Abstract
This chapter presents an overview of a more common pediatric cardiac lesion; tetralogy of Fallot. The authors provide a description of the anatomic variants that comprise the tetralogy spectrum in addition to a detailed pathophysiology discussion. The surgical/catheter-based interventions are presented in reference to the Tetralogy variant.
A four-week-old infant presents to the emergency room with agitation and cyanosis. His mother says he has been doing this for shorter periods of time for the past couple of weeks whenever he cries for food. These episodes resolve when she takes him in her lap and breastfeeds him. However, he started running a fever yesterday and had a few episodes of diarrhea this morning. And this current episode has been going on for several minutes and he is only getting more agitated and inconsolable.
Vital signs are: blood pressure unmeasurable due to worsening agitation as the cuff inflates, heart rate 180/min, respiratory rate 60/min, oxygen saturation reads 40–50% during times of agitation but improves when the patient is calm.
What Condition Are You Most Worried About? What Are Tet Spells?
This child is experiencing a hypercyanotic “tet” spell characteristic of tetralogy of Fallot (ToF).
Tetralogy of Fallot is the most common cyanotic congenital heart disease. Often, ToF is prenatally diagnosed or detected by the newborn congenital heart defect (CHD) screen. However, occasionally ToF may present with worsening spells of cyanosis, also known as hypercyanotic or “tet” spells, during which there is severe obstruction of blood flow to the lungs. In conditions that increase the pulmonary vascular resistance, more blood shunts from right-to-left through the VSD, resulting in worsening cyanosis. Tet spells are ominous indicating the acute need for palliative intervention to provide pulmonary blood flow and improve oxygenation.
What Is the Underlying Anatomy in Tetralogy of Fallot?
The tetralogy in ToF develops due to anterior deviation of the interventricular septum or infundibular tissue (Figures 64.1 and 64.2). The remainder of the tetralogy occurs following this movement. The anterior movement of the infundibulum compresses the right ventricular outflow tract (RVOT). This shift also results in an anterior malalignment VSD for which the aorta “overrides” or sits over the VSD. The compression of the RVOT ultimately results in compensatory right ventricular hypertrophy (RVH) (Figure 64.3).
Figure 64.1 Cardiac specimen highlighting the infundibulum region (dashed circle). The region between the aortic valve and right ventricular outflow tract (RVOT) is underdeveloped leading to anterior shift of structures.
Figure 64.3 Drawing of characteristic components of tetralogy of Fallot:
Identify the Anatomic Spectrum That Is Encompassed by Tetralogy of Fallot
Tetralogy of Fallot is a spectrum of anatomy and physiology manifestations, depending on the degree of the obstruction to the right ventricular outflow tract, and the subsequent pulmonary blood flow limitation. ToF can include varying degrees of pulmonary stenosis (mild to severe). In its most extreme form, there is complete pulmonary atresia in which no blood flows from the RVOT; hence the patient is dependent on a patent ductus arteriosus (PDA) to supply blood to the lungs. If not detected prenatally, these patients may present with worsening cyanosis and may be found to have a closing PDA on echo. This cohort of patients will benefit from prostaglandin infusion and usually undergo either a modified Blalock–Taussig shunt (BTs) or PDA stent placement in the catheterization laboratory, to provide stable pulmonary blood flow. After a few months the patient will undergo completion of the repair with a takedown of the shunt or stent, and closure of the VSD, and placement of a conduit from the right ventricle (RV) into the main pulmonary artery, to create normal septated anatomy.
Considering a slightly lower less degree of pulmonary outflow obstruction, there may be a very small amount of blood flow through the pulmonary valve but this may not be adequate in the absence of supplementation from the PDA. These ToF patients with severe pulmonary stenosis (PS) follow the course of the ToF with pulmonary atresia and undergo shunting or stenting initially, prior to complete septation and repair later in infancy.
Moving further along the spectrum, there are patients that may have mild to moderate obstruction initially and be able to manage without a PDA. However, as time passes the RV muscle hypertrophy just under the narrow pulmonary valve worsens and adds a dynamic component of RV outflow obstruction to the mix. This dynamic obstruction is worse when the patient is dehydrated and volume deplete, as the hypertrophied RV does not have adequate preload and generates dynamic obstruction from all the thick muscle bundles collapsed together. It is also worse when the heart rate is higher, and the RV is unable to relax completely in diastole and fill. Episodes of agitation and higher pulmonary vascular resistance or intrathoracic pressures would further limit the ability of the RV to push blood through the narrow outflow tract. Hence patients having a “tet” spell prefer to shunt blood across the VSD through the aorta, instead of being able to send it to the lungs. Thus, while they do not lose cardiac output, they become hypoxemic and cyanotic and more inconsolable and tachycardic, getting into a vicious cycle.
A final category of ToF, which manifests very differently from the usual cyanotic CHD, are ToF with pulmonary atresia and major aortopulmonary collateral arteries (MAPCAs). These MAPCAs originate from various spots on the descending aorta or from the aortic arch or its branches, and supply various segments of the lungs (Figure 64.4). The patients with MAPCAs will not present with cyanosis, rather they may have pulmonary overcirculation and manifest with tachypnea, tachycardia, and failure to gain weight normally. Their surgical planning is more precarious, as the MAPCAs may be physically far apart and difficult to unifocalize to a central main pulmonary artery. The MAPCAs also expose the pulmonary vascular bed to systemic blood pressures and predispose to development of pulmonary hypertension.
