Acids and Bases: Henderson Hasselbalch Equation

• Chapter focuses on acids and bases and how the body deals with them

Henderson Hasselbach Equation

• Illustrates the relationship between pH, bicarbonate (HCO3-), and partial pressure of carbon dioxide (pCO2)
• pH is calculated as 6.1 + log10([HCO3-] / (0.03 x pCO2))
• H+ + HCO3 forms H2CO3, which then forms CO2 and H2O

Normal Values

• pH: 7.35-7.45 (average 7.4)
• pCO2: 35-45 (average 40)
• HCO3: 22-26 (average 24)

HCO3 (Bicarbonate)

• Acts as a base
• Primarily regulated by the kidneys
• Made in the kidneys
• Can be removed from kidneys
• Affected by multiple disorders
• Changes occur over a long period (days)
• Can be a metabolic disorder

PCO2 (Partial Pressure of Carbon Dioxide)

• Acts as an acid
• Primarily regulated by the lungs
• Single disorder
• Either breathing fast or slowly dictates pCO2
• Respiratory disorder
• Changes occur rapidly (minutes)

Respiratory Regulation

• Hyperventilation decreases PaCO2
• Hypoventilation increases PaCO2

Renal Regulation

• Kidneys control acid-base balance by excreting either a basic or acidic urine
• Kidneys handle acid-base balance by excretion or regeneration of HCO3
• Kidneys excrete H+

Metabolic Acidosis (2 Causes)

• Hallmark sign is low serum bicarbonate (HCO3) with arterial pH below 7.35
• Loss of Bicarb (no anion gap): Bicarbonate (HCO3) decreases, Chloride (Cl) increases
• Too much Acid (anion gap): Bicarb combines with H+
• Conditions are often linked to increases in sodium, chloride, BUN, and creatinine

Anion Gap

• Calculation: (Na+) - (HCO3 + Cl-) = Anion Gap
• Example: 140 - (24 + 104) = 12
• Normal anion gap is 12
• An increased anion gap suggests a more acute clinical situation

Metabolic Acidosis Causes

• Anion Gap Metabolic Acidosis: Methanol, Uremia, DKA, Paraldehyde, Iron; INH, Lactic Acidosis, Ethylene Glycol, Salicylates
• Non-anion Gap Metabolic Acidosis: Diarrhea and Renal tubular acidosis, Carbonic Anhydrase inhibitors (medication that block HCO3 reabsorption), Acetazolamide, treat the underlying cause, fluid
• Tx: Fluid
• Treat underlying cause

Metabolic Alkalosis

• Serum pH is greater than 7.45 and Bicarb levels are greater than 26 mEq/L

• Occurs when HCO3- is increased

• May be due to:

  • A loss of acid: Vomiting, gastric suctioning, diuretics, hypokalemia
  • Accumulation of base: Administration of Bicarb

Respiratory Acidosis

• Caused by a decreased rate of breathing resulting in alveolar hypoventilation
• CO2 is retained, leading to CO2 excess (Hypercapnia)
• Can be acute or chronic
• Chronic cases examples: COPD (renal compensation occurs over days for chronic conditions)
• Causes include over sedation, head injury, and other factors

Respiratory Alkalosis

• Alveolar hyperventilation causes this
• Decreases CO2 leading to Hypocapnea
• Stimulated by increased breathing caused by fever, anemia, anxiety/panic disorder, mechanical ventilation, and pain

Mixed Acid Base

• Occurs when 2 primary disorders occur at the same time
• Most common in hospital patients and patients in the ICU

Compensation

• Respiratory Disorder: Respiratory Acidosis (Compensate) & Respiratory Alkalosis (Compensate)
• Metabolic Disorder: Metabolic Acidosis (Compensate) & Metabolic Alkalosis (Compensate)