Understanding arterial blood gases

Important lab values

• pH
  • Normal/neutral = $7.35$–$7.45$
  • Below $7.35$ = acidic
  • Above $7.45$ = alkaline

• Partial pressure of oxygen ($Pa{O}_2$)

  • Normal = $75$–$100$ mmHg
  • Below $75$ = hypoxemia
  • Above $100$ = hyperventilation

• Partial pressure of carbon dioxide ($PaC{O}_2$) – related to respiratory conditions

  • Normal = $35$–$45$ mmHg
  • Below $35$ = alkaline
  • Above $45$ = acidic

• Bicarbonate ($HC{O}_3^{-}$) – related to metabolic conditions

  • Normal = $22$–$26$ mEq/L
  • Below $22$ = acidic
  • Above $26$ = alkaline

Acid-base diseases

• Respiratory acidosis

  • Related diagnosis: COPD, pneumonia, atelectasis, anesthesia, drug overdose
  • Define: the lung does not efficiently remove carbon dioxide from the system, causing a buildup of carbon dioxide in the bloodstream
  • Lab values:
    • pH below $7.35$
    • $PaC{O}_2$ above $45$
  • Symptoms: hypoventilation, hypoxia, rapid-shallow breaths, decreased blood pressure, dyspnea, hyperkalemia, muscle weakness

• Respiratory alkalosis

  • Related diagnosis: anxiety attacks, pulmonary embolism, mechanical ventilation
  • Define: the lung excessively removes carbon dioxide from the system, leading to decreased amounts of carbon dioxide in bloodstream
  • Lab values:
    • pH above $7.45$
    • $PaC{O}_2$ below $35$
  • Symptoms: hyperventilation, tachycardia, hypokalemia, numbness/tingling of extremities, deep-rapid breathing, seizures, nausea, vomiting, light headedness, lethargy, confusion

• Metabolic acidosis

  • Related diagnosis: diabetic ketoacidosis, severe diarrhea, renal failure, sepsis (systemic shock)
  • Define: excessive fluid accumulation in the body leading to a buildup of acids and a reduction in bicarbonate in the bloodstream
  • Lab values:
    • pH below $7.35$
    • $HC{O}_3^{-}$ below $22$
  • Symptoms: headache, decreased blood pressure, hyperkalemia, muscle twitching, nausea, vomiting, diarrhea, changes in consciousness, Kussmaul respirations (compensatory hyperventilation)

• Metabolic alkalosis

  • Related diagnosis: severe vomiting, excessive diuretic use, excessive antacid use, excessive gastrointestinal suctioning
  • Define: the body has removed too much acid or retained too much bicarbonate, leading to an alkaline shift in the bloodstream
  • Lab values:
    • pH above $7.45$
    • $HC{O}_3^{-}$ above $26$
  • Symptoms: restlessness, tachycardia, compensatory hypoventilation, confusion, nausea, vomiting, tremors, cramps

Treatment of arterial blood gas disorders

There are three types of compensation:

Partially compensated arterial blood gases

Definition:

• pH is still outside the normal range, so the imbalance is not fully corrected.
• The opposing system has begun to respond, but not enough to normalize pH.

Example: partially compensated respiratory acidosis

Interpretation: Respiratory acidosis with partial metabolic compensation.

Fully compensated arterial blood gases

Definition:

• The pH is back within normal range ($7.35$–$7.45$) due to effective compensation.
• One component remains abnormal, indicating that a primary disorder still exists, but the body has corrected the pH.

Example: fully compensated metabolic acidosis

Interpretation: Fully compensated metabolic acidosis.