MS ABG Comprehensive Study Notes on ABG

ABG Interpretation

Acid-Base Imbalances

• Acidosis vs. Alkalosis: Determining whether the pH is low (acidic) or high (alkaline).
  • pH < 7.35: Acidosis
  • pH > 7.45: Alkalosis

Respiratory vs. Metabolic

• ROME Method: Respiratory Opposite, Metabolic Equal.
  • For respiratory issues, pH and $CO_2$ move in opposite directions.
  • For metabolic issues, pH and $HCO_3$ move in the same direction.

Compensation

• The body tries to compensate for acid-base imbalances.
• Compensation is indicated by the system that is not the primary problem moving in the opposite direction to correct the pH.
• Partial Compensation: All values (pH, $CO<em>2$, $HCO</em>3$) are abnormal.
• Full Compensation: pH is normal, but $CO<em>2$ and $HCO</em>3$ are abnormal.

Mixed Acid-Base Disorders

• Occur when both respiratory and metabolic disturbances are present.
• Example: Both $CO<em>2$ and $HCO</em>3$ indicate acidosis.
• In mixed disorders, the usual rules of compensation do not apply.
• No compensation occurs if both are acidosis, one needs to convert to alkalosis for compensation to start.

Uncompensated Disorders

• The compensating system is within normal range and not addressing the primary problem.
• Example: Metabolic acidosis with normal $CO_2$.

Examples

• Example 1: pH 7.34, $CO<em>2$ 47, $HCO</em>3$ 20 (All values abnormal)
  • pH: Acidic
  • $CO_2$: Acidic (High)
  • $HCO_3$: Acidic (Low)
  • Interpretation: Mixed acidosis (respiratory and metabolic).
• Example 2: pH 7.34, $CO<em>2$ 45 (Normal), $HCO</em>3$ 20 (Low)
  • pH: Acidic
  • $CO_2$: Normal
  • $HCO_3$: Acidic (Low)
  • Interpretation: Uncompensated metabolic acidosis.

Additional Notes

• ABG interpretation resources available in Moodle.
• Consistency in ABG interpretation method is key if it yields correct results.

Practice Problems

• Problem 1: pH 7.32 (Acidic), $CO<em>2$ 48 (Acidic), $HCO</em>3$ 24 (Normal), $PaO_2$ 78 (Low).
  • Interpretation: Uncompensated respiratory acidosis with hypoxemia.
• Problem 2: pH 7.47 (Alkalotic), $CO<em>2$ 45 (Normal), $HCO</em>3$ 33 (Alkalotic), $PaO_2$ 99 (Normal).
  • Interpretation: Uncompensated metabolic alkalosis.
• Problem 3: pH 7.2 (Acidotic), $CO<em>2$ 45 (Normal), $HCO</em>3$ 14 (Acidotic).
  • Interpretation: Uncompensated metabolic acidosis with hypoxemia.

Combined Disorders

• Mixed acidosis: Both respiratory and metabolic components are acidic.
• Mixed alkalosis: Both respiratory and metabolic components are alkaline.

Respiratory Acidosis

• Low pH, High $CO_2$: Often due to hypoventilation.
• Causes: Post-op abdominal surgery (shallow breathing), sedative or opioid overdose (decreased respiration).
• Associated with conditions causing CO2 buildup.
• Electrolyte imbalances: hyperkalemia, hypernatremia, hyper.
• Signs: Muscle twitching, convulsions (due to electrolyte abnormalities like low calcium), arrhythmias, flushed skin.

Respiratory Alkalosis

• High pH, Low $CO_2$: Often due to hyperventilation.
• Causes: Anxiety, nicotine use, high elevation, pain.
• Electrolyte imbalances: hypokalemia, hypocalcemia, hypotonyluremia, hypochluremia.
• Symptoms: Headache, vertigo, tinnitus.
• Compensation: Kidneys decrease hydrogen secretion and increase bicarbonate excretion.
• Treatment: Paper bag rebreathing (to inhale exhaled $CO_2$).

Metabolic Acidosis

• Characterized by: Gain of acid or loss of bicarbonate.
• Loss of bicarbonate occurs from stool (which is normally alkaline due to bile salts).
• Conditions: Colostomy/ileostomy with high output can lead to metabolic acidosis.
• Associated with DKA (Diabetic Ketoacidosis).
• Electrolyte Changes in DKA: Increased potassium levels.
• Other Causes: Alcoholism, starvation (ketone production).
• Starvation converts Proteins and fats to pyruvate acid.
• Kidney Failure: Kidneys produce bicarbonate in renal tubules; failure leads to acidosis.
• Hyperkalemia: Potassium shifts out of cells into the blood.
• Monitoring: DKA patients are often placed on telemetry to monitor for dysrhythmias due to hyperkalemia.
• Treatment: Monitor potassium levels, underlying cause, sodium drip.
• Chronic metabolic acidosis = low calcium levels.
• S/S Central nervous system depression.
• Most Common ABG Due to kidney failure
• High Aspirin Ingestion = acid.
  *
• Compensation: Lungs hyperventilate to reduce $CO_2$ (Kussmaul’s respirations).

Metabolic Alkalosis

• Associated with hypokalemia from diuretic use.
• What causes it?: Excessive vomiting or NG tube suctioning, Cushing's disease, kidney disease, or drugs containing baking soda
• Electrolyte imbalances: Hypokalemia and hypocalcemia.
• Electrolyte & S/s similar to respiratory acidosis & alkalosis.
• Compensation: Decrease in respiratory rate to retain $CO_2$.

Arterial vs. Venous Blood Gas

• Arterial blood gas (ABG) is more commonly used.
• Venous blood gas (VBG) is less common; used to assess unoxygenated blood.

Review Scenarios

• Opioid Overdose: Expected ABG = Respiratory acidosis with possible hypoxemia.
  • Assess LOC, respiratory rate, oxygen saturation, cardiac rhythm.
  • Treatment = Naloxone (Narcan) to reverse opioid effects, oxygen.
• Pulmonary Edema Treated with Loop Diuretics: Expected ABG = Metabolic alkalosis.
  • Assess potassium levels, watch for dysrhythmias, monitor for signs of hypocalcemia.
  • Treatment = Treat underlying cause, manage electrolyte imbalances.
• Pneumothorax: Expected ABG = Respiratory alkalosis.
  • Assess respiratory rate, heart rate, oxygen saturation, LOC (cerebral vasoconstriction).
  • Treatment = Paper bag rebreathing, treat fever if present, administer oxygen.
• Aspirin Poisoning: Expected ABG = Acidosis.
  • Assess LOC, electrolytes (hyperkalemia), monitor heart rhythm.
  • Treatment = Sodium bicarbonate, activated charcoal, manage hyperkalemia, fluids, possible hemodialysis.