Point-of-Care Echocardiography in the Difficult-to-Image Patient in the ICU: A Narrative Review

Abstract

OBJECTIVES:

The objective of this narrative review was to address common obstacles encountered in the ICU to acquiring quality and interpretable images using point-of-care echocardiography.

DATA SOURCES:

Detailed searches were performed using PubMed and Ovid Medline using medical subject headings and keywords on topics related to patient positioning, IV echo contrast, alternative subcostal views, right ventricular outflow tract (RVOT) hemodynamics, and point-of-care transesophageal echocardiography. Articles known to the authors were also selected based on expert opinion.

STUDY SELECTION:

Articles specific to patient positioning, IV echo contrast, alternative subcostal views, RVOT hemodynamics, and point-of-care transesophageal echocardiography were considered.

DATA EXTRACTION:

One author screened titles and extracted relevant data while two separate authors independently reviewed selected articles.

DATA SYNTHESIS:

Impediments to acquiring quality and interpretable images in critically ill patients are common. Notably, body habitus, intra-abdominal hypertension, dressings or drainage tubes, postoperative sternotomies, invasive mechanical ventilation, and the presence of subcutaneous emphysema or lung hyperinflation are commonly encountered obstacles in transthoracic image acquisition in the ICU. Despite these obstacles, the bedside clinician may use obstacle-specific maneuvers to enhance image acquisition. These may include altering patient positioning, respiratory cycle timing, expanding the subcostal window to include multilevel short-axis views for use in the assessment of RV systolic function and hemodynamics, coronal transhepatic view of the inferior vena cava, and finally point-of-care transesophageal echocardiography.

CONCLUSIONS:

Despite common obstacles to point-of-care echocardiography in critically ill patients, the beside sonographer may take an obstacle-specific stepwise approach to enhance image acquisition in difficult-to-image patients.

KEY POINTS

Question: Impediments to point-of-care echocardiographic imaging are commonly encountered in critically ill patients in the ICU. The aim of this narrative review was to provide a stepwise approach to how the bedside clinician may address these impediments.

Findings: Methods to enhance echocardiographic imaging in critically ill patients include optimizing patient positioning, using advanced methods to enhance subcostal window views, including for hemodynamic assessments, as well as the use of point-of-care transesophageal echocardiography.

Meaning: Advanced echocardiographic maneuvers can be used in the ICU to improve diagnostic evaluation with point-of-care echocardiography.

Point-of-care ultrasonography (POCUS) is an imaging modality used in the emergency department, wards, preoperative and postanesthesia care unit, and ICU settings that provides essential information for rapid real-time diagnosis as well as assessment of treatment responses in patients with life-threatening diseases. As such, POCUS has become the standard of care for use in caring for the critically ill (1–4). However, proper POCUS interpretation requires adequate transthoracic image acquisition. Impediments to transthoracic and abdominal echocardiographic windows often exist in critically ill patients due to body habitus, intra-abdominal hypertension, dressings or drainage tubes, postoperative sternotomies, invasive mechanical ventilation, the presence of subcutaneous emphysema, or lung hyperinflation (5) (Fig. 1). This narrative review will focus on point-of-care echocardiography in difficult-to-image patients and aims to highlight common problems encountered in the ICU followed by techniques to optimize image acquisition to acquire interpretable images by overcoming patient-specific factors or incorporating alternative views into a POCUS examination (Fig. 2).

Figure 1.

Schematic illustrating commonly encountered obstacles to transthoracic echocardiography. COPD = chronic obstructive pulmonary disease. Created with Biorender.com.

Figure 2.

Schematic illustrating maneuvers to improve image acquisition by specific obstacles. IVC = inferior vena cava, RVOT VTI = velocity time integral of the right ventricular outflow tract, SAX = short axis, TEE = transesophageal echocardiography.

BODY HABITUS

Obesity is highly prevalent in the ICU setting, affecting an estimated 20% of ICU patients (6), and may result in inadequate transthoracic images for bedside interpretation, particularly as weight exceeds 250–300 lbs (7). Ultrasound waves are attenuated by adipose tissue, thereby reducing image quality in obese patients. Additionally, there is often increased distance between the ultrasound transducer and heart. To improve image acquisition and quality, the bedside clinician can make some adjustments to help overcome some of these challenges. These adjustments rely on patient position, contrast administration, modified conventional POCUS views, and breath holds.

Patient positioning can play an important role in transthoracic echocardiography (TTE). Repositioning critically ill patients may be challenging for providers due to patient safety, particularly in the presence of obesity, an inability of the patient to assist, the presence of indwelling invasive devices, and other impediments. The left lateral decubitus position has been shown to improve parasternal and apical views by more than 12% by bringing the heart closer to the chest wall (8, 9). In obese patients, this will decrease the depth of penetration required for the ultrasound waves and ultimately decrease the amount of ultrasound wave attenuation. Repositioning often requires multiple providers and unilateral wedges placed under the patient can assist in achieving left lateral decubitus positioning. However, despite optimizing patient positioning, some obese patients will continue to have inadequate parasternal and apical windows. IV microbubble echo contrast has been shown to improve endocardial border opacification in critically ill patients with difficult transthoracic windows, allowing for improved evaluation of left ventricular (LV) function (5, 10, 11). However, IV echo contrast may not be readily available at most institutions for POCUS examinations. Alternatively, some obese patients may have adequate views from the subcostal window when compared with the parasternal and apical windows. Additional advanced echocardiographic assessments can be made by modifying the subcostal views which will be discussed further below. Subcostal views may also be improved by bending the patient’s knees enhancing abdominal muscle relaxation. A subset of patients may have transient quality images in the subcostal window throughout the respiratory cycle. During inspiration, the heart moves inferiorly toward the ultrasound transducer when placed in the subcostal position. If a patient is able to cooperate, the clinician may request a patient perform a full breath hold to improve subcostal image quality; alternatively, in mechanically ventilated patients, an inspiratory hold maneuver can be performed in an effort to improve image quality (12). Finally, while adequate image quality may still be challenging secondary to body habitus, presence of bowel gas, and postoperative states, some patients may still have obtainable dynamic assessments such as tricuspid annular plane systolic excursion (TAPSE), mitral annular plane systolic excursion or tissue Doppler imaging S’ that may provide valuable information about right ventricular (RV) and LV functions.

INADEQUATE PARASTERNAL AND APICAL WINDOWS

The use of TTE in the assessment of cardiac function and hemodynamics in critically ill patients is largely performed using the parasternal and apical windows. However, many patients in the ICU setting have impediments to adequate image acquisition at these locations (10, 13). Invasive mechanical ventilation and lung hyperinflation (as seen in chronic obstructive pulmonary disease and asthma) often interpose aerated lung parenchyma between the ultrasound transducer and heart leading to scattering and reflection of the ultrasound waves, low acoustic impedance, and a high attenuation coefficient (14). This may translate to visualization of reverberation artifacts with the pleural line and lung parenchyma. Patients receiving mechanical ventilation may have difficult parasternal or apical images as the addition of positive end-expiratory pressure may displace the heart inferiorly due to diaphragmatic flattening (15). Postoperative patients after thoracotomies or sternotomies may also have dressings and/or drainage tubes that obscure the parasternal or apical windows. Patients undergoing cardiopulmonary resuscitation (CPR) also often have inadequate parasternal views due to positioning of defibrillator pads and ongoing manual or mechanical chest compressions. In the absence of parasternal and apical views, the bedside clinician can gain additional information about cardiac performance by incorporating modified subcostal views.

Traditional subcostal views include the subcostal 4 and 5 chamber views and the subcostal inferior vena cava (IVC) view (eFig. 1A–C, http://links.lww.com/CCX/B295) (16, 17). These views provide vital information about the presence of pericardial pathology, global biventricular function, atrial pathology, valvulopathy, estimation of right atrial pressure, intravascular volume status, and volume responsiveness (15, 18–21). Qualitative assessments of RV and LV size and function and the presence of a pericardial effusion have shown strong agreement between subcostal and parasternal/apical views (22). In addition to traditional measurements taken from the subcostal window, advanced echocardiographic assessments may also be performed from this window in the absence of adequate parasternal and/or apical windows. For example, although base-to-apex RV function is often evaluated using TAPSE from the traditional apical four-chamber view (23, 24), the subcostal echocardiographic assessment of tricuspid annular kick can be used to evaluate RV systolic function from the subcostal window (25). This is similarly accomplished by placing M-mode over the tricuspid annulus and measuring excursion (Fig. 3A). Due to the change in orientation from the subcostal view, lower values are expected in normal RV function (about 2–3 mm lower than TAPSE values), with a value of greater than or equal to 1.6 cm indicative of normal RV function (25, 26). Alternatively, one can identify and mark the tricuspid annulus in systole and diastole and measure the distance between the two points, which has been shown to correlate well with traditional TAPSE measurements performed from the apical window (Fig. 3B) (27).

Figure 3.

Point-of-care ultrasonography (POCUS) images displaying: (A) Calculation of subcostal echocardiographic assessment of tricuspid annular kick (SEATAK) from the subcostal short-axis (SAX) view, which is calculated by placing M-mode over the tricuspid annulus in a SAX view from the subcostal window and measuring excursion, (B) Estimation of tricuspid annular plane systolic excursion (TAPSE) from the subcostal four-chamber view by measuring the distance between the tricuspid annulus in diastole and systole. RA = right atrium, RV = right ventricle.

The subcostal window can also be modified to obtain short-axis (SAX) cardiac views, providing additional information about cardiac performance in multiple planes. For example, the subcostal SAX view at the midpapillary level may be obtained by tilting the probe more cephalad from the subcostal IVC view (increasing the window into the thorax), and by fanning toward the right flank (Fig. 4). Just as with the parasternal SAX view, this view can be used to evaluate global LV function, including advanced echocardiographic assessments such as fractional area change, signs of RV dilation, septal flattening during end-systole and/or end-diastole to evaluate RV volume and/or pressure overload, as well as regional wall motion abnormalities (eFig. 2, A and B, http://links.lww.com/CCX/B295) (15).

Figure 4.

Schematic illustrating angle of insonation and fanning direction to obtain subcostal SAX views. Arrows indicate direction of fanning moving from papillary muscle level to aortic valve level. Created with Biorender.com.

The subcostal SAX view at the level of the aortic valve and RV outflow tract (RVOT) can also be obtained by fanning toward the right shoulder from the midpapillary level (Fig. 5, A and B). Assessment of the aortic, tricuspid, and pulmonary valves can be obtained from this view, including color flow Doppler to evaluate for valvular regurgitation and estimation of pulmonary artery pressures. Hemodynamic assessments may also be performed using the velocity time integral of the RVOT (RVOT VTI) (Fig. 5, C and D). Although traditionally the LV outflow tract (LVOT) VTI has been used to estimate stroke volume and ultimately cardiac output and cardiac index using the equation $\mathrm{S}\mathrm{V}=\pi \ast {\left(\frac{D_{\mathrm{L}\mathrm{V}\mathrm{O}\mathrm{T}}}{2}\right)}^2\ast \mathrm{V}\mathrm{T}{\mathrm{I}}_{\mathrm{L}\mathrm{V}\mathrm{O}\mathrm{T}}$, where SV is the stroke volume, D_LVOT is the diameter of the LVOT measured in the parasternal long axis, and VTI_LVOT is the LVOT VTI (16). However, in the absence of adequate views to obtain LVOT VTI Doppler assessment from the apical window, RVOT VTI may be used as an alternative measurement to infer stroke volume. Although the cutoff for normal LVOT VTI is greater than 17 cm (28, 29), the average RVOT diameter is both slightly larger and structurally different than that of the LVOT, and its cross-sectional area cannot be as readily determined in the same manner as for the LVOT. Therefore, the cutoff for normal is slightly lower for RVOT VTI, generally greater than 14 cm (30, 31). However, evidence suggests there is moderate agreement in comparing the RVOT VTI obtained from a subcostal SAX view and LVOT VTI in critically ill patients, and this view was obtained in 90% of patients (32). RVOT VTI has also been shown to correlate to cardiac index, with a cutoff of less than 9.5 cm highly predictive of cardiac index less than 2.2 L/min/m² and associated with increased mortality in critically ill patients with pulmonary embolism (33). Although directly estimating SV, cardiac output, and cardiac index using RVOT VTI may be challenging, RVOT VTI measured from the subcostal view can be used as a screening tool to identify patients who may be in low SV states. The use of RVOT VTI may be less reliable in patients for whom RV and LV stroke volume differ, for example, those with intracardiac shunts or severe valvular regurgitation.

Figure 5.

Point-of-care ultrasonography (POCUS) images displaying: (A) parasternal short-axis (SAX) aortic valve (AoV)/right ventricular outflow tract (RVOT) view, (B) subcostal SAX AoV/RVOT view, (C) subcostal SAX AoV/RVOT view with color-flow Doppler displaying flow away from the transducer through the RVOT, (D) calculation of RVOT velocity time integral (VTI) from the subcostal SAX AoV/RVOT view. LA = left atrium, PA = pulmonary artery, PV = pulmonic valve, RA = right atrium, RV = right ventricle, TV = tricuspid valve.

The bedside clinician can also enhance the POCUS examination from the subcostal region by obtaining the subcostal bicaval view by tilting the probe more cephalad from the traditional sagittal IVC view (Fig. 6A). This view may be of particular interest in the setting of determining cannula positioning during extracorporeal life support or superior vena cava pathology (15).

Figure 6.

Point-of-care ultrasonography (POCUS) images displaying: (A) subcostal bicaval view, (B) right midaxillary line transhepatic coronal inferior vena cava (IVC) view. ECMO = extracorporeal membrane oxygenation, LA = left atrium, LV = left ventricle, LVOT = left ventricular outflow tract, OxyRVAD = right ventricular assist device with membrane oxygenator, RA = right atrium, RV = right ventricle, SVC = superior vena cava.

In the specific case of POCUS during CPR, the subcostal region provides an optimal window to identify causes of pulseless electrical activity (specifically right heart strain and pericardial tamponade) (34, 35), presence of cardiac activity which has been associated with improved return of spontaneous circulation and survival to hospital discharge (36), and to monitor the adequate positioning of chest compressions (34). Furthermore, the use of the subcostal window has been demonstrated to be feasible in providing adequate and interpretable images during CPR pulse checks without prolonging no-flow time between CPR cycles (34, 35).

INADEQUATE SUBCOSTAL WINDOWS: THE INFERIOR VENA CAVA

The subcostal sagittal view of the IVC has become an essential component of the POCUS examination in critically ill patients. It can provide vital information regarding intravascular volume status and estimation of right atrial pressure in select patients, and while it has traditionally been used for evaluation of volume responsiveness prediction, accurate prediction using IVC can be fraught with challenges and there is significant heterogeneity in the literature regarding its use (18–21, 37–41). It may be more reasonable to use IVC dimensions as well as the extremes of its collapsibility or distensibility index to improve diagnostic accuracy to rule in/out a patient who may be volume responsive. For example, in one study, a cutoff of IVC collapsibility index in spontaneous breathing patients of 42% was 97% specific and only 31% sensitive for predicting fluid responsiveness; however, a cutoff of 20% would have improved sensitivity to 79% at the expense of specificity (42). Therefore, it is certainly worthwhile to include IVC measurements in the global context of the point-of-care echocardiogram. However, subcostal windows may be inaccessible in ICU patients in the setting of dressings, drains, and body habitus. In this scenario, if the right midaxillary window is accessible, a transhepatic view of the IVC in the coronal plane can be obtained (Fig. 6B). It should be noted that these measurements may not be interchangeable. A number of small single-center studies have been performed comparing subcostal and transhepatic assessments of the IVC as it relates to size, distensibility or collapsibility, and prediction of volume responsiveness (43–48). There is varying agreement amongst these studies, likely due to the nature of the IVC collapsing as an elliptical shape (43, 49). One study found that an IVC collapsibility index of 42% should be used to predict fluid responsiveness when using the transhepatic IVC view in spontaneously breathing patients (48). Though the transhepatic view of the IVC can be used in the absence of accessible subcostal windows, caution should be used when interpreting measurements at the bedside. Additionally, in patients with intra-abdominal hypertension, the IVC becomes a poor surrogate of intravascular volume status and volume responsiveness due to extrinsic compression and decreased venous return (50).

FUTURE DIRECTIONS: POINT-OF-CARE TRANSESOPHAGEAL ECHO

Despite all the maneuvers described above, there will still be a small subset of critically ill patients in the ICU who will not have any meaningful interpretable image for diagnostic and therapeutic interpretation. These patients, particularly those who are already sedated and mechanically ventilated, may benefit from point-of-care transesophageal echocardiography (TEE) to obtain vital information regarding cardiac and hemodynamic performance. Point-of-care TEE is feasible and safe in critically ill patients, although requires specific training and competence of the sonographer (51–53). TEE provides high-quality images without the impediments experienced with TTE. In addition, transesophageal echo is also advantageous in several scenarios. Postoperative cardiac surgery patients with sternotomy wounds and chest wall devices benefit from transesophageal echo in the setting of unexplained hypotension (54), particularly as localized cardiac tamponade, for example, a posterior pericardial hematoma with right atrial compression, may not be readily identified, even with adequate transthoracic views. Just as with TTE, TEE can also provide information about preload sensitivity and provides a superior view of the superior vena cava which can be used to assess volume responsiveness (55). In addition, TEE provides increased sensitivity for the evaluation of intracardiac shunts and valvular disorders, including vegetations, when compared with TTE (51). Other advantages of TEE over TTE, particularly in the presence of inadequate transthoracic windows, include identification of the cause of cardiac arrest during CPR (56), identification of aortic dissection and central pulmonary embolus, as well as real-time procedural guidance such as during extracorporeal membrane oxygenation cannulation, percutaneous RV assist device placement, and intra-aortic balloon pump placement (51). Given some of the limitations of TTE and additional benefits of TEE, there is increasing demand for training in TEE competency amongst general intensivists. Evidence suggests that point-of-care TEE in the ICU improves diagnostic yield beyond TTE, often changes management, and basic midesophageal and transgastric views are readily learned by trainees (52, 57). However, TEE does not come without risks. Contraindications include significant esophageal disease such as stricture or recent esophageal or gastric surgery. Severe complications may include oropharyngeal bleeding, esophageal bleeding, and esophageal perforation, although these are rare complications and occur in roughly 0.01–0.5% of cases (57, 58).

CONCLUSIONS

Bedside echocardiography provides vital information pertaining to rapid diagnoses and responses to therapeutic interventions for critically ill patients in the ICU. However, the bedside clinician may face several impediments to obtaining quality transthoracic images, namely body habitus, the presence of wounds, dressings, and drainage tubes, invasive mechanical ventilation, intra-abdominal hypertension, and hyperinflated lungs. A stepwise approach can be taken to improve image quality including patient positioning, timing of image acquisition during the respiratory cycle, the use of modified views from the subcostal window, the use of transhepatic coronal imaging for IVC assessment, administration of contrast, and ultimately point-of-care TEE in select patients (Fig. 2). Clinicians should use caution when interpreting modified views as further research is needed in these areas to correlate to advanced echocardiographic measurements obtained from traditional transthoracic parasternal and apical views.