| Electrolyte | Level | Replacement | Expected Rise | Recheck | Pearls |
|---|
Potassium
(Goal: 4.0โ5.0)
(ICU goal: 4.0โ4.5) |
3.5โ3.9 | KCl 40 mEq PO ร 1 | ~0.3 mEq/L per 10 mEq | Next AM BMP |
Always replete Mg first -hypoMg causes renal Kโบ wasting via the ROMK channel. Mgยฒโบ normally inhibits ROMK in the collecting duct. When Mgยฒโบ is low โ ROMK becomes uninhibited โ Kโบ pours into the urine โ refractory hypokalemia no matter how much Kโบ you give. Fix the Mgยฒโบ first, or the Kโบ won't stay. Max IV rate: 10 mEq/hr peripheral, 20 mEq/hr central. PO preferred if tolerating. Recheck Kโบ 2โ4h after IV repletion. |
| 3.0โ3.4 | KCl 40 mEq PO ร 2 doses (1h apart) or KCl 20 mEq IV ร 2 | ~0.6โ1.0 mEq/L | 2โ4h post-IV |
| < 3.0 | KCl 40 mEq IV ร 2โ3 (with continuous telemetry). PO supplement simultaneously. | Variable -recheck frequently | Q2h until > 3.0 |
Magnesium
(Goal: โฅ 2.0)
(ICU goal: โฅ 2.0) |
1.5โ1.9 | MgOxide 400 mg PO BID or MgSOโ 2g IV ร 1 | ~0.1โ0.2 per 1g IV | Next AM |
Diarrhea is the dose-limiting PO side effect. IV preferred in critically ill. 1g IV MgSOโ โ 8 mEq Mg. Renal excretion -use caution in CKD. For torsades โ 2g IV push.
Why Mg matters for Kโบ (ROMK channel): Mgยฒโบ normally blocks the ROMK channel in the collecting duct, preventing Kโบ secretion. Low Mgยฒโบ โ ROMK uninhibited โ kidney wastes Kโบ โ refractory hypokalemia. Also: low Mgยฒโบ causes PTH resistance โ refractory hypocalcemia. Always check Mgยฒโบ when Kโบ or Caยฒโบ won't correct. |
| < 1.5 | MgSOโ 4g IV over 4h | ~0.3โ0.5 | 2โ4h post |
Phosphorus
(Goal: 2.5โ4.5) |
1.5โ2.4 | NeutraPhos 2 packets PO (32 mmol) or Na/K-Phos 15 mmol IV over 2h | ~0.5โ1.0 mg/dL | Next AM |
IV repletion in CKD โ risk of hypocalcemia (CaPOโ precipitation). Check Caยฒโบ concurrently. Oral preferred unless severe or NPO. K-Phos contains potassium -check Kโบ first. |
| < 1.5 | Na/K-Phos 30 mmol IV over 4โ6h | ~1.0โ1.5 mg/dL | 2โ4h post |
Calcium
(Ionized goal: 1.1โ1.3) |
Mild (iCa 0.9โ1.1) | CaCOโ 1250 mg PO TID (with meals) + vitamin D | Gradual | Next AM |
Always check ionized Ca (not total -albumin confounds). Correct Mg first (hypoMg causes PTH resistance). IV CaClโ = 3ร more elemental Ca than Ca gluconate but vesicant (central line only). Avoid IV Ca if hyperphosphatemic (CaPOโ precipitation โ calciphylaxis). |
| Severe (iCa < 0.9, symptomatic) | Ca gluconate 2g IV over 20 min (peripheral OK) or CaClโ 1g IV (central only) | Transient | Q2h + telemetry |
๐ The Mgยฒโบ โ Kโบ โ Caยฒโบ Connection (ROMK Channel)
Low Mgยฒโบ causes THREE problems:
1. Refractory hypokalemia -Mgยฒโบ normally blocks the ROMK channel (Renal Outer Medullary Kโบ channel) in the collecting duct. Without Mgยฒโบ โ ROMK is wide open โ kidney wastes Kโบ into urine โ no amount of Kโบ repletion will stick until Mgยฒโบ is corrected.
2. Refractory hypocalcemia -Mgยฒโบ is required for PTH secretion and end-organ response. Low Mgยฒโบ โ PTH resistance โ Caยฒโบ won't correct.
3. Cardiac arrhythmias -Mgยฒโบ stabilizes cardiac membranes. Low Mgยฒโบ โ prolonged QT โ Torsades de Pointes, refractory Afib/RVR, digoxin toxicity.
Clinical rule: When Kโบ or Caยฒโบ won't correct despite adequate repletion โ check and replete Mgยฒโบ first.