2808NRS - Arterial Blood Gases Interpretation

Overview of Arterial Blood Gases (ABGs)

Bicarbonate Buffering System

• Key Concepts:

  • Maintains hydrogen ion levels, thus regulating pH.

  • Normal blood pH range: 7.35 to 7.45.

  • Equation involves carbon dioxide (CO2) and water forming carbonic acid which helps buffer pH.

  • Overview of CO2 production:

    • Produced by all cells during ATP generation from glucose and oxygen.

    • Transports CO2 to lungs for exhalation.

Roles of Lungs and Kidneys

• Lungs:

  • Regulate CO2 levels in blood (normal range: 35 to 45 mmHg).

  • High CO2 leads to decreased pH (acidic condition).

• Kidneys:

  • Regulate bicarbonate levels (normal range: 22 to 26 mmol/L).

  • Low bicarbonate indicates acidosis, while high indicates alkalosis.

Types of Acid-Base Imbalances

Respiratory Conditions

• Respiratory Acidosis:

  • High CO2 due to hypoventilation and resulting low pH.

  • Caused by sedatives, respiratory diseases (pneumonia, pulmonary oedema, COPD).

• Respiratory Alkalosis:

  • Low CO2 due to hyperventilation and resulting high pH.

Metabolic Conditions

• Metabolic Acidosis:

  • Low bicarbonate indicates excess hydrogen ions.

  • Causes include lactic acid production, renal failure, diarrhoea.

• Metabolic Alkalosis:

  • High bicarbonate suggesting loss of hydrogen or excessive base.

Analysing Arterial Blood Gases

Interpreting ABG Values

• Format: pH / CO2 / HCO3 / ROME (Respiratory Opposite / Metabolic Equal)

• Steps for Interpretation:

  1. Assess pH (normal: 7.35 - 7.45).

  2. Check CO2 levels (normal: 35 - 45 mmHg).

  3. Evaluate bicarbonate levels (normal: 22 - 26 mmol/L).

  4. Use ROME acronym:

     • Respiratory Opposite

     • Metabolic Equal

Example Interpretation

• Example ABG: pH 7.29, CO2 60, HCO3 28

  • pH indicates acidosis (down arrow).

  • CO2 above normal (up arrow).

  • HCO3 above normal (up arrow).

  • Conclusion: Respiratory acidosis (partially compensated) due to high CO2 and compensatory rise in HCO3.

  • Two arrows up Two down, Opposite so RO → 7.29 pH so partially compensated respiratory acidosis, hypoventilation, COPD, CO2 increase

• Example ABG: 7.5, 41, 98, 29

  • pH indicates alkalosis(up arrow).

  • paCO2 normal (-).

  • paO2 normal (-).

  • HCO3 up .

  • Two arrows up Two normal, metabolic cause same direction (?)

  • Metabolic alkalosis uncompensated (because CO2 is 41 and not compensating yet) - lost acid (vomiting loses HCL)

• Example ABG: 7.49, 24, 105, 22

  • pH indicates alkalosis(up arrow).

  • paCO2 low(down).

  • paO2 high (up).

  • HCO3 okay.

  • Two arrows up one normal one down, uncompensated respiratory alkalosis

  • Respiratory alkalosis - CO2 is getting rid of so less acid but expect HCO3 to compensate, Hyperventilation (in pain, anxious),

• Example ABG: 7.38, 30, 93, 15

  • pH normal (-).

  • paCO2 low(down).

  • paO2 normal(-).

  • HCO3 low (down).

  • Two arrows down two normal, metabolic acidosis (compensated because pH is in normal range)

  • Metabolic acidosis - in shock (producing lactic acid), ketogenesis, diarrhoea

Conclusion

• Importance of Monitoring ABGs: Essential to understand underlying causes of acid-base imbalances for clinical management. Understanding ROME assists in identifying the nature of the disturbance.