US COMBATTING COVID-19

 

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.