Acid-Base Balance and Arterial Blood Gases

Acid-Base Balance

• The body needs to maintain a steady balance between acids and bases to achieve homeostasis.
• Health problems such as diabetes, chronic obstructive pulmonary disease (COPD), and kidney disease can lead to an imbalance.

pH

• pH is a measure of H+ ion concentration.
• An increase in H+ concentration leads to acidity, while a decrease in H+ concentration leads to alkalinity.
• The normal pH range of blood is slightly alkaline, between 7.35 and 7.45.

Acid-Base Regulation

• There are three mechanisms to regulate acid-base balance and maintain a pH between 7.35 and 7.45:
  • Buffer system
  • Respiratory system
  • Renal system

Buffer System

• The buffer system is the primary regulator of acid-base balance.
• It acts chemically to change strong acids into weak acids or binds acids to neutralize them.
• The buffer system requires the respiratory and renal systems to be functioning adequately.

Carbonic Acid (H2CO3)/Bicarbonate (HCO3–) Buffer

• This is the major buffer system.
• The reaction is: HCl + NaHCO3 → NaCl + H2CO3.
• The carbonic acid (H2CO3) is broken down into water (H2O) and carbon dioxide (CO2).
• The lungs excrete CO2.

Other Buffer Systems

• Phosphate buffer system
• Protein buffer system
• Hemoglobin buffer system
• Cellular buffer system, which shifts H+ in and out of cells in exchange for potassium.

Respiratory System Regulation

• The respiratory system regulates acid-base balance through the reaction: CO2 + H2O → H2CO3 → H+ + HCO3–.
• The respiratory center in the medulla controls breathing.
• Increased respirations lead to increased CO2 elimination and decreased CO2 in the blood.
• Decreased respirations lead to CO2 retention.

Renal System Regulation

• The renal system conserves bicarbonate and excretes some acid through three mechanisms:
  • Secreting free hydrogen ions
  • Combining H+ with ammonia (NH3)
  • Excreting weak acids

Alterations in Acid-Base Balance

• Imbalances occur when compensatory mechanisms fail.
• Classification of imbalances:
  • Respiratory (CO2) or metabolic (HCO3–)
  • Acidosis or alkalosis
  • Acute or chronic

Blood Gas Values

• Arterial blood gas (ABG) values give objective information about:
  • Acid-base status
  • Underlying cause of imbalance
  • Body's ability to regulate pH
  • Overall oxygenation status

Normal Arterial Blood Gas Values

• pH: 7.35 - 7.45
• PaCO2: 35 - 45 mm Hg
• Bicarbonate (HCO3–): 22 - 26 mEq/L
• PaO2: 80 - 100 mm Hg
• SaO2: >95%
• Base excess: + 2.0 mEq/L

Interpretation of ABGs

• Look at each of the values
• Look at pH first
• Use ROME to determine respiratory or metabolic
• Determine if the patient is compensating
• Assess the PaO2 and O2 saturation

Acid-Base Mnemonic—ROME

• Respiratory Opposite (RO): Alkalosis ↑ pH ↓ PaCO2, Acidosis ↓ pH ↑ PaCO2
• Metabolic Equal (ME): Acidosis ↓ pH ↓ HCO3–, Alkalosis ↑ pH ↑ HCO3–

Case Studies

• Jeri, a 22-year-old female, with respiratory acidosis due to hypoventilation secondary to alcohol ingestion.
• Mayna, an 18-year-old female, with respiratory alkalosis due to hyperventilation secondary to anxiety and hysteria.