In case of VSD, do we have S2 wide splitting?

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ventricular septal defect S2 splitting auscultation

Usually no. In an uncomplicated VSD, S2 is generally normally split.

Small VSD: normal S2 split.
Moderate VSD with increased pulmonary flow: S2 may be narrowly split, often with a louder P2.
Pulmonary hypertension/Eisenmenger physiology: P2 becomes loud and S2 splitting may become very narrow or single.

A wide, fixed split S2 is classic for an ASD, not a VSD. MSD VSD findings

Why don't we have a two-wide split in case of VSD, as there is also blood flowing from left ventricle to right ventricle is present, right? So more amount of blood is entering into l- l- right ventricle. So a pulmonary valve has to be closed lately, whereas in l- left ventricle, the aortic valve has to close early, as it is having less amount of blood. Why isn't there?

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VSD why S2 not wide split left-to-right shunt ventricular systole physiology

Your reasoning is intuitive, but the key correction is:

Valve-closure timing is not determined simply by how much blood passes through a ventricle. It depends mainly on the timing of ventricular pressure fall relative to the great-artery pressure, plus the duration of ventricular ejection and pulmonary vascular “hangout” time.

Why VSD does not cause wide, fixed S2

In a VSD, the left-to-right shunt occurs mainly during ventricular systole because LV pressure exceeds RV pressure then.

So the extra blood:

LV → VSD → RV outflow/pulmonary artery during systole

It does not first enter the RV during diastole and substantially increase RV end-diastolic filling in the way it does with an ASD.

Thus, it does not reliably prolong the RV's mechanical systole enough to produce a broad delay in P2. The shunted blood is effectively added to pulmonary flow during ejection, not added as sustained RV diastolic preload.

Also, in a large nonrestrictive VSD, LV and RV systolic pressures become near-equal, so the two ventricles behave more like coupled chambers during systole. That does not produce the marked difference in ejection timing required for a widely split S2.

What happens to A2?

You are right that LV can partly eject through the low-resistance VSD into the pulmonary circulation. This can shorten effective LV ejection and tend to make A2 earlier.

But this effect alone does not necessarily produce an obviously wide S2, because P2 is not necessarily markedly delayed. In a moderate VSD, the usual finding is a narrowly split S2 with a relatively loud P2, rather than wide splitting. MSD's VSD examination summary describes normal splitting in small VSDs and narrow splitting with an accentuated P2 in moderate defects.

Contrast with ASD

In an ASD, the left-to-right shunt occurs mainly in diastole:

LA → RA → RV

Therefore, the RV is chronically volume-loaded before every systole. It ejects a persistently increased stroke volume through the pulmonary valve, contributing to delayed P2. Respiratory changes are also buffered by changing atrial shunting, so RV output changes little between inspiration and expiration. That makes the split both wide and fixed.

Defect	Main time of shunt	Primary volume-loaded chamber	Typical S2
ASD	Diastole	RV	Wide, fixed split
VSD	Systole	Pulmonary circulation, then LA/LV after recirculation	Usually normal or narrowly split; P2 may be loud

A useful exam rule:

ASD causes wide fixed splitting because it causes continuous RV diastolic volume overload.
VSD shunting is systolic, so it does not create the same persistent RV preload and delayed P2.

Wide S2 splitting generally reflects abnormal RV ejection timing, such as RBBB or RV outflow obstruction, rather than increased flow alone, as summarized in Fuster and Hurst's The Heart section on abnormal S2 splitting.

Why in VSD there is no left to right shunt in diastole?

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ventricular septal defect left to right shunt occurs systole not diastole ventricular pressure explanation

There can be a small left-to-right flow in diastole across a VSD. My earlier wording, “no diastolic shunt,” was too absolute.

The important point is that it is usually tiny and low velocity, so it has little hemodynamic effect.

Pressure difference drives shunt flow

Flow across a VSD depends on the instantaneous pressure gradient:

[ \text{Shunt flow} \propto P_{LV} - P_{RV} ]

Phase	LV pressure	RV pressure	LV to RV gradient	Result
Systole	about 120 mmHg	about 25 mmHg	Large	Large, fast LV-to-RV shunt
Diastole	low	low	Very small	Minimal, slow L-to-R flow

During diastole, both ventricles are relaxed and filling from their atria through open AV valves. LV diastolic pressure may be only slightly higher than RV diastolic pressure, so a small low-velocity LV-to-RV flow can occur. Doppler echocardiography may show it. Textbook of Clinical Echocardiography specifically notes that slight LV-over-RV diastolic pressure can produce low-velocity left-to-right flow in diastole.

But this is not comparable to the large systolic shunt because the systolic LV-RV pressure gradient is enormous. That is why a VSD produces a harsh holosystolic murmur, not a diastolic murmur.

Why this does not significantly preload the RV

Even if a little blood passes from LV to RV during diastole, it is usually too small to create the persistent RV diastolic volume loading seen in ASD.

In ASD, blood enters the RV pathway during every diastole:

[ LA \rightarrow RA \rightarrow RV ]

This creates significant RV filling before its next contraction.

In VSD, the clinically important excess pulmonary flow is created primarily in systole:

[ LV \rightarrow VSD \rightarrow RV/PA ]

The resulting extra blood goes through lungs, returns to the LA, then to the LV. Therefore, the main chronic volume overload in a significant VSD is more prominent in the LA and LV. The RV may face pressure/flow effects depending on VSD size and pulmonary vascular resistance, but it does not get the characteristic sustained diastolic preload pattern of ASD.

So the precise statement is:

VSD may have a small diastolic LV-to-RV flow, but the large, clinically important shunt is systolic because that is when the LV-RV pressure gradient is large.

That is why VSD does not give the typical wide fixed split S2 of ASD. Echocardiographic shunt physiology and VSD clinical findings are consistent with this distinction.

In case of congenital cyanotic heart diseases, what do you mean by increased or decreased blood flow? What do you want to do with this?

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cyanotic congenital heart disease increased decreased pulmonary blood flow classification radiograph

In this classification, “blood flow” means pulmonary blood flow (Qp):

The amount of blood reaching the lungs through the pulmonary arteries for oxygenation.

It does not mean the total blood flow to the body.

Why do we care?

In cyanotic congenital heart disease, deoxygenated blood reaches the systemic circulation. The amount of blood that can get to the lungs to pick up oxygen strongly affects:

How severe the cyanosis is
Chest X-ray pulmonary vascular markings
Whether the child tends toward cyanotic spells versus heart failure
The likely diagnosis and immediate treatment

1. Decreased pulmonary blood flow

There is obstruction to blood reaching the lungs, usually at the pulmonary valve or RV outflow tract.

Example: Tetralogy of Fallot

[ RV \rightarrow \text{pulmonary stenosis/RVOT obstruction} \rightarrow \downarrow \text{blood to lungs} ]

Blood cannot easily go to the lungs, so more deoxygenated blood crosses right-to-left into systemic circulation.

Consequences

More marked cyanosis
Often cyanotic spells in TOF
Chest X-ray: oligemic lung fields, meaning reduced pulmonary vascular markings or “dark” lungs
Usually no pulmonary overcirculation or early heart failure

Examples: TOF, pulmonary atresia, severe pulmonary stenosis, tricuspid atresia with pulmonary stenosis.

2. Increased pulmonary blood flow

A large volume of blood is reaching the lungs, but cyanosis occurs because of mixing of oxygenated and deoxygenated blood, or because circulation is arranged abnormally.

Example: transposition of great arteries with a VSD

A lot of blood may flow through the pulmonary circulation, but the systemic circulation can still receive mixed, insufficiently oxygenated blood.

Consequences

Cyanosis may be less dramatic than in low-pulmonary-flow lesions, though it can still be dangerous
Chest X-ray: plethoric lungs, meaning prominent pulmonary vascular markings
Pulmonary congestion and heart failure are more likely because the lungs receive too much flow

Examples: TGA with VSD, truncus arteriosus, unobstructed TAPVC, single ventricle without pulmonary stenosis.

High-yield contrast

Feature	Decreased pulmonary blood flow	Increased pulmonary blood flow
Blood reaching lungs	Too little	Too much
Lung X-ray	Oligemia, dark lung fields	Plethora, prominent vascular markings
Cyanosis	Often marked	Often milder, but variable
Heart failure	Less common initially	Commoner
Typical example	TOF	TGA with VSD / truncus

So, saying “cyanotic CHD with decreased pulmonary blood flow” means:

The baby is blue partly because too little blood is being sent to the lungs to become oxygenated.

Saying “cyanotic CHD with increased pulmonary blood flow” means:

Plenty of blood reaches the lungs, but because of abnormal mixing or abnormal connections, the blood delivered to the body is still inadequately oxygenated.

This classification is especially useful when reading a chest X-ray and narrowing the differential diagnosis. Radiographic classification of cyanotic CHD describes the increased versus decreased pulmonary vascularity approach; StatPearls' cyanotic heart disease review notes that chest radiography helps determine whether pulmonary blood flow is increased, normal, or decreased.

How can we differentiate the lesions of increased blood flow in cynotic congential heart disease

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cyanotic congenital heart disease increased pulmonary blood flow differentiate TGA TAPVC truncus single ventricle chest x ray

Differentiate them in steps. First identify a cyanotic child with pulmonary plethora on chest X-ray:

Prominent hilar vessels extending to lung periphery
Often cardiomegaly
Sometimes pulmonary edema or heart-failure signs

Then use age at presentation, severity of cyanosis, cardiac silhouette, murmur, ECG, and echocardiography.

High-yield approach

Lesion with increased pulmonary blood flow	Typical clue	Chest X-ray	Other useful clue
TGA	Severe cyanosis in first hours to days of life, often little murmur	Narrow superior mediastinum with oval cardiac shadow: “egg on a string”; pulmonary plethora, especially if VSD/PDA	Usually RVH in neonates. Urgent echo confirms ventriculoarterial discordance.
TAPVC, unobstructed	Cyanosis plus heart failure, usually after a few weeks	Pulmonary plethora and cardiomegaly; “snowman/figure-of-8” occurs in supracardiac TAPVC, usually later, not in a newborn	Fixed split S2 and flow murmur may occur. If obstructed, there is severe respiratory distress and pulmonary edema, and the classic snowman is absent.
Truncus arteriosus	Mild cyanosis with early, prominent heart failure	Cardiomegaly with pulmonary plethora; a right aortic arch supports the diagnosis	Single loud S2, bounding pulses/wide pulse pressure, ejection click or truncal regurgitation murmur.
Single ventricle, unrestricted pulmonary flow	Mild cyanosis plus heart failure	Cardiomegaly and pulmonary plethora	Anatomy is variable. Echo defines the single-ventricle anatomy and outflow obstruction, if any.
DORV without pulmonary stenosis	Cyanosis severity depends on VSD position and physiology	Usually pulmonary plethora, often cardiomegaly	May resemble TGA physiologically. Echo is required to define the relation of great arteries and VSD.
Tricuspid atresia without pulmonary stenosis	Cyanosis with variable heart failure	Pulmonary plethora if there is no PS	ECG clue: left-axis deviation is classic.
HLHS or other left-sided obstructive lesions	Shock, poor pulses, acidosis as ductus closes, sometimes differential saturation	Often cardiomegaly and pulmonary venous congestion	Weak lower-body pulses and systemic hypoperfusion are more striking than cyanosis.

Fast bedside distinctions

1. Very early, intense cyanosis with relatively quiet chest

Think TGA.

A baby may look very blue but not have major respiratory distress initially.
The murmur may be absent or soft.
CXR may show “egg on a string,” although this is not always present early.

2. Cyanosis plus marked respiratory distress and pulmonary edema

Think obstructed TAPVC.

This can resemble severe lung disease or sepsis.
The child is very sick, tachypneic, and hypoxemic.
Unlike unobstructed TAPVC, pulmonary venous obstruction causes pulmonary edema rather than simple pulmonary plethora.

3. Mild cyanosis + bounding pulses + early heart failure

Think truncus arteriosus.

One arterial trunk supplies systemic and pulmonary circulations, so pulmonary flow is excessive as pulmonary vascular resistance falls after birth.
A single loud S2 is a useful clue.

4. Cyanosis + heart failure, but no characteristic silhouette

Think single ventricle or DORV. These cannot be reliably distinguished from each other by examination or CXR alone. Echocardiography is essential.

A practical examination mnemonic

For cyanotic CHD with increased pulmonary blood flow, remember:

T-T-T-S-D
TGA
TAPVC
Truncus arteriosus
Single ventricle
DORV

Add: tricuspid atresia without pulmonary stenosis can also have increased pulmonary flow.

Important limitation

“Increased pulmonary flow” is not a fixed label for every patient with a given defect:

TGA with pulmonary stenosis may have decreased pulmonary flow.
TAPVC with venous obstruction produces pulmonary edema and may not look simply plethoric.
Single ventricle, DORV, and tricuspid atresia may have either increased or decreased pulmonary flow depending mainly on pulmonary stenosis/outflow obstruction.

So use this as a CXR-based physiologic classification, then confirm the anatomy by echocardiography with Doppler, which determines connections, shunt direction, and outflow obstruction. StatPearls' overview and this radiology review summarize these X-ray patterns and their role in narrowing the diagnosis.

photo-1789712069922.jpg

Explain this

analyse_media

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cyanotic CHD increased pulmonary blood flow TGA TAPVC chest xray egg on string snowman

Your notes are describing cyanotic congenital heart diseases with increased pulmonary blood flow (increased PBF or Qp).

Read the note as this flowchart

[ \text{Cyanotic CHD} \rightarrow \uparrow \text{PBF} \rightarrow \text{mixing/admixture lesions} ]

When a lot of blood goes to the lungs but the child is still cyanosed, the problem is usually mixing of oxygenated and deoxygenated blood or abnormal parallel circulations.

Because pulmonary blood flow is high:

Chest X-ray shows increased pulmonary vascular markings or pulmonary plethora
The infant may develop congestive cardiac failure (CCF/CHF): tachypnea, poor feeding, sweating during feeds, hepatomegaly, poor weight gain
Cyanosis is often less deep than in lesions with decreased PBF, but this is not always true

What “vascular marking/plethora” means

Normally, pulmonary arteries are seen near the hila and gradually taper toward the lung periphery.

With increased PBF:

Pulmonary arteries are enlarged
Vessels are more visible even in outer lung fields
Lungs look “busy” or plethoric

This is not lung fluid by itself. It means excess blood is being sent through the pulmonary circulation. If severe, that excess flow can then cause pulmonary venous congestion and edema.

The two branches shown in the note

The note says “different based on associated lesion.” That is a good way to remember the anatomy, but write it more precisely as follows.

1. TAPVC with an ASD/PFO for obligatory mixing

TAPVC

Total anomalous pulmonary venous connection/return means pulmonary veins do not drain into the left atrium. Instead, they drain into the systemic venous circulation, usually the right atrium via an abnormal route.

Thus, oxygenated blood coming from the lungs returns to the right side of the heart and mixes with deoxygenated systemic venous blood.

For blood to reach the left heart and body, it must cross an ASD or patent foramen ovale (PFO):

[ \text{Pulmonary veins} \rightarrow \text{right atrium} \rightarrow \boxed{\text{ASD/PFO}} \rightarrow \text{left atrium} \rightarrow \text{aorta} ]

So, the note’s:

“ASD ↓ TAPVC”

should be understood as:

TAPVC requires an atrial-level communication, ASD/PFO, for survival.

It does not mean an ASD causes TAPVC.

Chest X-ray

With unobstructed supracardiac TAPVC, there may be:

Cardiomegaly
Increased pulmonary vascular markings
The classic snowman sign or figure-of-8 sign

But this sign generally appears only after several months, not in a newborn. It is caused by enlarged vertical vein, innominate vein, and SVC forming the upper part of the “snowman.” The textbook library notes that the snowman appearance is rarely visible before about 4 months.

Important exception

If TAPVC is obstructed, the baby presents very early with severe cyanosis and respiratory distress. There is pulmonary edema, and the snowman sign is usually absent.

2. TGA with a VSD, ASD/PFO, or PDA for mixing

TGA

In D-transposition of the great arteries, the great vessels are switched:

[ RV \rightarrow aorta ] [ LV \rightarrow pulmonary artery ]

This produces two parallel circulations:

Deoxygenated systemic venous blood returns to RA → RV → aorta → body
Oxygenated pulmonary venous blood returns to LA → LV → pulmonary artery → lungs

Without communication between the two circuits, the condition is incompatible with life.

Mixing can occur through:

ASD/PFO
VSD
PDA

So the note’s:

“VSD ↓ TGA”

means:

A VSD is one route for mixing in TGA, and TGA with VSD often has increased pulmonary flow.

It does not mean every TGA has a VSD, or that VSD causes TGA.

Why TGA with VSD has increased PBF

The VSD permits significant mixing and often permits a large volume of blood to enter the pulmonary circulation. As pulmonary vascular resistance falls after birth, pulmonary flow increases further, leading to heart failure.

Chest X-ray

Typical classic description:

Egg-on-a-string or egg-on-side
Narrow superior mediastinum, the “string”
Increased pulmonary vascular markings

However, the CXR can initially be non-classic in a newborn. Echocardiography confirms the diagnosis.

Corrected one-page summary

Disease	Why cyanosis occurs	Why PBF is increased	Necessary/associated communication	Classic X-ray
TAPVC, unobstructed	Oxygenated pulmonary venous blood mixes with systemic venous blood on right side	All pulmonary venous return recirculates through right heart and lungs	ASD/PFO required for systemic output	Pulmonary plethora, cardiomegaly; snowman sign in supracardiac type later in infancy
TGA with VSD	Parallel circulations, with mixing of oxygenated and deoxygenated blood	VSD allows increased flow to pulmonary circuit	VSD, ASD/PFO, or PDA allows mixing	Egg-on-a-string, pulmonary plethora
Truncus arteriosus	One common arterial trunk receives mixed blood	Pulmonary arteries arise from common trunk without protective pulmonary stenosis	Usually associated VSD	Cardiomegaly, pulmonary plethora, often right aortic arch
Single ventricle/DORV without PS	Complete or substantial mixing	No significant obstruction to pulmonary outflow	Anatomy varies	Pulmonary plethora, often cardiomegaly

One correction to the handwritten sentence

The line:

“Due to mixing they have mild cyanosis”

is often but not universally correct.

Unobstructed TAPVC and truncus arteriosus may have mild to moderate cyanosis initially, with early CHF.
TGA can cause profound cyanosis from birth, especially if there is poor mixing through ASD, VSD, or PDA.
The degree of cyanosis depends on how much mixing occurs, pulmonary blood flow, and pulmonary venous obstruction.

The core memory point is:

Cyanosis + pulmonary plethora = think mixing lesion.
The main exam examples are TGA, unobstructed TAPVC, truncus arteriosus, single ventricle, and DORV without pulmonary stenosis.

The characteristic TGA and TAPVC radiographic patterns are supported in Tintinalli’s Emergency Medicine: TGA has an egg-shaped heart with narrow mediastinum and increased pulmonary markings, whereas TAPVC has cardiomegaly, increased markings, and the snowman sign.

What is mean by overriding of iota?

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Quick answer

You mean overriding of the aorta.

Normally, the aorta arises completely from the left ventricle (LV):

[ LV \rightarrow Aorta ]

In overriding aorta, the aorta is shifted rightward and sits over the ventricular septal defect (VSD). Therefore, it receives blood from both ventricles:

[ LV + RV \rightarrow \text{overriding aorta} ]

In Tetralogy of Fallot, because of pulmonary stenosis, right ventricular pressure rises. Deoxygenated blood from the RV can then cross the VSD and enter the aorta directly. This causes systemic desaturation and cyanosis.

Think of it as:

The aorta is “straddling” the VSD instead of being connected only to the LV.

The amount of overriding varies:

Mild overriding: aorta receives mostly LV blood.
Marked overriding: aorta receives substantial RV blood.
If the aorta arises almost entirely from the RV, it may be classified as double-outlet right ventricle (DORV).

, Overriding of Aorta.

Overriding of the aorta means the aorta is positioned partly over a VSD, rather than arising entirely from the left ventricle.

Normal:

[ LV \rightarrow Aorta ]

In Tetralogy of Fallot:

[ LV + RV \rightarrow \text{Aorta} ]

Because the aorta “straddles” the VSD, it can receive:

oxygenated blood from the LV, and
deoxygenated blood from the RV.

This mixing contributes to cyanosis, especially when pulmonary stenosis makes it difficult for RV blood to reach the lungs.

Pathology of tof

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anatomical diagram Tetralogy of Fallot VSD overriding aorta pulmonary stenosis right ventricular hypertrophy

This composite educational graphic illustrates the pathophysiology and diagnostic imaging of Tetralogy of Fallot (TOF). Panel A is an anatomical diagram depicting the four hallmark features of TOF: a large ventricular septal defect (VSD), pulmonary stenosis (subvalvular and valvular), right ventricular hypertrophy with a thickened muscular wall, and an overriding aorta that sits above the septal defect, receiving blood from both ventricles. Panel B is a diagnostic 2D echocardiogram in a parasternal long-axis view. It confirms the clinical findings illustrated in Panel A, showing the right ventricle (RV) situated superiorly and the left ventricle (LV) inferiorly. A prominent white arrow points to the large malaligned VSD. The aorta (Ao) is clearly visualized overriding the interventricular septum, demonstrating its communication with both the LV and RV. The left atrium (LA) is seen posterior to the LV. The image serves as a clinical correlation between conceptual cardiac defects and their real-time ultrasound presentation.

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Tetralogy of Fallot pathology embryology anterior deviation infundibular septum authoritative

Tetralogy of Fallot: pathology and pathophysiology

TOF is best understood as one primary developmental defect that produces the classic four findings.

Primary embryological defect

There is anterior and superior deviation of the conal (infundibular/outlet) septum.

This displaced septum causes:

Narrowing of the right ventricular outflow tract (RVOT)
Failure of normal ventricular septum alignment, producing a large malalignment VSD
The aorta shifts rightward and lies over the VSD, causing an overriding aorta

The fourth feature, RV hypertrophy, develops later because the RV pumps against RVOT obstruction.

The four components

1. Pulmonary stenosis / RV outflow tract obstruction

Usually obstruction is subvalvular (infundibular), from narrowing and hypertrophied muscle below the pulmonary valve. It may also be valvular, supravalvular, or involve pulmonary artery branches.

This is the most important determinant of severity.

[ \text{More RVOT obstruction} \Rightarrow \text{more RV} \rightarrow \text{LV shunting} \Rightarrow \text{more cyanosis} ]

2. Large VSD

Usually a large, nonrestrictive malalignment VSD.

Because it is large, RV and LV systolic pressures are nearly equal. Blood chooses the outlet with lower resistance.

If RVOT obstruction is mild: blood can go into pulmonary artery, often left-to-right shunt, little or no cyanosis. This is called pink TOF.
If RVOT obstruction is severe: blood is diverted through VSD into the aorta, right-to-left shunt, causing cyanosis.

3. Overriding aorta

The aorta “straddles” the VSD and receives blood from both ventricles:

[ LV + RV \rightarrow \text{overriding aorta} ]

Therefore, when RV blood is deoxygenated, it can enter the systemic circulation and cause cyanosis.

4. Right ventricular hypertrophy

The RV muscle becomes thick because it must pump against the RVOT obstruction and often against near-systemic RV pressure transmitted through the large VSD.

This produces the upturned cardiac apex on chest X-ray, contributing to the classic boot-shaped heart.

Central mechanism of cyanosis

[ \text{RVOT obstruction} \uparrow \Rightarrow \text{resistance to pulmonary flow} \uparrow \Rightarrow \text{RV pressure} \uparrow \Rightarrow \text{right-to-left flow through VSD} \uparrow \Rightarrow \text{deoxygenated blood enters aorta} \Rightarrow \text{cyanosis} ]

Thus, the VSD itself does not decide cyanosis. The degree of pulmonary stenosis/RVOT obstruction does.

Why “tet spells” occur

A hypercyanotic spell is an acute increase in right-to-left shunting, often triggered by crying, feeding, fever, or defecation.

Mechanisms include:

Sudden infundibular muscle spasm, worsening RVOT obstruction
Fall in systemic vascular resistance

Both direct more RV blood through the VSD into the aorta instead of the lungs.

Squatting raises systemic vascular resistance, reducing right-to-left shunting and improving cyanosis.

Associations

TOF is a conotruncal anomaly and can be associated with:

22q11.2 deletion syndrome (DiGeorge syndrome)
Right-sided aortic arch
Coronary artery anomalies
Pulmonary atresia in severe forms

The textbook description in Harrison’s Principles of Internal Medicine identifies anterior deviation of the conal septum as the structural basis for RVOT obstruction, VSD, overriding aorta, and RV hypertrophy.

Reason for ejection systolic murmur in tof

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Tetralogy of Fallot ejection systolic murmur RV outflow obstruction explanation

In TOF, the ejection systolic murmur is mainly due to right ventricular outflow tract obstruction, usually infundibular pulmonary stenosis with or without pulmonary valve stenosis.

During systole:

[ RV \rightarrow \text{narrow RVOT/pulmonary valve} \rightarrow PA ]

Blood is forced at high velocity through this narrowed passage, creating turbulence. Therefore the murmur is:

Systolic ejection, because flow across the RVOT occurs during ventricular ejection
Harsh, crescendo-decrescendo
Best heard at the left upper or mid-sternal border
May have a systolic thrill

Is the murmur from the VSD?

Usually, no.

The VSD in classic TOF is large and nonrestrictive. RV and LV pressures are almost equal, so there is little pressure gradient across the VSD and hence little turbulence. Thus, the VSD is usually “silent.”

In TOF, the murmur is generally from pulmonary stenosis/RVOT obstruction, not from the VSD.

Important clinical correlation

As RVOT obstruction becomes very severe:

[ \text{Pulmonary blood flow} \downarrow \Rightarrow \text{turbulent flow across RVOT} \downarrow \Rightarrow \text{murmur becomes softer or shorter} ]

At the same time, right-to-left shunting through the VSD increases, so cyanosis worsens.

Therefore:

A very cyanotic TOF child with a soft or disappearing murmur is more severe, not better.

This is seen during a hypercyanotic or “tet” spell. MSD Manual’s TOF description states that the murmur comes from pulmonary stenosis and becomes softer as obstruction becomes more severe.

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