CHAPTER 4
Bedside Ultrasound

Daisi Choi¹ and John M. Oropello²

¹ Weill Cornell School of Medicine, New York, NY, USA

² Icahn School of Medicine at Mount Sinai, New York, NY, USA

OVERALL BOTTOM LINE

Bedside US is a safe, non‐invasive diagnostic procedure that allows rapid evaluation of undifferentiated hypotension and identification of reversible causes of shock. It is also a useful tool for promptly excluding immediately life‐threatening emergencies.
The focused assessment using sonography for trauma (FAST) exam is the standard of care in the initial evaluation of trauma patients with hypotension or signs of shock. Until recently, there was no standardized sonographic approach for evaluating the critically ill medical patient.
In 2009, the American College of Chest Physicians produced a consensus statement describing competence in critical care US. The components of critical care US the intensivist should achieve competence in for routine ICU operations include the following:
- Critical care echocardiography.
- Pleural ultrasonography.
- Lung ultrasonography.
- Abdominal ultrasonography.
- Vascular ultrasonography: guidance of vascular access and diagnosis of venous thrombosis.

Indications

Initial evaluation of undifferentiated hypotension and shock.
Non‐invasive monitoring of hemodynamic status and following response to therapy.
Respiratory failure.
Cardiac arrest.
Vascular access.

Basic concepts

Ultrasound physics (Table 4.1)

Sound waves: series of mechanical pressure waves that require a medium to travel through.
US waves undergo attenuation, reflection, refraction, and scattering as they travel through tissue.
Acoustic impedance: resistance of tissue to passage of US waves.
Degree of reflection is determined by difference in acoustic impedance of two tissues at interface.
US image is formed from reflected echoes.

Table 4.1 Features of ultrasound physics.

Body tissue	Acoustic impedance	Degree of reflection
Air	Very low	High
Liver, blood, kidney	‘Average’ soft tissue	Low
Bone	Very high	High

Table 4.2 Probe types.

Linear array probe	Phased array probe	Large curvilinear probe
Commonly referred to as vascular probe High frequency (typically 5–10 MHz) Large footprint Excellent image resolution of superficial structures at expense of tissue penetration Use: vascular, lung (specifically pleura)	Commonly referred to as cardiac probe Small footprint; sound waves originate from single point and fan outward Low frequency (typically 1–5 MHz) Excellent tissue penetration at expense of image resolution Use: cardiac, lung, abdomen	Large footprint; sound waves originate from large area and fan outward Low frequency (typically 2–5 MHz) Excellent tissue penetration at expense of image resolution Use: abdomen

Ultrasound equipment

Transducer (probe): sends out US waves that pass through tissue; also senses sound waves reflected back to transducer.
Structures closest to transducer are displayed at top of US screen in ‘near field.’
All probes have an ‘indicator’ (typically a bump or groove) on one side of the transducer that corresponds to an index marker on the US screen. Types of probes: see Figure 4.1 and Table 4.2.
General radiology convention is to position the screen index marker on left side of screen, and ‘point’ the probe indicator to patient’s right side or head. This means images on left side of screen correspond to structures on patient’s right side or toward patient’s head, respectively.
Cardiologists use an opposite convention (discussed in more depth in Procedure section).
It is critical to confirm your probe orientation with gel prior to any US exam or procedure. Relative to you, with the probe placed just above the intended point of contact, tapping under the right side of the probe should result in movement on the right side of the ultrasound screen. If movement occurs on the left side of the screen, rotate the probe 180°.

Basic knobology

Depth: adjusts depth of field of view by increasing or decreasing depth of US beam. Increasing depth will visualize deeper structures and decreasing will enlarge superficial structures.
Gain: adjusts brightness of image by changing amplification of returning echoes.
Time‐gain compensation: adjusts gain at selective depths to account for tissue attenuation; echoes returning from deeper tissues will be weaker.
Freeze: creates ‘still’ or ‘frozen’ 2D images.
Modes:
- B‐mode (brightness): standard scanning mode using different shades of gray to provide structural information in 2D image.
- M‐mode (motion): temporal measurement of structures moving toward or away from probe.
- Color Doppler: distinguishes vascular from non‐vascular structures and shows direction of flow.

Basic terminology

Echogenicity: brightness (amplitude) of image.
Hyperechoic/echogenic: structure appears brighter/whiter by generating more echoes than surrounding tissue.
Hypoechoic: structure appears darker than surrounding tissue by generating few echoes.
Isoechoic: same brightness as surrounding tissue.
Anechoic: area appears black due to complete absence of echoes.

Artifacts of US imaging

Shadowing: partial or total reflection of US waves (gallstones, ribs).
Posterior enhancement: area behind anechoic fluid‐filled structures appears brighter (bladder).
Edge artifact: shadow formed by refraction of US wave at edge of rounded structure.
Mirror artifact: image of structure duplicated as US wave reflects off highly reflective surface (diaphragm).
Reverberation artifact: US wave bounces between two highly reflective surfaces (pleura).
Ring‐down artifact: appearance of needle tip as a hyperechoic structure casting a narrow shadow.

Procedure

Cardiac ultrasonography

Probe selection and orientation

Use phased array ‘cardiac‘ probe.
Conventional cardiology screen/probe orientation is to position the screen index marker on the right side of the screen (reverse of general radiology convention).

Scanning technique

There are four standard views (Figure 4.2).
Parasternal long axis. Position probe just left of sternum at the third or fourth intercostal space. When using conventional cardiology orientation (marker on right side of screen), point probe indicator towards patient’s right shoulder. If you prefer keeping the screen marker fixed on the left side while obtaining views consistent with conventional cardiology imaging, simply point probe in opposite direction towards patient’s left hip. Otherwise, images will be reversed.
Parasternal short axis. Rotate probe 90° from long axis view to obtain circular short axis view of left ventricle. For conventional cardiology orientation, this means pointing probe towards the patient’s left shoulder. For general radiology orientation, point probe towards patient’s right hip. Angling probe through short axis views allows visualization of different segments of the left ventricle, including apex, papillary muscles (mid‐section), mitral valve (base of heart), and aortic valve (‘Mercedes Benz’ sign).
Apical four chamber. Using same orientation as short axis view, slide probe leftward – lateral to nipple line (men) or inframammary crease (women) – to point of maximal impulse. Position probe so ventricular septum is in center of US screen. The left heart will be on right side of screen and vice versa.
Subxiphoid. Position probe just below subxiphoid and angle cephalad toward the patient’s left shoulder using the liver as an acoustic window. If using conventional cardiology orientation, point probe towards patient’s left side. Otherwise, point probe towards patient’s right side. Transition to evaluating the inferior vena cava (IVC) from this view.

Clinical application

Assess for pericardial effusion or tamponade:
- Effusion appears as anechoic area within pericardial space.
- Large effusions tend to wrap circumferentially around heart.
- Clotted blood or exudates appear more echogenic.
- If effusion is identified, observe closely for diastolic collapse of right heart which indicates tamponade physiology.
Determine global left ventricular (LV) function, systolic function and size:
- Contractility is best determined using parasternal views.
- ‘Good’ contractility: LV walls almost touch during systole and nearly obliterate ventricular cavity; anterior leaflet of mitral valve moves vigorously in parasternal long axis view.
- ‘Poor’ contractility: minimal wall movement or change in ventricular cavity between systole and diastole.
- Small ventricular cavity in hypovolemic conditions.
Assess for right ventricular (RV) strain (Figure 4.3):
- Classic sign of massive pulmonary embolus.
- RV dilation: RV size exceeds LV size.
- Paradoxical septal wall motion or ’D’ sign: best seen in parasternal short view; normal LV is circular but increased RV pressure flattens or bows interventricular septum into LV during diastole.
- McConnell’s sign: RV dysfunction with characteristic sparing of apex; often described as ‘invisible man jumping on trampoline at RV apex’.

BOTTOM LINE/CLINICAL PEARLS

Ribs block US waves and obscure views of heart. Make fine adjustments by rotating and angling cardiac probe so the US beam is parallel to ribs.
Epicardial fat pad may be mistaken for an effusion; it is most prominent anteriorly.
Some views will be difficult to obtain in individual patients. Turning patient into left lateral decubitus position may improve views.

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