Mechanics
MECHANICS OF VENTILATION
Elastic resistance
Lung volumes
Nonelastic resistance
Work of breathing
Anesthetic effects on pulmonary mechanics
elastic resistance of tissues and the gas-liquid interface ex. surface tension
-determines lung volume
-determines associated pressures under static conditions (no gas flow)
-stored potential energy supports the work needed to overcome the elastic resistance
nonelastic resistance to gas flow
-involved with frictional resistance to airflow and tissue deformation
-energy is the form of heat supports the work needed to overcome the nonelastic resistance
elastic resistance
-surface tension forces
-compliance
general elastic properties
chest elastic properties creates the tendency to expand outwardly
due to structural components that resist deformation
may also include chest wall muscle tone
lung elastic properties creates the tendency to collapse inwardly
effects of surface tension on the air-fluid interface within the alveoli
due to high content of elastin fibers clincal correlate ex. open pneumothorax
the chest will expand and the lungs will deflate
surface tension:
-the inherent tendency of the alveoli to collapse inward due to the gas-liquid interface
-alveoli act as little bubbles with greater tendency of smaller alveoli collapsing into larger alveoli
-alveolar collapse is directly proportional to surface tension ex. increased surface tension promotes alveolar collapse
-alveolar collapse is indirectly proportional to alveolar size ex. smaller alveolar size promotes alveolar collapse
pulmonary surfactant:
-produced in type II pneumocytes
-decreases alveolar surface tension therefore helps to prevent alveolar collapse
-ability to decrease surface tension is directly proportional to the concentration of surfactant within the alveoli
-smaller alveoli have greater concentration of surfactant therefore have a greater reducing effect of surface tension from surfactant
-larger overdistended alveoli have less concentration of surfactant therfore have a less reducing effect of surface tension via surfactant
-overall effect of surfactant is to prevent smaller alveoli to get smaller and larger alveoli from getting larger
compliance
-the inherent elastic recoil of the lung, chest wall and the total compliance of lung and chest wall combined
-elastic recoil of lung is inward and elastic recoil of chest wall is outward
compliance equals change in volume divided by change in distending pressure
supine position
-chest wall compliance is reduced because of compressing weight of the abdominal contents onto the diaphragm
LUNG COMPLIANCE nl: 150- 200 ml/cm H20
C(lung) = change in lung volume / change in transpulmonary pressure
factors affectings lung compliance
-lung volume
-pulmonary blood volume
-extravascular lung water
-pathological conditions (ex. fibrosis, inflammation)
CHEST WALL COMPLIANCE nl: 200 ml/cm H20
C(chest wall) = change in chest volume / change in intrathoracic pressure
TOTAL COMPLIANCE nl: 100 ml/cm H20
compliance of lung and chest wall combined together
1/C(total) = 1/ C(lung) + 1/C(chest wall)
elastic resistance of tissues and the gas-liquid interface ex. surface tension
-determines lung volume
-determines associated pressures under static conditions (no gas flow)
-stored potential energy supports the work needed to overcome the elastic resistance
nonelastic resistance to gas flow
-involved with frictional resistance to airflow and tissue deformation
-energy is the form of heat supports the work needed to overcome the nonelastic resistance
volumes: extent to which a lung can be inflated maximally
capacities: represents a combination of two or more volumes
Functional Residual Capacity
-lung volume at the end of normal exhalation
-inward elastic recoil of the lung equals the outward elastic recoil of the chest wall
-reference point for the beginning and ending of normal respiration
-measured using: NitrOgen wash-Out and Helium wash-In or body plethysmography
factors affecting FRC
-body habitus: height : Increased/decreased height associated with increased/decreased FRC respectively
obesity: increased obesity decreases FRC due to decreased chest wall compliance
-sex female: 10% reduction of FRC in comparison to males
-posture supine/prone associated with decreased FRC due to decreased chest wall compliance
greatest change in FRC occurs between angles of 0 to 60 degrees of inclination
lung disease restrictive lung disease associated with decreased lung compliance
Closing Capacity
-volume in which airways begin to close in dependant portions of the lungs
-small airways which lack cartilagenous support are dependant on radial traction
-radial traction: the elastic recoil of surrounding tissue to keep smaller dependant airways open
-decreased lung volumes are associated increased closing capacity due to decreased radial traction
-closing capacity at low lung volumes may lead to intrapulmonary shunting and hypoxemia
measurements of closing capacity
-evaluation of tracer gas (xenon 133)
-inhaled near residual volume
-exhaled from total lung capacity
approximate effects of age on closing capacity
supine position: closing capacity = FRC average age: 44
upright position: closing capacity= FRC average age: 66
FRC is affected by position
CC not affected by position
Vital Capacity nl: 60-70 ml/kg
-maximal volume of gas exhaled following maximal inspiration
factors affecting VC
-body habitus
-respiratory muscle strength
-chest-lung compliance
Tissue resistance
-primarily due to viscoelastic frictional resistance of tissues to gas flow
-often underestimated and may be accounted for upto half of the total airway resistance
-primarily performed by the inspiratory muscles (mostly diaphragm)
-expiration is essentialy passive during spontaneous ventilation
Three factors to overcome during ventilation
1. elastic recoil of both the chest and lung
2. frictional resistance to gas flow within the airways
3. tissue frictional resistance
inspiration
-approximately 50% of stored potential energy is released
-released stored potential energy is used to overcome the inspiratory airway resistance and the pulmonary elastic recoil
-increased inspiratory airway resistance is compensated for with increased work of breathing via inc. inspiratory muscle effort
expiration
-the stored potential energy us released in order to overcome expiratory airway resistance
-compensatory response to increased expiratory resistance is to increase lung volume
-increased lung volume due to compensatory response creates a greater elastic recoil energy stored to overcome exp. resistance
respiratory muscles
-require appoximately 2-3 % of oxygen requirements
-90% of the work generated by the respiratory muscles is inefficiently dissipated as heat (from elastic and airfow resistance)
-10% remaining work is used effeciently used for breathing
-certain pathological states can quickly deteriorate the coordination and contractility of the respiratory muscles
factors increasing the work of breathing
-increase in tidal volume: increases work to overcome the elastic resistance
-increase respiratory rate: increases work to overcome the airflow resistance
pts with reduced lung compliance: ex. restrictive lung disease (pulmonary fibrosis) generally have rapid, shallow breaths
pts with increased air resistance : ex.obstructive lung disease( emphysema) generally have slow, deep breaths
Anesthetic effects on pulmonary mechanics
-effects on lung volumes and compliance
-effects on airway resistance
-effects on work of breathing
anesthetic effects on lung volumes and compliance
Functional Residual Capacity
-15-20% reduction (approx. 400ml) upon induction of anesthesia added to additional reduction of FRC in supine position
-due to loss of diaphragmatic tone and rise of diaphragm allowing abdominal contents to elevate into chest cavity
-elevated diaphragm position decreases the lung volume and decreases both the chest and lung compliance
-depth of anesthesia is not related to the decrease in FRC
-decrease of FRC may last for days
-muscle relaxants do not significantly alter the FRC in an already anesthetised patient
Closing Capcity
-15-20% reduction (generally similar reduction as FRC)
anesthetic effects on airway resistance
-generally not obsereved
-increased airway resistance associated with a dec in FRC is offset by the bronchodilating effects of inhalational anesthetics
-increased airway resistance is more associated with pathological factors
factors assoc. with inc airway resistance
-posterior placement of the tongue
-laryngospasm
-bronchoconstriction
-secretions
-blood
-tumor within the airway
-small sized ETT
-malfunctioning valves
-breathing circuit obstruction
anesthetic effects on work of breathing
-increased work of breathing
-increased work of breathing provided with controlled mechanical ventilation
reasons for increased for of breathing (as described above)
-reduced lung and chest wall compliance
-increased airway resistance
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