In 1888, Fallot described a congenital heart defect composed of four characteristics: (1) large VSD, (2) right ventricular (RV) outflow obstruction, (3) overriding aorta, and (4) right ventricle hypertrophy (RVH). Broadly defined, TOF is a complex of anatomic malformations consisting of a large, misaligned conoventricular VSD; a rightward and anterior displacement of the aorta, such that it overrides the VSD; and a variable degree of subvalvular RVOT obstruction due to anterior, superior, and leftward deviation of the conal (infundibular) ventricular septum. In addition, abnormalities in the septal and parietal bands of the crista supraventricularis further exacerbate the infundibular RVOT obstruction. RVH is the result of chronic RVOT obstruction. The most common associated lesion is a right aortic arch with mirror image arch vessel branching (innominate artery gives rise to left carotid and left subclavian arteries; right carotid and right subclavian arise separately) present in 25% of patients. Two broad subsets of TOF exist: TOF with PS (TOF/PS) and TOF with pulmonary atresia. A third much less common type of TOF known as TOF with absent pulmonary valve will not be considered here.

TOF/PS is a disease with a complex shunt in which a communication (VSD) and a partial obstruction to RV outflow (RV infundibular and valvular stenosis) are present. In complex
shunts, the resistance to outflow is a combination of the resistance from the obstructive lesions and the PVR. If the resistance from the RV obstructive lesions is high, changes in PVR will have little effect on shunt magnitude and direction. In most patients with TOF/PS, there is a fixed and a dynamic component to RV outflow obstruction. The fixed component is produced by the valvular stenosis. The dynamic component is produced by variations in the caliber of the RV infundibulum. The pathophysiology present in TOF/PS is physiologic rightto-left (R-L) shunting induced by the presence of a VSD and RVOT obstruction. In addition, because the aorta overrides the VSD and the RV, desaturated systemic venous blood tends to stream out of the aorta, even in the presence of mild RVOT obstruction.

The SaO₂ is determined by the relative volumes and saturations of recirculated systemic venous blood and effective systemic blood flows that have mixed and reached the aorta. This is summarized in the following equation:

This is demonstrated in Figure 38.4 where the SaO₂ = [(98 × 0.5) + (65 × 0.5)] / 1 = 81. Notice that the patient has a Q_P:Q_S = 0.5:1.

Shunting is the process whereby venous return into one circulatory system is recirculated through the arterial outflow of the same circulatory system. Flow of blood from the systemic venous atrium (right atrium) to the aorta produces recirculation of systemic venous blood. Flow of blood from the pulmonary venous atrium (left atrium) to the PA produces
recirculation of pulmonary venous blood. Recirculation of blood produces a physiologic shunt. Recirculation of pulmonary venous blood produces a physiologic left-to-right (L-R), whereas recirculation of systemic venous blood produces a physiologic R-L shunt. A physiologic R-L or L-R shunt commonly is the result of an anatomic R-L or L-R shunt. In an anatomic shunt, blood moves from one circulatory system to the other through a communication (orifice) at the level of the cardiac chambers or great vessels. Physiologic shunts can exist in the absence of an anatomic shunt; transposition physiology is the best example.

Effective blood flow is the quantity of venous blood from one circulatory system reaching the arterial system of the other circulatory system. Effective pulmonary blood flow is the volume of systemic venous blood reaching the pulmonary circulation, whereas effective systemic blood flow is the volume of pulmonary venous blood reaching the systemic circulation. Effective pulmonary and effective systemic blood flows are the flows necessary to maintain life and are always equal, no matter how complex the lesions. Effective blood flow usually is the result of a normal pathway through the heart, but it may occur as the result of an anatomic R-L or L-R shunt.

Total pulmonary blood flow (Q_P) is the sum of effective pulmonary blood flow and recirculated pulmonary blood flow. Total systemic blood flow (Q_S) is the sum of effective systemic blood flow and recirculated systemic blood flow. Total pulmonary blood flow and total systemic blood flow do not have to be equal. Because Q_S (systemic cardiac output) tends to remain constant to supply end organs, a physiologic L-R shunt (pulmonary recirculation) causes pulmonary volume overload while a physiologic R-L shunt (systemic recirculation) allows Q_S to be maintained at the expense of SaO₂.

Calculation of Q_P:Q_S (the ratio of total pulmonary blood flow to systemic blood flow) is greatly simplified when the determination is made using low inspired concentrations of oxygen (fraction of inspired oxygen [FIO₂]). This allows the contribution of oxygen carried in
solution (PO₂ × 0.003) to be ignored. Failure to account for this component when determination of Q_P:Q_S is made using an FIO₂ of 1.0 will introduce substantial (100%) error. If the FIO₂ is low, the determination of Q_P:Q_S can be simplified to the following equation using just oxygen saturations:

(S_AO₂ – S_SVCO₂) / (S_PVO₂ – S_PAO₂)

where A = arterial, SVC = superior vena cava, PV = pulmonary vein that can be assumed to be 98% in the absence of significant pulmonary disease, and PA = pulmonary artery.

The pathophysiology of TOF with pulmonary atresia is similar to the single ventricle physiology. Single ventricle physiology describes the situation wherein complete mixing of pulmonary venous and systemic venous blood occurs at the atrial or ventricular level and the ventricle(s) then distributes output to both the systemic and pulmonary beds. As a result of this physiology, the following are observed:

This physiology can exist in patients with one well-developed ventricle and one hypoplastic ventricle as well as in patients with two well-formed ventricles. In the case of a single anatomic ventricle, there is always obstruction to either pulmonary or systemic blood flow as the result of complete or near complete obstruction to inflow and/or outflow from the hypoplastic ventricle. In this circumstance, there must be a source of both systemic and pulmonary blood flow to ensure postnatal survival. In some instances of a single anatomic ventricle, a direct connection between the aorta and the PA through a PDA is the sole source of systemic blood flow (hypoplastic left heart syndrome) or of pulmonary blood flow (PA with intact ventricular septum). This is known as ductal dependent circulation. In other instances of a single anatomic ventricle, intracardiac pathways provide both systemic and pulmonary blood flow without the necessity of a PDA. This is the case in tricuspid atresia with normally related great vessels, a nonrestrictive VSD and minimal or absent PS.

In certain circumstances, single ventricle physiology can exist in the presence of two wellformed anatomic ventricles. This is generally the result of atresia or near atresia of outflow from one of the ventricles. Examples include the following:

With single ventricle physiology, the SaO₂ will be determined by the relative volumes and saturations of pulmonary venous and systemic venous blood flows that have mixed and reached the aorta. This is summarized in the following equation:

The primary goal in the management of patients with single ventricle physiology is optimization of systemic oxygen delivery and perfusion pressure. This is necessary if end-organ (myocardial, renal, hepatic, splanchnic) dysfunction and failure are to be prevented. This goal is achieved by balancing the systemic and pulmonary circulations. The term balanced circulation is used because both laboratory and clinical evaluations have demonstrated that maximal systemic oxygen delivery (the product of systemic oxygen content and systemic blood flow) is achieved for single ventricle lesions when Q_P:Q_S is at or just below 1:1. Increases in Q_P:Q_S in excess of 1:1 are associated with a progressive decrease in systemic oxygen delivery because the subsequent increase in systemic oxygen content is more than offset by the progressive decrease in systemic blood flow and by diastolic hypotension due to runoff into the pulmonary circulation. Decreases in Q_P:Q_S below 0.7 to 0.8:1 are associated with a precipitous decrease in systemic oxygen delivery because the subsequent increase in systemic blood flow is more than offset by the dramatic decrease in systemic oxygen content.

The term pink Tet refers to any noncyanotic patient with TOF/PS or TOF with pulmonary atresia. In these patients, Q_P:Q_S is sufficiently high (Q_P:Q_S generally greater than 0.8:1 in the presence of a normal mixed venous saturation and pulmonary vein saturation) to maintain a deoxyhemoglobin concentration less than 5 g per dL (SaO₂ generally >80%). The designation
of “pink Tet” would apply to TOF/PS patients with minimal valvular and subvalvular PS and to all patients with TOF with pulmonary atresia where pulmonary blood flow is supplied from a large PDA and/or MAPCAs.

The occurrence of hypoxic spells in TOF patients may be life-threatening and should be anticipated in every patient with TOF/PS and any infundibular obstruction, even those who are not normally cyanotic. The peak frequency of spells is between 2 and 3 months of age; spells occur more frequently in severely cyanotic patients. The onset of spells usually prompts urgent surgical intervention, so it is not unusual for the anesthesiologist to care for an infant who is at great risk for spells during the preoperative period. The etiology of spells is not completely understood, but infundibular spasm or constriction plays a role. Crying, defecation, feeding, fever, and awakening all can be precipitating events. Paroxysmal hyperpnea is the initial finding. There is an increase in rate and depth of respiration, leading to increasing cyanosis and potential syncope, convulsions, or death. During a spell, the infant will appear pale and limp secondary to poor cardiac output. Hyperpnea has several deleterious effects in maintaining and worsening a hypoxic spell. Hyperpnea increases oxygen consumption through the increased work of breathing. Hypoxia induces a decrease in SVR, which further increases the R-L shunt. Hyperpnea also lowers intrathoracic pressure and leads to an increase in systemic venous return. In the face of infundibular obstruction, this results in an increased RV pressure and an increase in the R-L shunt. Treatment of a “Tet spell” is focused on increasing the pulmonary circulation and decreasing the R-L shunt. It includes the following: