Situation: A 34-year-old man who works outdoors comes to the emergency room with sudden, severe right flank pain spreading to the groin that began 2 hours ago. He is pacing, breathing rapidly at 32/min, and reports tingling of his fingers and around his lips. Temperature is 37.0 °C, blood pressure 138/86 mmHg, and pulse 104/min. His urinalysis shows red blood cells with negative nitrite and leukocyte esterase, and his creatinine is 0.9 mg/dL (80 µmol/L). A computed tomography (CT) scan shows a 4-mm stone in the lower ureter. Which explanation accounts for the tingling in his fingers and around his lips?

Alkalosis makes more calcium bind to albumin, lowering the ionized (active) calcium in the blood. Low ionized calcium increases nerve and muscle excitability, causing numbness and tingling of the fingers and around the mouth, muscle cramps, and, if severe, tetany with positive Chvostek and Trousseau signs.

Pathophysiology of perioral and digital tingling

The patient’s rapid breathing at 32/min in the setting of acute pain and anxiety produces a primary respiratory alkalosis. As carbon dioxide is blown off, the blood pH rises. This alkaline environment shifts the equilibrium of calcium binding: more calcium becomes attached to albumin, so the fraction of free, ionized calcium in the plasma falls even though total serum calcium remains unchanged.

Ionized calcium stabilizes voltage-gated sodium channels on nerve and muscle membranes; when its concentration drops, the threshold for depolarization decreases and spontaneous action potentials fire more easily. This increased neuromuscular excitability manifests first in the most sensitive areas—the fingers and the circumoral region—as tingling and numbness. If the alkalosis worsens, the same mechanism progresses to carpopedal spasm and overt tetany with positive Chvostek and Trousseau signs.

Key point! The tingling is not caused by hypoxemia, potassium shifts, or vasodilation. It is the direct consequence of hypocalcemia induced by alkalosis—a functional, not absolute, calcium deficit.

Why the other options do not fit

Option	Proposed mechanism	Why it is incorrect
1. Potassium moves out of cells into blood	Hyperkalemia from cellular shift	Respiratory alkalosis actually drives potassium into cells, lowering serum potassium; hyperkalemia causes weakness, not perioral tingling
2. Retained carbon dioxide dilates blood vessels	Hypercapnia with vasodilation	This patient is hyperventilating and blowing off CO2, not retaining it; vasodilation does not produce circumoral paresthesia
3. Low blood oxygen irritates peripheral nerves	Hypoxemic nerve irritation	Hyperventilation in a young healthy person maintains normal or elevated oxygen saturation; hypoxemia causes confusion and cyanosis, not selective perioral tingling
4. High blood pH lowers ionized calcium	Alkalosis-induced hypocalcemia	Correct—explains the classic distribution of tingling (fingers, around lips) and the risk of tetany

Clinical correlation with the renal stone presentation

The underlying problem is a 4-mm lower ureteral stone causing severe flank-to-groin pain. Pain is a powerful driver of hyperventilation. The urinalysis shows red blood cells with negative nitrite and leukocyte esterase, consistent with an uncomplicated stone rather than infection. Creatinine of 0.9 mg/dL (80 µmol/L) indicates preserved renal function.

In the emergency setting, recognizing that the tingling is a benign consequence of pain-driven hyperventilation—not a neurologic emergency—allows the nurse to focus on analgesia and coached breathing rather than unnecessary neurologic workup. Slowing the respiratory rate or breathing into a rebreather mask (when hypoxemia is excluded) restores carbon dioxide, normalizes pH, and rapidly reverses the paresthesia.

Watch out! Chvostek and Trousseau signs are elicited to confirm latent tetany when ionized calcium is low. In this scenario, they would be expected to be positive if tested, reinforcing the diagnosis of alkalosis-induced neuromuscular irritability rather than a primary parathyroid or electrolyte disorder.