High Yield PULMONOLOGY USMLE STEP 1 NOTES A+ latest

High Yield PULMONOLOGY USMLE STEP 1 NOTES EMBRYOLOGY The Lung bud which is also called the respiratory diverticulum is the outgrowth of the foregut. This forms during the 4th week of development. Lung Maturation Stages/Periods Pseudoglandular (5-16 weeks) Canalicular (16-26 weeks) Saccular (26 weeks to birth) Alveolar (after birth) Anatomy Bronchi → Hyaline cartilage Bronchioles → No cartilage. Terminal → respiratory Alveoli → Capillaries and gas exchange. Pseudoglandular Period In this period the lung resembles a gland. There is branching to level of terminal bronchioles. No respiratory bronchioles or alveoli are present. Fetal Respiration Fetal breathing movements occur in utero. The baby aspirates amniotic fluid which stimulates lung development and growth of respiratory muscles. Fetal respiration is important for growth during pseudoglandular phase. Canalicular Period In this period the terminal bronchioles divide and form respiratory bronchioles. Respiratory bronchioles divide into alveolar ducts. Survival after birth is possible at the end of this period. The airway lumens become larger. Type II pneumocytes form in this period. This pneumocyte produces surfactant to lower surface tension and keeps alveoli open. Saccular Period Terminal sacs (primitive alveoli) form. Capillaries multiply in contact with alveoli. Alveolar Period At birth, only about ⅓ of alveoli is present. Following birth there is increasing number of respiratory bronchioles and alveoli. There is continued lung development through age 10. Alveolarization: Airspaces are subdivided and new walls will be formed (septa) Bronchopulmonary Dysplasia Occurs in premature infants and is treated in the NICU. Surfactants, oxygen, mechanical ventilation. Oxygen toxicity and lung trauma. Alveolarization does not progress normally. Respiratory problems during infancy. This often improves during childhood. Pulmonary Hypoplasia This is seen in: Oligohydramnios (Potter’s sequence) Congenital diaphragmatic hernia - This is defective formation of pleuroperitoneal membrane - This leads to a hole in the diaphragm and abdominal organs herniate into chest. If there is herniation in utero it will lead to pulmonary hypoplasia. Often fatal. Bronchogenic Cysts Abnormal budding of foregut. Usually found in the mediastinum. Contain clear fluid but air is seen when it is infected. Bronchogenic cysts do not communicate with lungs. Lined by respiratory epithelium (columnar, ciliated). The walls contain cartilage which is a diagnostic criteria. Often asymptomatic. May lead to pneumonia, compression of airway. Pulmonary Vascular Resistance In utero - PVR is high - Canalicular stage: few/no pulmonary capillaries - Later stages: hypoxemia → vasoconstriction - Umbilical venous blood: PaO2 30 mmHg; O2 saturation = 80% - Only about 10% of cardiac output to lungs At birth - PVR falls significantly - 100% cardiac output through lungs ANATOMY Zones Conducting Zone - No gas exchange - Large airways: nose, pharynx, trachea and bronchi - Filters, warms and humidifies air Respiratory Zone - Gas exchange - Respiratory bronchioles, alveolar ducts and alveoli Mucous Secretions produced by respiratory tracts. The secretions are mostly glycoproteins and water. The mucus is secreted by goblet cells in bronchial walls. Protects against particulates, infection. Beating cilia move mucus to epiglottis → swallowed Alveoli Alveoli are small sacs that is responsible for gas exchange. It is surrounded by capillaries. Alveolar Cells: Pneumocytes Type 1 - Most common (97% of cells) - Thin for gas exchange Type 2 - Produce surfactant - Can proliferate to other cells - key for regeneration after injury Club Cells (bronchioles) - Surfactant - Responsible for detoxification Surfactant When you exhale the alveoli shrinks. The alveoli can collapse and this is called atelectasis due to decreased efficiency gas exchange. Surfactant allows alveoli to avoid collapse. Surfactants are secreted by type 2 pneumocytes. Mix of lecithins. Especially dipalmitoylphosphatidylcholine Fetal Lung Maturity Lungs are “mature” when enough surfactant are present. Occurs around 35 weeks. Lecithin-sphingomyelin ratio (L/S ratio) Both produced equally until approx 35 weeks. Ratio 2.0 in amniotic fluid suggests lungs mature Preterm delivery: betamethasone is used to stimulate surfactant production in lungs. Neonatal Respiratory Distress Syndrome Hyaline membrane disease. Can lead to atelectasis. There is severe hypoxemia/↑pCO2 (poor ventilation). Poorly responsive to O2 because lungs are collapsed (alveoli) and intrapulmonary shunting. Risk Factors - Prematurity - Maternal diabetes: high insulin levels decrease surfactant production - Cesarean delivery: lack of vaginal compression stress leads to reduced fetal cortisol and reduction in surfactant There are many complications in NRDS: 1. Bronchopulmonary dysplasia 2. Patent ductus arteriosus (hypoxia keeps shunt open) 3. Retinopathy of prematurity - Oxygen → free radical formation - Neovascularization in the retina - Retinal detachment → blindness Lobes of the Lung Aspiration Right lung is more common site of aspiration. The right bronchus is wider so there is less angle and more vertical path to lung. If patient is upright: Right inferior lobe to lower portion If supine: - Right inferior lobe → superior portion - Right upper lobe → posterior segment Diaphragm Caval Opening - T8 - IVC Esophageal hiatus - T10 - Esophagus, Vagus nerve Aortic Hiatus - T12 - Aorta, thoracic duct, azygos vein The diaphragm is innervated by C3, C4, C5 (phrenic nerve) Diaphragm irritation → “referred” shoulder pain - Classic example is gallbladder disease. - Also lower lung masses - Irritation can cause dyspnea and hiccups Cut nerve → diaphragm elevation, dyspnea. - “Paradoxical movement” → Moves up with inspiration - Can see on fluoroscopy (“sniff test”) Muscles of Quiet Respiration Diaphragm → inspiration Exhalation is passive with normal (“quiet”) breathing Inspiration (neck) - Scalenes → raise ribs - Sternocleidomastoids → raise sternum Exhalations (abdomen) - Rectus muscle - Internal/external obliques - Transverse abdominis - Internal intercostals Use of accessory muscles in respiratory distress