For Internal Medicine and Nephrology Residents

Acid-Base Disorders: Full Teaching Module

Built around the Henderson-Hasselbalch nomogram and a bedside algorithmic approach.

Syllabus

This module is arranged as a short bedside acid-base course. Use the calculator first if you want a rapid interpretation, then open the lesson that matches the pattern you are reviewing.

Foundations

Part I explains the equation and ratio. Part II places disorders on the map.

Bedside workflow

Part III walks through the algorithm. Part IV sorts the major disorder groups.

Application and synthesis

Part V, Part VI, Part VII, and the take-home section reinforce how to teach and apply the framework.

Teaching flow: equation -> map -> decision tree -> classification -> worked example -> physiology -> advanced concept -> take-home rules.

The Henderson-Hasselbalch Nomogram A programmatically generated nomogram showing measured pH on the vertical axis, PaCO2 on the horizontal axis, iso-bicarbonate curves, a normal reference point, and acute respiratory compensation lines.

Curved lines are generated from the Henderson-Hasselbalch equation and represent constant bicarbonate concentrations (mEq/L). The ordinate shows pH, the abscissa shows PaCO2, and the dashed lines show acute respiratory compensation. This original figure uses no third-party image asset.

Interactive Acid-Base Calculator

Enter the blood gas and chemistry values below. The interpretation updates automatically and then points you to the matching teaching section.

Optional if HCO3 and PaCO2 are entered
Required
Required
Optional, for anion gap
Optional, for anion gap
Part I - What the Henderson-Hasselbalch Equation Really Means

Part I - What the Henderson-Hasselbalch Equation Really Means

pH = 6.1 + log ( HCO3 / (0.03 * PaCO2) )

The Henderson-Hasselbalch equation describes the bicarbonate buffer system.

6.1 = pKa of carbonic acid

HCO3 = metabolic (renal) component

0.03 * PaCO2 = dissolved carbonic acid (respiratory component)

Key Teaching Principle: pH is determined by a ratio.

pH ~ HCO3 / PaCO2

The kidney controls the numerator (HCO3). The lung controls the denominator (PaCO2). All acid-base disorders are disturbances of this ratio.

If the ratio decreases -> acidemia

If the ratio increases -> alkalemia

Normal ratio: 24 / 40 = 0.6

Which produces pH ~ 7.40

Part II - Henderson-Hasselbalch Map (Disorder Positioning)

Part II - Henderson-Hasselbalch Map (Disorder Positioning)

Think of a 2-axis map:

X-axis = PaCO2

Y-axis = HCO3

Center point: PaCO2 = 40, HCO3 = 24

1. Metabolic Acidosis

HCO3 down

PaCO2 normal or compensatory down

Map position: Downward

2. Metabolic Alkalosis

HCO3 up

PaCO2 normal or compensatory up

Map position: Upward

3. Respiratory Acidosis

PaCO2 up

HCO3 normal or compensatory up

Map position: Right

4. Respiratory Alkalosis

PaCO2 down

HCO3 normal or compensatory down

Map position: Left

Mixed disorders occupy diagonal positions.

Low HCO3 + High PaCO2 -> Severe mixed acidosis

High HCO3 + Low PaCO2 -> Severe mixed alkalosis

Part III - Master Decision Tree (Algorithmic Approach)

Part III - Master Decision Tree (Algorithmic Approach)

Step 1

Determine acidemia or alkalemia

If pH < 7.35 -> Acidemia

If pH > 7.45 -> Alkalemia

If normal pH but abnormal HCO3 and PaCO2 -> Mixed disorder

Step 2

Identify Primary Disorder

Compare direction of pH and PaCO2:

  • If pH down and PaCO2 up -> Respiratory acidosis
  • If pH down and HCO3 down -> Metabolic acidosis
  • If pH up and PaCO2 down -> Respiratory alkalosis
  • If pH up and HCO3 up -> Metabolic alkalosis

Step 3

Check Compensation

Metabolic Acidosis:

Expected PaCO2 = (1.5 * HCO3) + 8 +/- 2

Metabolic Alkalosis:

Expected PaCO2 = 40 + 0.7 * (HCO3 - 24)

Respiratory Acidosis:

Acute: HCO3 rises 1 per 10 PaCO2

Chronic: HCO3 rises 3-4 per 10 PaCO2

Respiratory Alkalosis:

Acute: HCO3 falls 2 per 10 PaCO2

Chronic: HCO3 falls 4-5 per 10 PaCO2

If measured compensation does not equal expected -> Mixed disorder

Step 4

If Metabolic Acidosis - Calculate Anion Gap

AG = Na - (Cl + HCO3)

Normal AG ~ 12

If AG elevated -> High AG metabolic acidosis

If AG normal -> Non-gap metabolic acidosis

Step 5

Delta-Delta (High AG only)

Delta AG = AG - 12

Delta HCO3 = 24 - measured HCO3

  • If equal -> Pure AG acidosis
  • If Delta AG > Delta HCO3 -> Concurrent metabolic alkalosis
  • If Delta AG < Delta HCO3 -> Concurrent non-gap acidosis
Part IV - Complete Classification of Acid-Base Disorders

Part IV - Complete Classification of Acid-Base Disorders

I. Metabolic Acidosis

A. High Anion Gap

  • Lactic acidosis
  • Ketoacidosis (diabetic, alcoholic, starvation)
  • Uremia
  • Salicylates
  • Toxic alcohols
  • Pyroglutamic acid

Position on H-H Map: Low HCO3 -> downward shift

B. Non-Anion Gap

  • Diarrhea
  • Type I RTA
  • Type II RTA
  • Type IV RTA
  • Hyperchloremia

Low HCO3 with high chloride

II. Metabolic Alkalosis

A. Chloride responsive

  • Vomiting
  • NG suction
  • Diuretics (early)
  • Post-hypercapnia

B. Chloride resistant

  • Hyperaldosteronism
  • Bartter
  • Gitelman
  • Severe hypokalemia

Position: High HCO3 -> upward shift

III. Respiratory Acidosis

A. Acute

  • Opioids
  • CNS depression
  • Airway obstruction

B. Chronic

  • COPD
  • Obesity hypoventilation
  • Neuromuscular disease

Position: Rightward shift (PaCO2 up)

IV. Respiratory Alkalosis

A. Acute

  • Anxiety
  • Pain
  • Early sepsis
  • PE

B. Chronic

  • Cirrhosis
  • Pregnancy
  • Chronic hypoxemia

Position: Leftward shift (PaCO2 down)

V. Mixed Disorders

  1. Metabolic acidosis + Respiratory alkalosis (salicylates)
  2. Metabolic acidosis + Respiratory acidosis (cardiac arrest)
  3. Metabolic alkalosis + Respiratory acidosis (COPD + diuretics)
  4. Triple disorders (DKA + vomiting + hyperventilation)

These occupy diagonal positions on the H-H map.

Part V - Teaching Using the Equation (Control the Variables)

Part V - Teaching Using the Equation (Control the Variables)

HCO3 = 12

PaCO2 = 20

Ratio = 12 / 20 = 0.6

pH near normal.

Interpretation: Both numerator and denominator low.

Likely metabolic acidosis with respiratory compensation.

Check expected PaCO2:

1.5 * 12 + 8 = 26 +/- 2

Measured = 20 -> lower than expected -> respiratory alkalosis added.

Diagnosis: Mixed metabolic acidosis + respiratory alkalosis.

Part VI - Physiologic Understanding

Part VI - Physiologic Understanding

Key Concept: CO2 is acid. HCO3 is base.

  • The lungs remove acid in minutes.
  • The kidney regenerates base over hours-days.
  • Compensation never overcorrects.
  • If it appears to, a second disorder is present.
Part VII - Advanced Concept (Teaching Point for Senior Residents)

Part VII - Advanced Concept (Teaching Point for Senior Residents)

The Henderson-Hasselbalch equation is a descriptive equation.

It describes pH as a function of HCO3 and PaCO2.

It does not explain mechanism.

The Stewart approach reframes pH as determined by:

  1. PaCO2
  2. Strong ion difference (Na - Cl)
  3. Weak acids (albumin, phosphate)

Clinically, both approaches lead to the same bedside diagnoses when applied systematically.

Core Take-Home Message for Residents

Core Take-Home Message for Residents

  1. pH is a ratio problem.
  2. Always determine primary process first.
  3. Always calculate expected compensation.
  4. Always calculate anion gap in metabolic acidosis.
  5. If numbers do not fit, there is more than one disorder.