When muscle is starving: the ST segment and T wave
The ST segment is the flat stretch right after the QRS, where the whole ventricle is momentarily depolarized together. In a healthy heart it sits level with the baseline. When muscle is short of blood — ischemia — that segment lifts or sags. ST elevation (the segment rising above baseline) typically signals a fully blocked artery starving the full thickness of the wall, while ST depression (the segment dipping below) often means a partial or demand-driven shortage. The T wave may also flip and point downward.
When muscle has died: the Q wave
If an artery stays blocked, muscle can die — a myocardial infarction. Hours to days later, the dead, electrically silent patch leaves a lasting mark: a deep, wide Q wave. Because that scar generates no current, the lead overlooking it effectively sees the heart's electricity heading away, producing a downward Q. A pathological Q wave is therefore a footprint of old, completed injury, while ST elevation marks injury happening now.
This separates two big diagnoses you will hear. A STEMI is a heart attack with ST elevation, the picture of a fully blocked artery that needs the blockage opened urgently. An NSTEMI is a heart attack without ST elevation — the muscle is being injured, shown by ST depression, T-wave changes and a rise in blood markers, but the ECG alone does not show the full-thickness elevation pattern. The tracing and the blood tests are read together.
Common rhythms at a glance
A few rhythms have signatures distinctive enough to recognize once you know the waves. Use these as a first impression, then confirm carefully — patterns can overlap and look-alikes exist.
Quick visual cues (always confirm, never diagnose from one feature):
Normal sinus P before every QRS, regular, 60-100 bpm.
Atrial Irregularly irregular rhythm, NO clear
fibrillation P waves; baseline wavers. QRS usually narrow.
Ventricular Wide QRS, fast, regular, looks 'broad and
tachycardia ugly'; P waves lost. A medical emergency.
Complete heart P waves and QRS march at their OWN rates,
block totally unrelated (P-P regular, R-R regular,
but no fixed PR). Often slow.Atrial fibrillation is the classic “irregularly irregular” rhythm with no organized P waves — the atria quiver instead of beating. Ventricular tachycardia is a fast run of wide, regular complexes and is an emergency because it can collapse the pump. Complete heart block is the giveaway where atria and ventricles each keep their own steady beat, ignoring each other, because the signal can no longer cross between them. Recognizing these at a glance is the payoff of everything in this track — but recognizing a pattern is the start of care, not a substitute for it.