Unmasking the surgeons: the evidence base behind the use of facemasks in surgery

…The facemask has been used in surgical settings for over a hundred years; first described in 1897, at its inception, it consisted merely of a single layer of gauze to cover the mouth, and its primary function was to protect the patient from contamination and surgical site infection. This practice was substantiated, at the time, by a recent discovery which demonstrated that bacteria could be disseminated from the nose and mouth during normal conversation as observed by bacterial colony growth on strategically placed agar plates in theatres. In the 1940s and 1950s, antibiotics and aseptic technique came to the forefront of infection control strategies within the surgical setting. Until recently, it has remained unclear as to whether bacterial colony growth on an agar plate was a direct correlate of surgical site infections and also whether the purpose of the surgical mask has been superseded by more modern strategies of infection control.

In order to advocate the validity of an intervention in medicine, it must satisfy three levels of evidence: efficacy, effectiveness and cost-effectiveness. In the context of facemask, efficacy is whether masks prevent the propagation of droplets derived from the mouth and nose of the operating staff. Effectiveness is whether efficacy translates into a significant reduction in surgical site infection morbidity and mortality. And finally, cost-effectiveness determines whether the cost-to-benefit ratio of this effect would be desirable compared to an alternative course of action.

Intuition would suggest that facemasks offer a physical barrier preventing the emanation of droplets from the oral or nasal passages and therefore satisfy the efficacy requirement of the evidence ladder. However, there are a number of different hypotheses as to why this may not be the case. ‘Venting’ is a phenomenon whereby air leaks at the interface between mask and face which can act to disperse potential contaminants originating from the pharynx. The accumulation of moisture, during prolonged usage, may exacerbate this problem by increasing resistance to air flow through the filter itself. Moisture accumulation is also thought to facilitate the movement of contaminants through the material of the mask itself by capillary action. These bacteria can subsequently be dislodged by movement. Friction at the face/mask interface has also been demonstrated to disperse skin scales which can further contribute towards wound contamination.

In the modern era, there has also been a scarcity of experimental evidence to support the effectiveness of facemasks in the prevention of surgical site infections. The earliest retrospective studies failed to demonstrate any statistically significant improvement in surgical site infection rates following the use of masks. Indeed, the latest National Institute for Health and Care Excellence guidelines on the matter do not require operating staff to wear a mask in theatre. This decision was based primarily upon the findings of a Cochrane systematic review. This review was guided by the findings of two particular randomised/quasi-randomised control trials. The latest update of this review, which was amended after the publication of current National Institute for Health and Care Excellence guidelines, included one further study.

…Overall, we found very few studies and identified no new trials for this latest update. We analysed a total of 2106 participants from the three studies we found. All three studies showed that wearing a face mask during surgery neither increases nor decreases the number of wound infections occurring after surgery. We conclude that there is no clear evidence that wearing disposable face masks affects the likelihood of wound infections developing after surgery.

…More research in this field is needed before making further conclusions about the use of face masks in surgery.

see also:
https://www.isitzen.com/blog/2021/3/cochrane-mask-studies

the trolley problem

The trolley problem: should you pull the lever to divert the runaway trolley onto the side track?

Foot's original structure of the problem ran as follows:

Suppose that a judge or magistrate is faced with rioters demanding that a culprit be found for a certain crime and threatening otherwise to take their own bloody revenge on a particular section of the community. The real culprit being unknown, the judge sees himself as able to prevent the bloodshed only by framing some innocent person and having him executed.

The trolley problem is a thought experiment in ethics modeling an ethical dilemma. It is generally considered to represent a classic clash between two schools of moral thought, utilitarianism and deontological ethics. The general form of the problem is this:

There is a runaway trolley barreling down the railway tracks. Ahead, on the tracks, there are five people tied up and unable to move. The trolley is headed straight for them. You are standing some distance off in the train yard, next to a lever. If you pull this lever, the trolley will switch to a different set of tracks. However, you notice that there is one person on the side track. You have two options: 

Do nothing and allow the trolley to kill the five people on the main track.Pull the lever, diverting the trolley onto the side track where it will kill one person.

Which is the more ethical option? Or, more simply: What is the right thing to do?

droplets

…the reported number and size of released droplets vary significantly. In terms of the total mass of saliva, 1.1-6.7 mg of saliva were collected on a mask during a single cough, and 18.7 mg were collected while counting from 1-100.

There were 1-320 droplets per liter of exhaled air found for breathing, 24-23,600 found for coughing, and 4-600 found for speaking.

…individuals infected with influenza virus produce a significantly greater volume of aerosol during clinical illness compared with during the asymptomatic stage (P = .0143). This enhancement in aerosol generation during illness may play an important role in influenza virus transmission.

Two reviews

highlight the limited evidence base supporting the efficacy of face masks in reducing influenza virus transmission. They suggested that surgical masks may reduce infectiousness, rather than protect against infection, especially when airborne transmission is important. Influenza viruses (with sizes in the 80- to 120-nm range) and other viruses of similar size are capable of penetrating the mask in either direction. The N95 respirators are efficient in removing very fine droplet nuclei, but face masks are not. However, face masks, if worn by an infected person, can suppress the expired jets (Fig 2A) and reduce the close contact transmission via both the droplet-borne and short-range airborne routes.

growing evidence covid-19 is (aerosol) airborne

A group of 239 scientists from 32 countries have written an open letter to the World Health Organization arguing that covid-19 can be transmitted through the air. You might think we know that already, but most current guidance is based on the idea that covid-19 is transmitted via droplets expelled from an infected person’s nose or mouth. The thought is that these larger respiratory droplets quickly fall to the floor. That's the position the WHO has taken from early on in the pandemic, and that’s why we have been keeping at a distance from one other. However, the signatories of the open letter say the organization is underestimating the role of airborne transmission, where much smaller droplets (called aerosols) stay suspended in the air. These aerosols can travel farther than droplets and linger in an area even when an infected person has left.

What’s the evidence? The letter says multiple studies “have demonstrated beyond any reasonable doubt that viruses are released during exhalation, talking, and coughing in microdroplets small enough to remain aloft in air.” It says these microdroplets “pose a risk of exposure at distances beyond 1 to 2 m from an infected individual.” An early laboratory studycarried out by the US National Institutes of Health found that the coronavirus can linger in the air for up to four hours in aerosol form. The coronavirus was also detected in aerosols collected at two hospitals in Wuhan, China, according to a study published in Nature in April. And superspreading events add to the weight of evidence: for example, after a choir practice in the US nearly 50 people were infected even though they kept a safe distance apart.

The implications: If airborne transmission is a route for the spread of the virus, it could lead to changes in the current advice. It would suggest that social distancing may be insufficient, especially indoors. This may place yet more importance on mask-wearing around people who are not part of your household if you meet them indoors, even if you are distancing, and increasing ventilation in enclosed areas. It could make air-filtering systems more important to try to cut down on the recirculation of air. And it might mean health-care workers caring for coronavirus patients need the highest grade of mask—N95—to filter out the smallest droplets.

truth

The naked figure of Truth is held aloft by her father, Time, who with his scythe subdues Falsehood, in her fine apparel and dissembling mask, while the baleful figure of Discord and Envy recedes, protesting. The subject may have held a personal significance for the artist; the day after he completed it he committed suicide. Yet, as the collector Caylus recalled: ‘there is no sense of his alienation of spirit in the work, it is even one of his most beautiful cabinet pictures’.

Truth is the property of being in accord with fact or reality.[1] In everyday language, truth is typically ascribed to things that aim to represent reality or otherwise correspond to it, such as beliefs, propositions, and declarative sentences.

Truth is usually held to be the opposite of falsity. The concept of truth is discussed and debated in various contexts, including philosophy, art, theology, and science. Most human activities depend upon the concept, where its nature as a concept is assumed rather than being a subject of discussion; these include most of the sciences, law, journalism, and everyday life. Some philosophers view the concept of truth as basic, and unable to be explained in any terms that are more easily understood than the concept of truth itself.[2] Most commonly, truth is viewed as the correspondence of language or thought to a mind-independent world. This is called the correspondence theory of truth.

Various theories and views of truth continue to be debated among scholars, philosophers, and theologians. There are many different questions about the nature of truth which are still the subject of contemporary debates, such as: How do we define truth? Is it even possible to give an informative definition of truth? What things are truthbearers and are therefore capable of being true or false? Are truth and falsity bivalent, or are there other truth values? What are the criteria of truth that allow us to identify it and to distinguish it from falsity? What role does truth play in constituting knowledge? And is truth absolute, or is it merely relative to one's perspective?

Mathematics of Testing

Coronavirus Antibody Tests Have a Mathematical Pitfall

The accuracy of screening tests is highly dependent on the infection rate

With a test that is not 100 percent accurate, there are four possible outcomes for each individual:

• you are positive and test positive

• you are negative and test negative

• you are positive but test negative (a false negative)

• you are negative but test positive (a false positive)

If a test has a 95 percent specificity and a 95 percent sensitivity, that means it correctly identifies 95 percent of people who are positive and 95 percent of those who are negative. Even with very effective screening tests, depending on the infection rate in the population, and individual’s test result may not be reliable.

If a test with 95 percent specificityand 95 percent sensitivity is used in a community of 500 people with a 5 percent infection rate, the results look like this:

In this scenario, an individual who tests negative has a 99.8 percent chance of actually being negative. But an individual who tests positive has only a 50 percent chance of being positive.

If an equally accurate test is used on a group of 500 people with a 25 percent infection rate, the results may look like this:

In this scenario, and individual who tests negative has a 98.3 percent chance of actually being negative. And and individual who test positive has an 86 percent chance of actually being positive.

Sensitivity and specificity

Sensitivity and specificity are statistical measures of the performance of a binary classification test, also known in statistics as a classification function, that are widely used in medicine:

• Sensitivity (also called the true positive rate, the epidemiological/clinical sensitivity, the recall, or probability of detection[1] in some fields) measures the proportion of actual positives that are correctly identified as such (e.g., the percentage of sick people who are correctly identified as having the condition). It is often mistakenly confused with the detection limit[2][3], while the detection limit is calculated from the analytical sensitivity, not from the epidemiological sensitivity.

• Specificity (also called the true negative rate) measures the proportion of actual negatives that are correctly identified as such (e.g., the percentage of healthy people who are correctly identified as not having the condition).

The terms "positive" and "negative" do not refer to the value of the condition of interest, but to its presence or absence; the condition itself could be a disease, so that "positive" might mean "diseased", while "negative" might mean "healthy".