Special Report: Fighting The New Coronavirus
The anatomy of the first patient's body in China shows that the new coronavirus (Covid-19) still mainly attacks the lungs. In severe cases, patients often die from asphyxia. How did all this happen?
In an article published in China Science Communication, Xu Sijia of Kyoto University School of Medicine explained.
The new coronavirus epidemic is continuing, with mortality rates in critically ill patients exceeding 50%. Reports show that patients die more from respiratory failure.
Various studies have shown that the new coronavirus mainly damages the lungs.
The latest patient anatomy report (February 28, the first case) shows that although virus-induced pulmonary fibrosis (the "scar" left by self-repair after lung tissue damage) is not as severe as SARS, the exudative response but it's more obvious.
Hypoxia is the main cause of death in the lungs of critically ill patients who are surrounded by a large amount of mucus.
But how did mucus appear in the lungs, and what exactly happened? How does it affect breathing? Can't the doctors solve it?
Tree of Life
The problem starts with the anatomy of the lungs.
You may never see what your lungs look like, but you must have seen a tree. The leaves on top of those branches are the breathing part of the tree. The human lung is also like a tree. Its "leaf" is the "alveoli" at the end of each trachea. Looks like a bunch of grapes.
Human alveoli look like bunches of grapes
The alveoli are surrounded by capillaries. With each breath, the blood flowing around them removes fresh oxygen from the alveoli while exchanging carbon dioxide.
The alveoli are very small, each with a diameter of only 0.2 to 0.5 mm, but there are a large number of them. Together, the surface area in contact with air is very considerable.
There are about 600 million alveoli in the human body. If they are spread out, the area can reach 100 square meters. Adults need about 500 liters of oxygen per day.
When the virus infects the lungs through the respiratory tract, the inflammatory response causes the blood vessels in the lungs to dilate, and the permeability (substance permeability) of the blood vessel walls is enhanced. The protein-rich liquid in the blood will leak out of the blood vessels and block the blood vessels and alveoli Between, even into the alveolar cavity, hinder the diffusion of oxygen from the alveoli to the blood.
Infected tissue will be localized edema. Local edema is positive in the early stages of inflammation: exudate can dilute toxins, bring nutrients to immune cells that come to destroy pathogens (viruses), and take away metabolites.
At this time, the blood can still get oxygen from other healthy alveoli, and the patient's hypoxic symptoms will not be too obvious.
However, if the inflammation is aggravated and the affected area is too wide, large-scale fluid exudation will quickly aggravate the body's hypoxia.
The anatomy report of the patient's body showed that the new coronavirus mainly caused inflammatory reactions characterized by deep airway and alveolar damage, and the lesions were mainly exudative.
The lung tissue of the deceased was covered with a large amount of mucus, which prevented ventilation.
The alveoli, which could be retracted like balloons, seemed to be filled with paste inside and out, and then filled with water. Naturally, oxygen could not smoothly enter the blood.
Elderly or severely ill patients with chronic diseases, because of weak immunity, a large number of virus replication, poor therapeutic effect, inflammation more easily spread to the entire lung, leaving few areas that can effectively absorb oxygen. In this way, no matter how diligently the blood ran into the lungs, it was doomed to return. The whole body's hypoxia gradually worsens, and the patient's prognosis is worrying.
Alveoli surrounded by exudate in critically ill patients are like these balloons.
But the above are all happening inside the body, how can we know that the body is hypoxic?
One is faster breathing. When the amount of oxygen obtained by breathing is less than normal, the body will accelerate the breathing compensatoryly.
Normal people breathe 16 to 20 times per minute, while severe patients often breathe more than 30 times per minute.
This high-frequency breathing consumes a large amount of physical energy.
In addition, due to lack of oxygen, the heart has to send blood to the whole body to deliver oxygen more frequently. Accelerated heart rate is also common, further increasing the energy consumption of patients.
The other is decreased blood oxygen saturation. "Blood oxygen saturation" is a commonly used indicator to determine the degree of hypoxia in patients. In popular terms, it is the percentage of red blood cells filled with oxygen to all red blood cells.
From a physical point of view, arterial blood with higher oxygen content is bright red (not easy to absorb red light), while venous blood with hypoxia is relatively dark (easy to absorb red light).
Therefore, a blood oxygen saturation device which is clamped on a finger is commonly used in the clinic to test the "blood oxygen saturation". It emits red light at the fingertips and estimates the oxygen saturation in the bleeding by observing the redness of the subcutaneous arterial blood.
In a quiet state, normal people's fingertip blood oxygen saturation is not less than 95%, if it is less than 93%, it indicates that hypoxia is very serious.
However, this method also has limitations. If you encounter people with poor peripheral circulation or anemia (the measured oxygen saturation is normal, but the total number of blood cells is small, the body is still hypoxic), it may not be accurate.
Therefore, the most accurate test is to directly draw the arterial blood (such as the wrist pulse), which can also have other useful indicators, such as electrolytes, acid-base balance, and so on.
For patients with unknown or critical illness, a blood test is the most reliable.
Oxygen in need
What if the body is already hypoxic due to problems such as inflammation exudate?
It takes time for the inflammation to resolve. Even with anti-infection and anti-inflammatory treatments, it is not possible to remove the exudate at once like a towel twist. So you can only rely on oxygen.
Doctors usually adopt different levels of oxygen supply strategies according to the degree of hypoxia to protect patients' important organs and help them to survive the day when inflammation subsides and lung function recovers. Specifically, from light to heavy:
1) oxygen
The most common one is oxygen inhalation, also known as "oxygen therapy," which means administering oxygen to a nasal cannula or mask for those with mild symptoms. There is no risk in this method, except that the oxygen concentration is not well controlled, it is easy to leak air, and it will feel dry and cold when the air flow is large.
Therefore, high-flow oxygen inhalation equipment that can control the flow rate and has heating and humidifying functions is now commonly used to make patients more comfortable.
2) Non-invasive ventilator
When the symptoms of hypoxia are severe and cannot be corrected by oxygen inhalation, a ventilator is needed to assist breathing. Not so comfortable at this time.
A lighter one can use a non-invasive ventilator. This ventilator is also connected to the patient through a mask, which opens the airway by applying a certain pressure (air pressure) to help the lungs swell and reduce the patient's breathing fatigue.
Because the mask and the patient's face need to fit tightly, people with "thin skin" are prone to facial pressure injuries. In addition, you must remove it when drinking water, eating, vomiting, and expectoration to prevent foreign objects from being blown into the intake pipe.
Therefore, this method is only suitable for patients who are conscious and can cooperate calmly.
3) Invasive ventilator
For patients with severe hypoxia or unconsciousness who cannot cooperate with non-invasive ventilator, they have to use the machine to take over the patient's breath. That is invasive ventilator.
The doctor inserts a catheter from the patient's throat glottis, or cuts the trachea so that the ventilator's tube passes through the patient's airway.
These are traumatic procedures, but they are significant for critically ill patients.
Because no more gas is supplied through the nose and nose, the digestive tract is avoided, the risk of vomit and other accidental inhalation of the airway is reduced, and the secretions in the trachea are sucked out in time to keep the airway open.
As far as the current technology is concerned, the ventilator can already set the air pressure and oxygen flow rate by itself, and it can be said that the function is very powerful when the alveoli are opened, and the foot is given oxygen.
However, in fact, in the face of severely vulnerable alveoli, which are devastated by inflammation, it will eventually be constrained.
Simply put, ventilation cannot be increased indefinitely.
There is a term called "tidal volume", which means the amount of gas the human body exhales or inhales each time. Theoretically, the ventilator's air pressure is proportional to the tidal volume. The higher the air pressure, the more gas the patient inhales.
However, if the tidal volume is too large, it will force the alveoli to expand and contract greatly over and over again, which will cause physical damage to the alveoli and further increase inflammation. In severe cases, the alveoli may blow out.
In the past two decades, experts have summarized a lot of experience on how to protect the alveoli and improve the survival rate of patients under the ventilator.
The core conclusion is that using small tidal volume ventilation, that is, keeping the alveoli in an open state without excessive expansion with low pressure, can effectively avoid physical damage.
In this new type of coronary pneumonia treatment, "little tidal volume ventilation" is also one of the basic treatment strategies.
However, if the ventilation volume is too small, the patient's carbon dioxide will not be fully exhausted, and he will feel depressed.
Many awake patients will be unable to resist the ventilator and follow their own rhythm, making the breathing rhythm more chaotic.
To solve this problem, the doctor will use a sedative or muscle relaxant to help the patient relax if necessary, or simply anesthetize the patient and let them fall asleep completely.
This also brings some new problems, for example, sedatives may cause hypotension or respiratory arrest, so be careful with the dosage.
Anesthesia patients need to undergo very strict monitoring in the ICU (Intensive Care Unit), which requires high personnel, venue and equipment, which is usually difficult to guarantee in an outbreak.
Because a new type of coronavirus causes a large number of viscous exudates, it not only directly affects lung ventilation, but also brings risks to respiratory assistance. Experts suggest that while giving ventilation treatment, attention should also be paid to the dilution and dissolution of secretions.
Otherwise, improper pressurized ventilation may further squeeze the viscous secretions in the airways into the alveoli, which is difficult to discharge, further worsening the ventilation function.
"Artificial Lungs" ECMO
The above are all about oxygen assistance measures for the lungs (the lungs can still be used by default), but in super extreme cases, the lungs can no longer support gas exchange. This time, you can only skip the organ of the lung and start directly from the blood.
This is the "artificial lung", which is what we call ECMO (External Membrane Oxygenation Technology).
In this epidemic, ECMO was considered a successful lap. It was actually the first to replace the heart during heart surgery to maintain blood circulation in the body.
Because the heart and lungs always work together, one delivers blood to the body and one adds oxygen to the blood, and ECMO was later introduced into the treatment of critically ill patients.
By directly oxygenating the blood and excluding carbon dioxide from the body, it can replace the patient's cardiopulmonary function in the short term, and let the exhausted heart and lungs rest.
Although ECMO can gain some survival time for critically ill patients, it is not a treatment for the cause. If the patient's cardiopulmonary function is hopelessly restored, and ECMO is inseparable, treatment will be meaningless.
At present, the cost of ECMO is also very expensive. Since the core components and disposable consumables are mainly imported from abroad, the startup cost is as high as 60,000 to 70,000 yuan.
Although it can be used continuously after being turned on, the daily maintenance and medicine costs are at least thousands of yuan.
What's more, ECMO can also lead to fatal complications.
We know that blood clots outside the body. In order to prevent it from clotting in the ECMO line, anticoagulant drugs need to be added to the blood all the time.
Such blood transfusion into the body is bound to increase the risk of bleeding in the body, such as fatal intracranial bleeding.
At the same time, anticoagulant drugs will also disrupt the balance of coagulation factors in the body, causing abnormal hypercoagulability in the blood and causing thrombosis. If blood clots block blood vessels in vital organs, they can cause organ necrosis.
In cardiac surgery, ECMO only takes a few hours, and the risk is still manageable; while patients with severely impaired lung function generally use it for a long time, the risk of complications will increase.
Therefore, if the underlying cause cannot be corrected in time, which leads to irreversible damage to the lung function, ECMO is definitely not a long-term solution.
On February 29, China successfully completed the world's first double lung transplantation of a new type of coronary pneumonia, and rescued a patient who had saved other organ functions through a ventilator and ECMO, but whose lungs had been severely atrophied and fibrotic. Hope for more critically ill patients.
From the above, we can see that once the new type of coronary pneumonia develops into severe disease, treatment is very difficult.
In addition to the respiratory failure described in this article, large-scale inflammation has the risk of triggering an "inflammatory storm" and inducing damage to other organs, which is a major test for medical resources and costs.
Therefore, the best method is early intervention and early treatment, especially for the elderly who are susceptible to acceleration, or patients with underlying diseases, and they should intervene as soon as possible before the condition becomes critical to improve the treatment effect.
The new coronavirus epidemic has affected many people's lives, and panic in the face of the unknown is commonplace.
By understanding some medical knowledge, we hope to help us face fear more openly and better understand the development process of human medicine.