Respiratory distress syndrome: Pathology review

Definitions & Key takeaways

Respiratory distress syndrome (RDS) is a respiratory condition in which the alveoli collapse due to the deficiency of the surface-active substance called surfactant. Collapsed alveoli make it difficult to breathe and get enough oxygen. Acute respiratory distress syndrome happens when inflammation causes diffuse alveolar injury and pulmonary edema. This edema can wash away the surfactant coating the alveoli to the point where it causes the alveoli to collapse. There is also neonatal respiratory distress syndrome, which mostly affects premature infants, whose lungs are not fully developed and lack enough surfactant.

Symptoms of respiratory distress syndrome include rapid breathing, grunting, and flaring of the nostrils while breathing, as well as cyanosis (bluish color of the skin) and difficulty feeding. All forms of respiratory distress can lead to respiratory failure and death if not treated promptly. Treatment includes providing respiratory support, and administering surfactants, especially in neonatal respiratory syndrome. The use of corticosteroids before delivery may also be considered to improve lung function in some cases.

Chapters:

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Case Study0:00–1:20

Two people are admitted to the emergency department. Mike, a 55-year-old man, presents with shortness of breath, high fever, and cough.
A chest x-ray was ordered and it showed a right lower lobe infiltrate, which is suggestive of pneumonia. He was then started on IV antibiotics but the following day Mike became hypoxic and hypotensive.
Because his hypotension didn’t improve despite intubation, IV fluids, and vasopressors, he is diagnosed with septic shock.
Next, a repeat x-ray detected newly-developed bilateral alveolar opacities, heart echography ruled out heart failure, and arterial blood gas analysis revealed a PF ratio of 109 milligrams Mercury.
Then there was Dona, an infant delivered by cesarean section at 36 weeks’ gestational age, with an Apgar score of 9 at birth.
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.

Physiology1:20–2:10

But first, a bit of physiology. Normally, when you breathe in, the air reaches the alveoli, which are made up of two types of pneumocytes.
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. This let it act like droplets of oil that coats the inside of the alveoli, decreasing surface tension, so if it’s missing, the alveoli will collapse.
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.

ARDS2:10–3:33

ARDS is not a primary disease, as it is usually triggered by conditions like sepsis, aspiration, trauma, and pancreatitis.
Now ARDS starts when these conditions cause alveolar damage, and a high yield fact is that the injury triggers the pneumocytes to secrete inflammatory cytokines like TNF-alpha and interleukin 1.
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.
You’ll need to know that the main site of injury is the alveolar-capillary membrane, which becomes more permeable, causing fluid to move into the alveoli resulting in pulmonary edema.
This fluid can impair gas exchange, leading to hypoxemia. Furthermore, the edema can also wash away the surfactant coating the alveoli to the point where it can’t reduce surface tension anymore, and as a result, the alveoli collapse.
And finally, dead cells and protein-rich fluid start to pile up in the alveolar space and, over time, it forms these waxy hyaline membranes which look like a layer of glassy material.

Symptoms3:33–4:19

Individuals with ARDS present with serious symptoms and signs that require urgent investigation. The inflammation process and impaired gas exchange lead to fever, shortness of breath, tachypnea, chest pain, hypotension, hypoxia, and cyanosis.
More often than not, ARDS will lead to shock due to hypotension. The excess fluid in the lungs can cause a crackling sound called rales during auscultation, which is the sound of collapsed alveoli popping open with inspiration.
Keep in mind additional symptoms might provide clues to the underlying cause. For example, epigastric abdominal pain radiating to the back along with a history of gallstones indicate acute pancreatitis.

Diagnosis4:19–6:00

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. Second, and particularly high yield, a chest X-Ray or CT scan shows opacities or “white out” in both lungs, which is due to pulmonary edema.
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.
Another clue is the pulmonary capillary wedge pressure, which is measured by inserting a catheter into a small pulmonary arterial branch.
In heart failure, this is elevated because more blood remains in the left side of the heart and it prevents pulmonary venous return.
The blood backs up into the pulmonary vessels, and the increase in pressure pushes fluid into the interstitium of the lungs, resulting in edema.
In ARDS, the pressure is normal since the edema is caused by leaky capillaries instead of increased pressure. Treatment of ARDS ultimately comes down to treating the condition that triggered it.

Treatment6:00–7:16

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. Another important thing to watch out for is positive pressure ventilation can cause compression of pulmonary vessels which leads to pulmonary hypertension decreased pulmonary venous return.
This will reduce cardiac output and hypotension might worsen. Now, even with supportive care, the macrophages clean up old cell debris attract and activate fibroblasts, which are cells that secrete collagen and form scar tissue in the alveolar walls.
If there’s enough scar tissue, it can still lead to a decrease in lung compliance, or the ability of the lungs to expand and come back to its original size.
Remember that this means the individual will have residual symptoms all their lives. Next, neonatal respiratory distress syndrome is a disease of the newborn caused by a deficiency of surfactant.

Neonatal RDS7:16–8:09

This results in alveoli that can’t stay open and the newborn has to work hard to breathe, resulting in the development of progressive and diffuse atelectasis.
Now, there are some risk factors associated with the disease that are commonly tested. First, there’s prematurity, usually because the lungs are not mature enough to produce surfactant.
Next, maternal diabetes can cause increased insulin levels in the infant which interferes with surfactant production. C-section delivery is another cause.
Normal birth is a very stressful process for the infant and this increases glucocorticoid levels, which causes the pneumocytes to release more surfactant.
During a C-section, there’s no boost of glucocorticoids which can cause a deficiency of surfactant. For symptoms, at birth, the newborn might be asymptomatic because they receive oxygen via the umbilical cord before birth.

Symptoms8:09–8:56

A few hours later, initial clinical signs like dyspnea, tachypnea, tachycardia, and hypoxemia will develop. Eventually, respiratory failure will manifest and they baby will present with chest wall retractions, expiratory grunting, nasal flaring or nasal widening while breathing.
Eventually they might become cyanotic. Another fact you need to remember is that since the O2 pressure in the blood will be lower than normal, the ductus arteriosus might not close.
A small patent ductus arteriosus, or PDA, might not cause additional symptoms, but a large one might lead to heart failure.
Now, you’ll be able to diagnose neonatal respiratory distress syndrome by chest radiography or CT, where typical findings include low lung volume and the classic diffuse, reticulogranular ground glass appearance.

Diagnosis 8:56–10:01

This shows up as a contrast between the black, aerated alveoli and the white or gray ones where there’s alveolar atelectasis.
Another feature is air bronchogram where the air-filled bronchi appears dark in contrast to the surrounding white or gray atelectatic tissue.
Arterial blood gasses usually show hypoxemia and hypercapnia. Now, we can also assess lung maturity before the baby is born with amniocentesis, where a sample of amniotic fluid is drawn to measure the lecithin-sphingomyelin ratio in amniotic fluid, both of which are surfactant components.
A ratio under 1.5 is predictive of neonatal respiratory distress syndrome. Other tests include the foam stability index, and surfactant-albumin ratio, which are similar to the lecithin-sphingomyelin ratio test.

Treatment10:01–10:47

Regarding treatment, something essential to remember is that antenatal corticosteroid therapy should be administered to all pregnant individuals at 23 to 34 weeks gestation who are at increased risk of preterm delivery.
This is done to prevent or decrease the severity of the syndrome. In newborns without respiratory failure, nasal continuous positive airway pressure is the preferred initial intervention.
However, keep in mind that some complications of supplemental oxygen can include retinopathy of prematurity, intraventricular hemorrhage, and bronchopulmonary dysplasia.
If this fails, endotracheal intubation and intratracheal surfactant therapy is needed. All right, as a quick recap, Acute respiratory distress syndrome happens when inflammation causes diffuse alveolar injury and pulmonary edema.

Review10:47–11:50

The four criteria of ARDS are: it develops within a week, affects both lungs, causes the PF ratio to dip below 300 mmHg, and is not due to heart failure or other cardiac causes.
Treatment includes supplemental oxygen and mechanical ventilation. Neonatal respiratory distress syndrome is caused by a deficiency of surfactant.
Often this is due to prematurity, maternal diabetes, or delivery through C-section. Diagnosis is based on a clinical picture of the infant with the onset of progressive respiratory failure and chest radiography showing low lung volume, diffuse reticulogranular ground glass appearance, and air bronchogram.
Treatment begins with nasal continuous positive airway pressure. If this fails, endotracheal intubation and intratracheal surfactant therapy is needed.

Summary11:50–13:07

Now back to our cases, So Mike presents with acute onset of shortness of breath, high fever, and cough, which together with the chest x-ray showing a right lower lobe infiltrate, led to a diagnosis of pneumonia.
He was then started on intravenous antibiotics but his respiratory symptoms only got worse. The next day, Mike developed hypoxemia and septic shock despite appropriate treatment.
The x-rays detected newly-developed bilateral alveolar opacities, heart echography ruled out heart failure, and arterial blood gas analysis revealed a severely decreased PF ratio.
These four factors means Mike met all the criteria needed for an ARDS diagnosis. Dona was delivered prematurely by C-section but she initially had a good Apgar score.
However, soon after delivery, she develops signs of respiratory distress, tachypnea and subcostal retractions with nasal flaring, and tachycardia.
Given her history and risk factors, a diagnosis of neonatal respiratory distress syndrome should be high on the differential list.
This was actually confirmed by a chest x-ray showing the classic a reticulogranular ground glass appearance with air bronchograms.
Respiratory distress syndrome: Video and Causes | Osmosis