Acid-Base Balance Overview

Acid-Base Balance

• The body maintains a balance between acids and bases for optimal cell function.
• Acid-base homeostasis is a dynamic interplay of 3 processes: acid production, acid buffering, and acid excretion.
• Normal acid-base balance is maintained when acid production equals acid excretion.

Intended Student Learning Objectives

• Describe the processes that regulate acid-base balance.
• Interpret acid-base imbalances based on arterial blood gas (ABG) results.
• Identify risk factors for acid-base imbalances.
• Apply the nursing process when caring for patients with acid-base imbalances.
• Assess patients with specific acid-base imbalances.

Acid Buffering

• All body fluids contain buffers that work within seconds.
• Buffers are pairs of chemicals that work together to maintain the normal pH of body fluids.
• A buffer takes up excess H+ when there are too many.
• A buffer can release H+ when there are too few.
• The major buffer system in extracellular fluid (ECF) is the HCO3- buffer system, consisting of a lot of HCO3- and a small amount of carbonic acid (20:1 ratio).
• Other buffers include hemoglobin (Hgb), protein buffers, phosphate buffers, and cellular and bone buffers.
• Buffers normally keep the blood from becoming too acidic.

Acid Excretion

• The lungs excrete carbonic acid in the form of CO2 and H2O when you exhale.
• Rising carbon dioxide (PaCO2) levels trigger faster, deeper respirations to excrete the excess CO2.
• Falling PaCO2 levels trigger slower, shallower respirations.
• The kidneys excrete metabolic acids, secreting H+ into the renal tubular fluid and putting HCO3- back into the blood simultaneously.

Normal Ranges for ABG

• pH: 7.35-7.45
• PaCO2: 35-45 mmHg (4.7-6 kPa)
• HCO3- : 21-28 mEq/L (21-28 mmol/L)
• PaO2: 80-100 mmHg (10.7-13.3 kPa)
• SaO2: 95%-100%
• Base Excess/Deficit: -2 to +2 mEq/L

ROME Mnemonic

• Respiratory: Opposite relationship between pH and PaCO2
• Metabolic: Equal relationship between pH and HCO3-

Respiratory Acidosis

• Causes: Alveolar hypoventilation (lungs cannot excrete CO2, increasing carbonic acid). This can be caused by malfunctioning brainstem respiratory control, respiratory depressant drugs, head injury, acute asthma, airway obstruction, atelectasis (collapsed lung), COPD, pneumonia, or impaired neuromuscular functions.
• Symptoms: Decreased level of consciousness (confusion, lethargy, coma). Headache, lightheadedness, and dysrhythmias.
• Lab results: Low pH (<7.35), high PaCO2 (>45 mmHg/6 kPa), normal or high HCO3- (compensated).

Respiratory Alkalosis

• Causes: Alveolar hyperventilation (rapid or deep breathing) caused by stimulation of brainstem respiratory control, aspirin overdose, gram-negative sepsis, head injury, meningitis, or acute pain.
• Symptoms: Lightheadedness, numbness/tingling in extremities, excitement/confusion, dysrhythmias.
• Lab results: High pH (>7.45), low PaCO2 (<35 mmHg/4.7 kPa), normal or low HCO3- (compensated).

Metabolic Acidosis

• Causes: Increased metabolic acids (e.g., ketoacidosis from diabetes, starvation; hypermetabolic state; severe kidney disease; ingestion of acid precursors, circulatory shock, diarrhea, pancreatic fistula, or intestinal decompression).
• Symptoms: Decreased level of consciousness, increased respiratory rate and depth, abdominal pain, dysrhythmias.
• Lab results: Low pH (<7.35), normal PaCO2, low HCO3- (<21 mEq/L/21 mmol/L).

Metabolic Alkalosis

• Causes: Excessive bicarbonate intake (e.g., sodium bicarbonate administration), massive blood transfusion (liver converts citrate to HCO3-), loss of metabolic acids (e.g., excessive vomiting, gastric suction, increased aldosterone, or hypokalemia).
• Symptoms: Possible excitement/confusion followed by decreased level of consciousness, lightheadedness, numbness/tingling, dysrhythmias, muscle cramps.
• Lab results: High pH (>7.45), normal or high PaCO2 (if compensated), high HCO3- (>28 mEq/L/28 mmol/L).