AssessmentsAcute respiratory distress syndrome: Clinical practice
USMLE® Step 1 style questions USMLE
USMLE® Step 2 style questions USMLE
A 74-year-old Caucasian woman presents to her family doctor with a 1-day history of tarry, black stools. She is somewhat confused, but says she has passed multiple bowel movements with this appearance in the last 24 hours. Her past medical history is significant for atrial fibrillation, for which she takes warfarin. Her laboratory results demonstrate an INR of 10.1 and a hemoglobin value within the normal range. Her family doctor, suspecting an upper gastrointestinal bleed, sends the patient to hospital. In the hospital, she is administered vitamin K, coagulation factors, and fresh frozen plasma. Four hours later, the patient develops chest pain and dyspnea. Her chest X-ray is pictured below.
Which of the following is the most likely cause for her shortness of breath?
Content Reviewers:Rishi Desai, MD, MPH
The main site of injury in ARDS is the alveolar-capillary membrane.
Now, any damage to the alveolar epithelium or the capillary endothelium increases the permeability of the alveolar-capillary membrane, causing fluid to move into the alveoli.
Oxygen and carbon dioxide have to travel across this fluid, so it acts as a barrier against normal gas exchange.
The fluid also dilutes out the surfactant molecules coating the alveoli, and as a result the alveoli are less able to remain open and compliant, so they become stiff.
If the injury continues, the alveoli eventually collapse.
Now, the pulmonary edema from ARDS causes the same problems as pulmonary edema from congestive heart failure, but because the triggering events are different, the term non-cardiogenic pulmonary edema is often used for ARDS.
Now, ARDS is not a primary lung disease, rather it arises as a complication of a systemic injury that causes widespread inflammation which results in damage to the alveolar-capillary membranes within the lung.
The most common underlying systemic cause of ARDS is sepsis, which causes systemic inflammation in response to an infection.
But other insults include trauma, severe burns, near-drowning, disseminated intravascular coagulation or DIC, acute pancreatitis, massive blood transfusions, aspiration of gastric contents, and toxic smoke inhalation.
The list basically includes any serious injury that directly or indirectly affects the entire body.
So, as you would guess, individuals with ARDS are very sick, and have severe shortness of breath, tachypnea, hypoxemia, and diffuse crackles on auscultation of the chest. At that point, it’s important to look for an underlying cause, like a pneumonia, urinary tract infection, or an infected intravenous line - all of which might result in sepsis.
ARDS is clinically diagnosed by fulfilling the following criteria: the symptoms developed within 1 week of the suspected injury, a chest x-ray shows diffuse bilateral opacities, the symptoms are not fully explained by congestive heart failure, and the ratio of the partial pressure of oxygen in the arteries or PaO2, to the fraction of inspired oxygen, or FiO2 is less than 300 mmHg.
The first two criteria are usually straightforward.
First, you can measure the serum brain-natriuretic peptide, or BNP levels, which are elevated in congestive heart failure.
Third, you can insert a Swan-Ganz catheter and measure the pulmonary capillary wedge pressure, or PCWP, which is a reflection of the pulmonary venous or the left atrial pressure.
The PCWP is increased in heart failure, typically over 18 mmHg.
The PCWP increases because blood backs up into the atria and subsequently into the pulmonary veins, increasing the hydrostatic pressure in those veins.
In ARDS, BNP levels, the ejection fraction and myocardial relaxation, and PCWP are all normal.
Now, measurement of the PCWP is the most accurate in differentiating ARDS from heart failure, but it’s not routinely done because it’s invasive.
For criteria 4, an arterial blood gas or ABG is needed to calculate the PaO2 to the FiO2 ratio.
The PaO2 is measured in millimeters of mercury, while the FiO2 is measured as a decimal between 0.21, which is the FiO2 of the atmospheric air, and 1, which is the maximum FiO2 you can artificially give an individual.
In ARDS, the edema in the alveoli prevents adequate ventilation, and some blood ends up in the left atrium without being oxygenated.