Acid-Base Balance and Buffers

Acid-Base Regulatory Actions/Mechanisms

• Healthy body cells constantly produce acids, carbon dioxide, and hydrogen ions.
• The normal arterial pH ranges between 7.35 and 7.45.
• Maintaining this balance relies on consistent hydrogen ion production that is not excessive.
• Carbon dioxide loss through breathing should correlate with hydrogen ion production.
• The chemical equation: CO2 + H2O ↔ H2CO3 ↔ HCO3- + H+ represents the relationship between carbon dioxide, water, carbonic acid, bicarbonate, and hydrogen ions in maintaining acid-base balance.

Buffers

• Buffers are the first line of defense in controlling hydrogen ion concentration.
• Chemical buffers include bicarbonate in extracellular and intracellular fluid, and phosphate specifically in intracellular fluid.
• Protein buffers include albumin and globulins in extracellular fluid, plus hemoglobin in intracellular fluid.
• Protein buffers provide quick responses to changes and handle small fluctuations in hydrogen ion production during normal metabolic and health conditions.

Respiratory System

• The respiratory system is the second line of defense for acid-base balance.
• Carbon dioxide converts directly into hydrogen ions.
• "Blowing off" CO2 reduces hydrogen ion concentration, increasing pH.
• Carbon dioxide retention increases hydrogen ion concentration, decreasing pH.
• The respiratory system regulates pH controlled by the CNS.
• Central receptors respond to decreasing pH by increasing the respiratory rate.

Renal System

• The renal system acts as the third line of defense, responding strongly but requiring 48-72 hours for full effect.
• More bicarbonate is reabsorbed by the kidneys to decrease hydrogen ion concentration when the pH decreases.
• The renal system converts ammonia (NH3) into ammonium (NH4), trapping hydrogen ions and excreting them in urine.
• The pH is dependent on the ratio of kidney function (bicarbonate levels) to lung function (carbon dioxide levels).
• The kidney function (bicarbonate levels) response is slow but powerful
• The lung function (carbon dioxide levels) response is rapid but limited.

Key Values to Memorize

• Normal pH: 7.35-7.45; Higher numbers indicate alkalosis, lower ones indicate acidosis.
• Normal PaCO2: 35-45; Higher numbers indicate respiratory acidosis, lower numbers indicate respiratory alkalosis.
• Normal HCO3: 22-26; Higher numbers indicate metabolic alkalosis, lower numbers indicate metabolic acidosis.

Levels of Compensation

• Fully compensated: pH is within normal limits.
• Partially compensated: pH is not quite normal.

Compensation - Respiratory System

• Compensation occurs through the lungs.
• It usually "fixes" metabolic problems.
• Respiratory compensation causes an increased rate and depth of respiration.

Compensation - Renal System

• Compensation depends on a healthy kidney.
• It usually helps when lungs cannot do "enough".

Acidosis

• Acidosis is where more hydrogen ions are present than should be.

Actual Acid Excess

• Overproduction of hydrogen ions can result from conditions like diabetic ketoacidosis or seizures.
• Under elimination of hydrogen ions can result from respiratory or renal impairment.

Relative Acid Excess (Base Deficit)

• A normal amount of hydrogen ions may be present.
• Underproduction of bicarbonate ions can be related to pancreatitis or dehydration.
• Over elimination of bicarbonate ions can result from diarrhea.

Acidosis Labs

• pH: LOW, less than 7.35.
• Potassium: usually high.
• Respiratory Cause: CO2 high (greater than 45), HCO3 normal to high (compensating).
• Metabolic Cause: CO2 normal to low (compensating), HCO3 low.

Causes of Metabolic Acidosis - Overproduction of Hydrogen Ions

• Conditions leading to metabolic acidosis through overproduction include diabetic ketoacidosis, starvation, seizure activity, hypoxemia, ischemia, hypermetabolism, fever, heavy exercise.

Causes of Metabolic Acidosis - Under Elimination of Hydrogen Ions

• Conditions leading to metabolic acidosis through under elimination include renal failure, pancreatitis, liver failure, dehydration.

Causes of Metabolic Acidosis - Over-Elimination of Bicarbonate (Bicarb)

• Conditions leading to metabolic acidosis through over elimination include Diarrhea.

Causes of Respiratory Acidosis

• Respiratory Depression: Anesthesia, Drug overdose (especially opioids).
• Inadequate Chest Expansion: Muscle weakness, Pain.
• Airway Obstruction: Internal, External.
• Alveolar-capillary diffusion abnormalities: COPD, TB, Pneumonia, ARDS, PE.

Key Features of Acidosis - Cardiovascular

• Delayed electrical conduction, Bradycardia, Widened QRS complex, Prolonged PR interval, Hypotension, Thready peripheral pulses.

Key Features of Acidosis - Central Nervous System

• Depressed activity, Lethargy, Confusion, Stupor, Coma.

Key Features of Acidosis - Neuromuscular

• Hyporeflexia, Skeletal muscle weakness, Flaccid paralysis.

Key Features of Acidosis - Integumentary

• Warm flushed skin (metabolic acidosis), Pale, cyanotic dry skin (respiratory acidosis).

Key Features of Acidosis - Respiratory

• Kussmaul respirations = Deep and rapid, Not under voluntary control, Variable respiratory rate.

Interventions for Acidosis

• Correct the Underlying Problem.

Interventions for Acidosis - Respiratory Interventions

• Maintain patent airway.
• Prevent complications.
• Improve ventilation/oxygenation: Oxygen therapy, Ventilation support, Medication (Bronchodilators, Mucolytics, Anti-inflammatories).

Interventions for Acidosis - Metabolic Interventions

• Hydration/drug treatments are used for disease or problem.
• Give Bicarb only if HCO3 level is down.
• Continuous monitoring: ABG, Hemodynamic status, Skeletal system.

Alkalosis

• Alkalosis is a decrease in the hydrogen ion concentration.

Alkalosis - Actual Base Excess

• Overproduction of bicarb from oral ingestion or blood transfusion contributes to alkalosis.
• Under elimination of bicarb from renal impairment can cause alkalosis.

Alkalosis - Relative Base Excess (Acid Deficit)

• Underproduction of acid can result from NG suctioning or thiazide diuretics.
• Over elimination of acid may be caused by prolonged vomiting.

Alkalosis Labs

• pH: HIGH, greater than 7.45.
• Potassium: usually low.
• Calcium: usually low.
• Respiratory Cause: CO2 low, HCO3 normal to low (compensate).
• Metabolic Cause: CO2 normal to high (compensate), HCO3 high.

Key Features of Alkalosis - Cardiovascular

• Increased heart rate, Normal or low blood pressure, Increased digitalis toxicity.

Key Features of Alkalosis - Neuromuscular

• Hyperreflexia, Muscle cramping/twitching, Skeletal muscle weakness.

Key Features of Alkalosis - Respiratory

• Increased rate and depth in respiratory alkalosis, decreased respiratory effort in metabolic alkalosis.

Key Features of Alkalosis - Central Nervous System

• Anxiety, irritability, Paresthesia, Positive Chvostek's sign (Facial twitching), Positive Trousseau's sign (Hand and finger spasm in palmar flexion).

Interventions for Alkalosis

• Treat underlying cause.
• Monitor ABG and electrolytes closely.
• Safely- especially risk for falls: Muscle weakness or Hypotension.

How to Interpret an ABG

• Examine the pH to determine acidosis or alkalosis.
• Examine the CO2 to determine acidosis or alkalosis.
  • High equals acidosis
  • Low equals alkalosis
• Examine the HCO3 to determine acidosis or alkalosis.
  • Low equals acidosis
  • High equals alkalosis
• Determine what body system caused the problem.
  • Respiratory- CO2 matches pH
  • Metabolic- HCO3 matches pH