Arterial Blood Gas (ABG) Analysis & Acid-Base Balance

• Arterial Blood Gas Analysis (ABG) is important in understanding Acid-Base Balance

Normal Blood Gas Ranges

• pH should be between 7.35 and 7.45
• PaCO2 levels should be between 35 and 45
• Bicarbonate (HCO3) should be between 22 and 26.
• PaO2 should be between 80 and 100
• SaO2 should be greater than 95%

What Determines Acid Base Balance?

• Carbon dioxide (CO2) is an acid controlled by the lungs/respiratory system
• Breathing faster reduces acid levels, referred to as "blowing off acid".
• Breathing slower retains acid.
• Bicarbonate (HCO3) is a base that is controlled by the kidneys/metabolic system.
• Retaining bicarbonate makes the body more alkaline
• Lowering bicarbonate makes the body more acidic

Identifying the Primary Process

• pH, PCO2, and HCO3 levels are measured to determine acidemia or alkalemia

• Low pH indicates acidemia.

  • High PCO2 with low pH indicates respiratory acidosis.
  • Low HCO3 with low pH indicates metabolic acidosis

• High pH indicates alkalemia

  • Low PCO2 with high pH indicates respiratory alkalosis
  • High HCO3 with high pH indicates metabolic alkalosis

What Will The Gases Look Like?

• Respiratory Acidosis results in high CO2 levels; this can be caused by narcotic administration or retention of CO2 in COPD patients.
• Respiratory Alkalosis results in low CO2 levels; this can be caused by hyperventilation.
• Metabolic Acidosis results in low HCO3 levels; this can be caused by sepsis or MI.
• Metabolic Alkalosis results in high HCO3 levels; this can be caused by loss of GI fluids, NG suction, or gastric/enteral drains.

Blood Gas Interpretation

• Normal ranges: pH: 7.35 – 7.45, PaCO2: 35 – 45, Bicarbonate (HCO3): 22 – 26, PaO2: 80 - 100, SaO2: >95%
• Example 1: pH: 7.51, PaCO2: 30 mm Hg, HCO3: 24 mEq/L
• Example 2: pH: 7.26, PaCO2: 25 mm Hg, HCO3: 15 mEq/L

Blood Gas Interpretation: Compensation

• Respiratory system compensates for metabolic imbalances
• Metabolic system compensates for respiratory imbalances
• Compensation normalizes the pH but does not fix the original problem of the abnormal ABG
• Compensation can be partial or complete

Interpretation Example

• ABGs: pH: 7.46, pCO2: 50, HCO3: 33 indicates partially compensated metabolic alkalosis
  • pH: alkalosis
  • pCO2: elevated (acidosis?)
  • HCO3: alkalosis

ABG: Fully Compensated Examples

• ABG 1: pH: 7.45 (normal but leaning toward base), pCO2: 50 (acidic), HCO3: 33 (base)
  • This began as metabolic alkalosis and the lungs compensated fully by increasing the CO2 (acid).
• ABG 2: pH: 7.36 (normal but leaning toward acid), pCO2: 29 (base), HCO3: 20 (acid), pO2: 81 (normal)
  • This began as metabolic acidosis and the lungs compensated by "blowing off" CO2, thus getting rid of acid.

Arterial Blood Gas (ABG) analysis is crucial for understanding acid-base balance. Normal ranges for pH, PaCO2, bicarbonate (HCO3), PaO2, and SaO2 are essential for interpretation. CO2, controlled by the lungs, acts as an acid, while bicarbonate, managed by the kidneys, functions as a base.