Complete Electrolyte Management Guide
Sodium | Potassium | Bicarbonate
References: Harrison's 22E (2025), Goldman-Cecil Medicine, Tintinalli's EM, Schwartz's Surgery, Brenner & Rector's Kidney
PART 1: SODIUM DISORDERS
TOTAL BODY WATER (TBW) - How to Calculate
TBW varies with age, sex, and body composition (fat has almost no water):
| Population | TBW Factor | Example (70 kg) |
|---|
| Adult male | 0.6 x weight (kg) | 42 L |
| Adult female | 0.5 x weight (kg) | 35 L |
| Elderly male | 0.5 x weight (kg) | - |
| Elderly female | 0.45 x weight (kg) | - |
| Infant / child | 0.7-0.75 x weight (kg) | - |
| Neonate | 0.8 x weight (kg) | - |
Mnemonic for TBW factors - "Boys Are Bigger Babies"
- Boy (adult male) = 0.6
- Adult female = 0.5
- Baby (child) = 0.7
- Baby newborn = 0.8
HYPONATREMIA
Definition and Classification
Hyponatremia = serum Na+ < 135 mEq/L
Step 1 - Is it TRUE hyponatremia? Check osmolality first.
Serum Na+ < 135 mEq/L
|
v
Check Serum Osmolality
|
+----+----+----+
| | |
HYPEROSMOLAR NORMAL HYPO-OSMOLAR
(>290) (275-290) (<275)
| | |
Glucose Pseudo- TRUE Hyponatremia
Mannitol hyponatremia (excess free water)
Glycine (lipids, |
proteins) Go to Step 2
Correction for hyperglycemia:
Na+ corrected = Measured Na+ + 1.6 x [(glucose - 100) / 100] mEq/L
Step 2 - Classify by Volume Status
TRUE HYPONATREMIA (hypo-osmolar)
|
Check Volume Status
(JVP, skin turgor, BP, urine Na+)
|
+--------+--------+
| | |
HYPOVOLEMIC EUVOLEMIC HYPERVOLEMIC
(low vol) (normal) (edema)
| | |
Urine Na+ Urine Na+ Urine Na+
<20 mEq/L >40 mEq/L <20 mEq/L
| | |
Extrarenal SIADH CHF
GI losses Hypothyroid Cirrhosis
Burns Addison's Nephrotic
| Polydipsia Renal failure
Renal causes |
if Na >20 Restrict Na+
(diuretics, and water
CSW, RTA) + Diuretics
Mnemonic for Causes of Hyponatremia by Volume Status
HYPOVOLEMIC - "GADS" (you feel sad when you're volume-depleted):
- GI losses (vomiting, diarrhea)
- Adrenal insufficiency (Addison's)
- Diuretics (thiazides > loop)
- Sweat / skin losses / CSW (cerebral salt wasting)
EUVOLEMIC - "SCHIPT" (each cause SHIFTs water in):
- SIADH
- Cort deficiency (glucocorticoid)
- Hypothyroidism
- Inappropriate IV fluids
- Polydipsia (primary)
- Too little solute (beer potomania, tea-and-toast)
HYPERVOLEMIC - "CCR" (like Creedence Clearwater Revival - fluid overload is rock solid):
- CHF (congestive heart failure)
- Cirrhosis
- Renal failure / Nephrotic syndrome
Step 3 - Calculate Sodium Deficit
Formula (from Tintinalli's / Schwartz's Surgery):
$$\text{Na Deficit (mEq)} = [\text{Na}\text{desired} - \text{Na}\text{measured}] \times (\text{TBW factor} \times \text{weight kg})$$
Example: 60 kg adult male, Na = 118 mEq/L, target 125 mEq/L
- Na deficit = (125 - 118) x (0.6 x 60) = 7 x 36 = 252 mEq
Step 4 - Rate of Correction (CRITICAL!)
The Golden Rules (Harrison's 22E, Goldman-Cecil):
| Scenario | Max Correction Rate |
|---|
| Asymptomatic / chronic (>48 h) | ≤0.5 mEq/L/hr; max 8-10 mEq/L in 24 h |
| Symptomatic / acute (<48 h) | 1-2 mEq/L/hr UNTIL symptoms resolve |
| Absolute max in any 24 h | ≤12 mEq/L/day |
| Absolute max in any 48 h | ≤18 mEq/L/48 h |
| High-risk for ODS (alcoholic, malnourished, cirrhotic, K+ depleted) | ≤8 mEq/L/day |
Key Danger: Correcting too fast causes Osmotic Demyelination Syndrome (ODS) = formerly called Central Pontine Myelinolysis (CPM). Risk is highest in chronic hyponatremia.
Treatment by Clinical Scenario
1. Asymptomatic Hyponatremia (mild/moderate, Na 125-134):
- Correct underlying cause
- Water restriction (1-1.5 L/day for SIADH)
- Correct over 48 h using oral intake or isotonic saline
2. Severe Symptomatic Hyponatremia (seizures, coma, Na <120):
- Give 3% NaCl (hypertonic saline)
- Dose: 1-2 mL/kg/hr IV until symptoms resolve OR Na rises by 4-6 mEq/L
- Alternative: 100 mL bolus of 3% NaCl (repeat up to 2-3 times)
- Then SLOW DOWN - do not exceed 12 mEq/L in 24 h
3. Hypovolemic Hyponatremia:
- Volume replacement with 0.9% Normal Saline (NS)
- Restoring volume will suppress ADH naturally - watch for overcorrection!
4. SIADH (euvolemic):
- Water restriction first line
- If refractory: Vaptans (tolvaptan), demeclocycline, or urea
- Treat underlying cause (lung cancer, drugs, CNS)
5. Hypervolemic (CHF, cirrhosis):
- Sodium and water restriction
- Loop diuretics (furosemide)
Saline Concentrations Reference
| Solution | Na+ Content |
|---|
| 0.9% NS (normal saline) | 154 mEq/L |
| 0.45% NS (half-normal) | 77 mEq/L |
| 3% NaCl (hypertonic) | 513 mEq/L |
| Lactated Ringer's | 130 mEq/L |
HYPERNATREMIA
Definition
Serum Na+ > 145 mEq/L - ALWAYS means relative water deficit (too little water OR too much sodium)
Mnemonic for Causes: "Too Little WATER In = Too High Na+"
W - Water loss (insensible - fever, burns, sweating)
A - ADH absent or resistance (Diabetes Insipidus - central or nephrogenic)
T - Tube feeds / inadequate water intake in elderly/unconscious
E - Electrolyte loading (hypertonic saline, sodium bicarb excess, hyperaldosteronism)
R - Renal losses (osmotic diuresis - DM, mannitol, post-obstruction)
Algorithm for Hypernatremia
HYPERNATREMIA (Na+ > 145)
|
Always = Free Water DEFICIT
|
Check Urine Osmolality
+--------+--------+
| |
>700 mOsm/L <300 mOsm/L
(concentrated) (dilute)
| |
Extrarenal loss Diabetes Insipidus
(GI, skin, resp) (central or nephrogenic)
Check with DDAVP test
Free Water Deficit Formula
$$\text{Free Water Deficit (L)} = \text{TBW} \times \left[\frac{\text{Na}_\text{actual}}{140} - 1\right]$$
Example: 70 kg male, Na = 162 mEq/L
- FWD = (0.6 x 70) x [(162/140) - 1] = 42 x 0.157 = 6.6 L
Rate of Correction for Hypernatremia
| Setting | Rate |
|---|
| Acute (<24 h onset) | 1 mEq/L/hr (max 12 mEq/L/day) |
| Chronic / unknown | ≤0.5 mEq/L/hr; max 10 mEq/L/day |
Danger of too-fast correction: Cerebral edema, seizures, brain herniation (opposite problem from hyponatremia - brain swells when Na drops too fast)
Fluids Used for Hypernatremia Correction
| Situation | Fluid |
|---|
| Hemodynamically unstable | 0.9% NS first (resuscitate) |
| Stable, mild dehydration | 0.45% NS (hypotonic) |
| Pure water deficit (DI) | 5% Dextrose in Water (D5W) |
| Oral available | Free water via NG tube / oral |
Key Complications to Keep in Mind
| Electrolyte Error | Complication |
|---|
| Too-fast correction of hyponatremia | ODS / Central Pontine Myelinolysis |
| Too-fast correction of hypernatremia | Cerebral edema, herniation |
| Too-rapid hypertonic saline infusion | Volume overload, pulmonary edema |
| Hyponatremia + K+ depletion (uncorrected) | Worsens ODS risk |
| SIADH treated with NS | Can WORSEN hyponatremia (if urine is more concentrated than NS) |
| Thiazide + hyponatremia | Stop thiazide; can precipitate profound hyponatremia |
PART 2: METABOLIC ACIDOSIS & BICARBONATE MANAGEMENT
Step 1 - Confirm Metabolic Acidosis
- pH < 7.35
- HCO3- < 22 mEq/L
- Expected pCO2 compensation: pCO2 = (1.5 x HCO3) + 8 ± 2 (Winter's formula)
- If pCO2 < expected: added respiratory alkalosis
- If pCO2 > expected: added respiratory acidosis
Step 2 - Calculate Anion Gap (AG)
$$\text{Anion Gap} = \text{Na}^+ - (\text{Cl}^- + \text{HCO}_3^-)$$
Normal AG = 8-12 mEq/L (some labs: 6-14)
Correct for hypoalbuminemia:
$$\text{Corrected AG} = \text{Measured AG} + 2.5 \times (4 - \text{albumin g/dL})$$
Mnemonic for HIGH Anion Gap: "MUDPILES" (classic & direct)
| Letter | Cause |
|---|
| M | Methanol |
| U | Uremia (renal failure) |
| D | DKA / AKA (Diabetic / Alcoholic Ketoacidosis) |
| P | Propylene glycol / Paracetamol (late) |
| I | Isoniazid / Iron |
| L | Lactic acidosis |
| E | Ethylene glycol |
| S | Salicylates |
Mnemonic for NORMAL AG (Hyperchloremic) Acidosis: "USED CARP"
| Letter | Cause |
|---|
| U | Ureteroenterostomy |
| S | Small bowel fistula / diarrhea |
| E | Extra chloride (saline excess) |
| D | Drugs (acetazolamide, cholestyramine) |
| C | Carbonic anhydrase inhibitor |
| A | Addison's / Adrenal insufficiency |
| R | Renal Tubular Acidosis (RTA 1, 2, 4) |
| P | Parenteral nutrition (TPN) |
Step 3 - Calculate Bicarbonate Deficit
Formula (Goldman-Cecil Medicine):
$$\text{HCO}_3^- \text{ Deficit (mEq)} = (25 - [\text{HCO}3^-]\text{measured}) \times \text{weight (kg)} \times 0.5$$
(Some sources use 0.4-0.5 for the distribution factor - also written as weight/2)
Example: 70 kg patient, HCO3 = 10 mEq/L
- Deficit = (25 - 10) x 70 x 0.5 = 15 x 35 = 525 mEq
Sodium Bicarbonate Ampoules Available in Pakistan
| Formulation | Na+ content | HCO3- content | Volume |
|---|
| 8.4% NaHCO3 (standard) | 1 mEq/mL | 1 mEq/mL | 50 mL = 50 mEq |
| 4.2% NaHCO3 (neonatal/dilute) | 0.5 mEq/mL | 0.5 mEq/mL | 20-50 mL |
| 7.5% NaHCO3 | 0.89 mEq/mL | 0.89 mEq/mL | 50 mL = 44.6 mEq |
In Pakistan: The most commonly available in hospitals (tertiary/emergency) is 8.4% NaHCO3 50 mL ampoules (50 mEq per vial). These are hypertonic and must always be diluted before use.
How to Prepare for Infusion:
- Calculate deficit (formula above)
- Give half the deficit in the first 4-8 hours (do NOT give full deficit at once - risk of overcorrection, paradoxical CSF acidosis, hypokalemia, hypocalcemia, hypernatremia)
- Dilute 8.4% NaHCO3 in D5W or sterile water (NOT in NS - risks hypernatremia)
- For IV infusion, dilute to ~1.4% (isotonic): Add 50 mEq (50 mL of 8.4%) to 300 mL D5W = ~350 mL of 1.4% NaHCO3
- Infuse at 50-100 mL/hr (adjust based on deficit and recheck ABG in 2-4 h)
When to Give Bicarbonate
| pH | Action |
|---|
| pH > 7.2 | Treat underlying cause only; bicarb not needed |
| pH 7.1-7.2 | Consider bicarb if cause is non-lactic (RTA, diarrhea, hyperchloremic) |
| pH < 7.1 | Bicarb indicated (especially in hyperchloremic, RTA, or renal failure) |
| Lactic acidosis | Bicarb generally NOT recommended - treat underlying cause |
| DKA | Bicarb only if pH < 6.9 and severe; may worsen outcome otherwise |
Pakistan Ward Tip: In DKA and lactic acidosis, do NOT routinely give bicarb. This is a common ward mistake. Focus on IV fluids, insulin (for DKA), and treating the cause.
Rate of Sodium Bicarbonate Infusion
- Never bolus full dose - give as slow infusion
- Standard rate: 1-2 mEq/kg over 1-4 hours, then reassess ABG
- Maintenance infusion: 0.5-1 mEq/kg/hr
- In cardiac arrest (PEA/asystole refractory): 1 mEq/kg IV bolus (8.4%, undiluted)
- Neonates: Use 4.2% only; bolus NaHCO3 in neonates can cause intraventricular hemorrhage
Anion Gap Algorithm (Visual)
Metabolic Acidosis (HCO3- < 22, pH < 7.35)
|
Calculate Anion Gap
Na - (Cl + HCO3)
|
+----+----+
| |
HIGH AG NORMAL AG
(>12) (8-12)
| |
"MUDPILES" "USED CARP"
Methanol RTA / Diarrhea
Uremia Saline excess
DKA/AKA Addison's
Propylene Ureteroentero-
glycol stomy
Isoniazid
Lactic acid
Ethylene glycol
Salicylates
|
Check Delta-Delta Ratio (AG acidosis)
Delta ratio = (AG - 12) / (24 - HCO3)
<0.4: Pure normal AG acidosis
0.4-1: Mixed AG + normal AG acidosis
1-2: Pure AG metabolic acidosis
>2: AG acidosis + concurrent metabolic ALKALOSIS
PART 3: POTASSIUM DISORDERS
Key Physiology
- Normal serum K+ = 3.5-5.0 mEq/L
- 98% of body K+ is intracellular (ICF K+ ~140 mEq/L vs ECF ~4 mEq/L)
- Every 0.1 unit drop in pH raises K+ by ~0.5 mEq/L (shift from ICF to ECF)
- Insulin, beta-2 agonists, alkalosis all drive K+ INTO cells (lower serum K+)
HYPOKALEMIA (K+ < 3.5 mEq/L)
Mnemonic for Causes: "DIGGER" (K+ gets dug out of the body)
| Letter | Cause |
|---|
| D | Diuretics (loop + thiazide - most common) |
| I | Insulin excess / Insulin drip |
| G | GI losses (vomiting, diarrhea, NG suction, fistulas) |
| G | Glucocorticoids / Mineralocorticoid excess (Conn's, Cushing's) |
| E | Excessive beta-2 agonists (salbutamol, terbutaline) |
| R | Renal tubular disorders (RTA type 1 & 2, Bartter's, Gitelman's) |
Total Body Potassium Deficit
The correlation between serum K+ and total body deficit is non-linear and imprecise (Harrison's 22E, Comprehensive Clinical Nephrology):
| Serum K+ | Estimated Total Body Deficit |
|---|
| 3.0-3.5 mEq/L | ~150-300 mEq |
| 2.5-3.0 mEq/L | ~300-600 mEq |
| 2.0-2.5 mEq/L | ~600-1000 mEq |
| < 2.0 mEq/L | >1000 mEq |
Practical Formula (rough estimate):
$$\text{K deficit (mEq)} \approx (4.0 - \text{measured K}^+) \times (0.4 \times \text{weight kg})$$
(Factor 0.4 = approximate distribution volume)
Treatment of Hypokalemia
Step 1: Always check and correct magnesium first!
- Hypomagnesemia causes refractory hypokalemia (Mg is needed for K+ retention by the kidney)
- Give Mg sulfate 2g IV if Mg <0.8 mmol/L
Step 2: Route of replacement
| Severity | Route | Dose |
|---|
| Mild (K+ 3.0-3.5) | Oral KCl | 40-80 mEq/day in divided doses |
| Moderate (K+ 2.5-3.0) | Oral or IV | 60-120 mEq/day |
| Severe (K+ <2.5 or symptomatic) | IV KCl | Per formula; central or peripheral with ECG monitoring |
Step 3: IV KCl Rates (CRITICAL - most common ward mistake)
| Route | Maximum Rate | Maximum Concentration |
|---|
| Peripheral IV | 10 mEq/hr | 40 mEq/250 mL = 0.16% |
| Central venous line | Up to 20 mEq/hr (with ECG monitoring) | 40 mEq/100 mL |
| Emergency only | 40 mEq/hr maximum in ICU with continuous ECG | Only via central line |
NEVER give IV potassium as a bolus - FATAL CARDIAC ARREST
KCl Formulations Available in Pakistan
| Formulation | Concentration | Use |
|---|
| KCl 7.45% (ampoule) | 20 mEq in 10 mL (2 mEq/mL) | Must dilute before IV use |
| Slow-K tablets | 8 mEq / tablet | Oral |
| Sando-K effervescent | 12 mEq K + 8 mEq HCO3 / tablet | Oral |
| Kaon oral syrup | 20 mEq/15 mL | Oral |
| KCl in IV bags | Pre-mixed 20 mEq in 500 mL or 1000 mL | Ready to use |
Preparation from 7.45% ampoule (20 mEq/10 mL):
- For peripheral infusion: Add 20-40 mEq KCl to 500 mL NS or D5W (makes 40-80 mEq/L)
- Infuse at 83-125 mL/hr (= 10 mEq/hr)
- Oral is always preferred over IV when GI tract is functioning
ECG Changes in Hypokalemia vs Hyperkalemia
| K+ Level | ECG Changes |
|---|
| Mild hypokalemia (3.0-3.5) | Flat T waves, prominent U waves |
| Severe hypokalemia (<2.5) | ST depression, inverted T, prominent U (looks like prolonged QT) |
| Mild hyperkalemia (5.5-6.5) | Peaked (tented) T waves |
| Moderate (6.5-7.5) | Prolonged PR, widened QRS |
| Severe (>7.5) | Sine wave pattern, P waves disappear |
| Critical (>8.0) | VF / cardiac arrest |
HYPERKALEMIA (K+ > 5.0 mEq/L)
Mnemonic for Causes: "ARDU" (Urdu for "hard" - hard problem to fix)
| Letter | Cause |
|---|
| A | ACE inhibitors / ARBs / Aldosterone antagonists (spironolactone) |
| R | Renal failure (acute or chronic - most common) |
| D | Drugs: NSAIDs, trimethoprim, heparin, beta-blockers, digoxin toxicity |
| U | Under-the-skin (tissue breakdown): Rhabdomyolysis, hemolysis, burns, tumor lysis |
Also: Addison's disease, acidosis (each 0.1 pH drop = +0.5 mEq/L K+), pseudohyperkalemia (fist clenching, hemolysis in tube)
Treatment of Hyperkalemia - The 4-Step "CDRS" Approach
Step 1: C - Cardiac Membrane Stabilization
- Calcium Gluconate 10%: 10-20 mL IV over 2-5 min (or Calcium Chloride 10 mL of 10% - 3x more elemental Ca)
- Onset: 1-3 minutes; Duration: 30-60 minutes
- REPEAT in 5 min if ECG not improved
- First given when ECG changes are present (peaked T, widened QRS)
Step 2: D - Drive K+ into cells (redistribute)
- Insulin (regular/soluble) 10 units IV + 50 mL of 50% Dextrose (to prevent hypoglycemia)
- Or 10 units insulin in 100 mL of 25% dextrose over 15-30 min
- Onset: 15-30 min; Lowers K+ by 0.5-1.5 mEq/L
- Salbutamol (albuterol) nebulization: 10-20 mg (high dose)
- Onset: 30 min; Lowers K+ by 0.5-1.0 mEq/L (works additively with insulin)
- Sodium bicarbonate (if metabolic acidosis): 1 mEq/kg IV; modest K+ reduction
Step 3: R - Remove K+ from body
- Furosemide 40-80 mg IV (if some renal function)
- Ion-exchange resins:
- Patiromer or Sodium Zirconium Cyclosilicate (SZC) - preferred (Harrison's 22E)
- Calcium Resonium (older SPS) - oral 15-30g, or 30-60g retention enema
- Dialysis - most effective; needed in ESRD or refractory cases
Step 4: S - Sustained treatment / prevent recurrence
- Stop offending drugs (ACEi, ARB, K+-sparing diuretics, NSAIDs)
- Low potassium diet (<2 g/day)
- Treat underlying cause
Potassium Replacement Rate Summary Table
| Severity | Serum K+ | Route | Rate | Monitoring |
|---|
| Mild | 3.0-3.5 | Oral | 40-60 mEq/day | Repeat K+ in 24h |
| Moderate | 2.5-3.0 | Oral/IV | 10 mEq/hr peripheral | Repeat K+ in 6-8h |
| Severe | <2.5 | IV (central preferred) | 10-20 mEq/hr | Continuous ECG |
| Life-threatening arrhythmia | Any | IV central | Up to 40 mEq/hr ICU | Continuous ECG mandatory |
PART 4: CLINICAL MISTAKES - WARD & ER
Most Common Dangerous Mistakes
| Mistake | Why It's Dangerous | Prevention |
|---|
| Correcting Na+ too fast in chronic hyponatremia | ODS (central pontine myelinolysis) - irreversible | Never >10-12 mEq/L/24h |
| Correcting Na+ too fast in hypernatremia | Cerebral edema, seizures, herniation | Never >10 mEq/L/24h |
| Giving NS to SIADH patient | Worsens hyponatremia (NS Na 154, urine may be 600+ - net free water retention) | Use hypertonic saline or water restriction |
| IV potassium bolus | Cardiac arrest (VF/VT) | Always dilute; max 10 mEq/hr peripheral |
| Not checking Mg before replacing K+ | Refractory hypokalemia continues | Always replace Mg first |
| Bicarb in lactic acidosis/DKA (mild-moderate) | Paradoxical CNS acidosis; worsens outcome | Only use if pH <6.9 (DKA) or <7.1 severe |
| Not rechecking electrolytes after bicarb correction | Overshoot alkalosis, hypocalcemia (tetany), hypokalemia (K+ shifts into cells) | Recheck ABG + electrolytes in 2-4h |
| Not diluting 8.4% NaHCO3 | Extravasation necrosis; hypertonicity | Always dilute to 1.4% for infusion |
| Hyperkalemia = stop and give only calcium | Calcium only stabilizes membrane; must also remove K+ | All 4 steps needed |
| Diagnosing hyponatremia without checking osmolality | Misses pseudohyponatremia and treats inappropriately | Always check serum osmolality first |
| Thiazide diuretics continuing in hyponatremia | Thiazides block diluting segment - STOP them | Stop thiazide immediately |
PART 5: EXAM HIGH-YIELD QUESTIONS
FCPS Part 2 / MRCP / USMLE - Most Common Exam Scenarios
Sodium
| Scenario | Answer |
|---|
| Patient with SIADH - what fluid? | Water restriction; if severe: hypertonic saline |
| Max rate of Na correction in chronic hyponatremia | 10 mEq/L in 24h; 8 mEq/L if high-risk |
| Complication of rapid correction of hyponatremia | Osmotic Demyelination Syndrome (ODS/CPM) |
| Best test to differentiate CSW vs SIADH | Fractional excretion of urate (high in CSW only) |
| Old man, thiazide, low Na - what happened? | Thiazide-induced hyponatremia - impaired diluting segment |
| Hypernatremia in unconscious patient - formula | Free water deficit = TBW x [(Na actual/140) - 1] |
| Correct Na for hyperglycemia | Add 1.6 mEq/L per 100 mg/dL glucose above normal |
Potassium
| Scenario | Answer |
|---|
| ECG shows peaked T + wide QRS - first drug? | Calcium gluconate IV (membrane stabilization) |
| Hypokalemia not correcting despite replacement | Check serum magnesium - hypomagnesemia |
| Patient on digoxin + hypokalemia - danger | Digoxin toxicity worsened (K+ competes with digoxin at Na/K ATPase) |
| Periodic paralysis + hypokalemia in Asian male | Thyrotoxic periodic paralysis - treat with propranolol + K+ |
| K+ 6.5 + no ECG changes - first step? | Check for pseudohyperkalemia first (repeat with non-hemolyzed sample) |
| Max rate of peripheral IV K+ | 10 mEq/hr in 40 mEq/250 mL concentration |
| What drug causes hyperkalemia + metabolic acidosis + low renin/aldosterone? | ACE inhibitor (type 4 RTA / hyporeninemic hypoaldosteronism) |
Bicarbonate / Acid-Base
| Scenario | Answer |
|---|
| Anion gap >20 in alcoholic - cause? | Alcoholic ketoacidosis (AKA) |
| DKA + treated with fluids - now hyperchloremic acidosis, why? | Saline (Cl- load) - normal AG acidosis now |
| Winter's formula: HCO3 is 12 - expected pCO2? | (1.5 x 12) + 8 ± 2 = 24-28; if actual pCO2 < 24 = additional respiratory alkalosis |
| Bicarb dose formula (exam standard) | (25 - measured HCO3) x weight x 0.5 |
| Salicylate poisoning - acid-base pattern? | Mixed: High-AG metabolic acidosis + respiratory alkalosis |
| RTA type 1 feature | Urine pH cannot fall below 5.5 even in acidosis; hypokalemia; nephrocalcinosis |
| RTA type 4 feature | Hyperkalemia + metabolic acidosis; low aldosterone or resistance; urine pH <5.5 |
| Most common cause of high AG acidosis in hospital | Lactic acidosis |
SUMMARY FORMULA CARD
Cut-Out Quick Reference
SODIUM:
Na Deficit (mEq) = (Na_desired - Na_measured) × TBW
Free Water Deficit = TBW × [(Na_actual/140) - 1]
TBW: male=0.6, female=0.5, child=0.7, neonate=0.8
Max correction: ≤10 mEq/L/24h; ≤18/48h
BICARBONATE:
HCO3 Deficit (mEq) = (25 - HCO3) × weight × 0.5
Give HALF the deficit in first 4-8h
8.4% NaHCO3 = 1 mEq/mL (50 mL ampoule = 50 mEq)
Dilute in D5W to 1.4% before infusion
ANION GAP:
AG = Na - (Cl + HCO3) Normal = 8-12
Correct for albumin: +2.5 per 1 g/dL below 4
High AG = MUDPILES
Normal AG = USED CARP
POTASSIUM:
K+ Deficit ≈ (4.0 - K+) × (0.4 × weight)
IV max rate (peripheral): 10 mEq/hr
IV max rate (central ICU): 20-40 mEq/hr
NEVER bolus IV K+
Treat hyperkalemia: Calcium → Insulin/Dextrose → Remove (furosemide/resin/dialysis)
Always replace Mg with K+
References: Harrison's Principles of Internal Medicine 22E (2025, McGraw Hill); Goldman-Cecil Medicine International Edition; Tintinalli's Emergency Medicine: A Comprehensive Study; Schwartz's Principles of Surgery 11E; Brenner and Rector's The Kidney; Comprehensive Clinical Nephrology 7E; Morgan & Mikhail's Clinical Anesthesiology 7E.
