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Respiratory-First-Aid-For-The-Usmle-Step-1
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First Aid for the USMLE Step 1 2020, Thirtieth
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,HIGH-YIELD SYSTEMS
Respiratory
“There’s so much pollution in the air now that if it weren’t for our lungs, ` Embryology 660
there’d be no place to put it all.”
—Robert Orben ` Anatomy 662
“Freedom is the oxygen of the soul.” ` Physiology 664
—Moshe Dayan
` Pathology 671
“Whenever I feel blue, I start breathing again.”
—L. Frank Baum ` Pharmacology 686
“Life is not the amount of breaths you take; it’s the moments that take
your breath away.”
—Will Smith, Hitch
Group key respiratory, cardiovascular, and renal concepts together
for study whenever possible. Know obstructive vs restrictive lung
disorders, V̇/Q̇ mismatch, lung volumes, mechanics of respiration, and
hemoglobin physiology. Lung cancers and other causes of lung masses
are high yield. Be comfortable reading basic chest x-rays, CT scans, and
PFTs.
659
,660 SEC TION III RESPIRATORY `R̀ESPIRATORY—EMBRYOLOGY
``
RESPIRATORY—EMBRYOLOGY
Lung development Occurs in five stages. Initial development includes development of lung bud from distal end of
respiratory diverticulum during week 4. Every Pulmonologist Can See Alveoli.
STAGE STRUCTURAL DEVELOPMENT NOTES
Embryonic Lung bud trachea bronchial buds Errors at this stage can lead to
(weeks 4–7) mainstem bronchi secondary (lobar) tracheoesophageal fistula.
bronchi tertiary (segmental) bronchi.
Pseudoglandular Endodermal tubules terminal bronchioles. Respiration impossible, incompatible with life.
(weeks 5–17) Surrounded by modest capillary network.
Canalicular Terminal bronchioles respiratory bronchioles Airways increase in diameter.
(weeks 16–25) alveolar ducts. Surrounded by prominent Respiration capable at 25 weeks.
capillary network. Pneumocytes develop starting at 20 weeks.
Saccular Alveolar ducts terminal sacs. Terminal sacs
(week 26–birth) separated by 1° septae.
Alveolar Terminal sacs adult alveoli (due to 2° At birth: 20–70 million alveoli.
(week 36–8 years) septation). By 8 years: 300–400 million alveoli.
In utero, “breathing” occurs via aspiration
and expulsion of amniotic fluid vascular
resistance through gestation.
At birth, fluid gets replaced with air in
pulmonary vascular resistance.
Embryonic Fetal Postnatal
period period period
Alveolar
Saccular
Canalicular BIRTH
Pseudoglandular
Embryonic Surfactant
2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 2 4 6 8
Weeks Years
L
R
Congenital lung malformations
Pulmonary hypoplasia Poorly developed bronchial tree with abnormal histology. Associated with congenital diaphragmatic
hernia (usually left-sided), bilateral renal agenesis (Potter sequence).
Bronchogenic cysts Caused by abnormal budding of the foregut and dilation of terminal or large bronchi. Discrete,
round, sharply defined, fluid-filled densities on CXR (air-filled if infected). Generally
asymptomatic but can drain poorly, causing airway compression and/or recurrent respiratory
infections.
, RESPIRATORY `R̀ESPIRATORY—EMBRYOLOGY SEC TION III 661
Club cells Nonciliated; low columnar/cuboidal with secretory granules. Located in bronchioles. Degrade
toxins; secrete component of surfactant; act as reserve cells.
Alveolar cell types
Type I pneumocytes Squamous. 97% of alveolar surfaces. Thinly line
the alveoli (two black arrows in A ) for optimal
gas exchange.
Type II pneumocytes Cuboidal and clustered A . 2 (surface tension)
Collapsing pressure (P) =
A
2 functions: radius
Type II pneumocyte 1. Serve as stem cell precursors for 2 cell Law of Laplace—Alveoli have tendency to
types (type I and type II cells); proliferate collapse on expiration as radius .
during lung damage.
Type I 2. Secrete surfactant from lamellar bodies
(arrowheads in B )
Surfactant— alveolar surface tension,
B alveolar collapse, lung recoil, and
compliance.
Composed of multiple lecithins, mainly
dipalmitoylphosphatidylcholine (DPPC).
Synthesis begins ~week 20 of gestation
and achieves mature levels ~week 35.
Corticosteroids important for fetal surfactant
synthesis and lung development.
Alveolar macrophages Phagocytose foreign materials; release cytokines
and alveolar proteases. Hemosiderin-laden
macrophages (“HF cells”) may be found in
the setting of pulmonary edema or alveolar
hemorrhage.
Neonatal respiratory Surfactant deficiency surface tension Screening tests for fetal lung maturity: lecithin-
distress syndrome alveolar collapse (“ground-glass” appearance sphingomyelin (L/S) ratio in amniotic fluid
A of lung fields) A . (≥ 2 is healthy; < 1.5 predictive of NRDS), foam
Risk factors: prematurity, maternal diabetes (due stability index, surfactant-albumin ratio.
to fetal insulin), C-section delivery ( release Persistently low O2 tension risk of PDA.
of fetal glucocorticoids; less stressful than 3
vaginal delivery). Mature
Concentration (mg %)
Treatment: maternal steroids before birth; 15
2
in
ith
exogenous surfactant for infant.
L/S ratio
Lec
10 Transitional
Therapeutic supplemental O2 can result in
tio Sph 1
Retinopathy of prematurity, Intraventricular 5 L/S ra ingo
mye
lin Immature
hemorrhage, Bronchopulmonary dysplasia
(RIB). 20 26 30 35 40
Gestational age (wk)
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