Ultrasound in critical care

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1 Stephen Wilson Bsc MBChB MRCP FRCA Andrew Mackay MBChB, FRCA, EDIC, FFICM Matrix Reference 2C01 Key points Focused ultrasound (US) studies can supplement physical examination of critically ill patients. National guidelines now recommend the use of US for a number of practical procedures commonly performed in critical care. Full training and competence are essential before significant therapeutic decisions are made based on the US assessment. A record of focused US studies performed by an individual should be maintained in the form of a logbook. Stephen Wilson Bsc MBChB MRCP FRCA ST6 Anaesthesia Glasgow Royal Infirmary Glasgow UK Andrew Mackay MBChB, FRCA, EDIC, FFICM Consultant in Anaesthesia and Intensive Care Anaesthetic Department Victoria Infirmary Glasgow G42 9TY UK Tel: þ Fax: þ ntlandy@yahoo.com (for correspondence) 190 Although medical diagnostic ultrasonography has been used since the 1950s, advances in ultrasound (US) technology over the last decade have led to: (i) smaller and portable US machines, which means that US can come to the patient, providing an immediate advantage over other imaging modalities, such as computed tomography (CT); (ii) decreased cost of US technology, resulting in increased availability of US machines to areas outside the radiology department; (iii) improved probes achieving higher resolution images which are easier to interpret. Interest in US use in critical care continues to increase as more applications of this technology are appreciated. This growing interest is mirrored in other specialities, and in recognition of this, the Royal College of Radiologists published Ultrasound training recommendations for medical and surgical specialities. 1 This publication gives guidelines on how US training should be developed when being performed by the non-radiologist. The College of Emergency Medicine used these guidelines to develop its own curriculum and now US training is part of the core curriculum for all Emergency Medicine Trainees. With the publication of guidelines drawn up by a joint working party of the Association of Anaesthetists of Great Britain & Ireland, the Royal College of Anaesthetists, and the Intensive Care Society in November 2010, 2 it is inevitable that formal US training will become part of the anaesthesia curriculum. This article looks at the uses of US in the critically ill patient and training in US use for Anaesthesia and Intensive Care. US for assessment and diagnosis In the 1970s, radiologists and surgeons started describing the role of US examination for haemoperitoneum in patients who had sustained blunt abdominal trauma. This led to evaluation of US in trauma patients, specifically for the detection of haemoperitoneum and haemopericardium. This research culminated in the description of the Focused Assessment with Sonography for Trauma (FAST) exam, which has been included as part of the advanced trauma life support course since FAST examination is an example of a focused ultrasound assessment, a type of US examination performed in order to answer a specific clinical question, such as is there free fluid in the abdomen? or is there a pericardial effusion? This differs from traditional US examination where an often time-consuming scan is performed in order to generate a detailed report of the area being examined. A list of possible focused assessments relevant to intensive care can be seen in Table 1. Focused echocardiography Focused echocardiographic examination is significantly shorter in duration than traditional echocardiography. The goal of such an exam is to supplement the physical examination, and in the care of acutely ill patients, an approach that combines both physical examination and bedside echocardiography has proven to improve clinical diagnosis and management. 3 Transthoracic echocardiography (TTE) is noninvasive and more readily available than transoesophageal echocardiography and should thus be the initial modality of choice. Objectives of the examination should include: (i) assessment of left ventricular (LV) and right ventricular (RV) function, (ii) assessment of the pericardial space for effusion and tamponade, and (iii) assessment of the volume status. A detailed assessment of valvular function and of the great vessels (aorta and pulmonary artery) should not be part of basic echocardiography training; however, recognition of an abnormal scan should prompt a full echographic assessment by a trained practitioner. doi: /bjaceaccp/mks019 Advance Access publication 2 May, 2012 Continuing Education in Anaesthesia, Critical Care & Pain Volume 12 Number & The Author [2012]. Published by Oxford University Press on behalf of the British Journal of Anaesthesia. All rights reserved. For Permissions, please journals.permissions@oup.com

2 Table 1 The different anatomical areas where a focused US assessment can be performed. When performing the scan, the examiner should try to answer a specific question in order to arrive at a diagnosis Scan Questions being asked Diagnosis Echocardiography Is there a pericardial effusion? Pericardial effusion Are there signs of tamponade? Are the ventricles grossly Myocardial insufficiency dilated? Is there adequate ventricular function? Is the right ventricle dilated? Pulmonary embolism Is the IVC fixed and dilated? Is the left ventricle underfilled? Hypovolaemia Is the IVC collapsing on inspiration? Is there ventricular activity? Confirmation of cardiac arrest Thoracic ultrasound Is lung sliding present? Pneumothorax Are comet tail artifacts present? Is there fluid in the thorax? Pleural effusion Are A lines present? Pulmonary oedema Are B lines present? Abdominal Is there free fluid/blood in the Intraperitoneal fluid ultrasound abdomen? Is the abdominal aorta.3 cm Abdominal aortic aneurysm in diameter? Is there hydronephrosis? Post-renal obstruction Is the bladder distended? Is the anterior gallbladder wall Cholecystitis thickened? Is the common bile duct dilated? Are there gallstones? Does the patient have a sonographic Murphy sign? Lower extremity Does the common femoral vein Deep venous thrombosis fully compress? Does the popliteal vein fully compress? Ocular ultrasound Is the optic sheath diameter Raised intracranial pressure.5 mm? Transcranial What is the flow velocity? Cerebral vasospasm or Doppler What is the pulsatility index? cerebral artery obstruction A common use of echo in critical care is to help diagnose the cause of hypotension. Hypovolaemia can often be difficult to diagnose clinically. TTE may demonstrate a hyperdynamic LV, but visualization of the inferior vena cava (IVC) may be more helpful. The IVC can be visualized in the abdomen as it travels behind the liver. In a healthy subject breathing spontaneously, an inspiratory decrease in an IVC diameter of 50% can be observed as a result of cyclic changes in pleural pressure. This cyclic change in the vena cava diameter is abolished, when the vessel is dilated, for example, in cardiac tamponade, pulmonary embolism, and severe RV failure. In a mechanically ventilated patient, the respiratory changes in the IVC diameter are reversed, with an inspiration causing an increase in the diameter. Cyclic respiratory changes in the IVC diameter during mechanical ventilation can be observed only with a normal or low volume status. Recent studies have shown in mechanically ventilated patients exhibiting circulatory failure, with an absence of IVC diameter variation, further fluid boluses would not be of benefit in more than 90% of cases. An increase greater than 12% in the IVC diameter during lung inflation allows discrimination between responders and non-responders to volume loading, with a positive predictive value of 93% and a negative predictive value of 92%. 4 US of the IVC therefore can give a non-invasive parameter to evaluate volume loading. It remains to be seen whether this index is still reliable in patients with a significant increase in intra-abdominal pressure, which could limit IVC diameter variations. Although echocardiography is not recommended as a routine imaging test to diagnose suspected pulmonary embolism, echocardiographic findings in a massive pulmonary embolism can be diagnostic. The echographic findings of RV dilation, impairment of the RV free wall contraction, paradoxical septal wall motion, or dilation of the right pulmonary artery, in a patient with circulatory collapse, would be suggestive of massive pulmonary embolism. Prompt treatment with thrombolysis should then be considered. The 2010 ALS resuscitation guidelines recognize the possible advantage of echocardiography during cardiac arrest to detect potentially reversible causes, although no studies have shown that the use of this imaging modality improves outcome. The integration of echocardiography into advanced life support requires training in order to obtain a subcostal view within the 10 s pause for rhythm assessment. This ensures that interruption to chest compressions is minimized. Thoracic US Thoracic US was traditionally thought to be limited because air is not a good transducer of sound waves, and US examination of the normal aerated lung results in an image distorted by artifacts. However, recent work has shown that the presence or absence of these artifacts can correlate with pathology of the lung. Lung US can differentiate a variety of lung pathologies including pleural effusion, pneumothorax, atelectasis, pneumonia, and alveolar-interstitial syndrome (as seen in pulmonary oedema). Pleural effusions on chest X-ray are often missed or diagnosed as consolidation, or more worryingly, a white-out may be diagnosed as an effusion when the appearance is actually due to collapse/consolidation. US assessment of a pleural effusion can easily reveal a characteristic echo-free space between visceral and parietal layers (Fig. 1). Pneumothorax can also be diagnosed using US and has greater diagnostic utility than plain chest radiography in diagnosing a pneumothorax in the supine patient, where air lies anteriorly and can be difficult to see. 5 Diagnosis of pneumothorax on US is made when normal lung artifacts are absent and the normal movement of the visceral pleura with respiration is lost. Continuing Education in Anaesthesia, Critical Care & Pain j Volume 12 Number

3 example, whether a diagnosis can be made or excluded, or when further imaging modalities are required. Abdominal aorta Fig 1 Thoracic US revealing a large pleural effusion (E) that has displaced the lung. The diaphragm (D) and liver (L) are visualized. The depth from the skin to the fluid has been measured. Insertion of the needle at this site is not advised; given the proximity to the liver, a more superior approach should be marked. Abdominal US A number of focused US assessments can be performed on the abdomen. FAST scan As described above, the FAST scan was developed to aid in the diagnosis of blood/free fluid in the abdomen in patients who had suffered blunt trauma with the primary aim to rapidly direct appropriate operative interventions in unstable patients. The use of a FAST scan has replaced diagnostic peritoneal lavage (DPL), despite DPL being more sensitive in detecting small volumes of intraperitoneal blood. This is because a FAST scan is more specific, with a therapeutic laparotomy being performed in % of the patients with a positive FAST scan, compared with 74 94% of the patients with a positive DPL. 6 A therapeutic laparotomy is defined as a laparotomy that reveals pathology requiring surgical intervention. With an increasingly conservative approach to the treatment of splenic and liver injuries, the high-sensitivity and invasive nature of the DPL has become less useful. A negative FAST scan does not exclude injury as in both blunt and penetrating trauma, significant injury may not have associated free fluid, or free fluid may take time to develop. One study found that 22% of abdominal injuries in adults were not associated with free fluid. This increases to 37% in paediatric abdominal trauma. 6 Injuries less frequently associated with significant free fluid would include injuries to bowel and mesentery; the liver and spleen where bleeding may be contained by their capsules; and retroperitoneal injuries including renal and pancreatic injuries. Given the moderate sensitivity of the FAST scan, its use should be part of an algorithm when assessing a patient with abdominal trauma. The FAST scan gives a useful guide as to how other focused US assessments in critical care should be carried out. Information on sensitivity and specificity is required for each US assessment performed as this will guide the decision-making process, for Abdominal aortic aneurysms (AAAs) can present with varied symptoms and therefore US should be considered on patients presenting with back pain, flank pain, ureteric colic, syncope, abdominal pain, and gastrointestinal bleeding to rule out an AAA. The entire length of the abdominal aorta must be evaluated and shown to be less than 3 cm in diameter before the diagnosis can be excluded. If the US is positive for AAA, the decision for further imaging or immediate operative intervention will be governed by the clinical picture. The most common impediments to visualization of the aorta are bowel gas, obese habitus, and an uncooperative patient in pain. Measurement errors are also common. Renal and bladder The kidneys and the bladder are the most sonographically available organs. Post-renal (or obstructive) acute renal failure (ARF) can complicate the clinical situation of the patient in the intensive care unit (ICU) and a focused assessment of the kidneys and bladder can detect mild or moderate hydronephrosis or a distended bladder. Early detection is important as post-renal ARF can usually be corrected without the need for dialysis. The examination may sometimes reveal the pathological lesion responsible for the obstructive, although US has a very low specificity for identifying ureteric stones. Research of the use of US to monitor renal function in the critically ill is ongoing. Kidney blood flow can be evaluated with colour Doppler and it has been shown in the critically ill that renal vasoconstriction can reduce renal blood flow to 50% of normal. 7 In the future, it may be possible to document these changes in regional renal blood flow and provide prognostic information. Gallbladder US is considered as the first-line tool to diagnose disorders of the gallbladder and biliary tree. Cholelithiasis and any associated inflammation of the gallbladder (cholecystitis) may be confirmed with US. A gallbladder wall thickness of.3 mm is considered abnormal, although a number of disease processes can result in a diffuse gallbladder wall thickening. The normal common bile duct is usually,6 mm in diameter, although it can vary with age. If the diameter is.8 mm, there is evidence of obstruction. Acute acalculous cholecystitis (AAC) has been diagnosed with increasing frequency in ICU patients, probably as a result of treating more seriously ill patients for longer. The aetiology is believed to have an ischaemic basis and a gangrenous gallbladder may result. The US findings of AAC are the same as cholecystitis except for the absence of gallstone shadowing. 192 Continuing Education in Anaesthesia, Critical Care & Pain j Volume 12 Number

4 Other focused assessments Deep venous thrombosis The occurrence of deep venous thrombosis (DVT) is common among patients requiring prolonged mechanical ventilation in the ICU setting, despite the routine use of thromboprophylaxis, up to 20% in some studies. Bedside US of the lower extremity venous system using a simple compression technique with the probe can detect a DVT. Formal anticoagulation may then be appropriate to reduce the risk of pulmonary embolism. Leg US is also useful to help in the diagnosis of pulmonary embolism. British Thoracic Society guidelines state that the finding of a DVT by leg US in patients with a suspected pulmonary embolism is often sufficient to confirm the diagnosis; however, a normal leg US cannot exclude a pulmonary embolism. Ocular A focused assessment using ocular US to measure the optic nerve sheath diameter can be performed to look for raised intracranial pressure (ICP) which may be reflected through the nerve sheath as in papilloedema (Fig. 2). Research has shown that an optic nerve sheath diameter of.5 mm correlates with evidence of increased ICP shown on CT. 8 Transcranial Doppler Transcranial Doppler (TCD) is a non-invasive assessment of cerebral arterial supply. The flow velocity (FV) of cerebral arteries can be calculated using the Doppler probe at various acoustic windows of the skull. The pulsatility index (PI) can then be derived. The FV and PI values of the vasculature alter with different pathologies, such as vasospasm or ischaemic stroke. TCD can not only help in the diagnosis of disorders of the vasculature but also monitor the effects of therapeutic intervention. Neonatal fontanelle scanning Focused US is also an important tool in the paediatric intensive care, and many of the uses described in this article are appropriate for both adult and paediatric intensive care patients. In addition, the anterior fontanelle of the infant provides an additional acoustic window for the assessment of suspected intraventricular haemorrhage or hydrocephalus. Use of US for therapy and treatment US is also used before and during practical procedures in the critical care environment. Vascular access The National Institute of Clinical Excellence (NICE) recommends that central venous catheters (CVCs) are inserted using US guidance by practitioners who are appropriately trained. CVC insertion using US has already been discussed in this journal. Its use in long-stay patients is particularly relevant, given the increased risk of venous thrombosis in previously used sites. US can also be used to guide peripheral venous and arterial access when tissue oedema can make palpation of peripheral vessels impossible. Thoracentesis Studies have demonstrated that using US to guide thoracentesis results in a higher success rate than a landmark-based method, and also a lower incidence of pneumothorax. The British Thoracic Society has issued recent guidelines stating that thoracic US is now strongly recommended for all pleural procedures. A suitable site of needle insertion is identified where fluid is seen to persist throughout the respiratory cycle and where there is no incursion of the lung or diaphragm into the sonographic window. The depth from the skin to the fluid should be noted and the skin marked for the site of entry. Fig 2 Optic nerve sheath diameter (ONSD) in a healthy subject. The measurement is taken 3 mm posterior to the papilla. A direct communication has been documented between the subarachnoid space of the optic nerve and the chiasmal cistern of the brain. When ICP increases, cerebrospinal fluid flows towards the perineural subarachnoid space and results in expansion of the ONSD. Percutaneous tracheostomy US of the neck may identify structures at risk for haemorrhage, such as aberrant blood vessels. One study showed that based on prior US findings, the site of tracheal puncture had to be changed in 24% of the patients undergoing percutaneous tracheostomy. 9 Continuing Education in Anaesthesia, Critical Care & Pain j Volume 12 Number

5 Regional anaesthesia US-guided regional anaesthesia in intensive care can be an important tool to aid analgesia, especially when pain is contributing to respiratory failure. Training Full training and competence are essential before significant therapeutic decisions are made based on US assessment. As mentioned, guidelines for training in the use of US in anaesthesia and intensive care have been produced by a working party of the Association of Anaesthetists of Great Britain & Ireland, the Royal College of Anaesthetists, and the Intensive Care Society. The guidelines describe three levels of training with Level 1 training (core training) being the expected standard of knowledge and practice that anaesthetic and intensive care consultants should have in the future. It is proposed that for the completion of specialist training in anaesthesia and intensive care, Level 1 accreditation will be mandatory in the following: US for regional anaesthesia and US for vascular procedures and echocardiography, and optional in thoracic US and abdominal US. The training will be split into theoretical and practical domains. Theoretical training Preliminary theoretical training should be undertaken before using US in clinical situations and cover relevant anatomy, the physics of US, machine familiarization, image recording and reporting, artifacts, and the relevance of other imaging modalities to US. This training may be delivered locally, using either Level 1 practitioners with 12 months experience or higher level practitioners, or by distance learning using web-based packages. There are already national courses available that include theoretical training within their programme. Assessment after theoretical training will ensure that the trainee has understood and can apply this knowledge to clinical practice. Practical training Practical training should include both training with phantoms and models before using US in a clinical situation. Practical experience should be gained under the guidance of a named supervisor trained in US (who should at least be a Level 1 practitioner with at least 12 months experience). Practical training will be competencybased, rather than performance of a set number of procedures undertaken. However, to ensure optimal training and timely acquisition of the required competencies, practical training should involve frequent supervised examinations. A logbook of all training cases should be kept and ideally should include a number of more detailed case studies. Most US machines have the ability to store US images (still and video) and these images should form part of the logbook. Although these guidelines may not be implemented into the curriculum for anaesthesia for some time, they provide the trainee with a guide on how they should focus their current training in US. Maintaining a logbook of US experience will allow equivalence of training when this structured training programme is in place. Declaration of interest None declared. References 1. Board of the Faculty of Clinical Radiology. Ultrasound Training Recommendations for Medical and Surgical Specialities. London: Royal College of Radiologists, Joint working party of the Association of Anaesthetists of Great Britain & Ireland, the Royal College of Anaesthetists and the Intensive Care Society. Ultrasound in Anaesthesia and Intensive Care: A Guide to Training. London: The Association of Anaesthetists of Great Britain & Ireland, The Royal College of Anaesthetists, and The Intensive Care Society, Beaulieu Y. Specific skill set and goals of focused echocardiography for critical care clinicians. Crit Care Med 2007; 35: S Barbier C, Loubières Y, Schmit C et al. Respiratory changes in inferior vena cava diameter are helpful in predicting fluid responsiveness in ventilated septic patients. Intensive Care Med 2004; 30: Noble V, Nelson B, Sutingco AN. Manual of Emergency and Critical Care Ultrasound. New York, NY: Cambridge University Press, Rippey JCR, Royse AG. Ultrasound in trauma. Best Pract Res Clin Anaesthesiol 2009; 23: Barozzi L, Valentino M, Santoro A, Mancini E, Pavlica P. Renal ultrasonography in critically ill patients. Crit Care Med 2007; 35: S Blaivas M, Theodoro D, Sierzenski PR. Elevated intracranial pressure detected by bedside emergency ultrasonography of the optic nerve sheath. Acad Emerg Med 2003; 10: Kollig E, Heydenreich U, Roetman B, Hopf F, Muhr G. Ultrasound and bronchoscopic controlled percutaneous tracheostomy on trauma ICU. Injury 2000; 30: Please see multiple choice questions Continuing Education in Anaesthesia, Critical Care & Pain j Volume 12 Number