Trystan Casey
The cardiac cycle is a series of pressure changes within the heart, resulting in blood movement through different chambers. The cycle can be divided into systole and diastole, which correspond to cardiac muscle contraction and relaxation, respectively. Atrial systole is the contraction of cardiac muscle cells of both atria, which completes the diastole and precedes ventricular systole. So, what electrical event immediately precedes atrial systole?
| Characteristics | Values |
|---|---|
| Electrical event preceding atrial systole | P-wave deflection of a steady signal in an electrocardiogram (ECG) |
| Atrial systole | Contraction of cardiac muscle cells of both atria |
| Atrial systole duration | Approximately 100 ms |
| Atrial systole function | Completes ventricular diastole by finalizing the filling of both ventricles with blood |
| Atrial systole overlap | End of ventricular diastole |
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What You'll Learn
Electrical systole
The cardiac cycle involves four major stages of activity: isovolumic relaxation, inflow, isovolumic contraction, and ejection. The cardiac cycle is represented electrically by the electrocardiogram (ECG), which records electrical changes during the cycle. Electrical systole is the electrical activity that initiates atrial systole, which is the contraction of the atria that forces blood into the ventricles.
During systole, cardiac muscle contracts and generates pressure on the blood within the heart chambers, providing the energy needed to eject the blood from the chambers. Systole and diastole occur in both the right and left heart, but with very different pressures. Systole represents ventricular contraction and ejection, while diastole represents ventricular filling.
The P-wave on an ECG represents the summed electrical activity of the atrial contraction, which initiates atrial systole. The interval between the P-wave and the Q-wave indicates the time taken for the action potential to travel from the atrium to the ventricle through the AV-canal. Atrial systole lasts approximately 100 ms and ends prior to ventricular systole, as the atrial muscle returns to diastole. Atrial contraction contributes the remaining 20-30% of ventricular filling, which becomes significant in left ventricular hypertrophy or thickening of the heart wall.
Atrial fibrillation is an example of an electrical malady in the heart that can appear during atrial systole. It involves an electrically disordered but well-perfused atrial mass working in an uncoordinated fashion with a comparatively electrically healthy ventricular systole. This results in a compromised load that detracts from the overall performance of the heart, causing the ejection fraction to deteriorate by 10-30%.
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P-wave deflection
The P-wave is an integral part of an electrocardiogram (ECG) and represents the electrical depolarization of the atria of the heart. It is typically a small positive deflection from the isoelectric baseline that occurs just before the QRS complex. The P-wave is frequently biphasic in V1 (occasionally in V2), with the initial positive deflection corresponding to right atrial activation and the subsequent negative deflection denoting left atrial activation. The P-wave duration should be less than or equal to 0.12 seconds, and the amplitude should be less than 2.5 mm in the limb leads.
The P-wave morphology can provide important information about the status of the left atrium. For example, left atrial abnormalities such as hypertrophy and/or dilation are sometimes identifiable on ECG by the presence of broad, notched (bifid) P waves in lead II. Tall, peaked P waves in lead II can indicate right atrial enlargement, while P-wave inversion in the inferior leads suggests a non-sinus origin of the P waves.
The interval between the P-wave and the Q-wave, known as the PR interval, indicates the time taken for the action potential to travel from the atrium to the ventricle through the AV-canal or AV-node. A normal PR interval ranges between 0.12 seconds to 0.22 seconds. If the PR interval is less than 120 milliseconds (ms), it may indicate that electrical impulses are travelling between the atria and ventricles too quickly, which can be indicative of pathology.
The P-wave is also associated with atrial repolarization, which is the process of the atria returning to their resting state after contraction. In some cases, such as in hagfish, atrial repolarization appears as a Pr-wave that follows the P-wave and precedes the QRS-wave due to a prolonged AV-delay.
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Atrial contraction
The cardiac cycle is represented electrically by the electrocardiogram (ECG), which records the electrical changes during the cycle. The P-wave on the ECG represents the atrial contraction, specifically the depolarization of the atria. This is followed by the QRS-wave, which represents the contraction of the ventricles. The interval between the P-wave and the Q-wave indicates the time taken for the electrical impulse to travel from the atrium to the ventricle.
It is important to note that atrial contractions can sometimes occur prematurely, known as premature atrial contractions (PACs). These are extra heartbeats that disrupt the normal rhythm of the heart and can originate in one of the upper chambers of the heart, known as the atria. While PACs are typically benign and occur sporadically in healthy individuals, they can be more frequent in those with certain medical conditions and may be a risk factor for atrial fibrillation.
The evaluation and treatment of PACs involve a multidisciplinary approach, including medical history, physical examinations, diagnostic tests, and evidence-based management options. While most people do not require treatment, invasive procedures and medications may be necessary in some cases to manage the condition and reduce the risk of associated complications.
In summary, atrial contraction is a crucial phase of the cardiac cycle, ensuring the efficient filling of the ventricles with blood. This process is represented by the P-wave on an ECG and is followed by ventricular contraction. Premature atrial contractions can disrupt the normal rhythm, but they are typically benign and may not require treatment.
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Atrial systole
The cardiac cycle is the performance of the human heart from the beginning of one heartbeat to the beginning of the next. It consists of two main periods: diastole and systole. Diastole refers to the period when the heart muscle relaxes and refills with blood, while systole refers to the period of contraction and ejection of blood from the heart.
The cardiac cycle is represented electrically by the electrocardiogram (ECG). The P-wave represents the summed electrical activity of the atrial contraction, which is also known as atrial systole. The interval between the P-wave and the Q-wave indicates the time taken for the action potential to travel from the atrium to the ventricle through the AV-canal, the AV-delay. The electrical activity of the contraction of the ventricles is represented by the QRS-wave.
The cardiac cycle involves four major stages of activity: "isovolumic relaxation", inflow, "isovolumic contraction", and "ejection". Atrial systole is part of the ventricular diastole period, which includes the first two stages. During this time, blood returning to the heart flows through the atria into the relaxed ventricles. Atrial systole lasts approximately 100 ms and ends prior to ventricular systole, as the atrial muscle returns to diastole.
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Ventricular systole
The cardiac cycle is a series of pressure changes within the heart, resulting in blood movement through different chambers of the heart and the body. The cardiac cycle can be divided into periods of systole and diastole, which correspond to cardiac muscle contraction and relaxation, respectively. Systole and diastole occur in both the right and left heart, but with very different pressures.
The process of excitation-contraction coupling and cardiomyocyte contraction during this phase results in a rapid increase in intraventricular pressure. The atrioventricular valve closes once the intraventricular pressure exceeds atrial pressure. Following the closure of this valve and before the opening of the semilunar valve, ventricular pressure rapidly increases without a corresponding change in ventricular volume. This brief period is known as isovolumetric contraction.
Individual cardiomyocyte contractions are not exclusively isometric. Individual cardiac muscle fibres undergo a combination of eccentric (lengthening), isotonic (fixed tension), and isometric (fixed length) contractions, resulting in the heart becoming more spherical. The exact pattern of ventricular wall shortening and compression of the chamber is complex and designed to maximise efficiency. During systole, individual cardiac muscle fibres move synergistically to produce torsion and energy-conserving ventricular shape changes necessary for optimal systolic ejection.
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Frequently asked questions
The P-wave of an ECG represents the electrical activity that immediately precedes atrial systole.
The P-wave represents the depolarization of the sinoatrial node, which spreads an action potential across the atria, resulting in atrial systole.
The cardiac cycle is a series of pressure changes within the heart that result in blood movement through different chambers. It can be divided into systole (contraction) and diastole (relaxation).
The sinoatrial node, often known as the cardiac pacemaker, produces electrical impulses that stimulate atrial contraction by creating an action potential across the myocardium cells.
The P-wave represents the electrical activity that initiates atrial systole. Atrial systole is the contraction of the atria, which completes the diastole and fills the ventricles with blood.
