PATIENT SELECTION FOR
LUS DURING THE COVID-19 PANDEMIC
The role of LUS
during the COVID-19 pandemic is to identify characteristic sonographic
abnormalities as well as to support clinical decision making. Not all patients
with clinically suspected COVID-19 will warrant LUS, and appropriate patient
selection is essential to minimise unnecessary exposure of healthcare workers
(HCWs) to this virus. LUS should be performed after the medical history is
taken, when a specific clinical question arises and with a pretest probability
of COVID-19 diagnosis already in mind.
► The majority of patients who are clinically well and fit
for discharge are unlikely to benefit from LUS, as they will be managed based
on clinical appearance.
► In clinically well patients with risk factors for severe
COVID-19 (such as chronic lung disease, obesity, diabetes mellitus or
cardiovascular disease), abnormal LUS findings may identify a patient cohort
that would benefit from closer observation such as a home pulse oximeter and
remote monitoring.
► Critically ill patients should be resuscitated without
delay, and LUS is not useful for the primary diagnosis of COVID-19. Ultrasound
is useful in critically unwell patients to examine for other causes of
undifferentiated shock, for example, PE, cardiac tamponade or hypovolaemia,
thus avoiding anchoring bias in the midst of the current pandemic.
► Goal-directed focused cardiac ultrasound may help identify
left ventricular and right ventricular size and function in the case of
COVID-19 heart–lung complications, which include myocarditis, right-sided and
left-sided heart failure and PE.27 28
► Ultrasound can also be used to assess volume status and
guide fluid resuscitation where necessary.29
► Ultrasound can be
used to assist with emergency central or peripheral venous access.
LUS SCANNING
TECHNIQUE
In general,
principles and techniques of LUS are the same for patients with suspected
COVID-19 as they were in the preCOVID-19 era. Some modifications necessary for
patients with suspected COVID-19 will also be outlined. Transducer selection30
► Linear transducers (5–10 MHz) are better for visualising
superficial structures (figure 5). These may be used to view pleural line
irregularities, small superficial effusions, skip lesions and B-lines.
► Curvilinear transducers (2–7 MHz) may be better for
posterior and deeper or central pathology such as consolidation, hepatisation
and air or fluid bronchograms.
Optimising settings
► Optimise the depth of field of view so that the pleural
line is in the middle of the screen.
► Adjust the
transducer focal zone to the level of the pleural line for increased spatial
resolution.
► Turn off smoothing algorithms such as compounding and
tissue harmonic imaging filters to allow visualisation of lung artefacts. Most
lung presets will default to this mode.
► Record cine loop clips rather than still images to
visualise subtle pleural changes that may not appear on a single frame.
Transducer hold Hold the transducer close to the crystal matrix, between the
tips of the index finger and the thumb of the insonating hand (figure 5).
Fingers of the insonating hand should be spread out to stabilise the transducer
and hand position. Brace the insonating hand against the surface being scanned.
These techniques will facilitate small adjustments of the transducer and will
allow for greater probe stability and better quality images to be shown on the
screen. Scanning protocol Traditional lung scanning protocols suggest
evaluation of several anterior, lateral and posterior lung zones. Chinese
authors have described COVID-19 scanning using a 12-zone protocol (figure 6).6
Soldati et al30 have proposed a 9-zone protocol and associated scoring system
to quantify pulmonary involvement. It is possible to perform a focused study
(six chest zones) in less than 2min,31 and the Intensive Care Society has
endorsed this approach as part of the Focused Ultrasound in Intensive Care
(FUSIC) lung accreditation module (figure 6).32
Modifications to minimise exposure
risk COVID-19 changes are often found in postero-basal zones.6 30 It may be
quicker and safer for the point-of-care ultrasound provider to:
► Scan with the patient facing away from the operator to
minimise healthcare worker (HCW) exposure to droplets (figure 5). The
ultrasound machine may also become less contaminated if placed behind the
patient
Start by scanning the patient’s back using the linear
transducer in vertical orientation.
► Start medial to the scapula sliding inferior to the lower
rib border and moving laterally towards the posterior axillary line.
► Evaluate each rib
space first with the transducer in a vertical (crossing the ribs) orientation
(figure 5) then evaluate each rib space again with the transducer in a
horizontal orientation (between the ribs) especially if any abnormalities are
seen.
► Finish by scanning lateral zones of the lung in the
midaxillary line. Using the curvilinear probe here may be helpful (figure 5).
Cleaning and disinfection protocols
Strict adherence to decontamination strategies are vital to
prevent patient-to-patient COVID-19 transmission as well as patient-to-HCW
transmission. What follows are summary points drawn from a number of
international best practice standards33 34 and should be considered when using
ultrasound with suspected COVID-19 patients:
► Place a dedicated ultrasound machine in the COVID-19 ‘hot
zone’ of the ED.
► Wear standard personal protective equipment when performing
LUS and wear gloves when moving the machine between cubicles.
► Strip away ECG leads, gel bottles, extra buckets and
straps from the machine.
► Use a barcode scanner to enter patient details to
avoid further contact with the machine.
► Use the machine in battery mode; precharge at all times to
avoid use of cables.
► Use a touchscreen device rather than a keyboard,
cart-based system.
► Consider using a handheld device, for example, Lumify or
ButterflyIQ systems, with the advantage that the whole device can be placed
within a probe cover and images are uploaded to the cloud for remote reviewing.
► Consider use of a
transparent, disposable drape to cover the screen, cradle and cart of the
ultrasound machine.
► Use chlorhexidine/alcohol or soap-based wipes to clean transducer
heads, as well as the entire length of probe cables, screen and cart after
scanning.35 Wait for up to 3min ‘dry time’ after using disinfectant wipes
before using the machine again.
► Use a transducer sheath/probe cover for all high-risk
patients.
► Use single-use gel packets rather than gel bottles.
CONCLUSION
LUS appears promising as a comprehensive imaging modality in
clinically suspected or diagnosed COVID-19, when implemented mindfully and in
conjunction with other diagnostic modalities. LUS findings should be
interpreted alongside a careful history, physical examination and with pretest
probability in mind. Point-of-care ultrasound may help to identify the need for
further investigations or may guide the physician towards an alternative diagnosis.
Incorporating ultrasound into the evaluation of COVID-19 patients will depend
on available resources, expertise of personnel and logistic configurations
unique to each situation.
