First Aid USMLE STEP 1: Respiratory

How many periods does it take to develop the lung?

What are they?

5

1) Embryonic (weeks 4-7)

2) Pseudoglandular (weeks 5-16)

3) Canalicular (weeks 16-26)

4) Saccular (weeks 26-birth)

5) Alveolar (weeks 32-8 years)

Describe the embryonic stage

Lung bud-> trachea-> mainstem bronchi-> secondary (lobar) bronchi-> tertiary (segmental) bronchi

**Errors at this stage can lead to TE fistula

Describe the Pseudoglandular stage

Endodermal tubules-> terminal bronchioles. Surrounded by modest capillary network

**Respiration impossible, incompatible with life

Describe the Canalicular stage

Terminal bronchioles-> respiratory bronchioles-> alveolar ducts.

Surrounded by prominent capillary network

Describe the Saccular stage

Alveolar ducts-> terminal sacs.

Terminal sacs separated by primary septae. Pneumocytes develop.

Describe the Alveolar stage

Terminal sacs-> adult alveoli (d/t secondary septation).

In utero, "breathing" occurs via aspiration and expulsion of amniotic fluid-> increase in vascular resistance through gestation. At birth, fluid gets replaced w/ air-> decrease in pulmonary vascular resistance

**At birth: 20-70 million alveoli

**By 8 years: 300-400 million alveoli

What are the 2 congenital lung malformations & describe them

1) Pulmonary hypoplasia= poorly developed bronchial tree w/ abnormal histology usually involving the right lung. Associated w/ congenital diaphragmatic hernia, bilateral renal agenesis (Potter Syndrome)

2) Bronchogenic cysts= Caused by abnormal budding of foregut & dilation of terminal or large bronchi. Discrete, round, sharply defined & air-filled densities on CXR. Drain poorly & cause chronic infections.

What are Type I pneumocytes?

thin squamous cells present in the alveoli, functioning in optimal gas diffusion

Where are Type I pneumocytes found?

97% of alveolar surfaces. (line the alveoli)

Role & epithelium of Type I pneumocytes

squamous. Thin for optimal gas diffusion

How is collapsing pressure calculated?

P = (2 x surface tension) / radius

What is the function of Type II pneumocytes?

secrete pulmonary surfactant --> decrease alveolar surface tension; prevent alveolar collapse, decrease lung recoil & increase compliance

What type of cells, histologically, are Type II pneumocytes?

cuboidal

Do Type II cells originate from Type I cells, or are Type II cells progenitors for Type I cells?

Type II cells are progenitors for Type I cells. Type II cells can also give rise to other Type II cells.

When do Type II cells proliferate?

in LUNG DAMAGE

What is the Law of Laplace?

As the radius decreases upon expiration, alveoli have an increased tendency to collapse.

What does "atelectasis" mean, and how is it caused?

DEFINITION collapse of alveoli

CAUSES obstruction, compression, or contraction

--> damage to Type II pneumocytes --> loss of surfactant

NOTE Even reinflation may not return full function due to the loss of surfactant.

What is surfactant, chemically?

a complex mix of lecithins, most importantly DIPALMITOYLPHOSPHATIDYLCHOLINE

What are Clara (Club) cells?

nonciliated, columnar cells with secretory granules

What do Clara cells secrete?

a "watery" component of surfactant

What are the functions of Clara cells?

to secrete a component of surfactant, to degrade toxins, and to act as reserve cells

When does surfactant synthesis begin?

around week 26 of gestation

When are mature levels of surfactant reached?

around week 35 of gestation

If a child is born premature, is it likely that they will produce sufficient levels of surfactant? If not, what is the child at risk of developing?

no

atelectasis

What measurement indicates if a fetus has mature lung function?

lecithin : sphingomyelin above 2

This can be measured in the amniotic fluid.

What is the cause of neonatal respiratory distress syndrome?

inadequate surfactant --> increased surface tension --> alveolar sac collapse after expiration --> formation of hyaline membranes

What lecithin:sphingomyelin ratio in amniotic fluid is predictive of neonatal RDS?

ratio <1.5

With what is neonatal RDS associated?

prematurity: adequate surfactant levels are not reached until week 35

C-section: d/t lack of release of stress-induced steroids (fetal glucocorticoids) --> no increased synthesis of surfactant

maternal diabetes: increased fetal glucose-> increased fetal insulin-> decreased surfactant levels

What are the clinical features of neonatal RDS?

increasing respiratory effort after birth

tachypnea with use of accessory muscles

grunting

hypoxemia with cyanosis

CXR showing "ground-glass" appearance of lung

What are the complications of neonatal RDS?

(1) persistently low O2 tension --> hypoxemia --> increased risk of PDA, necrotizing enterocolitis

(2) Therapeutic supplemental oxygen--> increased risk of free radical injury (O2 can be toxic!) --> "RIB"

R= Retinopathy of prematurity

I= Intraventricular hemorrhage

B= Bronchopulmonary dysplasia

What is the treatment for neonatal RDS?

maternal steroids before birth;

artificial surfactant for infant

What is the order of structures in the Respiratory tree?

Trachea-> bronchi-> bronchioles-> terminal bronchioles-> respiratory bronchioles-> alveolar sacs

What does smoking do the epithelial lining of the trachea?

pseudo stratified ciliated columnar-> squamous (via metaplasia & now sputum cannot be cleared)

Where is the highest & lowest resistance in the Respiratory Tree?

Highest= medium-size bronchi (turbulent airflow)

Lowest= terminal bronchioles (high CSA)

What is the conducting zone?

the larger airways that warm, humidify, and filter air without participating in gas exchange (i.e. anatomic dead space)

What are the large airways of the conducting zone?

nose, pharynx, trachea, bronchi

What are the small airways of the conducting zone?

bronchioles and terminal bronchioles (large #'s in parallel-> least airway resistance)

To what level of the conducting zone will cartilage and goblet cells extend?

bronchi

To what level of the conducting zone will psuedostratified ciliated columnar cells extend?

terminal bronchioles

**clear mucus & debris from lungs (mucociliary escalator)

To what level of the conducting zone will smooth muscle cells extend?

terminal bronchioles

What is the respiratory zone?

the airways participating in gas exchange

What are the airways of the respiratory zone?

lung parenchyma; respiratory bronchioles, alveolar ducts, alveoli

What is the histology of the respiratory bronchioles?

cuboidal cells

What is the histology of the alveoli?

simple squamous cells

You see simple squamous cells on a histology slide. From what level of the respiratory system is the slide?

alveoli or alveolar ducts

You see psuedostratified ciliated columnar cells on a histology slide. From what level of the respiratory system is the slide?

terminal bronchioles or above

You see cartilage on a histology slide. From what level of the respiratory system is the slide?

bronchi or above

You see goblet cells on a histology slide. From what level of the respiratory system is the slide?

bronchi or above

You see cuboidal cells on a histology slide. From what level of the respiratory system is the slide?

respiratory bronchioles

Are cilia present in the respiratory zone?

no

Where in the respiratory system may macrophages be found?

alveoli-> clear debris & participate in the immune response

Which lung has three lobes?

right lung

Which lung has two lobes?

left lung; in place of the middle lobe, the lung accommodates the space necessary for the heart.

"Left Lung has Less Lobes

Which lung has a lingula?

left lung

**lingula is a tongue shaped portion of the left lung

Which lung is the more common site for inhaled foreign bodies and why?

right lung; right main stem bronchus is wider and more vertical

"Swallow a bite, goes down the right"

The relation of the pulmonary artery to the bronchus at each lung hilum is described by?

RALS: Right Anterior; Left Superior

If a patient aspirates a peanut while upright, where in the lungs will it be found?

inferior (AKA basilar) portion of the right inferior lobe

If a patient aspirates a peanut while supine, where in the lungs will it be found?

superior portion of the right inferior lobe OR posterior portion of the right upper lobe

What structures perforate the diaphragm at T8, T10, and T12, respectively?

T8= IVC

T10= esophagus, vagus nerve (CN 10)

T12= aortic (red), thoracic duct (white), azygous vein (blue)

"I 8 10 Eggs At 12"

What is the innervation of the diaphragm?

C3, C4, C5 (phrenic nerve)

--C3, 4, and 5 keep the diaphragm alive--

Where might pain from the diaphragm be referred?

shoulder (C5)

trapezius ridge (C3, C4)

Name the bifurcations for the common carotid, trachea & abdominal aorta

C4= common carotid

T4= trachea

L4= abdominal aorta

"biFOURcates"

In quiet breathing, what muscle is responsible for inspiration?

diaphragm

In quiet breathing, what muscle is responsible for expiration?

none (passive process)

In exercise, what muscles are responsible for inspiration?

external intercostals, scalenes, sternocleidomastoid

--inSpiration: external, Scalene, Scm--

In exercise, what muscles are responsible for expiration?

rectus abdominus

internal obliques

external obliques

transversus abdominis

internal intercostals

Graph: Normal Lung

What is the IRV?

Inspiratory Reserve Volume:

the air that can still be breathed in after normal inspiration

What is the TV?

Tidal Volume:

air that moves into lung with each quiet inspiration

What is the normal TV?

500

What is ERV?

Expiratory Reserve Volume:

air that can still be breathed out after normal expiration

What is RV?

Residual Volume:

the air in lung after maximal expiration

Which lung volume measurement cannot be read by spirometry?

RV (residual volume)

How is IC calculated?

Inspiratory Capacity = IRV + TV

How is FRC calculated?

Functional Residual Capacity = RV + ERV

Volume of gas in lungs after normal expiration; cannot be measured on spirometry

How is VC calculated?

Vital Capacity = IRV + TV + ERV

Maximum volume of gas that can be expired after a maximal inspiration

How is TLC calculated?

Total Lung Capacity = IRV + TV + ERV + RV

Volume of gas present in the lungs after a maximal inspiration; cannot be measured on spirometry

What is physiologic dead space?

anatomic dead space of conducting airways plus alveolar dead space (capable of gas exchange but no exchange occurs) in alveoli; volume of inspired air that does NOT take place in gas exchange

How is physiologic dead space calculated?

Vd = Vt x [(PaCO2 - PeCO2) / PaCO2]

"Taco, PAco, PEco, PAco"

Vd= physiologic dead space

Vt= Tidal Volume

PaCO2 = arterial PCO2

PeCO2 = expired air PCO2

What is the largest contributor of alveolar dead space?

apex of the lung d/t not enough blood flow

When is the physiologic dead space = anatomic dead space?

normal lungs

When is the physiological dead space grater than the anatomic dead space?

lung diseases w/ V/Q defects

What is pathologic dead space?

when part of the respiratory zone becomes unable to perform in gas exchange. Ventilation but no perfusion

Equation for Minute Ventilation

Total volume of gas entering lungs per minute

*Ve= VtRR

Equation for Alveolar Ventilation

Volume of gas per unit of time that reaches alveoli

*Va= (Vt-Vd)RR

What are the normal values for RR, Vd & Vt

RR= 12-20 breaths/min

Vd= 150 mL/breath

Vt= 500 mL/breath

There is a tendency for the lungs to _____ _____ and chest wall to ____ ______.

collapse inward

spring outward

At FRC, what is the system pressure?

atmospheric; the inward pull of the lung is balanced by the outward pull of the chest wall.

What determines the combined volume of the chest wall and lungs?

their elastic properties

At FRC, what is the airway pressure?

0

At FRC, what is the alveolar pressure?

0

At FRC, what is the intrapleural pressure?

negative (This prevents pneumothorax). PVR is at a minimum

What is compliance?

the change in lung volume for a given change in pressure

[C= V/P]

**higher compliance= lung easier to fill

**lower compliance= lung hard rot fill

In what processes does compliance decrease?

pulmonary fibrosis

pneumonia

pulmonary edema

**FRC decreases b/c the lungs are now exerting more inward collapsing pressure

What are the causes of pulmonary edema?

HEMODYNAMIC: increased vascular pressure, decreased oncotic pressure

MICROVASCULAR DAMAGE: infection

ARDS

DIC

UNCLEAR: neurogenic, high altitude

In what processes does compliance increase?

emphysema

normal aging

**FRC increases because the lungs don't do a good job of resisting the outward pull of the chest wall

Pressure-Volume Curves of Lung and Chest Wall

Does surfactant increase or decrease compliance?

increase

What happens to intra-thoracic volume when the lung collapses?

increases d/t unopposed chest expansion

Discuss PVR for extra alveolar vessels & alveolar vessels at RV & TLC

RV= extra alveolar vessels have highest PVR

TLC= alveolar vessels have highest PVR