23. Emergency Imaging

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Emergency Imaging

Edited by George Jelinek

23.1 Emergency department ultrasound

James Rippey, Adrian Goudie and Andrew Haig

Background

Clinical ultrasound followed developments from the use of sonar, where the principle that sound waves could be used to locate objects was developed. Initially, ultrasound machines were large and cumbersome, but advances in technology have improved image quality while reducing machine size, so that today, small machines are able to produce high quality images. As a result of this improved technology, ultrasound is now available to clinicians and can be performed at the bedside of patients. There was early adoption of the technique by obstetricians and gynaecologists worldwide and later by other specialties in Europe and Japan. Ultrasound is now used by many different specialties in most countries.

Clinician performed ultrasound has a different approach than formal diagnostic ultrasound, such as that performed in radiology departments. Clinician performed ultrasound is generally limited in scope and targeted to answering a specific question (such as ‘is there an abdominal aortic aneurysm’), rather than providing a full assessment of an anatomical area (Table 23.1.1). In this regard, it is often viewed more as an extension of the clinical exam, than a technique that competes with other imaging techniques (including formal ultrasound).

Table 23.1.1

Current indications for emergency ultrasound

Trauma (haemoperitoneum, haemopericardium, pneumothorax)

Abdominal aortic aneurysm

Early pregnancy complications

Biliary disease

Renal stones and hydronephrosis

Echocardiography in trauma and shock

Lung ultrasound in acute dyspnoea

Proximal DVT exclusion

Procedural

Musculoskeletal

The Australasian College for Emergency Medicine supports the use of bedside ultrasound by emergency physicians [1], as does the American College of Emergency Physicians, the College of Emergency Medicine in the UK and the International Federation for Emergency Medicine, where it is seen as a core skill required of all trainees. It is expected that with increasing experience the range of conditions for which ultrasound is used in emergency departments (EDs) will increase.

Basic physics of ultrasound [2]

Sound waves are mechanical waves that transmit energy through the vibration of particles. Ultrasound waves are defined as those that are above the usual range of human hearing (20–20 000 Hz). Current diagnostic ultrasound machines are based upon the pulse–echo principle, using pulses of sound waves at frequencies of 2–15 MHz which are reflected back. Processing of these reflected echoes creates the ultrasound data and image.

The ultrasound transducer converts electrical impulses into pulses of sound (via the piezoelectric effect) which are then directed into the body. As the sound wave travels through tissue, it gradually loses energy, termed attenuation. The degree of attenuation differs for different tissues and is also dependent on the frequency of the pulse wave. Upon reaching a tissue interface, some of the energy is reflected back as an echo, due to the differences in acoustic impedance (gel or other coupling material is used to minimize reflection at the probe/skin surface). This reflected echo then travels back through tissue, undergoing further attenuation, until it reaches the transducer, which converts the energy back to an electrical impulse. This returning impulse is then amplified and processed. The time taken for the pulse wave to travel to the tissue interface and back is converted into distance by using the average speed for sound in tissue. The intensity of the returning wave determines the brightness of the displayed pixel. The returning pulses from the different reflecting surfaces along the path of the ultrasound beam generate a single line of the ultrasound image. The ultrasound beam is steered across the field to generate the multiple lines of information that then form the 2D image (termed B-mode, for brightness modulation). Alternatively, if the direction of the beam is kept constant and the changing surfaces are mapped over time then an M-mode image is generated.

The degree of attenuation is dependent on the frequency of the sound wave, so higher frequency pulses undergo greater attenuation. They also have shorter wavelengths, which improve the resolution of the ultrasound beam (the ability to distinguish two separate objects close together). This leads to one of the most important trade-offs in ultrasound, between resolution and penetration. To obtain high resolution, a high frequency probe can be chosen, but these will be unable to image deep structures.

To form the image, the ultrasound machine makes certain assumptions about the ultrasound beam and sound impulse. Deviations from these behaviours will result in image artefacts, i.e. when the image does not represent the tissue accurately. There are many artefacts, most of which reduce the information available from the image. The most clinically important artefacts, however, can also be used diagnostically:

Shadowing – when all of the energy of the ultrasound pulse is reflected or absorbed at a surface (such as air or bone). In this situation, there will be no returning pulses from the tissue distal to the object. This creates a black area on the screen, known as an acoustic shadow. The presence of a shadow behind a brightly reflective surface can thus be used to diagnose a region of calcification, such as a calculus (Fig. 23.1.1). Stones and bones generally give ‘clean’ or black shadows, while gas gives ‘dirty’ or grey shadows due to the superposition of both shadow and reverberation artefact (Fig. 23.1.2).

Enhancement – when an area of interest (such as fluid in a cyst) absorbs less energy than the surrounding tissue, the pulses that have travelled through that area will have more energy than equidistant pulses that did not, resulting in a bright region deep to the area of interest on the image (Fig. 23.1.3). Enhancement is used to confirm the fluid-filled nature of lesions.

Transducer

Different ultrasound transducers are available varying in shape, frequency and the size of the contact area (termed footprint). Transducers may have a small footprint to fit into small areas, such as between ribs, from which the beam spreads in a large arc (e.g. a sector transducer). Alternatively, they may be larger with a flat or slightly curved surface where contact can be maintained, such as a linear probe. Special transducers have been designed for use within body cavities, such as transoesophageal, endovaginal and endo-anal probes. These transducers offer the advantage of reduced distance between the transducer and area of interest, which allows higher frequencies to be used resulting in improved resolution. Very high frequency transducers have been used for intravascular and superficial ocular scanning. Appropriate choice of transducer is important in ensuring the optimal image is obtained.

The scope of emergency department ultrasound

Extended focused assessment by sonography for trauma (EFAST) [36]

Descriptions of the use of ultrasound by clinicians to evaluate trauma patients appeared in the European literature in the 1970s. Reports have subsequently appeared from countries around the world and the technique is now well established. Initially limited to the abdomen and pericardium (FAST), the examination is now routinely extended to include the chest (EFAST). With relatively brief training and experience, non-radiologists are able to diagnose haemoperitoneum, pericardial effusions, pleural effusions and pneumothorax with a high degree of sensitivity and specificity, although accuracy does improve with experience.

Clinical examination in abdominal trauma can be difficult and unreliable. Diagnostic peritoneal lavage (DPL), ultrasound (FAST) and computed tomography (CT) have been used to evaluate further this group of patients. In most cases, FAST has replaced diagnostic peritoneal lavage as it is non-invasive and does not interfere with subsequent interpretation of CT images. CT scanning is highly accurate for diagnosing both free fluid and solid organ injury and is slightly less accurate for hollow viscus and diaphragmatic injury.

Studies of ultrasound scanning in trauma have reported varying sensitivity. Much of this variation is due to differences in the gold standard used for comparison and definition of ‘true positive’. Haemoperitoneum (on further imaging, surgical or post-mortem examination), organ injury and clinical stability have all been used in different studies. It must be remembered that the primary role of a FAST scan is to detect free fluid in the peritoneal or pericardial spaces, for which it has high sensitivity and specificity. Solid organ or retroperitoneal haemorrhage may be detected but, even in expert hands, the accuracy is much lower (with as many as two-thirds of injuries being missed). FAST has been shown to be reliable and useful in both pregnant and paediatric patients.

Technique

FAST scanning evaluates four regions for the presence of free fluid: (1) pericardial; (2) perihepatic; (3) perisplenic; and (4) pelvic (Fig. 23.1.4). The chest is then scanned to examine the pleural spaces posterolaterally for fluid and anteriorly to exclude pneumothorax. The technique is rapid, generally being completed in under 5 minutes.

Free fluid appears as an echolucent area (i.e. black) which is generally linear or triangular in shape in the most dependent area of the peritoneal or pericardial space, although blood clots may be seen as echogenic (grey) collections (Figs 23.1.523.1.7). While fluid is most commonly seen in the perihepatic space, all spaces should be examined before the result can be considered negative. Small amounts of fluid (<500 mL) may not to be detected.

When scanning the chest, the presence of lung sliding or lung pulse is an indication that the visceral and parietal pleura are in contact, excluding a pneumothorax at that point. The presence of a moving transition point between areas of lung sliding and absent lung sliding (the ‘lung point’ sign) is diagnostic of pneumothorax.

Limitations and pitfalls

Clinical implications and utility

The limitations of ultrasound in excluding all intra-abdominal injuries requiring laparotomy and the increasing use of conservative management of some injuries, even in the setting of intra-abdominal free fluid, has resulted in there being no universally accepted clinical algorithm based upon EFAST scan results. However, in this regard, EFAST scanning is no different to any other clinical, laboratory or imaging information of the trauma patient, the results of which are routinely used in combination to determine the management plan. Various algorithms incorporating EFAST scanning have been proposed, which generally incorporate haemodynamic stability and EFAST scan result, such as in Figure 23.1.8. Some algorithms incorporate a semiquantitative scoring system to estimate the amount of free fluid, with an increased volume of free fluid associated with greater need for therapeutic laparotomy. A positive abdominal EFAST scan is highly predictive of significant intra-abdominal injury and, based upon the clinical condition of the patient, generally indicates the need for CT or surgical exploration. A negative EFAST scan, stable haemodynamics and clinical observation have been shown to be highly accurate in excluding significant intra- abdominal injury. Some authors advocate serial EFAST examinations in stable patients, suggesting this can reduce the requirement for CT.

Similarly, for pneumothorax, the integration of EFAST findings with other clinical, laboratory and imaging findings will determine patient management. Conservative management of small pneumothoraces, even in the setting of positive pressure ventilation means that the ultrasound findings must be considered in the setting of the individual patient when management decisions are made.

In the Australasian setting, EFAST is generally accepted as fulfilling a complementary role to CT. Its portability and speed allow it to be used early in the evaluation of trauma patients (e.g. immediately after the primary survey) and this information is then incorporated with other clinical information to risk stratify the trauma patient to help to determine the requirement and timing for either laparotomy, thoracotomy or CT. Repeated examinations, particularly if the patient’s condition changes, can be valuable. Providing the limitations of the technique are not ignored, it can rapidly provide vital information to assist with patient management.

Abdominal aortic aneurysm [7,8]

Abdominal aortic aneurysms (AAA), defined as an aortic diameter>3.0 cm, are common, occurring in 1–9% of the population. Clinical assessment of the abdominal aorta is unreliable and may be especially difficult in the obese or unstable patient with abdominal pain. Clinical presentation of ruptured abdominal aortic aneurysm can be varied, with only 50% of patients demonstrating the classic presentation of hypotension, back pain and pulsatile mass. Other presentations may include abdominal, groin or flank pain, unexplained hypotension, syncope, haematuria or cardiac failure and AAA should be considered in any of these presentations.

Ultrasound is the primary mode of investigation of the abdominal aorta. Ultrasound performed by emergency clinicians has been shown to be rapid, highly sensitive and highly specific (>95%) in assessing aortic diameter. Ultrasound may occasionally detect rupture, but it is not reliable in excluding rupture. In addition to its utility in diagnosing AAA, ED ultrasound is very beneficial in rapidly excluding AAA in the wide variety of presentations listed above.

The risk of rupture of an AAA increases with the diameter. Although the risk of rupture if the aneurysm diameter is less than 4 cm is<0.5% per year and 1.5% per year for aneurysms 4.0–4.9 cm, rupture can still occur. Approximately 10% of ruptured aneurysms measure 5 cm or less.

Technique

The aorta should be identified anterior to the vertebral body and to the left of the inferior vena cava (IVC). It should be followed from the epigastric region to its bifurcation, just above the umbilicus, remembering that, in elderly patients, it may follow an ectactic course rather than following a strictly cranial–caudal course. It must be distinguished from both the superior mesenteric artery (SMA) (which runs anterior to the aorta) and the IVC (ensuring that the venous pulsation of the IVC is not mistaken for the arterial pulse of the aorta). Measurements should be taken both proximally and distally and, if an aneurysm is present, at the widest point. Measurements from both transverse and longitudinal planes should be taken. Measurements are taken from the outer wall to outer wall, including any mural thrombus (Fig. 23.1.9). If the renal arteries or SMA origin are identifiable then the relation to the aneurysm should be noted although, in the ED setting, this may not be possible. Any retroperitoneal haematoma or peritoneal free fluid should be noted.

Limitations and pitfalls

Clinical implications and utility

In the patient with ruptured AAA who is haemodynamically unstable, ED ultrasound allows rapid and accurate diagnosis within the resuscitation area. Rapid diagnosis of these patients is essential to achieve successful treatment. In the stable patient, whose presentation may be atypical, ED ultrasound provides a rapid means of excluding the diagnosis (e.g. in the elderly patient who presents with ‘renal colic’). If an AAA is detected in these patients, then further imaging will often be required to determine if the AAA is an incidental finding or the cause of the patient’s symptoms. If the AAA is an incidental finding then formal follow up should be arranged.

Early pregnancy [9,10]

Ultrasound is the primary imaging modality for early pregnancy and its complications [11]. In the emergency department setting, it is most commonly used for the pregnant patient with pain or bleeding. In addition to transabdominal scanning (TAS), transvaginal scanning (TVS) can be performed with patient consent using a specifically designed probe which places the transducer close to the pelvic organs and utilizes higher frequencies to produce images of much greater detail than TAS. It does not require a full bladder and should not be a painful procedure. TAS still has an important role, as it allows a broader field of view that allows better assessment of large amounts of free intraperitoneal fluid and may diagnose other causes of pain. Emergency physician performed ultrasound for early pregnancy complications has been shown to be safe and reduce the time patients spend in emergency departments.

Technique

TAS is performed initially, preferably when the patient has a full bladder, as the pelvic organs will be better visualized. The uterus is identified and examined in both longitudinal and transverse planes (recognizing that the longitudinal axis of the uterus may not necessarily be in a strictly sagittal plane). The endometrial thickness is noted and any fluid collections or gestational sac noted. The adnexa are examined to identify the ovaries and any masses. The pelvis is scanned for free fluid. The upper abdomen can be examined to estimate the volume of free fluid if seen. The kidneys can also be examined to identify any alternate diagnoses.

TVS is performed after the procedure is explained and consent is obtained. A chaperone should be present if the sonographer is male. The patient is asked to empty their bladder and the pelvis is elevated slightly off the bed using a foam wedge or similar. The probe is covered with a sterile sheath (e.g. condom) with gel placed inside and outside the sheath. The probe is gently inserted into the vagina and advanced. The uterus and adnexa are then examined in both longitudinal and transverse planes as in TAS. After the scan is complete the probe must be cleaned and disinfected.

Limitations and pitfalls

Clinical implications and utility

The primary aim of ultrasound in evaluating early pregnancy complications in the emergency department is to locate the gestational sac. Additional information should then be sought for the presence of free fluid, adnexal masses, embryonic size and embryonic cardiac activity (viability). The earliest ultrasound evidence of pregnancy is a small anechoic fluid collection surrounded by an echogenic ring, which can be seen on TVS at approximately 4.5 weeks. A pseudogestational sac (due to fluid within the endometrial cavity), however, can have very similar appearances. Definite signs that the sac is a true gestational sac appear at 5.5 weeks when the yolk sac can be visualized, or later when the embryo can be identified. A heartbeat may be visualized from 6.0 to 6.5 weeks onward. TAS will show the same features but 1–2 weeks later.

Pregnancies that are not identified by ultrasound are termed ‘pregnancy of unknown location’ (PUL). Most will either fail (miscarry or resolve spontaneously) or progress to normal pregnancy. However, 9–43% will eventually be identified as ectopic pregnancies (lower rates are seen in centres with more expert scanning ability as they have higher rates of definitively diagnosing ectopic pregnancy). Quantitative human chorionic gonadotrophin (HCG) levels have been used to determine when a gestational sac should be identifiable by ultrasound, termed the ‘discriminatory zone’. For TVS, this is usually 1500 IU and for TAS 4500 IU (varying between institutions and depending on expertise and equipment). Even though a normal pregnancy may not be expected to be seen in patients with β-HCG levels below these levels, ultrasound should still be performed as it may still show diagnostic findings. In particular, ectopic pregnancies often have lower β-HCG levels than normal pregnancies of corresponding gestation and may be seen, as can the presence or absence of free fluid, which is valuable for risk stratification.

All patients with pregnancy of unknown location require close follow up with serial HCG and repeat ultrasound.

If an intrauterine pregnancy is confirmed, the risk of ectopic pregnancy is very low in spontaneously conceived pregnancies. Heterotopic pregnancy is where both an intrauterine and extrauterine pregnancy coexist and occurs in up to 1:7000 pregnancies in spontaneous conceived pregnancies, but over 1:100 pregnancies in the setting of fertility treatment. Failure to visualize an intrauterine pregnancy may be due to early dates in a normal pregnancy, failed intrauterine pregnancy (including complete miscarriage) or ectopic pregnancy. Other ultrasound findings in ectopic pregnancy include non-specific findings, such as pelvic blood and adnexal mass (Table 23.1.2) [12]. Visualization of a gestational sac (with yolk sac or embryo) outside the uterus is diagnostic, but seen only in 8–26% of ectopic pregnancies (Fig. 23.1.10).

Table 23.1.2

Ultrasound findings of ectopic pregnancy

Ultrasound finding Accuracy (%)
Absent IUP 5
Any free fluid (no IUP) 50
Mod–large free fluid (no IUP) 60–85
Adnexal mass (no IUP) 75
Adnexal mass+free fluid (no IUP) 97
Ectopic pregnancy seen 100

IUP: intrauterine pregnancy.

Unusual forms of ectopic pregnancy include interstitial, cervical and scar ectopics. In these cases, a gestational sac may be seen, but not within the true uterine cavity. It is recommended that pregnancies that appear low or eccentric should be reviewed by expert sonographers.

Distinguishing between miscarriage and ectopic pregnancy when no adnexal mass has been identified can be difficult on ultrasound. However, if the clinical symptoms have settled, no free fluid is identified on ultrasound and no adnexal masses have been identified, then it is safe to observe or discharge the patient for formal ultrasound review the following day and subsequent follow up with repeat ultrasound and quantitative HCG (see Chapter 19.2).

If an intrauterine pregnancy is confirmed, the gestational age can be estimated by measuring the size of the embryo. Most machines will automatically calculate gestational age based upon this measurement. Cardiac activity can usually be identified by TVS once the embryo is approximately 5 mm (9 mm by TAS). Absent cardiac activity when the embryo is 7 mm or above suggests embryonic demise. Absent yolk sac or embryo on TVS when the gestational sac is 25 mm suggests an empty sac miscarriage (also referred to as a blighted ovum) [13]. Other sonographic signs of poor prognosis for continued pregnancy exist, but they are generally beyond the scope of emergency ultrasound.

Right upper quadrant/gallbladder [14]

Upper abdominal pain due to biliary disease is a common presenting complaint to emergency departments and includes biliary colic, choledocholithiasis, cholecystitis and ascending cholangitis. Many of the patients suspected of having acute cholecystitis will have alternate diseases and clinical examination is neither sufficiently sensitive nor specific for these patients. Ultrasound is the primary imaging modality for these patients, where it is used to detect the presence of gallstones (see Fig. 23.1.1), other sonographic signs of cholecystitis and bile duct obstruction. It is superior to both scintigraphy and CT for these patients.

Ultrasound has a high sensitivity and specificity for the identification of stones when performed by either radiology or ED staff. Some stones may, however, be missed and false-positive results also occur. The diagnosis of cholecystitis relies upon associated findings including sonographic Murphy’s sign, gallbladder wall thickening, gallbladder distension and pericholecystic fluid (Fig. 23.1.11). Gallbladder wall thickening and pericholecystic fluid are both non-specific findings and may be seen in other hepatic or generalized diseases as well as in acalculous cholecystitis. Fasting can cause gallbladder distension. The common bile duct, if visualized, should be measured and examined for stones, although this is technically more difficult and may be beyond the scope of a focused gallbladder examination.

Technique

The gallbladder (GB) is usually identified by scanning under the costal margin in a longitudinal plane. Positioning the patient in the left lateral decubitus position and/or deep inspiration may assist. The gallbladder should be scanned throughout its length in both longitudinal and transverse planes. The sonographic Murphy’s sign is assessed by pressing with the ultrasound probe over the gallbladder. Wall thickness should be measured and is normally less than 3 mm. Gallstones will appear as bright, echogenic masses with posterior acoustic shadowing. Stones tend to be mobile unless impacted in the gallbladder neck or cystic duct. Stones impacted in these positions are technically more difficult to detect and may be missed if not painstakingly searched for. Sludge and polyps will also appear echogenic but will not shadow. Whenever possible, the common bile duct (CBD) should be visualized, measured and followed; when dilated, distal CBD obstruction should be considered.

Limitations and pitfalls

Clinical implications and utility

In a patient with abdominal pain, the finding of gallstones with a positive sonographic Murphy’s sign is strongly predictive of cholecystitis. The more sonographic signs of cholecystitis that are seen, the more likely the diagnosis. However, asymptomatic gallstones are common and may therefore represent an incidental finding, especially if the sonographic Murphy’s sign is absent. In elderly, diabetic or critically ill patients, 5–10% of cholecystitis can be acalculous. In those patients thought to have biliary colic or cholecystitis, a negative ultrasound should prompt a search for alternative diagnoses or consideration of further imaging, either formal ultrasound or, if an alternate diagnosis is believed likely, CT.

Renal ultrasound

The primary focus of renal ultrasound in the emergency setting is the detection of hydronephrosis in the presence of acute renal failure or renal colic [15]. As experience increases, users can often detect renal calculi, particularly when located at the vesicoureteric junction (VUJ). The presence of a ureteric jet excludes complete ureteric obstruction on that side.

Technique

The kidneys are paired retroperitoneal organs lying on either side of the spine between T12 and L4. They have a convex lateral border and a concave medial border and hilum. The normal adult kidney is 9–12 cm in length, 2.5–4 cm thick and 4–6 cm wide. The kidney itself is composed of two distinct areas, the renal parenchyma and the renal sinus.

The adult kidney is scanned using a curvilinear 3.5–5 MHz transducer and a renal preset that provides the best contrast resolution and grey map for imaging the kidneys. The patient may be supine, although the kidneys are usually best seen with the patient in a lateral decubitus position. A combination of subcostal and intercostal approaches is often necessary to fully evaluate the kidneys. The kidneys should be imaged in at least two planes, including the sagittal or coronal plane and the transverse plane (Figs 23.1.12 and 23.1.13). On ultrasound, the kidney can be identified by its elliptical shape with a thin echogenic capsule. Normal renal parenchyma has slightly decreased or equal echogenicity relative to the hepatic or splenic parenchyma, although this is age dependent with it being comparatively hyperechoic in the elderly. The central renal sinus is particularly echobright due to the fat and fibrous tissue content. The renal pelvis and infundibulum are usually collapsed and not seen except in the setting of hydronephrosis when they become filled with urine, appearing anechoic. The bladder should be full and examined in both the sagittal and transverse planes to complete the study. It should be noted that an excessively full bladder may cause mild dilatation of the pelvicalyceal system, however, this will return to normal following micturition.

Ureteric jets occur when either ureter contracts propelling urine into the bladder. This occurs every 10–20 seconds in the euvolaemic patient with normal renal function and excludes complete obstruction on that side. The presence of a jet, however, does not exclude the possibility of a non-obstructive renal or ureteric calculus.

Hydronephrosis is the dilatation of the renal pelvis and calyces and may be secondary to an anatomical obstruction or may be functional in nature (such as with ureteric reflux). Obstructive hydronephrosis may be intrinsic or extrinsic. Depending on the level of obstruction, it may be unilateral or bilateral with or without associated hydroureter. When hydronephrosis is identified, the cause for the obstruction should be sought. Common intrinsic obstructive causes seen in the emergency department include obstructive or partially obstructive renal or ureteric calculi. Bladder outlet obstruction due to prostatic hypertrophy is another common cause of hydronephrosis. Extrinsic or invasive masses in the pelvis obstructing either the ureters or bladder outflow should also be considered.

Hydronephrosis may be described as mild, moderate or severe depending on the extent of dilatation of the renal collecting system:

Limitations and pitfalls

Clinical implications and utility

Ultrasound is less sensitive than plain films or CT in detecting renal calculi. Small stones may often be obscured by the echogenic renal sinus and be hard to detect if they have a weak posterior acoustic shadow. Having said this, stones in the kidney that are greater than 5 mm in size have been shown to be detected in experienced hands with 100% sensitivity sonographically [16]. Renal stones appear as bright, echogenic foci with distal acoustic shadowing and sometimes twinkle artefact when interrogated with colour Doppler. Ureteric calculi are difficult to visualize as they are often obscured by bowel gas in their retroperitoneal position. A normal appearing kidney and the failure to visualize a calculus therefore does not exclude a ureteric calculus that is non-obstructing or where hydronephrosis has not yet developed.

Deep vein thrombosis (DVT)

The primary focus of emergency department ultrasound in the assessment of DVT is in the diagnosis or exclusion of a proximal lower limb DVT.

The clinical assessment of DVT is unreliable and inaccurate [17,18]. Positive findings on sonographic examination of only 11% have been reported for patients referred for suspected acute DVT on the basis of clinical features [19].

Technique

Ultrasound is the imaging modality of choice for assessing for DVT. The technique relies primarily on B-mode imaging with intermittent venous compression, with the main diagnostic criteria used to exclude a DVT being complete collapse of the vein with apposition of the anterior and posterior walls of the vessel.

A broadband linear array transducer with a centre frequency of about 5 MHz is used to examine the femoral, popliteal and sometimes the calf veins. In larger patients, the curved linear array transducer with a centre frequency of 3.5 MHz (as used for abdominal studies) may be substituted. The curved linear array transducer is also used to examine the iliac veins. The machine should be configured to use the lower limb venous preset and the use of harmonic imaging may improve the contrast resolution between the vessel and surrounding tissue. Transducer compression of the interrogated vessel should be in the transverse imaging plane. Starting at the level of the groin, with the patient in a supine position, the common femoral vein is identified lying medial to the common femoral artery and the vein is compressed to demonstrate collapsibility extending distally in a stepwise fashion with the vein compressed every 2–3 cm (Fig. 23.1.14). Where thrombus is present in the vein, pressure with the transducer will not result in its collapse. The popliteal vein is best examined with the patient in a lateral or prone position with the knee slightly flexed. Colour and spectral Doppler may be used to supplement the findings of intermittent compression. Emergency physicians who had undergone standardized training to identify clot in the femoral or popliteal veins have shown an accuracy comparable to formal vascular studies [20].

Limitations and pitfalls

Clinical implications and utility

The accuracy of compression ultrasonography is highest in symptomatic patients, with studies comparing venography with compression ultrasound demonstrating an average sensitivity of 95% and specificity of 98% [21]. For proximal lower limb DVT, this technique has demonstrated sensitivity of up to 100% [22]. The use of colour and spectral Doppler to assess for vessel filling defects and flow patterns has not been shown to increase significantly the sensitivity for proximal DVT detection in the lower limb [2225]. It has also been suggested that an abbreviated technique, using only two compression points (the saphenofemoral junction and the lower popliteal vein) has adequate sensitivity, provided repeat examination is performed in 5–7 days [24,26,27]. The accuracy of ultrasound in detecting isolated calf DVT, especially when applied to bedside emergency ultrasound, is low with success rates as low as 40% reported [28].

Thus, the aim of focused emergency department ultrasound in the assessment of DVT is generally to confirm or exclude the presence of clot in the proximal deep veins of the lower limb. A negative compression ultrasound study of the proximal lower limb significantly reduces the likelihood of DVT and discharge from the emergency department without anticoagulation, with outpatient follow up for a definitive study can be considered [21,2931].

Emergency echocardiography

Focused use of echocardiography in the emergency department represents one of the most valuable uses of ultrasound in emergency medicine. Applications include its use in cardiac arrest, undifferentiated hypotension, suspected pericardial effusion and tamponade, chest pain, pulmonary embolus and ultrasound guided procedures. The use of cardiac ultrasound in emergency medicine is likely to increase significantly as more emergency physicians learn the technique and look to apply it increasingly in the clinical environment.

Echocardiography provides direct structural and functional information on cardiac structures only inferred by clinical examination, which has been shown to have limited accuracy, and with greater sensitivity and specificity than indirect tests, such as ECG and chest radiography [3234].

Technique

Modern general ultrasound machines can provide good quality transthoracic echocardiography capability. A broadband phased array transducer with a centre frequency of 2 MHz and a small footprint to improve access between the ribs should be used. Ideally, the patient should be positioned on their left side.

The full, standard echocardiographic examination includes parasternal long and short axis views obtained at the left sternal edge in the 2nd to 4th rib spaces, the apical 4-, 5-, 3- and 2- chamber views that are obtained at the cardiac apex and the subcostal views obtained from a subxiphoid position. The standard examination would involve 2-D assessment of cardiac structure and function using B-mode, supplemented by the use of colour and spectral Doppler to assess valvular function and measure chamber pressures using the windows described above.

Where an abbreviated, emergency bedside echocardiographic examination is being used to screen for the presence of tamponade, right ventricular dysfunction, left ventricular dysfunction and hypovolaemia alone, simply using B-mode and attaining the parasternal short and long axis views, the apical 4-chamber view and the subcostal view is generally enough.

In cardiac arrest, the subcostal view is used with the patient in the supine position. All preparations are made while CPR continues. During the 10-second pulse check, a loop is recorded and reviewed once CPR recommences. It is imperative that the time without CPR is minimized and echocardiography should not interfere with this.

Emergency physicians have been shown to assess accurately left ventricular function in the hypotensive patient [35].

Limitations and pitfalls

ent Good views may not be obtainable in a supine patient, especially if ventilated.

ent Not appreciating the limitations of focused and abbreviated emergency echocardiographic studies when compared to formal detailed echocardiographic studies.

ent Focused echocardiographic examination aims to detect for tamponade, right ventricular dysfunction, left ventricular dysfunction and hypovolaemia. It does not assess for diastolic dysfunction, regional wall motion abnormality, valvular dysfunction or aortic dissection, the detection of which generally take significantly more experience.

ent Confusing pleural and pericardial effusions.

ent Confusing pericardial fat pad and pericardial effusion.

ent Not appreciating the fluid causing tamponade is not always anechoic, particularly when exudative, purulent or haemorrhagic.

ent Not appreciating that a loculated clot or effusion may cause tamponade but not be seen, particularly in the postoperative patient

ent Not appreciating a normal echo does not exclude pulmonary embolism.

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