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Hypokalaemia: Causes, Deficit Estimation, and Treatment

Definition

• Mild: 3.0-3.4 mmol/L
• Moderate: 2.5-2.9 mmol/L
• Severe: < 2.5 mmol/L

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Causes

1. Intracellular Shift (K+ moves into cells)

2. Reduced Intake

• Rare alone; usually prolonged starvation ("tea and toast" diet)

3. Gastrointestinal Loss

• Vomiting - K+ lost in urine (secondary hyperaldosteronism from volume depletion) more than in vomitus
• Diarrhoea / laxative abuse - direct stool loss
• Intestinal drainage, bowel prep (colonoscopy prep)

4. Renal Loss (most common overall)

• Loop diuretics, thiazide diuretics (most common cause overall)
• Osmotic diuresis (hyperglycaemia, mannitol)
• Bartter syndrome, Gitelman syndrome
• Distal or proximal renal tubular acidosis
• Hypomagnesaemia (cisplatin, alcohol, diuretics)
• Surreptitious diuretic use

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Potassium Deficit Estimation

For every 1 mEq/L fall in serum K+, the total body deficit is approximately 200-400 mEq

• The formula may overestimate deficit in transcellular shift states (e.g. hypokalemic periodic paralysis, insulin overdose) - do NOT overcorrect these
• The formula may underestimate true deficit in chronic depletion
• Concurrent acid-base status alters the relationship: alkalosis drives K+ into cells, worsening apparent (but not necessarily actual) hypokalemia

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Treatment

Step 1: Treat the underlying cause

Step 2: Choose the potassium salt

• KCl (potassium chloride) - preferred; also corrects co-existing metabolic alkalosis (most common scenario)
• Potassium citrate - use in acidotic patients (e.g. renal tubular acidosis)
• Potassium phosphate - use when co-existing phosphate deficit
• Avoid potassium gluconate (non-reabsorbable anion - does not adequately replete the deficit)

Step 3: Choose the route

• Used when K+ > 3.0 mmol/L, patient is not vomiting, and haemodynamically stable
• Usual dose: 40-100 mmol/day in divided doses (max 40 mmol per single dose)
• Total daily dose should not exceed 200 mmol/24 hours (FDA)
• If K+ 3.0-3.4 mmol/L: Sando-K 2 tablets TDS = 72 mmol K/day
• Dietary potassium increase can be considered if K+ > 3.0 mmol/L

• Patient cannot take oral/enteral route
• Symptomatic hypokalaemia (paralysis, cardiac arrhythmias)
• Serum K+ < 3.0 mmol/L
• On IV aminophylline, DKA protocol

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How Much KCl Can Be Given IV?

Standard / Peripheral IV rates:

Key safety limits:

• If rate > 10 mmol/hr or K+ < 3.0 mmol/L, patient must be in a monitored setting
• Concentrations > 60 mmol/L must only be given via a central line
• Recheck serum K+ after each replacement course before giving further doses
• Do not use glucose-containing solutions for initial correction - insulin release will drive K+ further into cells

Total daily IV limits:

• Adults: typically 100-200 mmol/day IV
• Higher amounts may be needed in severe/ongoing losses but require specialist (nephrology/ICU) oversight and pharmacy input

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Monitoring During Treatment

• Serial serum K+ measurements (1-4 hourly in severe cases)
• ECG monitoring - watch for: U waves, T-wave flattening, ST depression, prolonged QU interval
• Urine output (renal function must be confirmed before giving K+)
• Serum Mg²+ - hypomagnesaemia perpetuates hypokalaemia and must be corrected simultaneously; K+ replacement will fail if Mg²+ is not replaced
• Avoid glucose-containing solutions

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Special Situations

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Critical Clinical Scenario: K+ 1.2 mEq/L + No Respiratory Effort

This is a medical emergency. K+ of 1.2 mEq/L with absent respiratory effort = hypokalaemic respiratory paralysis with respiratory arrest. Resuscitation and potassium replacement must happen simultaneously.

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Immediate Assessment (First 60 Seconds)

Why no respiratory effort?

• No diarrhoea / no vomiting = GI loss excluded
• The cause is likely: renal loss, transcellular shift, or a disorder not yet identified

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Step-by-Step Emergency Management

STEP 1 - Secure the Airway IMMEDIATELY

Call resuscitation team / code blue

• Bag-mask ventilate the patient right now
• Prepare for emergency endotracheal intubation (RSI)
• Connect to mechanical ventilation - the respiratory muscles will not recover until K+ is substantially corrected (this may take hours)
• Do NOT wait for labs before ventilating - airway first

STEP 2 - Monitoring (simultaneous with airway)

STEP 3 - Urgent Bloods

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STEP 4 - Estimate the Potassium Deficit

For every 1 mEq/L fall in serum K+, body deficit = 200-400 mEq

Estimated deficit = 2.8 × 200 to 2.8 × 400 = 560 to 1120 mEq total

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STEP 5 - IV Potassium Replacement Protocol

• Use 0.9% NaCl as the vehicle - NEVER glucose (glucose triggers insulin release, which drives K+ into cells and worsens hypokalaemia)
• Concentration > 60 mmol/L = central line only
• Peripheral veins can safely receive up to 40 mmol/L at max 10 mmol/hr

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STEP 6 - Correct Magnesium Simultaneously

~50% of patients with severe hypokalaemia have hypomagnesaemia. Potassium replacement is refractory until Mg²+ is replete. Magnesium is an essential cofactor for K+ uptake into cells.

• Magnesium sulphate 2g (8 mmol) IV over 15-30 mins, then consider infusion if Mg²+ is low
• Even if you don't have the Mg²+ result yet, empiric Mg²+ replacement is reasonable in this setting
• Also reduces arrhythmia risk independently

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STEP 7 - ECG Monitoring and Arrhythmia Management

• MgSO4 2g IV bolus immediately
• Defibrillate if pulseless VF
• Overdrive pacing if recurrent torsades
• Continue aggressive K+ replacement
• Avoid drugs that prolong QT (class IA, IC, III antiarrhythmics)

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STEP 8 - Find the Underlying Cause

No GI cause → Think:
├── Renal K+ wasting (most common)
│   ├── Diuretics (loop/thiazide) — check medication list
│   ├── Primary hyperaldosteronism
│   ├── Bartter / Gitelman syndrome
│   ├── RTA type 1 or 2
│   └── Hypomagnesaemia-induced renal wasting
├── Transcellular shift (no true deficit)
│   ├── Insulin excess / glucose load
│   ├── Beta-2 agonist (salbutamol, bronchodilators)
│   ├── Thyrotoxic hypokalemic paralysis
│   ├── Familial hypokalemic periodic paralysis
│   └── Barium / hydroxychloroquine toxicity
├── Excess mineralocorticoid activity
│   ├── Cushing syndrome / exogenous steroids
│   └── Liquorice / carbenoxolone / chewing tobacco
└── Rare: prolonged starvation + catabolism

• Urine K+ < 20 mEq/L (or TTKG < 2) = extrarenal loss or transcellular shift
• Urine K+ > 40 mEq/L (or TTKG > 4) = renal K+ wasting (diuretics, hyperaldosteronism, Bartter, RTA)

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STEP 9 - ICU Care and Ongoing Monitoring

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Summary Card

K+ 1.2 mEq/L + No respiratory effort

1. AIRWAY → Bag-mask → RSI → Mechanically ventilate
2. ACCESS → 2x IV (large bore) + Central venous catheter
3. MONITOR → Continuous ECG + cardiac telemetry
4. BLOODS → K+, Mg²+, phosphate, Ca²+, ABG, renal function, urine K+
5. REPLACE → KCl 20-40 mmol/hr via central line in 0.9% NaCl
6. Mg²+ → MgSO4 2g IV empirically → prevents refractory hypoK
7. AVOID → Glucose solutions, drugs prolonging QT
8. RECHECK → K+ every 1-2 hours, adjust infusion
9. CAUSE → Find it (urine K+, aldosterone/renin, TFTs, medication review)
10. WEAN VENT → Only after K+ > 3.0 and neuromuscular function recovering

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Correction Protocol: K+ 1.2 mEq/L | Central Line + Ryle's Tube | Ca²+/Mg²+ Normal

Recap of the Deficit

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Phase 1 - Emergency IV Correction via Central Line (Hours 0-2)

IV KCl infusion via central line:

40 mmol/hr is the absolute maximum rate ever used - only justified here because K+ = 1.2 with respiratory arrest. This rate requires ICU, continuous cardiac monitoring, and nurse 1:1.

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Phase 2 - Dual Route: IV + Enteral via Ryle's Tube (Hours 2-12)

IV (central line) - reduced rate:

Enteral via Ryle's tube (simultaneous):

Oral/enteral potassium has a safety advantage: the GI mucosa absorbs it more slowly than IV, buffering against sudden surges. Using both routes gives faster total repletion without exceeding safe IV rates.

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Phase 3 - Maintenance Correction (Hours 12-48)

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Hour-by-Hour Summary Table

Expected total K+ correction time: 24-48 hours to reach and sustain 4.0 mEq/L. Replenishing intracellular stores takes days even after serum K+ normalises.

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Critical Rules for This Patient

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When to Recheck and When to Stop IV K+

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Quick Summary

K+ 1.2 | Central line + Ryle's tube | Ca, Mg normal

PHASE 1 (0-2h):  IV 40 mmol/hr via central (KCl in 0.9% NaCl)
                 Recheck K+ at 1h and 2h

PHASE 2 (2-12h): IV 20 mmol/hr via central
               + NG 40 mmol q6h via Ryle's tube
                 Recheck K+ every 2-4 hrs

PHASE 3 (12-48h): IV 10 mmol/hr (step down)
                + NG 40-60 mmol q6h
                  Target K+ = 4.0 mEq/L

TOTAL estimated: 600-1000 mmol over 24-48 hrs
MAX safe IV rate: 40 mmol/hr (central only, continuous ECG)
VEHICLE: 0.9% NaCl ALWAYS — never glucose