đź“š CONTENT
Learning Objectives
By the end of this chapter, learners will be able to:
1.Understand the basic principles of acid-base physiology and regulation.
2.Interpret arterial blood gas (ABG) results to diagnose primary acid-base disorders.
3.Identify compensatory mechanisms for each primary disorder.
4.Calculate and interpret the anion gap in metabolic acidosis.
5.Recognize and manage common acid-base disturbances.
8.1 Introduction to Acid-Base Physiology
Maintaining a stable pH in the extracellular fluid (ECF) is crucial for normal physiological function. The body tightly regulates pH within a narrow range (7.35-7.45) through buffer systems, respiratory compensation, and renal compensation. Acid-base disorders occur when these regulatory mechanisms are overwhelmed or dysfunctional.
pH Scale
•Acidosis: pH < 7.35
•Alkalosis: pH > 7.45
Key Components of Acid-Base Balance
•Acids: Substances that donate H+ ions (e.g., H2CO3, lactic acid).
•Bases: Substances that accept H+ ions (e.g., HCO3-).
•Buffers: Systems that resist changes in pH by reversibly binding H+ ions (e.g., bicarbonate buffer system, phosphate buffer system, proteins).
Henderson-Hasselbalch Equation
pH = pKa + log ([HCO3-] / [0.03 x PCO2]) This equation highlights the relationship between pH, bicarbonate (metabolic component), and partial pressure of carbon dioxide (PCO2, respiratory component).
8.2 Primary Acid-Base Disorders
There are four primary acid-base disorders, each with a characteristic change in pH, PCO2, and HCO3-.
1. Metabolic Acidosis
•Definition: Primary decrease in HCO3- leading to a decrease in pH.
•Causes: Increased acid production (lactic acidosis, ketoacidosis), decreased acid excretion (renal failure), or bicarbonate loss (diarrhea, renal tubular acidosis).
•Compensation: Respiratory compensation (hyperventilation to decrease PCO2).
•Anion Gap (AG): Used to differentiate causes of metabolic acidosis.
•AG = Na+ – (Cl- + HCO3-) (Normal AG: 8-12 mEq/L)
•High Anion Gap Metabolic Acidosis (HAGMA): Accumulation of unmeasured anions.
•MUDPILES mnemonic: Methanol, Uremia, Diabetic Ketoacidosis, Paraldehyde, Iron/Isoniazid, Lactic Acidosis, Ethylene Glycol, Salicylates.
•Normal Anion Gap Metabolic Acidosis (NAGMA) / Hyperchloremic Metabolic Acidosis: Loss of bicarbonate or gain of chloride.
•USED CARP mnemonic: Ureteroenterostomy, Saline infusion, Endocrine (hyperparathyroidism), Diarrhea, Carbonic anhydrase inhibitors, Adrenal insufficiency, Renal tubular acidosis, Pancreatic fistula.
2. Metabolic Alkalosis
•Definition: Primary increase in HCO3- leading to an increase in pH.
•Causes: Loss of H+ (vomiting, nasogastric suction), excess HCO3- administration, or volume contraction (diuretics).
•Compensation: Respiratory compensation (hypoventilation to increase PCO2).
•Chloride-responsive vs. Chloride-resistant:
•Chloride-responsive: Responds to saline administration (e.g., vomiting, diuretics).
•Chloride-resistant: Does not respond to saline (e.g., hyperaldosteronism).
3. Respiratory Acidosis
•Definition: Primary increase in PCO2 leading to a decrease in pH.
•Causes: Hypoventilation (e.g., COPD exacerbation, opioid overdose, neuromuscular disease, severe asthma).
•Compensation: Renal compensation (increased HCO3- reabsorption and H+ excretion).
4. Respiratory Alkalosis
•Definition: Primary decrease in PCO2 leading to an increase in pH.
•Causes: Hyperventilation (e.g., anxiety, pain, fever, hypoxia, pulmonary embolism, mechanical ventilation).
•Compensation: Renal compensation (decreased HCO3- reabsorption and H+ excretion).
8.3 Arterial Blood Gas (ABG) Interpretation
ABG analysis is essential for diagnosing and managing acid-base disorders. The key values to assess are pH, PCO2, and HCO3-.
Steps for ABG Interpretation:
1.Assess pH: Is it acidemic (<7.35), alkalemic (>7.45), or normal (7.35-7.45)?
2.Assess PCO2: Is it the primary respiratory component? (Normal: 35-45 mmHg)
•If pH and PCO2 move in opposite directions, it’s a respiratory disorder.
3.Assess HCO3-: Is it the primary metabolic component? (Normal: 22-26 mEq/L)
•If pH and HCO3- move in the same direction, it’s a metabolic disorder.
4.Check for Compensation: Is the non-primary component moving in the expected direction to compensate?
•Acute vs. Chronic: Compensation differs based on acuity.
5.Calculate Anion Gap (if metabolic acidosis): To identify the cause.
6.Check for Mixed Disorders: If compensation is inappropriate or if there are multiple primary disorders.
Expected Compensations:
•Metabolic Acidosis: PCO2 decreases by 1.2 mmHg for every 1 mEq/L decrease in HCO3-.
•Metabolic Alkalosis: PCO2 increases by 0.7 mmHg for every 1 mEq/L increase in HCO3-.
•Respiratory Acidosis (Acute): HCO3- increases by 1 mEq/L for every 10 mmHg increase in PCO2.
•Respiratory Acidosis (Chronic): HCO3- increases by 3-4 mEq/L for every 10 mmHg increase in PCO2.
•Respiratory Alkalosis (Acute): HCO3- decreases by 2 mEq/L for every 10 mmHg decrease in PCO2.
•Respiratory Alkalosis (Chronic): HCO3- decreases by 4-5 mEq/L for every 10 mmHg decrease in PCO2.
8.4 Common Acid-Base Disturbances in Nephrology
1. Metabolic Acidosis in CKD
•Cause: Reduced renal acid excretion and decreased bicarbonate reabsorption.
•Management: Oral bicarbonate supplementation to maintain HCO3- >22 mEq/L.
2. Renal Tubular Acidosis (RTA)
•Definition: Disorders characterized by normal anion gap metabolic acidosis due to defects in renal acid excretion or bicarbonate reabsorption.
•Types:
•Type 1 (Distal RTA): Defect in H+ secretion in the distal tubule. Associated with hypokalemia, nephrolithiasis.
•Type 2 (Proximal RTA): Defect in HCO3- reabsorption in the proximal tubule. Associated with Fanconi syndrome.
•Type 4 (Hyperkalemic RTA): Defect in aldosterone production or action, leading to impaired K+ and H+ excretion. Associated with hyperkalemia.
3. Metabolic Alkalosis with Volume Contraction
•Cause: Often due to vomiting or loop diuretic use, leading to volume depletion and increased HCO3- reabsorption.
•Management: Volume repletion with normal saline.
đź“Š SUMMARY
Key Points on Acid-Base Disorders
1.pH Homeostasis: Maintained by buffers, respiratory, and renal systems.
2.Primary Disorders: Metabolic Acidosis (low HCO3-), Metabolic Alkalosis (high HCO3-), Respiratory Acidosis (high PCO2), Respiratory Alkalosis (low PCO2).
3.Compensation: Body attempts to normalize pH by adjusting the non-primary component.
4.ABG Interpretation: Systematic approach using pH, PCO2, HCO3-.
5.Anion Gap: Differentiates causes of metabolic acidosis (HAGMA vs. NAGMA).
6.Renal Role: Kidneys are crucial for long-term acid-base balance, especially in chronic disorders.
ABG Interpretation Quick Guide
•pH < 7.35: Acidosis
•pH > 7.45: Alkalosis
•PCO2 (35-45 mmHg): Respiratory component
•HCO3- (22-26 mEq/L): Metabolic component
Common Nephrology-Related Disorders
•Metabolic Acidosis in CKD: Due to impaired acid excretion.
•Renal Tubular Acidosis (RTA): Specific defects in renal acid-base handling.
đź’Ž CLINICAL PEARLS
ABG Interpretation Pearls
1.ROME: Respiratory Opposite, Metabolic Equal – a mnemonic for remembering the direction of pH and PCO2/HCO3- changes.
2.Delta-Delta Gap: In high anion gap metabolic acidosis, calculate the delta-delta gap (ΔAG – ΔHCO3-) to identify co-existing metabolic disorders.
3.Compensation Rules: Memorize the expected compensation rules to determine if a disorder is simple or mixed.
Metabolic Acidosis Pearls
1.MUDPILES: Always consider these causes for high anion gap metabolic acidosis.
2.Diarrhea: The most common cause of normal anion gap metabolic acidosis due to bicarbonate loss.
3.CKD and Acidosis: Metabolic acidosis in CKD often worsens as GFR declines; early bicarbonate supplementation can be beneficial.
Metabolic Alkalosis Pearls
1.Volume Status: Crucial for differentiating chloride-responsive (volume depletion) from chloride-resistant (mineralocorticoid excess) metabolic alkalosis.
2.Vomiting: A common cause of metabolic alkalosis due to loss of gastric acid.
Respiratory Disorders Pearls
1.Acute vs. Chronic: Distinguish acute from chronic respiratory disorders based on the degree of renal compensation.
2.Hyperventilation: Often a sign of underlying metabolic acidosis or anxiety.
RTA Pearls
1.Type 1 RTA: Associated with hypokalemia and nephrolithiasis.
2.Type 4 RTA: Associated with hyperkalemia and often seen in patients with diabetes or adrenal insufficiency.
🖼️ VISUAL MATERIALS
ABG Interpretation Flowchart
A comprehensive flowchart guiding the systematic interpretation of Arterial Blood Gas results.
Key Diagrams
•Henderson-Hasselbalch Equation Breakdown: Visualizing the relationship between pH, PCO2, and HCO3-.
•Anion Gap Calculation and Differential Diagnosis: A table or diagram illustrating HAGMA and NAGMA causes.
•Acid-Base Nomogram: A graphical tool for visualizing acid-base relationships and identifying disorders.
🎯 MULTIPLE CHOICE QUESTIONS
Question 1
Which of the following pH values indicates alkalosis? A) 7.30 B) 7.38 C) 7.42 D) 7.50
Answer: D) 7.50 Explanation: A pH greater than 7.45 indicates alkalosis.
Question 2
A patient presents with a pH of 7.25, PCO2 of 60 mmHg, and HCO3- of 24 mEq/L. What is the primary acid-base disorder? A) Metabolic acidosis B) Metabolic alkalosis C) Respiratory acidosis D) Respiratory alkalosis
Answer: C) Respiratory acidosis Explanation: The low pH and high PCO2 indicate a primary respiratory acidosis.
Question 3
Which of the following is a common cause of high anion gap metabolic acidosis? A) Diarrhea B) Renal tubular acidosis C) Lactic acidosis D) Normal saline infusion
Answer: C) Lactic acidosis Explanation: Lactic acidosis is one of the causes of high anion gap metabolic acidosis (MUDPILES mnemonic).
Question 4
In a patient with chronic respiratory acidosis, what is the expected renal compensatory response? A) Decrease in HCO3- reabsorption B) Increase in HCO3- reabsorption C) Increase in H+ secretion D) Decrease in PCO2
Answer: B) Increase in HCO3- reabsorption Explanation: In chronic respiratory acidosis, the kidneys compensate by increasing bicarbonate reabsorption to buffer the excess acid.
Question 5
Which type of renal tubular acidosis (RTA) is characterized by hyperkalemia and a defect in aldosterone production or action? A) Type 1 (Distal) RTA B) Type 2 (Proximal) RTA C) Type 3 RTA D) Type 4 (Hyperkalemic) RTA
Answer: D) Type 4 (Hyperkalemic) RTA Explanation: Type 4 RTA is associated with hyperkalemia due to impaired potassium and hydrogen excretion, often related to aldosterone deficiency or resistance.
Question 6
Which of the following is the formula for calculating the anion gap? A) Na+ + Cl- + HCO3- B) Na+ – (Cl- + HCO3-) C) Cl- – (Na+ + HCO3-) D) HCO3- – (Na+ + Cl-)
Answer: B) Na+ – (Cl- + HCO3-) Explanation: The anion gap is calculated as the difference between measured cations (Na+) and measured anions (Cl- and HCO3-).
Question 7
A patient with severe vomiting is likely to develop which acid-base disorder? A) Metabolic acidosis B) Metabolic alkalosis C) Respiratory acidosis D) Respiratory alkalosis
Answer: B) Metabolic alkalosis Explanation: Severe vomiting leads to a loss of gastric acid (H+), resulting in metabolic alkalosis.
Question 8
Which of the following is a common cause of respiratory alkalosis? A) Opioid overdose B) COPD exacerbation C) Anxiety/Hyperventilation D) Neuromuscular disease
Answer: C) Anxiety/Hyperventilation Explanation: Hyperventilation, often due to anxiety, pain, or fever, leads to excessive CO2 exhalation and respiratory alkalosis.
Question 9
What is the normal range for serum bicarbonate (HCO3-) in mEq/L? A) 18-22 B) 22-26 C) 26-30 D) 30-34
Answer: B) 22-26 Explanation: The normal range for serum bicarbonate is typically 22-26 mEq/L.
Question 10
Which of the following is a key compensatory mechanism for metabolic acidosis? A) Hypoventilation B) Increased renal bicarbonate excretion C) Hyperventilation D) Increased renal acid reabsorption
Answer: C) Hyperventilation Explanation: The respiratory system compensates for metabolic acidosis by increasing ventilation (hyperventilation) to blow off CO2 and reduce carbonic acid.
🎤 POWERPOINT PRESENTATION
[Link to interactive presentation slides covering all Acid-Base Disorders concepts with visual aids and animations]