Cardiac Ultrasound for the Nephrologist: Know thy Heart to Know thy Kidneys

Abstract

Kidney disease patients have a high prevalence of cardiovascular morbidity and mortality. It can be challenging to adequately assess their cardiovascular status based on physical examination alone. Cardiac ultrasound has proven to be a powerful tool to accomplish this objective and is increasingly being adopted by the non-cardiologists to augment their skills and expedite clinical decision making. With the advent of inexpensive and portable ultrasound equipment, simplified protocols, and focused training, it is becoming easier to master basic cardiac ultrasound techniques. After a short course of training in focused cardiac ultrasound, nephrologists can quickly and reliably assess ventricular size and function, detect clinically relevant pericardial effusion and volume status in their patients. Additional training in Doppler ultrasound can extend their capability to measure cardiac output, right ventricular systolic pressure, and diastolic dysfunction. This information can be instrumental in effectively managing patients in inpatient, office, and dialysis unit settings. The purpose of this review is to highlight the importance and feasibility of incorporating cardiac ultrasound in nephrology practice, discuss the principles of basic and Doppler ultrasound modalities and their clinical utility from a nephrologist’s perspective.

“A 72-year-old woman with hypertension and end stage kidney disease (ESKD) on intermittent hemodialysis presented with shortness of breath and fatigue. Physical exam was significant for bibasilar crackles and laboratory results showed serum potassium of 6.7 mEq/L. She underwent hemodialysis on two consecutive days. During the second session, her blood pressure dropped from 100/53 mm Hg to 85/53 mm Hg, and her heart rate increased from 89 to 130 beats/min. She was started on vasopressors with a further drop in blood pressure to 64/39.¹”

INTRODUCTION

The clinical case above highlights the inherent connection between the heart and kidneys. Malfunctioning of either deeply impacts the other.² The prevalence of cardiovascular diseases among patients with CKD is extremely high.³ It is common for a nephrologist to encounter challenging situations like intradialytic hypotension, uncontrolled hypertension, volume overload, difficulty in adjusting dry-weight, chest pain during dialysis, and at times, life-threatening emergencies such as cardiac tamponade and arrest.

Until recently, nephrologists have relied primarily on physical examination to assess their patients’ cardiovascular and hemodynamic status. However, recent studies have shown that lung crackles and peripheral edema very poorly reflect interstitial lung edema and intravascular volume status in patients.^4,5 Cardiac auscultation, similarly, has limited accuracy in diagnosing and grading most valvular abnormalities.⁶ Ultrasound is the most portable form of cardiac imaging. It is extremely safe, involves no ionizing radiation, and can be used for serial examination in patients of all age groups. Incorporating ultrasound as a part of the physical exam can equip the nephrologists to reliably and rapidly diagnose, triage, and treat various cardiovascular conditions in their patients.

The utility of cardiac ultrasound in nephrology has thus far been limited due to the complexity of diagnostic protocols, high cost, lack of proper training/equipment, and time needed to acquire multiple images for interpretation. With the widespread adoption of “Focused Cardiac Ultrasound (FoCUS)” to answer clinically relevant questions by non-cardiologists, there has been a surge in simplified protocols utilizing affordable and portable ultrasound systems. Among a subset of nephrologists, however, there is still a lack of enthusiasm and adequate training to integrate cardiac ultrasound in their day-to-day practice.

The purpose of this review is to outline the principles of FoCUS useful for the practicing nephrologist in inpatient, office, and dialysis unit settings. We will also review the principles of advanced echocardiographic techniques that can be helpful in specific clinical scenarios.

FOCUSED CARDIAC ULTRASOUND FOR THE NEPHROLOGIST

Mastering the fundamentals of FoCUS requires an understanding of cardiac anatomy, imaging windows, and views. The training focuses on making the learners comfortable with image acquisition, interpretation, and integrating this information into clinical practice. These objectives can be best achieved by interactive didactics and hands-on training done simultaneously or within a reasonable timeframe of each other to maximize learning.

The Technique of Cardiac Ultrasound

Understanding the basics of ultrasound physics is important to acquire and interpret cardiac ultrasound images. A detailed discussion of various ultrasound systems and transducers is available elsewhere in this issue. Here, we will focus on the fundamentals of acquiring and interpreting cardiac images using a ‘phased array’ transducer. The ‘all-in-one” transducers employ a circuit that simulates a piezoelectric crystal allowing it to produce a broader range of frequencies. Multiple organ systems, including the heart, can be imaged using this transducer.

Most standard phased array transducers have a frequency of 1–5 MHz with an imaging depth up to 35 cm.⁷ It is primarily used for imaging heart and inferior vena cava but may be used for lungs and abdominal structures as well. It has a small footprint (20 × 15 mm) ideal to maneuver between ribs and a high frame rate (>100/sec) to capture images of the moving heart. For these reasons, it is the preferred transducer for the cardiac ultrasound.

According to the commonly used cardiology convention, the screen orientation marker or ‘dot’ indicating the leading edge of the ultrasound array is located to the right side of the monitor screen (Fig 1). As per the ‘dot matches the dot’ principle, the phased array orientation marker points towards the structures displayed on the right side of the screen. The operators need to acquaint themselves with various transducer movements; sliding, rocking, sweeping, fanning, and rotating to optimize image acquisition (Supplement Fig 1).⁸

Figure 1:

Basic focused cardiac ultrasound views with probe positioning and normal ultrasound images: A, parasternal long-axis (PLAX); B, parasternal short-axis (PSAX); C, apical 4-chamber (A4C); and D, subcostal 4-chamber (S4C). Abbreviations: AV, aortic valve; ICS, intercostal space; IVS, interventricular septum; LA, left atrium; LV, left ventricle; RA, right atrium; RV, right ventricle.

There are 3 commonly used imaging windows for cardiac ultrasound (parasternal, apical, and subcostal). They can be combined with the 4 imaging planes (long axis, short axis, 4-chamber, and 2-chamber) to yield the various traditional views used in comprehensive echocardiography.⁹ We recommend that the cardiac structures be imaged in at least 2 imaging planes. Parasternal and subcostal windows are typically easier to master than the apical window. For FoCUS purposes, we recommend 5 most commonly used views for nephrologists to achieve proficiency and answer the majority of clinical questions (Fig 1, Table 1).

Table 1.

Imaging Views for Focused Cardiac Ultrasound

Imaging Views	Patient Position	Probe Position	Marker ‘Dot’ Orientation	How to Optimize Image Quality?	Ideal View	Structures Seen	Uses
Parasternal Long Axis (PLAX)	Supine or left lateral decubitus	Left sternal border, 3rd or 4th ICS (range 2–5 ICS)	Pointing towards patient’s right shoulder (10 o’clock position)	Left lateral decubitus position; end inspiration or end-expiration images; slight rotation may open LV cavity	AV and MV visible and slightly right to the center of image	AV, MV, LV, LVOT, RVOT, pericardium, ascending and descending aorta	LV size and function; AV/MV function; left atrial size; circumferential pericardial effusion
Parasternal Short Axis (PSAX) at Mid-ventricular Level	Supine or left lateral decubitus	Left sternal border, 3rd or 4th ICS (range 2–5 ICS)	Pointing towards patient’s left shoulder (2 o’clock position)	Center PLAX view at MV, rotate clockwise 90 degrees; five different imaging planes can be obtained by tilting the probe from base of the heart to apex; mid-ventricular or papillary muscle level is most useful for FoCUS purposes	Both papillary muscles should be visible and symmetric; circular LV cavity	LV, papillary muscles, interventricular septum, RV, pericardium	Global LV function; RWMA; shape/function of interventricular septum in setting of RV dilatation
Apical 4-Chamber (A4C)	Supine or left lateral decubitus	LV apex (Usually located in the 5th ICS close to mid-clavicular line)	Pointing towards patient’s left side (2 o’clock position)	Locate apex by palpation or slide transducer towards apex from a PSAX view followed by tilting the beam towards right shoulder; use rocking to align interventricular septum in the center of image; use rotation to visualize LV/RV cavities in appropriate longitudinal sections	All 4 chambers (LV, RV, LA, RA) should be visible with interventricular septum in the center	LV, RV, LA, RA, AV, MV	RV systolic function and size in relation to LV; global LV systolic function; MV/TV evaluation; pericardial effusion
Subcostal 4-Chamber (S4C)	Supine	Under xiphoid process, ultrasound beam is directed upward behind the sternum	Pointing towards patient’s left side (3 o’clock position)	Probe needs to be flattened and pressed firmly; bend patient’s knees; breath-hold at deep inspiration; sliding transducer slightly to the right side to use liver as the acoustic window may help in case of bowel gas	All 4 chambers should be visible along the long axis of heart	LV, RV, LA, RA, AV, MV, and pericardium	RV size and function; global LV function; pericardial effusion and tamponade; useful view in acutely ill patients, may obtain images without interrupting CPR; preferred view in COPD and mechanically ventilated patients due to downward displacement of diaphragm
Subcostal IVC (Supplemental Fig 4)	Supine	Under xiphoid process	Pointing towards patient’s head (12 o’clock position)	Use liver as the acoustic window; IVC can be distinguished from aorta by its thin walls, collapsibility, and communication with hepatic vein and right atrium	Uniform diameter; draining into right atrium	IVC, hepatic vein, right atrium	Guide fluid management; estimate RA pressure; tamponade physiology

Abbreviations: AV, aortic valve; CPR, cardio-pulmonary resuscitation; ICS, intercostal space; IVC, inferior vena cava; LA, left atrium; LV, left ventricle; LVOT, left ventricle outflow tract; MV, mitral valve; RA, right atrium; RV, right ventricle; RVOT, right ventricular outflow tract; RWMA, regional wall motion abnormalities.

Clinical Utility of Basic Cardiac Ultrasound

Similar to other diagnostic tools in medicine, clinical information gathered from FoCUS is most useful when it is performed to answer a specific clinical question with a pre-test probability in mind. The basic cardiac views explained above can be used to quickly answer the commonly encountered clinical questions in nephrology, most importantly qualitative assessment of LV function, right ventricular (RV) size/function, and pericardial effusion.

Left ventricular systolic function

LV systolic dysfunction (LVSD) has important diagnostic and prognostic implications for nephrologists. CKD is an independent risk factor for impaired LV systolic and diastolic function in children.¹⁰ Reduced cardiac peak performance and cardiac functional reserve have been described in adult patients with asymptomatic CKD in absence of pre-existing cardiac disease or diabetes.¹¹ LVSD as defined by ejection fraction <50% is seen in approximately 1 in 4 patients with ESKD.¹² All-cause mortality increases six-fold in ESKD patients with LVSD.¹³ Thus, serial measurements of LV systolic function in CKD and dialysis patients can provide valuable insight into their clinical course and impact management decision such as timely expert referral, volume management, and optimizing pharmacotherapy.

In the acute setting, determination of LV systolic function is equally important, especially in those with undifferentiated shock and AKI.¹⁴ An acute worsening of LV systolic function can explain the worsening renal function in patients with cardio-renal syndrome. Among patients with ST-elevation myocardial infarction (STEMI) undergoing coronary intervention, LVSD is a strong and independent predictor of AKI.¹⁵

FoCUS is more reliable than the physical exam, laboratory parameters, and chest x-ray for detecting LVSD and acute decompensated heart failure.¹⁶ FoCUS relies on the qualitative assessment of LV function by the visual method instead of quantitative measurements that can be time-consuming, need additional training and may not be as accurate in these scenarios.¹⁷ Qualitative assessment of LV function is sufficient for FoCUS purposes and skills needed for visual estimation of LV systolic function can be acquired by non-cardiologists with focused training. Studies have shown that after a limited experience of 20 practice studies, internal medicine residents were able to interpret reduced LV systolic function with both sensitivity and specificity above 90%.¹⁸

Qualitative assessment of LV systolic function can be performed in all 4 views mentioned in Table 1; however, parasternal long-axis (PLAX) and parasternal short-axis (PSAX) views are most useful for this purpose. In the PLAX view, LV systolic function can be visually estimated by observing the distance between the anterior leaflet of the mitral valve and interventricular septum at end-diastole, also known as E-Point Septal Separation (EPSS, Supplemental Fig 2). An EPSS cut-off of 7mm has a 100% sensitivity and over 50% specificity to identify severe LVSD.¹⁹ LVSD can also be estimated by the degree of LV wall thickening (normal ~ 40%) and fractional shortening of the LV cavity during systole. Based on visual estimation, LV systolic function can be broadly divided into four categories: hyperdynamic, normal, reduced, and severely reduced (Video 1).

It is important to assess LV systolic function by different views, as both PLAX and apical 4-chamber (A4C) views only show anteroseptal and inferolateral walls of LV. PSAX is an excellent view to visualize all four walls of LV for estimation of true global LV function, but it should be obtained at the mid-ventricular papillary muscle level to avoid over or underestimation of LV systolic function. It is possible to evaluate regional wall motion abnormalities (RWMA) with PSAX, but it requires additional expertise. Subcostal 4-chamber (S4C) view in isolation is frequently the only view available in emergent situations like cardio-pulmonary resuscitation and can yield important diagnostic information such as underlying cardiac tamponade. Only septal and lateral walls are visualized in this view. If S4C is the only view available, consider including a subcostal short axis view to assess global LV function. A subcostal short-axis view can be obtained from the S4C view by rotating the probe counterclockwise.

Right ventricular function

Right heart failure is caused by right-sided pressure/volume overload or direct RV injury from trauma or infarction. It can be an unrecognized cause of cardio-renal syndrome.²⁰ Pulmonary hypertension (PH) and right heart failure (RHF) may result in decreased eGFR by central venous congestion and decreasing cardiac output as a result of leftward bowing of interventricular septum (IVS).²⁰ Right ventricular dysfunction has been associated with the development of AKI in critically ill patients, heart transplant recipients, patients with decompensated heart failure and pulmonary hypertension.^21–23

Cardiac ultrasound is a readily available tool to accurately estimate RV size/function and IVS kinetics. It can help evaluate acutely ill patients with undifferentiated respiratory failure and shock and guide treatment decisions regarding inotropic support and fluid management. Administering IV fluids in a patient with RV dilation and bowing of IVS may compromise LV filling worsening the shock state.²⁴

Assessing RV size and function using FoCUS primarily depends on comparing its size, shape, and wall thickness with LV and evaluating IVS kinetics in all possible views. In a normal heart, RV walls are thinner than LV, and the RV chamber size is approximately two-thirds of LV. In the PSAX view, RV is crescent-shaped (Fig 1B), while it is triangular in the A4C view (Fig 1C). These characteristics are altered in the presence of acute or chronic RV dysfunction, dilatation, and hypertrophy.

Only the right ventricular outflow tract (RVOT) is visible in PLAX view limiting its utility in assessing RV function; however, severe dilatation and hypokinesis of RV may be diagnosed with the help of this view (Fig 2A, Video 2). PSAX view allows direct comparison of RV size and shape with LV. Flattening of IVS and enlarged RV in this view results in a “D” shaped LV cavity (Fig 2B, Video 2) suggesting RV pressure (flattening during systole) or volume overload (flattening during diastole).²⁵

Figure 2:

Right ventricular dilation suggesting increased right heart pressures: A, parasternal long-axis view with RV >LV and IVS bowing; B, parasternal short axis view with ‘D-shaped’ LV cavity; C, apical 4-chamber view with cardiac apex dominated by RV suggesting severe RV dilation. Abbreviations: IVS, interventricular septum; LV, left ventricle; RV, right ventricle.

A4C is the most informative view to diagnose global RV dysfunction. It allows a side-by-side comparison of both ventricles. It also allows visualization of IVS motion and bowing. Similar to LV, qualitative estimation of RV dilation and systolic function is preferred by visual method for FoCUS purposes. Recognizing the “true” view vs. atypical or foreshortened view when assessing RV size and function is important. RV size is categorized as normal (<2/3 of LV size), moderately dilated (>2/3 of LV), or severely dilated (RV >LV, apex dominated by RV, Fig 2 A–C, Video 2) based on visual assessment. As the RV dilates, its shape changes from crescentic to circular in PSAX view, and triangular to ovoid in A4C view. RV primarily contracts longitudinally and its systolic function can be estimated to be normal, reduced, or severely reduced by observing the motion of tricuspid annulus also called tricuspid annular plane systolic excursion or TAPSE (Supplemental Fig 3) for quantitative estimation (normal range 22–24 mm, RV systolic dysfunction <16 mm).²⁶ Advanced users can utilize A4C view for measuring TAPSE, Doppler flow evaluation of tricuspid valve, and estimation of pulmonary pressure.

In the S4C view, triangular RV is seen in the near-field view. Like A4C, it’s a good view for a side-by-side comparison of ventricles size and shape, especially when the A4C view is difficult to obtain. S4C is the preferred view for measuring RV wall thickness due to its location being perpendicular to the ultrasound beam. In acute RV failure, wall thickness is generally <0.5cm. When RV failure becomes chronic, the walls become hypertrophied with a thickness >1 cm.

Pericardial effusion

Pericardial effusion is defined as an accumulation of fluid greater than the normal 50 mL of the physiological amount in the pericardial space.²⁷ The etiology for pericardial effusions is diverse and includes malignancy, trauma, infection, inflammatory and auto-immune conditions.²⁷ The prevalence of asymptomatic pericardial effusion in ESKD patients has been reported to be as high as 62% and it can be a result of additional factors such as accumulation of uremic solutes and inadequate dialysis.²⁸ Based on these factors, pericarditis or pericardial effusion in advanced CKD patients has been broadly categorized into uremic and dialysis-associated pericarditis.²⁹ Uremic pericarditis has been described to occur because of the accumulation of toxic metabolites before or within 8 weeks of starting kidney replacement therapy (KRT).³⁰ Whereas dialysis-associated pericarditis is thought to be due to fluid overload and metabolic abnormalities resulting from inadequate long-term KRT.³⁰ Pericardial effusion has been noticed more frequently in ESKD patients starting emergent hemodialysis.³¹

Pericardial or cardiac tamponade is a deadly complication of pericardial effusion with an estimated incidence of 3.1% in non-ESKD patients with pericarditis and 10–20% in patients with uremic or dialysis-associated pericarditis.^32,33 It can be difficult to diagnose tamponade by history and physical exam in kidney disease patients as they have a gradual accumulation of pericardial fluid. They usually present with vague complaints, appear weak, and may not have the classical findings of Beck’s triad (hypotension, jugular venous distension and muffled heart sounds) as seen in trauma patients with the rapid accumulation of fluid.^32,34

FoCUS allows rapid, reliable, and non-invasive diagnosis of pericardial effusion and tamponade by non-cardiologists with >95% accuracy when compared with conventional echocardiography.^35,36 Identification of pericardial effusion at the bedside can be very helpful for nephrologists. It expedites management by initiating or intensifying dialysis and allowing timely referral for drainage of a large effusion. A diagnosis of cardiac tamponade and emergent ultrasound-guided pericardiocentesis may be lifesaving.

Pericardial effusion can be identified as an anechoic (black) space between the hyperechoic (bright white) visceral and parietal layers of the pericardium. In a supine patient, free-flowing pericardial fluid initially accumulates posteriorly in the dependent portion of the pericardial sac and then becomes more circumferential as the volume of effusion increases.³⁷ Posteriorly located small pericardial effusions can be seen in the far-field of PLAX view as an anechoic space traversing anterior to the descending thoracic aorta. The location of fluid in relation to the aorta is important to distinguish pericardial effusion from a left pleural effusion which is seen deeper or posterior to the descending thoracic aorta (Fig 3A).

Figure 3:

Circumferential pericardial effusion: A, parasternal long-axis view showing effusion in near and far-field, co-existing pleural effusion can be differentiated from pericardial effusion from its relative location with DTA (not visible, the approximate location is shown), and pericardium; B, parasternal short axis view with circumferential pericardial effusion and pleural effusion; C, subcostal 4-chamber view with large pericardial effusion and the diastolic collapse of RV free wall suggestive of tamponade physiology. Abbreviations: DTA, descending thoracic aorta; LV, left ventricle; RV, right ventricle.

In the S4C view, the anechoic space of pericardial effusion is initially seen in the near field, and then circumferentially as more fluid accumulates (Fig 3C). It is important to distinguish a pericardial effusion from an epicardial fat pad, which will appear more isoechoic and will not change with the patient’s position like a free-flowing pericardial effusion. Similarly, complicated effusions with purulent material, blood, thrombus, fibrin, and cellular debris may not appear anechoic and could be misinterpreted as epicardial fat or myocardium. Peritoneal fluid (ascites) is present adjacent to the diaphragm and can be misinterpreted as a pericardial effusion in the S4C view; however, ascites will not be circumferential or visible in other views.

The size of the pericardial effusion can be estimated by measuring the maximum dimension of anechoic space during diastole. Small effusions are typically <1 cm, moderate 1–2 cm and large effusions are >2 cm.³⁷ Large effusions typically require drainage; however, the hemodynamic significance of a pericardial effusion also depends on its rate of accumulation, and fluid characteristics (serous vs. hemorrhagic). A rapidly accumulating or loculated small to moderate effusion in penetrating cardiac injuries may result in higher pericardial pressure than RA and RV leading to tamponade and hemodynamic compromise.^7,34

Cardiac tamponade should be suspected in any hemodynamically unstable patient with circumferential pericardial effusion. In cases of pulseless electrical activity (PEA) arrest, the incidence of pericardial effusion has been noted to be as high as 67%.³⁸ Important echocardiographic signs of tamponade are RA and RV collapse during their respective diastolic phases when the chamber pressure is lowest, IVC dilatation >2 cm with <50% respiratory variation, or a large pericardial effusion with swinging heart (Video 3).³⁷ IVC plethora has excellent sensitivity (97%) and negative predictive value for ruling out cardiac tamponade.³⁷

Clinical and echocardiographic evidence of tamponade is a true medical emergency. Intravenous fluids and vasoactive agents can be used as temporizing interventions in hypotensive patients; however, emergent pericardiocentesis is of paramount importance for definitive management. A surgical evaluation may be warranted for purulent effusions and traumatic hemopericardium.

Valvular pathologies

Patients on long-term hemodialysis are more likely to develop hemodynamically relevant aortic and mitral valve disorders due to alterations in calcium metabolism.³⁹ Acute conditions for which screening can be helpful for nephrologists include unexplained pulmonary edema and heart failure resulting from the new onset or worsening valvular dysfunction. A detailed evaluation of valvular pathologies is beyond the scope of FoCUS; however, basic two-dimensional (2-D) ultrasound with color-flow Doppler can help screen for severe mitral or aortic regurgitation that can significantly impact management and warrants referral for comprehensive echocardiography.

Basic 2-D FoCUS may detect major valvular abnormalities including calcification, large vegetations, flail leaflets, thickening, and tethering, but these findings should be used with caution and only in the appropriate clinical context. Color-flow Doppler is the cornerstone of detecting valvular regurgitation in FoCUS; however, it requires a good understanding of its key concepts and consideration to avoid misinterpretation.

BEYOND THE BASICS: ADVANCED CARDIAC ULTRASOUND FOR THE NEPHROLOGIST

Competency in FoCUS can be attained by understanding the basic principles of 2-D echocardiography after a short course of training.⁴⁰ However, mastering advanced cardiac ultrasound (ACUS) techniques can be more challenging and time-consuming. It necessitates image acquisition skills of a trained echocardiographer as well as image interpretation and clinical application knowledge similar to a cardiologist.⁴¹ While FoCUS is increasingly being recognized as a mandatory skill among several disciplines, ACUS remains an optional area of expertise for non-cardiologists. Depending on their interest and scope of practice, many nephrologists may find it useful to become proficient in ACUS.

We have briefly outlined the basic principles of ACUS, their limitations, and clinical utility for a practicing nephrologist. A more detailed review is out of the scope of this article. For interested learners, a comprehensive review can be found in the standard echocardiography texts.^17,42,43

The Doppler Principle

In ACUS, Doppler is considered the language of blood flow. As per the Doppler principle, the frequency of a returning wave changes when the object reflecting the wave (blood or myocardium) and the source of the wave (transducer) are moving in relation to each other. A positive Doppler Shift or increased frequency of the returning ultrasound wave is observed when the blood flow is towards the transducer, and a negative Doppler shift or decreased frequency of the returning ultrasound wave is seen when it is moving away. Depending on the type of Doppler (spectral or color), positive Doppler shift is seen as an upward deflection or red color, while negative Doppler shift as downward deflection or blue color (Fig 4). A popular mnemonic to remember this principle is BART (Blue Away; Red Towards).⁴⁴

Figure 4:

The Doppler principle in cardiac ultrasound. Doppler shift is maximum in A & E when blood flow is parallel to the direction of the ultrasound beam, and minimum in C when they are perpendicular to each other.

As shown in Fig 4, in addition to informing about the direction of blood flow the Doppler shift can also provide information about its velocity. As is apparent in the figure, the Doppler shift depends on the cosine of the ‘Angle of Insonation’ – the angle between the direction of the ultrasound beam and blood flow. As you recall, the cosine of 90° is 0, whereas the cosine of 0° is 1. Therefore, the Doppler shift has a negative correlation with the cosine of the angle of insonation. This is a critical concept as a high angle of insonation can lead to large underestimations of the Doppler shifts and associated blood flow velocities.

Types of Doppler

There are two basic types of Doppler ultrasounds – continuous wave and pulsed wave. As the name suggests, the continuous wave Doppler is continuously sending and receiving the ultrasound waves, whereas the pulsed wave Doppler alternates between transmission and receipt of these waves. This basic difference between these modalities provides opportunities for use in different clinical contexts. The continuous wave Doppler measures all velocities across the path of the ultrasound wave. It can measure very high velocities generated by turbulent flows across the diseased valves. In contrast, the pulsed wave Doppler measures the blood velocity at a very specific location between the pulsed wave Doppler gates on the ultrasound screen. It, however, is unable to measure high velocities due to the phenomenon of ‘aliasing’.⁴⁵ It is therefore used to measure velocities at specific locations, for example, at LV outflow tract to measure stroke volume. Color-flow and tissue Dopplers are two special variants of pulsed-wave Doppler that are frequently used in ACUS.

Clinical Utility of Advanced Cardiac Ultrasound

The utility of ACUS in nephrology practice lies in the fact that it is readily available, cost-effective, and findings are interpreted by clinicians who are well aware of patients’ clinical course. Real-time image acquisition and interpretation can be performed serially for patients on renal replacement therapy in inpatient and outpatient settings. ACUS has several practical applications pertinent to a nephrologist such as measurement of cardiac output, volume responsiveness, right ventricular systolic pressure (RVSP), LV diastolic dysfunction, tamponade physiology, hemodialysis induced regional wall motion abnormalities^46,47, and measuring transvalvular flow for assessing dynamic abnormalities during ultrafiltration.⁴⁸ In addition to ACUS, an understanding of Doppler principles has wider applications in nephrology including vessel identification for dialysis catheter placement, arteriovenous fistula assessment, diagnosing systemic venous congestion, and measuring renal and splenic artery resistive indices during hemodynamic assessment.⁴⁴

A study that explored the feasibility of basic and advanced cardiac ultrasound by trainees in the emergency room found that advanced cardiac ultrasound adds less than 7 minutes to the total time when performed by junior residents.⁴⁹ This was down to 4 minutes when performed by fellows. Therefore, with appropriate training and practice, these additional views can be performed in specific situations without significant burden. A detailed overview of these applications is out of the scope of this review.

LIMITATIONS OF CARDIAC ULTRASOUND

Cardiac ultrasound findings can be non-specific and are most useful when used to answer a specific clinical question in a specific clinical scenario. Lack of hands-on practice and interpretation of poor quality, off-axis images may lead to incorrect conclusions negatively impacting clinical decision making. Nephrologists work in several settings including hospital, office, and dialysis unit, and may not have readily available ultrasound equipment at all locations. Not all hand-held devices may have optimal image quality due to smaller screen size, lack of resolution, and features to adjust image quality. Finally, the specific training and maintenance of skill standards for FoCUS and ACUS still need to be determined. They should be utilized in conjunction with the physical exam and the right clinical context. Any doubtful or significant clinical findings should be confirmed with a referral for comprehensive echocardiography.

CLINICAL SUMMARY

• Cardiovascular diseases are highly prevalent in CKD patients and knowledge of focused cardiac ultrasound enables nephrologists to reliably assess ventricular size and function, pericardial effusion, and volume overload.

• Basic principles and applications of cardiac ultrasound can be learned effectively and used as a part of the physical exam by non-cardiologists after a short course of training.

• Additional expertise in Doppler modalities may help evaluate cardiac output, right ventricular systolic pressure, left ventricular diastolic dysfunction, and tamponade physiology.

CONCLUSION

FoCUS is increasingly being recognized as an essential diagnostic tool across several clinical disciplines, including nephrology. Our introductory case describes a not so uncommon scenario in a nephrologist’s practice who cares for patients with advanced cardiac comorbidities. Coming back to our case, a FoCUS exam was performed revealing a large pericardial effusion, plethoric IVC, and diastolic collapse of RV free wall suggestive of tamponade physiology. The diagnosis was further supported by ACUS showing >25% mitral inflow variation. An urgent pericardial drain was placed at the bedside with drainage of over 600 mL of pericardial fluid with immediate improvement in her blood pressures. Aggressive ultrafiltration in the setting of an underlying large pericardial effusion likely precipitated cardiac tamponade in this case.¹ It was further worsened after the initiation of vasopressors due to the reduced time available for ventricular filling. This simple case emphasizes the importance of adding FoCUS and ACUS to nephrologists’ armamentarium in the fight against kidney diseases.

Supplementary Material

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Supplementary Figure 1: Transducer movements: Sliding is motion along the long-axis (Y axis) of the transducer across the body surface with a consistent angle; rocking is motion along the long-axis of the transducer with a fixed point on body surface and a changing angle; sweeping is motion along the short-axis (X axis) of the transducer across the body surface with a consistent angle; fanning is motion along the short-axis of the transducer with a fixed point on body surface and a changing angle; and rotating is moving the transducer along the vertical (Z axis) in a clockwise or counter-clockwise direction.

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Supplementary Figure 2: Left ventricular systolic function visual assessment by E-Point Septal Separation (EPSS): A, parasternal long-axis view in end-systole; B, parasternal long-axis view in end-diastole showing EPSS measurement, EPSS >1 cm is suggestive of left ventricular systolic dysfunction. Abbreviations: ALMV, anterior leaflet of mitral valve; IVS, interventricular septum; LA, left atrium; LV, left ventricle; LVOT, left ventricle outflow tract; RV, right ventricle.

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Supplementary Figure 3: Tricuspid annular plane systolic excursion (TAPSE): A, apical 4-chamber view showing positioning of M-mode curser at the tricuspid valve annulus; B, M-mode tracing with TAPSE measurement, TAPSE <16 mm is suggestive of right ventricular systolic dysfunction. Abbreviations: LA, left atrium; LV, left ventricle; RA, right atrium; RV, right ventricle.

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Supplementary Figure 4: Normal inferior vena cava. Abbreviations: IVC, inferior vena cava, RA, right atrium.

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Video 1: Parasternal long-axis view showing normal, reduced, and severely reduced left ventricular systolic function. Notice difference in the movement of the anterior leaflet of mitral valve in relation to inter-ventricular septum and inward motion of ventricular walls.

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Video 2: Right ventricular dilation suggestive of increased right heart pressures shown in parasternal long axis, parasternal short axis, and apical 4-chamber views.

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Video 3: Cardiac tamponade with swinging heart and right ventricular free wall collapse during diastole.