Welcome to The Plague Pit – issue 17

I’ve written a lot about oxygen so far. Given that COVID-19 is first a disease of the respiratory system, that seems fair. In issue 13 (Physiology and Hairstyles), I talked about how COVID-19 increases the work of breathing, even as it decreases the amount of oxygen getting into the blood from the lungs.

The relationship between breathing and oxygen is even more complicated, though. As the work of breathing goes up, so does the amount of oxygen required by the muscles that do the breathing – the diaphragm and the intercostal muscles.

Muscle man

Some of the most famous experiments connecting muscular activity and oxygen consumption were conducted by British scientist, Archibald (‘A.V.’) Hill. These followed his studies on heat production in muscle, which won him a share of the 1922 Nobel Prize in Physiology or Medicine. [1]

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 In 1926, Hill spoke about his work in a series of children’s Christmas lectures at the Royal Institution – ‘Living Machinery’. The series was published a few years later [2]. Here’s one of the more striking images – more to come in the next issue.

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At the end of the last issue, I talked about the clinical reasons why COVID-19 patients end up being intubated and mechanically ventilated.

Intubation and ventilation sounds like a major step, but before the pandemic, it was an absolutely routine procedure conducted by anaesthetists on more than a million unconscious UK patients undergoing surgery every year. [3]

Deliberate paralysis

There are a number of reasons why this alarming-sounding technique is used during operations. For some procedures, surgeons like the anaesthetist to give drugs that ‘relax’ – actually, paralyse – muscles overlying the organ of interest. Paralysis reduces muscle tension, making tissues easier to stretch or manipulate.

I’ve talked before about laparascopic, or ‘keyhole’, operations. For these, surgeons inflate the abdominal cavity with carbon dioxide gas. The rectus abdominis muscle – your six-pack – makes the subsequent surgery very difficult unless it is paralysed with a neuromuscular blocker (NMB). These drugs act at the neuromuscular junction. This is the gap between the end of the motor nerve (the cell which carries ‘instructions’ from the brain in electrical form) and the muscle cell.

Neuromuscular physiology and pharmacology is fascinating. It covers everything from rare diseases to insecticides and nerve gas and led to more UK Nobel Prizes.

Procedures other than laparoscopy sometimes go better with a paralysed patient, too. Surgeons operating down a microscope like the operative site to be absolutely still. Or, at least, they like the regular and consistent respiratory movements that come with mechanical ventilation. NMBs are used in most anaesthetics for brain surgery, for example. Ventilation has other advantages here, such as good control of the level of carbon dioxide in the blood.

NMBs don’t discriminate much between different muscles – though they don’t affect smooth muscle (more on that some other time). Inconveniently, then, they paralyse the diaphragm as well as the rectus abdominis. That means anaesthetists who give NMBs have to manage the business of breathing for the patient, using mechanical ventilation. That is greatly assisted by inserting an endotracheal tube to make an airtight seal between the ventilator and the lungs.

Some aspects of intubation and ventilation are common to elective surgical patients and emergency COVID-19 ones. Neither type of patient can breathe effectively for themselves – drugs in one case, disease in the other. The kit for intubation is similar in both cases, though the technique is a bit different. Intensive care ventilators are more sophisticated than their anaesthetic counterparts (the same is not true of the doctors).

During the pandemic, we have been ventilating many patients on anaesthetic machines.

The main difference between ventilating surgical patients and COVID-19 patients, though, is that anaesthetists can reverse the process in the first group whenever they like. In COVID-19 patients, the virus calls the shots.

What about COVID-19?

So which COVID-19 patients gets to be intubated and ventilated?

I’ve mentioned the clinical factors that precipitate this decision before – hypoxia, hypercarbia, exhaustion, cardiac arrest. But other considerations apply too, particularly where resources are limited.

Hospital doctors were very familiar with these dilemmas, long before COVID-19 came along. Will you use your last ventilator for a hypoxic 95-year old with a myocardial infarction (‘heart attack’), unable to communicate and totally dependent before admission, with low quality of life and little chance of recovery? Will you make the same choice when you know there also is a young multiple trauma victim in Casualty, previously completely fit, who requires immediate surgery with postoperative ventilation?

The key to decisions like this is objectivity – and there are well-established principles to help intensive care doctors make them. There is often some leeway, too, in large hospitals. Patients around the hospital can be re-assessed and transferred between wards to ensure everyone, including new arrivals, receives a level of care that does not exceed what is required. (I wrote briefly about the levels of care that might be applied to COVID-19 patients in my last post: https://plaguepit.com/speculative-letter-about-ecmo/

In the early stages of the pandemic, it was expected that this sort of ‘triage’ would be crucial. The National Institute for Clinical Excellence published a COVID-19 guideline at the end of March. https://www.nice.org.uk/guidance/ng159

In the guideline, the Clinical Frailty Scale (CFS) plays a major role in decisions on ICU admission. The scale – from 1-9 inclusive – was created as a ‘global clinical measure of fitness and frailty’ about ten years ago by geriatricians at Dalhousie University, Canada. In a COVID-19 context, it refers to the patient’s condition before hospital admission.

As you see, NICE advises ward doctors to hold advance discussions with the ICU team on whether critical care/ventilation would be appropriate if a COVID-19 patient is deterioriating but has a CFS >4.

I reckon most sixth-form students have a CFS of one. Most teachers? From one to four inclusive……

The Good Soldier

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(1) Bassett DR. Scientific contributions of A. V. Hill: exercise physiology pioneer. J Appl Physiol 2002;93: 1567–1582.

(2) Hill A V. Living Machinery. London: G. Bell and Sons; 1934.

(3) Woodall NM, Cook TM. National census of airway management techniques used for anaesthesia in the UK: first phase of the Fourth National Audit Project at the Royal College of Anaesthetists. Br J Anaesth. 2011 Feb;106(2):266-71.