Five step technique to blood gas analysis

Interpreting the results of an arterial blood sample to determine a patient’s acid-base status and respiratory gas exchange is a key component in the management of any acutely unwell patient. The real value of an ABG comes from its ability to provide an almost real time reflection of the physiology of your patient, allowing for rapid management of any abnormalities. Below you will find the 5 step technique to interpreting arterial blood gas (ABG) results:

  • Step 1: How is the patient?

    The first question that you should always ask is "how is the patient?"

    You must also be aware of their past medical history, current drug regimen and current oxygen therapy. This history and background will assist in interpreting the result.

  • Step 2: Is the patient hypoxemic?

    To establish this we look at the PaO2, Partial pressure (tension) of oxygen (O2) in arterial blood (a)

    Normal range PaO2: 11-13 kPa

    PaO2 should normally be >11 kPa on AIR in a healthy patient. If it is less than this then your patient is hypoxic

    PaO2 <8 kPa on air then the patient is severely hypoxic

    Please don’t take your patients off supplementary oxygen for ABGs when they are unwell.

  • Step 3: How is the pH - acidotic, normal, or alkalotic?

    pH stands for Power of Hydrogen this is a negative log of hydrogen ion (H+) activity

    This tells us how acid or alkaline our blood is

    Small changes in pH can have significant effects on the human body

    Normal Range is pH 7.35- 7.45

    Acidotic pH <7.35

    Alkalotic pH >7.45

    Now we need to think about what is making the pH either acidotic or alkalotic. Generally the causes can be either:

    Respiratory: caused by an imbalance of CO2 - CO2 is acidic.

    Metabolic: caused by an imbalance of the bicarbonate (HCO3-) - bicarb is alkalotic.

  • Step 4: What has happened to the PaCO2?

    • The PaCO2 is the partial pressure (tension) of carbon dioxide (CO2) in arterial blood (a)

    • Normal range of PaCO2: 4.7- 6 kPa

    Looking at the PaCO2 will help to quickly rule in or out a respiratory element to explain any acid/ base changes.

    When the PaCO2 cannot explain changes to the pH we then look for a metabolic cause by moving on to the next step to check bicarbonate levels or base excess.

  • Step 5: What has happened to the base excess or bicarbonate?

    • Bicarbonate ( HCO3-) is the body's naturally occurring buffer. As a base it helps to ‘mop up’ acids (H+), playing a vital role in acid – base homeostasis.

    • The normal range for HCO3- : 22-26 mmol/L-1.

    • Base excess is also a marker of how much bicarbonate is in the body. The number represents the amount of strong acid required to restore a litre of blood to normal pH (7.4). The normal Range for base excess is -2 to +2 mmol/l-1

    AGAIN WE QUESTION:

    Is the HCO3- normal or abnormal?

    If it is abnormal does it fit with the rest of the information, e.g. low bicarbonate and acidosis or high bicarbonate and alkalosis?

  • Compensation

    Our bodies are very smart and have developed several ways to maintain homeostasis.

    RESPIRATORY ACIDOSIS/ ALKALOSIS (changes in CO2) can be metabolically compensated by increasing or decreasing the HCO3- in an attempt to move the pH closer to normal range.

    METABOLIC ACIDOSIS/ ALKALOSIS (changes in HCO3-) can be compensated by the respiratory system either retaining or “blowing off” CO2 in an attempt to move the pH closer to the normal range.

    Rates of compensation vary and are not always instant. The respiratory system can compensate relatively fast by increasing/ decreasing ventilation, adjusting respiratory rate and tidal volume to increase or decrease the level of CO2. The metabolic system can take several days to increase or decrease the amount of bicarbonate in the body. If you see evidence of metabolic compensation for respiratory disease this may have happened over several days if not longer (e.g. chronic CO2 retention in COPD).

Summary

ABG interpretation steps:

  1. How is the patient, this will provide valuable tools to help with interpretation

  2. Is the patient hypoxic

  3. Is the pH acidotic pH <7.35, or alkalotic pH >7.45

  4. Determine the respiratory component. Is the CO2 normal or abnormal, can this explain the pH; if not is the driver metabolic?

    Respiratory acidosis: pH < 7.35, with high PaCO2 > 6kpa.

    Respiratory Alkalosis: pH > 7.45, with low PaCO2 < 4.7 kPa

  5. Determine the metabolic component. Is the HCO3 normal or abnormal?

    Low HCO3 (<22mmol/L) is likely to indicate a metabolic acidosis, it can also be a compensation for respiratory alkalosis. High HCO3 (>26mmol/L) is likely to be compensation for respiratory acidosis, commonly seen in chronic respiratory disease.

Respiratory acidosis

Metabolic acidosis