Principles of Critical Care Ultrasonography

Phillips Perera, Laleh Gharahbaghian, Thomas “Tom” Mailhot, Sarah R. Williams, and Diku P. Mandavia

BACKGROUND

Traditionally, clinicians have divided shock into four distinct categories. In each category, there are several subtypes (Table 6.1).

Rapidly determining the type of shock state in the critically ill patient and initiating the appropriate resuscitative measures can lower patient mortality.^1,2 With the decreased reliance on invasive monitoring tools for shock assessment, focused bedside ultrasonography, or ultrasound (US), has evolved to become a key means for evaluation. As US allows for the rapid assessment of both the anatomy and physiology of the shock patient, multiple resuscitation protocols have been created.

The major resuscitation US protocols in critically ill medical and trauma patients include ACES,³ BEAT,⁴ BLEEP,⁵ Boyd ECHO,⁶ EGLS,⁷ Elmer/Noble Protocol,⁸ FALLS,⁹ FATE,¹⁰ FAST,¹¹ extended FAST,¹² FEEL resuscitation,¹³ FEER,¹⁴ FREE,¹⁵ POCUS (FAST and RELIABLE),¹⁶ RUSH-HIMAP,¹⁷ RUSH (pump/tank/pipes),^18–20 Trinity,²¹ and UHP.²² These algorithms have many similar components but differ in the sequence of exam performance (Table 6.2).

The RUSH protocol, named for Rapid Ultrasound in Shock, offers one easily remembered and comprehensive resuscitation protocol first to identify the shock state and then to monitor targeted therapy.

SOCIETY SUPPORT FOR FOCUSED ULTRASOUND IN CRITICALLY ILL PATIENTS

The use of focused US, including the individual components of the RUSH exam, has been supported by the major emergency medicine organizations. These organizations include the American College of Emergency Physicians (ACEP), the Society for Academic Emergency Medicine, and the Council of Emergency Medicine Residency Directors (CORD).^23–26 Critical care societies have endorsed both training in and the clinical use of bedside US. US has become an increasingly important diagnostic modality for this specialty.^27–30 In 2010, an important collaborative paper was published jointly between the American Society of Echocardiography (ASE) and ACEP that endorsed focused echocardiography (echo) for a defined set of emergent conditions.³¹ These exam indications and goals include the core exam components of the RUSH exam (Tables 6.3 to 6.5).

In addition, other components of the RUSH exam, including the FAST, lung, aorta, and deep venous thrombosis (DVT) US exams, are supported by ACEP as core applications for use by the emergency physician.²³

PERFORMANCE OF THE RUSH EXAMINATION: BASIC CONCEPTS

Ultrasound Probe Selection

A phased array probe at 2 to 3 MHz is used for the cardiac and thoracic components of the exam. A curvilinear probe at 2 to 3 MHz can be used for the abdominal components (FAST and aorta). A linear array probe at 8 to 12 MHz is used for the more superficial vascular components (DVT, internal jugular (IJ) veins).

Ultrasound Presets

The heart moves rapidly in reference to other body structures. For this reason, selection of a high frame rate on the US machine settings will allow for optimal imaging. This is done by selecting the cardiac preset, which is preloaded on most current US machines. The abdominal preset is best for the FAST and aorta exams. The vascular or venous preset is best for the DVT and IJ vein exams.

B-Mode Ultrasound

The RUSH US exam utilizes modalities that can image both critical anatomy and physiology.³² This is done by first employing two-dimensional B-mode imaging. B-mode imaging projects the body as a continuum of color in the gray spectrum, termed echogenicity. Echogenicity results from the fact that the US probe first acts as a transducer that sends sound waves into the body. The sound waves then penetrate into the body, traveling a distance until they are bounced back to the probe. Different tissues will have varying resistance to the movement of sound. Higher-density (hyperechoic) structures will reflect an increasing amount of the sound back to the probe, resulting in a brighter appearance (i.e., a calcified heart valve, diaphragm). Fluid-filled structures (hypoechoic or anechoic) will allow for increased propagation of sound through the body, leading to a darker appearance (i.e., blood, body fluids) (Fig. 6.10).

M-Mode Ultrasound

M-mode or “motion” mode illustrates an “ice pick” image of movement across a defined anatomical axis in relation to time. This generates a gray-scale illustration of movement over time that can be used to easily document motion on a static image (Figs. 6.11 and 6.12).

Doppler Ultrasound

Doppler US allows for the evaluation of motion within the body. The Doppler shift is defined as the movement of body structures relative to the position of the US probe. A positive Doppler shift results from structures (such as blood cells) moving toward the probe and a negative Doppler shift from movement away from the probe. The Doppler shift can be interpreted in several imaging modalities, two of which are discussed below.

Color-Flow Doppler

This modality demonstrates directionality of flow both toward and away from the probe and is often used in echo and vascular applications. Movement toward the probe results in a shorter frequency of sound. It is traditionally represented as red on the US image. Movement away from the probe results in a longer frequency of sound and is typically represented as blue. The scale that displays the color-flow Doppler setting should be set high (>70 cm/s) for echo to best capture the fast flow of the blood traveling through the heart. A lower scale can be used for the evaluation of the aorta and other vascular applications (DVT, IJ veins).

Pulsed-Wave Doppler

Pulsed-wave Doppler allows for assessment of flow velocity in a waveform that identifies the specific speed of blood flow over time. This modality is often used in advanced echo to define the velocity of blood flow through cardiac valves.

Orientation of Indicator on Machine and Probe

Historically, there has been practice variation between different US exams with regard to the orientation of the indicator dot on the screen and the marker on the probe. The reason for this being that the first widespread applications used in emergency medicine practice, such as the FAST and OB/GYN exams, were oriented based on traditional radiology practice, with the US screen indicator dot oriented to the left. Emergency medicine-practiced echo was therefore configured similarly. This differs from traditional cardiology practice, where the indicator dot is oriented to the right on the US screen. Despite this difference, the standard practice has been to orient the US probe (at a 180-degree variance, depending on screen orientation), so that the cardiac images obtained with the screen indicator dot on either side display the heart in the same configuration. In this chapter, the probe orientation for all RUSH exam components, including the cardiac views, will be described with the screen indicator dot located to the left side. This convention avoids having to flip the screen marker dot between different exams.

THE RUSH EXAM: PROTOCOL COMPONENTS

The RUSH exam involves a 3-part bedside physiologic patient assessment, which is simplified as “the pump,” “the tank,” and “the pipes.”

RUSH STEP 1: THE PUMP

Clinicians caring for the patient in shock should begin with assessment of “the function of the pump,” which is a goal-directed echo exam looking specifically for:

In addition, other cardiac pathology may be detected on bedside echo. A confirmatory test should generally be ordered if more advanced pathology is seen on bedside US, in accordance with the joint ACEP/ASE guidelines. The information gained by this exam can also allow a better assessment of the need for an emergent cardiac procedure. If indicated, US can then allow more accurate guidance of both the pericardiocentesis procedure and placement of a transvenous pacemaker wire.

Performance of the Echocardiography Examination

There are three traditional windows used for performance of cardiac US. These are the parasternal (long- and short-axis views), subxiphoid, and apical views (Fig. 6.1).

The Parasternal Long-Axis View

Patient Position

This view can be performed with the patient in a supine position. Turning the patient into a left lateral decubitus position will often improve this view by moving the heart away from the sternum and closer to the chest wall. This displaces the lung from the path of the sound waves.

Probe Position

The probe should initially be positioned just lateral to the sternum at about the third intercostal space. The probe position can then be adjusted for optimal imaging by moving the probe up or down one additional intercostal space. The probe indicator should be oriented toward the patient’s left elbow (Fig. 6.2).

Anatomic and Sonographic Correlation

The parasternal long-axis view will visualize three cardiac chambers and the aorta. The right atrium is not seen from this view. Optimally, the parasternal long-axis images have both the aortic and mitral valves in the same view. The aortic valve and aortic root can be visualized as the area known as the left ventricular outflow tract (Fig. 6.3).

Parasternal Short-Axis View

Probe Position

This view is obtained by first identifying the heart in the parasternal long-axis view and then rotating the probe 90 degrees clockwise. The probe indicator dot is aligned toward the patient’s right hip (Fig. 6.4).

Anatomic and Sonographic Correlation

The short-axis view visualizes the left and right ventricles in cross section and is known as the ring, or doughnut view, of the heart (Fig. 6.5). The traditional view is of the left ventricle at the level of the mitral valve, which appears as a “fish mouth” opening and closing during the cardiac cycle. Visualizing the heart as a cylinder with the US beam cutting tangentially through different levels, one can look as far inferiorly as the apex of the left ventricle and superiorly to the level of the aortic valve.

To best evaluate left ventricular contractility, the probe is moved inferiorly to the level of the papillary muscles, allowing confirmation of the assessment taken from the parasternal long-axis view. In addition, cardiologists routinely evaluate for segmental wall motion abnormalities on this view. If the probe is angled superiorly and medially from the above location, the aortic valve and right ventricular outflow tract will come into view. The aortic valve should appear as the “Mercedes-Benz sign” with a normal tricuspid configuration. A calcified bicuspid valve that may be prone to stenosis and pathology can be identified here.³³

Subxiphoid Window

Patient Position

This view is performed with the patient supine. Bending the patient’s knees will relax the abdominal muscles and can improve imaging.

Probe Position

Place the probe just inferior to the xiphoid tip of the sternum, with the indicator oriented toward the patient’s right side (Fig. 6.6). Flattening and pushing down on the probe will aim the US beam up and under the sternum to best image the heart. If gas-filled stomach or intestine impedes imaging, one can move the probe to the patient’s right while simultaneously aiming the probe toward the patient’s left shoulder, to utilize more of the blood-filled liver as an acoustic window.

Anatomic and Sonographic Correlation

The liver, which will be seen anteriorly, will act as the acoustic window to the heart from the subxiphoid view, allowing all four cardiac chambers to be seen. Because of the superior ability to visualize the right side of the heart from the subxiphoid window, it is often employed when close assessment of these chambers is needed (Fig. 6.7).

Apical Window

Patient Position

Roll the patient into the left lateral decubitus position to bring the heart closer to the lateral chest wall, and obtain optimal imaging from this view.

Probe Position

Palpate the point of maximal impulse on the lateral chest wall and place the transducer at this point. This is generally just below the nipple line in men and under the breast in women. For the apical view, the probe marker will be oriented toward the patient’s right elbow (Fig. 6.8).

Anatomic and Sonographic Correlation

The apical window allows for detailed assessment of the sizes and movements of all four cardiac chambers in relation to one another (Fig. 6.9). The apical four-chamber view is the first traditional view from this window. The optimal views from this position have both the mitral and tricuspid valves in the image. From this position, the probe can then be angled more superiorly to obtain the apical 5-chamber view. The “5th chamber” will be the aortic valve and aortic outflow tract in the middle of the image.

RUSH STEP 1A: ASSESSMENT OF CARDIAC CONTRACTILITY

Background

A relatively high percentage of critical patients may have compromised cardiac function contributing to their shock state, which may be diagnosed with bedside echo.³⁴ Published studies have demonstrated that emergency physicians with focused training can accurately evaluate left ventricular contractility.³⁵

Qualitative Evaluation of Left Ventricular Contractility

Evaluating motion of the left ventricular walls by a visual estimation of the volume change from diastole to systole provides a qualitative assessment of contractility.^34–36 A ventricle that has good contractility will have a large-volume change between the two cycles (Fig. 6.10), while a poorly contracting heart will have a small percentage change. The poorly contracting heart may also be dilated in size. Based on these assessments, a patient’s contractility can be broadly categorized as being normal, mildly to moderately decreased, or severely decreased. A fourth category, known as hyperdynamic, can be seen in advanced hypovolemia or in distributive shock states. The heart will have small chambers and vigorous, hyperkinetic contractions with the endocardial walls almost touching during systole.

Semiquantitative Means for Assessment of Contractility

Fractional Shortening

M-mode can be used to graphically depict the movements of the left ventricular walls through the cardiac cycle. In the parasternal long-axis view, the M-mode cursor is placed across the left ventricle beyond the tips of the mitral valve leaflets at about the midventricle area. The resulting tracing allows a two-dimensional length-based measurement of the chamber diameters over time. Fractional shortening is calculated according to the following formula:

where ESD is the end-systolic diameter, measured at the smallest dimension between the ventricular walls, and EDD is the end-diastolic diameter, where the distance is greatest (Fig. 6.11).

In general, fractional shortening above 35% to 40% correlates to a normal ejection fraction.³⁷ Compared to the comprehensive volumetric assessment required for measuring ejection fraction, fractional shortening is a semiquantitative method for determining systolic function that is relatively fast and easy to perform.³⁸

E-Point Septal Separation

Motion of the anterior leaflet of the mitral valve in the parasternal long-axis view can also be used to assess left ventricular contractility. In the early diastolic phase of a normal contractile cycle, the anterior mitral leaflet can be observed to fully open to a position close to the septal wall. This is with the caveat that mitral valve abnormalities (stenosis, regurgitation), aortic regurgitation, and extreme left ventricle hypertrophy are not present. Early diastolic opening of the mitral valve is represented on M-mode US as the E-point. The distance measured between the E-point, representing the position of the fully open mitral valve, and the septum is known as the E-point septal separation or EPSS.³⁹ To measure the EPSS, the M-mode cursor is placed over the tip of the anterior mitral valve leaflet. In a normal contractile state, the EPSS will be <7 mm, as the mitral valve will almost approximate the septum during early diastolic filling.^39–41 As left ventricular contractility decreases, diastolic flow through the valve will diminish. This results in decreased mitral valve opening to a position relatively farther from the septum and a corresponding increase in the EPSS (Fig. 6.12). Further research is ongoing to determine the accuracy of correlation between EPSS and fractional shortening.⁴²

RUSH STEP 1B: DIAGNOSIS OF PERICARDIAL EFFUSION AND CARDIAC TAMPONADE

Pathophysiology

Published studies have documented that pericardial effusions may be found relatively commonly in critical patients presenting with acute shortness of breath, respiratory failure, shock, and cardiac arrest.^43,44 Fortunately, the literature also indicates that emergency physicians with focused echo training can accurately identify effusions.⁴⁵ Pericardial effusions may result in hemodynamic instability as the pressure in the pericardial sac acutely increases, resulting in reduced cardiac filling.⁴⁶ Acute pericardial effusions (as small as 50 cc) may result in tamponade. This pathology may quickly compromise the trauma patient. Conversely, in chronic conditions, the pericardium may slowly stretch to accommodate large effusions over time without tamponade.⁴⁷

Sonographic Appearance of Pericardial Effusions

Pericardial effusions are generally recognized by a dark, or anechoic, appearance. However, inflammatory or infectious conditions may result in effusions with a brighter, or more echogenic, appearance. In addition, traumatic pericardial effusions will take on a more echogenic appearance over time as blood clots.

Grading Scale for the Size of Pericardial Effusions

One scale for describing the size of the effusion is shown below (Table 6.6).⁴⁸

Specific Echocardiographic Windows for Evaluating Pericardial Effusions

Parasternal Long-Axis View

Size and Location of Effusions

Smaller effusions will first layer posteriorly behind the heart. As effusions grow in size, they will surround the heart in a circumferential manner, moving into the anterior pericardial space.⁴⁷ Most effusions are free flowing in the pericardial sac. However, occasionally loculated effusions may occur. These typically occur in postoperative cardiac surgery patients and in inflammatory conditions.⁴⁹

Differentiation of Pleural from Pericardial Fluid

The critical landmarks for detection of a pericardial effusion are the descending aorta and the posterior pericardial reflection. The descending aorta will appear as a circle directly behind the left atrium, posterior to the mitral valve (Fig. 6.13). The posterior pericardial reflection will be identified as a hyperechoic structure immediately anterior to the descending aorta. First, select the appropriate depth of the US image, so that the descending aorta and pericardial reflection are adequately visualized posteriorly on the screen. Pericardial effusions will be located anterior to the descending aorta and above the posterior pericardial reflection (Fig. 6.13). In contrast, pleural effusions will be located posterior to the descending aorta and below the posterior pericardial reflection (Fig. 6.14). To further confirm the presence of a left pleural effusion, the probe can be moved to a lateral position on the chest wall as for the FAST views and aimed above the diaphragm to visualize the lower thoracic cavity (Fig. 6.17).

Pericardial Fat Pad

A pericardial, or epicardial, fat pad may at times be confused with a pericardial effusion. The typical location for this structure is in the area just deep to the near-field pericardial reflection and anterior to the heart. The fat pad often has a classic appearance, with an interspersed speckling of bright, or hyperechoic, regions. From the parasternal views, an isolated anterior “echo-dense” structure is more suggestive of a fat pad and not of an effusion. For an effusion to be visualized anteriorly on the parasternal views, a circumferential effusion would usually be present (with the exception of the presence of a rarer loculated pericardial effusion). From the subxiphoid view, the fat pad would be seen closer to the probe, located just beneath the near-field pericardial reflection and anterior to the heart.

Subxiphoid View

Size and Location of Effusions

Because the subxiphoid window is taken from a position inferior to the heart, small effusions will typically first layer out with gravity along the near-field pericardial reflection. This is especially noted in cases where the patient has been in an upright position. Larger effusions will spread to surround the heart circumferentially.

Differentiation of Pericardial Effusion from Ascites

Ascites may be confused with a pericardial effusion. To help differentiate between the two, ascites will be seen nearer to the probe, anterior to the near-field pericardial reflection, outside the pericardial sac, and surrounding the liver within the abdominal cavity. In contrast, a pericardial effusion is located posterior to the near-field pericardium, adjacent to the heart, and within the pericardial sac.

Echocardiographic Diagnosis of Cardiac Tamponade

Ultrasound Findings

As pericardial effusions accumulate, the pressure in the pericardial sac rises and will first compromise the lower pressure circuit of the right heart. This is best recognized sonographically as an inability of these chambers to fully expand during the relaxation phase of the cardiac cycle. Cardiac tamponade is thus classically defined on US as diastolic collapse of either the right atrium or the right ventricle. While both right heart chambers should be evaluated, diastolic collapse of the right ventricle is a more specific finding. This is because as tamponade progresses, the right atrium may take on an appearance of a “furiously contracting chamber” with hyperdynamic contractions. This can at times make differentiation of atrial systolic contraction from diastolic collapse more difficult.

Diastolic collapse of the right ventricle in tamponade is best understood as a spectrum of US findings, from a subtle serpentine deflection of the wall to complete chamber compression (Fig. 6.15).⁵⁰ One important pitfall to this general diagnostic strategy is seen in the patient with pulmonary hypertension, where diastolic collapse of the right heart may occur late in the disease process.

Advanced Strategies in the Identification of Tamponade

There are several more advanced strategies used to document diastolic compression of the right heart in tamponade.⁵¹ The first is to attach an EKG monitoring lead to the US machine to allow for simultaneous display of both the US and electrical phases. Systole will be identified immediately following the QRS, and diastole will follow later in the electrical cycle, just prior to the next P–QRS complex. Slowing the video down and scrolling through the echo with simultaneous attention to both the EKG phase and the US will allow discrimination of systolic from diastolic movements of the right atrium and ventricle. In tamponade, right atrial diastolic collapse may be noted first, occurring directly after atrial systole and early after the QRS complex. Right ventricular diastolic collapse will be noted later as tamponade progresses. This will be seen on EKG later in the electrical cycle and just before the following P–QRS complex.

Evaluation of the inferior vena cava (IVC) may also be performed to confirm tamponade physiology. A dilated, or plethoric, IVC without respiratory collapse implies tamponade.⁵² A more advanced exam using Doppler US allows for one of the most sensitive tests to evaluate for tamponade. From the apical 4-chamber view, color-flow Doppler can first be used to identify the flow of blood through the tricuspid and mitral valves. Pulsed-wave Doppler is then used to identify the augmented respiratory variation in the flow velocities across these valves, which is noted in tamponade. In inspiration, an increase in blood flow through the tricuspid valve and a decrease in flow through the mitral valve will be seen. Flow variations >25% across the tricuspid valve and >15% across the mitral valve are considered abnormal.⁵³

Ultrasound Guidance of Pericardiocentesis

In cases of cardiac tamponade and shock, an emergent pericardiocentesis is generally indicated. Emergency physicians have classically been taught the subxiphoid approach for pericardiocentesis. However, a large review from the Mayo Clinic included 1,127 pericardiocentesis procedures and found that the optimal position for placement of the needle was the apical position in 80% of patients.⁵⁴ The subxiphoid approach was only chosen in 20% of these procedures, due to the interposition of the liver. US allows for accurate guidance of the pericardiocentesis needle and guidewire into the pericardial sac. In addition, agitated saline can be used as a form of US contrast to confirm proper needle placement in the pericardial space.^55,56

RUSH STEP 1C: ECHOCARDIOGRAPHY IN PULMONARY EMBOLUS AND EVALUATION FOR RIGHT VENTRICULAR ENLARGEMENT

Background

While a CT scan is typically thought of as the current diagnostic standard for pulmonary embolism, focused echo can identify one of the more serious complications of this disease, right ventricular strain. This finding correlates with a poorer prognosis and the need for more immediate treatment.^57,58 Right ventricular enlargement on focused echo may also suggest this pathology in the undifferentiated patient presenting in shock, potentially leading to more timely diagnosis and treatment.

Echocardiography Literature for Pulmonary Embolus

Studies have previously evaluated the use of echo for the diagnosis of pulmonary embolus, specifically looking for the presence of right ventricular enlargement due to acute cardiac strain. The documented sensitivity of this test in all patients with pulmonary embolus is only moderate. Therefore, echo cannot be used to rule out a pulmonary embolus, especially in those patients who are hemodynamically stable. However, identification of right ventricular enlargement can be of increased diagnostic utility in cases of hypotension with suspected thromboembolic disease, where it will have a higher specificity and positive predictive value.^59–64

The traditional treatment of patients with a pulmonary embolus has been with anticoagulation. However, more recent guidelines recommend the combined use of anticoagulants and fibrinolytics in cases of severe pulmonary embolism.^65–68 This is defined as the presence of acute right heart strain and clinical signs and symptoms of hypotension, severe shortness of breath, or altered mental status.

Echocardiographic Findings of Hemodynamically Significant Pulmonary Embolism

Parasternal Views

The relative sizes of the left and right ventricles can be evaluated from this window. A normal ratio of the right to the left ventricle is defined as 0.6:1 with a greater than 1:1 ratio indicating right ventricular dilatation.^69,70 A higher relative ratio, combined with deflection of the interventricular septum from right to left, indicates the right ventricular strain that may be seen in a severe pulmonary embolus. In acute right ventricular strain, the chamber wall will typically be thin, due to the lack of time for compensatory hypertrophy. Conversely, in cases of chronic pulmonary strain seen in conditions of long-standing pulmonary artery hypertension, the right ventricle will compensate with hypertrophy. This will result in a thicker wall, typically measuring >5 mm.^48,71 These findings can allow the clinician to further differentiate the US findings of acute from chronic right heart enlargement. On the parasternal short-axis view, the interventricular septum may be seen to bow from right to left with high right-side pressures. This can result in a finding known as the left ventricular “D-shaped cup,” or “D-sign,” as the septum is pushed down and away from the right ventricle (Fig. 6.16).⁷²

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