The simplest method of measuring flow is to use a thermodilution catheter. This method is a variation on the dye dilution method using temperature decrement to determine blood flow. However, the method is only applicable in the absence of intracardiac shunting when there is a mixing chamber between the proximal and distal ports. Since most children with congenital heart disease do have either right-to-left, left-to-right, or bidirectional shunting, thermodilution is not an appropriate method of determining cardiac output in these patients.

More often, the Fick principle is used for calculation of flow. The essence of the Fick principle is that flow can be calculated by the use of a measurable indicator, which is added or subtracted at a known rate. In practice, the indicator is oxygen.

F = unknown flow rate (ml/min)

I₁, I₂ = indicator concentrations (mg/ml) at the two ends of the flow pathway

R = rate at which indicator is added or subtracted (mg/min)

F (ml/min) = R (mg/min)/I₂ − I₁ (mg/ml)

In a hemodynamic study, the unknown flow rate is systemic or pulmonary blood flow, the indicator is oxygen, and the rate of change of the indicator is the oxygen consumption. Oxygen content is determined by multiplying the hemoglobin concentration times the spectrophotometrically measured percentage of oxygen saturation of hemoglobin; dissolved oxygen must be included in the calculation when patients receive supplemental oxygen. Oxygen consumption is either measured or assumed on the basis of age and heart rate, using standardized tables. The equation for calculation of pulmonary blood flow is as follows:

Q_P = O₂ consumption/(pulmonary vein O₂ content – pulmonary artery O₂ content)

and the equation for calculation of systemic blood flow is as follows:

Q_S = O₂ consumption/(aortic O₂ content − venous O₂ content).

Effective flow describes the quantity of deoxygenated blood that flows through the pulmonary circuit, which is also equal to the volume of oxygenated blood that flows through the systemic circuit. Q_P effective equals Q_S effective and is calculated by taking the difference between the mixed systemic and pulmonary venous oxygen content and dividing that into the oxygen consumption:

Q_eff = O₂ consumption/(pulmonary venous O₂ content – mixed venous O₂ content)

Shunt flow can be thought of as the ineffective flow in the system: the left-to-right shunt is the volume of oxygenated blood flowing through the lungs and is equal to the total pulmonary flow minus the effective pulmonary flow (Q_P – Q_eff). The right-to-left shunt is the volume of deoxygenated blood flowing through the systemic circulation and is equal to the total systemic flow minus the effective systemic flow (Q_S – Q_eff).

Because of the great variation in size among pediatric patients, flows are always indexed to body surface area.

Resistance is the change in mean pressure divided by the flow. Thus, pulmonary vascular resistance (R_p) is the difference between the pulmonary venous and pulmonary arterial pressures divided by the total pulmonary flow.

R_p = (mean pulmonary vein pressure − mean pulmonary arterial pressure)/Q_P

and the systemic vascular resistance is the difference between the systemic arterial and right arterial pressures divided by systemic flow.

R_s = (mean arterial pressure – mean right atrial pressure)/Q_S

The resulting units, liters per minute per millimeters of mercury, are termed Wood’s units, after the cardiologist Paul Wood. As with flows, vascular resistance is indexed to body surface area in pediatric patients.

In the cardiac catheterization laboratory, gradients across stenotic valves or vessels are generally measured as “peak-to-peak” gradients. This measurement is often inaccurately referred to as the PSEG or peak systolic ejection gradient. The peak-to-peak gradient is a different entity than the maximal instantaneous gradient that is estimated noninvasively by Doppler studies. Whether a gradient is expressed as peak-to-peak, maximal instantaneous gradient, or mean gradient, it is not a
meaningful number unless one has some idea of the amount of flow that is producing the gradient. For example, a patient with critical aortic stenosis who is in low cardiac output may have a relatively low measured gradient across a severely narrowed valve orifice.

The severity of pulmonary valve stenosis is graded according to the peak-to-peak gradient across the valve (trivial, <25 mmHg; mild, 25 to 49 mmHg; moderate, 50 to 79 mmHg; severe, >80 mmHg). In general, the presence of a peak-to-peak gradient of ≥40 mmHg is an indication for treatment. Pulmonary insufficiency is difficult to assess quantitatively in the cardiac catheterization laboratory. A ventricularized pulmonary artery tracing—that is, one in which the diastolic pressure approximates that of the right ventricle—indicates severe pulmonic insufficiency.

Although cardiologists will generally grade aortic stenosis in adults according to the calculated effective valve area, in the pediatric population indications for intervention are based on catheter-measured peak-to-peak systolic pressure gradients. Because gradients across a stenotic valve will vary depending on flow and heart rate, cardiac output, left ventricular function, and mitral and aortic valve insufficiency all must be evaluated to arrive at an accurate assessment of left ventricular outflow tract obstruction. Left ventricular outflow tract obstruction may be subvalvar, valvar, or supravalvar.

Aortic valve insufficiency is assessed in a semiquantitative manner in the cardiac catheterization laboratory by angiography. A wide arterial pulse pressure not only may represent important aortic valve insufficiency but may also be seen with aggressive afterload reduction.

Isolated tricuspid stenosis is extremely rare; more often, tricuspid stenosis is seen in the context of right heart hypoplasia and pulmonic stenosis or atresia. Tricuspid stenosis cannot be assessed in the presence of pulmonic atresia because there will be little or no flow across the tricuspid valve in these cases. The presence of a patent foramen ovale that allows right-to-left shunting or right ventricular diastolic dysfunction (both of which are common in right heart hypoplasia) makes assessment of the degree of tricuspid stenosis problematic. Complete evaluation requires balloon occlusion of the atrial communication and simultaneous measurement of right atrial and right ventricular pressures; this may not be technically feasible particularly in neonates and small infants (e.g., the atrial communication is too large to be occluded). However, the maneuver may be quite useful in evaluating right heart competency in older children noting any resultant changes in right atrial pressure, cardiac index, and heart rate with balloon occlusion.

Angiographic assessment of tricuspid insufficiency can be difficult because the catheter in the right ventricle of necessity crosses the tricuspid valve; if the catheter stents the valve open or if some of the holes of the catheter are in the right atrium at the time the injection is made, artifactual insufficiency will result. In addition, ectopic beats will often be associated with induced atrioventricular valve insufficiency.

Mitral stenosis is generally graded in terms of calculated valve area by cardiologists treating adults and in terms of anatomy and valve gradient by cardiologists treating children. Both the mean valve gradient (arrived at by digitizing the area between the left atrial or pulmonary artery wedge pressure tracing and the left ventricular pressure tracing during the period when the mitral valve [MV] is open) and the gradient between the a-wave and the left ventricular end-diastolic pressure as well as the absolute left atrial pressure are generally considered in assessing mitral stenosis. Because gradients are meaningful only when considered in association with flow, cardiac output must be assessed to appropriately interpret the severity of obstruction. A mean left atrial pressure >25 mmHg is generally associated with moderate-to-severe reactive pulmonary hypertension.

Mitral insufficiency is assessed in a semiquantitative manner by angiography. This is more reliably accomplished than in the case of the tricuspid valve because the catheter with which the picture is taken usually does not cross the MV and the shape of the left ventricle is such that it accommodates a catheter more easily than does the right. However, entanglement of the catheter in the MV apparatus or stimulation of ventricular premature contractions can result in induced mitral insufficiency.

Catheter-directed treatments have assumed a major role in the care of patients with congenital cardiovascular defects. These techniques have replaced surgery as the primary mode of therapy in some instances. In many others, optimal treatment combines staged transcatheter and surgical intervention. It is, therefore, essential that the congenital heart surgeon has a working knowledge of interventional cardiology. This section introduces current catheter interventions for patients with congenital cardiovascular defects, high-lighting therapies particularly useful in combination with surgery.

Catheter interventions for congenital heart defects are quite varied, but in simplistic terms can be classified into: (1) procedures to create or enlarge communications and vessels (septostomies, dilations, and stents); (2) procedures to close vessels or defects (device closures, vascular occlusions); and (3) other (thrombectomy, vascular retrievals). Another commonly performed intervention, radiofrequency ablation, is more appropriately dealt with as part of a discussion of cardiac dysrhythmias.

Balloon atrial septostomy described in 1966 by Rashkind and Miller for palliation of cyanosis in D-transposition of the great arteries is often considered the first intervention for congenital heart defects. Even with the use of prostaglandin E1, many infants with transposition require septostomy procedures to ameliorate compromising cyanosis. The procedure is also performed to relieve left atrial hypertension in infants with left atrioventricular valve stenosis or atresia. The basic technique has changed little since first described. A septostomy catheter is advanced through either umbilical or femoral vein into the right atrium and across the foramen ovale. The balloon is filled with fluid and the catheter pulled quickly across the foramen tearing septum primum. Balloon septostomy is often performed at the bedside under echocardiographic guidance (Fig. 74.1).

In many situations, balloon septostomy is not the best way to create an atrial communication. Balloon septostomy is ineffective in patients over 6 weeks of age because the atrial septum is too thick. In most newborns with HLHS and restrictive or intact atrial septum, balloon septostomy is not feasible or ineffective because the left atrium is too small or posterior deviation of septum primum is present. In these populations, an atrial communication is best created by other means. The intact atrial septum may be traversed either by the standard transseptal puncture technique (Brockenbrough procedure), or with the aid of a specially designed radiofrequency perforation system (Nykanen Catheter, Baylis Medical Corp., Mississauga, ON). Once the atrial septum has been crossed, a hole may be created by static balloon angioplasty, or in the case of the very thick atrial septum, endovascular stent deployment. When the procedure is performed for relief of cyanosis in patients with mixing lesions and left atrial outlet obstruction (such as the newborn with HLHS born with intact atrial septum), the goal should be the creation of an unrestrictive atrial communication. In other circumstances (i.e., augmentation of systemic blood flow in patients with right heart failure, or for left atrial decompression in patients on extracorporeal support for left heart dysfunction), smaller atrial communications suffice. In fact, when these techniques are used to palliate right heart failure (transient right ventricular dysfunction after repair of tetralogy of Fallot, or primary pulmonary hypertension, or fenestration creation for Fontan failure), the procedure results in systemic arterial desaturation; thus, the size of communication created must be carefully titrated to arterial oxygen saturation. The goal is to create a restrictive ASD to augment systemic output without excessive cyanosis. In tetralogy of Fallot or pulmonary hypertension patients, this is best accomplished by graded balloon angioplasty of the atrial septum. In Fontan patients requiring fenestration creation, the optimal strategy depends on the type of Fontan. It may include the use of standard or cutting balloon dilation (lateral
tunnel-type Fontan), or transseptal puncture followed by bare metal or covered stent deployment (extracardiac Fontan).