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ECG Graph

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ECG Graph

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ECG - Graph

electrocardiogram/ECG, EKG

Reflects and records the electrical activity of the heart muscle; details regarding the physiology of electrical conduction are addressed elsewhere.
The movement of action potentials through cardiac muscle cells produces extracellular signals that the ECG records.
A typical ECG comprises the following:
Waves, which are visible as movement above or below baseline voltage.
— A typical ECG comprises 5 waves, labeled P, Q, R, S, and T.
Segments represent time spent at baseline.
Intervals include both segments and waves.
— For example, the ST interval includes the distance encompassing the ST segment and the T wave.

Key electrochemistry definitions:

Depolarization occurs when the membrane potential becomes more positive.
Repolarization occurs when the membrane potential returns to negative.
– Recall that the normal resting potential of ventricular cardiac cells is approximately -90 millivolts.

Normal conduction pathway of electrical signals through the heart:

  1. Sinoatrial (SA) node is the pacemaker of the heart; it sets the heart's rhythm.
    — It sends electrical signals throughout the atria, and to the atrioventricular (AV) node.
  2. The AV node then transmits the signal to the bundle of His (aka, AV bundle).
  3. From here, signals travel through the right and left bundle branches to the apex of the heart.
  4. The Purkinje fiber network spreads the electrical signals throughout the cardiac muscle cells of the ventricles.

Notice that this arrangement ensures that ventricular depolarization and subsequent contraction begins at the apex and moves towards the atria; this pathway moves blood out of the ventricles, whereas depolarization and contraction that began at the AV node, for example, would only squeeze blood to the bottom of the ventricles!

Idealized ECG recording of the electrical events of a single cardiac cycle:

  • The horizontal axis tracks time from 0 to 1.4 seconds.
  • The vertical axis tracks voltage from -0.6 to 1 millivolt.

P wave

  • At 0.2 seconds, a small positive "wave"
    — Reflects the period of atrial depolarization; atrial contraction occurs during the latter part of the P wave.

QRS Complex

  • Peaks at 0.4 seconds.
    — Reflects the period of ventricular depolarization; a wide QRS complex indicates impaired conduction within the ventricles, as in bundle branch block.
    — Obscures atrial repolarization.

T wave

  • Wider and taller than the P wave.
    — Reflects the period of ventricular repolarization.

Electrical and mechanical details:

  • The sinoatrial node fires just before the P wave; the ECG does not record this event, but recall that the SA node is the pacemaker, and sends the electrical signals that initiate the P wave.
  • The PR interval begins at the start of the P wave and ends at the start of the QRS complex.
    — The PR segment is a sub-set of this interval, and encompasses the time between the end of the P wave and the onset of the QRS complex.
    — Within the PR interval, the AV node fires, sending the electrical signal through the bundle of His, bundle branches, and to the Purkinje fibers (notice that this occurs specifically during the PR segment.).
    — The duration of the PR interval is clinically important; PR intervals lasting longer than 0.12 - 0.20 seconds may indicate AV conduction block, which we learn about elsewhere.
  • The ST segment begins after the QRS complex and ends at the onset to the T wave.
    — Ventricular contraction begins during the QRS complex and continues through the ST segment
    – More specifically, isovolumetric contraction begins during the QRS complex, and the ST segment reflects the period of ventricular ejection of blood into the great vessels (see a portion of a Wigger's Diagram).
  • Occasionally, an additional wave, the U wave, will appear after the T wave.

Clinical correlations

  • A prominent U wave may reflect bradycardia (slow heart rate), hypokalemia, ischemia, or effects of antiarrhythmic drugs.
  • The ECG can be used to determine heart rate:
    — Calculate the PP interval, which is the distance between P waves of successive cardiac cycles, or the RR interval, which is the distance between successive R waves.

Misc. Info

Be aware that, unlike in action potential graphs, the shape of the wave does not indicate depolarization or repolarization – notice that both the P wave and T wave are positive (rise above the baseline), but the P wave reflects depolarization and the T wave reflects repolarization. This is because the ECG only shows the direction of current flow relative to the lead's axis.

Full-Length Text

  • Here we will learn about the electrocardiogram (ECG), which reflects the electrical activity of the heart muscle; details regarding the physiology of electrical conduction are addressed elsewhere.
  • To begin, start a table, and denote that an ECG comprises the following:
    • Waves, which are visible as movement above or below baseline voltage; a typical ECG comprises 5 waves, labeled P, Q, R, S, and T.
  • Between the waves are segments, which simply represent time spent at baseline.
  • Intervals include both segments and waves; for example, the ST interval includes the distance encompassing the ST segment and the T wave.

#Let's also remind ourselves of key electrochemistry definitions:

  • Depolarization occurs when the membrane potential becomes more positive (due to movement of sodium and potassium across the membrane, as discussed elsewhere).
  • Repolarization occurs when the membrane potential returns to negative (due to movement of potassium ions across the membrane).
    • Recall that the normal resting potential of ventricular cardiac cells is approximately -90 millivolts.
  • The movement of action potentials through cardiac muscle cells produces extracellular signals that the ECG records.

Before we draw an ECG, let's illustrate the normal conduction pathway of electrical signals through the heart.

  • Outline the heart, and label the atria and ventricles.
  • Now, show the sinoatrial (SA) node, which is the pacemaker of the heart; it sets the heart's rhythm.
  • Show that, upon firing, it sends electrical signals throughout the atria, and to the atrioventricular (AV) node.
    • The AV node then transmits the signal to the bundle of His (aka, AV bundle).
  • From here, signals travel through the right and left bundle branches to the apex of the heart,
  • Then through the Purkinje fiber network, which spreads the electrical signals throughout the cardiac muscle cells.
  • Notice that this arrangement ensures that ventricular depolarization and subsequent contraction begins at the apex and moves towards the atria; this pathway moves blood out of the ventricles, whereas depolarization and contraction that began at the AV node, for example, would only squeeze blood to the bottom of the ventricles!

With this in place, let's show an idealized ECG recording of the electrical events of a single cardiac cycle.

  • Set up the axes:
    • The horizontal axis tracks time from 0 to 1.4 seconds; the vertical axis tracks voltage from -0.6 to 1 millivolt.
  • Now, where Time and Voltage equal 0, draw a horizontal line; at 0.2 seconds, draw a small positive "wave" and label it the P wave, and indicate that this this reflects the period of atrial depolarization.
  • Then, in our graph, indicate a zig-zag-like shape that peaks at 0.4 seconds; label the Q, R, and S waves, which comprise the QRS complex, which reflects the period of ventricular depolarization.
    • A wide QRS complex indicates impaired conduction within the ventricles, as in bundle branch block.
  • Draw another horizontal segment, then indicate the T wave, which is wider and taller than the P wave; it reflects the period of ventricular repolarization.
  • Finally, draw a horizontal segment, and, at 1.2 seconds, draw another P wave to indicate that the cardiac cycle starts again.

Now, let's return to the beginning of the graph to fill in the electrical and mechanical details.

  • First, indicate that the sinoatrial node fires just before the P wave; the ECG does not record this event, but recall that the SA node is the pacemaker, and sends the electrical signals that initiate the P wave.
  • In our heart diagram, write that atrial contraction occurs during the latter part of the P wave.
  • Now, in the ECG recording, indicate the PR interval, which begins at the start of the P wave and ends at the start of the QRS complex.
    • The PR segment includes a sub-set of this time, encompassing only the time between the end of the P wave and the onset of the QRS complex.
    • Within the PR interval, the AV node fires, sending the electrical signal through the bundle of His, bundle branches, and to the Purkinje fibers (notice that this occurs specifically during the PR segment.).
    • The duration of the PR interval is clinically important; PR intervals lasting longer than 0.12 - 0.20 seconds may indicate AV conduction block, which we learn about elsewhere.
  • Next, write in our diagram that atrial repolarization is obscured by the much larger QRS complex; thus, it is not typically visible as a discrete event on the ECG.
  • Label the ST segment, which begins after the QRS complex and ends at the onset to the T wave.
    • In our heart diagram, write that ventricular contraction begins during the QRS complex and continues through the ST segment; more specifically, isovolumetric contraction begins during the QRS complex, and the ST segment reflects the period of ventricular ejection of blood into the great vessels (we discuss these events in detail, elsewhere).
  • Occasionally, an additional wave, the U wave, will appear after the T wave.
    • As a clinical correlation, indicate that a prominent U wave may reflect bradycardia (slow heart rate), hypokalemia, ischemia, or effects of antiarrhythmic drugs.
  • Finally, denote in our table that the ECG can be used to determine heart rate, and remind ourselves that heart rate is the number of cardiac cycles per minute.
    • Two common methods include calculation of the PP interval, which is the distance between P waves of successive cardiac cycles, or the RR interval, which is the distance between successive R waves.
  • As a final note, be aware that, unlike in action potential graphs, the shape of the wave does not indicate depolarization or repolarization – notice that both the P wave and T wave are positive (rise above the baseline), but the P wave reflects depolarization and the T wave reflects repolarization.
    • This is because the ECG only shows the direction of current flow relative to the lead's axis.