Transthoracic Echocardiography: Normal Two-Dimensional and Doppler Imaging

Abstract

Transthoracic echocardiography (TTE) provides the intensivist with a tool that has potential to give a rapid, noninvasive assessment of the hemodynamic status of the critically ill patient at the bedside. Even though critically ill patients have specificities that can make TTE images suboptimal, recent technological advances have allowed interpretable images to be obtained in the vast majority of patients, and TTE should be considered a first-line approach. The echocardiographic modalities with their respective indications for use in the intensive care unit (ICU) are described in this chapter. In addition, the stages involved in performing a TTE study are explained together with practical tips for optimizing imaging quality. The principles of Doppler imaging and indications are also reported.

TTE uses five standard views for hemodynamic evaluation, namely parasternal long- and short-axis views, apical four- and five-chamber views, and the subcostal view of the heart (with examination of the inferior vena cava).

The limitations of TTE in the ICU relate to the patient, the pathology, and the ability of the operator (both technical and in interpretation of the study). When TTE is inconclusive, the transesophageal approach should be considered.

Transthoracic echocardiography (TTE) provides the intensivist with a tool that has potential to give a rapid, noninvasive assessment of the hemodynamic status of the critically ill patient at the bedside. Information obtained may be used to assess cardiac anatomy and physiology. TTE may operate as a monitoring device and be employed to investigate the response to therapeutic intervention. Although TTE images in the critically ill may be suboptimal compared with those in the outpatient setting, the use of harmonic imaging and new upper-end echocardiographic platforms have improved two-dimensional imaging [1–6], and they are usually sufficient for diagnosis, particularly when assessing hemodynamics. Where images are nondiagnostic, transesophageal echocardiography (TEE) should be performed.

There is increasing support for both comprehensive and focused TTE in the investigation, management, and monitoring of the critically ill patient. Given the accessibility of the technique and the lack of risk to the patient (other than misinterpretation of the findings), TTE should be considered the first echocardiographic modality in the intensive care unit (ICU). Recent technological advances have allowed progressive miniaturization of machines. The capabilities vary from fully comprehensive to those capable only of two-dimensional imaging with limited facility for measurements and Doppler function [7–9]. For use in ICU settings, the ultrasound system must have electrocardiographic (ECG), high-quality, two-dimensional imaging, all Doppler-mode capacities, and facilities for recording and subsequent review of scans, ideally in a digital format. In this chapter, the ECG modalities with their respective indications for use in the ICU will be described. The stages involved in performing a TTE study will be explained together with practical tips for optimizing imaging quality.

1 Knobology

1.1 Ultrasound

Ultrasound is generated by piezoelectric crystals that vibrate when compressed and decompressed by an alternating current applied across the crystal. The same crystals can act as receivers of returning, reflected ultrasound where the vibrations induced by the ultrasound pulse are then used to generate images on the ultrasound machine. Ultrasound can be refracted, reflected, or attenuated, allowing us to image structures owing to its differential interaction with adjacent media. Thus, where adjacent structures have similar acoustic properties, ultrasound may not be able to differentiate between them (an example is hematoma of the liver).

Ultrasound is a sound wave, comprising waves of compression and decompression of the transmitting medium (e.g., air and water) traveling at a fixed velocity. As a longitudinal wave, ultrasound is described by its amplitude, wavelength, and cycle duration. Some of these features are key to understanding how to set up an ultrasound machine for optimal imaging and explain some of the artifacts that may confound the inexperienced user. One of the important differences between ultrasound and audible sound is that at higher frequencies, ultrasound tends to move more in straight lines, like electromagnetic beams, and be reflected and focused like light beams. Ultrasound is reflected by small objects (because of its shorter wavelengths) and does not propagate easily in gaseous media. This is clinically relevant in echocardiography since the heart shares the thoracic cavity with the lungs. Accordingly, there are only certain “windows” where the heart may be visualized easily with ultrasound. Although ultrasound and audible sound differ in some important physical properties, the basic principles are the same. The relationship between the wavelength, velocity, and frequency is shown in the equation below, where λ is the wavelength, V the velocity of ultrasound through tissues, and f the frequency of the transducer:

$$ \lambda =\text{V}/f $$

As the velocity of ultrasound is relatively fixed in soft tissues, the resolution (ability to distinguish accurately between two points) of the images is determined by manipulation of the other two factors. Thus, a higher-frequency probe will result in shorter wavelength and better axial resolution. However, owing to its interaction with tissues, a higher frequency will be more attenuated and reduce penetration of the ultrasound beam. Spatial (axial and lateral) and temporal resolution (discerning discrete events in time in a moving image) determine the image quality. The principles of axial and lateral resolution are shown in Fig. 2.1. Axial resolution cannot be better than two wavelengths, in practice approximately 0.5 mm using TTE. Temporal resolution is determined by the frame rate and, in turn, by the number of pulses per scan line, the number of scan lines per sector, and the sector depth and width. Thus, reducing the depth and width of the scan will improve the temporal resolution.

Fig. 2.1

Axial and lateral resolution in transthoracic echocardiography (apical fourchamber view). Axial resolution (a) is the ability to discern two structures parallel to the ultrasound beam, and lateral resolution (b) is the ability to discern two structures perpendicular to the beam. Axial resolution is optimal with high-frequency short-wavelength ultrasound, whereas lateral resolution is optimized by focusing the ultrasound beam to the zone of interest

1.2 Basic Settings

The main challenges are to select the correct probe, adjust the settings, and optimize image quality in order to provide the best resolution for the imaging performed.

• 1. Probe selection: select the highest-frequency cardiac probe that will enable imaging of the heart with adequate resolution given the depth required (usually 3.5 MHz in an adult).

• 2. Set the depth according to what needs to be imaged (e.g., the subcostal view may need increased depth, particularly in obese patients), but avoid wasted depth as this will reduce the temporal resolution (Fig. 2.2).

  Fig. 2.2

  

  Effects of depth setting on image acquisition. (a) The depth setting is excessive, and so the image is small, occupying only half of the available sector. In addition, the focus (yellow arrow to the left of the imaging sector) is set beyond the acquired image, thus limiting the lateral resolution of the image. (b) The depth is inadequate. Here, the image extends beyond the sector, which means that the posterior wall of the left ventricle and the descending aorta are not visible. It would not be possible to exclude a pericardial collection from this image, as the pericardial space is not visualized. (c) The depth setting is correct, and the whole of the available cardiac image is seen in the imaging sector, including the pericardium (arrows) and descending aorta (Ao). Abbreviations: RV, right ventricle; LV, left ventricle; LA, left atrium

• 3. Set the sector width as narrow as possible to allow appropriate interrogation of the image (Fig. 2.3).

  Fig. 2.3

  

  Effects of sector width and colorization on image resolution. In the image on the left (parasternal long-axis view) the sector width is maximal, taking in most of the cardiac structures visible from this view. In the image on the right, sector width has been reduced, thus improving resolution. Further colorization improves the ability to discern tissues, particularly where the acoustic properties are similar. Abbreviations: RV, right ventricle; LV, left ventricle; LA, left atrium; Ao: thoracic aorta

• 4. Focus: to maximize lateral resolution, focus on the zone of interest (indicated by dot/mark on the side of the sector).

• 5. Optimize the gain: imaging in the critical care environment can mean that ambient lighting is high. This results in inappropriately high gain settings for optimal image acquisition since high gain will reduce the resolution (Fig. 2.4). The usual range is 50–70.

  Fig. 2.4

  

  Effects of gain setting on image acquisition. In the image on the left, the borders between black and white (and hence resolution) are difficult to discern as the gain settings are too high. In the image on the right, the gain settings allow better definition and, hence, resolution. Abbreviations: RV, right ventricle; LV, left ventricle; LA, left atrium; RA, right atrium

• 6. Time-gain compensation: attenuation of more distant structures means that the gain for these will need to be increased using the time-gain compensation controls. Newer machines already take this into account, and no adjustment is required.

• 7. Compression: this refers to the logarithmic manipulation of the dynamic range of received signal amplitude. Increasing compression gives a reduced dynamic range of amplitudes (and a “softer” image), whereas reduced compression leads to a wider range of amplitudes and a more contrasted image.

1.3 Artifacts

An artifact is a misrepresentation of anatomy or physiology either as a result of poor setting of the ultrasound machine or as a result of the intrinsic properties of the ultrasound. The major artifacts experienced are as follows: echo dropout due to excessive reflection (Fig. 2.5), reverberation, mirror/duplication, gain, and side-lobe artifacts.

Fig. 2.5

Artifact from acoustic hyper-reflectivity/high density. In this parasternal long-axis view, a mitral annuloplasty ring has been placed, resulting in high echo reflectivity. As a result, little or no ultrasound passes behind, resulting in black, echo-free areas (arrows). Abbreviations: RV, right ventricle; LV, left ventricle; LA, left atrium; Ao: ascending aorta

2 Ultrasound Modalities

Different ultrasound modalities can be utilized (Table 2.1). Although there is a tendency to move toward newer imaging techniques, the challenges in obtaining adequate images in the ICU patient mean that often older techniques may provide more robust data since the potential for artifacts is lessened. In the following sections, the ECG modalities will be described, together with examples from the critical care setting.

Table 2.1 Modalities of echocardiography and their applications in the intensive care unit

Table 2.2 Hemodynamic assessment using transthoracic echocardiography

2.1 M-Mode Echocardiography

The most basic ECG modality still used in the critically ill is M-mode (motion-mode). M-mode is derived by displaying the A-mode (amplitude-mode) images over time (Fig. 2.6). The temporal resolution of this modality means it is superior in demonstrating rapidly moving structures and timing events accurately within the cardiac cycle (Fig. 2.7). M-mode is mostly used for measuring the wall thickness and chamber size of the left ventricle (LV) as well as LV fractional shortening (Fig. 2.8). It also allows measurement of the systolic displacement of the tricuspid valve annulus used as a parameter of right ventricular (RV) function (Fig. 2.9).

Fig. 2.6

The left of the figure represents the ultrasound pulse (P), which is then reflected from tissues at three different distances – a, b, and c (which is oscillating) – and two different echogenicities (a = c > b). Of note, the reflected ultrasound is dependent on the incident ultrasound and also the echogenicity of the tissues. Hence, the reflected pulse from c is of smaller amplitude than that from a, despite identical echogenicity. A-mode (amplitude mode) displays the reflected impulse in terms of an amplitude; B-mode (brightness mode) displays this on a gray scale; when this is displayed over time; M-mode (motion mode) presents the oscillatory movement of c as a wave (adapted from Stoylen with permission)

Fig. 2.7

M-mode recordings of a normal heart obtained in the parasternal long-axis view. A minaturized two-dimensional image is seen at the top of each panel. The x-axis is time (sweep speed 100 mm/s) and the y-axis distance (cm). From the base to the apex of the heart, M-mode allows precise examination of anatomical structures along the scanning line: the left atrium and ascending aorta (upper panel; 1, opening of the aortic valve); the mitral valve and basal portion of the left ventricle (middle panel; 2, opening of the mitral valve); the two ventricles (lower panel). Abbreviations: RV, right ventricle; LV, left ventricle;LA, left atrium; Ao, ascending aorta

Fig. 2.8

M-mode measurements of the left ventricular cavity and wall thickness of a normal heart in the parasternal long-axis view. The scanning line is positioned perpendicular to the left ventricular walls, immediately below the mitral valve. Measurements are performed using the leading edge-to-leading edge technique. Most frequently performed measurements are the end-diastolic thickness of the interventricular septum (a) and posterior wall (b), and the end-diastolic diameter (c) and end-systolic diameter (d) of the left ventricular cavity. Fractional shortening is calculated as end-diastolic minus end-systolic left ventricular diameter divided by end-diastolic diameter. Abbreviations: RV, right ventricle; LV, left ventricle

Fig. 2.9

M-mode of the long-axis function of the lateral wall of the right ventricle taken across the tricuspid annulus from the apical four-chamber view. The total excursion (magnitude) is known as tricuspid annular plane systolic excursion (TAPSE)

2.2 Two-Dimensional Echocardiography

To display a two-dimensional representation of the heart, repeated sweeps of M-mode scans are performed electronically or mechanically, and these are reconstructed to provide real-time two-dimensional images of the scanned anatomical structures. Since each sector scan forms one frame, the temporal resolution is limited by the depth of scanning and the number of scan lines (sector width) because the speed of ultrasound in the tissue is usually fixed. Thus, temporal resolution of two-dimensional imaging is less than that provided with M-mode echocardiography.

2.3 Doppler Echocardiography

Doppler ultrasound is used to determine the velocity and direction of blood (or tissue) toward or away from the ultrasound probe, first by measuring the Doppler shift. The Doppler shift (F_d) is the difference between the originating (F_o) and received (F_r) ultrasound frequency:

$$ {F}_{\text{d}}={F}_{\text{r}}-{F}_{\text{o}} $$

Thus, where blood flows toward the transducer, the frequency of the returning signal is increased, and when blood flows away the frequency is reduced. From the Doppler shift (F_d), the velocity (c) is calculated as follows (F_o, originating ultrasound frequency, cosθ cosine of the angle of incidence):

$$ {F}_{\text{d}}\times c/2\times {F}_{\text{o}}\times \mathrm{cos}\theta $$

In order to minimize error in recording velocities of the targets (red blood cells, myocardial tissue), the probe should be as parallel as possible to the target being measured because the higher the angle of incidence, the greater the error. Practically, the velocity can only be underestimated. There will be no signal with a crossing angle of 90°, the error being < 6% if the crossing angle is < 20° (Fig. 2.10).

Fig. 2.10

Effect of incident angle of ultrasound probe on velocity. On the left of the figure, the relationship between measured velocity (m/s) and calculated peak gradient pressure (mmHg) is shown. To the right of the figure, the incident angle between the ultrasound and the tissue being interrogated is shown in red (reproduced from Stoylen with permission). Thus, where the incident angle increases, the inaccuracy of the calculation of peak gradient pressure also increases, and its value is progressively underestimated (Modified from Feigenbaum et al. [10] with permission)

2.3.1 Continuous-Wave Doppler

Here, two separate elements of the probe continuously emit and receive ultrasound, with a miniaturized image available to help the practitioner orientate the ultrasound beam. Continuous-wave (CW) Doppler will therefore measure all the velocities along the selected length of the cursor, from highest to lowest, and the peak measured will be the peak velocity along the length chosen (Fig. 2.11). Thus, CW Doppler has excellent resolution of velocity, but not position. The commonest use for CW Doppler in the critically ill is in estimation of peak pulmonary artery pressure (Fig. 2.12), using the simplified Bernoulli equation and a measure of the right atrial pressure (either from invasive monitoring, clinical assessment, or ultrasound assessment of the inferior vena cava).

Fig. 2.11

Continuous-wave (CW) Doppler across the left ventricular outflow tract and aortic valve. The flow is high velocity and moving from the left ventricle to the ascending aorta, down, away from the probe. Peak velocity is arrowed, and using the modified Bernoulli equation, the peak pressure drop from the left ventricle to the aorta has been calculated (top left of figure) and corresponds to 134 mmHg. Velocity is shown on the y-axis in meters/second. A miniaturized two-dimensional reference image is shown at the top of the figure, demonstrating the correct positioning of the Doppler beam

Fig. 2.12

CW Doppler with the Doppler positioned across the tricuspid valve in the apical four-chamber view to measure the maximal velocity of the jet of the tricuspid regurgitation. The flow is high velocity and moving from the right ventricle to the right atrium during systole, away from the probe. The peak velocity between the right ventricle and atrium is arrowed. Velocity is shown (m/s) on the y-axis. Using the modified Bernoulli equation, the echo machine converts this maximal velocity to a pressure difference (seen in top left-hand corner). In the absence of pulmonary stenosis, systolic right atrioventricular pressure gradient directly reflects the systolic pulmonary artery pressure. A miniaturized two-dimensional reference image is shown at the top of the figure, demonstrating the correct position of the CW Doppler beam

2.3.2 Pulsed-Wave Doppler

Here, the Doppler pulse is sent from the probe, and the Doppler shift measured at a set time, which determines the depth at which sampling occurs. As the Doppler shift is small compared with the ultrasound frequency, multiple repetitions of Doppler pulses are sent in the same direction. This ensures depth resolution, but at half the pulse-repetition frequency, the signal direction becomes ambiguous (Nyquist limit). Accordingly, pulsed-wave (PW) Doppler is not useful at measuring high-velocity signals, but it is ideally suited for measuring relatively low-velocity signals at the level of specific anatomical locations, as required for the determination of stroke volume for example (Fig. 2.13).

Fig. 2.13

Pulsed-wave (PW) Doppler with the sample volume positioned in the left ventricular outflow tract, just below the aortic valve. The flow is laminar and moving from the left ventricle to the ascending aorta, down, away from the probe. Peak velocity is arrowed, and tracing around the Doppler envelope (for example, in measurement of velocity–time integral) is shown as a dashed white line. A miniaturized two-dimensional reference image appears at the top of the figure, demonstrating the correct positioning of the PW Doppler

2.3.3 Color Doppler

Color Doppler is based on PW Doppler technology, using multiple sample volumes along multiple planes and color mapping for velocity/direction data. The color map may be changed, but usually the BART (blue away, red toward) scale is used to represent the mean velocity and direction of circulating blood flow for that area. Color Doppler mapping is superimposed on each two-dimensional frame, allowing visual estimation in real time of both the velocity and direction of blood flow within the region of interest (Fig. 2.14). Where the two-dimensional images are poor, color Doppler will be unreliable. As with PW Doppler, aliasing occurs when the Nyquist limit is exceeded. This appears on two-dimensional images as a mosaic of colors. Provided that the Nyquist limit is adequately set, the presence of extended aliasing usually reflects high-velocity and turbulent flows secondary to an underlying cardiac abnormality (Fig. 2.15).

Fig. 2.14

Two-dimensional parasternal long-axis view of a normal heart with color Doppler oriented over the mitral valve during diastole. Note the blue laminar flow across the mitral valve to the left ventricle. Abbreviations: RV, right ventricle; LV, left ventricle; Ao: ascending aorta; LA, left atrium

Fig. 2.15

Apical four-chamber view in a patient with postacute myocardial infarction, using color Doppler over the mitral valve and left atrium. With a Nyquist limit set at 58 cm/s, a mosaic of colors indicates the presence of a high-velocity regurgitant jet consistent with a relevant mitral regurgitation (arrowed). Abbreviations: RV, right ventricle; RA, right atrium, LV, left ventricle; LA, left atrium

3 TTE Views

There are five standard views used in TTE (Fig. 2.16). All are usually easy to obtain in most patients, and where one view is challenging, often another view will provide excellent images. The common order of TTE views when systematically screening a patient is as follows: parasternal long- and short-axis views, apical four- and five-chamber views, and subcostal view of the heart and examination of the inferior vena cava. The suprasternal view is rarely used in critically ill patients. The order or number of views may change, however, depending upon the circumstances of the echocardiogram. For example, during resuscitation, only the subcostal view may be performed in order to facilitate chest compressions.

Fig. 2.16

The basic five transthoracic echocardiography views. 1: parasternal long-axis view; 2: parasternal short-axis view; 3: apical four-chamber view; 4: subcostal view; 5: suprasternal view

In the outpatient setting, the patients are normally turned to their left side, with the left hand placed behind the head in order to move the heart forward and increase the intercostal distance, thus facilitating positioning of the probe between the rib spaces. In the critically ill, this is often impractical, and the patient usually has to be imaged lying supine. Further challenges are presented by the presence of drains, dressings, intercostal and mediastinal tubes, surgical wounds, positive-pressure ventilation, pulmonary pathology, and a rapidly changing pathophysiological environment. Where inadequate views are obtained or for specific indications, TEE should be considered (see Chaps. 1 and 3).

3.1 Parasternal Long-Axis View

The transducer is placed in the second left intercostal space, close to the sternal border, with the marker pointing to the patient’s right shoulder (Fig. 2.17). This long-axis view of the heart allows evaluation of the LV size and systolic function, regional motion of both the septal and posterior LV walls, size of the left atrium, mitral and aortic valves with the initial portion of the ascending aorta, and anterior and posterior pericardium (Fig. 2.17). Ideally, the interventricular septum should be aligned horizontally to allow accurate measurement of chamber dimensions using M-mode (Figs. 2.17 and 2.18). Where this is not possible, care must be taken in obtaining measurements of chamber dimensions, most often using electronic calipers on a freeze frame and ensuring measurement strictly perpendicular to the cardiac walls. Finally, color Doppler mapping of the aortic and mitral valves is used to demonstrate any significant valvular regurgitation.

Fig. 2.17

Parasternal long-axis view. Hand and probe positioning are shown for a patient in the supine position (left panel). The probe is in the second intercostal space, just lateral (left) to the sternal border, with the indicator pointing toward the right shoulder of the patient (arrowed). The corresponding two-dimensional image provides valuable information on left cardiac cavities and valves (right panel). Abbreviations: RV, right ventricle; LV, left ventricle; Ao: thoracic aorta; LA, left atrium; i, interventricular septum; ii, posterior wall; iii: aortic valve; iv, mitral valve (modified from FEEL-UK with permission)

Fig. 2.18

Hand and probe positioning for the parasternal short-axis view in a supine patient. The probe is in the second intercostal space, just lateral (left) to the sternal border, with the green indicator light pointing toward the left shoulder of the patient (arrowed in green). To move between short-axis views of the aortic valve, mitral valve, and left ventricle, the probe is angled as shown (white broken arrow) (modified from FEEL-UK with permission)

3.2 Parasternal Short-Axis View

From the parasternal long-axis position, with the aortic valve in the center of the image, the probe is rotated clockwise such that the marker is now pointing toward the left shoulder of the patient (Fig. 2.18). Angling up shows the aortic valve and allows pulmonary PW, CW, and color Doppler to be performed (Fig. 2.19). Mild pulmonary regurgitation is common and almost always seen where a pulmonary artery catheter is placed. By tilting the probe perpendicular to the chest wall, more inferior angulation allows for a short-axis view of the mitral valve (Fig. 2.20) and a more apically angled probe enables the LV dimensions to be imaged (Fig. 2.21).

Fig. 2.19

Two-dimensional parasternal short-axis echocardiographic view of the heart. The probe is angled to image the base of the heart with the aortic valve and right ventricular inflow and outflow tracts (left panel). Color Doppler over the pulmonary valve reveals a small diastolic jet of pulmonary regurgitation (right panel, arrow). Abbreviations: Ao, aortic valve; tr, tricuspid valve; RV, right ventricular outflow tract; pulm, pulmonary valve; PA, main pulmonary artery; LA, left atrium; RA, right atrium

Fig. 2.20

Two-dimensional parasternal short-axis view angled at the level of the mitral valve. The two leaflets of the mitral valve are shown closing during systole (left panel) and fully opened at end diastole. Abbreviations: A, anterior leaflet of the mitral valve; P, posterior leaflet of the mitral valve; RV, right ventricle

Fig. 2.21

Two-dimensional parasternal short-axis view angled at the level of the left ventricular papillary muscles. The two ventricles are shown in their short axis and the ventricular septum is well depicted (left panel). This view allows the analysis of regional wall motion of the left ventricular walls supplied by the three main coronary arteries (right panel; the endocardial and epicardial borders are outlined). Abbreviations: LV, left ventricle; RV, right ventricle; i, interventricular septum; ii, anterolateral wall; iii, posterior wall

3.3 Apical four- and Five-Chamber Views

The probe is positioned at the apex of the heart (judged by palpation and echo imaging) with the marker pointing to the patient’s left axilla (Fig. 2.22). Ideally, on viewing the image, the septum should be aligned down the center, and the LV and atrium will be seen to the right, and the RV and atrium to the left (Fig. 2.22). From this position, the ventricular inlet dimensions can be measured and PW Doppler used to assess ventricular filling (see Chap. 16). It should be noted that good Doppler information may be obtained even in the presence of suboptimal two-dimensional images. CW Doppler should then be used to assess atrioventricular valve regurgitation. This should be performed even in the absence of regurgitation on color Doppler mapping since CW Doppler is more sensitive. Where tricuspid regurgitation is detected, the measurement of its maximal velocity may be used to estimate the peak pulmonary artery pressure (Fig. 2.12). Finally, color Doppler should be used to interrogate the two atrioventricular valves in order to detect the presence of an underlying valvulopathy (Fig. 2.15).

Fig. 2.22

Apical four-chamber view obtained in a supine patient. The probe is positioned at the apex of the heart, with the green indicator pointing toward the left axilla of the patient (left panel, arrow). To view the left ventricular outflow tract and aortic valve, the probe is angled so the ultrasound beam is directed superiorly (white, broken arrow). The two-dimensional apical four-chamber view depicts the two ventricles and atria (upper right panel), while the apical five-chamber view also includes the left ventricular outflow tract and aortic valve (lower right panel). Abbreviations: LV, left ventricle; RV, right ventricle; LA, left atrium; RA, right atrium; Ao, aortic valve; i, interventricular septum; ii, lateral wall; iii, apex; iv, interatrial septum (modified from FEEL-UK with permission)

Tilting the probe superiorly will reveal the LV outflow tract and aortic valve in the center of the image, giving the five-chamber view (Fig. 2.22). Color Doppler allows the identification of turbulent flow associated with an aortic valvular disease or an LV outflow tract obstruction. In the absence of relevant aortic valvulopathy, PW Doppler should be performed immediately below the valve (Fig. 2.13) to measure LV stroke volume (see Chap. 5). Where aortic stenosis is suspected, CW Doppler may be used to measure peak velocities and velocity–time integral as a reflection of maximal and mean transvalvular gradients, respectively.

3.4 Subcostal View

The probe is positioned in the subcostal area with the marker pointing to the right and with the operator’s hand on top of the transducer to allow horizontal access to the heart through the subcostal window (Fig. 2.23). Where other views have not been optimal, the subcostal view may provide good image quality, particularly in patients with respiratory disease and those with positive-pressure ventilation. Counterclockwise rotation will open up the inferior vena cava (Fig. 2.23). The vessel has to be visualized in a true long axis together with the inferior vena cava/right atrial junction. The abdominal aorta may also be imaged from this view.

Fig. 2.23

Subcostal view. To obtain the subcostal view, the hand is positioned on top of the probe to allow the ultrasound beam to be directed superiorly, under the ribs toward the heart, with the green indicator pointing to the left upper quadrant of the patient (left panel, arrow). Careful positioning of the probe allows depiction of the four chambers of the heart (upper right panel). To visualize the inferior vena cava, the probe is rotated counterclockwise such that the green indicator light is pointing upward (left panel, white broken arrow). This allows imaging of the inferior vena cava in its long axis and the junction with the right atrium (lower right panel). Abbreviations: RA, right atrium; RV, right ventricle; LV, left ventricle; LA, left atrium; IVC, inferior vena cava; L, liver; i, suprahepatic vein (modified from FEEL-UK with permission)

3.5 Additional Echocardiographic Views

In some examinations, additional information is required that necessitates further views. In the echocardiography laboratory, these views are generally performed on all patients, but they are infrequently required and obtained in the ICU setting.

3.5.1 Suprasternal View

To examine the aortic arch (for example, in aortic dissection), the probe may be positioned in the suprasternal notch, with the patient’s neck slightly extended and the marker toward the right shoulder (Fig. 2.24). The probe should be kept as horizontal as possible to allow access to the retrosternal window. Color, PW, and CW Doppler are useful to confirm direction of flow, exclude holodiastolic flow reversal in the descending aorta, and exclude coarctation (Fig. 2.25). This view may, therefore, be performed in all patients with dissection, aortic regurgitation, and congenital heart disease. The aortic arch may not be seen in patients who are intubated owing to interposition of the endotracheal tube. Here, images may be obtained from either the left or right supraclavicular areas and angling accordingly.

Fig. 2.24

Suprasternal view. The probe is positioned in the suprasternal notch, angled so the ultrasound beam is lined up with the aortic arch, and with the indicator pointing toward the right shoulder of the patient (left panel, arrow). This views depicts the aortic arch and the takeoff of supraaortic vessels. Abbreviations: Ao, aortic arch; i, inominate artery, ii, left carotid artery; iii, left subclavian artery

Fig. 2.25

Suprasternal view with the PW Doppler sample volume positioned in the descending aorta. The flow is laminar and moving down, away from the probe. Peak velocity is arrowed. A minaturized two-dimensional reference image is shown at the top of the figure, demonstrating the correct positioning of the PW Doppler. Abbreviations: i, ascending aorta; ii, descending aorta

3.5.2 Apical two- and Three-Chamber Views

These views are obtained by a counterclockwise rotation of the probe when positioned in the apical four-chamber view. They allow evaluation of the inferior, lateral, and anterior walls of the LV. The views are useful in evaluating regional wall-motion abnormalities and in detailed evaluation of the mitral valve.

4 Practical Use in the ICU Setting

In the ICU, echocardiography may be used as a diagnostic tool for monitoring the response to interventions or therapeutic maneuvers [11,12], as an extension to the clinical examination [13–15], and as an adjunct to investigation and diagnosis in the periresuscitation period [16,17].

Although many authorities propose that a full study should be performed as important new findings might be missed, under certain circumstances, focused should be considered. For example, during cardiopulmonary resuscitation, a comprehensive ECG study is usually not feasible. Here, focused studies designed to exclude obvious treatable causes for the periarrest state may be performed without interference using advanced life-support protocols. These findings may lead to a significant change in immediate management and possibly survival [16]. In the periresuscitation period, a limited ECG study may be performed to exclude obvious pathology, assess ventricular wall thickness and contractility, assess chamber dimensions, and image the pleura in a systematic manner [8,15]. In certain centers, such focused studies are used as an extension to the daily physical examination in the ICU; where an abnormality is seen, a comprehensive study is requested [18].

Echocardiography may be used to answer relatively simple hemodynamic questions in a noninvasive manner, both for diagnosis and in the monitoring of any therapeutic intervention [19]. The indications and main modalities in hemodynamic assessment of the critically ill are shown in Table 2.2.

5 Limitations of TTE in the ICU Setting

The limitations of TTE in the ICU relate to the patient, the pathology, and the ability of the operator (both technical and in interpreting the study). Patient factors include immobility, the effects of positive-pressure ventilation, the presence of dressings and drains, and obesity – all of which limit the diagnostic windows and increase the challenge to the operator. Pathological factors include those related to ICU support: changing hemodynamics, inotropic support, sedation, oxygen and carbon dioxide tensions, and those related to confounding factors from the underlying disease process (i.e., tamponade postcardiac surgery or endocarditis on a prosthetic valve). The greatest limitation, however, is the competence of the operator in adequately acquiring and interpreting TTE images.

In the ICU setting, performing a high-quality TTE is challenging and, as such, should not be regarded as somehow easier than TEE. With the transesophageal approach, obtaining images is simpler and the quality of images superior. Accordingly, image interpretation may be less challenging. There is, however, a risk associated with performing TEE, and the sedation/anesthesia required may change the hemodynamic findings significantly (for example, in mitral regurgitation). Thus, all intensivists should be able to perform TTE as a first-line ECG investigation. With both TTE and TEE come the challenges of interpretation of echocardiography in the context of the critically ill patient. The majority of research regarding ventricular function and valvular pathology derives from the non-ICU setting, and the relevance therefore to the critically ill is questionable. Further, many normal values may not, in fact, be normal in the context of the ICU patient.

6 Conclusion

TTE provides noninvasive, real-time, bedside assessment of the critically ill patient, offering unparalleled anatomical and hemodynamic information. Although images are often not perfect, the ability of echocardiography to yield accurate hemodynamic Doppler data, even in the presence of suboptimal image quality, means it is extremely useful in the ICU setting. Although gaining expertise in ICU echocardiography is both time-consuming and challenging and there is the potential for findings to change rapidly, the advantages of having a noninvasive window on cardiac function are undeniable.