Cardiac Catheterization and Fetal Intervention

Introduction and Overview

Catheterization in the field of congenital cardiology continues to improve understanding of disease and expand options for therapy. Diagnostic information previously obtained exclusively through catheterization is now routinely acquired through use of a host of diverse and highly sophisticated, noninvasive diagnostic imaging modalities. Invasive hemodynamic measurement and angiographic assessment, however, remain a mainstay in the comprehensive evaluation of the patient with complex heart disease. Furthermore, the interventional role of cardiac catheterization in the management of congenital heart disease continues to evolve, and in conjunction with surgical improvements, presents expanding opportunities for therapeutic intervention. Thus, the catheterization laboratory is increasingly like an operative suite, and in many cases of “hybrid” suites, it incorporates functions of a more traditional cardiac operating room.

Ongoing advances in transcatheter technology ensure that the historical border zone between transcatheter and open therapies will continue to shift and likely broaden. Thus, the team of cardiac surgeons and cardiologists who deal with congenital heart disease will benefit from a contemporary awareness of the capabilities and limitations of their partners' work. What follows is a brief summary of the basic hemodynamic and angiographic information acquired through cardiac catheterization and a rough sketch of the currently practiced procedures. The remainder of the chapter provides a survey of the current state of transcatheter therapies, from valvuloplasty to valve replacement, including angioplasty and defect closure using implantable devices. It also provides an overview of the experience to date with fetal cardiac intervention.

Role of Diagnostic Catheterization

A primary role of the catheterizer, since the earliest days of invasive physiologic assessment of congenital heart disease, has been to safely obtain the data necessary to formulate a complete and accurate anatomic and hemodynamic understanding of the patient. This role as an investigator of physiology remains a defining role of the best invasive cardiologists; a thorough diagnostic catheterization constitutes a valuable part of almost all studies performed, even in the current era of intervention. An equally important role is to assist in planning management and to perform necessary transcatheter interventions as indicated, part of the integrated care of the patient with congenital disease. The ideal care of the most anatomically complex patients will likely require an orchestrated series of both open surgical and catheter-based procedures to achieve good, long-term, functional outcomes. A third, and more recent, role of the interventional cardiologist is to provide definitive therapy via a minimally invasive route, for some highly selected cardiac defects, as an alternative to surgery.

For decades, traditional biplane angiographic imaging constituted an important component of anatomic assessment of congenital heart disease. Expertise in acquisition and interpretation of these images secured the role of angiography in the preoperative investigation of patients with most major anatomic defects. The enormous volume of information provided by this modality has, fortunately, been exhaustively catalogued by Freedom and colleagues. Preoperative anatomic information is now routinely and comprehensively acquired by noninvasive imaging modalities, primarily echocardiography, and increasingly, magnetic resonance imaging (MRI) or computed tomography (CT), as discussed in Chapter 106 . Valvar anatomy is beautifully displayed using transesophageal or three-dimensional echocardiography; ventricular volumes and function are quantified using MRI. Vascular abnormalities, such as aortic arch anomalies, or the complex venous patterns in heterotaxy syndrome, can similarly be well rendered through CT or MRI reconstruction. These modalities will no doubt continue to undergo improvements in ease of acquisition, image resolution, and postprocessing capabilities, likely avoiding the need for diagnostic angiography in all but rare cases. With currently available technology, vascular beds in which angiography remains the gold standard include pulmonary vessels beyond the hilum, and the coronary arterial bed, particularly in younger and smaller patients with highly atypical variants ( Fig. 107-1 ).

FIGURE 107-1, Right ventricular angiography in a neonate with pulmonary atresia and intact ventricular septum. Injection in the right ventricle opacifies the entire coronary artery system and documents right coronary ostial atresia.

With the changing, and increasingly interventional, role of catheterization, the need for integration of three-dimensional anatomic information into the procedure has driven the development of “overlay” technologies and transfer/display of non-angiographic imaging data into the catheterization laboratory. As a natural evolution, the techniques and technology to acquire three-dimensional angiographic data sets have developed and matured to the point of incorporation into clinical practice in many centers. For patients who may also have indications for direct hemodynamic measurement and are candidates for intervention, this alternative provides the advantage of giving the interventionalist highly directed, immediate, as-needed, anatomic information at the time of catheterization. This advantage obviously needs to be considered in the context of the associated radiation exposure but will certainly warrant the angiographic information in some cases. The feasibility of MRI-guided cardiac catheterization has been shown, and MRI-guided interventions have been performed in experimental settings. To date, limitations have been posed by the availability of adequate MRI-compatible intravascular tools and by the processing speed of MRI systems, which lack the real-time feedback of fluoroscopy.

Much hemodynamic information can also be derived from echocardiography and MRI, as is discussed in great depth in Chapter 106 . In some cases, there is no substitute for directly measured pressures that must be obtained invasively. When the best possible estimate of pulmonary vascular resistance is critical—as in the case of (1) pulmonary hypertension, (2) pretransplant evaluation, or (3) preoperative risk assessment for single ventricle palliation—cardiac catheterization is performed despite the availability of other methods. Similarly, even with a myriad of noninvasive indicators of diastolic function, direct measurement of the end-diastolic pressure is not infrequently an indication for catheterization. In the case of both pulmonary vascular assessment and measurement of filling pressures, the catheterization procedure also provides the opportunity to perform maneuvers to understand physiologic responses to intervention, such as administration of nitric oxide, expansion of intravascular volume, infusion of inotropes, or manipulation of pacing parameters. Fortunately for the invasive cardiologist, cardiac catheterization is a highly interactive process, and so as observations are made, hypotheses can be tested, disruptions such as test occlusions can be imposed, or interventions can be undertaken.

For surgical patients, the operating surgeon ultimately decides whether the accuracy and completeness of preoperative diagnostics is sufficient to plan and proceed with the operation with the best possible outcome expected. For patients with complex disease or those with atypical management, early and steady involvement of the surgeon in the preoperative evaluation, including the conduct of the catheterization, is invaluable. A firm grasp of the advantages and limitations of data gained through all available techniques is critical, as is the ability to resolve apparently conflicting findings and place the appropriate weight on specific measurements or interpretations.

Although a series of preoperative hemodynamic catheterizations have been a staple of management for single ventricle disease, it is increasingly argued that many of these studies may be of marginal benefit and may not warrant the risk of an invasive procedure and radiation exposure. Most clinically well patients with favorable findings of noninvasive evaluation are likely to undergo successful bidirectional Glenn operation, or even Fontan, without excess of morbidity and without the benefit of a preoperative catheterization. The role of hemodynamic catheterization in this setting is ultimately determined by the reliability of noninvasive imaging modalities, primarily echocardiography, to screen for and accurately identify potential problems, such as pulmonary arterial distortion or arch obstruction, which should be addressed either in the catheterization laboratory or at the time of operation. Also, it is incumbent on the catheterizer to demonstrate the usefulness of some interventions that might occur at preoperative catheterization. The routine scheduling of catheterization before a Glenn or Fontan procedure is likely to receive increasing scrutiny in the future, and the added value of these studies will need to be defended.

Hemodynamic Assessment

A full hemodynamic data set allows estimation of systemic and pulmonary blood flow, measurement of intracardiac and intravascular pressures with evaluation of pathologic gradients, and calculation of systemic and pulmonary vascular resistances. Two important vulnerabilities of traditional hemodynamic assessment in the catheterization laboratory are the designation of oxygen consumption, which is most often assumed, and the assumption of a steady state. Both of these limitations are more pronounced when dealing with more extreme or labile circulations. Ideally, the primary hemodynamic data can be obtained during catheterization under conditions that approximate each patient's baseline condition. The routine use of general anesthetic may pose some difficulty in this regard.

Flows

Systemic blood flow, or cardiac index, is commonly estimated either by a thermodilution technique or by using the Fick method. Calculation of flow using the Fick method is based on the principle that the total uptake (or release) of a substance by an organ is the product of blood flow to that organ and the difference between the concentrations of an indicator substance in the arteries and veins leading into and out of that organ. When calculating the systemic blood flow, the whole of the systemic tissues are considered the organ, and oxygen is considered the indicator. By measuring the hemoglobin concentration, the percent oxygen saturation of hemoglobin, and the partial pressure of oxygen in a given sample of blood, the oxygen content can be calculated. The oxygen contents of aortic and mixed systemic venous blood samples serve as the necessary concentrations of indicator, and the patient's oxygen consumption represents the total uptake of indicator in a given unit of time (1 minute). The systemic blood flow, or cardiac index (Qs), can be calculated according to the following equation:

Qs (L / \min / m^{2}) = \frac{O_{2} consumption ({mL O}_{2} / \min / m^{2})}{\begin{matrix} systemic arterial - systemic \\ {venous O}_{2} content ({mL O}_{2} / dL) \times 10 \end{matrix}}

$Qs (L / \min / m^{2}) = \frac{O_{2} consumption ({mL O}_{2} / \min / m^{2})}{\begin{matrix} systemic arterial - systemic \\ {venous O}_{2} content ({mL O}_{2} / dL) \times 10 \end{matrix}}$

Although methods exist for measuring oxygen consumption during catheterization, most catheterization laboratories assume a level of oxygen consumption based on the patient's age, sex, and heart rate.

Pulmonary blood flow can be similarly calculated by calculating the oxygen content of pulmonary arterial and venous blood. With the systemic and pulmonary blood flow rates in hand, various shunts can be calculated, and the ratio of pulmonary to systemic blood flow, or Qp : Qs, can be determined. A rapid bedside calculation of Qp : Qs can be obtained by simply using the following equation:

Qp : Qs = \frac{aortic saturation - mixed venous saturation}{\begin{matrix} pulmonary venous \\ - pulmonary arterial saturation \end{matrix}}

$Qp : Qs = \frac{aortic saturation - mixed venous saturation}{\begin{matrix} pulmonary venous \\ - pulmonary arterial saturation \end{matrix}}$

Pressures

At catheterization, pressure measurements are obtained through fluid-filled catheters, which are vulnerable to various errors, ranging from an inappropriate zero level, to catheter entrapment with waveform distortion. Ideal catheters for pressure transduction provide free communication between the environment external to the catheter tip and the lumen of the catheter. Thus, larger-bore catheters, or catheters with multiple end or side holes, yield the most accurate depiction of the intracardiac waveform. A complete set of pressure measurements includes pressure waveforms recorded in all cardiac chambers and great vessels. When the left atrium is not entered directly, a pulmonary capillary wedge tracing is recorded in lieu of a left atrial trace. Systolic and diastolic pressures are noted in arterial vessels, whereas mean pressures are commonly noted in atrial or venous tracings. In the ventricle, the systolic and end-diastolic pressures are specified. Gradients can be documented by single catheter pullback or by simultaneous recordings in adjacent chambers or vessels using multilumen catheters or multiple catheters. When interpreting pressure gradients to determine the severity of obstructive lesions, the general principle of Ohm's law as it applies to hemodynamic variables must be considered, and the gradient should be considered in the context of the flow across the lesion.

Resistances

Having measured pressures and calculated flows, the resistance across a vascular bed can be calculated by relating the mean pressure change (ΔP) across that vascular bed to its flow. In the following equation,

R = \frac{Δ P}{Q}

$R = \frac{Δ P}{Q}$

will thus be applied as

PVR = \frac{TPG}{Qp},

$PVR = \frac{TPG}{Qp},$

where TPG is the transpulmonary gradient, which is equal to the mean pulmonary artery pressure minus mean pulmonary venous pressure, and PVR is pulmonary vascular resistance. The pressures are measured in millimeters of mercury (mm Hg), and flows are typically indexed and expressed in liter/min/m ² . Thus resistance is commonly expressed as mm Hg/liter/min/m ² , or indexed Wood units.

Sedation and Anesthesia

There is wide institutional variability in approaches to sedation for catheterization. Stimuli associated with catheterization include local pain related to percutaneous placement of access sheaths and more visceral pain related to intracardiac or intravascular manipulation and interventions. Provoked ectopy or arrhythmia, even if hemodynamically stable, may be disturbing to awake patients. Older, cooperative children without major hemodynamic compromise can, for the most part, undergo a routine catheterization safely and comfortably with conscious sedation and a spontaneous airway. In preparation for the procedure, oral benzodiazepines can be administered as anxiolytics before placing an intravenous line. Once intravenous access is established, a combination of benzodiazepines and opioids works well for most children who undergo catheterization without general anesthesia. Bolus doses of these drugs may be administered over the course of the procedure, or a continuous infusion may be preferable, depending on the nature and duration of the case. Alternative agents that have been used successfully with appropriate oversight are ketamine in conjunction with midazolam, and propofol infusion.

In the current era, general inhalational anesthetic is administered for most pediatric cases, to minimize the potential for patient discomfort. Some interventional cases warrant general anesthetic almost regardless of the tolerance or cooperativity of the patient, because of the anticipated duration of the procedure, high-risk nature of the intervention, necessity of patient immobility, and/or the need for both airway and hemodynamic control. Most infants and young children who are likely to be undergoing interventional procedures are most safely catheterized under general anesthesia. In addition, individuals who require uncomfortable additional impositions, such as placement of a transesophageal echocardiographic probe for monitoring device closure of atrial septal defects (ASDs), or placement of internal jugular or subclavian catheters, are less likely to be distressed if under a general anesthetic.

Very rarely, the induction of general anesthesia or use of inhalational anesthetic may, in and of itself, be destabilizing, as can be the case in children with left ventricular (LV) outflow obstruction and/or coronary compromise, or those with severe restrictive physiology. Patients with severe heart failure who rely on endogenous catecholamine levels for maintenance of circulation can also be at risk with typical anesthetic induction techniques. Consultation with, and anticipatory inclusion of, members of a dedicated cardiac anesthesia team before catheterization serves the best interests of the patients and their caregivers and contributes to successful completion of the procedure.

Vascular Access

Standard Arterial and Venous Access

For most catheterization procedures, intravascular access is established and maintained in both an artery and a vein. Even when a single venous catheter may provide access for a full hemodynamic data set, an arterial catheter is often placed for continuous intraprocedural monitoring. Placement of an arterial line in these cases should be weighed against the risk of compromised postprocedural perfusion of the extremity. Highly focused cases in stable, healthy subjects (e.g., ASD closure, pulmonary valvuloplasty) are usually performed with only a venous line, as are many pulmonary hypertension studies and routine cardiac biopsies. In some emergent clinical situations, when there is a premium on speed of intervention, the case should not be delayed while placing “elective” access, but rather the catheterizer should be creative about the most expedient access possible to complete the procedure.

Vascular access is typically established in the femoral vessels, as the consistent anatomy, superficial course, and relative sizes of these vessels allow straightforward percutaneous entry. The side-by-side placement of femoral venous and arterial lines allows the operator to stand over a well-circumscribed field that can easily be kept sterile and allows the patient to be continuously audited for catheter displacement or bleeding. Given the placement of standard anteroposterior and lateral cameras in a biplane lab, the femoral sites are also the most ergonomically favorable. Approach to the heart via the femoral vein and the inferior vena cava allows entry to almost all necessary catheter sites, including complex trans-septal courses that can be difficult from a superior approach. At the conclusion of the procedure, catheter removal and control of the vessels in the groin with manual compression is relatively easy, and the procedure is well tolerated by individuals of all ages. In pediatric catheterization, manual compression is most often applied to achieve hemostasis. In larger patients, especially those who have had large caliber sheaths in the femoral vessels, the use of percutaneous site closure systems can significantly reduce the time to hemostasis and the risk of late rebleeding.

Alternate Routes of Access

For some catheterizations, alternative sites of access are electively chosen. In newborns, part or all of the study can often be carried out from the umbilical vessels. The umbilical vein approach is ideal for balloon atrial septostomy in infants with transposition of the great arteries, and the use of this access site spares the femoral vein from potential injury by the relatively large sheath. Compared with standard femoral access, umbilical access makes complex venous catheter manipulation more difficult, and greater attention must be paid to maintaining sheath positioning. Only simple manipulation of the arterial catheter is possible, and catheter exchange should be limited, as the tortuous abdominal course from the umbilical artery to the aorta can predispose the patient to arterial trauma.

Access from the jugular or subclavian vein may be planned in some cases because of anatomic factors, such as the presence of a Glenn anastomosis, when there is typically no other navigable pathway to the pulmonary arteries. Non-femoral sites may also be electively chosen in smaller patients who require larger access sheaths, for example, for device delivery. The jugular or subclavian vein is typically larger than the femoral vein, and if both communicate in a usual way with the right atrium, these brachiocephalic vessels may be an alternative, or even preferred, route. In rare cases, transhepatic access is electively chosen to provide a short, direct course to the atrial septum or pulmonary veins for defect closure or creation, or for pulmonary venous sampling or interventions.

Increasingly, alternative (to the femoral vessels) access is used out of necessity, because the femoral vein or artery is occluded. Brachiocephalic veins may be used primarily when prior procedures or indwelling catheters have caused vascular compromise and loss of the femoral venous access. Similarly, a percutaneous transhepatic route can be used safely, even in infants. Importantly, we and others have found that in many cases of suspected or known loss of femoral vessels, taking a more aggressive approach with the use of specialized guidewires, catheters, and fluoroscopic guidance will often allow reestablishment or enlargement of femoral vascular continuity with the central circulation. This enables not only cardiac catheterization from these routes but placement of long-term indwelling lines. Long-term patency remains an issue and may be improved with placement of intravascular stents or antithrombotic therapy.

The cardiac surgeon may be asked to assist in a number of ways to facilitate the access for a catheterization procedure. In a backup capacity, surgical cutdown may be necessary when attempts at necessary percutaneous access fail. Most interventionalists have limited experience with these procedures and will require the assistance of a surgeon. This may involve cutdown on extremity vessels (femoral, radial, axillary) or on neck vessels. Surgical cutdown on the carotid artery can greatly facilitate aortic valve dilation in small infants, allowing for larger-caliber dilating balloons to be easily introduced and often for easier retrograde passage through a highly obstructed valve.

Beyond extremity or neck cutdowns, the surgeon and interventionalist can work together to conduct procedures with a variety of alternative surgical exposures. A limited thoracotomy can enable a better angle of approach to the right atrium for device closure of large ASD, or a more controlled site of direct LV access for aortic valve delivery, or external control/reduction and visualization of the right ventricular (RV) outflow tract during placement of a transcatheter pulmonary valve. The hybrid stage I procedure is performed with open chest access to deliver the ductal stent directly from the main pulmonary artery and perform pulmonary arterial bands simultaneously. With the increasing use of catheter-based devices, many of which require relatively large delivery systems, surgical access to large intrathoracic vessels or even direct cardiac access, will likely become a more frequent opportunity for creative interaction between interventionalist and surgeon.

Interventions

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