20. Electrolyte Derangements, Acid-Base Disorders, and Toxicities

Toxicological ingestions and profound acid-base derangements represent reversible electrical crises where surface electrocardiography directs immediate antidotal therapy.

1. Sodium Channel Blocker Toxicity (Tricyclic Antidepressants, Flecainide, Cocaine)

Electrocardiographic Triad of Sodium Channel Blocker Toxicity:

Terminal R Wave in Lead aVR $> 3.0\text{ mm}$ (or $R/S \text{ ratio in aVR} > 0.7$).
QRS Duration Prolongation:
- $\text{QRS} > 100\text{ ms} \implies 34\%$ risk of generalized seizures.
- $\text{QRS} > 160\text{ ms} \implies 50\%$ risk of life-threatening ventricular arrhythmias (VT/VF).
Right Axis Deviation of the Terminal 40 ms of the QRS complex.

Antidotal Therapy: Immediate IV Sodium Bicarbonate ($8.4\%$): $1-2\text{ mEq/kg}$ rapid IV push repeated until QRS narrows and pH reaches $7.45-7.55$, followed by continuous infusion ($150\text{ mEq}$ in $1\text{ L } D5W$).

2. Digitalis Effect vs Digitalis Toxicity

Digoxin Effect (Therapeutic): "Scooped" sagging ST depression (Salvador Dali's mustache pattern) in leads with tall R waves; shortened $QTc$; prominent U waves.
Digoxin Toxicity (Pathological): Intracellular calcium overload generates delayed afterdepolarizations $\to$ Ventricular Bigeminy, Bidirectional VT, Junctional Escape Tachycardia, and complete AV block.
Specific Antidote: Digoxin Immune Fab (DigiFab) calculated based on serum digoxin level or number of ingested tablets.

3. Calcium & Magnesium Abnormalities

Severe Hypercalcemia: Shortened $QTc$ with absent ST segment, abrupt T-wave takeoff, hypertension $\to$ treated with IV Normal Saline hydration + IV Bisphosphonates (Zoledronic acid) + Calcitonin.
Severe Hypocalcemia: Prolonged isoelectric ST segment $\to$ IV Calcium Gluconate $10\%$.