Acute Respiratory Distress Syndrome
What in the World is ARDS?
Have any of you heard a doctor refer to a patient as having “ARDS?” The first time I heard the term ARDS, I was unfamiliar with the pathophysiology. Now, I work in a medical ICU and frequently take care of patients who are experiencing this respiratory distress disease process. I would like to share with you what I have learned.
Acute Respiratory Distress Syndrome: What is It?
ARDS is a life-threatening form of acute lung injury. There is an initial insult to the alveoli usually caused by either alveolar or vascular damage. This increases the permeability of the capillaries surrounding the alveoli and causes protein-rich fluid to fill the alveolar sacs. This also results in damage to the alveolar cells decreasing surfactant production leading to alveolar collapse. If your alveoli are full of fluid and collapsed, how in the world are you going to get the oxygen that you need? You can see how a patient’s respiratory status can decline very quickly!
According to Medscape, the diagnosis of ARDS is defined by the ratio of the partial pressure of oxygen in the patient’s arterial blood (PaO2) to the fraction of oxygen in the inspired air (FiO2). In ARDS, the PaO2/FiO2 ratio is less than 200, and in acute lung injury, it is less than 300. It is amazing to watch someone admitted to your unit go from a state of respiratory distress to being on maximum support on the ventilator in a matter of a few hours.
Alveoli of a healthy patient (above right) and an ARDS patient (below right)
What Causes It?
ARDS can be caused by direct or indirect insult. According to NYU Langone Medical Center, the direct causes include chest trauma, aspiration of stomach contents, obstructed airways, high altitude disease, tuberculosis, oxygen toxicity, radiation, cardiopulmonary bypass, burns, and breathing smoke, chemicals, or salt water. Indirect causes include severe infection, massive blood transfusion, pneumonia, shock, burn, head trauma, pancreatitis, and overdoses of alcohol or certain drugs.
The most common cause of ARDS that we saw in my ICU this last winter was the swine flu. Many of our patients did not survive which made for a very depressing season. They would require maximum ventilator support for weeks! Family and staff would watch as the patients slowly went into multi-system organ failure despite our interventions.
What Do We Do About It?
There are many interventions we perform for these patients. They are very time intensive and patient status requires constant monitoring. Our main goal is to increase oxygenation. How do we do that?
Increase that PEEP! The vast majority of ARDS patients require mechanical ventilation with a large amount of ventilatory support. One of the settings that can be adjusted depending on the patient is the pulmonary end expiratory pressure (PEEP). Ever wonder how you can get the air knocked out of you? There is a reserve of air left in your lungs even when you have exhaled as much as you can. This is the PEEP. In patients with ARDS, the PEEP is turned up just about as high as you can go without causing a pneumothorax (which can happen). This helps to keep those alveoli inflated. There is always a risk of causing barotrauma (damage) to your alveoli when you increase the PEEP as high as is necessary to maintain oxygenation. We also give low tidal volumes in an attempt to decrease damage to the alveoli.
Can’t oxygenate? Paralyze! Patients are sedated while on such high ventilator settings. Despite this, patients can still be breathing out of sync with the ventilator. In these extreme cases, patients will be placed on a continuous paralyzing IV medication. Our standard medication is Cys-Atricurium. This allows the patient’s body to be completely relaxed and let the ventilator do the work. In order to be sure the patient is at an adequate paralyzed level, we perform a “train-of-four” every few hours which provides four small shocks to the patient. Research “train-of-four” for an interesting read. We don’t want our patients overparalyzed…who knew there was such a thing?!
So What Happens to These Patients?
If a patient survives ARDS, there is a high chance they will have developed pulmonary fibrosis. One patient I cared for, who despite having overcome the acute phase of ARDS, could not maintain their oxygenation because they developed severe fibrosis. It often requires lifelong medications and oxygen therapy to manage fibrosis.
