Key Points
- ECGs display voltage changes across the cardiac cycle, captured as waveforms, segments, and intervals.
- Waveforms represent depolarization or repolarization events.
- Segments are baseline connections between waveforms, usually representing electrically silent phases.
- Intervals measure the timing of conduction across the cardiac cycle.
- Understanding these elements is essential for identifying ischemia, conduction delays, electrolyte abnormalities, and arrhythmias.
Waveforms: The Heart's Electrical Signatures
Each waveform corresponds to a discrete phase of cardiac depolarization or repolarization. They are described by:
- Amplitude (voltage magnitude)
- Duration (time in ms)
- Morphology (shape and direction of deflection)
Key Waveforms
- P wave: atrial depolarization
- QRS complex: ventricular depolarization
- Q, R, S waves (individual deflections)
- T wave: ventricular repolarization
- Delta wave: pre-excitation (WPW)
- J point & ST elevation/depression: ischemia/occlusion markers
- J waves (Osborn): hypothermia, hypercalcemia
- Epsilon waves: ARVC
- U wave: hypokalemia, bradycardia
Dive deeper into the key waveforms here:
Segments: Connecting Waveforms
Segments are electrically neutral baselines between active depolarization/repolarization. They are critical for ischemia and injury evaluation.
Key Segments
- PR segment: AV nodal conduction
- ST segment: ventricular isoelectric period (diagnostic for ischemia, OMI, pericarditis, hyperkalemia)
- TP segment: true baseline reference
Dive deeper into the key segments here:
Intervals: Timing Conduction
Intervals include waveforms + segments, reflecting conduction across chambers.
Key Intervals
- PR interval: onset of atrial depolarization to onset of ventricular depolarization (120–200 ms). Prolonged = AV block; shortened = pre-excitation.
- QT interval: ventricular depolarization + repolarization. Prolonged = risk of torsades; shortened = hypercalcemia.
Dive deeper into the key intervals here:
By analyzing waveforms, segments, and intervals, clinicians can glean essential information about the heart’s electrical function, helping to diagnose a wide array of cardiac conditions and ensuring accurate and effective patient care.
KEY CLINICAL PEARLS:
- ST segment and J point changes are the most time-sensitive markers of occlusion MI.
- PR interval abnormalities help distinguish AV nodal disease (benign) from infranodal disease (high-risk).
- QT interval is critical to assess in any patient with syncope, drug toxicity, or electrolyte disturbance.
- Compare segments to the TP baseline, not the PR, when evaluating ST deviations.
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