Welcome to LY Med, where I go over everything you need to know for the USMLE STEP 1, with new videos every day.
Follow along with First Aid, or with my notes which can be found here: https://www.dropbox.com/sh/fa2307zt7970c19/AADE66sTZnvNjYpMR3ueKDhHa?dl=0
This video will transition us from restrictive lung diseases to emergency lung conditions. A restrictive lung disease AND emergency lung condition is ARDS - or acute respiratory distress syndrome. This is diffuse bilateral inflammation of your lungs and is seen in sick patients from a variety of etiologies, some including sepsis, shock, aspiration, pneumonia, and pancreatitis. When there is inflammation, it causes vascular permeability and pulmonary edema. Also damage to the alveoli can lead to hyaline thickening as well as damage to the pneumocytes can lead to fibrosis and loss of surfactant. CXR will show diffuse opacities. Now you must make sure this edema is from the lungs and not from heart failure. Some labs that show this include PaO2/FiO2 ratio and PCWP measurement. Abnormal findings in these three investigations leads to the diagnostic criteria of ARDS! Treatment is via low-tidal volume ventilation and PEEP.
Now you can also have fluid in your pleural cavity- we call this pleural effusion. Usually pleural fluid gets reabsorbed by your lymphatics. Too much can overwhelm our ability to reabsorb it. Chest x-ray will show blunting of the costophrenic angle. Now what causes this increase in fluid? Causes include changes in capillary pressures, like increase in hydrostatic pressure, or decreased oncotic pressure like in HF, nephrotic syndrome or cirrhosis. This type of fluid is called transudate. Another mechanism of fluid is during inflammation. Inflammatory cells cause permeability, release a ton of proteins and cytokines and these can increase fluid. This is call exudate and is seen in infections, TB, and cancer. Lastly, another mechanism is leaky lymphatics or chylothorax. This fluid is high in triglycerides. How can we tell if the fluid is transudative or exudative? Well we''ll do a thoracentesis and look for proteins, in particular LDH. In transdates, LDH of the effusion will be low. In exudates, LDH of the effusion will be high.
Next topic! Let's talk about pneumothorax. This is when there isn't fluid in your pleural space, but air instead. This air can push the lung away from the pleura and signs include unilateral chest pain and expansion. Causes include the popping of pleural blebs. This makes a hole in the lung, which equalizes the negative pressure and your lung collapses. This is called spontaneous pneumothorax. Your trachea will move towards the collapsed lung. Another cause of pneumothorax is tension pneumothorax. Again you have a hole, but in particular, a valve which allows air to build and push your lungs and trachea and IVC away. This causes the trachea to deviate away from the pneumothorax, as well as JVD and hemodynamic collapse. Treatment is via decompression.
Our last topic is on pulmonary embolisms! A pulmonary embolism is a blood clot in the lungs and is usually from a clot from the superficial deep veins of the legs. The Virchows triad of clots include blood stasis, hypercoagulability, and endothelial damage - all of which predispose you to clots, deep vein thrombosis, and pulmonary embolus! We want to catch the DVT while it's still in the legs. Presentation will show leg pain and swelling and ultrasound will show a clot. Treat with anticoagulation like heparin and warfarin. If it's too late, you may get a PE, and signs include chest pain, tachycardia and tachypnea. Confirm with a CT scan and anticoagulate. Now there are some special causes of pulmonary embolisms. These include:
Fat embolism: often due to long bone fractures and bone marrow may leak and lead to a fat embolism. Particular signs include petechiae and neurological decline.
Air emboli: in particular nitrogen. Seen in deep sea divers and with patients who have indwelling lines.
Bacteria: seen in infective endocarditis, especially in right heart tricuspid involvement (seen in IV drug users).
Amniotic fluid: seen in pregnancy and obstetric patients. This causes wide spread inflammation similar to sepsis. Patients get DIC, ARDS, and mortality rate is high.
Done with this topic - moving on!
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