Atrial Contractions on ECG

When the heart's atria contract, a depolarization or contraction wave is created. A P-wave on an ECG tracing represents it. The electrical activity, which starts in the sinoatrial (SA) node and spreads through the atria, causes contraction. In a healthy person, this originates at the (SA) node and migrates to other parts of the heart.

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The P wave helps us know about the heart’s health. An abnormal P-wave indicates atrial enlargement or other abnormalities in the atria. Interpreting a P-wave helps us diagnose atrial arrhythmias and assess overall atrial function.

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In this blog, we will learn about atrial contractions and how the electrical impulse travels from the atria to the ventricles. We will also learn how atrial contraction is important for diagnosing heart conditions, assessing electrical activity, and determining treatment for patients with heart diseases.

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Anatomy and Physiology of the Atria

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Anatomy of the atria:-

The atria, or the upper chambers, receive blood from the body and lungs. The right atrium receives deoxygenated blood, while the left atrium receives oxygenated blood. Efficient atrial function ensures proper blood circulation throughout the body.

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The left atrium is concealed behind the right atrium, whereas the right atrium forms a major upper part of the sternocostal surface of the heart. A fibromuscular wall separates the right and left atria. The atrioventricular septum separates the ventricles of the same side from the atria.

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Conduction in atria:-

• Role of the sinoatrial node: It is located in the right atrium near the opening of the superior vena cava and is the heart’s natural pacemaker. It maintains the heart's normal function and initiates its electrical impulses.

• Role of the atrial muscles: The atrial muscles are specialised to conduct electrical impulses and cause contraction. Bachmann’s bundle helps to conduct the electrical impulses from the right to the left atrium.

•  Pathways of atrial conduction: The electrical impulses travel from the sinoatrial node to the atrioventricular (AV) node. From here, the signals progress to the ventricles to ensure coordinated heartbeats.

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Atria and its role in cardiac hemodynamics:-

Atria ensures blood flows continuously into the heart even when the lower chambers contract. It is possible because they have

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• No valves in the veins that allow the blood to flow in without interruption.

• During atrial contraction, venous return continues with minimal interruption due to the elastic nature of the atria and the absence of valves at their inlets.

• Atrial relaxation begins just before and continues even during ventricular contraction. It allows the atria to fill with blood from the veins without interruption.

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As a result, the atria help the heart pump 15 - 25% more blood. The atrial contraction, also called the atrial kick, pushes more blood into the ventricles. Its main benefit is preventing any slowdown in blood returning to the heart.

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Role of atrial contraction:-

It begins with activating the heart's upper chambers, the atria. It causes the atria to squeeze and push more blood into the lower chambers, i.e, the ventricles through the walls. Because of the direction of contraction and movement of the incoming blood, blood can't flow backwards.

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At rest, this contraction fills about 10% of the left ventricle as most of the blood flows in passively. When the heart rate increases, the atrial contraction fills up to 40% of the left ventricle, also known as the Atrial Kick.

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The sinus node, located in the upper right corner of the right atrium, is where the electrical impulses start. These impulses contract the heart in an organized and rhythmic manner, determining the heart rate.

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When the atria contract, the blood gets pushed into the ventricles, which then pump it into the rest of the body. The atrial contractions may be classified broadly into:-

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• Normal contractions are a part of the regular cardiac cycle.

• Premature atrial contractions occur when the atria contract before the next normal heartbeat, making it skip a beat or experience an extra beat.

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Characteristics of a Normal sinus P wave:-

The first positive deflection on the ECG is a P-wave. It represents atrial depolarization:-

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1. Morphology

• Smooth contour.

• Monophasic in lead II.

• Biphasic in V1.

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2. Axis

• Normal P wave access is between zero degrees and +75 degrees.

• P waves should be upright in leads I, II, and inverted in aVR.

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3. Duration

• < 0.12s (120 ms or 3 small squares).

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4. Amplitude

• < 2.5mm (0.25 mV) in the limb leads.

• <1.5mm (0.15mV) in precordial leads.

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Inferior leads (II, III, and aVF) and V1 reveal atrial abnormalities because P waves are most prominent in them.

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Atrial Contraction and Its Relationship to the P Wave:-

• Atrial contraction proceeds from the right to the left side of the heart.

• The P Wave is the summation of the right and left atrial waveforms.

• The first third of the P wave corresponds to the right atria, while the final 1/3 corresponds to the left atria. The middle third is a combination of the two.

• The right and left atrial waveforms move in the same direction to form a monophasic P wave.

• In V1, the right and left waveforms move in opposite directions to produce a biphasic P wave with an initial positive deflection indicating right atria activation. A subsequent negative deflection indicates left atrial activation.

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Lead V1 reveals abnormalities in each atrium because of the separation of right and left atrial electrical impulses. The variations in P waveforms in other leads also help us identify abnormalities.

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Premature Atrial Contractions 

They occur when the atria contract before the next normal heartbeat, making it skip a beat or experience an extra beat. Premature atrial contractions (PACs) have other names, such as atrial premature complexes or atrial premature beats.

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The sinoatrial node controls the heartbeat during normal sinus rhythm. PACs occur when the atria contract before the sinoatrial node, triggering a premature heartbeat. The exact cause of PAC is not clear. It is usually asymptomatic and is noticed only with 24-hour monitoring. It is perceived as skipped beats or a jolt in the chest. 

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Symptoms 

• Skipping or pounding of a heartbeat.

• Palpitations.

• Followed by anxiety and shortness of breath.

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Causes

• Change in the blood volume (during dehydration) in the heart.

• Injury to the heart.

• Coronary heart disease or hypertrophic cardiomyopathy.

• Electrolyte imbalance.

• Tiredness.

• Excess Caffeine intake.

• Alcohol.

• Using tobacco.

• Stress.

• Recreational drugs.

• Medications for heart and mental health.

• Thyroid problems.

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Risk factors

It can occur in any individual, but it usually occurs in people who are:-

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• Tall.

• Pregnant.

• Have a cardiovascular disease.

• Physically inactive.

• Have diabetes.

• Have COPD.

• Smoke.

• Do not drink sufficient water.

• Highly stressed.

• Have poor sleep habits.

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Classification of PACs

• Unifocal, arising from a single ectopic focus, each PAC is identical.

• Multifocal, they arise from different foci and have different P wave morphologies.

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PACs often repeat patterns:-

• Bigeminy, every other beat is a PAC.

• Trigeminy, every third beat is a PAC.

• Quadrigeminy, every fourth beat is a PAC.

• Couplet: two consecutive PACs.

• Triplet: three consecutive PACs.

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ECG Features of PAC:-

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1. Abnormal P Wave

The P wave represents atrial contraction, which differs from the normal sinus P wave and originates from a spot other than the SA node.

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2. Normal QRS complex

The QRS complex remains narrow and normal when < 120ms.

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3. Post-extrasystolic pause

After a PAC, a delay may occur before the next normal heartbeat, which is known as a noncompensatory pause. This means that the interval between the beat before the PAC and the beat after the PAC is less than twice the normal RR interval.

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4. Conduction pattern

If a PAC occurs very early, the conduction may occur abnormally through the ventricles, often resembling right bundle branch block.

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5. Blocked PAC

Sometimes, PAC doesn’t lead to a ventricular contraction, leading to an absent QRS complex on the ECG.

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Clinical Significance of PAC

Most often, PACs are asymptomatic, but they also predict atrial fibrillation. Some important clinical implications are:-

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• Increased risk of atrial fibrillation.

• Increase the risk of stroke by indicating atrial abnormalities that predispose to atrial fibrillation.

• Frequent PACs cause changes in the atrial structure and the development of atrial cardiomyopathy.

• Patients who experience PAC have a higher risk of hospitalisation and can have high mortality due to underlying heart disease or arrhythmia. PACs don’t cause high mortality but are markers for increased risk.

• Such patients develop subclinical heart diseases like atrial enlargement.

• PACs can trigger or worsen arrhythmias like atrial flutter and ventricular arrhythmias, but their role in ventricular arrhythmias is uncommon.

• Having a sensation of skipping a beat affects the quality of life.

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Conclusion

Your heart skipped a beat, which must have surprised you, but it is too true to be ignored. While it may not warrant attention occasionally, an increased frequency could indicate an underlying health condition. Long-standing PACs require evaluation, and there can be no better place to reach out than Gauze.