Mosby's ECG: Interpreting the 12-Lead ECG

Rhythm

Measuring atrial and ventricular rhythms

- Regular?

- Irregular?

If variations are less than 0.04 seconds (1 small box), consider the rhythm regular.

If the difference between the longest and shortest P-P intervals is greater than 0.12 seconds (3 small boxes), consider sinus arrhythmia.

Rate

Measuring atrial and ventricular rates

- Tachy?

- Brady?

Count 6 second tracing and multiply by 10.

Count large or small boxes.

Use the sequencing method.

P Waves

Present?

- Location: should precede every QRS; if after QRS, consider junctional or ventricular origin

- Amplitude: should be no more than 2.5 mm high

- Duration: should be no more than 0.10 seconds

- Configuration: should be smooth, rounded, uniform

- Positive in lead II

Absent? Probably afib.

PR Intervals

Duration: should be 0.12 - 0.20 seconds in adults

- if longer, 3rd degree AV block

- if shorter, ectopic origin or accessory pathway

Constant

- if variable, consider AV block

QRS Complexes

Duration: should be 0.06 - 0.10 seconds

- if longer, aberrant conduction, block, or ventricular

Configuration

- leads I, II, and III should be positive

Q-wave duration should be less than 0.04 seconds.

Q-wave amplitude should be less than 25% of R wave.

ST Segments

Amplitude: 0.04 seconds, less than 1 mm

T Waves

Location: follow ST segment

Amplitude: should be 5 mm or less

Configuration: rounded, smooth, asymmetrical, upright in leads I, II, V2-V6

- if tented or peaked, consider myocardial injury

- if symmetrical, consider ischemia, electrolyte imbalance, nervous system disorder

- if inverted, consider myocardial ischemia

- if notched or pointed, consider pericarditis

QT Intervals

Duration: varies; usually 0.36 - 0.40 seconds

U Wave

Not always there.

If there, consider electrolyte imbalance.

If negative, consider ischemic heart disease.

Flutter or Fibrillation

Flutter = sawtooth, regular

Fibrillation = craziness, irregular

Ischemia

Heart's demand for oxygen exceeds supply.

Causes

- atherosclerosis

- vasospasm

- thrombosis

- embolism

- tachycardia

- severe hypotension

- aortic valve disease

S/S

- angina

- ST depression

- T wave inversion

Injury

If ischemia is not resolved, it will lead to injury.

Infarction

Death of myocardial cells.

Serious business.

Most commonly due to thrombus blocking the coronary artery.

Q Wave

Small, septal Q waves typically appear in leads I, aVL, V5, and V6.

Pathologic Q Wave

- indicates necrosis

- more than 0.04 seconds

- more than 25% of the following R wave

Key Leads for Infarction

Right Coronary Artery

- lead II

- lead III

- aVF

Left Coronary Artery

- lead I

- aVL

- V1-V6

R-Wave Progression

As the precordial electrodes move left, the R wave becomes taller and more positively deflected.

Poor R-wave progression means:

- LBBB

- left ventricular hypertrophy

- severe COPD (emphysema)

- old anteroseptal or anterior infarction

Anterior Wall Infarction

Leads

- V3

- V4

ECG Changes

- ST elevation

- tall T waves

- pathologic Q wave

Complications

- left ventricular dysfunction

- heart failure

- cardiogenic shock

Lateral Wall Infarction

Leads

- I

- aVL

- V5

- V6

ECG Changes

- ST elevation

- reciprocal changes in leads II, III, and aVF

- AV block

Inferior Wall Infarction

Leads

- II

- III

- aVF

ECG Changes

- ST elevation

- reciprocal changes in leads I and aVL

Septal Infarction

Leads

- V1

- V2

ECG Changes

- absence of normal septal R waves

- absence of normal septal Q waves in leads I, II, III, aVF, and V4-V6

- ST elevation and tall T waves in V1 and V2

- R and LBBB

- 2nd Degree AV block type II

- complete AV block

Posterior Wall Infarction

Leads

- no leads directly view this wall

- reciprocal changes may be viewed in anterior view

ECG Changes

- ST depression in V1-V3

- tall R waves in V1-V3

Right Ventricle Infarction

Leads

- II

- III

- aVF

ECG Changes

- when ECG changes suggest acute inferior wall MI

Symptoms

- JVD

- liver tenderness

- pulmonary edema

- hypotension

Bundle Branch Fascicles

Anterior Fascicle

- anterior and lateral walls of LV

- thin and vulnerable to disruptions in impulse transmission

Posterior Fascicle

- posterior and inferior walls of LV

- short, thick, rarely disrupted, dual blood supply

Septal Fascicle

- midseptum

ECG Characteristics

View Lead V1

- LBBB may have a notch

Complete RBBB or LBBB

- QRS complex is greater than 0.12 seconds

Incomplete RBBB or LBBB

- QRS complex is 0.10 - 0.12 seconds

Wide QRS complex or delay

- QRS complex is wide but no BBB pattern is present

Right Bundle Branch Block

Septum is activates by the left posterior fascicle and is depolarized from left to right.

Then the LV is stimulated and depolarizes from right to left.

Then the RV is depolarized, causing a second R wave (R') in lead V1

Left Bundle Branch Block

Alters septal activation.

Causes RV to depolarize before the LV.

Atrial Englargement

Right Atrium

- P wave is abnormally tall at the upslope

- tall, peaked, normal duration

- 2.5 mm or more in leads II, II, and aVF

- possibly biphasic in lead V1

Left Atrium

- P wave may be notched

- 0.10 seconds or more

- may be biphasic in V1

Ventricular Enlargement

Right Ventricle

- reversed R-wave progression in precordial leads

Left Ventricle

- deeper-than-normal S waves and small R waves in V1 and V2

- taller-than-normal R waves and small S waves in V5 and V6

Hypercalcemia

- prolonged PR interval

- prolonged QRS complex

- shortened QT interval

Hypocalcemia

- long, flattened ST segments

- prolonged QT intervals

Hypermagnesemia

- prolonged PR interval

- prolonged QRS complex

- elevated T wave

Hypomagnesemia

- diminished voltage of P waves and QRS complexes

- flattened T wave

- widened QRS

- prolonged QT

- prominent U wave

Hyperkalemia

- peaked T wave

- widened QRS

- prolonged PR

- flattened ST

- flattened or absent P

Hypokalemia

- depressed ST

- flattened T wave

- prominent U wave