Why is this review important?
Although the importance of managing spinal anaesthesia-associated hypotension during caesarean section is well appreciated, there continues to be some debate over relative efficacy of interventions, whether vasopressors or fluid-loading.
Fitzgerald at al.'s review and network meta-analysis quantitatively compares a complete range of interventions across 109 studies.
What did they show?
Vasopressors were more effective at preventing intra-operative hypotension than fluid infusion techniques alone. Although there was no statistically significant difference in the incidence of hypotension among metaraminol, phenylephrine or noradrenaline/norepinephrine, metaraminol appeared the most effective (OR 0.04-0.26) and ephedrine the least (0.09-0.85). [vs Norepinephrine (OR 0.06-0.28), Phenylephrine (OR 0.11-0.29)]
Similarly, nausea and vomiting incidence was lower with vasopressors than other interventions. Phenylephrine more commonly caused bradycardia than other pressors, and ephedrine more commonly tachycardia. There was no significant difference in reactive hypertension.
The most effective way of preventing and managing maternal intraoperative hypotension is, as international guidelines already assert, alpha-agonist vasopressors. Using more complex protocols, such as phenylephrine or norepinephrine infusions, does not appear to offer benefit over metaraminol. Fluid infusions are at best a secondary intervention.
Vasopressors are the most effective method for preventing maternal hypotension, nausea & vomiting after spinal anaesthesia, with metaraminol possibly the best.
N95 respiratory masks should be prioritised for high-risk aerosol-generating procedures because they do not appear to offer general healthcare workers greater protection from influenza or other respiratory illness than surgical masks.
Lockhart et al. provide a considered exploration of COVID-19 infection-control issues specific to anaesthesiologists, proposing an additional third category of personal protective equipment (PPE).
Give it to me in point form!
They propose three PPE types:
- Droplet & contact precaution PPE: surgical mask, gown, gloves.
- General airborne, droplet & contact PPE: addition of N95 respirator mask and eye protection.
- PPE for high-risk aerosol-generating medical procedures: addition of gown neck protection and double gloves.
Why should I take notice?
The Canadian view on PPE is tempered by both their current significant COVID burden, and their experience of the 2003 SARS pandemic which infected 257 Canadians, 20% of whom were healthcare workers. Much of our PPE evidence is based upon SARS. This article emphasises the importance of PPE for anaesthesiologists and their airway assistants.
On airborne spread?
Unfortunately much of what we did not know about respiratory spread and SARS in 2005 persists today:
Although this observation [about lack of knowledge of SARS infectious droplets] was made 15 years ago, basic questions regarding nosocomial spread during the SARS epidemic, and now the COVID-19 pandemic, have yet to be answered.
Absence of evidence however, should not imply evidence of absent airborne spread.
The role of airborne particles in the spread of COVID-19 remains unclear, although Lockhart notes the infamous case of Hong Kong's Amoy Gardens housing complex in the 2003 SARS outbreak, resulting in 187 cases – likely via airborne spread.
Endotrachial intubation has been shown in several studies to be a high-risk procedure for healthcare worker infection. Considering this the authors reccomend a third level of PPE, adding:
- AAMI level-2 gown, incluidng neck protection, noting that the neck is a high-risk area for contamination in simulation studies.
- Double gloves that overlap the sleeve, noting that the gown-glove interface is a common PPE failure site, and that Verbeek's 2020 Cochrane review concluded that there was less contamination vs single gloving (RR 0.36).
- Only allow presence of essential staff in room during AGP.
- Provide access to shower resources for staff after high-risk AGP.
- Do not ‘‘MacGyver’’ homemade combinations of PPE.
- Doffing is a high-risk critical moment, that should not be rushed, distractions should be minimised, and use a doffing supervisor. Pay attention when donning to ease later doffing.
- Masks should be the last item removed.
Lockhart emphasises that there is no ideal PPE, but by focusing on consistent protection at known high-risk interactions (ie. intubation) safety improvements can be made.
When considering whether COVID could have significant airborne transmission, note that during the SARS pandemic at least one large Hong Kong outbreak (187 cases) was very likely due to airborne infection: Yu 2004 (NEJM).
Airway procedures associated with an increased risk of SARS transmission to HCWs include tracheal intubation (OR 6.6), non-invasive ventilation (OR 3.1), tracheotomy (OR 4.2) and manual ventilation (OR 2.8).
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.
- 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"
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.
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. ↩
Acknowledging the difficulty of balancing PPE supply and demand, the tone of this review tends to be biased toward hopeful but unproven assumptions that airborne transmission is not significant. There is considerable concern that this is in fact not true.
When infection of a potentially fatal disease is occurring among frontline healthcare workers, a more cautious posture is warranted, along with greater acknowledgement of the uncertainty inherent in these recommendations.
It is unclear whether either chest compressions or defibrillation is an aerosol generating event and whether there is significant transmission risk for resuscitators.
Public Health England's 'New and Emerging Respiratory Virus Threats Advisory Group (NERVTAG)' concluded:
It is biologically plausible that chest compressions could generate an aerosol, but only in the same way that an exhalation breath would do. No other mechanism exists to generate an aerosol other than compressing the chest and an expiration breath, much like a cough, is not currently recognised as a high-risk event or an AGP.” NERVTAG also stated that it “does not consider that the evidence supports chest compressions or defibrillation being procedures that are associated with a significantly increased risk of transmission of acute respiratory infections.”