Anat and Physio 2: ECG

What is ECG

What is an ECG (electrocardiogram)

A graphic recording of the heart’s electrical activity during the cardiac cycle.

• Electrodes detect electrical impulses and display them as waveforms.

• ECGs assess heart rate, rhythm, conduction, and abnormalities but do not directly measure mechanical pumping.

What is a 12-lead ECG? → Records the heart’s electrical activity from 12 different views (6 frontal and 6 transverse/chest to help identify rhythm abnormalities and the location and extent of myocardial damage

5 properties of myocardial cells

• Excitability (electrical): Responds to electrical stimulation.

• Automaticity (electrical): Spontaneously generates electrical impulses.

• Conductivity (electrical): Transmits impulses between cells.

• Contractility (mechanical): Contracts in response to electrical stimulation.

• Refractoriness (electrical): Period when a cell cannot respond normally to another stimulus; protects against sustained contraction.

Intrinsic Conduction Pathway

SA node → AV node → Bundle of His → Right and left bundle branches →Purkinje fibers.

1. SA node: (normal pacemaker) In the R. Atrium, initiates the heartbeat and sets the pace (60-100 BPM)

2. AV node: Between atria and ventricles, delays impulse (0.1 sec) for ventricular filling (40-60 BPM)

3. Bundle of HIS (AV Bundle): carries AV node impulses into the interventricular septum

4. R and L Bundle Branches: Carry impulses toward each ventricle

5. Purkinje fibers: Rapidly spread impulses through ventricles; 15–40 bpm

Automaticity, Escape vs. ectopic pacemaker

An escape pacemaker takes over and escapes sinus domination from SA node when the normal pacemaker fails; an ectopic pacemakeris an abnormal impulse-generating site.

Mechanics of the ECG

Electrode Placement Frontal Leads

a) Standard limb leads: Leads I, II, and III are bipolar leads that measure voltage between a negative and positive electrode.

• Lead I: Right arm (−) → Left arm (+).

• Lead II: Right arm (−) → Left leg (+).

• Lead III: Left arm (−) → Left leg (+).

• Einthoven’s triangle: The three limb leads form a triangle around the heart. Einthoven’s law: Lead II = Lead I +Lead III.

b) 12-lead ECG: Uses 10 electrodes to create 12 electrical views: I, II, III, aVR, aVL, aVF, and V1–V6.

Chest lead placement:

• V1: 4th intercostal space, right sternal border.

• V2: 4th intercostal space, left sternal border.

• V3: Halfway between V2 and V4.

• V4: 5th intercostal space, left midclavicular line.

• V5: Left anterior axillary line, level with V4.

• V6: Left midaxillary line, level with V4 and V5.

ECG graph paper:

a) Paper

• Horizontal axis = time; vertical axis = voltage.

• Small box: 0.04 sec horizontally; 0.1 mV vertically.

• Large box (5 small boxes): 0.20 sec horizontally; 0.5 mV vertically.

• 5 large boxes = 1 second; 25 small boxes = 1 second.

b) Heart-rate calculation

• 6-second method: QRS count × 10; useful for irregular rhythms.

• 3-second method: QRS count × 20.

• Large-box method: 300 ÷ large boxes between R waves.

• Small-box method: 1500 ÷ small boxes between R waves.

ECG Waves, Intervals, Segments A

P wave	Atrial depolarization	<0.12 sec
PR interval	Atrial-to-ventricular conduction time	0.12–0.20 sec
QRS complex	Ventricular depolarization; atrial repolarization is hidden	0.04–0.10 sec*
ST segment	Ventricles are depolarized; normally near baseline	Variable
T wave	Ventricular repolarization	Variable
QT interval	Total ventricular depolarization and repolarization	0.36-0.44 Sec

Systole vs. Diastole

Systole: Ventricular contraction; follows ventricular depolarization.

Diastole: Ventricular relaxation and filling

ECG Interpretation

1. Rhythm: Compare R–R intervals. Equal intervals = regular; varying intervals = irregular.

2. Heart rate: Normal adult rate is 60–100 bpm. Use the 6-second method for irregular rhythms or the 300/1500 method for regular rhythms.

3. P:QRS ratio: Check whether P waves and QRS complexes have a consistent relationship. Normal = 1:1.

4. PR interval: Measure from the beginning of P to the beginning of QRS. Normal = 0.12–0.20 sec.

5. QRS interval: Measure the duration of ventricular depolarization. Your manual lists 0.04–0.10 sec.

Common ECG Abnormalities and Clinical Conditions

1. Heart (AV) blocks

Type	ECG finding	Key point
First-degree	PR > 0.20 sec; every P followed by QRS	Delayed conduction
Second-degree Type I (Mobitz I)	PR progressively lengthens, then QRS drops	Progressive delay
Second-degree Type II (Mobitz II)	Intermittent dropped QRS without progressive PR lengthening	May progress to complete block
Third-degree	P waves and QRS complexes are independent	Complete AV dissociation; may require pacing

2. AFib

• Cause: Chaotic atrial electrical activity from multiple impulses.

• ECG: No distinct, consistent P waves; irregularly irregular R–R intervals.

• Effect: Atria quiver instead of contracting effectively.

• Main risk: Blood clots and stroke due to blood pooling in the atria.

3. ST- segment changes and myocardial infarcation

• Normal ST segment: Near the isoelectric baseline.

• ST elevation: May indicate acute myocardial injury; in the appropriate setting, can indicate STEMI.

• ST depression: May indicate myocardial ischemia (reduced blood flow).

• Myocardial ischemia: Inadequate blood and oxygen supply to the heart muscle.

• Myocardial infarction (MI): Death of heart muscle tissue due to prolonged ischemia.

4. VFib

• Cause: Chaotic ventricular electrical impulses.

• ECG: Disorganized waveform with no identifiable, organized P waves or QRS complexes.

• Effect: Ventricles quiver instead of pumping, producing essentially no cardiac output.

• Treatment: Immediate CPR and defibrillation.