Invasive Procedures

A. Indications

Accurate diagnosis of most CHDs does not require diagnostic catheterization. With improved capability of noninvasive imaging tools such as echo and color flow Doppler studies and radiologic techniques such as cardiac magnetic resonance imaging and cardiac computed tomography, many cardiac problems are adequately diagnosed and managed without cardiac catheterization studies. Indications for these invasive studies vary from institution to institution and from cardiologist to cardiologist. The following are some circumstances that suggest the need for cardiac catheterization.

• 1.

  To perform balloon procedures for angioplasty (with or without stent placement), valvuloplasty, or balloon atrial septostomy in patients with lesions amenable to these procedures

• 2.

  To perform pulmonary valve replacement with a tissue valve (Melody valve and Edwards SAPIEN valve) that is sutured to a stent and is deployed percutaneously

• 3.

  To determine accurate pressure gradients in combined lesions of AS and AR or PS and PR, or multiple levels of obstruction

• 4.

  To assess pulmonary hypertension and its responsiveness to vasodilator therapy

• 5.

  To calculate pulmonary vascular resistance as a preoperative study or in the setting of low-flow lesions, such as seen in patients after bidirectional Glenn operation or after complete Fontan operation

• 6.

  To determine details of pulmonary vascular supply, the aortopulmonary collateral supply, and the coronary artery anatomy in patients with pulmonary atresia with intact ventricular septum or pulmonary atresia with complex ventricular anatomy

• 7.

  To find answers to postoperative problems such as excessive desaturation after a B-T shunt or bidirectional Glenn operation, or when excessive aortopulmonary collateral is suspected

• 8.

  To assess post-transplantation vasculopathy and to obtain endomyocardial biopsy for rejection identification in cardiac transplantation patients

• 9.

  To assess cardiomyopathy or myocarditis

• 10.

  To assess coronary circulation in some cases of Kawasaki disease

B. Sedation

A number of sedatives have been used by different institutions with equal success rates. Smaller doses of sedatives are usually used in cyanotic infants. General anesthesia is usually used, especially when an interventional procedure is planned.

Among the sedatives that have been used are chloral hydrate, diphenhydramine, meperidine (Demerol), promethazine (Phenergan), chlorpromazine (Thorazine), ketamine, and morphine. It should be kept in mind that ketamine has important hemodynamic effects; it increases the SVR and blood pressure.

C. Hemodynamic Values and their Calculations

Pressure and oxygen saturation values for normal children are shown in Fig. 6.1 . During cardiac catheterization, cardiac output, cardiac shunt, and vascular resistance are routinely calculated.

• 1.

  Flows (cardiac output) are calculated by the Fick formula:

  $\text{Pulmonary}\text{flow}\left(\text{Qp}\right)=\frac{{\mathrm{VO}}_2}{C_{\mathrm{PV}}-C_{\mathrm{PA}}},$
  $\text{Systemic}\text{flow}\left(\text{Qs}\right)=\frac{{\mathrm{VO}}_2}{C_{\mathrm{AO}}-C_{\mathrm{MV}}},$

  where flows are in liters per minute, VO ₂ is oxygen consumption in milliliters per minute, and C is oxygen content in milliliters per liter at the various positions: the PV, PA, aorta (AO), and mixed systemic venous blood.

  Oxygen consumption is either directly measured during the procedure or estimated from a table (see Appendix, Table A.6 ). Oxygen content (milliliters per 100 mL of blood) is derived by multiplying oxygen capacity by percent saturation. Oxygen capacity (milliliters per 100 mL of blood) is the total content of oxygen that hemoglobin contains when it is 100% saturated (1.36×hemoglobin in grams per 100 mL). Normal systemic flow (or pulmonary flow in the absence of shunt) is 3.1±0.4 L/min/m ² (i.e., cardiac index).

• 2.

  The magnitude of the shunt is calculated as follows:

  $\text{Left}-\text{to}-\text{right}\left(L-R\right)\text{shunt}=\text{Qp}-\text{Qs}$
  $\text{Right}-\text{to}-\text{left}\left(R-L\right)\text{shunt}=\text{Qs}-\text{Qp}$

  In pediatrics, the ratio of pulmonary to systemic flow (Qp/Qs), which does not require an oxygen consumption value, is often used. The ratio provides information on the magnitude of the shunt. Patients with an L-R shunt greater than 2:1 are usually candidates for surgical or percutaneous intervention.

• 3.

  Vascular resistances are calculated by using formulas derived from Ohm’s law (R = ΔP/Q)).

  $\text{PVR}=\frac{\text{Mean}\text{PA}\text{pressure}–\text{mean}\text{LA}\text{pressure}}{\mathrm{Qp}}$
  $\text{SVR}=\frac{\text{Mean}\text{PA}\text{pressure}–\text{mean}\text{LA}\text{pressure}}{\mathrm{Qp}}$

  The SVR varies between 15 and 30 units/m ² . The normal PVR is high at birth but reaches near-adult values (1 to 3 units/m ² ) after 2 to 4 months. The normal ratio of PVR/SVR ranges from 1:20 to 1:10.