (metabolic), or the respiratory system. The four classifications of pH disorders are metabolic alkalosis, metabolic acidosis, respiratory alkalosis and respiratory alkalosis.

Blood gas interpretation & calculations
Respiratory System in Ph Balance (thorax x-ray of the lungs image by JoLin from Fotolia.com)

Normal Arterial Blood Gas Readings

Three readings provide the information necessary to classify the source of pH imbalance:

PaCO2: Respiratory--Measurement of partial pressure of carbon dioxide in the lungs. The normal range is from 35 to 45mm/Hg (millimetres of mercury). Readings above 45 are acidic and below 35 are alkaline.

HCO3: Metabolic--Measurement of bicarbonate from the renal system. The normal range is 22 to 26 mEq/L (milliequivalents per litre). Readings below 22 are acidic and above 26 are alkaline.

PH: Alkalotic, Acidic or Normal--Measurement of Arterial blood PH.

Blood gas interpretation & calculations
Renal System in PH Balance (urine image by Stef Run from Fotolia.com)

Alkalosis or Acidosis

According to the self-learning packet "Interpretation of the Arterial Blood Gases," Step 1 of interpreting the readings is "to determine if the blood is within normal range." Readings above 7.45 are alkalotic, and readings below 7.35 are acidic."

Blood gas interpretation & calculations
PH of Blood Acidic, Alkalotic or Normal (button question symbol blue neon icon image by Andrey Zyk from Fotolia.com)

Metabolic or Respiratory

The self-learning packet explains that Step 2 and 3 are to determine whether the respiratory system or the renal system is responsible for the imbalance. Determine which system is causing the imbalance by matching the pH of the blood to either the renal or respiratory system.

Excessive carbon dioxide/CO2 in the respiratory system makes the blood more acidic but a deficit makes the blood more alkaline; excessive bicarbonate/HCO3 in the renal system makes the blood more alkaline but a deficit makes the blood more acidic. The body system that matches the blood pH imbalance is the system responsible for the imbalance.

Blood gas interpretation & calculations
Matching PH of Arterial Blood to Metabolic or Respiratory Cause (test results being analysed image by Keith Frith from Fotolia.com)

Calculating the ABG

Dysfunctions of the renal system are classified as metabolic. Metabolic alkalosis is indicated if the pH of the arterial blood is greater than 7.45 and the HCO3 measurement is greater than 26 mEq/L.

Metabolic acidosis is indicated if the pH of the arterial blood is less than 7.35, and the HCO3 measurement is greater than 26 mEq/L.

Dysfunctions of the respiratory system are classified as respiratory. Respiratory alkalosis is indicated if the pH of the arterial blood is greater than 7.45, and the PaCO2 measurement is less than 35mm/Hg.

Respiratory acidosis is indicated if the pH of the arterial blood is less than 7.35, and the PaCO2 measurement is greater than 45mm/Hg.

Blood gas interpretation & calculations
Analysing and Interpreting Arterial Blood Gas Readings (hand s image by Andrey Kiselev from Fotolia.com)

Compensation

If the body attempts to correct the pH imbalance, the blood gas readings of the system that is functioning normally may be outside of the normal range, but the body system with a blood gas reading that matches the pH of the blood is causing the imbalance while the other system is trying to correct the imbalance.

Blood gas interpretation & calculations
Compensation to Return the PH Balance to Normal (balance image by YvesBonnet from Fotolia.com)

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