09/11/2025
✅ Basic ECG – Easy Explanation
1. What the ECG waves mean
• P wave → Atria contract (atria depolarize)
• QRS complex → Ventricles contract (ventricles depolarize)
• T wave → Ventricles relax (ventricles repolarize)
2. ECG paper basics
• 1 small box = 0.04 sec
• 1 large box (5 small boxes) = 0.20 sec
• Up–down height measures voltage (1 mm = 0.1 mV)
✅ 3-Lead ECG Placement
• White → Right arm/chest
• Black → Left arm/chest
• Red → Left leg/thigh
• MCL1 → Modified chest lead placed near V1 position (helps monitor arrhythmias)
✅ 12-Lead Chest Electrode Placement
These are the V1–V6 chest leads:
• V1: 4th intercostal space, right of sternum
• V2: 4th intercostal space, left of sternum
• V3: Between V2 and V4
• V4: 5th intercostal space, mid-clavicular line
• V5: Same level as V4, anterior axillary line
• V6: Same level as V4 & V5, mid-axillary line
These leads help check different parts of the heart (septal, anterior, lateral).
✅ Heart’s Electrical Pathway (How electricity flows)
1. SA node (natural pacemaker)
2. Atria contract → P wave
3. AV node (slight delay)
4. Bundle of His → bundle branches → Purkinje fibers
5. Ventricles contract → QRS
6. Ventricles relax → T wave
✅ Intervals You Should Know
• PR interval → Time from atrial start to ventricular start
• QRS duration → Time ventricles take to contract
• ST segment → Ventricles just finished contracting
• QT interval → Entire process of ventricle contraction & relaxation
✅ Quick Memory Tips
• P = Push (atria push blood down)
• QRS = Squeeze (ventricles pump out blood)
• T = Tired (ventricles relaxing)
• White on right, smoke (black) over fire (red) for lead placement