14.6.14 Transposition of the great arteries (TGA)

TGA is the most common cyanotic heart disease at birth. It accounts for 5-7% of all congenital heart disease cases and mainly affects boys. TGA entails complete discordance of the ventriculoarterial junction, with the aorta stemming from the anatomically right ventricle acting as the systemic ventricle and the pulmonary artery stemming from the morphologically left ventricle. In dextro-TGA (D-TGA or classic TGA), the two vessels run parallel – the aorta is anterior and the pulmonary artery is posterior. The aortic valve is located in front and to the right of the pulmonary valve (Figure 14.64). 



Figure 14.64: Diagram showing transposition of the great arteries (TGA). A: normal position of the aorta and pulmonary artery. B: position of vessels in TGA. C: long-axis view of the aorta and PA, which appear parallel instead of crossing at 45°. The anatomically right ventricle is hypertrophied since it acts as a systemic ventricle (sub-aortic).

In levo-TGA (or L-TGA), the aorta and PA are also parallel, although side-by-side in the frontal plane. L-TGA is combined with ventricular discordance. This combination is called congenitally corrected TGA (see below).

In D-TGA, the systemic and pulmonary circulations operate in parallel rather than in sequence. Survival is only possible if a patent ductus arteriosus, ASD or VSD (present in 30-40% of cases) provides a mixture of venous and arterialised blood. Children with an intact septum are deeply cyanotic. LVOT obstruction (30% of cases) and coronary artery anomalies are not uncommon (see Figure 14.73) and complicate surgical reconstruction. Since PVR falls in the first 2 weeks of life, the LV rapidly loses mass and hypotrophies. In contrast, the RV hypertrophies since it is connected to SVR. The flow rate and direction of shunts is dependent on their position and on PVR. Shunting is normally R-to-L through the VSD and patent ductus arteriosus, but L-to-R through the ASD [15].

Neonates with TGA and insufficient shunting are cyanotic from birth (SaO₂ < 60%) and are unable to survive unless surgery is performed early. The prognosis for such procedures has been significantly improved by prenatal diagnosis [5,12].
 

Subsequent survival of the infants is ensured by several surgical procedures. These are generally performed in the first weeks or months of life.
 

Arterial switch is currently given preference over other procedures. Since it involves anatomical reconstruction, cardiac function is normal provided that the operation is performed sufficiently early to prevent irreversible involution of the LV connected to the PA's low-pressure system. With the exception of 3% perioperative mortality, life expectancy is virtually normal (97% at 10 years) [13,19]. Potential sequelae include dilation of the aortic root with pulmonary or aortic valve leakage (30% of cases), myocardial ischaemia (10% of cases) and obstruction of the RVOT or LVOT [9,18,19].



Figure 14.65: Mustard procedure for TGA: a complex patch positioned between the two atria (interatrial baffle) redirects systemic venous blood to the sub-pulmonary ventricle (LV) and arterialised blood to the sub-aortic ventricle (RV). The systemic ventricle is the anatomically right ventricle. A: the pulmonary veins (PV) drain into a posterior venous atrium (PVA) connected to the sub-aortic RV. B: the inferior vena cava (IVC) drains into the lower part of a superior venous atrium (SVA) connected to the sub-pulmonary LV. In the same way, the superior vena cava drains into the upper part of the SVA [15].



Figure 14.66: Jatene procedure (arterial switch): switching of the great arteries by reconnecting the PA to the RV and the aorta to the LV. A: transection of the aorta and excision of the coronary arteries. B: the PA is excised and the coronary arteries are reimplanted in the proximal part of the PA connected to the LV. C: The PA is positioned anteriorly to the aorta (Lecompte manoeuvre). The proximal PA is anastomosed to the distal aorta. The aorta is unclamped. D: the sites from which the coronary arteries have been removed are patched and the proximal aorta, which is connected to the RV, is anastomosed to the distal PA [5]. This reconstruction involves a risk of myocardial ischaemia. 




Figure 14.67: Rastelli procedure: if TGA is associated with a VSD and an obstruction of the LVOT, the VSD may be closed with a patch (green line) positioned so that blood from the LV is directed towards the aorta. The proximal PA is ligated and a valved conduit (green/blue arrow) is built between the RV and PA. The venous blood (blue arrow) and arterial blood (red arrows) are separated and directed respectively to the PA conduit and the aorta.




Figure 14.68: Damus-Kaye-Stansel procedure: the proximal PA is anastomosed end-to-side to the ascending aorta and the distal part is sutured. A valved conduit is positioned between the RV and distal PA. Venous blood (blue arrow) is separated from arterial blood (red arrows). The prevailing pressure in the aorta keeps the aortic valve closed (it must be competent) [4].

Anaesthesia for arterial switch

At 1-3 weeks from birth, neonates with TGA exhibit peculiar features that increase surgical risk.
 

Based mainly on high doses of fentanyl/sufentanil, anaesthesia must be sufficiently deep to reduce VO₂, but has little impact on the distribution of pulmonary and systemic flow, as the Qp/Qs ratio is determined by the patient's anatomy. The reduction in preload and contractility linked with anaesthesia and the increase in intrathoracic pressure through positive pressure ventilation may considerably reduce the mixture of venous and arterialised blood at atrial level, especially in children who do not have a VSD. This leads to significant arterial desaturation requiring hyperoxic ventilation (FiO₂ 1.0) and normocapnia. Monitoring comprises SaO₂, ScO₂, an arterial catheter (umbilical, radial or femoral), a percutaneous central line, possibly a transthoracic catheter in the LA (inserted at the end of CPB) and transesophageal echocardiography. At this age, there is a high risk of a TEE probe interfering with ventilation and increasing peak airway pressure. The commercially available mini-probe should ideally prevent this risk. The aim of TEE is to continuously monitor the degree of filling, biventricular function, segmental contractility, the transanastomotic gradients of the PA and aorta, insufficiency of the neoaortic valve, and any residual defect.

In the absence of a VSD, the LV may be insufficiently developed to take on the systemic afterload, and may require significant inotropic stimulation (dobutamine, epinephrin-milrinone) or assistance during weaning from CPB (ECMO). If present, a VSD maintains a significant load on the LV, which prevents it from shrinking and facilitates the patient's transition off bypass. Target haemodynamics are: satisfactory cardiac output (normal SaO₂ and SvO₂, absence of metabolic acidosis), systemic arterial pressure of 50-75 mmHg and LAP of 4-6 mmHg. Any manipulation that might increase PVR or lower SVR are therefore avoided [15]. It should be assumed that ventricular dysfunction or failure to wean the patient from CPB is caused by ischaemia until proved otherwise, as coronary artery reimplantation is the procedure associated with the highest rate of complications [18]. Early extubation (< 6 hours) may be considered if the situation is quiet [1]. Perioperative complications such as hypoxaemia, acidosis, low flow or prolongation of CPB are directly associated with delayed neurocognitive development [3]. Since surgery is performed before PVR normalises, a pulmonary hypertensive crisis is always possible. In this case, the RV may also fail and require inotropic support (dobutamine, milrinone) and reduced afterload (pulmonary hypertension treatment with PGE1, NO^•, etc) (see Table 14.7).

Handling of the coronary arteries increases the risk of ischaemia – it is essential to monitor the ST segment and segmental contractility by TEE to assess whether specific treatment is required: increase in arterial pressure (norepinephrin), nitroglycerin (1-2 mcg/kg/min), or surgical revision of the reimplantation of the relevant coronary artery. A vicious circle of ischaemia and ventricular dilation may very quickly take hold, each intensifying the other.
 

 

 

Congenitally corrected TGA

If atrioventricular discordance is combined with ventriculoarterial discordance, survival is possible since the systemic and pulmonary circulations are back in sequence. This is the case in congenitally corrected transposition of the great arteries. In this anomaly, the PA and aorta are transposed as in TGA. However, the ventricles are also inverted. The aorta is positioned to the left of the PA (L-TGV) and the two vessels are side-by-side in the frontal plane. Consequently, the blood follows a physiological sequence along the crossed chambers: RA → LV → PA → lungs → LA → RV → Ao. The coronary artery system is inverted with the RCA on the left and the left main trunk on the right (Video and Figure 14.69).


Video: Congenitally corrected transposition of the great arteries; the TGA, not visible on this view, is doubled by a transposition of the ventricles: the LV is on the right side and the RV on the left; RV is recognisable by the presence of a septal papillary muscle, and by the septal insertion of the tricuspid valve below the insertion of the mitral valve.



Figure 14.69: Congenitally corrected transposition of the great arteries (TGA) or L-TGA. An atrioventricular discordance is combined with the ventriculoarterial discordance (TGA). The RA is connected to a mitral valve (MV), an anatomically left ventricle (LV), and the PA. Oxygenated blood is returned to the LA by the pulmonary veins. The LA drains through a tricuspid valve into an anatomically right ventricle, which is connected to the aorta. Although the patient is not cyanotic, his systemic ventricle is a right ventricle, which fails in about 20 years’ time.

Such patients are not cyanotic. However, the anatomically right ventricle is in a sub-aortic position, and therefore must function as a systemic ventricle. It dilates, develops severe tricuspid insufficiency (TI) and becomes insufficient around the age of 20-25 years [6]. The condition is often diagnosed at this stage (see Chapter 15 - Congenitally Corrected TGA). Corrected TGA is only symptomatic in children if the RV fails prematurely or cyanosis develops due to a VSD with stenosis of the LVOT (present in 40-50% of cases) leading to R-to-L shunting. Complete AV block is common as the anatomy of the atrioventricular junction is abnormal [15].

Two surgical options are available depending on the type of associated lesion (VSD, obstructed LVOT, tricuspid insufficiency) and symptom progression.

The key criteria for anaesthetic management are RV dysfunction, TI severity and conduction blocks. While reducing SVR (systemic vasodilator, isoflurane) facilitates RV ejection and limits tricuspid insufficiency, SVR should be maintained at sufficient levels to ensure coronary perfusion. PiCCO™ is very useful for haemodynamic monitoring. TEE is essential for assessing ventricular function and blood volume.

 

 

 

© BETTEX D, BOEGLI Y, CHASSOT PG, June 2008, last update February 2020