ECG Flashcards
P wave
Atrial depolarization (sum of R + L)
Smaller amplitude due to less total muscle mass
QRS Complex
Ventricular depolarization; occurs simultaneously as atrial repolarization, obscuring the signal
Upward deflection of R wave corresponds to phase 0 of action potential; isoelectric ST segment links QRS to T wave and corresponds to phase 2 of the action potential (long plateau)
Normal duration is 0.06-0.10s
T wave
Repolarization of the ventricles, corresponding to phase 3 of the action potential
T wave (hyperpolarization moving away from the lead) should deflect in the same direction as QRS (depolarization moving toward the lead)
QT Interval
Total duration of ventricular depolarization and repolarization
PR interval
Spans from the onset of the P wave to the onset of the QRS complex
Represents AV node conduction time
Normal duration is 0.12-0.20s
ECG paper speed
25 mm/s
ECG chronology - small vs. large squares
Thin lines (small squares) are spaced 0.04s apart
Thick lines (large squares) are spaced 0.2s apart
Unipolar leads
Measure the difference in electrical potential between one point on the body and a virtual reference point set at 0 electrical potential, located in the center of the heart
aVR, aVL, aVF
V1-V6
Bipolar leads
Measure the difference in electrical potential between two different points on the body
Standard Limb Leads I, II, and III
ST depression - Interpretation
Caused by ischemia due to sudden high oxygen demand in the presence of a fixed coronary obstruction
Resting ST segment is normal but during exercise there is a ST depression due to transient ischemia
T wave inversion - Interpretation
Caused by ischemia due to acute coronary artery obstruction during low oxygen demand; ischemia may be transient or result in tissue injury
Recall that normally T waves deflect in the same direction as QRS complex
ST elevation - Interpretation
Sign of transmural cardiac injury in an acute coronary syndrome, usually acute myocardial infarction
Significant Q wave - Definition & Interpretation
Defined as a Q wave that is:
- Greater than or equal to 1/4 the amplitude of the R wave
- Greater than or equal to one small box (0.04s) wide
- Seen in at least 2 leads reflecting the same region of the ventricle
Absence of normal transmural vector produces a negative deflection in leads over infarcted tissue
Lateral Leads
I and AVL
Inferior Leads
II, III, and AVF
Right Chest Leads
V1 and V2
Monitor the RV
Left Chest Leads
V5 and V6
Monitor the LV
ECG features of transmural myocardial infarction
ST elevation with Q waves
ECG features of subendocardial myocardial infarction
ST depression without Q waves
QT Interval - Definition & Causes
Defined as a QT interval that is more than half of the RR interval
Caused by: Hypocalcemia, hypokalemia, hypomagnesemia Class 1A or 3 anti-arrhythmic drugs Hypothermia Congenital Long QT Syndrome
ECG features of Left Ventricular Hypertrophy
Big R waves in L-sided leads:
I, aVL, V5, V6
ECG features of Right Ventricular Hypertrophy
Big R waves in R-sided leads:
V1, V2
ECG features of Hypercalcemia
Shortened QT interval
ECG features of Hypocalcemia
Prolonged QT interval
U wave
A small, variable wave following the T wave; thought to represent repolarization of the Purkinje fibers
Sinus Tachycardia
Regular, fast heart rate > 100bpm with normal P and QRS features
Commonly occurs during exercise or emotional stress; increased cardiac oxygen demand may precipitate angina in patients with coronary artery disease
No treatment usually needed; may be treated with beta blockers, if severe
Sinus bradycardia
Regular, slow heart rate ( seen in the elderly
Treatment with atropine or cardiac pacemaker
1st degree AV block
Defined as a PR interval > 0.2 seconds; interpreted as delayed conduction through the AV node
Caused by drugs (B-blockers, digitalis) and conduction system disease
2nd degree AV block
Some P waves conduct normally to the ventricles but others do not and therefore are not followed by R waves
Rate may be too slow to support adequate CO, resulting in syncope or confusion requiring the use of a pacemaker
Caused by conduction system disease and high vagal tone
3rd degree AV block
P waves and QRS show regular rhythm but occur at different rates with P rate > QRS rate
Caused by AV node failure secondary to severe conduction system disease; may cause syncope or sudden death, usually requires a pacemaker
Atrial Flutter
P waves flutter at rate of 240-320 bpm; pulse may be regular or irregular and ventricular rates vary but are typically rapid
Some risk of embolic stroke due to clot in the left atrium; treated with Warfarin for anti-coagulation and B-blockers, Ca2+ channel blockers, or Digoxin for rate control
Atrial Fibrillation
Irregular ventricular rhythm without P waves
Risk of embolic stroke due to atrial thrombi, heart failure due to rapid heart rate and loss of atrial kick
Treatment: Anti-coagulation, cardioversion (electrical or medical), and rate control with drugs
Atrial Tachycardia
Characterized by rapid HR (>180 bpm) with narrow QRS complexes and abnormal P waves
Terminated by adenosine infusion; recurrence prevented by ablation of re-entry pathway
Junctional Rhythm
Regular rhythms arising from the AV node; usually characterized by a narrow QRS without P waves, which are buried within the QRS signal
When P waves are present they are usually inverted because the wave is conducted upward from the AV node rather than downward from the SA node
Ventricular Tachycardia
Ectopic ventricular focus conducted by slow myocardium; caused by fibrosis, infiltrate, dilation - long path allows re-entry
Characterized by repetitive wide-abnormal QRS without preceeding P wave
Lead I
Standard (Bipolar) Limb Lead
L. arm (+)
R. arm (-)
Lead II
Standard (Bipolar) Limb Lead
L. foot (+)
R. arm (-)
Lead III
Standard (Bipolar) Limb Lead
L. foot (+)
L. arm (-)
aVF
Augmented (Unipolar) Limb Lead
L. foot (+)
aVR
Augmented (Unipolar) Limb Lead
R. arm (+)
aVL
Augmented (Unipolar) Limb Lead
L. arm (+)
V1 & V2
Precordial Chest Leads
R. ventricle + septum
V3 and V4
Precordial Chest Leads
Septum
V5 and V6
Precordial Chest Leads
L. Ventricle
Evolution of ECG in transmural myocardial infarct
- Peaked T wave (“hyperacute T wave”)
- T-wave inversion - sign of ischemia
- ST elevation - sign of transmural, ischemic injury occurring within 1-2 hours of insult
- Q wave + ST elevation + T inversion - typical of transmural infarcts
OR
- ST depression with no Q wave - typical of subendocardial infarcts
Atrial Premature Beats
P wave occurs too early following the last T wave and looks slightly different from other P waves; QRS is normal
Mostly benign
Premature Ventricular Contraction (PVC)
Depolarization originates in the ventricle and contraction occurs via contractile myocytes
Characterized by wide QRS signal without P waves
Common in normal subjects; treatment usually not required
