Critical Care Echocardiography: A Primer for the Nephrologist

Abstract

Critical Care Echocardiography refers to the goal-directed use of use of transthoracic or transesophageal echocardiography and represents one of the most common applications of Critical Care Ultrasound. Critical Care Echocardiography can be performed at the point-of-care, is easily repeated following changes in clinical status, and does not expose the patient to ionizing radiation. Nephrologists who participate in the care of patients in the intensive care unit will regularly encounter Critical Care Echocardiography as part of the decision-making and bedside management of ICU patients. The four primary indications for CCE are the characterization of shock, evaluation of preload tolerance, volume responsiveness, and the serial hemodynamic assessment to evaluate response to therapeutic interventions. This manuscript provides an overview of the anatomical structures that are routinely assessed in basic Critical Care Echocardiography, describe how these findings are incorporated into the clinical assessment of critically ill patients, and an introduction to some common applications of advanced Critical Care Echocardiography.

Introduction

Critical Care Ultrasound (CCUS) has become increasingly used in the intensive care setting, likely driven by the increasing quality, portability, and affordability of ultrasound devices. CCUS is now considered a standard of care for real-time guidance of many diagnostic and therapeutic procedures and has an ever-expanding list of indications in the management of critically ill patients. Critical Care Echocardiography (CCE) refers to the goal-directed use of transthoracic (TTE) or transesophageal echocardiography (TEE) and represents one of the most common applications of CCUS.¹ In contrast to comprehensive echocardiography, CCE is designed to answer a focused clinical question and monitor response to therapy.² CCE can be performed at the point-of-care, is easily repeated following changes in clinical status, and does not expose the patient to radiation. Nephrologists who participate in the care of patients in the intensive care unit (ICU) will regularly encounter CCE as part of the decision-making and bedside management of ICU patients.

As critical care providers become more facile with CCE, the level of complexity of bedside assessment has evolved substantially. As a result, CCE accreditation has been divided in two levels of practice; basic and advanced CCE.² Intensivists can use CCE in a range of common clinical scenarios, depending on their level of training, the clinical environment and wider institutional practices. The four primary indications for CCE are the characterization of shock, evaluation of preload tolerance, volume responsiveness, and the serial hemodynamic assessment to evaluate response to therapeutic interventions. This manuscript will provide an overview of the anatomical structures that are routinely assessed in basic CCE, describe how these findings are incorporated into the clinical assessment of critically ill patients, and an introduction to some common applications of advanced CCE.

Assessment of myocardial structure and function

One of the most immediate uses of CCE is to rapidly evaluate the structure and function of the heart and its associated structures. Although advanced CCE provides a more detailed assessment, basic CCE can provide essential information in the management of critical care patients.

Left Ventricle

Qualitative and semi-quantitative evaluation of the left ventricular ejection fraction (LVEF) is quickly learned and can be performed quickly and accurately.^3–8 Quantitative measurement of LVEF is more time-consuming, requires considerably more expertise, advanced machinery, and is less suitable for point-of-care CCE.

LVEF can be visually estimated in each of the cardinal windows through assessment of the endocardial excursion and the degree of myocardial thickening during systole, sometimes simplified as a unidimensional fractional shortening.^9–11 End Point Septal Separation (EPSS) is the distance between the anterior leaflet of the mitral valve and the ventricular septum in the parasternal long-axis view and it provides a semi-quantitative measure of LVEF, with a value of <7mm correlating closely with an LVEF of >50% (Figure 2a).^5,7,12 Although frequently assessed in the critically ill patient, LVEF is not equivalent to cardiac output (a dangerous misconception) and the LVEF can be affected by structural heart disease such as septal hypertrophy or valvular disease, as well as by cardiac load status, particularly hypovolemia or high-output states.^13–15

Figure 2:

Selection of 2-dimensional echocardiographic still images depicting ideal and corresponding advanced measurement technique. 2a: End-Point Septal Separation – parasternal long transthoracic echocardiogram (TTE) view with corresponding M-mode imaging; 2b: Left Ventricular Outflow Tract Velocity Time Integral (LVOT VTI) – apical five chamber TTE view with pulsed wave Doppler imaging over the LVOT; 2c: Assessment of superior vena cava variation – transesophageal echocardiogram view with corresponding M-mode imaging; 2d: Mitral valve inflow Doppler – apical four chamber transthoracic echocardiogram (TTE) view with corresponding pulsed wave Doppler at the level of the mitral valve leaflet tips.

Advanced CCE approaches allow for quantitative calculation of stroke volume (SV) and cardiac output (CO). LV outflow tract (LVOT) velocity time integral (VTI) can be measured by pulsed wave (PW) Doppler at the LVOT (Figure 2b). However, care must be taken to capture an accurate PW Doppler signal, as there are many potential sources of error for the novice scanner. Using the LVOT VTI and aortic valve area, SV and CO can readily be calculated repeatedly with clinical interventions and are valuable in the diagnosis of shock.¹⁶

Right Ventricle

Right ventricular (RV) dysfunction can be identified promptly and accurately by critical care physicians; this is especially important in conditions where the presence of RV dysfunction can fundamentally alter triage decisions, prognosis, and the clinician’s approach to volume resuscitation.^17,18 RV function can be assessed in a number of ways: basic CCE focuses on qualitative measures of RV function, whereas advanced CCE also incorporates quantitative approaches. Critical care physicians are accurate in the diagnosis of RV dysfunction and dilation in acute pulmonary embolism, with a negative predictive value of >90% for RV dilation and >85% for RV systolic function performed an average of 21 hours before comprehensive cardiac echocardiography.¹⁹ The potential advantage of CCE over traditional echocardiographic assessment is the low barrier to serial assessments with any change in clinical status.

RV size and function can be assessed in each of the cardinal cardiac windows. This assessment ideally includes both quantitative measures such as the tricuspid annular plane systolic excursion (TAPSE), and qualitative measures, such as RV diameter to LV diameter ratio of >1, RV basilar hypokinesis with apical sparing (so called McConnell’s sign), or paradoxical septal motion.^20–22However, RV assessment can be challenging due to the anterior position of the RV and the risk of angular errors in assessment of RV size in the apical 4-chamber and subxiphoid views. Additionally, evidence suggests that basic CCE assessment of RV is at risk of false positives and a tendency to overestimate pathology severity.²³

Pericardium

Assessment for a pericardial effusion and tamponade physiology remains one of the undisputed strengths of CCE. The presence of effusion can be diagnosed rapidly, accurately, and with minimal training in CCE.^24,25 Markers of tamponade physiology in basic CCE include IVC dynamics, RA systolic collapse, and RV diastolic collapse, but all are subject to diagnostic performance limitations.²⁶ The presence of tamponade physiology can be determined more definitively with the presence of pulsus paradoxus (PP) on physical examination or by using advanced CCE if PP cannot be measured through Doppler interrogation of the tricuspid valve. CCE diagnosis of cardiac tamponade uses advanced CCE approaches and involves comparison of inspiratory and expiratory flow through the mitral and tricuspid valves. In general, an expiratory increase in the tricuspid valve E-wave peak velocity ≥40–50% or decrease in mitral valve peak E-wave velocity is considered compatible with a diagnosis of tamponade.²⁶

Inferior Vena Cava (IVC)

Frequently one of the first techniques learned by new echo sonographers, the relative value of IVC dynamics to care of the critically ill patient is an area of controversy.^27,28 Achievable in at least 80% of critically ill patients, valuable information about the filling pressures on the right side of the heart can be learned from this assessment.^16,29,30 However, it is crucially important that IVC dynamics are understood in the context of the clinical scenario and not overly simplified to be a visual representation of volume tolerance or volume responsiveness.

Resuscitation

Initial assessment of shock states

Early and accurate assessment of shock etiology is essential, which requires a multidimensional approach. Clinical examination remains the foundation of diagnosis, management, and triage but is hampered by inter-observer variation and poor correlation with patient hemodynamics.^31,32 The additional information provided by CCE improves diagnostic accuracy and can aid with prognosis and guide initial therapy.^33,34 This information, however, should be combined with laboratory values, other imaging modalities, and additional POCUS studies such as lung ultrasonography.³⁵

CCE provides information to the critical care physician that physical examination alone cannot (Table 1) and is widely considered the first-line diagnostic modality, allowing for noninvasive and accurate diagnosis of shock state.^36–38Multiple protocols have been developed to rapidly assess and diagnose undifferentiated shock patients, with a selection provided in Table 2. Such protocols are quick to learn and can add valuable diagnostic information to initial evaluation in shock.

Table 1:

Physical examination, basic CCE and advanced CCE findings in different shock states.

Shock State	Clinical Exam	Pathology	Basic CCE	Advanced CCE
Cardiogenic	• Cold skin • Elevated JVP • Pulmonary rales • Narrow pulse pressure • Murmur	Myocardial infarction	• B-lines on LUS • Plethoric IVC • Plethoric IJV • Reduced LV EF	• Low LV VTI • Change in LV EF calculated • Wall motion abnormalities characterized • Lack of SVC respiratory variation (TEE)
		Severe diastolic dysfunction		• Abnormal transmitral Doppler • Abnormal mitral annular tissue Doppler • Elevated PASP • Abnormal pulmonary venous Doppler (TEE)
		Acute valvular regurgitation	• Regurgitation visualized on color flow Doppler	• Severity of valvular pathology assessed and graded
Distributive	• Warm skin • Wide pulse pressure	Septic shock Adrenal insufficiency Anaphylactic shock	• Hyperdynamic LV • Collapsed IVC	• Normal or high LV VTI • Identification of source (endocarditis)
Obstructive	• Cold skin • Elevated JVP	Pulmonary embolism	• Hyperdynamic LV • Plethoric IVC • RV dilation	• Pulmonary embolism identified (TEE) • Clot in transit identified • Reduced TAPSE • Elevated PASP • RVOT VTI notching
		Cardiac tamponade	• Pericardial effusion	• Tamponade physiology identified
		Obstructive cardiomyopathy		• SAM observed • LVOT gradient calculated
		Pneumothorax	• Plethoric IVC • Absence of lung sliding
Hypovolemic	• Cold skin • Low JVP Narrow pulse pressure	Hemorrhagic shock Diarrhea and other volume loss	• Hyperdynamic LV • Papillary apposition • Collapsed IVC Free fluid in abdomen	• Reduction in end-diastolic area • High SVC variation (TEE)

JVP: jugular venous pressure; IJV: internal jugular vein; IVC: inferior vena cava; LUS: lung ultrasound; PASP: pulmonary artery systolic pressure; RV: right ventricle; RVOT: right ventricular outflow tract; TEE: transesophageal echocardiogram; TAPSE: tricuspid annular plane systolic excursion; SAM: systolic anterior motion; SVC: superior vena cava; LVOT: LV outflow tract; VTI: velocity time integral.

Table 2:

Selected protocols developed for the assessment and diagnosis of patients in undifferentiated shock.

Protocol name	Indication	Echocardiographic views	Additional POCUS views
RUSH 54	Undifferentiated shock and hypotension in the ED	PLAX A4C	Morison’s and splenorenal (for peritoneal fluid) Bladder Abdominal aorta and IVC Lung sliding
SIMPLE 25	Undifferentiated shock	PLAX PSAX A4C SSX	Abdominal aorta and IVC
ACES 55	Non-traumatic hypotension in the ED	“Cardiac view”	IVC Abdominal aorta Pelvic view Right and left upper quadrants (for pleural/peritoneal fluid)
SHoC 56	Hypotension	Primary views: SSX and PLAX Additional views: A4C and PSAX	Lung views IVC
UHP 57	Undifferentiated shock	SSX	Morison’s pouch Abdominal aorta

A4C: apical-4-chamber; PLAX: parasternal long axis; PSAX: parasternal short axis; SSX: Subxiphoid view.

Resuscitation:

Once the likely etiology of shock has been identified, serial ultrasound assessment allows the critical care practitioner to assess dynamic hemodynamics using a series of “windows” into the body. In contrast to diagnostic echocardiography, which provides a detailed but static assessment of myocardial and valvular function, CCE includes lung ultrasound (LUS) and abdominal POCUS. It can be used for serial assessment of hemodynamic parameters to monitor response to therapeutic interventions.

Prediction of hemodynamic response (volume responsiveness)

Volume responsiveness is defined as a sustained increase in stroke volume or cardiac output (CO) by 10–15% after administration of a 500ml fluid bolus.³⁹ Many shock states will benefit from fluid administration through augmentation of preload, increased CO, and improved end-organ perfusion. However, the hemodynamic response to volume resuscitation is highly heterogenous: estimates are that only about 50% of patients in the ICU are fluid responsive.³⁹Given the growing evidence base for the harms associated with overly aggressive volume repletion, predicting volume responsiveness has become incredibly important.⁴⁰ A thorough understanding of the patient physiology is required – Figure 1, reproduced from Miller and Mandeville, 2016, outlines the hemodynamic changes observed during positive pressure ventilation.³⁸

Figure 1:

Reproduced with permission from Miller and Mandeville: The graphics demonstrate the pressure changes during the cardiac cycle with positive pressure ventilation. During inspiration, a rise in intrathoracic pressure is transmitted to the pericardium and increases transmural right ventricular pressure, resulting in dilation of the inferior vena cava and decreased superior vena cava diameter. Right ventricular stroke volume falls, while compression of the pulmonary vasculature forces blood into the left ventricle, increasing left ventricular stroke volume. After the pulmonary transition time, the left ventricle receives decreased blood and stroke volume falls. This effect is exaggerated in low circulating volume, and attenuated in volume overload or when either ventricle is failing..³⁸

The prediction of volume responsiveness using CCE has undergone an evolution in the past decade: traditional reliance on a single static parameter has been superseded by multimodal assessment and dynamic measures of hemodynamic response.^41,42 However, even dynamic assessment of IVC and SVC dynamics or cardiac chamber sizes have relatively limited diagnostic capability (although notably greater than that of physical exam).^16,31,32 Improved accuracy requires a detailed understanding of many basic and advanced ultrasound principles, including spectral Doppler and B-line imaging.

Cardiac output:

Qualitative and semi-quantitative assessment of systolic ventricular function using B-mode and M-mode is simple to perform for even novice sonographers.^7,8,12 However, systolic ventricular function is not synonymous with volume responsiveness or tolerance.

While the acquisition of Jour LVOT VTI, either by TTE or TEE, is more complex than the basic B- or M-mode assessment of ventricular function, it allows for dynamic assessment of cardiac output using spectral Doppler for assessment of SV/CO. Directional assessment of changes in CO after a fluid load maneuver - such as respiration, passive leg raise (PLR), or fluid bolus - provides the most useful indicator of volume responsiveness and a single measurement of CO is less useful.⁴³ Dynamic assessment of changes in SV and IVC variation, either by calculating respiratory variation or assessing the effect of a PLR can identify preload sensitive patients with high sensitivity (77 to 100 percent) and specificity (88 to 99 percent).⁴⁴

In a series of 540 patients in a multi-center trial, Phillpe Vingon and colleagues demonstrated that an increase in LVOT Vmax of ≥10% following passive leg raise (PLR) predicted volume responsiveness with a sensitivity of 79% and specificity of 64% and performed particularly well in the sub-population of patients with elevated lactate not from cardiogenic or obstructive shock.¹⁶

Grey Zones:

While commonly reported in the research studies, the concept of using a single cut-point for whether or not a patient is likely to be volume responsive fails to account for the huge variability in physiology found in the ICU. The reality is that the decision to give additional fluid to a patient is made on a case-by-case basis, accounting for multiple parameters - including the findings of CCE assessment – and through Bayesian reasoning. Vignon and colleagues were able to identify different thresholds for optimized sensitivity and specificity in assessment of IVC, SVC, and LVOT Vmax – which provide essential and information to the bedside resuscitationist that is much more easily interpreted (Table 3).

Table 3:

Performance of echocardiographic indices (LVOT - Left ventricular outflow tract; IVC: inferior vena cava; SVC: superior vena cava) in predicting volume responsiveness at thresholds identified for optimal sensitivity and specificity.

Dynamic parameter	Threshold	Performance at Threshold Value	Threshold Value for Optimized Sensitivity	Performance at Threshold Value	Threshold for Optimized Specificity	Performance at Threshold Value
LVOT Vmax variation	≥10%	Sensitivity 79% Specificity 64%	7%	Sensitivity 90% Specificity 39%	18%	Sensitivity 29% Specificity 90%
IVC variation	≥8%	Sensitivity 55% Specificity 70%	3%	Sensitivity 74% Specificity 36%	18%	Sensitivity 28% Specificity 90%
SVC variation	≥21%	Sensitivity 61% Specificity 70%	4%	Sensitivity 89% Specificity 25%	31%	Sensitivity 43% Specificity 90%

Adapted from Vignon et. al ¹⁶.

Volume tolerance:

Over the past few years, there has been increasing recognition of the harms of IV fluid resuscitation, which range from pulmonary edema to renal failure.⁴⁰ While the question of volume responsiveness is centered around whether or not the patient’s cardiac output will increase in response to a fluid bolus, the core question of volume tolerance concerns the risks posed to the patient by additional fluids. Rather than using ultrasound to predict volume tolerance, CCE allows the resuscitationist to dynamically assess a patient’s volume tolerance over the course of their resuscitation and to adjust the choice of therapy based on the risk posed to the patient by further fluids. In contrast to volume responsiveness, there are no widely accepted criteria used to identify patients who are likely to be volume tolerant, rather the CCE practitioner can use serial assessment of end-organ manifestations of volume intolerance.

CCE can be used to detect extravascular lung water, which manifests as a B-profile on lung ultrasonography. These findings can precede development of hypoxia or infiltrates seen on chest radiography, are readily attainable, and can be performed serially to monitor response to resuscitation. Lung ultrasonography and other end organ measures of volume overload have significant utility beyond the scope of this document and are described in detail in multiple other sources.

Advanced Applications:

Although less commonly encountered in daily practice, there are several applications of CCE that are used by many intensivists which may be important for the nephrologist.

Cardiac Output:

Filling pressures:

CCE can provide the clinician with a measure of the filling pressures, referred to as preload, in both the right and left sides of the heart. Assessment of the IVC diameter using B-mode is relatively simple to obtain and provides an indication of right atrial (RA) filling pressures – however despite the simplicity of attaining images, IVC assessment is hampered by technical limitations, low inter-rater reliability, and inaccuracy due to a range of patient-level factors.⁴⁵ A meta-analysis of 17 studies of the performance of IVC assessment reported a pooled sensitivity of 0.63 and specificity of 0.73, with worse performance in spontaneously ventilating patients.⁴⁶ The use of “grey zones” to guide interpretation of IVC distensibility/collapse may increase the utility of this parameter in clinical practice (Table 3). RA filling pressures can also be determined via TEE assessment of the superior vena cava (SVC) (Figure 2c).¹⁶ Advanced assessment of cardiac filling pressure need not stop at an assessment of the IVC: left-sided pressures can be assessed with mitral valve inflow patterns as would be performed during comprehensive echocardiography.

Analogous to the RA filling pressure, assessment of left-sided preload requires assessment of the left atrial (LA) pressure. This requires an understanding of the principles of diastolic dysfunction and skill in the use of Doppler methods such as Tissue Doppler (TDI) and mitral inflow pulse wave (Figure 2d). Integration of LA pressure evaluation with lung ultrasound can help to determine the etiology of pulmonary edema and abnormal mitral inflow patterns can be used to predict extubation failure.⁴⁷ However, LA filling pressure estimations can be affected by a range of clinical factors, including vasopressors, positive pressure ventilation, and pre-existing diastolic dysfunction or valvular disease.

TEE:

As one of the emerging elements of advanced CCE over the past few decades, the use of TEE has expanded from its traditional indications to include hemodynamic evaluation of patients in acute care environments. Multiple studies have demonstrated the feasibility of intensivist-performed TEE using focused protocols and have demonstrated that this modality is both safe and clinically impactful in the management of critically ill patients.^48,49 Common applications of focused TEE in critically-ill patients include assessment of circulatory failure, hemodynamic monitoring, evaluation of unexplained hypoxemia, and cardiac arrest.⁴⁸ In these settings, TEE is performed exclusively on intubated patients when TTE images are inadequate or cannot provide needed clinical data.

VEXUS:

More recently, protocols have been developed to systematically assess for end-organ venous congestion. The Venous Excess Ultrasound (VEXUS) protocol uses a combination of IVC diameter and evaluation of the Doppler profiles of the renal, hepatic, and portal veins.⁵⁰ While still an experimental approach, a profoundly abnormal VEXUS can be a useful tool to determine the probability of venous congestion – especially if contributing to acute kidney injury.

De-resuscitation:

Although there is not yet a robust evidence base to guide volume removal in critically ill patients, the physiological principles that underpin decisions around resuscitation can guide the removal of fluid through diuresis or dialysis. Monnet and colleagues demonstrated that patients who were predicted to be volume responsive after a PLR were more likely to have hypotension during intermittent renal replacement therapy.⁵¹ The underlying physiologic rationale is that if the patient’s CO increases with increased preload, they are also likely to decrease their CO in response to volume removal and preload reduction. Establishing volume responsiveness in this setting generally requires either invasive monitoring, specialized noninvasive CO monitors, or operator experience with Doppler-derived CO measurements.

Future Directions:

The field of CCE is expanding rapidly, and is likely to continue to do so with the introduction of affordable handheld devices, inclusion in a range of clinical guidelines, and an ever-increasing evidence base.

Conclusion

CCE has become established as an integral component in the evaluation, diagnosis and management of the critically ill patient. Understanding the concepts and limitations of CCE can be extremely beneficial to the nephrologist practicing in the ICU environment.

Clinical Summary:

• Critical Care Echocardiography is a useful tool in the evaluation of thecritically ill patient

• Understanding the applications and diagnostic capability of Critical Care Echocardiography is important for the nephrologist consulting in an intensive care unit, or providing primary critical care