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ABGs Basics

pH: The Picky Princess

• pH = "picky princess" - it wants things its way.

• The pH scale ranges from 0 to 14, with 7 as neutral.

  • Less than 7 is acidic.

  • Greater than 7 is alkaline (basic).

• Pure water/blood is around 7.

• Examples of common items on the pH scale:

  • Doctor Pepper and sodas are acidic.

Acid-Base Balance

• Metabolic processes maintain a balance between acids and bases in the body.

• Lungs:

  • Think respiratory (PaCO2)

  • Exhale CO2 (CO2 is an acid).

• Kidneys:

  • Think metabolic (HCO3)

  • Involve HCO_3 (bicarbonate, a base).

• The body works to stay compensated.

• Losing stomach acid (e.g., through vomiting or NG tube suction) leads to losing acid, resulting in becoming more basic (alkalosis).

Memorizing ABG Values

• Lab ranges will be provided in exam questions, but they may not always be helpful if multiple values are off.

• Normal pH: 7.35 - 7.45

  • pH less than 7.35: Acidosis (acidity going down).

  • pH greater than 7.45: Alkalosis (kicking pH up).

• "Princess" (pH) determines the problem, so acidic pH means acidosis is part of the answer.

• Test-taking strategy: Identify if pH is acidic or alkalotic to narrow down answer choices.

CO_2 (Carbon Dioxide)

• Normal CO_2: 35 - 45

• CO_2 is an acid; the more you have, the more acidic you are.

  • CO_2 greater than 45: Acidosis.

  • CO_2 less than 35: Alkalosis.

• Think lungs/respiratory when you see CO_2.

HCO_3 (Bicarbonate)

• Normal HCO_3: 21 - 28

• Some resources may say 22-26, but 21-28 is the standard for this course and board exams.

• HCO_3 is a base; the more you have, the more alkalotic you are.

  • HCO_3 greater than 28: Alkalosis.

  • HCO_3 less than 21: Acidosis.

Additional Values (Not Required for Now)

• PaO2 normal: 80 - 100 (hypoxemia if less than 80)

• SaO2 normal: 95 - 100

ABG Interpretation Steps

1. Look at the pH first (low or high).

2. Assess PaCO_2 (low, normal, or high).

3. Assess HCO_3 (low, normal, or high).

4. Match the pH imbalance with either CO2 or HCO3 imbalance.

5. If the issue is CO2, think respiratory; if it's HCO3, think metabolic.

Quick Method: ROME

• ROME = Respiratory Opposite, Metabolic Equal.

• Respiratory: pH and CO_2 arrows will move in opposite directions.

• Metabolic: pH and HCO_3 arrows will move in the same direction.

Practice Problems

Example 1

• pH: 7.30 (Low = Acidic).

• CO_2: 40 (Normal).

• HCO_3: 19 (Low = Acidic).

• Since HCO_3 is low (acidic), it's metabolic acidosis.

Example 2

• pH: 7.30 (Low).

• CO_2: 50 (High).

• HCO_3: 24 (Normal).

• Arrows are opposite (pH down, CO_2 up) = Respiratory.

• Low pH = Acidosis.

• Result: Respiratory Acidosis.

Example 3

• pH: 7.50 (High).

• CO_2: 40 (Normal).

• HCO_3: 30 (High).

• Equal arrows = Metabolic.

• High pH = Alkalosis.

• Result: Metabolic Alkalosis.

Respiratory Alkalosis

• pH > 7.45 and CO_2 < 35

• Hyperventilation is a key cause (breathing off too much CO_2).

• Other causes: anxiety, high altitudes, pregnancy, fever, hypoxia, early pulmonary emboli.

• Asthma:

  • Early stages: Respiratory Alkalosis (breathing fast).

  • Late stages (if untreated): Respiratory Acidosis (tiredness, poor gas exchange).

• COPD: Typically Respiratory Acidosis (retaining CO_2).

Metabolic Alkalosis

• Loss of gastric juices (e.g., vomiting, NG tube) removes stomach acid.

• Other causes: potassium-wasting diuretics, overuse of antacids.

• Treatment: Address the underlying cause (e.g., antiemetics for vomiting, fluids, treat the flu).

Lab Trick

• "If it comes out the as=idosis".

• Diarrhea = Metabolic Acidosis.

• Vomiting = Metabolic Alkalosis.

Respiratory Acidosis

• Low respiratory rate/hypoventilation (not exhaling enough CO_2).

• Causes: drug overdose, chest trauma, airway obstruction.

Metabolic Acidosis

• Diarrhea (losing base).

• Organ failure (kidney, liver).

• Sepsis.

• Diabetic ketoacidosis.

• Treatment: Address the underlying cause (e.g., Narcan for drug overdose, CPR if not breathing, fluids for diarrhea).