Ventilation Flashcards
Force expiratory volume (FEV1)
The volume of gas exhaled in 1 second by forced expiration from full inspiration
Vital capacity
The total volume of gas that can be exhaled after a full inspiration
Vital capacity measures with forced expiration vs. slower exhalation
Vital capacity measured with forced expiration may be less than that measured with a slower exhalation (so specify that it is forced vital capacity)
Normal ratio of FEV1 to FVC
- approximately 80%
- but will decrease with age
FEV1/FVC in obstructive disorders (i.e. COPD)
-the ratio is reduced
FEV1/FVC in restrictive disorders (i.e. pulmonary fibrosis)
-ration unchanged/increased
Use of FEV1/FVC tests
- valuable in assessing the efficacy of bronchodilator drugs (carry out before and after administration of drug)
- valuable in assessing the progress of disease
FEV1/FVC in patient with bronchospasm
-both values are usually increased
Forced expiratory flow
-the middle half (by volume) of a forced expiration is marked and its duration is measured
FEF 25-75% = volume (l)/ time (s)
Correlation FEF25-75% and FEV1
Generally close in patients with obstructive pulmonary disease (although changes in FEF 25-75%) are often more striking
What is vital capacity a measure of
Stroke volume
Causes of stroke volume reduction
1) Diseases of the thoracic cage and acute injuries
2) Diseases affecting the nerve supply to the respiratory muscles or disease of the muscles themselves
3) Abnormalities of the pleural cavity
4) Disease of the lung itself
5) Increased pulmonary blood volume
6) Space occupying lesins (cysts)
7) Disease of airways that cause them to close prematurely during expiation (asthma, bronchitis)
Diseases of the thoracic cage reducing stroke volume
1) Kyphoscoliosis
2) Ankylosing spondylitis
Abnormalities of the pleural cavity causing reduction in stroke volume
1) Pneumothorax
2) pleural thickening
Diseases of the lung reducing stroke volume
1) Pulmonary fibrosis (reduce distensibility)
Cause of increased pulmonary blood flow causing reduction in stroke volume
-left heart failure
What is the forced expiratory volume (and FEF 25-75%) affected by
- affected by airway resistance during forced expiration
- increase resistance will reduce these measures
Causes of airway resistance
1) Bronchoconstriction
2) Structural changes in the airways
3) Obstructions within the airways
4) Destructive processes in the lung parenchyma (interfere with the radial traction that normally holds the airways open)
Example causes of bronchoconstriction (2)
- asthma
- inhalation of irritants (cigarette smoke)
Example structural changes in airways (1)
-chronic bronchitis
Example obstruction within airways (2)
- foreign body
- excess bronchial secretions
Simple model of factors that may reduce the ventilatory capacity
1) Those that reduce stroke volume
2) Those that increase airway resistance
Expiratory flow-volume curves -how changes in obstructive and restrictive patterns
a) obstructive pattern
-decreased flow rate at larger lung volumes (maximal expiration begins and ends at abnormally high lung volumes)
b) restrictive pattern
-decreased flow rate at smaller volumes
(maximal expiration begins and ends at low lung volumes)
Limit of expiratory flow-volume curve
- impossible to get values outside of the normal curve (no matter how change respiration) - -> flow rate will not increase beyond the envelope
- factor that is limiting the maximum flow rate = dynamic compression of the airways
Consequences of dynamic compression of the airways
1) limits flow rate during a forced expiration
2) causes flow to be independent of effort
3) may limit flow during normal expiration in some patients with COPD
4) is a major factor limiting exercise in COPD
Explanation of cause of dynamic compression
- Initially (at rest) pressure in mouth, airways and alveoli = atm while pressure in intrapleural space is 5cm H2O below atm
- Inspiration –> diaphragm lowers –> pressure in intrapleural space becomes more negative (at end of inspiration pressure is 8 cm H2O) –> this is the pressure expanding the airway (alveoli inflate and pressure moves from positive –> negative down into alveoli)
- With forced expiration (diaphragm pushed back up) both the intrapleural pressure and alveolar pressure rise
- But the pressure in the airways does not increase as much due to the pressure drop ecause of flow
So get a pressure difference of 11 cm whcih tends to close the airways (i.e. intrapleural pressure pushing on airways)
THEREFORE FLOW DETERMINED BY DIFFERENCE BETWEEN ALVEOLAR PRESSURE AND THE PRESSURE OUTSIDE THE AIRWAYS AT THE COLLAPSE POINT (aka staling resistor effect)
Static recoil pressure
The pressure difference between alveolar pressure and the pressure outside the airways
Depends only on lung volumes and compliance and is independent of expiratory effort
Factors contributing to low flow rate in patients with obstructive long diseases
1) Thickening of walls of the airways and excessive secretions in the lumen due to bronchitis –> increasing flow resistance
2) Number of small airways reduced because of destruction lung tissue
3) Reduced static recoil pressure due to breakdown of elastic alveolar walls
4) Impaired traction from parenchyma due to loss alveolar walls –> collapse more easily than should
Factors contributing to low flow rate in patients with restrictive lung diseases
- lung static recoil high
- reduced compliance of lung makes volumes of small so absolute flow rate is reduced
What determines the flow rate when airways collapse during a forced expiration
Determined by the resistance of the airways up to the pint of collapse
Where does airway collapse occur
-at the point where pressure inside the airways is equal to intrapleural pressure (equal pressure point)
What happens to the equal pressure point as airway resistance increases
Pressure is lost more rapidly and the collapse point moves moves into more distal airways
Normal equal pressure point
In the vicinity of the lobar bronchi early in a forced expiration???
Peak expiratory flow rate
-maximum flow rate during a forced expiration starting from total lung capacity
Measuring peak expiratory flow rate
-estimated with a portable peak flow meter
Inspiratory flow-volume curve -what affects it
-useful in detecting upper airway obstruction –> will flaten the curve because maximum flow is limited
Tests for uneven ventilation
1) Single breath nitrogen test
2) multibreath nitrogen washout
Single breath nitrogen test
-measuring nitrogen concentration at the mouth piece when patent takes a vital pacity inspiration of oxyen and then exhales slowly as far as he can to residual volume
Four phases of single breath nitrogen test
1) Pure oxygen is exhaled from the upper airways, nitrogen concentration = 0
2) Nitrogen concentration rises rapidly as the anatomic dead space is washed out by alveolar gas
3) Third phase = alveolar gas - tracing only small upward slope (alveolar plateau)
4) Rise in nitrogen concentration
