Split S2

Key takeaways

Split S2 belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Split S2 to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Split S2 from memory before moving on to harder problems.

Reference excerpt

A split S2 is a finding upon auscultation of the S2 heart sound. It is caused when the closure of the aortic valve (A2) and the closure of the pulmonary valve (P2) are not synchronized during inspiration. The second heart sound (S2) is caused by the closure of the aortic and pulmonic valves, which causes vibration of the valve leaflets and the adjacent structures. The aortic valve closes slightly before the pulmonic, and this difference is accentuated during inspiration when S2 splits into two distinct components (physiological splitting). During expiration, the pulmonic valve closes at nearly the same time as the aortic, and splitting of S2 cannot be heard. Exercise increases the intensity of both the aortic and pulmonic components of S2, whereas deep inspiration increases the intensity of the pulmonic component only.

Physiological split During inspiration, the chest wall expands and causes the intrathoracic pressure to become more negative (akin to a vacuum). The increased negative pressure allows the lungs to fill with air and expand. While doing so, it also induces an increase in venous blood return from the body into the right atrium via the superior and inferior venae cavae, and into the right ventricle by increasing the pressure gradient (blood is being pulled by the vacuum from the body and towards the right side of the heart). Simultaneously, there is a reduction in blood volume returning from the lungs into the left atrium (the blood wants to stay in the lungs because of the vacuum surrounding the lungs, and PVR is lower because of lung expansion). Since there is an increase in blood volume in the right ventricle during inspiration, the pulmonary valve (P2 component of S2) stays open longer during ventricular systole due to an increase in ventricular emptying time, whereas the aortic valve (A2 component of S2) closes slightly earlier due to a reduction in left ventricular volume and ventricular emptying time. Thus the P2 component of S2 is delayed relative to that of the A2 component. This delay in P2 versus A2 is heard as a slight broadening or even "splitting" of the second heart sound; though it is usually only heard in the pulmonic area of the chest because the P2 is soft and not heard in other areas. During expiration, the chest wall collapses and decreases the negative intrathoracic pressure (compared to inspiration). Therefore, there is no longer an increase in blood return to the right ventricle versus the left ventricle and the right ventricle volume is no longer increased. This allows the pulmonary valve to close earlier such that it overlaps the closing of the aortic valve, and the split is no longer heard. It is physiologically normal to hear a "splitting" of the second heart tone in younger people, during inspiration and in the "pulmonary area", i.e. the second ICS (intercostal space) at the left edge of the sternum.

Steps Chest wall expands during inspiration Intrathoracic pressure becomes more negative to form a vacuum Venous return from the body to the right heart increases, venous return from the lungs to the left heart decreases

Analysis of pressure According to Harrison's Principles of Internal Medicine, "Normally, blood pressure falls during inspiration (equal or less than 10 mmHg), due to an increase in blood flow into the right ventricle with displacement of the interventricular septum to the left, decreasing left ventricular filling and cardiac output". The pressure in the right ventricle tries to open the pulmonary valve. The pressure in the pulmonary artery tries to close the pulmonary valve. The higher pressure will "win". Hence, the closure of the pulmonary valve (P2) will be delayed since the pressure in the right ventricle is increased in inspiration, opposing the pressure in the pulmonary artery and keeping it open longer than in expiration. The change in A2 is not that evident. Thus P2 appears after A2 in inspiration.

Pathological split The different types of split S2 can be associated with medical conditions:

Split during inspiration: normal. (See above) Wide splitting: seen in conditions that delay RV emptying (pulmonic stenosis, right bundle branch block). Delay in RV emptying causes delayed pulmonic sound (regardless of breath); it is an exaggeration of normal splitting sounds. Split during expiration: Reverse splitting (paradoxical splitting) indicates pathology due to delay of aortic valve closing. Aortic stenosis, hypertrophic cardiomyopathy, left bundle branch block (LBBB), and a ventricular pacemaker could all cause a reverse splitting of the second heart sound. Split during both inspiration and expiration: If splitting does not vary with inspiration, it is termed a "fixed split S2" and is usually due to a septal defect, such as an atrial septal defect (ASD). The ASD creates a left to right shunt that increases the blood flow to the right side of the heart, thereby causing the pulmonary valve to close later than the aortic valve independent of inspiration/expiration. A bundle branch block either LBBB or RBBB, (although RBBB is known to be associated only with S1 split), will produce continuous splitting but the degree of splitting will still vary with respiration. When the pulmonary valve closes before the aortic valve, this is known as a "paradoxically split S2". On physical exam, paradoxical splitting is appreciated as increased splitting on expiration relative to inspiration, versus normal splitting where inspiration will increase splitting. It is seen in conditions that delay left ventricular emptying (e.g., aortic stenosis, left bundle branch block).

References

Illustrations

Worked examples

Example 1 — a first encounter with Split S2

Start with the simplest possible case. Write down what Split S2 claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Split S2 before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Split S2 ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Split S2

In research: Split S2 appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Split S2 in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Split S2 is common in secondary-school and first-year university syllabi. It links to neighbouring topics Cardiology, so understanding it makes those chapters shorter.
In everyday life: Look for Split S2 outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Split S2 in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Split S2 means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Split S2 out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Split S2 in simple terms?

A split S2 is a finding upon auscultation of the S2 heart sound. It is caused when the closure of the aortic valve (A2) and the closure of the pulmonary valve (P2) are not synchronized during inspiration.

Why does Split S2 matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Split S2?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Split S2.