Blood Gas Reading

Introduction

Arterial Blood Gas (ABG) readings are a very useful, quick investigation that are able to tell you:

• The baseline status of a patient
• If interventions are helping the patient (repeats)
• A differential with the ABG pattern the patient has

It is therefore a key skill to be able to interpret these readings.

There are up to 5 components of any ABG reading- the following are their reference ranges:

(THEY DON’T NEED TO BE MEMORISED- YOU JUST HAVE TO KNOW WHAT THEY MEAN!)

• pH: 7.35 – 7.45
• PaCO₂: 4.7 – 6.0 kPa || 35.2 – 45 mmHg
• PaO₂: 11 – 13 kPa || 82.5 – 97.5 mmHg
• Base excess (BE): -2 to +2 mmol/L (Remember Base=Alkali)
• HCO₃– (bicarbonate): 22 – 26 mEq/L (A chemical base- is the same thing as BE)

---

Oxygen

Before interpretation- it is important to look at and immediately act on a decrease in PaO₂ -HYPOXAEMIA. As this is the most immediately life threatening condition to the patient.

A patient is considered hypoxaemic when PaO₂<10 kPa.

A patient is in respiratory failure when this hypoxaemia is severe at <8 kPa.

This is covered further in: Non-Specific Conditions.

---

Key Facts

There are key facts that should be known that will make interpretation more intuitive:

• There are two homeostatic components that change the acid-base balance in the body:
  • Respiratory – via the lungs
    • CO₂ dissolves in water (and blood) to make things more acidic – (think of pollution causing acid rain)
    • The lungs affect CO₂ concentrations in the blood – can make the blood more or less acidic by breathing at different rates
  • Metabolic via the kidneys
    • HCO₃/BE (bicarbonate) dissolves in water (and blood) to make things more alkaline/basic
    • The kidneys affect the concentrations of bicarbonate- so can make the blood more or less alkaline by getting rid of bicarbonate to urine at different rates.
• These components are like two opposite ends of a set of scales. – If one side changes – to maintain pH balance the other side will try to change in the other way.

---

Interpretation

There are 3 steps in interpretation:

1- What is pH saying?

---

2- Is anything driving a change in pH?

There are 2 causes- with their own list of differential diagnoses behind them:

• Respiratory -Change in PaCO₂
  • If ↓pH and PaCO₂↑ -RESPIRATORY ACIDOSIS [Caused by anything ↓respiratory rate]
    • Respiratory depression (drugs such as opiates)
    • Asthma
    • COPD
    • Iatrogenic (not enough mechanical ventilation)
  • If ↑pH and PaCO₂↓ – RESPIRATORY ALKALOSIS [Caused by anything ↑respiratory rate]
    • Panic Attacks – (hyperventilation)
    • Pain
    • Pulmonary Embolism (PE)
    • Pneumothorax
    • Iatrogenic (too much mechanical ventilation)
• Metabolic– Change in HCO₃/BE
  • If If ↓pH and HCO₃/BE↓ -METABOLIC ACIDOSIS
    • Diabetic ketoacidosis
    • Lactic acidosis
    • Renal failure
    • Diarrhoea
  • If ↑pH and HCO₃/BE ↑ -METABOLIC ALKALOSIS
    • Vomiting
    • Loop and Thiazide Diuretics
    • Heart Failure
    • Nephrotic Syndrome

---

3- Is there anything driving compensation – trying to make the pH normal again?

If a component is deranged- then the opposite component will try to compensate to get the blood back to a normal pH:

• METABOLIC COMPENSATION:
  • In a respiratory acidosis – pH= low, PaCO₂=high – to compensate the kidneys would increase HCO₃/BE in blood to increase pH back to normal.
  • In a respiratory alkalosis- pH=high, PaCO₂=low – to compensate the kidneys would do the opposite to decrease pH back to normal.

• RESPIRATORY COMPENSATION
  • In a metabolic acidosis- pH=low, HCO₃/BE=low – to compensate the lungs would work harder to blow off CO₂ in blood to increase pH back to normal.
  • In a metabolic alkalosis-pH= high, HCO₃/BE=high- to compensate the lungs would do the opposite to increase CO₂ in blood to decrease pH back to normal.

---

Worked Example

Often, there are other electrolytes given, but focus on what you know. – pH is essentially a measure of H+ concentration, a lower value meaning a higher concentration of H+.

In this scenario there is blood loss via vomit (haematemesis) which is causing the decrease in HCO₃ causing a clear acidic pH in the blood – A METABOLIC ACIDOSIS

Additionally due to the decrease in PCO₂ there is a PARTIAL RESPIRATORY COMPENSATION where there is an increased respiratory rate to blow off more CO₂ (explaining the additional increase in O₂) in an attempt to decrease the H+ to normal.