An EKG tracing consists of repeating patterns that correspond to specific electrical events in the heart. The EKG captures the various waves, segments, and intervals that represent different stages of the cardiac cycle. But what do these terms mean?
Waves
Waves represent the electrical activity associated with the depolarization or repolarization of the heart chambers.
• P Wave: Indicates atrial depolarization, which triggers the atria (upper chambers) to contract.
• QRS Complex: Reflects ventricular depolarization, causing the ventricles (lower chambers) to contract.
• T Wave: Represents ventricular repolarization, the process of the ventricles resetting electrically for the next heartbeat.
Sequence: The heart’s electrical cycle follows this pattern—P wave, brief pause, QRS complex, short pause, T wave. This sequence repeats with each heartbeat.
Segments
Segments are the straight lines (normally isoelectric lines- i.e, they are relatively horizontal and correspond to the “baseline” of the EKG) between waves and represent periods without electrical activity.
• PR Segment: The line from the end of the P wave to the beginning of the QRS complex. It reflects the delay at the atrioventricular (AV) node.
• ST Segment: The line from the end of the QRS complex to the beginning of the T wave. It indicates when the ventricles are depolarized.
While in a normal EKG, these segments should be at baseline, they may be elevated or depressed in certain pathologies. So, when trying to determine what the actual baseline of the EKG is when evaluating a pathology, the TP Segment should be used.
• TP Segment: The line from the end of the T wave to the beginning of the P wave. This part of the EKG typically has no electrical activity even in most pathologies and so, is considered the baseline of an EKG.
Intervals
Intervals include one or more waves plus the connecting segments, representing the duration of specific electrical events.
• PR Interval: Extends from the beginning of the P wave to the beginning of the QRS complex. It represents the time from the start of atrial depolarization to the start of ventricular depolarization.
• QRS Interval: Measures the duration of the QRS complex alone, indicating how quickly the ventricles depolarize.
• QT Interval: Spans from the beginning of the QRS complex to the end of the T wave. It reflects the total time for ventricular depolarization and repolarization.
Electrical Activity vs. Mechanical Contraction
While the EKG displays the heart’s electrical activity, it does not directly show the mechanical contraction (the actual pumping action).
• Electrical Activity: Initiates muscle contractions through electrical impulses.
• Mechanical Contraction: The physical response to electrical stimulation, pumping blood throughout the body—a process known as excitation-contraction coupling.
Important: Normal electrical activity usually leads to effective mechanical contractions, but not always.
Pulseless Electrical Activity (PEA)
Sometimes, electrical signals occur without effective mechanical contractions. This condition is known as pulseless electrical activity (PEA).
Characteristics of PEA:
• The EKG may display normal or near-normal electrical patterns.
• The heart fails to contract effectively.
• There is no palpable pulse or blood circulation.
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