Author: John P Sloan / Editors: Tim Harris, Eric VanDenBergh, Jonathan Leung / Reviewer: James Ward / Code: U8, RP3, SLO6 / Published: 30/12/2021
Bedside, focused or point-of-care sonography (PoCUS) using portable machines is now well established in emergency medicine. Recent improvements in ultrasound technology mean that machines are smaller and are able to deliver higher quality images than ever before. Many machines have echocardiography capability, and emergency physicians (EPs) are finding this aids information gathering in many clinical settings. Focused echocardiography can also be used during a cardiac arrest and fits into the ALS algorithm during a pulse check. A designated sonographer can perform a rapid assessment and obtain information which can point to correctable reversible pathology.
Evidence suggests that EPs are able to master the skills of basic echocardiography sufficiently to improve patient care in the resuscitation setting.
Introduction – Effect On Management
Focused echocardiography can be used to predict the likelihood of successful return of spontaneous circulation (ROSC). True pulseless electrical activity (PEA) is when co-ordinated electrical activity is seen on the cardiac monitor but there is no cardiac movement on echocardiography. Alternatively, pseudo-PEA is when there is co-ordinated electrical activity, no palpable central pulse however there is cardiac activity on echocardiography. The absence of co-ordinated cardiac activity proves to be an excellent predictor of failure to regain spontaneous circulation (negative predictive value 97%). In addition, ELS has been shown to alter management in 78% of cases3.
In view of this timely availability of critical information, and it’s influence on management, ELS is part of Level 1, or ‘core’ ultrasound training. This e-learning session provides the necessary theoretical training (alongside the other 6 created by the College) to begin hands-on practical training.
Introduction – Accuracy Of The Pulse Check
Time is of the essence when managing peri-arrest patients. The team constantly seeks information which might influence the minutes which lie ahead. ELS allows direct visualisation of cardiac motility, and this transforms the 10 second pulse check into a more valuable assessment. In fact evidence demonstrates the relative unreliability of the pulse check. This observation elevates ELS into an obligatory facility during cardiac arrest.
It is essential that the person obtaining the image limits their scan to no more than 10 seconds in order to minimise interruptions between chest compressions. If the scanner is unable to obtain an adequate image, then simultaneously feel for a central pulse.
An elementary echo view is more reliable than a manual pulse check in cardiac arrest.
For the Emergency Physician learning core ultrasound, the use of echocardiography involves handling a new transducer becoming familiar with new marker positions and recognising new patterns. It is therefore one of the more challenging aspects of core ultrasound training.
Acquiring the broader skills of echocardiography clearly requires considerable training and experience. However, the very limited application of Echo in Life Support (ELS) is much easier to learn and apply in the ED resuscitation room. This is because ELS is focused, does not include complex measurements and seeks very specific qualitative information. The application of ELS is exclusively in cardiac arrest, and peri-arrest settings.
It is important to appreciate that the skills acquired by EPs when being trained in ELS are basic echocardiography skills and practitioners should not try to use echocardiography for aspects they are not trained for e.g. diagnosis of valvular pathology, measurements of ejection fraction.
Looking at another setting with critically ill patients, intensive care units, emergency echo represents an entry level in overall echocardiographic competence. See diagram below.
US Residency Programmes
The exact period of training needed to gained ELS competency is unclear. In the US, a study to assess general EM US competency looked at 149 residency programmes. The consensus was that, in general terms, 150 scans were required.
This was not based on any evidence, however, and in fact only 65 residency programmes responded. The implication is that the majority of programmes were a little more flexible.
In fact using simple scan numbers is considered to be educational flawed. The GMC has commented on this regarding surgical skills and has taken the view that competency is the goal, not the number of practical procedures conducted.
How Long Does It Take To Train?
In fact we know that training EPs appears to be effective and relatively rapid. Evidence exists to show that short periods of training can give outcomes that are very valuable.
A recent study suggested that web based learning followed by an intensive 3 hour training period resulted in reliable ELS skills in EPs.
Emergency physicians, who had been trained to use focused echo in 50 advanced life support (ALS) scenarios, have been shown to gain adequate views in 94% of cases. Most importantly 90% gained these within the 10 second pulse check window.
The College of Emergency Medicine takes the view that theoretical training is essential (eg this e-learning session), and following this, practical workplace training should take place, followed by a workplace based assessment (“Triggered Assessment”). Absolute numbers of scans are not considered important, but rather the demonstration of appropriate skills.
The majority of EPs gain an informative cardiac view within the 10 second pulse check window.
The workplace based assessment, leading to practical sign-off, is known as the triggered assessment because the trainee triggers this when he or she is ready. The areas assessed are shown to the right.
Some errors create an automatic fail, such as incorrect transducer orientation. Nonetheless, with suitable coaching a trainee should be able to master the simple ELS views readily.
Phased Array Transducer
In order to learn basic ELS, one has to become familiar with a different transducer to those used in FAST, AAA or vascular access.
While the familiar curvilinear transducer can give good sub xiphoid views, it is difficult to achieve the other cardiac views using this transducer. A phased array transducer is required.
The phased array transducer was developed in the 1970s. It consists of many piezoelectric crystals that can transmit/receive independently at different times. By varying the timing, for instance by pulsing the elements one by one in sequence along a row, the beam can be steered electronically. The beam is swept like a search-light through the tissue being examined, and the data from multiple beams are put together to make a visual image showing a slice through the object. The key benefit of the phased array transducer is its small footprint. In a cardiac preset it also optimizes the machine settings to give you the best possible image production particular for moving vascular structures.
This transducer typically has a frequency range of between 1-5 MHz. The evolutionary pathway of echocardiography has been different than conventional ultrasound. When the cardiac preset is selected the default marker position is to the left i.e. the marker on the probe should be orientated to the left side of the patient and the marker indicator on the screen image will be opposite to what you would normally expect.
What Views Are Required?
A good Sub xyphoid (SX) view must be obtained. This requires increasing the machine depth to around 20cm, and holding the transducer almost flat against the indented epigastrium. Remember that the heart is an anterior structure and too deep at probe angulation is a common mistake.
Besides this, for the purposes of ELS, one other view should be obtained. This is usually the parasternal long axis view (PLAX). The machine depth settings should be around 14-16cm for this.
Alternatively the parasternal short axis view (PSAX) can be obtained, using similar depth settings.
An apical view is an alternative second view, but is possibly more difficult in a supine patient and often requires rotating the patient onto their left side by 45 degrees.
Finally the IVC must be visualized for collapse as it is an ultrasonic CVP.
Basic anatomy indicates that lung often over-lies much of the myocardium, and lung (ie air) is the enemy of ultrasound it will not pass through it. Therefore, in both parasternal views stay very close to the sternum. Rolling onto the left side also helps, if the clinical condition of the patient allows. A common mistake is to allow the conscious patient to rest their hands above their head. This augments the rib cage capacity and inflates any lung overlying the myocardium, making views more difficult.
Assuming the machine settings are set to the cardiac pre-set, relatively good views should be obtained. During the pulse check view, start a 10 second clip recording. Spend the 10 seconds acquiring the best images possible. During the next 2 minute cycle whilst CPR is on-going, review the images and relay your findings to the team leader.
Els requires the acquisition of a sub xiphoid view, plus one other cardiac view, together with an assessment of the ivc.
Sub Xiphoid (1)
The initial view should be a transverse sub xiphoid (SX) four-chamber view. This answers four key questions:
- Is there co-ordinated cardiac activity?
- Is there a pericardial effusion?
- What is the relative right and left ventricular chamber size?
- What is overall LV function like?
Sub Xiphoid (2)
This view is obtained with either the phased array transducer (marker at 3 oclock). If you do not have the benefit of a phased array transducer then a traditional FAST sub-xiphoid view with a curvilinear transducer can be obtained.
The key to obtaining a good view is to place the handle of the probe flat on the skin under the xiphisternum, with the probe pointing towards the patient’s chin and then to press the flattened probe down gently towards the bed at maximal inspiration. An increased depth, often over 20cm, is required initially to appreciate the correct angle followed by optimal adjustment of depth.
The subcostal view needs increased depth settings, usually over 20cm, and a different handling of the probe.
Parasternal Long Axis
The next view should be a parasternal long axis (PLAX) view. Ideally this requires a phased array transducer. To obtain the PLAX view the dot on the transducer points along the axis of the heart, which is towards the patients right shoulder. In actual fact, the axis is probably more towards the right mid-clavicular line. The transducer is placed tight against the sternum in either the third of fourth intercostal space (try both). The image produced is a slice through the left ventricle, from apex to base (the valve annulus region). At the base the image plane intersects the mitral and aortic valves as well as the left atrium and aorta.
This view is obtained less frequently as in many patients the left semi-lateral position is needed. The marker is to the patients left and the probe axis attempts to follow the cardiac axis.
An inferior vena cava (IVC) view is very similar to the longitudinal aortic view, but lies to its right, and the transducer needs to look more into the chest. With a curvilinear transducer the marker faces cranially, as in the aortic view. Using a phased array transducer (in a cardiac preset) the marker faces caudally. This longitudinal subxiphoid window views the IVC as it passes posterior to the liver and into the heart.
IVC View (2)
The IVC is identified lying posterior to the liver receiving hepatic veins anteriorly before it passes through the diaphragm and into the right atrium. The point to identify for interpretation (more later) is 2cm distal to the atrio-caval junction.
Interpreting The Sub Xiphoid View (1)
In this view the right ventricle (RV) is seen as the closest chamber through the window of the left lobe of the liver. Beyond is the left ventricle (LV) and more distant are both atria.
This view is excellent for pulse check imaging as it shows global motility, the possibility of a pericardial effusion, the RV/LV ratio and basic LV function.
The end-systolic RV/LV ratio should be around 0.5, with the LV indenting the RV into a bean shape, or a letter D . If this D is reversed with the RV indenting the LV it is strongly indicative of high right sided pressures. In the acute setting this may suggest pulmonary embolus. In this case it is important to quickly look at the IVC for signs of distension. RV/LV ratios > 1 are associated with adverse clinical outcomes.
Interpreting The Sub Xiphoid View (2)
RV/LV ratios in healthy controls in one study revealed a mean of 0.51 (95% confidence interval, 0.48-0.54). Meanwhile those with known pulmonary hypertension had ratios of 1.47 (mean, 95% confidence interval, 1.25-1.70). The RV/LV ratio correlated significantly with pulmonary artery pressure.
The rv/lv ratio correlates with pulmonary artery pressure and the normal should be around 0.5. Ratios above 1 are associated with adverse clinical outcomes
Parasternal Long Axis
The parasternal long axis view (PLAX) is excellent for assessing the posterior pericardium, where very early effusions will be identified. The left atrium (LA), mitral valve (MV), left ventricle (LV), left ventricular outflow tract (LVOT) and aortic valve (AV) are all seen very clearly. The right ventricular outflow tract (RVOT) is closest to the transducer.
The movement of the mitral valve leaflets should be dynamic and energetic and the anterior leaflet should almost touch the interventricular septum in diastole. If not the myocardial motility is sub-optimal. Similarly if the movement of the mitral valve leaflets are hyper-dynamic and the left ventricle completely collapses each cycle, then this suggests the patient is hypovolaemic and would benefit from an urgent fluid challenge.
Parasternal Short Axis
The parasternal short axis view (PSAX) enables assessment of the concentricity of LV motility. This is particularly useful in the ischaemic heart. At the level below the mitral valve the left ventricle should be contracting in a concentric fashion and if a finger is placed in the ventricular cavity, the walls should all thicken and move towards the finger.
The indented right ventricle (RV) is clearly seen.
In addition the posterior pericardium is seen (for early effusion).
To obtain the parasternal short axis (PSAX) view, find the PLAX view. Once you have a good image, rotate the probe 90 degrees clockwise so that the marker is pointing towards the left shoulder
The IVC View (1)
IVC Kissing Contact
As already stated, the point of assessment (POA) is 2cm distal to the atrio-caval junction. In a haemodynamically normal, spontaneously ventilating patient, the IVC collapses slightly on inspiration. In fact, in such patients, a pronounced ‘sniff’ results in the walls at the POA just touching, so called ‘kissing contact’.
This is reversed in a mechanically ventilated patient where there is an increased diameter in the abdominal IVC during inspiration.
The IVC view (2)
The IVC view is of great value in the acute assessment of shock and shortness of breath. As an example, in healthy blood donors the measurement of the IVC diameter is a reliable indicator of blood loss, with even small amounts (450 ml) causing a mean decrease in IVC diameter of 5 mm.
Conversely, a distended IVC suggests a high preload or could be an indicator of an obstructive cause of shock e.g. cardiac tamponade, pneumothorax or pulmonary embolism.
The IVC view (3)
Changes in diameter correlate with changes in intrathoracic and intra-abdominal pressure. A collapse index is calculated as the change in diameter between inspiration and expiration divided by the maximal diameter. It may be more useful to measure trends of IVC diameter and collapsibility in response to fluid resuscitation; however, there is evidence for cut-off values which can indicate an underfilled or overfilled status. A maximal IVC diameter of 2 cm with collapse of 40-50% suggests a pressure of >10 mm Hg
Although there is conflicting evidence on which exact IVC measurements equate to right atrial pressures. In patients being invasively ventilated, the distensibility index is used which is the difference between the maxium and minimum IVC diameter divided by the minimum IVC diameter. A distensibility index of >18% can suggest a patient will benefit from further fluid resuscitation
The IVC view (4)
M Mode of IVC
Most EPs do not calculate the collapse index, as a more qualitative approach is equally useful in the acute setting. A dilated non-collapsing IVC implies that the patient is well filled or over filled, or that there is an obstructive aetiology, whereas a narrow fully collapsing IVC suggests an under-filled patient where aggressive fluid resuscitation is required, and points towards hypovolaemia as the likely cause. It is important to remember that these rules about the IVC are rules of thumbs and that there are circumstances where they will not apply. In a fit, healthy person, the IVC collapse can be greater than half due to the mechanics of respiration on the thorax. Patients with right sided heart failure can have a fixed & dilated IVC despite being hypovolaemic
If it is required to calculate the collapse index, this is best carried out using M mode. M stands for measurement (B for brightness). Using M mode calculations at the POA of minimal and maximal diameter can be made across the proximal IVC.
A dilated non-collapsing IVC implies that the patient is well filled or over filled, or that there is an obstructive aetiology, whereas a narrow fully collapsing IVC suggests an under-filled patient.
- Moving away from the sternum on parasternal views
- Probe orientation confusion
- Failure to increase the depth setting in the sub-xiphoid view
- Too deep a transducer angle in the sub-xiphoid view
- Allowing the patient to place their hands above their head
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