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.
Before delving into the adjustments, it is essential to grasp the standard settings. These standardized settings are universally recognized and offer a consistent framework for interpreting EKGs across various machines and clinical environments. Typically, EKG machines generate calibration markers, also referred to as standard calibration pulses or signals, either at the beginning or the end of the strip. These markers serve as visual references for both speed and amplitude.
Standard Calibration Pulse
• Appearance: A rectangle or square wave that is 10 mm tall (vertical) and 5 mm wide (horizontal).
• Intervals Appear Shorter: Time intervals seem shortened.
Amplitude Variations
a. Increased Amplitude (20 mm/mV)
Calibration Marker:
• Height Doubles: The marker is 20 mm tall and 5 mm wide.
• Waveform Appearance:
• Stretched Vertically: Waveforms are taller.
• Voltage Appears Higher: Amplitudes seem increased.
b. Decreased Amplitude (5 mm/mV)
Calibration Marker:
• Height Halves: The marker is 5 mm tall and 5 mm wide.
• Waveform Appearance:
• Compressed Vertically: Waveforms are shorter.
• Voltage Appears Lower: Amplitudes seem decreased.
Quick Tips to Recognize Calibration Variants
Examine Calibration Markers First
• Location: Usually at the beginning or end of each lead or rhythm strip.
• Shape and Size: Changes in the marker’s dimensions directly indicate calibration adjustments.
Look for Printed Calibration Information
• Margins and Headers: Calibration settings are often printed in these areas.
• Common Notations:
• Speed: “25 mm/s”, “50 mm/s”, “12.5 mm/s”
• Amplitude: “10 mm/mV”, “20 mm/mV”, “5 mm/mV”
Analyze Waveform Characteristics
Unusual Waveform Size:
• Too Wide: Suggests increased speed.
• Too Narrow: Suggests decreased speed.
• Too Tall: Indicates increased amplitude.
• Too Short: Indicates decreased amplitude.
When and Why to Adjust EKG Speed
Analyzing Fast Heart Rates (Tachycardia)
Why Adjust?
• Improve Waveform Clarity: In cases of tachycardia (heart rates above 100 bpm), the EKG waveforms can appear compressed, making it difficult to distinguish individual components like the P wave, QRS complex, and T wave.
How to Adjust?
• Increase Speed to 50 mm/second: Doubling the speed stretches out the waveforms horizontally, allowing for better visualization of each cardiac cycle.
At 50 mm/second:
• 1 small square = 0.02 seconds
• 1 large square = 0.10 seconds
How we calculate the heart rate changes accordingly also: Heart Rate = 3000 / Number of small squares between R-R intervals.
Clinical Benefits:
• Detailed Rhythm Analysis: Enhanced clarity aids in identifying arrhythmias, conduction blocks, or other abnormalities that might be obscured at standard speed.
Evaluating Slow Heart Rates (Bradycardia)
Why Adjust?
• Conserve Paper and View More Cycles: In bradycardia (heart rates below 60 bpm), the waveforms are spread out over a longer period, which can make it challenging to view multiple cardiac cycles on one page.
How to Adjust?
• Decrease Speed to 12.5 mm/second: Halving the speed compresses the waveforms horizontally, allowing more cycles to fit on a single strip.
At 12.5 mm/second:
• 1 small square = 0.08 seconds
• 1 large square = 0.40 seconds
Accordingly, Heart Rate = 750 / Number of small squares between R-R intervals.
Clinical Benefits:
• Long-Term Rhythm Observation: Useful for observing patterns over time, such as in patients with suspected intermittent heart block.
Impact of Speed Adjustment on EKG Interpretation
Understanding Time Intervals
• Standard Speed (25 mm/second):
• 1 small square = 0.04 seconds
• 1 large square (5 small squares) = 0.20 seconds
When and Why to Adjust EKG Amplitude (Voltage)
Detecting Low Voltage QRS Complexes
Why Adjust?
• Enhance Small Waveforms: In some patients, especially those with obesity, pericardial effusion, or pulmonary emphysema, the voltage of the QRS complexes may be abnormally low.
How to Adjust?
• Increase Amplitude to 20 mm/mV: Doubling the amplitude vertically enlarges the waveforms, making small deflections more discernible.
At 20 mm/mV:
• 1 mm = 0.05 mV
• Waveforms appear twice as tall
Clinical Benefits:
• Improved Detection: Enhances the ability to detect and measure small voltage changes, aiding in accurate diagnosis.
Preventing Waveform Overlap in High Voltage Situations
Why Adjust?
• Avoid Clipping of Waveforms: In cases of hypertrophy or abnormal conduction, the QRS complexes may have very high voltage, causing them to overlap or go off the EKG paper.
How to Adjust?
• Decrease Amplitude to 5 mm/mV: Halving the amplitude compresses the waveforms vertically, ensuring the entire waveform fits within the recording.
At 5 mm/mV:
• 1 mm = 0.2 mV
• Waveforms appear half as tall
Clinical Benefits:
• Accurate Measurement: Prevents distortion of waveforms, allowing for precise measurement of amplitudes and intervals.
Impact of Amplitude Adjustment on EKG Interpretation
Understanding Voltage Measurements
• Standard Amplitude (10 mm/mV):
• 1 mm (small square vertical) = 0.1 mV
Practical Considerations
Always Document Calibration Changes
• Prevent Misinterpretation: Failure to note adjustments can lead to incorrect diagnoses due to miscalculations of heart rate and waveform measurements.
Understand the Clinical Context
• Tailor Settings to Patient Needs: Adjustments should be made based on the specific clinical scenario and the patient’s condition.
Recalibrate Formulas and Measurements
• Adapt Calculations Accordingly: Be mindful that standard formulas and normal values apply to standard calibration settings.
An electrocardiogram (EKG or ECG) is a vital tool that records the electrical activity of the heart. Before diving into interpreting heart rhythms and patterns, it’s essential to understand the EKG paper itself. This guide will help you grasp the basics of the EKG grid, including how time and voltage are measured, so you can accurately read and interpret EKG strips.
The EKG Paper Grid
The EKG paper features a grid of horizontal and vertical lines forming small and large squares. This grid helps measure time on the horizontal axis and voltage on the vertical axis.
The grid is composed of large squares with a bold outline (measuring 5 mm by 5 mm) and each large square is made up of 1mm by 1mm small squares with a lighter outline.
Time Measurement on the Horizontal Axis
The horizontal axis represents time, allowing you to determine the duration of electrical events in the heart.
• Each small square equals 0.04 seconds (40 milliseconds).
• Each large square (5 small squares) equals 0.2 seconds.
• Five large squares equal 1 second of elapsed time.
Example: Counting 10 large squares horizontally would represent 2 seconds of heart activity.
Voltage Measurement on the Vertical Axis
The vertical axis measures voltage, indicating the electrical potential generated by the heart’s activity.
• Each small square equals 0.1 millivolts (mV).
• Each large square equals 0.5 mV.
• Two large squares vertically represent 1 mV of electrical activity.
Calibration and Standard Settings
Before interpreting an EKG, always check the calibration settings, usually indicated at the beginning or end of the recording.
• Calibration Marker: Often, a rectangle measuring 1 cm in height and 5 mm in width is printed at the start of the EKG strip to confirm standard settings. This indicates that the EKG is done with a standard calibration- 1 cm height indicates that on the EKG, a 10mm (or 1cm) deflection reflects a 1 millivolt amplitude. On the other hand, a 5 mm width indicates that the EKG is printed at 25 mm/ second (so, the 25 cm standard EKG reflects the electrical activity of the heart over 10 seconds).
Note: Calibration ensures that time and voltage measurements are accurate. Settings can be adjusted, so always verify calibration to avoid misinterpretation.
Length of a Standard EKG Strip
A typical EKG strip represents 10 seconds of heart activity.
• Length of a standard EKG: Approximately 50 large squares or 250 small squares horizontally.
This length provides enough data to assess the heart’s rhythm and detect many abnormalities.
Conclusion
Understanding the EKG grid is the first step in analyzing the heart’s electrical activity. By knowing how to measure time and voltage using the grid’s squares, you can begin to interpret the rhythms and patterns displayed on an EKG. Stay tuned for our next post, where we’ll explore the different waves, segments, and intervals that make up the EKG tracing.
