CHAPTER 17
Point‐of‐Care Ultrasound for Congenital Heart Disease Patients

Alan F. Riley¹, Kriti Puri², and Adam C. Adler³

¹ Assistant Professor of Pediatrics, Division of Cardiology, Associate Director of the Cardiac Patient Care Unit, Texas Children’s Hospital, Baylor College of Medicine, Houston, TX, USA

² Assistant Professor of Pediatrics, Sections of Pediatric Critical Care Medicine and Cardiology, Department of Pediatrics, Baylor College of Medicine, Houston, TX, USA

³ Associate Professor of Anesthesiology, Department of Anesthesiology, Perioperative and Pain Medicine, Texas Children’s Hospital, Baylor College of Medicine, Houston, TX, USA

Introduction

Use of point‐of‐care ultrasound (POCUS) in acute care practice has expanded to include a variety of applications from ultrasound‐assisted physical examination to assessment of hemodynamic states as well as guidance for procedures. Within anesthesia practice, POCUS may be used to guide management at all stages of perioperative care [1]. While the list of applications for POCUS within anesthesia practice is growing, this chapter focuses on the cardiac and lung POCUS with an emphasis on patients with congenital heart disease (CHD). See Chapter 13 for a discussion of ultrasound‐guided vascular access and Chapter 24 for a presentation of ultrasound‐guided regional anesthesia.

What is POCUS?

POCUS is the systematic application of ultrasound performed at the bedside to evaluate specific structures, generally using a limited number of ultrasound views [2]. Clarity of language is paramount when describing the various ultrasound modalities examining the heart. Ultrasound evaluation of the heart has become a continuum of imaging devices and strategies ranging from limited evaluations with miniaturized handheld devices to comprehensive imaging of the heart with large cart‐sized devices on wheels with the highest resolution medical ultrasound technologies [3]. Some consider the very limited evaluations of the ultrasound of the heart to be more like an extension of the physical examination rather than a stand‐alone imaging test [4]. In many ways, these distinctions are still being created in modern cardiac imaging, but standard echocardiography remains the most comprehensive ultrasound‐based evaluation of cardiac anatomy and function [5]. Cardiac POCUS and focused cardiac ultrasound seem to be the best terms, which are currently being used interchangeably to describe more targeted cardiac ultrasound evaluations. For consistency, the authors are using the term cardiac POCUS.

During this imaging strategy, a bedside clinician (most often a noncardiologist) uses ultrasound to examine the heart in a limited fashion with a goal to identify a limited number of specific ultrasonographic signs that are which could be associated with the clinical situation of interest or a specific pathology [6]. While cardiac POCUS is almost always performed with a smaller, more portable ultrasound machine with limited functionality, it should be defined by the imaging technique rather than the machine used [7, 8]. Since cardiac POCUS has narrowed diagnostic targets with focused questions, it is important to clearly distinguish it from comprehensive standard echocardiography during medical documentation or communication between providers. Confusion can be created if ambiguous terms, such as “bedside echo or limited echo,” are used to describe clinician‐performed cardiac POCUS. The term echocardiography (or similar forms) should be reserved for more comprehensive evaluations, usually performed by a cardiac sonographer and often include quantitative measurements. Whereas cardiac POCUS should be limited to a few predefined views where the clinician can make qualitative assessments. With non‐precise language, the assumptions of broader diagnostic targets or evaluations can occur creating misunderstanding or false reassurances. See Chapters 15 and 18 for further discussion of comprehensive echocardiography.

Pediatric cardiac POCUS

The evidence supporting the accuracy of cardiac POCUS by noncardiologists in pediatrics comes predominantly from small single center studies with mostly structurally normal hearts [9–13]. The identification of severely depressed systolic function and larger effusions are the most reliable findings [14]. Given how cardiomyopathy and severely depressed ventricular systolic function can present in pediatrics with insidious onset and/or with misleading chief complaints, cardiac POCUS seems to have a valuable role to play in the early identification of severely depressed left ventricular systolic function [15]. However, borderline systolic dysfunction is the area of significant weakness [9, 16]. Some argue that borderline systolic dysfunction may be out of the scope of pediatric cardiac POCUS [13]. Users of pediatric cardiac POCUS should be keenly aware of this weakness of the modality. This is particularly important in pediatrics since borderline depressed function in a previously healthy child is often a harbinger of rapidly progressive cardiogenic shock, especially in infectious myocarditis or inflammatory disorders [17, 18]. There is scant evidence supporting cardiac POCUS in more complex CHD, and its use in this population should be limited to those with extensive training and experience in CHD training and/or pediatric echocardiography. Additionally, the use of cardiac POCUS as a screening tool for CHD is likely counterproductive and it is likely contraindicated for this purpose [14].

The diagnostic targets for pediatric cardiac POCUS are typically limited to the assessments of ventricular size, qualitative ventricular systolic function, right ventricular systolic pressure, and the pericardial space [14]. Evaluation of diagnostic targets is performed in a qualitative or semiquantitative manner, typically using an “eyeball” method [2, 8, 14]. These assessments are usually binary, i.e., is there pathology present or is there not? Declaration of uncertainty is often the correct approach in POCUS if the diagnostic targets are not well visualized, particularly given the inherent nature of POCUS. Cardiac POCUS image acquisition and interpretation are performed rapidly (less than 5 minutes) and usually in suboptimal imaging conditions. Quality improvement work in pediatric transthoracic echocardiography has shown that the conditions in which POCUS is performed – incomplete examinations outside the echocardiography laboratory, in poor ambient light conditions via lower end machines with increased acoustic artifacts – are the exact setup for diagnostic errors [19]. Therefore, a low threshold for referral for standard echocardiography seems reasonable when using cardiac POCUS to clarify equivocal imaging or to evaluate persistent symptoms after an unrevealing cardiac POCUS. Inability to diagnose disease using substandard imaging equipment in suboptimal imaging conditions does not prove the absence of disease.

Basic physics of POCUS

The ultrasound technology for POCUS is based on the same principles of physics as routine ultrasonography. The top of the ultrasound probe contains crystals (piezoelectric crystals) in various configurations based on the type of probe and its intended clinical use. The ultrasound machine transmits an electrical impulse, which results in vibration of these crystals. It is this vibration that emits a sound wave that travels into the adjacent tissue. Sound is absorbed, scattered, and reflected by the tissues. Sound reflected by the tissues returns to the probe where it results in vibration of the crystals. This mechanical vibratory energy is converted to electrical energy, which is processed to create an image.

Ultrasonic waves are refracted and reflected differently by tissues based on their various densities. The speed at which these sound waves are traveling when returning to the probe is converted to an image that is color coded in grayscale. The depth of the structure is determined by the time it takes from the wave transmission until the reflected sound wave returns. Image color may be darkened or lightened using the gain function of the machine. In contrast, air does not conduct sound waves (including ultrasonic waves) well. This limits the abilities of cardiac POCUS during situations involving pneumomediastinum or pneumothorax. Additionally, lung ultrasound relies more on observing artifacts and scatter from lung structures that generate certain ultrasonographic signs indicative of normal and pathologic conditions.

Most modern machines are equipped with complex software that is designed to reduce artifacts which is of benefit when scanning solid structures (e.g., the heart and liver). However, when scanning lung tissue, this feature should be turned off as artifact is vital to image interpretation. Many machines have pre‐settings in which image quality is optimized for the structure of interest.

Ultrasound machines have probes of different configurations, sizes, and with a wide range of frequencies. The frequency of the ultrasonic wave emitted is inversely proportional to the amplitude of the wave and hence the resolution. Therefore, a higher frequency probe, with a lower amplitude, is able to delineate structures more finely. On the other hand, waves with higher amplitude attenuate more slowly as they travel through tissue. Therefore, a higher frequency, lower amplitude probe is able to penetrate to a lower depth in the body, while a lower frequency probe is able to penetrate to a greater depth, e.g., in obese habitus or deeper structures. Depending on the brand of ultrasound equipment, a range of frequencies may be available with capabilities between 3 and 8 Hz. Cardiac POCUS is typically performed using a phased array probe while lung POCUS relies on a linear array probe (Figures 17.1 and 17.2). Providers should consider the size (space between ribs) of the patients and object of interest (e.g., lung or heart) when selecting a probe.

Performing cardiac POCUS

Basic cardiac POCUS relies on four main views. These are generally performed while the patient is supine with the exception of the apical four‐chamber view that benefits from left lateral decubitus positioning. Each ultrasound machine has a specific method for indicating the probe side also known as the orientation marker, which is located on the side of the probe.

Parasternal long‐axis view

The parasternal long‐axis (PLAX) view is obtained with the probe in the second left intercostal space with the orientation marker facing toward the patient’s right shoulder (10 o’clock position). The top of the screen depicts the anterior aspect, and the right and left of the screen are directed toward the feet and head of the patient, respectively. Visualized in this view are left atrium, left and right ventricles, aortic and mitral valves, and left ventricular and a portion of the right ventricular outflow tracts (Figure 17.3).

Photo depicts varying sizes of phased-array probes for use in cardiac point-of-care ultrasound. — **Figure 17.1** Varying sizes of phased‐array probes for use in cardiac point‐of‐care ultrasound. Arrows indicating the orientation marker.

Parasternal short‐axis view

The parasternal short‐axis (PSAX) view is obtained by rotating the probe 90° clockwise from the PLAX view, so the orientation marker points toward the patient’s left shoulder (2 o’clock). This view may be optimized by moving the probe up or down one rib space. Visualized in this view are the right and left ventricles, the cardiac apex, and the mitral valve (Figure 17.4).

Apical four‐chamber view

The apical four‐chamber view is obtained by placing the probe in the fourth or fifth intercostal space along the anterior axillary line with the orientation marker facing toward the left (3 o’clock position). Visualized in this view are the right and left atria and ventricles and the atrioventricular valves (Figure 17.5).

Subcostal four‐chamber and inferior vena cava views

The subcostal four‐chamber view is obtained with the probe in the subxiphoid space, with the probe facing the patient’s left axilla and orientation marker on the oriented at 2–3 o’clock position (Figure 17.6). To obtain the inferior vena cava (IVC) view, the probe is rotated counterclockwise, so that it is vertical with the orientation marker facing straight up. Visualized in the subcostal four‐chamber view are the right and left atria and ventricles and the atrioventricular valves. Visualized on the IVC view is the liver, intrahepatic IVC with entrance into the right atrium as well as the hepatic veins (Figure 17.7).

Photo depicts linear-array probes for use in lung point-of-care ultrasound. — **Figure 17.2** Linear‐array probes for use in lung point‐of‐care ultrasound. Arrows indicating the orientation marker.

Schematic illustration of demonstration of the parasternal long-axis view including probe placement (upper left) with red dot denoting orientation marker, scanning plane (upper right) and accompanying diagram (bottom left) and ultrasonographic appearance (bottom right). — **Figure 17.3** Demonstration of the parasternal long‐axis view including probe placement (upper left) with red dot denoting orientation marker, scanning plane (upper right) and accompanying diagram (bottom left) and ultrasonographic appearance (bottom right). RV = right ventricle, LA = left atrium, LV = left ventricle, MV = mitral valve, TV = tricuspid valve, AV = aortic valve, Ao = aorta, LVOT = left ventricular outflow tract, RVOT = right ventricular outflow tract. Dao = descending aorta. Illustration by A. Adler, MD.

Individual structure assessment using cardiac POCUS

In the following section, the common structures evaluated during cardiac POCUS are discussed. A key requirement of POCUS, and specifically cardiac and lung assessments, is for the provider to recognize normal from abnormal. This requires deliberate training and review of images, so that the sonographer is able to distinguish and identify ultrasonographic signs of pathology from normal sonoanatomy.

Assessment of the left ventricle

Systolic dysfunction—Perhaps, the most common use of POCUS in children, including those with CHD, involves the assessment of systolic function. This may be prompted by new onset hypotension in the perioperative period, presentation in shock and concern for cardiogenic etiology, or new onset ventricular arrhythmias. This assessment permits further clinical decision‐making including choice of vasoactive agent, next steps of resuscitation, possible need for activation of mechanical support team, or need for further advanced investigations. All typical cardiac POCUS views can be employed to assess left ventricular systolic function, but PSAX and PLAX views are more commonly employed views to utilize eyeball or quantitative methods. It is important to note that children who have undergone ventricular septal defect repair with patch will have dyskinesis of the part of the interventricular septum that is composed of patch material. The nuances of this cannot be worked out on cardiac POCUS and would need a formal echocardiogram. Also, operators must recognize that cardiac POCUS will give information only about systolic dysfunction and is not intended for diastolic dysfunction.

Qualitative or eyeball method assessment via ultrasound is best supported for the evaluation of left ventricular size and systolic function. While the method is inherently subjective, it compares quite well with more objective parametric measures of left ventricular systolic function when employed by experienced users [20]. For the novice user, the qualitative assessment of ventricular systolic function can be broken down into the evaluation of how well the ventricular walls are thickening and how well the ventricular walls are symmetrically moving towards the center of the cavity [21]. Precise estimation or quantification of ventricular ejection fraction is out of scope for cardiac POCUS, but novel users should know when evaluating the ventricular wall “squeeze” toward the center that the left ventricle normally ejects about 55–70% of its contents with each contraction. The decrease in chamber size is largely created by the thickening of ventricular walls during contraction. Making sure that the ventricular walls are actually contracting is important, because there can be misleading scenarios with poor imaging windows where the heart is moving or “rocking” on the screen, but there is ineffectual ventricular wall contraction and the chamber size decrease is difficult to evaluate.