14.6.10 Tetralogy of Fallot

Tetralogy of Fallot (TOF) accounts for 5-10% of congenital heart diseases. It is the most common cyanotic heart disease. Over 95% of children who benefit from surgery reach adulthood [5]. TOF is caused by a conotruncal anomaly with an anterior-right shift of the conal septum and underdevelopment of the right infundibulum. The embryonic parietal band is misaligned and abnormally positioned resulting in obstruction of the right ventricular outflow tract (RVOT) and a wide subaortic VSD [9,14]. The malformation combines four anomalies (Figure 14.56 and Figure 14.57).
 

Video: Membranous VSD located under the aortic valve, and typical of tetralogy of Fallot.


Video: Long-axis view of the large aortic valve overriding the membranous VSD in a case of tetralogy of Fallot; the right ventricle is severely hypertrophied.


Video: Long-axis view of the right ventricular outflow tract, pulmonary valve and pulmonary artery in a case of tetralogy of Fallot; RVOT presents a massive concentric hypertrophy with a tiny lumen; pulmonary valve and PA are severely hypotrophied.



Figure 14.56: Diagram of tetralogy of Fallot. The aortic valve overrides a wide VSD. The RV is hypertrophied (RVH), although the largest diameter of its chamber is not visible in this cross-sectional plane. The pulmonary valve and pulmonary artery are stenosed.

In some cases, an ASD is added to the other lesions, which is described as pentalogy. In 10% of cases, a coronary malformation (e.g. circumflex artery originating from the right coronary artery) causes a vessel to cross the anterior face of the RVOT. Several other malformations may be associated with TOF: stenosis of the distal trunks of the PA, multiple VSDs, dextroposition of the aortic arch, pulmonary agenesis, major aortopulmonary collateral arteries (MAPCA, see below). There is a whole range of lesions similar to those of the TOF. Ten percent of all congenital heart disease patients experience some degree of pulmonary stenosis. Their anatomy is similar to Fallot cases, although without a VSD or displacement of the aorta. They are not cyanotic at rest. Tetralogy cases with minor pulmonary stenosis enabling normal oxygenation and a Qp/Qs ratio close to 1/1 are sometimes described as pink Fallot. At the other end of the spectrum, complete pulmonary atresia causes deep cyanosis.



Figure 14.57: Transesophageal echocardiographic images of tetralogy of Fallot. A: 5-chamber view at 0° of the aorta opening in a overriding position over both ventricles. The arrow shows the VSD. The aortic valve receives blood from the LV and RV. The RV is hypertrophied. B: long-axis view at 120° of the aortic root. The valve overrides the VSD (arrow). The RV wall is > 1.2 cm thick and its trabeculations are hypertrophied. C: pulmonary muscular stenosis in the right outflow tract. Diameter 0.44 cm (arrow). The pulmonary valve and artery are hypoplastic. D: atresia of the pulmonary valve, whose opening is minuscule (arrow) followed by a hypoplastic and tubular pulmonary artery.

While pulmonary valve stenosis is fixed as it is caused by hypoplasia, RVOT stenosis is dynamic as this part is entirely muscular. Any increase in contractility or reduction in preload reduces its diameter, intensifies stenosis and impedes pulmonary blood flow. Sympathetic stimulation exacerbates cyanosis (blue spells), and therefore children are often administered β-blockers. This infundibular spasm gives rise to cyanotic episodes with hyperventilation requiring immediate treatment [10].

The spontaneous development of aortopulmonary collaterals during growth enables some children with TOF to grow up normally due to the pulmonary flow supplied by these L-to-R shunts (Figure 14.58). However, this is a delicate balance since pulmonary flow is dependent on systemic flow – it increases if SVR rises but falls in the event of hypotension. If the collaterals are small, PAP remains low and the child becomes cyanotic with the slightest exertion. In contrast, if the collaterals are large, the child risks developing pulmonary hypertension. Moreover, the collaterals impose volume overload on the LV, which must provide both the systemic and pulmonary flow.

Children with TOF are cyanotic because the aorta receives a mixture of venous blood from the RV and arterialised blood from the LV, and because pulmonary flow is insufficient. Pulmonary flow and pressure are low due to pulmonary stenosis, which protects the lungs from hypertension. If systemic vascular resistance (SVR) rises, pulmonary flow rises, the R-to-L shunt decreases, and cyanosis is reduced. SaO₂ directly reflects pulmonary flow. Since the RV is subjected to systemic pressure, it becomes progressively dysfunctional. 



Figure 14.58: Collateral flow in a case of tetralogy of Fallot. The flow is visible between the large collateral originating from the descending aorta and a branch of the pulmonary artery (PA). It is a continuous systolic and diastolic flow, since systemic pressure is constantly higher than pulmonary pressure. Diastolic arterial pressure is low as the vessels continuously direct flow into the low-pressure pulmonary vascular bed. It rises if pulmonary hypertension takes hold. If sufficient in number, these collaterals are able to ensure normal pulmonary flow at rest.

Surgical treatment

In the nineteen seventies, two-stage surgery was generally performed on these patients. An L-to-R shunt was first performed to increase pulmonary blood flow: Blalock-Taussig shunt (subclavian artery → right or left pulmonary artery), Potts shunt (descending aorta → left PA) or Waterston shunt (ascending aorta → right PA). The shunt’s size determined its flow rate. This initial neonatal surgical procedure was followed by total correction between the ages of 5 and 10 years. Unfortunately, this strategy maintained hypoxia and pressure overload on the RV for too many years. It also caused deformity of the pulmonary arteries and hampered their normal development.

Total correction is currently performed between the ages of 2 and 10 months, which avoids neonatal surgery while also eliminating cyanosis and RVH sufficiently early, normalising pulmonary flow, and encouraging growth of the pulmonary vascular tree [4,11]. Surgery consists of separating the systemic and pulmonary circulations and removing the blockage from the pulmonary artery root (Figure 14.59) [6].
 

The VSD is approached through a transatrial and trans-tricuspid route. The pulmonary valve is accessed via the PA. Avoiding a ventricular incision improves function and limits the risk of arrhythmia [12]. Widening of the RVOT entails a risk of pulmonary insufficiency if the patch is too large or of persistent stenosis if it is too restrictive. Correction is therefore based on a compromise between minimal regurgitation and minimal residual gradient. The maximum accepted gradient is 20-40 mmHg [17]. A flow rate of over 3 m/s, a gradient of over 40 mmHg or a RVsystP/LVsystP ratio > 0.5 are considered excessive. When resecting a dynamic hypertrophy of the outflow tract, the residual gradient may be functional rather than anatomical. It is reduced by rebalancing sympathetic tone (beta blockade), loading conditions, and Hb levels. Following total correction, SaO₂ must be > 98%. The surgical mortality rate for total correction performed at a young age is < 2% and the survival rate is > 95% at 36 years [1,5]. However, many children require a second operation in adulthood due to pulmonary insufficiency or, less commonly, stenosis, or to implant a larger conduit. Only 10-15% of patients who undergo delayed primary correction during adolescence require subsequent reoperation. However, these children are the rare cases who have survived with their malformation for some fifteen years.



Figure 14.59: Surgical correction of tetralogy of Fallot. The VSD patch divides the flows by guiding systemic venous blood to the PA and arterialised blood to the aorta. A patch enlarging the RVOT and pulmonary valve removes the pulmonary and subpulmonary stenosis. The arrow shows the point at which a valved conduit may be inserted between the RV and PA if a patch is not feasible.

Unfortunately, correction may allow a number of major sequelae [7,12,15].
 

Tetralogy of Fallot and pulmonary agenesis

In this combination, there is no connection between the RV and PA – the RVOT is a dead end. This agenesis is combined with variable underdevelopment of the pulmonary artery, which is compensated by patent ductus arteriosus flow at birth. In extreme cases, the entire pulmonary flow is provided by a system of collaterals originating from the aorta without any link to the atretic pulmonary vascular tree (MAPCA, major aortopulmonary collateral arteries). R-to-L shunting through the VSD causes a complete mixture of venous and arterialised blood in the LV.

If possible according to patient's anatomy, surgical correction entails closing the VSD and connecting the RV to the PA using a valved conduit. In cases of MAPCA, unifocalisation is performed, whereby the collaterals are detached from the aorta and reimplanted in a conduit originating from the RV. The VSD is occluded. These children are managed in a similar way to those with univentricular physiology (see Ventricular Hypoplasia) [10].

Tetralogy of Fallot and absent pulmonary valve

This differs from the previous situation by a valveless connection between the RV and PA. Regurgitation is massive, the RV is dilated and the increase in PA pulsatility during foetal life causes aneurysmal dilation of the pulmonary vessels combined with bronchial compression and tracheomalacia. Correction involves reducing the size of the PA, closing the VSD, and creating a pulmonary valve (allograft). Management of such children involves maintaining contractility of the volume-overloaded RV and lowering PVR to limit pulmonary insufficiency (hyperventilation, low intrathoracic pressure). Any factors that may induce pulmonary vasoconstriction must be avoided.

Anaesthesia for palliation

If pulmonary flow is significantly reduced, the child cannot wait until the age of a total correction and must undergo temporary palliation in the form of a modified Blalock shunt. Instead of anastomosing the subclavian artery directly to the ipsilateral pulmonary artery, a 3-4 mm Goretex™ shunt is interposed between the two vessels on the opposite side to the aortic arch. This maintains normal perfusion in the arm and adjusts pulmonary flow based on the diameter of the tube.

Management is aimed at increasing SVR with systemic vasoconstrictors (norepinephrin, phenylephrine) to increase flow through the shunt and force VSD flow in a L-to-R direction. Beta stimulation is contraindicated as this would increase the dynamic component of the right infundibular stenosis. Surgery is performed by thoracotomy: it requires low-dose heparinisation (100 U/kg) and retraction of the lung, which reduces gas exchange. Once the shunt is open, systemic diastolic pressure falls (Figure 14.60).


Video: Systolo-diastolic flow in the left subclavian artery used for a Blalock shunt (short-axis view of the distal aortic arch).


Video: Systolo-diastolic continuous flow in a Blalock shunt between the subclavian artery and the pulmonary artery.

The palliative shunt imposes volume overload on the LV and inotropic support is often necessary. The required SpO₂ is between 80% and 85%. A higher value indicates that pulmonary flow is excessive. However, even with acceptable SaO₂, unilateral pulmonary oedema may develop on the side of the shunt. Continuation of positive-pressure ventilation for 24 hours postoperatively is recommended.



Figure 14.60: Flow during construction of a Blalock-Taussig shunt. A: flow in the subclavian artery before the shunt is constructed. The flow is solely systolic as in any systemic artery. B: after connection to the pulmonary artery, the flow becomes systolo-diastolic as in a LR shunt. Diastolic systemic arterial pressure consequently falls.

Anaesthesia for total correction

There are two objectives for managing a total correction – reducing the shunt and increasing pulmonary flow (see Table 14.5).

A hypoxic episode (blue spell) may occur at any time. If not addressed in time, it may lead to cardiovascular collapse. Under general anaesthesia, the hypoxic crisis manifests as arterial desaturation, tachycardia, a reduction in arterial pressure, and loss of murmur on auscultation. Treatment must be immediate in the event of deep desaturation [10].
 

Anaesthesia is based on a fentanyl-midazolam or fentanyl-sevoflurane combination. Induction may be performed with a mask (sevoflurane) or intravenously (ketamine) provided that there is no ventricular dysfunction or risk of blue spells. In haemodynamically unstable cases, the main agents should ideally be fentanyl (50-75 mcg/kg) or sufentanil (5.0-7.5 mcg/kg) with midazolam or etomidate as a back-up. Cyanosis is combined with coagulation defects and platelet inhibition. It potentiates systemic inflammatory response, limits haemodilution during CPB (risk of hypoxaemia) and forces to keep a high Ht for going off-bypass.

Several major problems are predominant postoperatively [10].
 

If palliation has previously been performed by means of a systemic-pulmonary shunt or if large aortopulmonary collaterals are present, pulmonary perfusion is dependent on satisfactory systemic pressure. Any arterial vasodilation lowers the shunt flow, thereby also reducing pulmonary flow – cyanosis increases. These children therefore also benefit from increased SVR. Diastolic arterial pressure is chronically lowered if the shunt is flowing satisfactorily, since pressure in the pulmonary vascular tree is constantly diverting blood flow. Since the LV must ensure systemic and pulmonary perfusion, it functions in a situation of volume overload, bordering on failure.

 

 

 

 

 

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