A general practitioner’s guide to the daily use of point of care ultrasound in small animals
Author: Søren R. Boysen, DVM, DACVECC
Veterinary Point of Care Ultrasound (VPOCUS) is a sonographic examination of the patient performed by the attending clinician, often applied as an extension of the physical examination. The goal of VPOCUS is to identify the presence or absence of a finite number of specific findings, often with the aim to quickly direct further diagnostic procedures and/or initiate lifesaving interventions. It is important to realize that VPOCUS involves a different sonographic skill set (easily mastered by general practitioners) than the skills required to complete a comprehensive or limited sonographic evaluation performed by a dedicated specialist (requires years of specialized training/experience). VPOCUS is often an invaluable test performed at the point of care to confirm uncertain findings of the physical exam, identify essential underlying conditions in the unstable patient, and/or provide sonographic guidance to improve the success and safety of many diagnostic procedures, particularly when time is of the essence.
Most applications of VPOCUS try to answer simple questions, ideally with binary (i.e. yes/ no) answers. For example:
Is there free fluid in the pleural space?
Is there free fluid pericardial space?
Is there free fluid in the peritoneal space?
Is there a pneumothorax?
Is the left atrium enlarged?
Is the caudal vena cava distended or collapsed?
Are the lungs wet or dry?
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VPOCUS skills are rapidly becoming essential to the clinical development of today’s veterinarians and similar to human clinician guidelines, a minimum skill-set is likely to become mandatory for all graduating small animal veterinarians. Although VPOCUS techniques are rapidly expanding and new applications are being applied annually, this article will focus on the more commonly used and validated small animal veterinary techniques: AFAST, TFAST and Lung Ultrasound.
As mentioned, there are many applications of VPOCUS, which are expanding annually. To date, the most common indications to perform VPOCUS exams include, but are not limited to:
Any acutely collapsed or unstable patient.
Any patient sustaining blunt or penetrating trauma.
Any patient presenting in respiratory distress.
Any patient with an acute abdomen or acute abdominal pain.
Any patient suspected to have free abdominal fluid, pleural or pericardial effusions, or pneumothorax.
Any post-operative patients that have become unstable, are at-risk for dehiscence, hemorrhage or infection.
Any patient with an unexplained anemia.
Any patient suspected of a coagulopathy.
Any patient in cardiac arrest.
Patients that require urine volume to be estimated in the absence of a urinary catheter.
Patients suspected to have ileus.
Patients with suspected but unconfirmed pyometra.
Patients suspected but unconfi rmed anaphylaxis.
Patients with suspected but unconfirmed congestive heart failure.
Additionally, a number of guided procedures benefit from the assistance of ultrasound which include thoracentesis, abdominocentesis, pericardiocentesis, modified diagnostic peritoneal lavage and vascular access/sampling. These techniques are not covered in this article.
The most common VPOCUS techniques used in small animal patients include Focussed Assessment with Sonography for Triage (FAST) exams of the abdomen and thorax (often referred to as AFAST and TFAST respectively) and lung ultrasound (see below for techniques). Each of these examinations has its own set of objectives, but are complimentary when used together in patients, and it is generally advised to complete all three in any patient.
A designated VPOCUS machine is highly recommended. In today’s market, a portable ultrasound machine that can be brought to the patient is essential. Alternatively, in the absence of a portable machine, the VPOCUS machine should be stationed in the resuscitation area (leave the machine here)! It is considered malpractice to interrupt resuscitative efforts or to transport an unstable patient requiring VPOCUS examinations out of the treatment area; bring the machine to the patient, not the patient to the machine!
An ultrasound machine with a curvilinear probe (micro-convex) capable of B-mode is recommended. The frequency setting most often used range from 5 MHz setting in larger dogs (> 20 kg) and a 7 MHz for smaller patients (Dogs and cats ≤ 20 kg). Having a linear array probe is helpful for vascular access techniques and can be used to assess lung gliding (see lung ultrasound and TFAST for information regarding the glide sign). A phased array probe is rarely used in VPOCUS, however, if it is available it can be used in smaller patients where obtaining a good window of the heart is difficult due to interference from the ribs (rib shadowing).
Materials and preparation:
Patients generally do not require the fur to be shaved to perform commonly used VPOCUS exams. The fur is parted and alcohol applied over the desired site. Occasionally, if image quality is poor (e.g. thick undercoat in some Northern dog breeds), or greater detail is desired (e.g. cardiac imaging) shaving of the fur may improve the sonographic image.
In the majority of cases alcohol is used as the sole acoustic coupling agent. However, to enhance image acquisition, ultrasound conducting gel may be used alone or applied following the application of alcohol. Be sure to smooth out gel with your fingers before performing the sonographic exam to prevent air bubbles from forming (important if patients aren’t shaved as air bubbles may occur within the gel/fur medium which interferes with image quality). Avoid alcohol in patients that may require electrical defibrillation or electrocautery. Alcohol hand based gels can also be used.
Depending on the desired scan and specific question to be asked, patients are scanned in either sternal, standing or lateral (left or right) recumbency. If the patient is scanned in lateral recumbency, it is often rolled into a sternal position to obtain gravity dependent views of the VPOCUS exams (see below for details).
DO NOT place the patient in dorsal recumbency to perform VPOCUS examinations as this may cause cardiopulmonary patients to decompensate or even arrest.
Abdominal FAST scan (AFAST)
With the patient positioned in right or left lateral recumbency (most common positions), or in sternal (with the hind legs turned toward the sonographer to allow the subxiphoid and bladder sites to be accessed), or standing, a systematic four-view protocol is used (See figure 1a, Boysen et al, 2004). The four-views of the AFAST exam have been named by external land marks and for the target organs imaged at each position. The 4 sites evaluated include;
- The subxiphoid or diaphramaticohepatic (DH) view which is obtained by placing the probe at an approximate 45 degree angle just caudal to the xiphoid process (where the ribs first come together at the manubrium ventrally). Tip the tail of the probe towards the caudal aspect of the patient. The subxiphoid view allows evaluation of specific target structures including the liver lobes, the gallbladder (hypoechoic), the hepatodiaphragmatic interface (visualized a white hyperechoic curvilinear line separating the abdominal and thoracic cavities), the heart and pericardial space as well as the pleural space (Figure 1b and 1c). It is also possible to assess the caudal vena cava at this site as it transverses the diaphragm, which is used to assess patient volume status (not covered in this article).
- The left paralumbar or splenorenal (SR) view assesses the left kidney, the spleen (including the tail of the spleen, left body wall and the areas surrounding these structures (Figure 1d)).
- The midline view over the bladder or cystocolic (CC) view is obtained just adjacent to midline in the caudal abdomen near the pelvis and aims to identify the urinary bladder, particularly the apex, and it’s surrounding areas (Figure 1e).
- The right paralumbar or hepatorenal (HR) view assesses the area located between the liver and right kidney, the right kidney, small intestine and right body wall and it’s surrounding areas (Figure 1f, Boysen and Lisciandro, 2013).
Although the order of scanning is not critical, a systematic approach should be developed. The author prefers starting at the subxiphoid location followed by the urinary bladder view, then the non-gravity dependent view (when patients are in lateral) and lastly the gravity dependent view (Boysen et al, 2004).
In the interest of time (emergent unstable patient) the ultrasound probe is generally oriented in a longitudinal/orthogonal plane to start with. If fluid is identified, the probe is moved to the next AFAST site. That said, if practitioners want to become more familiar with ultrasound beyond VPOCUS, it is recommend that all sites be assessed in the longitudinal and transverse orientations, particularly if results are equivocal in the longitudinal plane.
Figure 1a, AFAST: To perform an AFAST exam the patient can be placed in right or left lateral recumbency or can be scanned in sternal. Left lateral recumbency is shown in this figure. The 4 sites to be evaluated include the subxiphoid or diaphragmatic-hepatic (DH) site (1), the right paralumbar or hepato-renal (HR) site (2), off midline over the bladder or cysto-colic (CC) site (3) and the left paralumbar or splenorenal (SR) site (4). A variation of the technique whereby a "flash" scan in place of the gravity dependent view (site 4 with the dog in left lateral recumbency or site 3 with the dog in right lateral recumbency) can be substituted if the objective is to identify free fluid and kidney assessment is not vital. At each site, the ultrasound probe is initially placed longitudinally to the underlying organs and fanned through an angle of 45° and moved 2.5 cm in cranial, caudal, left, and right directions. Adapted from Veterinary Emergency and Critical Care, 3rd ed. Mathews, 2017, Lifelearn, Guelph, Ontario, Canada; with permission”.
Figure 1b: Classic long axis view of the liver at the subxiphoid site. Liver, diaphragm, stomach, hepatic vessel, portal vessel. Image courtesy Dr. Serge Chalhoub (with permission).
Figure 1c: Liver and gallbladder identified at the subxiphoid site
Figure 1d Left paralumbar or spleno-renal view: showing kidney (long axis) and spleen, two target organs identified at this site.
Figure 1e Midline bladder or cystocolic view: Sonographic image of the bladder on longitudinal view. Note that the bladder fills most of the image. Adjust the depth setting on the machine so that the bladder fills most of the screen. Make sure you can identify the apex and trigone regions of the bladder and are able to differentiate the colon from the dorsal bladder wall. Image courtesy Serge Chalhoub, with permission.
Figure 1f Right paralumbar or hepatorenal view: showing the liver and right kidney (longitudinal) with surrounding areas.
Interpretation of AFAST examination
Free fluid appears hypoechoic (black and tends to form triangles or sharp angles between organs and is not contained within structures/walls,
Figure 1g Positive free fluid at the left paralumbar site: Free fluid is seen surrounding the spleen and dorsal to the left kidney. Note how uncontained free fluid forms sharp angles as it abuts against abdominal structures.
- Centesis to obtain a sample of fluid is recommended as it is not possible to determine the type of fluid by sonography alone; it may be comprised of transudates, modified transudates or exudates.
- Ultrasound guided centesis helps prevent accidental aspiration of abdominal organs and/or hollow organ aspiration (e.g. intestines/bladder).
- Early identification of free abdominal fluid and sonographic guided abdominocentesis allows for expedited therapy based upon cytology results and/ or fluid biochemical analysis.
- An Abdominal Fluid Score (AFS) has been used in dogs presenting in lateral recumbency following trauma. The AFS is calculated by recording the number of AFAST sites that are positive for the presence of free fluid.
a. AFS 1:is positive for free fluid at one site;
b. AFS 2: positive at any 2 sites;
c. AFS 3: positive at any 3 sites;
d. AFS 4: positive in all 4 sites.
- An increase in the AFS suggests ongoing intra-abdominal fluid accumulation (most often hemorrhage in the case of trauma) and further patient evaluation/serial AFAST and AFS monitoring is warranted (be ready for a blood transfusion)
- A decrease in the AFS suggests resorption of fluid (and arrested hemorrhage in the case of hemoabdomen).
Thoracic FAST scan (TFAST)
The TFAST scan is performed with the patient positioned in sternal or standing (preferred positioning for patients presenting with respiratory distress) or can be performed with the patient in right or left lateral recumbency (common when combined with the AFAST exam). The TFAST consists of a five-point evaluation of the thorax with the objective of identifying and assessing specific structures (Figure 2a). The five views consist of 1) the subxiphoid or diaphragmatic hepatic (DH) view, shared view used in both AFAST and TFAST techniques, 2) the chest-tube sites (CTS) located bilaterally on either side of the chest wall between the 7-9th intercostal spaces, and 3) the pericardial sites (PCS) located bilaterally on either side of the thorax over the heart between the 5-6th intercostal spaces (Lisciandro et al, 2008). If the patient is positioned in lateral recumbency it will need to be rolled into a sternal position in order to access the gravity dependent CTS site (both PCS sites can usually be obtained with the patient in lateral recumbency, although it may be easier to obtain the gravity depend views after the patient has been rolled into sternal). The probe is held perpendicular to the ribs (or can be placed within the intercostal space with the probe orientated parallel to the ribs) and held motionless at the CTS sites to assess the pleural-pulmonary interface (PPline) for the presence of a glide sign. At the PCS it is important to obtain both transverse and longitudinal views and to fan the probe through 45 degree.
Figure 2a: Place the probe perpendicular to the ribs at the left and right chest tube site (CTS) (1), the left and right pericardial sites with the probe in both longitudinal and transverse orientation to the heart (PCS) (2), and the subxiphoid site with the probe initially placed in a longitudinal orientation (3). In this figure the dog is in sternal recumbency with the hind end shifted into a more right lateral position. This allows both side of the chest to be evaluated while still leaving access to the subxiphoid site. From Veterinary Emergency and Critical Care, 3rd ed. Mathews, 2017, Lifelearn, Guelph, Ontario, Canada, with permission.
Interpretation of TFAST findings: At the CTS sites the adjacent ribs create hypoechoic shadows (rib shadows) which are used as landmarks (‘bat’ sign or ‘gator’ sign). In most patients, A-lines (artifacts appearing as horizontal hyperechoic lines) are the result of air reverberation artifact and can be identified in cases with or without pneumothorax (Figure 3). They are therefore not helpful in detecting or ruling out pneumothorax (Boysen and Lisciandro, 2013). Detection of pneumothorax requires dynamic assessment of the pleuralpulmonary (PP) line and the search for a glide sign. A glide sign is created by the normal dynamic motion of the PP interface sliding back and forth along the thoracic wall during respiration. The presence of a glide sign rules out pneumothorax, while an absent glide sign suggests pneumothorax. Although the presence of a glide sign rules out pneumothorax it lacks specifi city (it is not always easy to detect). To increase the specificity and confirm the presence of pneumothorax it is helpful to seek out the ‘lung point’. The ‘lung point’ is identified by sliding the ultrasound probe ventrally along the chest wall (between ribs), searching for the recurrence of the glide sign. The position where the lung resumes contact with the chest wall and glide sign is restored is referred to as the ‘lung point’. On the sonographic image this will appear as a glide sign only being present in a portion of the image, while the remaining portion has no visible glide sign.
Figure 3: Sonographic image obtained when the ultrasound probe is placed perpendicular to the ribs at the chest tube site (CTS) The ribs appear as the curvilinear white lines to either side of the image with rib shadowing (RS). The first white line that appears distal to the rib, connecting the two ribs, is the pleural line (identified by the long white arrow). This is the area that is assessed for the back and forth shimmering or glide sign. The reverberation artifact that causes the pleural line to be repeated in the far field of the image are known as A-lines (short arrows). Note that the pleural line and A -lines are both present in patients with normal peripheral lung and patients with pneumothorax. It is the back and forth motion, the glide sign, along the pleural line that differentiates normal peripheral lung (glide sign present) from patients with a pneumothorax (glide sign absent). From Veterinary Emergency and Critical Care, 3rd ed. Mathews, 2016, Lifelearn, Guelph, Ontario, Canada, with permission.
It is also possible to identify B-lines (also called ultrasound lung rockets) during the TFAST exam (Figure 4). These are hyperechoic lines that extend from the PP interface into the far field and oscillate back and forth with respirations and pass though/ obliterate A-lines (Lisciandro et al, 2014). B-lines are believed to result from a small volume of fluid being located adjacent to air in the outer 1-3 mm of the lung. The presence of an increased number of B-lines indicate interstial-alveolar pulmonary pathology at the probe location. They are also very helpful at ruling out pneumothorax at that location of the chest (their presence rules out pneumothorax). It should be noted that occasional B-lines exist in dogs and cats with radiographically normal lungs and no clinical evidence of respiratory disease. Therefore, B lines are not considered pathologic unless ≥ 3 lines are present in any single sonographic view of the lungs. Frequent and coalescing B-lines indicate pulmonary pathology that has reached the lung surface, with the severity correlating with the number of B lines. The differential diagnosis for B lines is similar to the differential diagnosis for interstitial alveolar patterns noted on thoracic radiographs (Boysen and Lisciandro, 2013).
Figure 4: Sonographic image obtained when the ultrasound probe is placed perpendicular to the ribs at the chest tube site (CTS) in a patient in respiratory distress with crackles noted on auscultation. The ribs appear as the curvilinear white lines to either side of the image with rib shadowing (RS). The first white line that appears distal to the rib, connecting the two ribs, is the pulmonarypleural line (long white arrow). In patients with interstitial/alveolar disease (e.g. pulmonary edema, contusions, etc.) vertical white lines known as B-lines (B) may be noted. These originate at the pulmonarypleural line, extend to the far field of the image, obliterating A-lines, and will move back and forth with respirations similarly to the glide sign. From Veterinary Emergency and Critical Care, 3rd ed. Mathews, 2016, Lifelearn, Guelph, Ontario, Canada, with permission.
Figure 5: Pleural effusion noted at the pericardial site of the TFAST exam.
The PCS sites are evaluated for pleural (Figure 5) and pericardial effusions by placing the probe over the heart between the 5th – 6th intercostal spaces. The heart, pericardial sac and pleural space may be visualized in these locations bilaterally. Free pleural or pericardial fluid appears hypoechoic and may consist of transudates, modified transudates or exudates. To further assess the type of fluid present and allow early goal direct therapy, ultrasound-guided collection through thoracentesis or pericardiocentesis is recommended. There are several point of care applications that can be used to assess the heart, although these techniques require more practice and will not be covered in detail at this time. Similar to repeated AFAST scans, serial TFAST examinations permit subjective trending of effusion volume particularly after resuscitation efforts and fluid therapy.
Several lung ultrasound protocols have been validated as a point of care test in small animal patients. The two that are used most often include the VetBLUE ultrasound protocol that assesses 4 focal sites on either side of the chest and a more comprehensive but equally rapid intercostal sliding lung ultrasound protocol.
Figure 6 VetBLUE lung ultrasound: Four sites are evaluated on each hemi-thorax: upper third of the thorax at the 9th intercostal space, or dorsal caudal lung region (1), 6th intercostal space in the middle third of the thorax or peri-hilar region (2), lower third of the thorax near the costochondral junction at the 6th to 8th intercostal space or middle lung region (3), and the lower third of the thorax near the costochondral junction at the 3rd to 5th intercostal space or cranial lung region (4). The probe is placed at each site and initially moved 1-2 rib spaces cranially and caudally to rapidly look for B lines. In larger dogs, the probe can also be moved 1-2 cm dorsally and ventrally. If B lines are visualized, the probe is held stationary and the number of B-lines is recorded. If no B lines are found, the probe is held stationary and the presence/absence of a glide sign is evaluated. At the middle lung site (3), the probe is initially placed over the 4th-6th intercostal space just above the level of the costochondral junction; if the heart obscures the field of view and prevents visualization of the lung field, the probe is moved caudally 1-2 rib spaces until the heart is no longer visible and the lung can be evaluated. At the cranial lung site (4), the probe is initially placed over the 4th-6th intercostal space 1-3 cm (depending on the size of the patient) above the costochondral junction so that the heart is visible; the probe is then moved cranially one rib space at a time until the heart is no longer visible and the lung can be evaluated. The patient’s forelimb may need to be pulled cranially to facilitate probe positioning at this site. Record the presence of pleural effusion, presence or absence and number of B lines as well as the presence or absence of a glide sign at each site. From Veterinary Emergency and Critical Care, 3rd ed. Mathews, 2017, Lifelearn, Guelph, Ontario, Canada; with permission”.
Figure 7: Lung sliding ultrasound protocol. The ultrasound probe is placed within the intercostal space (either perpendicular or parallel to the ribs) starting at the caudal dorsal lung/diaphragm interface. The probe is then slowly moved ventrally between the ribs to detect the presence of B lines until the diaphragm is encountered ventrally. If B lines are found, the probe is held stationary and the number of B lines are counted at that site before the scan is continued. The regions of lung with increased B lines (and specific numbers of B lines noted) are recorded. If B lines are coalescing throughout the lung field the regions effected should be recorded. The probe is advanced a rib space and then moved dorsally until the lumbar muscles are encountered. The process is repeated until the entire hemi-thorax is scanned. The procedure is then repeated on the opposite side of the chest in a similar manner. Note: If pneumothorax is a concern, the probe should be held stationary at the caudal dorsal sites prior to beginning the sliding technique.
Interpretation: Lung Ultrasound findings
- Lung ultrasound scans are designed to rapidly detect pulmonary and pleural pathology in cats and dogs, through the detection of several key findings which include; the presence or absence of pleural effusion (see TFAST), the presence or absence of a glide sign (see TFAST), presence or absence of B lines, and the presence or absence of additional pathology including lung consolidation (not covered in this article).
- Lung ultrasound is very sensitive at detecting interstitial/alveolar pathology via the identification of B lines; reverberation artifacts originating from the pleural line extending to the edge of the far field image that move to-andfro with inspiration and expiration (See TFAST B line description).
- The presence of 1-3 B lines at a single probe site can be normal in healthy dogs and cats.
- Multiple B lines (> 3 per ultrasound field), and/or coalescing B lines may indicate different underlying pathology depending on their number and distribution.
- When B-lines are numerous they create a B-pattern (coalescing B lines), which is indicative of advanced interstitial-alveolar disease.
- B lines also definitively rule out the possibility of pneumothorax.
- Patients that have a negative VPOCUS scans that fail to stabilize or have persistent clinical signs often benefit from serial exams.
- A negative VPOCUS scan does not exclude internal injury or pathology. Pathology located more than a few mm within the lung that does not extend to the periphery is unlikely to be seen with sonography. VPOCUS scans are also focussed exams targeting specific structures and may miss pathology in areas not assessed within the regions scanned.
- Patients that are panting or have rapid shallow respirations can be difficult to assess for a glide sign if B lines are not present.
- The glide sign is only visible during the dynamic phases of inspiration and expiration and disappears between breaths (static phases of respiration and during periods of apnea). Single lung intubation (i.e. left or right main stem bronchi intubation) will result in an absent glide sign in the non-intubated lung.
- Movement of your hand, the probe, or the patient may cause a false positive glide sign; keep your hand, the patient, and the ultrasound probe still when looking for the glide sign. False positives on AFAST are possible when normal hypoechoic abdominal structures are interpreted as free fluid. Structures commonly mistaken for free fluid include the gall bladder, common bile duct, hepatic veins, caudal vena cava and occasionally the GI wall and/or GI contents. Using both transverse and longitudinal views helps avoid the misinterpretation of normal abdominal structures.
- Altering the sonographic window depth and focus may aid in the sensitivity of identifying small volume effusions during the AFAST examination.
Point of Care Ultrasound in humans has been found to be more sensitive and specific for the detection of effusions when compared to radiography, and is comparable to that of computed tomography (CT scan). Point of Care Ultrasound may be performed by general practitioners with minimal prior ultrasound experience, is rapid, can be performed during initial stabilization of patients, is non-invasive, and has been proven to improve patient outcome. Early detection of free fluid allows for cytological evaluation following abdominocentesis, thoracentesis or pericardiocentesis, and may direct additional diagnostics and therapeutics. Minimal patient preparation is required and can be performed in animals not stable for radiographic positioning or general anesthesia required for CT scan. With minimal training, VPOCUS techniques can be incorporated into general and emergency practice.
About the author
Dr. Søren Boysen obtained his veterinary degree in 1996 (WCVM), completed a small animal internship at the 1998 (UPEI), and a residency in 2003 (Tufts University, Massachusetts). He is a diplomate of the American College of Veterinary Emergency and Critical Care (ACVECC) and has taught at several Canadian Veterinary Colleges. He is currently a professor at the University of Calgary. Extensively published, and a recipient of numerous teaching excellence awards, he has become an internationally recognized speaker. He is perhaps most recognized for his contribution to the advancement of small animal point of care ultrasound having developed the original Focussed Assessment of Sonography for Triage (FAST) exam in veterinary patients, adapted point of care ultrasound protocols for use in non-trauma patients, and is actively involved in point of care ultrasound research assessing intravascular volume status and response to fluid therapy. He continues to work on improving hands on training workshops targeted at improving point of care ultrasound use by nonspecialist practitioners that can be used on a daily basis.
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