Pulmonary Mechanics

Question 1

Lung is Negative Pressure Pump

Answer 1

• the lung is like a balloon can be inflated artifically by raising the pressure inside or like normal breathing by reducing the pressure on the outside
• during normal quiet breathing the contraction of the diaphragm exerts an expansive force on the intrapleural space- the space outside of the lung but within the chest wall
• the resultant pressure in the intrapleural space decreases, the drop in intrapleural pressure causes the lung to inflate
• when the diaphragm stops contracting, and starts to relax, then the passive elastic recoil of the lung results in deflation
• when resp muscles relaxed the pressure in intrapleural space is less than atmospheric pressure by about 5 cm H2O
• with wound the intrapleural pressure rises up to atmosperic pressure and the lung collapses. Mainttenance of lung inflation depends critically on the maintenance of neg pressure in the space between lung and chest wall. Atmospheric pressure in intrapleural space is pneumothorax

Question 2

Eupnea

Answer 2

• quiet breathing (with a ventilation of about 7/5 L/min), the diaphragm may be the only active inspiratory muscle, enlarging the thoracic cavity by moving downward by as much as 10 cm
• expiration is due to the passive recoil of the lung and the chest wall without the participation of expiratory muscles
• thus during eupneic breathing inspiration is active while expiration is passive

Question 3

Hyperpnea

Answer 3

• active breathing during exercise, inspiration is aided by the contraction of the external intercostals, which lift the ribs upwards and outward to expand the chest cavity to accommodate the larger volumes of air
• during strenuous exercise the accessory muscles of the chest and neck are used to reduce the resistance to airflow
• when ventilation exceeds about 40 L/min, expiration is aided by contraction of the internal intercostals which depress the ribs downward and inward
• four major abdominal muscles contract to increase abdominal pressure to force the diaphragm upward, thus increasing the positive alveolar pressure which expels air out of the lung. Thus during hyperpneic breathing, both inspiration and expiration are active

Question 4

Muscles of Inspiration

Answer 4

• Diaphragm: primary muscle of inspiration
• Accessory: Sternomastoid- lifts sternum scalene- lifts upper ribs, others- facial, neck, head
• External Intercostals: lifts ribs up and forward; increases anterior-posterior dimension of chest cavity; adds tone to chest wall

Question 5

Muscles of Expiration

Answer 5

• Internal Intercostals- lower ribs
• abdominal: depresses lower ribs; compresses abdominal cavity and pushes up diaphragm
• rectus abdominus, external oblique, internal oblique, transversus abdominus

Question 6

Tachypnea

Answer 6

-more rapid than normal

Question 7

Hypoventilation

Answer 7

• muscular dystrophy, respiratory muscle paralysis

- leads to alveolar hypoxia and hypercapnea respiratory acidemia

Question 8

Hyperventilation

Answer 8

• anxiety, panic attack

- breathing faster than required for oxygenation leads to alveolar hypocapnea respiratory alkalosis

Question 9

Alveolar Pressure

Answer 9

• Palv, varies during the breathing cycle
• air movement requires a pressure gradient
• during inspiration Palv < Patm
• during expiration Palv > Patm
• if the breath is held at any lung volume with no air moving, and with the mouth and glottis open, then Palv= Patm

Question 10

Intrapleural Pressure

Answer 10

• Ppl is the pressure in the space outside the lung but within the chest wall
• the intrapleural space is fluid-filled and is only about 10 microns thick, holding a few ml in volume
• the lung is in close apposition to the inner chest wall; the movements of the two elastic vessels are lubricated by fluid in the intrapleural space
• contraction of the diaphragm exerts an expansive force on the intrapleural space, thus decreasing its pressure, making it more negative, and acting to inflate the lung
• can measure esophageal balloon provides a good estimate of changes in Ppl because the esophagus is a thin walled tube which has little tone

Question 11

External pressure

Answer 11

• usually atmospheric pressure, or Patm, which is constant during the respiratory cycle
• f weight is placed on the chest, then the external pressure on the chest wall is greater than Patm and is referred to as body surface pressure

Question 12

Transmural Pressures

Answer 12

-Ptm
-across an elastic vessel is defined as internal minus external pressure
Ptm= Pinternal - Pexternal

Question 13

Lung pressure

Answer 13

• Pl is the transmural pressure across the lung and is the difference between alveolar and intrapleural pressures
• sets the degree of lung inflation and must always be positive to maintain inflation
• Pl = Palv- Ppl

Question 14

Chest wall pressure

Answer 14

• Pc
• the transmural pressure across the chest wall, and is the difference in pressure between the intrapleural space and atmospheric or body surface pressure
• sets degree of inflation of chest wall
• negative at rest
• Pc= Ppl- Palv

Question 15

Total transmural pressure

Answer 15

• also called relaxation pressure, is the total transmural pressure across the lung and the chest wall
• the total pressure is the difference between alveolar and atmospheric pressure and is equal to the sum of the lung presure and the chest wall pressure
• Pt= Palv- Patm = Pc + PL
• at rest the positive outward lung pressure is balanced by the inward passive elastic recoil pressure of the lung
• the negative inward chest wall pressure is balanced by the passive outward elastic recoil pressure of the chest wall

Question 16

Static compliance

Answer 16

-determines what particular volume the lung and chest wall will assume for a given transmural pressure when the elastic vessels are at mechanical equilibrium with no air moving

Question 17

Compliance

Answer 17

• is estimated from the inverse slope of pressure volume curve
• C= change in volume/ change in pressure
• total compliance is the inverse slope of the pressure volume curve for the lung plus chest wall

Question 18

How to measure compliance

Answer 18

• after being attached to a spirometer through a breathing tube connected to a pressure gauge the patient is instructed to inspire, then hold the breath with the glottis open, and then relax the chest muscles and diaphragm against a weighted spirometer
• have to relax since passive elastic recoil is being measured
• the weight maintains lung inflation while the muscles are relaxes

Question 19

Total compliance

Answer 19

• the total compliance, Ct is related to the lung and chest wall compliances
• since the lung and chest wall are in series, the transmural pressures across them are additive
• Pt = Pl +Pc
• for a given change in volume, change in Pt = change in Pl + change in Pc
• then, Pt/V = Pl/V +Pc/V
• 1/Ct= 1/Cl + 1/Cc
• the compliance of a single lung would be 1/2 of both lungs since change in volume is 1/2 for the same pressure

Question 20

Lung compliance

Answer 20

• Cl= change in volume/ change in Pl
• abnormally decreased total compliance could be either to the properties of the lung or the chest wall
• for example, a decreased total compliance of a barrel-chested coal miner might be due to fibrosis of the lung, or simply to the chest wall physique
• it is thus desirable to measure the lung and chest wall compliance seperately

Question 21

Chest Wall Compliance

Answer 21

-Cc= Change in volume/ change in pressure c

Question 22

functional residual capacity

Answer 22

• FRC
• the lung volume when Pt is zero
• the lung is at FRC at the end-expiratory position
• this mechanical equilibrium position of the respiratory apparatus occurs at about 36% of vital capacity
• at FRC Pc= -Pl

Question 23

Intrapleural pressure graph

Answer 23

• intrapleural pressure becomes positive when the muscles are relaxed and the weight is placed onto the spirometer
• with larger degrees of inflation, Ppl becomes more positive

Question 24

Compliance Emphysema

Answer 24

• lung disease associated with cigarette
• plasma normally contains an enzyme that digests connective tissue, and an inhibitor that regulates the rate of digestion
• contains an inhibitor of the inhibitor alpha1- antitrypsin, lung CT is digested by proteases
• smokers suffer from destruction of alveolar septae, merging of adjacent alveoli, and the formation of large blebs with overall loss of alveolar surface area
• loss of elastic recoil of lungs and increased compliance is characteristic
• so that at max inflation the lungs exert little recoil
• exhalation

Question 25

Pneumoconioses

Answer 25

• related lung diseases induced by inhalation of dust asbestoes, coal, silica, and other toxic material particles
• these materials induce the formation of granulomatous and fibrous tissue in the lungs leading to a decrease in compliance, or a stiffer lung
• inspiration is difficult and the work of breathing is increased
• changes in compliance also cause changes in total lung capacity and functional residual capacity. Increased Cl will increase FRC and TLC while decreased Cl will decrease FRC and TLC

Question 26

Pulmonary Surfactant

Answer 26

• contains insoluble lipoprotein with a large proportion of dipalmitoyl lecithin
• lowers the surface tension of lung and increases compliance
• a deficiency of surfactant increases the surface tension of alveoli and thus increase their elastic recoil and tends to deflate the lung
• filling lung with saline eliminates the air liquid interface and thus abolishes the recoil pressue due to surface tension leaving only that component due to elastic fibers- recoil pressure of saline is much less than air filled lung

Question 27

Respiratory distress syndrome

Answer 27

• also called hyaline membrane disease
• condition of lung immaturiry affecting premature infants
• don’t have surfactant and unable to keep their lungs inflated because high surface tension causes their lung to collapse after each breath
• can use continuous positive airway pressure until surfactant is synthesized
• without it the alveolar epithelial membrane becomes permeable to plasma proteins, which leak into the alvoli and impart a glassy translucent hyaline appearance to the lung tissue

Question 28

Stabilization of Lung by Surfactant

Answer 28

• helps to stabilize the alveoli and prevent atelectasis (partial collapse of lung)
• P= 2T/r (t is surface tension)
• a small bubble has a larger pressure than an adjecent larger bubble, and the small bubble would tend to merge with the large bubble
• however, the surface tension of alveoli with surfactant increases with increasing inflation volumes, and thus acts to stabilize alveolar structures