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☕️ The Abnormal waves seen in the precordial leads of the ECG tracing represents which of the following mechanical events?

☕️ Morning Quiz 114

Author

Dr. Shoaib Ahmad Baig
April 13, 2025

Morning Quiz ☕️ 

Patient: 34-year-old male
Chief Complaint: Routine health check-up

Relevant History:

  • 🏃‍♀️ Training for marathon, hydrates exclusively with electrolyte solutions

  • 💊 Takes "electrolyte supplements" from another runner

👨‍👩‍👧 Family History: Hypertension (both parents), father died of MI

Vitals:

  • HR: 55 bpm

  • BP: 120/88 mmHg

  • Temp: 37.6°C (99.6°F)

  • RR: 16/min

Physical Exam: Normal
ECG Findings: Abnormal waves in precordial leads

The abnormal waves seen in the precordial leads of the ECG tracing represents which of the following mechanical events?

A. Ventricular depolarization

B. Period between ventricular depolarization and repolarization

C. Ventricular repolarization

D. Atrial depolarization

E. Atrial repolarization

Correct Answer 🎯: C. Ventricular repolarization

Precordial Leads (V1-V6) detects electrical activity from the ventricles.

⚙️

👇 Marathon training

⬇

↑ Physical exertion → ↑ Sweating

⬇

↑ Fluid loss → ↑ Electrolyte loss (Na+, K+, Mg2+)

⬇

Excessive Electrolyte Intake (Especially Potassium)
⬇
↑ Serum Potassium (Hyperkalemia)
⬇
Altered Cardiac Myocyte Excitability

⬇

K⁺ gradient across membrane is disrupted

(↑ K⁺ → ↓ K⁺ gradient across membranes → Faster phase 3 repolarization → Tall, symmetric T waves )
⬇
ECG Changes:

Peaked T Waves (Earliest Sign)

⛰️ "Towering T waves"

 Due to accelerated repolarization from ↑ extracellular K⁺

(Think: "T for Tall & Toxic K⁺")

⬇

Ventricular Repolarization Abnormality (Correct Answer: C)

(Like a sudden voltage surge at the end of a heartbeat)

PR prolongation, QRS widening (if severe)

Pro - tip

❌ When it comes to treatment of Hyperkalemia,  First see the ECG .

If -

ECG has no changes, then insulin/glucose or Kayexalate is the first step—but if ECG changes are present, calcium comes first!

Because

Peaked T waves → widened QRS →Sine wave → risk of V-fib/asystole.

If You Give Insulin/Glucose First → Insulin drives K⁺ into cells → but takes 15–30 minutes to work→ still electrically unstable → high risk of deadly arrhythmias


💡 USMLE loves to test if you’ll give calcium before other treatments when ECG is abnormal.

Follow this Visual chart for such questions-

“ So, Calcium is given first for ECG changes, then it should lower K⁺. Right ? “

Nope! 

It’s a cardiac protector.

A "defibrillator for the cell membrane"—it doesn’t fix the high potassium, but it buys time to treat it.

IV Calcium (Ca²⁺)

(works in 1-3 minutes)

⬇

Binds sodium channels → ↑ threshold potential (-60 mV)

⬇

RMP (-70mV) vs threshold (-60 mV) gap restored

⬇

Cardiac membrane stabilized → ECG improves

"Which calcium form is correct then?" 

(Was Asked in one of the Step 2 CK forums as well)

→ Both gluconate & chloride work, but gluconate is more common (chloride is stronger but riskier for tissue damage)

Differential Table ⚖️

Options Autopsy 🔬

Practice Questions ⏳️ 

Your Brother in This Struggle

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Dr. Shoaib Ahmad