Resting Ventilation and Lung Mechanics

Question 1

Intrapleural Pressure in Lung Mechanics

Answer 1

• Pleura: thin lining along the lung and chest wall
  
  • Pleural space: theoretical space
  • Allows for easy movement of the lung during respiration
• Normally subatmospheric pressure created by elastic recoil of the lung and chest wall in opposite directions
  
  • Normally -3 to -5cm H2O

Question 2

Expiratory Muscles

Answer 2

At rest

• no muscles (at rest expiration is passive)

Exertion, Forced Expiration

• Abdominal wall muscles
• Internal intercostal muscles
  
  • Pull the ribs downward and inward

Question 3

Total Compliance

Answer 3

• Total Compliance of a person is dependent on Lung Compliance and Chest Wall Compliance
• Chest wall deformities and obesity can lead to reduction in chest wall compliance

Question 4

Elastance

Answer 4

Elastance: A measure of the tendency of a hollow organ to recoil toward its original dimensions upon removal of a distending or compressing force. It is the reciprocal of compliance.

Question 5

Pneumothorax Management

Answer 5

Small and asymptomatic: observation

Symptomatic moderate to large: Chest Tube

Tension : medical emergency, needle decompression 2nd intercostal space in the mid clavicular line

Question 6

Pressures in Respiration

Answer 6

• Atmospheric Pressure = 0
• Alveolar Pressure = fluctuates between 1 to -1
• Intrapleural Pressure = negative
• Transpulmonary Pressure (Alveolar Transmural Pressure) = positive

Question 7

Transpulmonary pressure

Answer 7

• difference between alveolar pressure and intrapleural pressure
• P_tp = P_alv-P_ip
• Pressure that keeps the lungs open
  
  • Usually positive value: P_alv is close to 0 and P_ip is negative
  • If it equals 0, the lung will collapse

Question 8

Inspiratory Muscles

Answer 8

Diaphragm

• During inspiration, active contraction occurs, forcing abdominal contents downward (1-2 cm) and forward; increases thoracic cavity size

External Intercostal muscles

• Connect adjacent ribs and slope downward and forward
• In contraction, ribs are pulled upward and forward increasing the thoracic cavity
• Therefore, act as a muscle of inspiration, particularly during exercise

Accessory muscles

• Include scalene muscles and sternocleidomastoid which elevate the first two ribs and sternum
• Primarily used in exercise to assist with inspiration

Question 9

Surfactant Role in Surface Tension

Answer 9

• Surfactant: phospholipid secreted by Type II alveolar epithelial cells (85% lipids and 15% protein)
• Acts similar to a detergent and reduces surface tension at the air- fluid interface
• Reduces elastic inward recoil of the lung
• Reduces hydrostatic pressure in the tissue outside the capillary
  
  • Important in the prevention of pulmonary edema
• Lack of surfactant seen in premature neonates
  
  • Not fully functional until seventh month of gestation or later
  • Results in Infant Respiratory Distress Syndrome
  • FKR Neonatal respiratory distress syndrome is caused by insufficient production of surfactant, which increases alveolar surface tension and can lead to pulmonary collapse.

Question 10

Surface Tension in Lung Mechanics

Answer 10

• Elastic tendency of fluid surface to acquire the least surface area possible
• Generated by cohesive forces between molecules of liquid
• Inward force acting at the air-liquid interface leading to collapse
• Pressure is determined by Laplace’s Law: Pressure = 2xsurface tension / radius of alveoli
  
  • If surface tension were constant, the pressure in a smaller alveoli would be much greater than larger alveoli
    
    • This would lead to air moving into larger alveoli promoting lung collapse
    • This would lead to an unstable system
• Tends to promote collapse

Question 11

Transpulmonary Pressure in Lung Mechanics

Answer 11

• Pressure difference across the whole lung
• Difference between alveolar pressure and intrapleural pressure
  
  • Ptp = Palv – Pip
• Pressure that keeps the lung open
  
  • Is usually positive value: Palv is close to 0 and Pip is negative
  • If it equals 0, the lung will collapse

Question 12

Definition of Respiration

Answer 12

• Movement of oxygen into the lung and carbon dioxide out of the lung
• The movement of inspired air driven by negative intrathoracic pressure generated by muscles

Question 13

Functional Residual Capacity

Answer 13

• volume of air remaining in the lung at the end of expiration in normal tidal breathing
• Outward Recoil of the Chest Wall = Inward Recoil of the Lung
• Thus at FRC, Alveolar Pressure equals Atmospheric pressure and air no longer passively fills the alveoli

Question 14

Tension Pneumothorax

Answer 14

• In a tension pneumothorax, a one-way valve effect lets air enter but not exit the pleural space, which can put pressure (i.e. tension) on the mediastinum. This causes hemodynamic instability, as well as a mediastinal shift and tracheal deviation away from the affected lung.
• Clinical diagnosis: Hemodynamic instability due to elevated intrapleural pressure impairing venous return to the heart
• Air enters the pleural space and is trapped during expiration

Question 15

Innervation of Respiratory System

Answer 15

Diaphragm: phrenic nerve (Cervical nerve roots 3-5)

External and internal intercostals: intercostal nerves originating from the spinal cord at the same level

Question 16

Changes in pressures during expiration and inspiration for:

• Lung volume (tidal volume)
• intrapleural pressure
• alveolar pressure
• airflow
• transpulmonary pressure

Answer 16

Question 17

Key Factors in Lung Mechanics

Answer 17

• Elastic Recoil
• Surface Tension
• Alveolar Interdependence
• Intrapleural Pressure
• Lung Compliance

Question 18

Alveolar Interdependence in Lung Mechanics

Answer 18

• Structural support of an individual alveolus by surrounding alveoli through an elastic tissue network
• As one alveolus attempts to collapse, surrounding alveoli support preventing collapse
• Negative intrapleural pressure is transmitted from adjacent alveoli to more centrally located alveoli

Question 19

Lung Compliance in Lung Mechanics

Answer 19

• Relationship of lung volume and transpulmonary pressure
• Ease with which the lung is distended for a given force
• C = change in volume / change in pressure
• At low lung volumes, the lung is highly compliant but as reaching TLC, lung is less compliant
• Slopes are different in expiration and inspiration – known as hysteresis
• Thought to be related to the effect of surfactant on surface tension
• Poorly compliant lungs have a shallow slope
  
  • Seen in Interstitial Lung Disease due to deposition of collagen
• Highly compliant lungs have a steep slope
  
  • Seen in emphysema due to destruction of elastin fibers

Question 20

• What happens when intrapleural pressure is increased?
• How is Pip created?
• What are situations in which Pip is increased?

Answer 20

What happens when intrapleural pressure is increased?

• we could see lung collapse

How is Pip created?

• Elastic recoil of the chest outward and elastic recoil of the lung inward

What are situations in which Pip is increased?

• Forced Exhalation
• Stiff chest wall (↓ chest wall compliance)
• Fluid or Air in the pleural space
• Pleural Effusion
• Pneumothorax

Question 21

What Happens During Expiration

Answer 21

• Inspiratory muscles relax.
• Thoracic volume decreases as chest wall collapses
• Intrapleural pressure to become less negative
• Transpulmonary pressure decreases
• Alveoli collapse due to decreased alveolar transmural pressure difference and increased alveolar elastic recoil
• Decreased alveolar volume increases alveolar pressure above atmospheric pressure, thus establishing a pressure difference for airflow out
• Air flows out of the alveoli until alveolar pressure equilibrates with atmospheric pressure

Question 22

Elastic Recoil in Lung Mechanics

Answer 22

• Tendency for a structure to return to its natural state
• Chest wall: structure of the thoracic cage directs energy outward; increases volume keeping lungs open;natural elastic recoil to move outward
• Lung: distention of the alveoli creates an inward directed energy; decreases volume; natural elastic recoil to move inward
  
  • Pulmonary parenchyma composed of elastin and collagen fibers
  • Tendency of a tissue to return to its natural state when stretched is known as elastanceWhen chest wall elastance and lung elastance equal each other, this is known as Functional Residual Capacity (FRC)
    
    • This is seen at end expiration of normal tidal breathing

Question 23

Pressure-Volume Relationships in Respiration

Answer 23

• Air entry into the lungs occur because a pressure difference is created to reduce alveolar pressure below atmospheric pressure (O cm H20)
• Negative Pressure Breathing
• Based on Boyle’s law (P1V1 = P2V2): as volume increases, pressure decreases – causes a pressure gradient to get air to flow into our body
• Thus, when muscle contraction occurs, intrathoracic volume increases leading to a decrease in intrathoracic pressure and air enters the alveoli

Question 24

Pneumothorax; definition, symptoms, exam

Answer 24

Definition

• a collection of air in the pleural space, between the lung and chest wall

Symptoms

• Chest pain, self limited
• Dyspnea
• Asymptomatic

Exam

• A pneumothorax presentation can be remembered with the mnemonic P-THORAX:
• Pleuritic chest pain
• Tracheal deviation
• Hyperresonance
• Occurs suddenly
• Reduced breath sounds (and dyspnea)
• Absent fremitus (asymmetric chest wall)
• X-rays showing collapse​

Question 25

Primary Spontaneous Pneumothorax (PSP) vs. Secondary Spontaneous Pneumothorax

Answer 25

Primary

• A primary spontaneous pneumothorax (PSP) is due to a rupture of a subpleural bleb
• No known lung disease
• Rupture of apical subpleural blebs due to shear force
• Patients tall, young, smokers; males
• history of PSP

Secondary

• due to an underlying pathology (e.g., COPD, trauma, infection)
• Assocated with chest wall trauma: penetrating (GSW) or non-penetrating (broken rib)
• can also be iatrogenic, due to a thoracocentesis, positive pressure ventilation, or errors in subclavian central line placement

Question 26

What Happens During Inspiration

Answer 26

• Diaphragm contracts.
• Thoracic volume increases as the chest wall expands (Boyle’s Law)
• Intrapleural pressure becomes more negative.
• Transpulmonary pressure increases.
• Alveoli expand in response to the increased transmural pressure difference.
• This increases alveolar elastic recoil.
• Alveolar pressure falls below atmospheric pressure as the alveolar volume increases, thus establishing a pressure difference for airflow.
• Air flows into the alveoli until alveolar pressure equilibrates with atmospheric pressure