Discussion

The potential treatment issues and clinical implications of variations in tetralogy of Fallot arch anatomy are as follows:

1. 1. Management of vascular ring with trachea and oesophageal compression: Tetralogy of Fallot cases with double aortic arch typically require division of the non-dominant arch (Figure 1). Our series included eight such patients (0.3%). A right aortic arch with aberrant left subclavian artery with left ductus produces a vascular ring, potentially resulting in tracheal and oesophageal compression, and will also typically require division of the ductus (Figure 2).^{Reference Knight and Edwards17} Though the prevalence of right aortic arch in the general population is 0.1%, in tetralogy of Fallot it approaches 25%. Among these patients, 14% have a vascular ring producing anatomy (10.6% of right aortic arch cases in our series), potentially requiring division. A right aortic arch with retro-oesophageal aortic segment (circumflex aorta) also causes a vascular ring, but was not observed in our series.^{Reference Knight and Edwards17}

2. 2. Planning vascular access for interventional procedures: Common origin of the innominate artery and left common carotid artery or origin of left common carotid artery from the innominate artery is seen in humans (Liechty’s type-2, type-4, type-7, type-8, type-11, type-13, and type-14), which is sometimes referred to as bovine arch or bovine branching pattern, a misnomer.^{Reference Layton, Kallmes, Cloft, Lindell and Cox16} In cattle, a single great vessel originates from the aortic arch (as the aortic arch is at a fair distance from the thoracic inlet). This brachiocephalic trunk gives rise to bilateral subclavian arteries and a bi-carotid trunk, which then bifurcates into right and left common carotid arteries (true bovine arch). This anatomy has not been reported in humans. Common origin of the common carotid arteries (common carotid trunk) is a relative contraindication for a carotid approach for a ductal stent (for both direct surgical exposure and percutaneous approaches). Guide wire-induced dissection or post-procedure pseudoaneurysm could lead to neurological damage. Totally, 103 (3.8%) of our patients had a common carotid trunk, 100 of which were associated with left aortic arch, and only three with right aortic arch. The prevalence is significantly less than that reported in the normal population with common carotid trunk, which is 29.5%.^{Reference Liechty, Shields and Anson3}

3. 3. Suitability for ductal stenting: Ductal stenting is probably contraindicated in tetralogy of Fallot with right aortic arch with aberrant left subclavian artery and left ductus, as stenting this ductus may exaggerate the tracheo-oesophageal compression. It is better to avoid ductal stenting in tetralogy of Fallot with a double aortic arch for the same reason, though there are isolated case reports of ductal stenting in such lesions.^{Reference Tola, Ergul, Saygi, Ozyilmaz, Guzeltas and Odemis18}

4. 4. Avoiding subclavian steal: Type 3 right aortic arch (isolation of left subclavian artery in right aortic arch) is known to cause subclavian steal syndrome.^{Reference Shuford, Sybers and Edwards14,Reference Knight and Edwards17,Reference Antia and Ottesen19,Reference Maranhao, Gooch, Ablaza, Nakhjavan and Goldberg20} Type 3 right aortic arch can occur in 0.5% of tetralogy of Fallot patients (Figure 3). A modified Blalock Taussig shunt to this subclavian artery is probably contraindicated as it may not supply adequate flow and also could exaggerate the steal.

5. 5. Siting of monitoring lines for surgical procedures: In type 3 right aortic arch (isolation of left subclavian artery), monitoring line placements in the left radial artery during a surgical procedure will give erroneous readings.^{Reference Hastreiter, D’Cruz, Cantez, Namin and Licata7} We encountered 14 (0.5%) patients in our series with this anatomy. A subclavian artery arising as the first branch of the aortic arch does not cause a vascular ring, but there are implications when there is associated tetralogy of Fallot. Twelve (0.5%) of our cases had this type of anatomy. Out of these 12, 11 patients had right aortic arch with left subclavian artery as the first branch and one had left aortic arch with right subclavian artery as the first branch. If recognised preoperatively, the arterial line should preferably be placed in the contralateral radial or femoral arteries, because this vessel may be dissected, looped, and/or retracted during the operation.^{Reference Krishnan, Theodore, Kiran and Neelakandhan21}

6. 6. Management of cardioplegia delivery and myocardial protection: 0.45% of our cases had subclavian artery arising as the first branch of aortic arch. When it arises very proximally in the arch, this vessel should be dissected and looped, as mentioned. If the aortic cross-clamp is applied distally to its origin, before delivery of cardioplegic solution, the vessel should be snugged to prevent possible cardioplegia runoff and inadequate myocardial protection.^{Reference Krishnan, Theodore, Kiran and Neelakandhan21} And as stated above monitoring lines should not be placed on the ipsilateral radial artery.

7. 7. Considerations for trans-cervical cannulation for cardiopulmonary bypass and extra corporeal life support: A common origin of bilateral common carotid arteries (common carotid trunk) is a relative contraindication to trans-cervical veno-arterial extra corporeal life support. In neonates, the risk of neurological injury is similar to those with a normal branching pattern, but the incidence of neurological injury in infants and children is not known.^{Reference Lamers, Rowland, Seguin, Rosenberg and Reber22} One hundred and three of 2684 (3.8%) patients had a common origin of bilateral common carotid arteries in our series. We did not have a true bovine arch in our series.^{Reference Layton, Kallmes, Cloft, Lindell and Cox16} Emergency neck vessel cannulation might be required during redo sternotomy in infants and children for re-entry related cardiac laceration. In cases of common origin of bilateral common carotid arteries, soon after sternal re-entry the arterial (inflow) cannula should be relocated to the aorta.

8. 8. Avoidance of confusion during dissection: Knowledge of variants of aortic arch branches is required to avoid confusion during angiography and surgical dissection, especially for redo surgeries in which anatomical definition is difficult due to intra-pericardial adhesions.^{Reference Boechat, Gilsanz and Fellows23}

9. 9. Additional arch issues: Aorto pulmonary window and coarctation of aorta are known associations in patients with tetralogy and may require additional procedures for correction.^{Reference Perdreau, Houyel and Baruteau10,Reference Crawford, Watson and Joransen12,Reference Kothari, Rajani and Shrivastava13} We had one patient each with aorto-pulmonary window (Figure 4) and coarctation (Figure 5) with tetralogy of Fallot in our series. The importance of aorto-pulmonary window in tetralogy of Fallot is that a relatively high pulmonary blood flow may prevent obvious cyanosis, leading to delayed presentation, and severe pulmonary arterial hypertension. The post-operative course can be complicated, especially when there is need of a transannular patch.

10. 10. Systemic to pulmonary artery shunts: Knowledge of the aortic arch anatomy and branching pattern is of paramount importance in constructing a systemic to pulmonary artery shunt. Misinterpretation of branching patterns leading to complications has been reported.^{Reference Idhrees, Cherian, Menon, Mathew, Dharan and Jayakumar24} When a tetralogy of Fallot patient presents with refractory cyanotic spells and requires emergency surgery, the chances of this might increase.

    1. A. Classical Blalock-Taussig shunt: Ideally, the Blalock-Taussig shunt should be constructed on the side opposite the aortic arch, and the subclavian artery must be divided at its first branch to prevent kinking of subclavian artery origin.^{Reference de Leval, McKay, Jones, Stark and Macartney25,Reference Litwin and Fellows26} A classical Blalock-Taussig shunt is, however, rarely used in the current era.

    2. B. Modified Blalock-Taussig shunt: This shunt can be performed via either thorax and to either branch pulmonary artery.^{Reference de Leval, McKay, Jones, Stark and Macartney25} Factors other than arch anatomy may dictate the best approach (e.g., need for ligation of ductus, presence of azygous continuation of inferior vena cava, need for aorto pulmonary collateral ligation, etc.). Failure to recognise the isolated left subclavian artery (type 3 right aortic arch) in tetralogy of Fallot may result in an unsuccessful attempt to create a left Blalock-Taussig shunt.^{Reference Luetmer and Miller27} Failure to recognise an aberrant subclavian artery might result in construction of the shunt between carotid artery and the ipsilateral branch PA (Figure 2).

    3. C. In cases of tetralogy of Fallot with double aortic arch, in which total correction may not advisable (e.g. very early presentation with tracheobronchial compression due to complete vascular ring), then division of the vascular ring with systemic to pulmonary artery shunt could be performed dividing the non-dominant arch, anastomosing it to the pulmonary artery.^{Reference Waterston, Pohl, Kallfelz and Kreutzberg11,Reference Chowdhury, Venugopal and Kothari28}

    4. D. Modified systemic to pulmonary artery shunts are advised in variations of aortic arch anatomy (complex CHD with decreased pulmonary blood flow with a right- or left-sided aortic arch and right-sided descending thoracic aorta, double aortic arch). The palliation provided by these shunts has been satisfactory, with predictable growth of pulmonary arteries, insignificant distortion in the great majority, and easy take-down.^{Reference Chowdhury, Venugopal and Kothari28}

    5. E. There are case reports in which an aberrant right subclavian artery was used for the construction of a systemic to pulmonary artery shunt with good outcome.^{Reference Legault, Camilleri, Bailly, Brazzalotto, Lusson and de Riberolles29,Reference Yamaguchi, Obo, Oshima, Ohashi, Hosokawa and Tachibana30} A modified Blalock-Taussig shunt between the aberrant subclavian artery and branch pulmonary artery usually does not pose any problem in palliation of cyanosis. In our series, we had 153 cases of tetralogy of Fallot with aberrant subclavian artery (73 patients of right aortic arch with aberrant left subclavian artery and 80 patients of left aortic arch with aberrant right subclavian artery). Totally, 123 patients underwent intra cardiac repair (121 primary corrections, and two post-shunt repairs, with shunts done elsewhere, both inadvertently constructed between carotid artery and ipsilateral branch pulmonary artery). Thirty patients had systemic to pulmonary artery shunts. Out of these thirty patients, seven had shunts between the aberrant vessel and ipsilateral branch pulmonary artery. Even the shunt from the aberrant vessel has led to good pulmonary artery growth (three had total correction and remaining four patients in follow-up have a McGoon ratio of approximately 2).

11. 11. Significance of persistent fifth arch: Persistent fifth aortic arch is a congenital anomaly that consists of an abnormal vessel originating from the distal ascending aorta proximal to the innominate artery, which is a derivative of the fifth branchial arch vessel. This embryological variation has been classified into four different types based on its connections (systemic to systemic, systemic to pulmonary, pulmonary to systemic, and combined). This variation has been seen in tetralogy of Fallot.^{Reference Holmes, Holmes, Berman and Yabek31–Reference Gupta, Gulati and Anderson33} We had two (0.07%) patients with a persistent fifth arch (both had left aortic arch type-1 as the primary arch); both of our cases had systemic-to-systemic connections.

12. 12. Significance of Kommerell diverticulum: Aberrant subclavian artery without an intracardiac lesion is associated with a Kommerell diverticulum. This typically causes indentation of the oesophagus, and when large can cause tracheal compression as well. There are reports of spontaneous dissection involving the diverticulum. In tetralogy of Fallot, during fetal life the flow through the ductus arteriosus is diminished or a ductus arteriosus does not develop; therefore, the dorsal root contralateral to the arch regresses completely or remains small, and there is no diverticulum-like origin of the aberrant left subclavian artery.^{Reference Velasquez, Nath, Castaneda-Zuniga, Amplatz and Formanek34} Out of 153 tetralogy of Fallot patients with aberrant subclavian artery in our series, only one had a Kommerell diverticulum (Figure 6).

Figure 1. Tetralogy of Fallot with double aortic arch, with dominant left arch (true vascular ring), 1 – ascending aorta, 2 – dominant left arch, 3 – right arch, 4 – left pulmonary artery, 5 – right pulmonary artery, 6 – left atrium, 7 – right atrium (enormously dilated due to severe tricuspid regurgitation), arrow – hypertrophied right ventricle, broken arrow – left ventricle.

Figure 2. Tetralogy of Fallot post modified Blalock-Taussig shunt (arrow) with right aortic arch and aberrant left subclavian artery and left sided ductus causing a vascular ring (*). 1 – ascending aorta, 2 – left common carotid artery, 3 – aberrant left subclavian artery, 4 – left pulmonary artery. The modified Blalock-Taussig shunt is between left common carotid artery and left pulmonary artery.

Figure 3. (a) Tetralogy of Fallot (post right ventricular outflow tract stent) with right aortic arch and mirror image branching, 1 – left innominate artery, 2 – right common carotid artery, 3 – left pulmonary artery, 4 – right pulmonary artery, arrow – right vertebral artery, broken arrow – right subclavian artery. (b) Tetralogy of Fallot with right aortic arch and isolated left subclavian artery (arrow) where ductus is arising from left pulmonary artery and continuing as left subclavian artery, 1 – ascending aorta, 2 – left common carotid artery, 3 – right common carotid artery, 4 – main pulmonary artery.

Figure 4. Tetralogy of Fallot with aorto pulmonary window (arrow), with enormously dilated pulmonary arteries and its branches. 1 – ascending aorta, 2 – left pulmonary artery, 3 – hypertrophied right ventricle, 4 – left ventricle.

Figure 5. Tetralogy of Fallot with cervical aortic arch. (a) Tortuous left-sided descending aorta (arrow) without gradient across the arch (pseudo-coarctation), (b) right-sided descending aorta with coarctation (bracket). The rarity of combined right- and left-sided obstruction is evident, with only one patient of tetralogy of Fallot having a coarctation in this series (0.04%).

Figure 6. Tetralogy of Fallot with right-sided aortic arch (a) and aberrant left subclavian artery (arrow) with Kommerell diverticulum (b). Aberrant subclavian artery associated with Kommerell diverticulum is a rarity in tetralogy of Fallot with only one patient out of 153 in our series (0.04% overall).