Module 3: Understanding the ECG Waveform
1.Components of the ECG Waveform
The ECG waveform consists of several key components:
P wave: Atrial depolarization (when the atria contract).
QRS complex: Ventricular depolarization. Q wave (first negative deflection), R wave (positive deflection), S wave (negative deflection following R).
T wave: Ventricular repolarization (when the ventricles relax).
U wave: Often small, follows the T wave; thought to represent “late” repolarization of Purkinje fibers or papillary muscles. Not always seen.
2. Intervals and Segments
PR interval: From start of P wave to start of QRS complex. Includes atrial depolarization and conduction through the AV node.
PR segment: The flat line between end of P wave and start of QRS (the “delay” in AV node etc).
ST segment: From end of QRS (J‑point) to start of T wave. Normally isoelectric (flat), representing period between ventricular depolarization and repolarization.
QT interval: From beginning of QRS complex to end of T wave. It reflects total time for ventricular depolarization + repolarization.
3. Normal Values and Durations
| Component / Interval / Segment | Normal Duration / Amplitude / Comment |
|---|---|
| Heart Rate | 60–100 beats per minute (adult, resting) elentra.healthsci.queensu.ca+1 |
| P‑wave | Duration < 0.11‑0.12 seconds (≈ 80‑120 ms) NCBI+2Medscape+2 Amplitude < ~2‑2.5 mm in limb leads ecg.utah.edu+1 |
| PR interval | 0.12‑0.20 seconds (120‑200 ms) mdnxs.com+2Medscape+2 |
| QRS duration | ≤ 0.10‑0.12 seconds (100‑120 ms) normally NCBI+2elentra.healthsci.queensu.ca+2 |
| ST segment | Usually isoelectric (flat); small deviations ok but not large elevation/depression. Duration approx. ~0.08 seconds (80 ms) often cited. Medscape+2mdnxs.com+2 |
| T wave | Usually 160 ms in some sources; symmetry is asymmetrical rise & slower fall; matches direction of QRS in many leads. Medscape+1 |
| QT interval | Varies with heart rate; in males usually ≤ ~0.44‑0.45 sec; in females slightly higher. At heart rate ~60 bpm, QT ≤ ~420‑440 ms is often considered normal. Medscape+2The Cardiovascular+2 |
| U wave | Small, often < 1/4 amplitude of T‑wave; not always seen. Most visible in V3‑V4 leads. The Cardiovascular |
ECG Waveform များ၏ အစိတ်အပိုင်းများ
P wave: နှလုံး၏ နှလုံးအဖေါ် (atria) depolarization ဖြစ်ပြီး atria များတွင် စစ်စစ်အတွင်း လျှပ်စစ်လှုပ်ရှားမှု ဖြစ်သည်။
QRS complex: Ventricular depolarization ဖြစ်သည်။ Q wave (ပထမဆုံး အနက်ရောင်အတိုင်း), R wave (အပေါ်ကဖျား), S wave (R အပြီးတွင် အနက်ရောင်) များပါဝင်သည်။
T wave: Ventricular repolarization ဖြစ်ပြီး ventricles များ သက်သာလာချိန် လျှပ်စစ်ပြန်လည်ပြောင်းလဲမှုကို ဆိုလိုသည်။
U wave: မကြာခဏ မမြင်ရသောအချို့သော မိုးလေးသောလှုပ်ရှားမှုဖြစ်ပြီး Purkinje fiber များ သို့မဟုတ် papillary ကြက်သားများ၏ late repolarization ကို ကိုယ်စားပြုသည်။
၂။ Interval များနှင့် Segment များ
PR interval: P wave ရဲ့ စတင်မှုမှ башлап QRS complex စတင်မှုအထိ။ Atrial depolarization နှင့် AVနဲ့ conduction ကို ပါဝင်သည်။
PR segment: P wave ပြီးချိန်၊ QRS စတင်ရာမတိုင်ခင် flat line ဖြစ်သောအစိတ်အပိုင်း။ AV node မှာ နားရပ်ခြင်း အစရှိသောရပ်တန့်မှုများဖြစ်သည်။
ST segment: QRS အဆုံး (J‑point) မှ T wave စတင်မချင်းအထိ။ Usually လုံးဝတည်ငြိမ်သော (isoelectric) ဖြစ်သည်။ Ventricular depolarization နှင့် repolarization ကြားချိန်ကို ကိုယ်စားပြုသည်။
QT interval: QRS စတင်မှုမှ T wave အဆုံးအထိသောအချိန်။ Ventricular depolarization နှင့် repolarization အားလုံးကို တစ်ဆိုင်တည်းတိုင်းတာသည်။
Q: What is the significance of a P wave that is longer than 0.12 seconds?
A: It suggests atrial enlargement or slowed atrial conduction; may be seen in left atrial enlargement, atrial scar, or atrial dilation.
Q: Why must QT interval be corrected for heart rate (QTc)?
A: Because QT interval duration shortens at higher heart rates and lengthens at lower heart rates; comparing raw QT without adjustment may misidentify normal vs abnormal.
Q: What if the QRS duration is more than 0.12 seconds?
A: This suggests intraventricular conduction delay (bundle branch block) or ventricular origin of beat (e.g. wide complex tachycardia).
Q: Is the U wave always present in ECGs?
A: No. Many people don’t have a visible U wave; it becomes more visible at slower heart rates; if present, should be small.
Q: What is normal shape of T wave?
A: Rounded, asymmetrical (slower ascent, steeper descent), positive in most leads except some like aVR; inverted T waves may indicate pathology or be a normal variant.
Q: How to measure the ST segment properly?
A: From the J‑point (end of QRS) to the beginning of T wave; observe whether isoelectric baseline; look for any elevation or depression relative to baseline.
Q: What causes QT prolongation?
A: Causes include certain medications (antiarrhythmics, some antibiotics, psychiatric meds), electrolyte disturbances (hypokalemia, hypomagnesemia), congenital syndromes, bradycardia.
Q: Are there variations in normal ECG intervals between men and women?
A: Yes. Women tend to have slightly longer QT intervals on average; age also influences; so gender‑specific cutoff values are used for QTc.
Q: How does heart rate affect interval durations?
A: At higher heart rates, intervals (like QT) shorten; PR interval may shorten slightly; waveform morphology may change. At low heart rates, intervals lengthen.
Q: What is considered a dangerous QTc value?
A: Often > 450 ms in men or > 460 ms in women is considered prolonged; values above ~500 ms carry higher risk of torsades de pointes or arrhythmia.
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