Tetralogy of Fallot

Tetralogy of Fallot tetralogy classically include: right ventricular obstruction, RV hypertrophy, and a VSD with overriding aorta.
Two broad subsets of T4 exist:

• RV obstruction in most patients is due to infundibular stenosis due to hypertrophy of the subpulmonic muscle (crista ventricularis). Infundibular obstruction may be increased by sympathetic tone and therefore, dynamic; this obstruction is likely responsible for the hypercyanotic spells observed in very young patient. The anatomy can almost never be clearly delineated by two dimensional echo alone. Cardiac catheterization and/or MI angiography are necessary to delineate collateral anatomy.
• 20-25% of patients also have pulmonic stenosis. the pulmonic valve is often bicuspid or less commonly atretic.it is important to point out that the valvular obstruction is a fixed obstruction. the anantomy the lesion can almost always be definitely delineated by two dimensional echo.Cardiac catheterization is rarely necessary or indicated.

 Patho-physiology:-There is Rt to Lt shunting induced by the presence of a VSD and Rt Vent outflow tract obstruction in addition because the aorta overrides the VSD and RV, desaturated systemic venous blood tends to stream out the aorta.
Clinical presentation:-1. Decreased pulmonary blood flow cause lung oligemia and arterial hypoxemia manifested by cyanosis.2. 2ry polycythemia due to chronic hypoxia.3. Hypercyanotic spells may be life threatening and should be anticipated in every patient. The frequency of spells is between 2-3 months of age. the aetiology of spells is not completely understood, but infundibular spasm or constriction play a role where sudden spasm decrease pulmonary blood flow and increase RV pressure causing R-L shunt and increase arterial hypoxemia. It can be precipitated by crying defecation, feeding, fever and awakening. Paroxysmal hyperpnea is the initial finding, there is an increase in rate and depth of respiration, leading to increasing cyanosis and potential syncope, convulsion or death. hyperpnea increase oxygen consumption through the increase work of breathing. it also lower intra-thoracic pressure and lead to an increase in systemic venous return which results in an increase in an increased RV pressure and increase R-L shunt.
Treatment of tet spells:-
1. Administration of 100% O2.
2. Compression of the femoral artery or placing the patient in a knee chest position (increase SVR and reduce R-L shunt).
3. Administration of morphine 0.05-0.1 mg/kg.
4. Administration of 15-30 ml/kg of crystalloid(enhancing preload will increase heart size which may increase the the diameter of RVOT)
5-adminstration of NaHco3 to treat the sever metabolic acidosis that can be seen during the spell.Correction of acidosis well helps normalize SVR and reduce hyperpnoea
.6-Phenylephrine 5-10 ug/kg iv or 2-5 ug/kg/m infusion can be used to increase SVR and reduce R-L shunting (B agonists are absolutely contraindicated).
7-administration of propanolo 0.1mg/kg or esmolol 0.5 mg/kg followed by infusion of 50-300 ug/kg/m may reduce infundibular spasm.
8-manual compression of the abdominal aorta will increase SVR; this maneuver is particularly effective for the anaesthetized patient.
9-extracorporial membrane oxygenation resuscitation is another alternative in refractory episodes. 4
The-Pink tet
 term pink tet refers to any non-cyanotic patient with T4 where pulmonary blood flow is supplied from a large PDA or major aorto-pulmonary collateral arteries.
MANAGEMENT OF T4:
I-Palliative shun tPalliative shunt procedures to increase pul. Blood flow can be used. It involve creation of a systemic to pulmonary arterial shunt analogous to a patent PDA. These shunt produce L-R shunt and increase pulmonary blood flow.The shunt can be summarized as follow:-
WATERSTON SHUNT:Side to side anastomosis between the ascending aorta and the right pulmonary artery.
APOTTS SHUNT:Side to side anastomosis between the descending aorta and the left pulmonary artery.Both are of historic interest only)
CENTERAL SHUNT:Placing a synthetic tube graft between the ascending aorta and the main or branch pulmonary artery.
BLALOCK TAUSSIG SHUNT:End to side anastomosis of Rt or Lt subclavian artery to the ipsilateral pulmonary artery
MODIFIED BLALOCK TAUSSIG SHUNT:Interposing a length of Goretex tube graft 3.5-4 mm between the subclavian or innominate artery and the branch of pulmonary artery.
II-Definitive Repair:Complete correction involves closure of the VSD, removal of obstructing infundibular muscle, and pulmonic valvulotomy or valvuloplasty.
ANESTHETIC MANAGEMENT
 Anesthetic Goals:1. To maintain intra-vascular volume.2. To maintain SVR(avoid histamine release V.D.-volatile agents).3. To avoid increase in RVR (acidosis, hypoxia, hypercapnea, hypothermia, excessive airway pressure).
Premedication: Premedication before induction can be used to facilitate a number of objectives:Midazolam 1 mg/kg orally in infants and younger children.In older children ketamine 7-10 mg/kg orally in combination with midazolam 1 mg /kg.
Induction:An alternative to iv induction in infants and neonate with difficult peripheral iv access is IM induction with.· Ketamine  3-5 mg/kg · Succinylcholine 5mg/kg· Glycopyrolate 10 ug/kg. this technique provides prompt induction and immediate control of the airway with tracheal intubation.
Analternative technique combines:Ketamine 4-5 mg/kg 10 ug/kg IM 1mg/kg

Maintenance:Maintenance of anesthesia is often achieved with N2O combined with Ketamine.N2O concetration should be limited to 50% to allow increased F1O2 which theoretically could decrease PVR.The use of an opioid or Benzodiazepines may be also considered during maintenance of anesthesia but the dose and rate of administration must be adjusted to minimize decreased SVR and BP.Muscle paralysis may be provided with pancoronium in view of its ability to maintain SVR and BP.Ventilation should be controlled but it must be appreciated that excessive positive airway pressure may adversely increase the resistance to pulmonary blood flow.Care must be taken to avoid infusion of airway through IV tubing because it could lead to systemic air embolism.a Agonists such as phenylephrine must be available to treat undesirable decrease in SVR and BP.
Post Correction Complications:Ventricular cardiac dysrrhythmia are common in patient following surgical correction.They often develop AF or AF ,RBBB is common.Pulmonic regurgitation may develop eventually leading to RV hypertrophy and dysfunction.An aneurysm may form at the site where the RV outflow tract was repaired.