A&P 18.5 - Part 3

ECG waves intervals and abnormalities

Electrocardiography and Normal Waves

What an ECG records

Electrical currents generated in the heart spread throughout the body and can be detected by an electrocardiograph. An electrocardiogram, or ECG, is the graphic record of heart activity. It combines action potentials from nodal and contractile cells at a given time; it is not the tracing of one cell’s action potential.

• Electrodes: Placed at different sites on the body surface.

• Typical 12 leads: The text describes three bipolar leads measuring voltage differences between the arms or an arm and a leg, plus nine unipolar leads. Together they provide a comprehensive picture of electrical activity.

Figure 18 16 shows skin electrodes attached to a patient and a typical tracing. The round device near the patient’s left shoulder reads pacemaker data. The tracing has a small P wave, a sharp QRS complex, and a broader T wave, with the major intervals labeled. In the QRS complex, Q is the small downward deflection before the tall upward R wave; S is the downward deflection after R.

P wave

• Event: Atrial depolarization as the wave spreads from the SA node through the atria.

• Duration: About 0.08 second.

• Contraction: The atria contract approximately 0.1 second after the P wave begins.

QRS complex

• Event: Ventricular depolarization, which precedes ventricular contraction.

• Duration: About 0.08 second on average.

• Shape: Depolarization paths through ventricular walls continually change, changing current direction. Depolarization time for each ventricle also depends on its relative size.

T wave

• Event: Ventricular repolarization.

• Duration: About 0.16 second.

• Shape: Repolarization is slower than depolarization, so the T wave is broader and lower in amplitude than the QRS complex.

Atrial repolarization occurs during ventricular excitation. Its wave is normally hidden by the much larger QRS complex.

Intervals and segments

• P R interval: About 0.16 second from the start of atrial excitation to the start of ventricular excitation. Includes atrial depolarization and contraction and passage through the conduction system. If the Q wave is visible, the interval may be called P Q.

• S T segment: The entire ventricular myocardium is depolarized while ventricular cell action potentials are in their plateau phases.

• Q T interval: About 0.38 second from the beginning of ventricular depolarization through ventricular repolarization.

ECG Sequence and Abnormal Findings

Figure 18 17 in words

• 1 Atrial depolarization: The SA node initiates the wave, producing the P wave.

• 2 AV nodal delay: Atrial depolarization is complete and the impulse pauses at the AV node.

• 3 Ventricular depolarization: Begins at the apex and produces the QRS complex. Atrial repolarization occurs at the same time.

• 4 Depolarization complete: The ventricular myocardium is fully depolarized.

• 5 Ventricular repolarization: Begins at the apex and produces the T wave.

• 6 Repolarization complete: The ventricles have completed repolarization.

The colored heart regions show where depolarization or repolarization is spreading, while the highlighted portion of the tracing shows the matching ECG event. Electrical events precede the mechanical contraction and relaxation they trigger.

What changes in the ECG can indicate

In a healthy heart, the ECG’s pattern and timing tend to remain consistent. Changes can alert clinicians to damage, disease, or conduction-system problems.

• Enlarged R wave: Hints at enlarged ventricles.

• Elevated or depressed S T segment: The text associates these changes with cardiac ischemia, meaning insufficient blood flow.

• Prolonged Q T interval: Indicates abnormal repolarization and an increased risk of ventricular arrhythmias.

Figure 18 18 in words

• Normal sinus rhythm: The figure identifies this as the normal ECG trace.

• Junctional rhythm: The SA node is nonfunctional, P waves are absent, and the AV node paces the heart at 40 to 60 beats per minute.

Comparing the two cardiac cell types

In pacemaker cells, spontaneous slow depolarization reaches threshold, and calcium entry causes the action potential’s rising phase. In contractile cells, fast sodium entry causes the rapid rise, followed by a prolonged calcium-dependent plateau. Potassium leaving the cell contributes to repolarization in both types.