AssessmentsRespiratory distress syndrome: Pathology review
USMLE® Step 1 style questions USMLE
A 1-day-old boy is brought to the intensive care unit from the nursery due to increased work of breathing. The patient was born at 31 weeks to a mother with a history of multiple preterm deliveries, polysubstance abuse and HIV. His temperature is 38°C (100.4°F), pulse is 215/min, respirations are 76/min, blood pressure is 60/41 mmHg, and oxygen saturation is 85% on room air. Physical exam shows tachypnea, nasal flaring, and subcostal retractions. Administration of supplemental oxygen and positive pressure ventilation improve the patient's oxygen saturation to 95%. Blood glucose is 95 mg/dL. Chest x-ray and laboratory results are shown below:
Reproduced from: Wikipedia
|Blood Gases, Serum|
Content Reviewers:Yifan Xiao, MD
Contributors:Victoria Cumberbatch, Evan Debevec-McKenney, Daniel Afloarei, MD, Ursula Florjanczyk, MScBMC
Two people are admitted to the emergency department.
A few hours after delivery, she develops tachypnea, chest wall retractions with nasal flaring, and tachycardia.
Aside from increased work of breathing, her physical examination findings are normal.
A chest x-ray was ordered and it showed diffuse reticulogranular ground glass appearance with air bronchograms.
Now, both people are in respiratory distress.
But first, a bit of physiology.
First, type I pneumocytes are thin, and have a large surface area that that facilitate gas exchange.
More important for the exams are the type II pneumocytes, which are smaller, thicker and have the ability to proliferate in response to lung injury.
They are in charge of making a fluid called surfactant which contains various phospholipids.
These cells also act like stem cells, meaning they can give rise to type I cells and type II pneumocytes.
Ok so acute respiratory distress syndrome, or ARDS, is characterized by rapid onset of widespread inflammation in the lungs which can lead to respiratory failure.
This subsequently leads to neutrophil recruitment, and they will release toxic mediators, like reactive oxygen species and proteases, which will damage the lungs even more.
Individuals with ARDS present with serious symptoms and signs that require urgent investigation.
More often than not, ARDS will lead to shock due to hypotension.
Keep in mind additional symptoms might provide clues to the underlying cause.
Diagnosis of ARDS is typically made when the individual presents all of the next four criteria, which you should definitely remember for your exams.
First, the symptoms have to be “acute” meaning an onset of one week or less.
The third is what’s called the PF ratio. It’s the partial pressure of oxygen in the arterial blood divided by the percent of oxygen in the inspired air, also called the fraction of inspired oxygen.
In ARDS, gas exchange is defective so the PF ratio is below 300 mmHg, and the lower this ratio gets, the more severe the condition.
Fourth, the respiratory distress must not be due to cardiac causes, like heart failure.
Often this is assessed by using an echocardiogram to look for evidence of heart failure, like an ejection fraction below 55% in systolic heart failure, and abnormal relaxation of the myocardium in diastolic heart failure.
Treatment of ARDS ultimately comes down to treating the condition that triggered it.
However, the most important initial step is supportive care, like supplemental oxygen or mechanical ventilation.
A high yield fact to remember is that it’s vital to maintain positive end-expiratory pressure, which is where the pressure in the lungs is kept slightly above atmospheric pressure, even after exhalation, because this prevents the alveoli from collapsing.
It’s also good to have low tidal volumes to prevent over-inflation of the damaged alveoli.
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