The electrical signals recorded on an electrocardiogram (ECG) occur before the mechanical processes of contraction and relaxation during the cardiac cycle.
A cardiac action potential originates in the SA node and spreads throughout the atria and the AV node in approximately 0.03 seconds. This results in the P wave in an ECG and triggers atrial contraction. The action potential is then briefly slowed at the AV node, allowing the atria to contract and fill the ventricles with blood before ventricular systole begins.
Following the delay, the action potential is rapidly propagated through the AV bundle (Bundle of His), then into the right and left bundle branches, and finally through the Purkinje fibers into the ventricles. This results in the QRS complex seen in the ECG, which signals the depolarization of the ventricles. Concurrently, atrial repolarization occurs, although this is usually obscured in an ECG by the more prominent QRS complex.
Ventricular contraction begins soon after the QRS complex appears, pushing blood upwards toward the semilunar valves. Ventricular repolarization follows, starting at the apex and spreading throughout the ventricular myocardium, and is marked by the T wave in an ECG.
Finally, ventricular diastole begins, allowing the ventricles to relax. Following a brief period where all cardiac fibers are relaxed, the cycle resumes with the appearance of another P wave.
The electrical signals recorded on an electrocardiogram or ECG occur before the mechanical processes of systole and diastole during the cardiac cycle.
The P wave signifies the spread of cardiac action potential from the sinoatrial node to the atrial contractile cells.
Approximately 0.1 seconds post the P wave, atrial systole begins, lasting until the P-Q interval.
About 0.2 seconds after the P wave, the QRS complex shows up, indicating that the cardiac potential has reached the ventricular contractile cells, moving down the septum to the apex, then upwards.
The ventricular systole begins after the QRS complex, spanning the entire S-T interval. Simultaneously, the atria return to diastole.
Next, 0.4 seconds after the P wave, the T wave emerges, signifying ventricular repolarization. Shortly after the T wave, the ventricles begin to relax.
By 0.6 seconds, the ventricles are completely relaxed. The next 0.2 seconds mark a brief period where both the atria and ventricles are relaxed.
Another P wave appears at 0.8 seconds, initiating atrial contraction and restarting the cycle.
繁體中文
