2.19 Pleural Effusions: Transudate vs Exudate

Pulmonary system review for the USMLE Step 1 Exam

• Pleural effusion is an abnormal accumulation of fluid in the pleural cavity.
• Pleural effusions can be characterized as either exudative or transudative based on the content of the extra fluid.
• Exudative effusions have high protein and lactate dehydrogenase content, while transudative effusions have low protein and lactate dehydrogenase content.
• Pleural exudates are commonly caused by lung infections, lung cancer, and inflammatory diseases.
• Pleural transudates are caused by increased hydrostatic pressures or decreased oncotic pressure within the lung capillaries that force fluid into the pleural cavity.
• Transudative effusions are not caused by inflammation, so lactate dehydrogenase content is not expected to be high.
• Proteins are too big to move between cells, so transudative effusions have low protein content.

2.18 Digital Clubbing

Pulmonary system review for the USMLE Step 1 Exam

• Digital clubbing is the enlargement of the ends of fingers and toes due to growth of connective tissue between the nail matrix and the distal portion of the digit.
• It is defined quantitatively as an increased angle between nail bed and nail plate (> 180°).
• Digital clubbing is caused by an interaction between platelets, megakaryocytes, and the growth factors platelet derived growth factor (PDGF) and vascular endothelial growth factors (VEGF).
• Diseases associated with digital clubbing include lung cancer, cystic fibrosis, idiopathic pulmonary fibrosis, bronchiectasis, and an empyema.
• Digital clubbing is usually not associated with asthma or COPD.

2.17 Pickwickian Syndrome

Pulmonary system review for the USMLE Step 1 Exam

• Pickwickian Syndrome is also called obesity hypoventilation syndrome
• Characterized by daytime hypoventilation leading to hypercapnia
• Often occurs in individuals with a BMI over 30
• Excess fat puts pressure on lungs and reduces ability to fully inflate
• Reduction in ventilatory drive may also cause hypercapnia
• Typical patient presents with obesity, daytime sleepiness, and symptoms consistent with obstructive sleep apnea
• Diagnosis confirmed by arterial blood gas
• Short term treatment with positive airway pressure therapy
• Long term treatment involves lifestyle changes to lose weight
• CPAP machine supplies positive pressure to reduce CO2 build up
• Weight loss relieves symptoms by reducing excess weight on lungs.

2.16 Sleep Apnea

Pulmonary system review for the USMLE Step 1 Exam

• Sleep apnea is the repeated severe decrease or cessation of airflow into the lungs for more than 10 seconds during sleep, which causes the person to stop breathing for more than 10 seconds while they’re sleeping.
• There are two types of sleep apnea: obstructive sleep apnea and central sleep apnea.
• Obstructive sleep apnea occurs when the airway collapses during sleep, and it is more likely to be associated with snoring, gasping, and choking.
• Central sleep apnea occurs when breathing cessations are caused by decreased central nervous system respiratory drive, and it is more likely to be associated with Cheyne-Stokes respirations.
• Sleep apnea can cause hypertension, pulmonary hypertension, cardiac arrhythmias, and even sudden death.
• The most common symptom patients will present with is daytime sleepiness, and obesity is associated with obstructive sleep apnea.
• Diagnosis relies heavily on testing, and the gold standard for diagnosis of sleep apnea is a sleep study (a laboratory polysomnography).

2.15 Pulmonary Hypertension

Pulmonary system review for the USMLE Step 1 Exam

• Pulmonary hypertension is elevated blood pressure within the pulmonary arteries at rest, defined as a mean pulmonary artery pressure above 20 mmHg.
• Primary pulmonary hypertension is high blood pressure in the pulmonary arteries that is not caused by some underlying disease, while secondary pulmonary hypertension is high blood pressure caused by an underlying condition.
• Common underlying conditions for secondary pulmonary hypertension include heart failure, chronic thromboembolic disease, lung disease, and autoimmune disease.
• Pulmonary hypertension can lead to right heart failure and eventually death.
• Treatment for pulmonary hypertension involves treating the underlying disease and using drugs such as PDE-5 inhibitors, endothelin receptor antagonists, and prostacyclin to dilate the vessels in the lungs and lower the pressure within the pulmonary vessels.

2.14 Acute Respiratory Distress Syndrome (ARDS)

Pulmonary system review for the USMLE Step 1 Exam

• Acute respiratory distress syndrome (ARDS) is an acute, diffuse, inflammatory form of lung injury that is life-threatening for patients.
• ARDS is a response from the lungs to severe injury, most commonly caused by sepsis, but can also be caused by pneumonia, trauma, drowning, toxin inhalation, and pancreatitis.
• The lung injury from sepsis causes systemic inflammation, damaging lung tissue and causing diffuse alveolar damage, reducing surfactant production and causing alveolar edema.
• Patients with ARDS will present with progressive cough, progressive dyspnea, and hypoxia, and usually deteriorate quickly, 12-24 hours after symptom onset.
• Diagnosis of ARDS is based on imaging findings of bilateral lung infiltrates seen as white-out on chest X-ray and ground glass opacities on CT, as well as a decreased Pao2/Fio2 ratio to less than 300.
• Patients with ARDS are placed on mechanical ventilation with low tidal volume and positive end expiratory pressure (PEEP) to keep the alveoli open at the end of the respiratory cycle.
• Treatment of ARDS involves addressing the underlying cause of the disease, such as sepsis or pneumonia.

2.13 Sarcoidosis

Pulmonary system review for the USMLE Step 1 Exam

• Sarcoidosis is a multisystem inflammatory disorder with non-caseating granulomas in various organs
• Most commonly affects the lungs and causes restrictive lung disease
• Classic patient is a young black female in her 20-30s, but can affect any gender or race
• Symptoms vary depending on which organs are affected, and can include uveitis, skin lesions, joint pain, and swollen lymph nodes
• Bilateral hilar lymphadenopathy is a classic chest X-ray finding
• Restrictive lung pattern on spirometry with normal FEV1/FEV ratio and decreased total lung capacity
• Bells palsy can be a rare symptom caused by inflammation of the facial nerve (CN VII) and external compression by granulomatous mass
• Non-progressive sarcoidosis doesn't need treatment, but patients with respiratory symptoms or bells palsy can be given immunosuppressants like corticosteroids or methotrexate.

3.21 H. Pylori

Microbiology review for the USMLE Step 1 exam

• Helicobacter Pylori (H. Pylori) is a gram-negative, comma-shaped bacteria that causes chronic gastritis
• Estimated that 50% of people worldwide are infected with H. Pylori; higher in developing countries, lower in developed countries
• More common in populations with lower socioeconomic status and in crowded conditions with poor hygiene
• Spread through oral-oral or fecal-oral routes
• Colonizes the antrum of the stomach
• Produces urease that converts urea into carbon dioxide and ammonia, helping it survive in the acidic environment of the stomach
• Secretes toxins that cause inflammation of the gastric epithelium and lead to gastritis and ulcer formation
• Most people with H. Pylori never have symptoms, but some may present with dyspepsia, abdominal pain, nausea, vomiting, gastritis, or peptic ulcer disease
• Duodenal ulcers often associated with H. Pylori or heavy NSAID use
• Diagnosis can be made through urea breath test, stool antigen test, or endoscopy with gastric biopsy
• Treated with a triple drug regimen of a proton pump inhibitor, clarithromycin, and amoxicillin or metronidazole (triple therapy)
  • PCAM

3.20 Vibrio Species

Microbiology review for the USMLE Step 1 Exam

• Vibrio species are gram negative comma-shaped rods with a single flagellum, allowing them to move quickly
• Two main species that cause human infections: Vibrio cholerae and Vibrio vulnificus
• Vibrio cholerae causes foodborne disease called cholera through cholera toxin secretion, spread through the fecal-oral route in developing countries
• Cholera is characterized by watery, mucinous diarrhea, if left untreated can lead to hypovolemic shock and high mortality
• Treatment involves rapid rehydration and electrolyte replacement, antibiotics may be used for severe cases
• Vibrio vulnificus causes severe skin infections acquired through exposure of open wounds in marine environments
• Symptoms include cellulitis, swelling, blister formation, ulceration, and hemorrhagic bullae

3.19 Campylobacter Jejuni

Microbiology review for the USMLE Step 1 exam

• Campylobacter jejuni is a gram-negative, comma-shaped bacteria that causes diarrheal illness
• It is the most common cause of bacterial diarrhea in the US
• It can be distinguished from other gram-negative, comma-shaped bacteria by its desired culture temperature of 42C/107F
• It is mostly acquired through consumption of raw milk, undercooked poultry, and contaminated water
• 90% of cases in the US occur during the summer months, likely due to increased grilling of poultry
• Symptoms include high fever, rigors, dizziness, body aches, diarrhea (more than 10 stools a day), abdominal cramping, and nausea
• Complications include Guillain-Barré syndrome (GBS) and reactive arthritis
• Diagnosis is typically made through a stool culture
• Treatment typically includes supportive care and replenishing fluids and electrolytes, with antibiotics (macrolides) used for more severe cases.