Anaesthesia
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In this review, Wilson, Norton, Young & Collins challenge the overly-simplistic view that SARS-CoV-2 transmission risk can be easily divided between droplet-contact and aerosol precautions.
Why is this important?
Many national societies have policies on Personal Protective Equipment (PPE) guided by classification of COVID exposure into aerosol-generation procedures (AGP) or other exposures. Although founded in some evidence, there are questions as to whether PPE shortage and availability also drives these recommendations. Widespread concern over healthcare worker (HCW) infection is understandable, given that during SARS 20% of infections were among HCWs.
Understanding the science behind respiratory particle generation and transmission helps to inform our understanding of how best to use limited PPE.
On the science of respiratory shedding
Aerosol generation is important because virus inhalation and deposition in small distal airways may be associated with greater infection risk and disease severity. Wilson et al. describe three mechanisms of aerosol generation:
- Laryngeal activity - talking, coughing, sneezing.
- High velocity gas flow - eg. high-flow oxygen
- Cyclical opening & closing of terminal airways.
Notably, the clinically features of COVID itself make all three high-risk mechanisms more likely. Additionally various studies show that even talking and tidal volume breathing produce large numbers and size ranges of respiratory droplets.
Exposure relative risk is primarily about proximity and exposure duration
Further, considering retrospective data form SARS HCW infections involving various procedures (eg. intubation, HCW infection RR 4.2; oxygen mask manipulation RR 9; urinary catheterisation RR 5), Wilson et al. propose that healthcare work risk can be considered:
infection risk ∝ 𝑏 × 𝑣 × 𝑡 / 𝑒
Where: 𝑏 = breathing zone particle viable virion aerosol concentration, 𝑣 = minute volume of healthcare worker, 𝑡 = time exposed , 𝑒 = mask efficiency
And on intubation:
"...[other] healthcare workers should stand over 2 m away and out of the direct exhalation plume. During a rapid sequence intubation muscle relaxation should be protective as coughing will be prevented and high airway gas flow and expiratory output will terminate. When expiratory flow is ended ... aerosol particles should start settling in the airways. The forces generated in gentle laryngoscopy are unlikely to cause aerosol formation."
"...[there is] limited evidence to suggest AGPs cause an increase in airborne healthcare worker transmission as this has not been studied. The few studies to sample pathogenic airborne particles in relation to procedures show no increase with the majority of AGPs."
Bear in mind...
Much of the evidence guiding our understanding of SARS-CoV-2 transmission is founded on understanding and research focusing on the 2003 SARS pandemic (SARS-CoV-1) and influenza research. Although sharing similarities, "...each has its own infective inoculum and aerosol characteristics."
What's the bottom-line?
Transmission of SARS-CoV-2 should be conceptualised as a spectrum of risk where time exposed may be the dominant factor and droplet-airborne spread is a complex continuum of varying probability of infection. Many 'non-AGP' events could in fact be higher risk than those traditionally considered AGP, such as intubation.
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As COVID-19 disease escalates globally, optimising patient outcome during this catastrophic healthcare crisis is the number one priority. The principles of patient blood management are fundamental strategies to improve patient outcomes and should be given high priority in this crisis situation. The aim of this expert review is to provide clinicians and healthcare authorities with information regarding how to apply established principles of patient blood management during the COVID-19 pandemic. ⋯ We discuss how preventative and control measures implemented during the COVID-19 crisis could affect the prevalence of anaemia, and highlight issues regarding the diagnosis and treatment of anaemia in patients requiring elective or emergency surgery. In addition, we review aspects related to patient blood management of critically ill patients with known or suspected COVID-19, and discuss important alterations of the coagulation system in patients hospitalised due to COVID-19. Finally, we address special considerations pertaining to supply-demand and cost-benefit issues of patient blood management during the COVID-19 pandemic.
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Ultrasound imaging of the lung and associated tissues may play an important role in the management of patients with COVID-19-associated lung injury. Compared with other monitoring modalities, such as auscultation or radiographic imaging, we argue lung ultrasound has high diagnostic accuracy, is ergonomically favourable and has fewer infection control implications. ⋯ This narrative review provides a summary of evidence and clinical guidance for the use and interpretation of lung ultrasound for patients with moderate, severe and critical COVID-19-associated lung injury. Mechanisms by which the potential lung ultrasound workforce can be deployed are explored, including a pragmatic approach to training, governance, imaging, interpretation of images and implementation of lung ultrasound into routine clinical practice.
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Why do we need another PPE review?
This review contextualises the PPE issues with their (relatively low quality) evidence base, focusing particularly on anaesthesia given that this is a high-risk occupational group. Coming from both a UK expert and journal, the recommendations should be carefully considered in terms of the UK's severe COVID outbreak and PPE supply issues.
Important takeaways?
- The significance of airborne transmission, in particular the infectivity of airborne viral particles beyond 1 meter, is uncertain.
- PPE should be seen as an important and essential part of a larger safety system.
- Intubation is a high-risk procedure for aerosol generation. A ventilated negative pressure room and airborne-precaution PPE is recommended. Ventilation (frequency of air-exchange) is likely more important than negative pressure.1 Chinese evidence suggests COVID transmission at intubation is low with appropriate PPE, although there is wide variability in extremes of PPE used along with post-exposure disinfection (eg. showering).
- High-flow nasal oxygen and supraglottic airway (eg. LMA) placement may also be aerosol generating.
- Most risk of transmission from sneezing and coughing is probably droplet and contact, rather than airborne, although the science behind these questions are complex and uncertain. Evidence attempting to answer these questions is often from non-clinical settings.
- Fluid-resistant surgical masks when worn by staff may reduce transmission by at least 80%. Superiority of respirator masks (eg. P2,P3,N95) is not yet reliably supported by evidence.
- Cook highlights two main PPE problems: 1. PPE supply; 2. Inappropriate use of PPE (using higher level than required).
- PPE should be simple to remove (doff) after use, to reduce contamination risk. Cook notes that Canada's SARS experience highlighted increased risk of self contamination with more complex PPE.
On specific levels of PPE
- Contact precautions (gloves & gown) are recommended when in vicinity of COVID positive patient but not within 2 meters.
- Droplet precautions (+ mask & eye protecting) are recommended within 2 meters of patients.
- Airborne precautions (+ FFP3 respirator mask) are only recommended for aerosol generating procedures (AGP). However classification of procedures as AGP or not is only loosely evidence based.
"Public Health England recommends airborne precautions are used in ‘hot spots’ where aerosol generating procedure are regularly performed, if any suspected COVID-19 patients are present – these include intensive care unit, operating theatre, emergency department resuscitation bays and labour wards where mothers are in stage 2 or 3 of labour"
(Interesting that two recent meta-analyses found no evidence of benefit of N95 masks vs surgical masks for healthcare workers: Bartoszko 2020 & Long 2020.)
Hang on...
The elephant in the room is that the lack of PPE supply appears to be the main driver of the rapidly-changing PPE recommendations.
PPE choices need to be made in consideration of the spectrum of risk, hazard and cost, acknowledging different risk profiles depending on location, procedure and individual clinicians.
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It's worth highlighting that negative pressure confers no protection on those in the room, it's purpose is to prevent escape of contagion to areas outside the room. ↩
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In December 2019, a cluster of atypical pneumonia cases were reported in Wuhan, China, and a novel coronavirus elucidated as the aetiologic agent. Although most initial cases occurred in China, the disease, termed coronavirus disease 2019, has become a pandemic and continues to spread rapidly with human-to-human transmission in many countries. This is the third novel coronavirus outbreak in the last two decades and presents an ensuing healthcare resource burden that threatens to overwhelm available healthcare resources. ⋯ Based on the Chinese experience, some 19% of coronavirus disease 2019 cases develop severe or critical disease. This results in a need for adequate preparation and mobilisation of critical care resources to anticipate and adapt to a surge in coronavirus disease 2019 case-load in order to mitigate morbidity and mortality. In this article, we discuss some of the peri-operative and critical care resource planning considerations and management strategies employed in a tertiary academic medical centre in Singapore in response to the coronavirus disease 2019 outbreak.