Overview

ARDS, or acute respiratory distress syndrome, is a type of lung injury in which fluid accumulates in the lungs, preventing the transfer of inhaled oxygen to the bloodstream. Oxygen levels must be maintained at certain levels or the patient will not be able to survive. The syndrome is well known in medical circles, but it came to the public's attention during the COVID-19 pandemic. The severity of the disease is diagnosed using the "Berlin Definition".¹ 

It is classified into mild, moderate, and severe according to the oxygen level in the blood. This helps doctors decide on medical treatment. There is a 'New Global Definition of ARDS' ², but this is only a further development of the Berlin Definition.

In severe cases, medical ventilation is urgently required. It prevents natural breathing and provides oxygen-rich air at a certain pressure. This is usually done while the patient is sleeping on their back. However, in the course of using the treatment, doctors have realised that the effect of mechanical ventilation is much more successful when the patient is lying flat on their stomach, with their back facing upwards.. This position goes back to the 1970s.³

What happens to the lungs in ARDS?

The small air sacs in our lungs, called alveoli, are where the air we breathe and the blood come together. They are no more than two thin layers. For various reasons, such as pneumonia, pancreatitis, blood transfusion, aspiration, opioid overdose, etc., these membranes have either been damaged or fluid has accumulated between them. ⁴This slows the diffusion, the transfer of oxygen into the bloodstream. Mechanical ventilation not only provides sufficient oxygen but also supplies constant high oxygen pressure to the small air sacs, the alveoli, preventing them from collapsing.⁴

How does prone positioning help with ARDS?

Several trials and meta-analyses clearly show that prone positioning saves lives in ARDS. But how does this happen? There are two possible explanations. The first is that gravity determines where blood collects. This is similar to everyday experience: if you stand on your feet for too long, your legs swell because of the pooling of blood. The same is true for the lungs. If someone is lying on their back, the blood tends to pool at the back of the lungs. 

However, in ARDS, most of the oxygen loss occurs at the back of the lung because the weight of the lung on the air-filled alveoli makes the back alveoli much smaller than the front alveoli. The second opinion is that by prone positioning, the distribution of the air in the lungs changes, because of the gravity and weight of the lung. And the trial proves that the second one is more plausible than the first one.⁵ There is a definition in the literature called 'baby lung', which means the part of the lung that contributes effectively to gas exchange. The aim of treatment, whether mechanical ventilation or prone positioning, is to protect and, if possible, enlarge the baby's lung.⁶ The heart itself puts weight on the lungs when you are lying on your back. Prone positioning removes this weight from the lungs. We can think of our lungs as sponges, and for the most effective gas exchange, the maximum volume is desired. It is important to emphasise that mechanical ventilation itself is a stress factor for the air sacs themselves. By applying constant pressure, the membranes or air sacs are iatrogenically damaged by the medical staff. The level of surfactant, which helps to reduce the friction, is reduced by mechanical ventilation.⁷ Prone positioning distributes the stress evenly and protects the baby's lungs. Therefore, patients should be kept prone at least for 12 hours. Other studies suggest that the prone position should last at least 16 hours or more.⁸

When to start proning?

For patients on mechanical ventilation, the ESICM (European Society of Intensive Care Medicine) strongly recommends that proning be started as soon as the patient is stabilised. During the COVID-19 pandemic, patients who were not using mechanical ventilation were placed in a prone position. Whether this affects in-hospital mortality or all-cause mortality is only being studied in ARDS with COVID-19 patients. Data is scarce, although there is a weak recommendation to position patients prone to reduce intubation. Whether it helps with mortality remains to be seen.⁷ It is also worth remembering that the more hypoxic the patient is, i.e., the less oxygen there is in the blood, the more you will benefit from proning. Proning should be started as early as possible, especially in severe ARDS.

What are the possible disadvantages of proning?

Turning an adult patient from supine to prone or vice versa requires 4 or 5 skilled people. It has been reported that during the turning process, the central line, which is a large blood line inserted into one of the major blood vessels, might be removed unplanned. Patients were more often extubated, so mechanical ventilation was removed during turning.

Due to the position of the endotracheal tube, the tube entering the airway could be obstructed by the pressure exerted by the patient. 

In addition, because pressure is applied to the same side of the body for more than 12 hours, these patients are more prone to pressure sores, and facial edema is another side effect.³ Breast injuries, genital injuries, corneal abrasions, or even vision loss might also happen.¹⁰

In some cases, blood circulation may stop during proning. This is not because of the position itself, but because of the course of the disease. One might think that the position is an obstacle, but it is possible to resuscitate even when the patient is prone.

When should it not be used?

The only time it is wrong to give the prone position is when the patient has a concomitant spinal injury. Protecting the integrity and stabilisation of the spine is more important for mortality than prone positioning during ventilation.⁹ In some cases, doctors should weigh up the benefits and harms before deciding to prone patients.

If the patient's blood circulation is not working properly, or if there is a pelvic or other large bone fracture. If there is an open wound in the abdominal area, etc. Patients with rheumatoid arthritis should not be turned until a collar has been applied. Massive obesity and late pregnancy are no obstacle if the positioning is done properly.⁹ Intracranial pressure may increase during the Proning procedure. It should be closely monitored, and the position of the head and neck should be protected. This is especially important in older people with vascular problems.¹⁰

Summary

Respiratory distress syndrome (ARDS) is a condition in which the oxygenation of the blood is obstructed either by damage to the air sac membranes or by fluid accumulation between the membranes. It is classified as mild, moderate, or severe depending on the level of oxygenation of the blood. In this group of patients, mechanical ventilation can be life-saving. 

The principle of the mechanism is to apply a constant pressure, so that the air sacs will remain open,  as oxygen will be supplied. Patients are sedated and laid on their backs. There is evidence that keeping patients in the prone position reduces mortality, especially in severe ARDS. The prone position relieves pressure on the air sacs, which are closer to the back, thus increasing their ventilation. The heart itself puts pressure on the lungs underneath, so the prone position also relieves the pressure and the symptoms. 

It should be used as soon as possible and, if possible, for more than 12 hours a day. During the COVID-19 period, it was also used in patients without mechanical ventilation and was helpful to some extent. The procedure itself can be challenging and requires skilled caregivers. It cannot be used in some cases, such as patients with spinal injuries.