The first published thoracic point-of-care ultrasound (POCUS) exam in veterinary patients was a technique developed for assessment of trauma patients; thoracic focused assessment with sonography for trauma (TFAST). Since then, several refinements and numerous techniques have been published to improve the accuracy of TFAST.
The original TFAST technique, was adapted for use in triage and tracking of critical patients but the technique remained the same; placement of the transducer bilaterally at two sites on the thorax; the 7–9th intercostal space on the dorsolateral thoracic wall (chest drain site) and the 5-6th intercostal space on the ventrolateral thoracic wall (pericardial site). A 5th site, the diaphragmatic-hepatic (subxiphoid) view, was later added, which is the same site used for abdominal POCUS, with the depth increased to assess the thoracic region beyond the diaphragm. Because TFAST was developed to assess only the pleural and pericardial spaces, and is very limited at detecting lung pathology, subsequent POCUS techniques were developed to assess greater lung surface area by scanning multiple sites on each hemithorax. These techniques include a four quadrant technique; the elbow and sixth intercostal space act as limits to the quadrants and four sites are scanned on each side of the hemithorax, Vet BLUE; four sites are scanned on each side of the hemithorax corresponding to the caudodorsal lung region, perihilar lung region, middle lung region, and the cranial lung region, vertical sliding ultrasound technique; the sonographically scannable lung surface of each hemithorax is examined by sliding the probe from dorsal to ventral between each intercostal space, and a horizontal sliding technique; each hemithorax is scanned in the dorsal, middle, and ventral thirds by sliding the probe between the cranial (forelimb/thoracic inlet) and caudal (abdominal curtain sign) sonographic lung margins. Other techniques have been developed to answer clinically driven questions; the ABCDE protocol is a trauma-specific protocol to determine the full extent of injuries secondary to trauma. This technique includes an extended thoracic and abdominal ultrasound including assessment of the circulation as well as scanning of the airway and measurement of the optic nerve sheath diameter. The thoracic scanning technique in this protocol is the horizontal sliding technique described above. Another iteration of this technique can be seen in the PLUS protocol, which was developed to better assess the caudal border of the lung where air is most likely to accumulate and the ventral most regions of the thorax where fluid is most likely to accumulate. In this protocol the probe is turned parallel to the ribs at the pericardio-diaphragmatic window in an attempt to further improve the detection of small volume pleural effusions.
The original TFAST protocol was found to have moderate agreement with CT for the detection of the pleural effusion and poor agreement for detection of pneumothorax. Similar findings were found when the Vet BLUE protocol was compared to CT; pneumothorax was not detected in 2/3 animals confirmed to have pneumothorax using CT. Pleural effusion was identified in most animals it was detected in using CT, but it was less accurate for detection of site-specific pleural effusion. The PLUS protocol has the potential to improve accuracy of diagnosing a pneumothorax as the scanning protocol involves evaluating a novel ultrasound finding; the abdominal curtain sign present at the caudal border of the lungs. PLUS also uses sonographically identifiable borders to ensure the most caudo-dorsal sites of the thorax are assessed, where free gas is likely to accumulate in the sternal/standing patient. Although there are no studies comparing the PLUS protocol to CT, a small study comparing PLUS protocol to Vet BLUE in detection of pleural effusion found these protocols to be comparable in the detection of moderate-large volume pleural effusions, but the PLUS protocol was superior in the detection of small volume pleural effusion.
Most veterinary studies exploring the utility of thoracic POCUS for the detection of lung pathology follow the Vet BLUE technique. This protocol has been found to be superior to radiographs for the detection of pulmonary contusions in small canine trauma population when using CT as the reference standard and has high specificity but low sensitivity for detection of alveolar-interstitial syndrome (AIS, defined as presence of ≥3 B lines). Thus, detection of lung pathology using Vet BLUE is likely significant but relevant findings may also be missed. By altering the definition of AIS to include the presence of any B lines as abnormal the sensitivity of the protocol to detect AIS increases. Although there are no veterinary studies comparing other lung ultrasound protocols with CT, it may be assumed that those protocols with increased number of scanning sites would have a greater accuracy for detection of lung pathology. There is evidence to suggest the number of B lines detected in healthy cats and dogs does increase as the lung surface area scanned increases. VetBLUE detects B lines in roughly 10% of healthy cats and dogs, while the vertical sliding protocol is reported to detect B lines in up to 50% of healthy cats. The other protocols fall between these two values. However, one of the key advantages of emergency ultrasound compared to other imaging modalities is its time efficiency. Increasing the number of scanning sites invariably will lead to longer scan times, which leaves the question unanswered; does scanning of additional sites provide additional clinical information? Literature from the human emergency and critical care setting suggests scanning more than six sites does not provide additional clinical information, particularly when pathology is diffuse. A single abstract involving 25 dogs demonstrated an extended lung ultrasound protocol may detect more lung pathology than VetBLUE. It may prove that different protocols are required depending on what pathology needs to be ruled in or ruled out, how diffuse the pathology is likely to be, and depending on how stable the patient is. Studies exploring the relative merits of techniques with additional scanning sites, or different sites than currently used in veterinary patients are needed.
When considering what technique is “best” for use in the emergency and critical setting there are various factors to consider including the experience of the person performing the technique, the ultrasound machine, and transducers available and most importantly the clinical question. The technique used should be tailored based on the clinical question you are attempting to answer when performing point-of-care ultrasound. If your question is “does my patient have a pneumothorax?” then you may prefer to choose a protocol that assesses the caudal lung border, such as the PLUS protocol to improve the chance of detecting a pneumothorax. If your clinical question is “does my markedly dyspneic patient have diffuse pulmonary parenchymal disease, such as cardiogenic pulmonary oedema?” you may choose a protocol that involves scanning a minimum number of sites as quickly as possible. If your clinical question is “does my mildly dyspneic patient have focal parenchymal disease, such as isolated pulmonary nodules?” you may choose a protocol that involves scanning as much lung surface as possible, such as the sliding lung technique.
The scanning technique chosen considerably affects the sensitivity of point-of-care thoracic ultrasound for the detection of lung pathology and pleural space disease. Veterinary ultrasound protocols have not been validated in regard to what is considered ideal in terms of number of scanning sites, but it is likely that there is no one size fits all. Like all diagnostic tests, thoracic POCUS should be tailored to the individual patient and the technique chosen is the one, or combination of techniques that best help answer the clinical question for the individual patient.
References
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