Abstract
Bedside ultrasonography has increasingly become a central part of critical care in recent times as a modality for procedural guidance and patient assessment. Despite the longstanding role of ultrasonography in medicine, recent advances in ultrasonographic technology have increased machine portability and ease of use and facilitated the accessibility of ultrasonography to intensivists. With regard to vascular access, ultrasonographic guidance has demonstrated superiority to traditional landmark methods in terms of first-pass success and has reduced complications in central venous and peripheral arterial catheterization. Additionally, it has become an important adjunct in the guidance of other percutaneous procedures such as thoracentesis, pericardiocentesis, and paracentesis. Bedside ultrasonography also facilitates the timely use of the technology in the intensive care unit for patient assessment. In addition to being useful for assessing effusion spaces as suggested by its utility in drainage procedures, it can evaluate for pathologies already well described in the existing ultrasonography literature. Examples include lung consolidation, diaphragm paresis, severely depressed cardiac function, and intravascular volume depletion among others. Rapid deployment of ultrasonography at the intensive care unit bedside also facilitates the development of new applications, such as the assessment of pneumothorax, cardiac arrest for reversible causes, and end-organ perfusion in shock. Ultrasonography has long been an important modality in patient care for procedural guidance and diagnosis. As its ease of use progressively increases, intensivists can expect new innovations in its bedside use and improved understanding of acute physiology in the critically ill pediatric cardiac patient.
Key Words
vascular access, paracentesis, pericardiocentesis, shock, echocardiography
With improving image quality and increasingly available portable machines, bedside ultrasonography has become central to the provision of high quality pediatric critical care. Bedside ultrasonography has been used for a number of years in the adult intensive care unit (ICU) and has become more common in pediatric ICUs (PICUs) for a variety of procedures. Ultrasonographically guided procedures, such as paracentesis and placement of arterial lines, have been shown to have lower complication rates and higher success rates. Recently bedside ultrasonography has become increasingly widespread as a diagnostic tool that can be used emergently in the PICU. Bedside ultrasonography helps intensivists to make timely decisions without exposing children to radiation. Bedside ultrasonography also permits serial examinations, which are useful in the PICU, where a child’s clinical status may be quickly evolving. With increasing exposure to bedside ultrasonography for a variety of different procedures and monitoring techniques, intensivists in the cardiac PICU can expand the use of this exceptionally valuable tool for patient management.
A spectrum of machines is available to the pediatric intensivist in the cardiac PICU. Full diagnostic platforms may have a variety of phased array transducers, transesophageal echocardiography capabilities, or advanced postprocessing features. In contrast, ultraportable handheld machines may be rapidly mobilized for emergencies even in confined spaces ( Fig. 35.1 ). Each machine class possesses inherent benefits and limitations for PICU workflow, and decisions on which type to deploy in a PICU depend upon the anticipated clinical needs of the patient. For the applications described in this chapter the authors recommend an ultrasonography machine capable of good two-dimensional imaging with color Doppler and M-mode imaging capabilities. Recommended transducers include a pediatric phased array transducer primarily for diagnostic applications and a linear array transducer for procedural applications. Though the phased array transducer is best suited to cardiac imaging because of its sector-shaped imaging field, low frequency for deep structure imaging, and excellent temporal resolution of moving structures, alternatives may include curvilinear or microconvex transducers or a linear array probe with a trapezoid imaging feature to provide a wide field of view of deep structures ( Fig. 35.2 ). The authors also recommend having multiple linear array transducers of different sizes to accommodate different procedural applications. A linear array that operates at frequencies toward 15 MHz and greater generates higher-resolution images of shallow structures for peripheral venous access in small children. A lower-frequency linear transducer capable of reaching 7 MHz or lower may be used for imaging a deep femoral vessel in an adolescent with obesity or contractures.
Vascular Access
Obtaining central or peripheral access is a common procedure in any pediatric cardiac ICU. Although these procedures are performed daily, complications from failed attempts and delays in care remain a concern. Given the increasing availability of ultrasonography, many intensivists now obtain vascular access under ultrasonographic guidance, which decreases the rate of failure and complications in both pediatric and adult patients. It is unclear how many intensivists use ultrasonography for vascular procedures in pediatrics, but in the adult population, estimates range from 28% to 73% depending on site. Intensivists cite several barriers to more frequent use of ultrasonography, such as limited availability of the ultrasonography machine, perceived increased time required for the procedure, and concern that they will lose the skills required to place lines using only anatomic landmarks.
Ultrasonography allows for direct visualization and facilitates the clinician’s understanding of patient anatomy and physiology. With advances in image quality, arteries and veins are often distinct in appearance, even on portable ultrasonography. The walls of the artery appear thicker due to the muscular tunica media. The vessel resists compression and may pulsate. In contrast, the vein has a thinner wall and is easily compressible. In some clinical scenarios the vein may pulsate when the vessel runs in close proximity to an artery or in the setting of significant pulmonary hypertension. Compressibility is typically the most specific means of identifying the vein; however, if distinguishing the vessels remains difficult, the intensivist may use Doppler ultrasonography to determine the flow. Color Doppler information should be interpreted in context with other findings because inappropriate gain, scale, frequency, or other settings may affect the intensivist’s ability to detect flow. It is important to remember that typically blue colors denote flow away from the probe and red colors denote flow toward the probe, not venous or arterial blood ( Fig. 35.3 ). After determining the artery and vein, the intensivist should trace the vessels both distally and proximally to identify whether branching, tortuosity, stenosis, or thrombosis might interfere with cannulation.
Ultrasonography can be used for site selection, which is crucial in children with critical cardiac disease due to unique vessel anatomy either congenitally acquired or from prior procedures. For example, ultrasonography may be used to locate vessels for instrumentation if the subclavian and jugular veins must be avoided to preserve the vascular architecture. In addition to traditional ultrasonographically guided cannulation in the internal jugular and femoral vein sites, use of ultrasonography has been described in identification of the axillary, subclavian, and peripheral upper extremity veins for cannulation with central venous catheters and peripherally inserted central catheters. An ultrasonographic survey of multiple sites before a vascular access procedure can help with site selection for a vessel that is patent and most easily accessible.
Ultrasonography may be used for either static evaluation of the vessel anatomy before cannulation or for real-time evaluation during needle insertion. There are two primary methods for real-time needle guidance. The first is to hold the probe transverse to the plane of the vessels to allow for continuous visualization of the vein and any neighboring arterial structures. However, this method requires the operator to repeatedly advance the needle and then find the needle tip. The inexperienced operator may be more likely to advance the needle too deeply using this method. A Pythagorean approach has been posited recommending that the operator first identify the depth of the target vessel, insert the needle at a 45-degree angle, then proceed from distal to proximal at an insertion site distal to the ultrasonography probe at a distance equivalent to the depth of the vessel from the probe face ( Fig. 35.4 ). With this arrangement the operator could theoretically anticipate the needle intersecting the plane of the ultrasonography transducer in the vicinity of the vessel. The authors of this chapter recommend following the needle’s tip to the vessel because this method facilitates active guidance of the needle for first-pass success. Another alternative is to hold the probe longitudinal to the plane of the vessel of interest, allowing for a continuous view of the needle entering the selected vessel. Although this method results in better visualization of the needle, there is a higher chance that the probe will come out of plane with the vessel. This may lead to frustration and cannulation of the inappropriate vessel and is recommended for a more advanced user.
Site-Specific Considerations
Using ultrasonography during placement of a femoral central venous catheter allows direct visualization and may help the operator better position the leg to achieve separation of the two vessels ( Fig. 35.5 ). Although there is limited data available in pediatrics, some studies have demonstrated decreased failure rates, a reduction in arterial puncture, and higher first-attempt success when catheters are placed under ultrasonography. More data exist in the literature for adults. Although there is also slightly increased first-attempt success, a Cochrane Library review of available literature found no difference was appreciated in arterial puncture or other complications for adults or children.
The data for internal jugular and subclavian venous catheters in pediatrics is similarly limited. Ultrasonographic guidance has been shown to decrease the rate of failure and arterial puncture but may increase the time to cannulation. Unfortunately, there are no pediatric trials comparing ultrasonographically guided placement to using anatomic landmarks, but in adults ultrasonographic guidance has been shown to result in fewer failures, fewer hematomas, and a reduction in arterial punctures.
In the pediatric cardiac ICU, umbilical arterial and venous catheters are common devices for vascular access in the newborn with congenital heart disease. During catheter placement a sector imaging transducer or linear array is placed over the subxiphoid region in the sagittal orientation to capture the inferior vena cava (IVC) longitudinally in the view of the transducer. The approach of the umbilical venous catheter is visualized through the umbilical vein diagonally approaching the inferior cavoatrial junction from the surface. Insertion depth can be confirmed with the tip reaching the junction before the first verification radiograph ( Fig. 35.6A ). Similarly, insertion of an umbilical arterial catheter can be confirmed by aligning the probe with a longitudinal view of the descending aorta, allowing for simultaneous confirmation of tip placement at the level of the diaphragm (see Fig. 35.6B ). Studies have demonstrated that ultrasonography is superior to x-ray examination when determining umbilical catheter position. Use of real-time ultrasonographically guided placement decreases the average time required to place an umbilical catheter. Although cardiac patients were excluded from these studies, in the patient with normal abdominal vascular anatomy an operator could anticipate similar performance of ultrasonographic guidance for arterial or venous catheter placement.
Arterial Access
Arterial catheterization may also be performed with either static or real-time ultrasonographic guidance in a manner similar to venous catheterization. Studies have demonstrated decreased incidence of hematomas, as well as decreased time to and increased success rate of cannulation compared with using palpation or Doppler. The usefulness of ultrasonographic guidance may be more pronounced in smaller children, who tend to be more difficult to cannulate.
Although the literature remains complicated and somewhat conflicted on the benefits of ultrasonographic guidance during vascular access, ultrasonography remains a noninvasive means of potentially improving outcomes without significant associated risks.
Cardiac
Echocardiography is a mainstay of diagnosis and management in the pediatric cardiac patient, with an emphasis on detailed, highly optimized, and complete imaging of the heart. Bedside ultrasonography complements the physical examination and the formal echocardiogram, permitting the intensivist to have real-time, focused diagnostic imaging. Although bedside ultrasonography may never replace diagnostic echocardiography by a pediatric cardiology service, the focused cardiac ultrasonography examination allows the intensivist to serially examine a patient and answer specific management questions, including evaluation of volume status, qualitative assessment of cardiac function, and evaluation of a pericardial effusion. In several limited series, bedside ultrasonography has modified the diagnosis and management of pediatric patients in the PICU.
The tenets underlying cardiac evaluation using bedside ultrasonography are derived from diagnostic echocardiography. Selection of probe size may be important for visualization and may be limited by what is available in each cardiac PICU. When available, smaller, higher-frequency phased array probes are well suited for infants and small children because of their higher near-field image resolution and ability to maintain skin contact over a range of angles on a smaller thorax. Larger phased array probes are more likely to contain a single crystal advantageous for imaging, are capable of imaging deeper structures at lower frequency, and are appropriate for older children and young adults. Ultrasonography machines tend to use cardiac presets in which the screen indicator appears to the right of the screen, in contrast to the radiology conventions in which the indicator appears on the left of the screen. In this section on cardiac imaging, position will be described presuming screen indicator placement on the right side of the screen. The basic cardiac windows used by the cardiac intensivist are similar to those used by pediatric echocardiography services, including the parasternal, apical, and subcostal windows. Differences may occur between specialties in the orientation of the cardiac views. By convention, many pediatric echocardiography laboratories orient images obtained in the subcostal and apical position with the probe at the bottom of the screen, in contrast to parasternal and suprasternal views, in which the probe remains at the top of the screen. The pediatric intensivist, in keeping with many adult echocardiography services, may maintain the probe at the top of the screen in all transthoracic views for ease of operation among operators ( Fig. 35.7 ). In the cardiac PICU, patient cardiac morphology naturally spans a wide spectrum of potential variants. Imaging of the heart is best performed when safe for the patient and the intensivist understands the patient’s cardiac anatomy, has a clear clinical or academic question to answer, and reviews the results collaboratively with diagnostic echocardiography services.
Qualitative Evaluation of Cardiac Function and Cardiac Views
The parasternal window offers the intensivist a qualitative means of assessing a variety of aspects of cardiac function. The parasternal view is generally obtained between the third and fourth intercostal space with the probe close to the sternum. In the short-axis view the indicator is pointed toward the patient’s left shoulder. The parasternal short-axis view at the level of the papillary muscles ( Fig. 35.8A ) visualizes the ventricle at the point where radial myocardial movement is most apparent. This permits the intensivist to qualitatively evaluate ejection fraction and identify asymmetric movement of any areas of the circular left ventricle (LV). A noncircular, or D -shaped, LV during diastole or through the entire cardiac cycle suggests right ventricular (RV) dysfunction. Additionally, thickening of the RV wall may be appreciated. Visualization of other structures in the heart is possible by fanning the beam from the midpapillary region toward the mitral valve to the level of the aortic valve. At the aortic valve level, neighboring structures that can be visualized include the left atrium, the left coronary artery, the atrial septum at the 7 o’clock position, and the tricuspid valve at the 9 o’clock position (see Fig. 35.8B ). Aortic valve opening in the setting of extracorporeal membrane oxygenation (ECMO) suggests that LV function is sufficient to overcome afterload and eject some quantity of stroke volume forward. Maintaining the same probe location and rotating the transducer counterclockwise 90 degrees such that the indicator is directed to the patient’s right shoulder places the probe in the parasternal long-axis view. This view is aligned across the major axis of the LV, showing the left heart in continuity from the left atrium through the mitral valve, LV, aortic valve, and aortic root ( Fig. 35.9 ). The parasternal long-axis view allows for qualitative evaluation of LV function. When the intensivist is concerned about poor cardiac output, the parasternal long-axis view allows the intensivist to rapidly evaluate the LV outflow tract, opening of the aortic valve, left atrium size, and excursion of the mitral valve leaflets. Additionally, pleural effusion versus pericardial effusion may be delineated based upon the presence of effusion below or above the descending aorta, respectively.
The apical four-chamber view is obtained with the probe placed in the area of the apex with the indicator directed toward the left flank, usually in the 2 to 3 o’clock position in patients with normal situs. Images of the heart are obtained with the transducer beam directed from the apex toward the atria. Optimally the view captures the four chambers of the heart with maximal visualization of the chambers and the semilunar valves ( Fig. 35.10 ). Images may be difficult to obtain in a patient with large lung volumes, such as a person receiving positive pressure ventilation or who has lung hyperinflation. When available, this view permits qualitative assessment of relative chamber size, cardiac function, and aortic valve opening. In this view, regurgitation of the semilunar valves may be appreciated using Doppler modalities to assess flow on the atrial side of the mitral and tricuspid valve. Tilting the probe more shallowly so that the left ventricular outflow tract and aortic valve are visualized facilitates assessment of outflow tract blood velocity with pulsed wave or continuous wave Doppler. Aortic valve opening can be assessed in scenarios in which LV function is severely depressed, as in ECMO.
The subcostal view can also characterize the four chambers of the heart simultaneously and facilitates imaging of the atrial and ventricular septum. In a patient with normal situs the transducer is placed in the area below the xiphoid process and aimed at the left scapula. The indicator is oriented toward the patient’s left flank or at the 3 o’clock position. The view is adjusted to optimize visualization of the four chambers of the heart ( Fig. 35.11 ). Frequently an overhand grip and shallow angle of insonation facilitate positioning the probe nearly parallel to the skin. The view may be difficult to obtain in a child who has eaten or who has a gas-filled stomach from assisted ventilation or gastroparesis. However, the subcostal view may be the only view available in a child whose parasternal views are unobtainable due to an open chest or multiple bandages after cardiac surgery. Because the probe is oriented inferiorly to the heart, gravity dependent pericardial effusions are readily imaged here. Flow across septal defects is also more readily visualized from this plane because the ultrasonography beam is parallel to their flow.
Intravascular Volume Status
Intensivists have expended considerable effort to identify noninvasive ultrasonographic means of assessing fluid status and volume responsiveness. Within the pediatric cardiac population, assessing volume responsiveness before administration has broad applications because careful fluid management is critically important in these children. Adult studies have suggested that the collapsibility of the IVC is an effective surrogate for volume status. The IVC diameter is evaluated 2 to 3 cm distal to the cavoatrial junction in adults, and the variance between inspiration and expiration is compared ( Fig. 35.12 ). Variance greater than 12% to 18% in an intubated, paralyzed patient has been correlated with volume responsiveness. The effect of positive airway pressure on IVC dynamic changes reflecting volume status is also closely tied to intrathoracic pressures from positive pressure ventilation. A study performed by Lin and colleagues revealed that anesthesia induction and intubation altered the performance of the IVC as a surrogate for fluid responsiveness. Induction itself increases venous capacitance and decreases apparent IVC diameter. However, increased intrathoracic pressures decrease respiratory variation in the IVC over the respiratory cycle and can increase the relative diameter of the IVC in relation to other intraabdominal structures. Additionally, pediatric IVC measurements have not been shown to consistently correlate with central venous pressure. Therefore it is difficult to assess the functional utility of the measurement. The IVC to aorta ratio is another measure that can evaluate hypovolemia and hypervolemia ( Fig. 35.13 ). This ratio eliminates the issue of age-related size variability of the IVC. Intensivists rarely need another indicator of overall fluid status in addition to clinical and laboratory indicators, and the utility of the ratio to evaluate for volume responsiveness has not yet been studied. Although there is significant potential for both IVC collapsibility and the IVC to aorta ratio, there are factors affecting hemodynamics that may be more complex in the child with a congenital heart defect. A intensivist using ultrasonography for an examination of overall function should consider the absolute effects of cardiac dysmorphology in these assessments.
