Pulmonary Mechanics I Flashcards
Describe the mechanical basis of breathing & give the breathing pressure volume relationship
PV = Constant
In contrast, during normal quiet breathing, or eupnea, the contraction of the diaphragm exerts an expansive force on the intrapleural space*–the space outside of the lung but within the chest wall & pleura. The resultant pressure in the intrapleural space decreases. The drop in intrapleural pressure causes the lung volume to inflate (since PV = Constant). When the diaphragm stops contracting, and starts to relax, then the passive elastic recoil of the lung results in deflation.
Thus unlike a balloon (where increase in volume increases pressure), the lung during normal breathing is a negative pressure pump. Lung inflation results from a drop in pressure outside of the lung instead of the development of a positive pressure inside the lung. Therefore, the lung is a negative pressure pump.
Define eupnea & state if inspiration & expiration are passive or active.
normal quiet breathing. During eupnea, the diaphragm may be the only active inspiratory muscle, enlarging the thoracic cavity by moving downward. 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.
Define hyperpnea & state if inspiration & expiration are passive or active.
During hyperpnea, or active breathing during exercise. Both tidal volume and respiratory rate increae.
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. 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.
Define tachypnea
More rapid breathing than normal so increased respiratory rate but normal tidal volume.
Define hypoventilation. Does it cause acidosis or alkalosis?
Hypoventilation occurs when ventilation decreases & is inadequate to perform needed gas exchange (oxyhenation). By definition it causes an increased concentration of carbon dioxide (hypercapnia) and respiratory acidosis.
Occurs in muscular dystrophy or respiratory muscle paralysis.
Define hyperventilation. Does it cause acidosis or alkalosis?
Hyperventilation means breathing faster than required for oxygenation leads to alveolar hypocapnia (reduced CO2) & respiratory alkalosis.
Occurs during anxiety or panic attacks.
Describe the static compliance curves for the lung, the chest wall, and the total system
Static compliance 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.
C = V/P, it is the slope of a PV plot where volume is on Y axis & P is on X axis
Describe how lung compliance is altered in patients with fibrotic lung disease
Pneumoconioses are related lung diseases induced by inhalation of dust asbestos, coal, silica, and other toxic mineral particles. These materials induce the formation of granulomatous and fibrous connective tissue in the lungs leading to a decrease in compliance, or a “stiffer” lung. Inspiration is difficult and the work of breathing is increased.
How does cigarette smoke contribute to the development of emphysema & how it affects lung compliance
Emphysema is a lung disease associated with cigarette smoking. Plasma normally contains an enzyme called trypsin that digests connective tissue, and an inhibitor called a1-Antitrypsin that regulates the rate of digestion. Cigarette smoke contains an inhibitor of a1-Antitrypsin so the trypsin protease is free to digest the lung.
Thus 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 the lungs and increased* lung compliance is characteristic of severe emphysema, so that at maximum inflation, the lungs exert little recoil pressure. With chronic severe emphysema, exhalation is difficult and death can result.
Describe how tissue elasticity and pulmonary surfactant both contribute to pulmonary compliance in normal individuals and in premature infants who lack pulmonary surfactant
Increased tissue elasticity increases compliance
Surfactant increases compliance by decreasing surface tension
Respiratory distress syndrome, or hyaline membrane disease, is a condition of lung immaturity afflicting premature infants. These neonates, deficient in pulmonary surfactant, are unable to keep their lungs inflated because the high surface tension causes their lung to collapse after each breath. Infant mortality decreased dramatically after the role of surfactant in pulmonary compliance was recognized. Continuous positive airway pressure is applied with respirators to keep the lungs of neonates inflated until the infants begin to synthesize surfactant and breathe normally.
Describe how pulmonary surfactant stabilizes alveoli
Compliance equation C = V/P
1) Surfactant coats the alveoli & decreases the surface tension (decreases pressure), which increases compliance compared to a lung with no surfactant. Surfactant increases the slope (V/P). Surfactant reduces the pressure difference needed to allow the lung to inflate.
2) Surfactant stabilizes the alveoli & helps prevent alveolar collapse: As the alveoli increase in size, the surfactant becomes more spread out over the surface of the liquid. This increases surface tension effectively slowing the rate of expansion of the alveoli, which decreases pressure.
Surfactant lowers the surface tension of the alveoli (especially @ low lung volumes) and thereby prevents collapse of the alveoli during expiration (lower lung volume).
See pg. 34
What variable in the Compliance equation C = V/P truly determines compliance?
Pressure, because it it the pressure increase that determines the magnitude of compliance.
Describe the roles of the principle respiratory muscles by inspiration & expiration. What is the mnemonic for the intercostals?
Inspiration
- diaphragm (primary)
- external intercostals
- sternocleidomastoid
- upper ribs
- sternum scalene
- facial, head, & neck
mnemonic: externals in, internals out
Expiration
- abdominals (during exercise or hyperpnea)
- internal intercostals
- lower ribs
Intrapleural pressure
Intrapleural pressure, or Ppl is the pressure in the space outside the lung but within the chest wall. The intrapleural space is a thin, fluid-filled space.
Describe what happens to alveolar pressure during inspiration, expiration, & holding breath.
Remember that air simply flows from high pressure to low pressure.
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. (The glottis opens and closes the entrance to the larynx and trachea, while the esophagus is always open to the mouth.)
Atmospheric (external) pressure
The external pressure is usually atmospheric pressure, or Patm, which is constant during the respiratory cycle. However, if a 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.
