Tetralogy of Fallot (TOF)

Tetralogy of Fallot (TOF), first separated from other causes of “maladie bleue” by Étienne-Louis Arthur Fallot in 1888, is one of the more common cyanotic congenital heart defects. Its anatomic spectrum runs remarkably wide — from hearts with only minimal aortic override and mild pulmonary stenosis, to the most severe end, which effectively constitutes pulmonary atresia with VSD. TOF belongs, along with double-outlet right ventricle, transposition of the great arteries, and truncus arteriosus, to the broader family of neural-crest-derived conotruncal defects — a shared developmental origin worth keeping in mind when these lesions cluster together.

One Anatomic Mechanism, Not Four Independent Features

TOF is conventionally taught as four separate findings — VSD, overriding aorta, RVOT obstruction, and RV hypertrophy — but this framing obscures the actual anatomy. The diagnostic hallmark is a single finding: antero-cephalad deviation of the outlet septum relative to the rest of the muscular ventricular septum. Everything else follows from this one deviation:

• The deviated outlet septum is malaligned relative to the rest of the ventricular septum — this malalignment is itself what creates the VSD, and is a general feature of any VSD opening into the right ventricular outlet, with or without TOF.
• Because the outlet septum is malaligned, the aortic valve — rather than sitting entirely over the LV — overrides the ventricular septum, attached in part to right ventricular structures.
• The same deviated outlet septum, positioned antero-cephalad, directly narrows the subpulmonary outflow tract, and hypertrophied septoparietal trabeculations lining the infundibulum add to this obstruction.
• Chronic pressure overload from that obstruction is what secondarily produces RV hypertrophy.

Seeing TOF this way — one primary lesion with three downstream consequences — makes the anatomic variability across individual hearts much easier to understand than treating all four as independently variable findings.

The VSD

Nearly always large and non-restrictive (rare exceptions occur when tricuspid valve tissue partially occludes it). In roughly four-fifths of cases, the defect is perimembranous — its postero-inferior border formed by fibrous continuity between the aortic, mitral, and tricuspid valve leaflets, sometimes duplicated into a “membranous flap.” In the remaining fifth, muscle separates the aortic and tricuspid valves at that same border (from fusion of the ventriculo-infundibular fold with the septomarginal trabeculation’s posterior limb) — this muscular separation also happens to protect the conduction tissue in that subset. In some populations (more frequently described in patients from East Asia and South America), the outlet septum is completely absent and the VSD is doubly committed and juxta-arterial instead — many classify this variant within the TOF diagnostic category as well.

Aortic Override Is a Spectrum, Not a Fixed Feature

The degree of rightward aortic deviation varies continuously: at one end, override is minimal and the aorta remains mostly connected to the LV (a concordant ventriculoarterial connection); at the other, the aortic valve attaches almost entirely within the right ventricle, which is by definition a double-outlet right ventricle. TOF and DORV aren’t categorically distinct diagnoses so much as points on the same anatomic continuum.

Multilevel RVOT Obstruction

Obstruction in TOF is often multilevel, and recognizing each contributing level matters for surgical planning:

• Infundibular — the dominant mechanism, from the deviated outlet septum itself, often compounded by hypertrophied septoparietal trabeculations. The infundibulum is more often elongated than normal, though occasional hearts have a hypoplastic outlet septum and a correspondingly short infundibulum instead.
• Apical trabecular — hypertrophy of the septomarginal trabeculation’s apical portion can produce a “two-chambered right ventricle” variant, which can also occur independent of TOF’s specific infundibular anatomy.
• Valvar — from fusion and doming of the leaflets, or a stenotic two- or three-leaflet valve, layered on top of the infundibular narrowing.

Physiology and Cyanosis

The degree of cyanosis is determined primarily by the severity of RVOT obstruction and the resulting magnitude of right-to-left shunting at the VSD — presentation genuinely ranges from no cyanosis at all (“pink TOF”) to significant cyanosis that may require ductal patency to maintain adequate pulmonary blood flow, a ductal-dependent physiology in the most severe forms.

Hypercyanotic (“tet”) spells result from right-to-left shunting at the VSD, triggered by either dynamic infundibular obliteration (a spasm-like narrowing of the already-obstructed outflow tract) or acute shifts in pulmonary versus systemic vascular resistance. Squatting is the classic compensatory maneuver, raising systemic vascular resistance and reducing the right-to-left shunt.

Clinical Findings

• Auscultation — the murmur heard is from the pulmonary stenosis, not the VSD itself (a large, non-restrictive VSD with equalized ventricular pressures typically doesn’t generate a loud murmur on its own). A single S2 is typical, reflecting an inaudible or absent pulmonary closure component; an early ejection sound may be present, and a thrill is often palpable at the left sternal border.
• ECG — right ventricular hypertrophy and right axis deviation.
• Chest X-ray — the classic “boot-shaped” heart (RVH with an upturned apex), reduced pulmonary vascular markings from diminished pulmonary blood flow, and a right aortic arch in roughly a fifth of cases — a figure worth anchoring to precisely, since it’s sometimes cited inconsistently elsewhere.

Associated Anomalies

• Atrial septal defect (typically secundum-type) is seen in a striking 86% of cases — genuinely closer to the rule than the exception. See Ventricular Septal Defect for the isolated-lesion anatomy this builds on.
• Coronary artery anomalies — the single most important to identify preoperatively is the anterior interventricular artery (LAD) arising from the right coronary artery, coursing across the anterior wall of the infundibulum. Missing this anomaly risks transecting the vessel during infundibular muscle resection or transannular patch placement.
• Right aortic arch — roughly a fifth of cases, without functional significance on its own, though its branching pattern is worth documenting.
• Absent pulmonary valve syndrome — a specific variant usually associated with an absent arterial duct. The mechanism is genuinely instructive: with a normal, patent duct in fetal life, even a severely regurgitant, rudimentary pulmonary valve doesn’t cause pathologic branch pulmonary artery dilation, because the duct decompresses the circuit. Without a duct, massive to-and-fro flow through the incompetent valve in utero drives aneurysmal dilation of the branch pulmonary arteries instead — occasionally severe enough to compress the airways.
• Left superior vena cava to the coronary sinus in up to 11% of cases, and other systemic/pulmonary venous anomalies, more often when associated with heterotaxy syndrome.
• Non-confluent pulmonary artery origins, branch pulmonary artery stenosis, and aortic arch branching variations are all recognized, less common associations worth a deliberate look.

Echocardiographic Findings

A complete preoperative evaluation is genuinely extensive, and getting it organized systematically matters as much as covering each individual finding. A useful practical tip for neonates: they often sleep soundly after feeding, which can allow a comprehensive, detailed exam without sedation — worth taking advantage of, since sedation itself carries added risk in TOF with moderate stenosis (a hypercyanotic spell can be provoked).

Preoperative Assessment: Key Elements

Priority elements for a complete anatomic and functional exam, by 2D/3D imaging and color/spectral Doppler: cardiac position and presence or absence of the thymus (absence is itself worth noting, given the 22q11 association above); visceral and atrial situs and full segmental/conotruncal analysis; pulmonary and systemic venous connections and any ASD; AV valve morphology and function; VSD morphology, specifically differentiating malalignment, doubly committed subarterial (absent conal septum), and AV canal types, ruling out additional muscular VSDs, and assessing shunt restriction and direction; degree and morphology of RVOT obstruction (conal septal size/position, plus any additional muscular obstruction as in a double-chambered RV); pulmonary valve annular diameter and morphology; main and branch pulmonary artery size and flow, ruling out anomalous branch origin or course (such as a left pulmonary artery sling) and screening for branch hypoplasia, discrete stenosis, or discontinuity — especially important after ductal closure; subaortic and aortic valve morphology and function; the ductus arteriosus’s origin (from the aortic arch or a brachiocephalic artery), ruling out additional pulmonary blood flow sources such as aortopulmonary collaterals and distinguishing a true ductus from a MAPCA; aortic arch sidedness and branching, screening for vascular rings; and coronary artery origin, branching, and flow, specifically ruling out a coronary-to-pulmonary-artery fistula, anomalous LCA origin from the main pulmonary artery, and — critically — an anomalous LAD origin from the RCA (with or without a dual LAD, or other large branches crossing the RVOT) that could complicate transannular patch or conduit placement.

A Systematic, View-by-View Approach

• Subxiphoid views are the preferred starting window for organized, systematic imaging. The initial long-axis sweep demonstrates visceral and atrial situs, systemic venous connections, the coronary sinus, and atrial septal morphology; the descending aorta is seen in cross-section to the left of the spine and, through the sweep, can be seen crossing to the right of the thoracic spine — a specific normal landmark worth confirming. The VSD with aortic override, and subpulmonary infundibular hypoplasia or stenosis, are also seen here; the right pulmonary artery is often identified traversing left-to-right above the atrial mass, while the left pulmonary artery and ductus are usually better shown with color Doppler. The subxiphoid short-axis view defines the atrial septum, right upper pulmonary vein entry into the left atrium, VSD number/size/location, RV hypertrophy, and additional RV muscle bundles (relevant to double-chambered RV) — a cleft mitral valve or complete AV canal defect is also well seen in this plane. A modified right anterior oblique subxiphoid view is of exceptional benefit specifically in TOF: it highlights the subpulmonary infundibulum, including both the size of the conal septum and its displacement, and is also useful for identifying additional muscle bundles contributing to RVOT obstruction in the double-chambered RV variant.
• Apical views demonstrate the AV valves, the VSD, and the overriding aortic valve, and can help identify other associated anomalies such as left-sided obstructive lesions, as well as visualizing the mechanism behind an atypically restrictive VSD when present.
• Parasternal imaging may be genuinely limited if the thymus is absent — the thymus normally provides a reliable acoustic window, so its absence (itself a 22q11-associated finding) can directly hamper this part of the study. The parasternal long-axis view shows the overriding aorta and, critically, fibrous continuity between the aortic and mitral valves — the specific finding that distinguishes TOF from its related double-outlet RV variant. Tilting the transducer toward the left shoulder from this same view brings the pulmonary valve, main pulmonary artery, and proximal right pulmonary artery into view, allowing direct measurement of the pulmonary annulus and main pulmonary artery. The parasternal short-axis view further defines VSD morphology, subpulmonary obstruction, infundibular hypertrophy, and conal septal deviation — an absent conal septum with the VSD extending to a hypoplastic pulmonary valve annulus is specifically what defines a doubly committed subarterial defect in TOF.
• Ductal views are best obtained from a high parasternal window in the sagittal plane, or from the suprasternal notch long-axis view. Distinguishing a true ductus arteriosus from an aortopulmonary collateral can be genuinely difficult, since both can arise from the same location on the proximal descending thoracic aorta — the distinguishing feature is where the vessel terminates: a ductus arteriosus ends at the proximal branch pulmonary artery origin, while a collateral typically inserts more distally, at or beyond the hilum. This distinction has become more clinically important with the rise of ductal stenting as an initial palliation option, and imaging the exact site of ductal insertion also helps anticipate the risk of peripheral pulmonary artery stenosis or discontinuity after the duct eventually closes.
• “Crossed” pulmonary arteries are a specific, recognizable variant: diagnosed when the right pulmonary artery originates inferiorly from the left aspect of the main pulmonary artery, while the left pulmonary artery originates more superiorly from the right aspect.
• High left and right parasternal views are used to evaluate the main and branch pulmonary arteries, and it’s important to align the Doppler beam perpendicular to whichever branch is being interrogated by adjusting the window for each side separately — high left (subclavicular) windows generally work best for the main and left pulmonary arteries, and high right windows for the right pulmonary artery. A specific, quantified finding worth knowing: a branch pulmonary artery diameter Z-score below −2.5 had 88% sensitivity and 100% specificity for the presence of one or more MAPCAs in one study — larger branch pulmonary arteries make concurrent MAPCAs correspondingly less likely.
• Suprasternal notch views assess arch sidedness and branching pattern, a retroaortic innominate vein, partially anomalous pulmonary venous connection to a systemic vein, double aortic arch, and vascular rings, supplemented by the high parasternal views above. A left SVC connecting to the coronary sinus, or directly to the left atrium via an unroofed coronary sinus, is also best seen from this view or from a left parasternal sagittal window.

Doppler Assessment of RVOT Obstruction

The level of obstruction is best localized by sequential pulsed Doppler interrogation, starting from within the RV cavity and working toward the main pulmonary artery — rather than a single Doppler sample assumed to capture the dominant gradient. In severe stenosis, a lower-frequency transducer setting may be needed to demonstrate antegrade flow at all. Color Doppler across the atrial septum usually shows left-to-right shunting even with severe pulmonary stenosis, though bidirectional or right-to-left atrial shunting can be present in more severe cases — worth checking rather than assuming a fixed shunt direction.

Coronary Artery Imaging: A Deliberate, Targeted Sweep

See Coronary Arteries for the general approach to imaging coronary origins and courses, and for the pitfalls that apply to every coronary study. Screening for an anomalous LAD arising from the RCA and crossing the RVOT — the single most surgically consequential coronary finding — requires a careful, deliberate sweep from the standard parasternal long-axis reference view, angling toward the left shoulder, rather than a passing glance. High left parasternal transverse cuts and a modified apical four-chamber view angled anteriorly are additional helpful positions; prominent crossing conal branches or a dual LAD may be seen in these same sweeps, and are occasionally identifiable from the most coronal extent of the subxiphoid long-axis sweep as well. A rare single coronary artery, or an anomalous LCA origin from the pulmonary artery, can also be diagnosed this way — but it’s worth remembering that turbulent pulmonary flow itself can complicate or obscure this specific assessment, so coronary screening deserves its own deliberate attention rather than being folded into general RVOT imaging.

Transesophageal and 3D Echocardiography

TEE is useful for adults, poor transthoracic acoustic windows, or preoperative planning. The relevant views are midesophageal, not transgastric: the ME four-chamber view confirms the VSD and RV hypertrophy, and the ME right ventricular inflow-outflow and ME aortic valve short-axis views — not a transgastric view — are what characterize the pulmonary valve and RVOT obstruction, consistent with how TEE is used to assess the RVOT and pulmonary valve in other lesions. 3D echocardiography adds detailed spatial visualization of the VSD and aortic override relationship, valuable for surgical planning.

Treatment Considerations

• Primary surgical repair in infancy is now the standard approach — VSD patch closure (directing LV blood to the aorta) combined with relief of the RVOT obstruction, either by infundibular muscle resection with or without pulmonary valvotomy, or by placement of a right-ventricle-to-pulmonary-artery conduit.
• A transannular patch is used when the pulmonary annulus itself is significantly hypoplastic. It effectively relieves the obstruction but sacrifices valve competence entirely — this essentially guarantees free pulmonary regurgitation as a direct structural consequence of the repair, not an occasional complication of it. Many patients repaired this way eventually need pulmonary valve replacement, typically in their late teens or twenties; the precise timing for this remains genuinely debated.
• A Blalock-Taussig (or similar) shunt — historically the default first step for most patients — is now reserved for specific higher-risk scenarios (very small pulmonary arteries or annulus, prematurity, other major comorbidities) rather than being a routine bridge to later repair, now that primary infant repair is standard.
• Cyanotic spells are managed acutely with oxygen, a knee-chest position (the in-bed equivalent of squatting), and beta-blockade to relax the infundibular spasm.

Postoperative Follow-Up

Intraoperative TEE assessment of repair adequacy has its own specific view requirements: residual RVOT obstruction is best assessed from the transgastric longitudinal and midesophageal RVOT imaging planes, and pulmonary valve function and the right pulmonary artery from the appropriate midesophageal windows — though the left pulmonary artery is often genuinely difficult to visualize from standard TEE views, worth anticipating rather than assuming equally good visualization of both branches. RV systolic pressure can be estimated from Doppler interrogation of tricuspid regurgitation, the RVOT, or a residual VSD, but the interrogation angle must be carefully optimized, and estimates should be confirmed by direct intraoperative pressure measurement when there’s genuine uncertainty.

A structured postoperative echocardiographic assessment — intraoperative and at every subsequent follow-up study — should specifically cover:

1. RV dilation or dysfunction
2. Residual pulmonary outflow obstruction
3. Pulmonary regurgitation severity — see Pulmonary Regurgitation for the broader grading framework
4. Peripheral pulmonary artery stenosis — genuinely easy to miss on echo, and a specific cause of elevated RV systolic pressure that can otherwise be puzzling if this branch-level obstruction isn’t actively sought; also associated with increased risk of life-threatening arrhythmia and heart block
5. Aortic root size and aortic regurgitation
6. RV systolic pressure
7. Residual VSD shunt

Right-sided heart failure symptoms can develop with progressive RV dysfunction or significant tricuspid regurgitation, and atrial or ventricular arrhythmias are a recognized long-term concern requiring ongoing surveillance well beyond the immediate postoperative period.

Clinical Importance

TOF’s lifelong course is shaped as much by the specific repair technique used in infancy — particularly whether a transannular patch was needed — as by the original anatomy itself, which is why the postoperative surveillance checklist above matters as much as the original diagnostic workup. See Pulmonary Valve Stenosis for the isolated-lesion picture of the RVOT obstruction TOF combines with the other three features, and Right Ventricle Evaluation for how the resulting hypertrophy and, later, volume overload are assessed.