Nephrology — L5: Acid Base Disorders

📚 Lecture Overview

This lecture covers the fundamentals of acid-base disorders, including buffer systems, physiological pH regulation, and the four primary types of acid-base disturbances. Mastering these concepts is essential for interpreting arterial blood gases (ABGs) and managing critical clinical scenarios like DKA, respiratory failure, and poisonings.

🎯 Key Concepts & Definitions

Acid: A substance that releases hydrogen ions ($H^+$) into a solution.
Base: A substance that can bind to hydrogen ions.
pH: The measure of hydrogen ion concentration; normal physiological range is 7.35–7.45.
Anion Gap (AG): The difference between measured cations and anions, representing unmeasured negative ions in the blood; normal is roughly 12 mEq/L (or 16 mEq/L if potassium is included).
ROME: A memory tool for ABG interpretation (Respiratory Opposite, Metabolic Equal).

📖 Main Content

1. Acid-Base Buffer Systems

Buffers are the first line of defense against changes in pH, working within minutes to days.
- Body Fluid Buffers: Includes the carbonic acid-bicarbonate buffer system ($CO_2 + H_2O \rightleftharpoons H_2CO_3 \rightleftharpoons H^+ + HCO_3^-$), protein buffers (hemoglobin, albumin), and phosphate buffers.
- Lungs: Regulate pH within hours by altering ventilation rate and volume to retain or eliminate $CO_2$.
- Kidneys: Regulate pH within days by secreting $H^+$ ions (primarily as $NH_4^+$) and reabsorbing bicarbonate ($HCO_3^-$).
- Bones: Dissolve during long-standing metabolic acidosis to release calcium carbonate and phosphate to buffer excess acid.

2. Metabolic Acidosis

Characterized by a low serum bicarbonate level ($HCO_3^-$) due to acid addition or bicarbonate loss.
- High Anion Gap Metabolic Acidosis (HAGMA): Caused by acid addition. Remember the mnemonic MUDPILES (Methanol, Uremia, Diabetic Ketoacidosis, Paraldehyde, Isoniazid, Lactic Acidosis, Ethylene Glycol, Salicylates).
- Normal Anion Gap Metabolic Acidosis (hyperchloremic): Caused by $HCO_3^-$ loss, $H^+$ excretion failure, or $Cl^-$ administration (e.g., severe diarrhea, fistulas, renal tubular acidosis, carbonic anhydrase inhibitors, excess normal saline).
- Clinical Findings: Compensatory hyperventilation (Kussmaul breathing), decreased cardiac contractility, and peripheral vasodilation leading to shock in severe cases.
- Treatment: Treat underlying cause. $NaHCO_3$ therapy is controversial and reserved for severe acidemia ($pH < 7.1$).

3. Metabolic Alkalosis

Characterized by high serum bicarbonate levels ($HCO_3^-$).
- Chloride-Responsive ($U_{Cl} < 20$ mEq/L): Vomiting, NG suction, chloride-losing diarrhea, post-diuretic therapy, and alkali administration. Treated with volume expansion using normal saline.
- Chloride-Unresponsive ($U_{Cl} > 20$ mEq/L): Bartter syndrome, Gitelman syndrome, primary aldosteronism, Liddle syndrome, and excess mineralocorticoid effects. Treated by addressing the underlying cause.
- Clinical Findings: Hypopnea, weakness from concomitant hypokalemia, and neuromuscular excitability (tetany and seizures) due to hypocalcemia.

4. Respiratory Acidosis & Alkalosis

Respiratory Acidosis: Caused by hypoventilation leading to hypercapnia ($CO_2$ retention). Causes include COPD, asthma, CNS depression (narcotics, benzodiazepines), severe obesity, and neuromuscular disorders. ABG shows low pH, high $Pco_2$, and high $HCO_3^-$.
Respiratory Alkalosis: Caused by hyperventilation reducing $Pco_2$. Causes include hypoxia, CNS-mediated disorders (anxiety, pain, stroke, salicylates), and pulmonary embolisms. ABG shows high pH, low $Pco_2$, and low $HCO_3^-$.

5. 6-Step ABG Interpretation Approach

Internal Consistency: Check using the Henderson-Hasselbalch equation or $[H^+] = 80 - decimal of pH$.
Acidemia vs Alkalemia: $pH < 7.35$ is acidemia; $pH > 7.45$ is alkalemia.
Respiratory vs Metabolic: Use ROME (Respiratory is Opposite, Metabolic is Equal).
Compensation: Check expected compensation formulas to identify mixed disorders.
Anion Gap: Calculate $AG = [Na^+] - ([Cl^-] + [HCO_3^-])$, correct for albumin if needed. Check osmolar gap if toxic ingestion is suspected.
Delta Ratio ($\Delta AG / \Delta HCO_3^-$): Should be 1.0 to 2.0 in uncomplicated HAGMA. $< 1.0$ indicates concurrent normal AG acidosis; $> 2.0$ indicates concurrent metabolic alkalosis.

📊 Visual Learning

flowchart TD A[ABG Sample] --> B{Check pH} B -->|< 7.35| C[Acidemia] B -->|> 7.45| D[Alkalemia] C --> E{Check PCO2 and HCO3} E -->|Opposite direction| F[Respiratory Acidosis] E -->|Equal direction| G[Metabolic Acidosis]

mindmap root("Metabolic Acidosis MUDPILES") "M" "Methanol" "U" "Uremia" "D" "Diabetic Ketoacidosis" "P" "Paraldehyde" "I" "Isoniazid" "L" "Lactic Acidosis" "E" "Ethylene Glycol" "S" "Salicylates"

graph LR A[Primary Disorder] --> B[Lungs compensate for metabolic] A --> C[Kidneys compensate for respiratory] B --> D[Works within hours] C --> E[Works within days]

💡 Important Points to Remember

Physiological pH: Strictly maintained between 7.35 and 7.45.
Buffers Timeline: Fluids work in minutes, Lungs in hours, Kidneys in days, and Bones in long-standing acidosis.
Anion Gap Formula: $AG = Na^+ - (HCO_3^- + Cl^-)$, normal is roughly 12 mEq/L.
Albumin Correction: Anion gap decreases by 2.5 mEq/L for every 1 g/dL reduction in serum albumin.
MUDPILES: The go-to mnemonic for high anion gap metabolic acidosis.
Chloride-Responsive vs Unresponsive: Differentiated by urinary chloride ($U_{Cl}$) threshold of 20 mEq/L in metabolic alkalosis.
ROME Mnemonic: Respiratory Opposite ($pH \uparrow, Pco_2 \downarrow$), Metabolic Equal ($pH \uparrow, HCO_3^- \uparrow$).
Delta Ratio Range: Normal uncomplicated HAGMA delta ratio is 1.0 to 2.0.
Common Mistake: Assuming a normal pH means there is no acid-base disorder; always check $Pco_2$ and $HCO_3^-$.
Hypocalcemia in Alkalosis: Severe alkalosis increases protein binding of calcium, leading to lower ionized calcium and tetany.

⚠️ Common Exam Questions & Traps

Exam Trick 1: Giving a patient with a normal pH ($7.40$) who actually has a severe mixed disorder (e.g., concurrent respiratory alkalosis and metabolic acidosis). Students often mistakenly skip calculating compensation when the pH is normal.
Exam Trick 2: Confusing normal anion gap metabolic acidosis (hyperchloremic) causes (like diarrhea) with high anion gap causes (like DKA). Always check if the anion gap is provided or needs calculation.
Common Trap: Forgetting to adjust the anion gap for hypoalbuminemia. If albumin is low, a "normal" calculated AG might actually be an elevated AG masking a metabolic acidosis.
MCQ Pitfall: Mixing up chloride-responsive vs chloride-unresponsive metabolic alkalosis etiologies (e.g., attributing vomiting to a chloride-unresponsive state when it is chloride-responsive).

📝 Quick Review Checklist

I can explain the components and timelines of acid-base buffer systems
I understand the difference between high anion gap and normal anion gap metabolic acidosis
I can define and apply the MUDPILES mnemonic for HAGMA
I know how to differentiate chloride-responsive and chloride-unresponsive metabolic alkalosis
I can state the compensation rules for respiratory and metabolic disorders
I can calculate and interpret the Anion Gap and Delta Ratio
I know how to use the ROME rule for ABG interpretation
I understand the clinical findings of tetany due to hypocalcemia in alkalosis

📚 Nephrology — L5: Acid Base Disorders