The cardiac catheterization laboratory can play an important diagnostic and/or therapeutic role in the management of children in the pediatric intensive care unit.
For patients with cardiac disease whose critical care course is not progressing as expected, early exploration for diagnosis of unsuspected or residual defects or hemodynamic derangements by cardiac catheterization is sometimes warranted.
To maximize the utility of catheterization, it is important to understand procedural benefits and limitations.
Effective communication between care teams is important to ensure safety and to maximize procedural benefits.
Cardiac catheterization provides an important diagnostic and therapeutic option to enhance the management of children in the critical care environment. Since first used in the 1940s, the role of cardiac catheterization has continued to evolve. With improvements and better understanding of noninvasive imaging, cardiac catheterization is rarely used as the initial diagnostic modality for cardiac disease. However, it continues to provide valuable diagnostic information in select clinical scenarios. Moreover, an increasing repertoire of interventional approaches has expanded the potential therapeutic role of cardiac catheterization as a less invasive method for treating certain cardiac diseases in children. As with all clinical studies or tests, it is important to understand and interpret catheterization results within the context of the study limitations and environment obtained. Furthermore, to maximize therapeutic benefit from cardiac catheterization, the critical care team must understand appropriate timing, indications, and limitations of catheter-based interventions. In the appropriate setting, comprehensive hemodynamic data, angiography, and therapeutic interventions from a catheterization can help the pediatric critical care team in managing complex patients with cardiac disease.
Catheterization laboratory environment
Cardiac catheterization laboratories are often remote from the intensive care unit (ICU) and rarely configured to accommodate critical care personnel and equipment. Safe and effective performance of a cardiac catheterization requires planning and multidisciplinary coordination between the ICU and catheterization lab staff, respiratory therapy, perfusion, and anesthesia. Safety starts with transportation of the critically ill child from the ICU to the catheterization laboratory, a process that should be carefully planned in advance, particularly for patients requiring mechanical circulatory or ventilator support. Once in the laboratory, space may be limited; therefore ergonomics should be considered in advance in case of emergencies that might require access to the patient. Application of all invasive and noninvasive monitoring should occur prior to full sterile draping of the patient, sometimes even in the ICU prior to transport. Adequate sedation and anesthesia during cardiac catheterization are essential to facilitate acquisition of meaningful hemodynamic data and to facilitate interventional procedures. For many interventional procedures, sedation may be appropriate; however, for prolonged procedures or those with the potential for significant hemodynamic compromise, general anesthesia is preferable. Ideally, for critically ill patients, a dedicated pediatric cardiac anesthesia team should manage sedation or anesthesia, mechanical ventilation, and medication administration during the procedure. Communication between the anesthesia and cardiology teams is crucial to optimize procedural safety and to facilitate obtaining accurate hemodynamic data, as these data can be affected by changes in oxygenation, ventilation, and hemodynamic state.
Diagnostic cardiac catheterization
Diagnostic cardiac catheterization assesses the blood flow, shunting, and resistance through the cardiac system, and can provide dynamic intracardiac and intravascular imaging. In patients with complex cardiac anatomy, pulmonary hypertension, unexplained cardiac or pulmonary hemodynamic compromise, or with concerns for important residual or recurrent anatomic lesions after cardiac surgery, physiologic data from catheterization may provide important diagnostic information that cannot be assessed by less invasive means. However, it is necessary to understand the limitations and sources of error in the catheterization laboratory in order to appropriately interpret and apply the data obtained. Cardiac catheterization provides a hemodynamic assessment at a moment in time, often under sedation, that may differ from the dynamic changes in a wakeful or agitated state. The hemodynamics may change over time with disease progression or interventions. Furthermore, errors can be introduced when hemodynamic parameters, such as blood flow, shunts, and resistance, are calculated. For these reasons, providers should interpret hemodynamic data with caution and with thoughtful consideration of the limitations.
Differences in oxygen saturations across vascular beds are used to calculate blood flow and shunting in the cardiovascular system. Saturations are typically obtained in the systemic veins, across the chambers of the right heart, and in the pulmonary arteries (PAs). Systemic saturations and, when accessible, pulmonary venous saturations, are also a standard part of a comprehensive diagnostic cardiac catheterization. In patients with hemodynamically significant left-to-right shunting lesions (i.e., atrial or ventricular septal defects), shunting of oxygenated blood into the deoxygenated circuit will result in a step-up in saturations across the right side of the heart. Conversely, in patients with right-to-left shunts (e.g., tetralogy of Fallot) or complete mixing lesions (e.g., single-ventricle heart defects), shunting will result in systemic desaturation. When interpreting saturation data, it is important to recognize that there can be significant sampling variability. For example, sampling in proximity to the low-extraction renal blood return will result in higher saturations than sampling in close proximity to the high-extraction hepatic or coronary sinus blood return.
A full pressure assessment of the right side of the heart consists of direct pressure measurements in the branch PAs, main PA, right ventricle, and right atrium. Pressure assessment of the left side of the heart consists of direct pressure measurements in the left ventricle, ascending aorta, and descending aorta. For patients with an atrial septal defect (ASD) or patent foramen ovale (PFO), a direct left atrial pressure may be obtained. For patients with an intact atrial septum, direct left atrial pressure can be obtained only by performing a transseptal puncture; therefore it is typically estimated by obtaining a pulmonary capillary wedge pressure (PCWP). Fig. 30.1 demonstrates normal intracardiac and PA pressures.
When stenosis is present, the degree of obstruction can be quantified by the pressure gradient across the lesion. For ventricular and arterial pressures, the peak systolic gradient typically estimates the degree of stenosis. For venous pressures, or when measuring pressure gradients across the atrioventricular valve, the mean gradient is more commonly used to estimate stenosis.
The two most commonly used methods to estimate blood flow through the cardiovascular system are thermodilution and the Fick method. In a structurally normal heart without significant valve regurgitation or intracardiac shunting, thermodilution can be used to assess the cardiac output. The thermodilution catheter has a proximal port that sits in the right atrium as well as a distal thermistor that is positioned in the PA and measures the change in blood temperature following a cold saline injection through the proximal port. In a structurally normal heart, the estimated blood flow using thermodilution is equivalent to the cardiac output since the pulmonary (Qp) and systemic (Qs) blood flows are equal (Qp:Qs = 1). Since most patients with congenital heart disease have some degree of intracardiac shunting lesion and/or valvar insufficiency, thermodilution cannot be used to measure cardiac output, and the Fick method is preferred. Table 30.1 lists simplified equations for calculating flows, shunts, and resistance by the Fick method. The Fick method estimates blood flow by measuring the change in oxygen content across a vascular bed relative to the rate of oxygen extraction or delivery quantified by the patient’s oxygen consumption (V o 2 ). , The oxygen content of the blood is composed of oxygen bound to hemoglobin (represented by the oxygen saturation), and the concentration of dissolved oxygen (represented by the partial pressure of oxygen [P o 2 ]). The dissolved oxygen content is low in patients breathing room air and can be omitted from the equation. When supplemental oxygen is used, the dissolved oxygen concentration increases and must be accounted for in the calculation by adding 0.0032 × P o 2 to the denominator of the equation.
|Cardiac output (Qs) (L/min/m 2 )|
|Pulmonary blood flow (Qp) (L/min/m 2 )|
|Pulmonary vascular resistance (WU × m 2 )|
|Systemic vascular resistance (WU × m 2 )|
Estimating the ratio of blood flowing through the pulmonary vascular bed to blood flowing through the systemic vascular bed is important for many congenital heart lesions. Commonly referred to as Qp:Qs, this ratio can be calculated by dividing the difference between the aortic and mixed venous saturations by the difference between the pulmonary vein and PA saturations (see Table 30.1 ; assuming that Fi o 2 = 21%). Quantifying the degree and direction of shunting through the cardiovascular system can help to inform clinical management decisions, including expected clinical course and/or need for surgical referral.
Based on the principle of Ohm’s law, the change in pressure through a system is equal to the flow of blood multiplied by the resistance to flow. This equation is rearranged such that measured and estimated values obtained during the catheterization are used to determine vascular resistance (see Table 30.1 ). The equation uses the change in mean pressures across a vascular bed and assumes equal blood flow across all vascular beds. For this reason, one cannot accurately calculate pulmonary vascular resistance in the setting of branch pulmonary stenosis or pulmonary vein stenosis without knowing the percentage of blood flow to each lung. This can be accomplished with supplemental data from a lung perfusion scan or cardiac magnetic resonance imaging (MRI) but requires a more complicated equation for calculating resistance in a parallel system.
Imaging of the cardiovascular system in the catheterization laboratory is performed via fluoroscopy with timed injections of iodinated contrast to visualize the intraluminal surfaces of the heart and vessels in real time. Catheterization laboratories specializing in congenital heart disease typically use biplane angiography systems, allowing for selection of a variety of imaging angles as well as simultaneous imaging in two planes. Over the last decade, rotational angiography has become a new diagnostic tool in the pediatric catheterization laboratory to generate three-dimensional images that assist in identifying pathology, guiding interventions, and monitoring for complications.
Due to advances in noninvasive imaging, cardiac catheterization is no longer the first-line diagnostic modality for congenital heart disease. In fact, most patients with congenital cardiac defects do not require a cardiac catheterization prior to surgical intervention, assuming an expected preoperative course. However, there are numerous clinical scenarios in which a diagnostic catheterization is indicated to provide supplemental and/or definitive diagnostic data for the patient. In 2011, Feltes et al. published a scientific statement from the American Heart Association that summarizes indications for cardiac catheterization and intervention in pediatric cardiac disease. Table 30.2 summarizes indications for diagnostic catheterization. Before proceeding with an invasive catheterization, it is recommended that a complete noninvasive imaging evaluation be completed. It is important to note that the guidelines do not provide an exhaustive list of potential indications, and many are supported by expert consensus only. Additionally, a cardiac catheterization is an invasive procedure with associated risks. For these reasons, open communication between intensivists, interventionalists, and potentially surgeons is key to providing high-quality patient care that minimizes harm.