Acid base fluid balance imbalance
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Acid-Base Fluid Balance & Imbalances
Fundamentals
The body maintains blood pH between 7.35-7.45 through three main buffer systems working together:
- Chemical buffers - bicarbonate (HCO3-/H2CO3), proteins, phosphates (immediate, seconds)
- Respiratory compensation - alters CO2 via ventilation (minutes)
- Renal compensation - adjusts HCO3- secretion/reabsorption (hours to days)
The central equation is the Henderson-Hasselbalch relationship:
pH = pKa + log([HCO3-] / [H2CO3])
Or, practically: pH depends on the ratio of HCO3- (metabolic) to PCO2 (respiratory)
Normal values:
| Parameter | Normal Range |
|---|---|
| pH | 7.35-7.45 |
| PaCO2 | 35-45 mmHg |
| HCO3- | 22-26 mEq/L |
| Base excess | -2 to +2 |
The 4 Primary Acid-Base Disorders
Acid-Base Map (Diagnostic Tool)
The acid-base map: plot your patient's PaCO2 (x-axis) against pH (y-axis). Points falling within the labeled zones indicate simple disorders; points outside zones suggest mixed disorders. - Roberts and Hedges' Clinical Procedures in Emergency Medicine
1. Metabolic Acidosis
- Definition: Low pH + low HCO3-
- Mechanism: Strong acids added to blood or loss of base (e.g., diarrhea, ketoacidosis, lactic acidosis, renal failure, toxic ingestions)
- Compensation: Increased ventilation (Kussmaul breathing) -> decreased PCO2
- Formula: Expected PCO2 = (1.5 × HCO3-) + 8 ± 2 (Winters' formula)
- OR: ↓PCO2 = 1.3 × ↓HCO3-
Causes classified by Anion Gap:
| Type | Anion Gap | Mechanism | Examples |
|---|---|---|---|
| High AG acidosis | >12 mEq/L | Unmeasured anions accumulate | Lactic acidosis, DKA, uremia, salicylates, methanol, ethylene glycol (mnemonic: A MUDPILE CAT) |
| Normal AG (hyperchloremic) acidosis | 8-12 mEq/L | Cl- replaces lost HCO3- | Diarrhea, renal tubular acidosis (RTA), saline infusion, urinary diversions |
Anion Gap formula:
AG = Na+ - (Cl- + HCO3-) -- Normal: 8-12 mEq/L
Note: Correct for hypoalbuminemia - AG falls ~2.5 mEq/L for each 1 g/dL decrease in albumin below normal. - NKF Primer on Kidney Diseases, 8e
2. Metabolic Alkalosis
- Definition: High pH + high HCO3-
- Mechanism: Loss of acid (vomiting, nasogastric suction) or gain of base (excessive alkali, diuretic-induced Cl- depletion)
- Compensation: Decreased ventilation -> increased PCO2
- Formula: ↑PCO2 = 0.6 × ↑HCO3-
Common causes:
- Vomiting / NG suctioning (HCl loss)
- Loop/thiazide diuretics (Cl- and K+ depletion)
- Mineralocorticoid excess (Conn's syndrome, Cushing's)
- Post-hypercapnia (chronic CO2 retention with renal HCO3- retention)
- Excessive antacid or bicarbonate intake
Saline-responsiveness is a key clinical distinction:
- Saline-responsive (urine Cl- < 20): volume depletion, vomiting
- Saline-resistant (urine Cl- > 20): hyperaldosteronism, severe K+ depletion
3. Respiratory Acidosis
- Definition: Low pH + high PCO2
- Mechanism: Hypoventilation -> CO2 retention
- Compensation: Renal retention of HCO3- (slow, 3-5 days for full compensation)
| Acute | Chronic | |
|---|---|---|
| HCO3- rise per 10 mmHg PCO2 increase | +1 mEq/L | +3-4 mEq/L |
Common causes:
- CNS depression (opioids, anesthetics, sedatives)
- Neuromuscular disease (myasthenia gravis, Guillain-Barre, ALS)
- Airway obstruction (COPD, asthma exacerbation)
- Chest wall restriction (severe kyphoscoliosis, obesity hypoventilation)
- Pneumothorax, severe pneumonia
In anesthesia: acute respiratory acidosis from hypoventilation shows dramatic pH fall, elevated PaCO2, and only a slight HCO3- rise (1 mEq/L per 10 mmHg PCO2); base excess should be zero in purely acute cases. - Miller's Anesthesia, 10e
4. Respiratory Alkalosis
- Definition: High pH + low PCO2
- Mechanism: Hyperventilation -> CO2 washout
- Compensation: Renal excretion of HCO3-
| Acute | Chronic | |
|---|---|---|
| HCO3- fall per 10 mmHg PCO2 decrease | -2 mEq/L | -5 mEq/L |
Common causes:
- Anxiety, pain, fever
- Hypoxia (stimulating peripheral chemoreceptors)
- Sepsis (early stage - most common cause of respiratory alkalosis in ICU)
- Pregnancy (progesterone stimulates respiration)
- Liver failure (CNS ammonia stimulation)
- Iatrogenic: excessive mechanical ventilation
Systematic Approach to ABG Interpretation
Step-by-step:
- Check pH - < 7.35 = acidosis; > 7.45 = alkalosis
- Identify the primary process - Is the PCO2 or HCO3- directionally consistent with the pH change?
- pH low + PCO2 high = respiratory acidosis
- pH low + HCO3- low = metabolic acidosis
- pH high + PCO2 low = respiratory alkalosis
- pH high + HCO3- high = metabolic alkalosis
- Calculate expected compensation (use formulas in table below)
- Compare actual vs. expected - if they don't match, a mixed disorder is present
- Calculate anion gap if metabolic acidosis is present
- Check delta-delta ratio if high AG acidosis: (AG - 12) / (24 - HCO3-) -- ratio 1-2 = pure high AG; < 1 = concurrent normal AG; > 2 = concurrent metabolic alkalosis
Compensation Formulas Summary
| Primary Disorder | Expected Compensation |
|---|---|
| Metabolic acidosis | PCO2 = (1.5 × HCO3-) + 8 ± 2 OR ↓PCO2 = 1.3 × ↓HCO3- |
| Metabolic alkalosis | ↑PCO2 = 0.6 × ↑HCO3- |
| Respiratory acidosis (acute) | ↑HCO3- = 1 mEq/L per 10 mmHg ↑PCO2 |
| Respiratory acidosis (chronic) | ↑HCO3- = 3-4 mEq/L per 10 mmHg ↑PCO2 |
| Respiratory alkalosis (acute) | ↓HCO3- = 2 mEq/L per 10 mmHg ↓PCO2 |
| Respiratory alkalosis (chronic) | ↓HCO3- = 5 mEq/L per 10 mmHg ↓PCO2 |
Sources: Roberts & Hedges' Clinical Procedures in Emergency Medicine; NKF Primer on Kidney Diseases, 8e; Miller's Anesthesia, 10e
Mixed Acid-Base Disorders
When measured compensation does not match predicted values, a mixed disorder exists. Clinically important combinations:
| Mixed Disorder | Clinical Context |
|---|---|
| Metabolic acidosis + respiratory alkalosis | Salicylate toxicity, sepsis, hepatic failure |
| Metabolic alkalosis + respiratory acidosis | COPD with vomiting or diuretic use |
| Metabolic acidosis + metabolic alkalosis | Renal failure with vomiting |
| Mixed respiratory + metabolic acidosis | Cardiac arrest, severe COPD exacerbation |
Fluid Balance and Acid-Base Connections
Fluid compartment disturbances frequently cause or accompany acid-base disorders:
| Fluid Imbalance | Associated Acid-Base Effect | Mechanism |
|---|---|---|
| Volume depletion | Metabolic alkalosis (contraction alkalosis) | HCO3- concentration rises as volume decreases; aldosterone increases H+ secretion |
| Normal saline infusion | Hyperchloremic (non-gap) metabolic acidosis | Excess Cl- dilutes HCO3- and displaces it |
| Diarrhea | Non-gap metabolic acidosis | Direct HCO3- loss in stool |
| Vomiting / NG suction | Metabolic alkalosis | HCl loss from stomach |
| Renal failure / oliguria | High AG metabolic acidosis | Retention of sulfates, phosphates, organic acids |
| Mannitol / hypotonic fluids | Dilutional (non-gap) metabolic acidosis | Dilution of serum HCO3- |
Clinical Pearls
- Compensation is never complete - the pH always remains on the side of the primary disorder. If pH is fully normal, suspect a mixed disorder.
- Renal compensation is slow (3-5 days) vs. respiratory compensation (minutes to hours).
- Osmolal gap (measured Osm - calculated Osm; normal 5-10 mOsm/kg): elevated gap in high-AG acidosis suggests methanol, ethylene glycol, isopropyl alcohol, or ethanol.
- In COPD patients, the "normal" PCO2 is elevated - always check baseline values before labeling as pure respiratory acidosis.
- Hypoalbuminemia lowers the expected anion gap; correct before concluding AG is normal.
- Metabolic alkalosis inhibits respiration to retain CO2, but this response is limited because the resulting hypoxia will stimulate ventilation again - PCO2 rarely exceeds 55-60 mmHg as compensation.
Primary sources: Roberts & Hedges' Clinical Procedures in Emergency Medicine; NKF Primer on Kidney Diseases, 8e; Ganong's Review of Medical Physiology, 26e; Miller's Anesthesia, 10e; Harrison's Principles of Internal Medicine, 22e (2025)
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ABG Quick Reference
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What's included in the PDF:
- Normal ABG Values - pH, PaCO2, HCO3-, base excess, PaO2, SaO2 with acidosis/alkalosis thresholds
- Step-by-Step Interpretation - 5-step systematic approach including delta-delta ratio
- Compensation Rules - All 6 formulas (metabolic acidosis/alkalosis, acute/chronic respiratory acidosis/alkalosis) in one clean table
- Common Causes - Side-by-side with all 4 disorders
- Anion Gap - Formula, A MUDPILE CAT mnemonic, albumin correction, and osmolal gap
- Mixed Disorders - 5 clinically important combinations with identifying clues
- Fluid-Acid-Base Connections - How each fluid imbalance causes specific acid-base disorders
- Clinical Pearls - 7 high-yield tips (compensation ceiling, COPD baseline, saline-responsive alkalosis, Winters' formula check, etc.)
Color-coded throughout: red tones for acidosis disorders, blue tones for alkalosis, with source citations at the footer.
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