Frictional opposition occurs only when the system is in motion.
Frictional opposition to ventilation has the two components:
1. tissue viscous resistance
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2. airway resistance.Tissue Viscous Resistance: the impedance of motion (opposition to flow)
caused by displacement of tissues during ventilation that includes the lungs,
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rib cage, diaphragm, and abdominal organs.
The frictional resistance is generated by the movement of each organ surface
sliding against the other (e.g., the lung lobes sliding against each other and
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against the chest wall).
Tissue resistance accounts for only approximately 20% of the total resistance
to lung inflation.
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In conditions : obesity, pleural fibrosis, and ascites, the tissue viscousresistance increases the total impedance to ventilation.
Airway Resistance (flow resistance)
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- Resistance to ventilation by the movement of gas through the airways.? accounts for approximately 80% of the frictional resistance to ventilation.
? -is usually expressed in units of cm H2O/L/sec:
R= P/ V
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? Airway resistance in healthy adults ranges from approximately 0.5 to 2.5 cm
H2O/L/sec.
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? To cause gas to flow into or out of the lungs at 1 L/sec, a healthy person needsto lower his alveolar pressure 0.5 to 2.5 cm H2O below atmospheric pressure.
Measurement of Airway Resistance
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? Airway resistance is the pressure difference between the alveoli and the mouth
divided by a flow rate. Mouth pressure is easily measured with a manometer.
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Alveolar pressure can be deduced from measurements made in a bodyplethysmograph.
Factors Determining Airway Resistance
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Lung volume : Like the extra-alveolar blood vessels , the bronchi are supported by the radialtraction of the surrounding lung tissue, and their caliber is increased as the lung expands.
As lung volume is reduced, airway resistance rises rapidly.
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West, John B. Respiratory physiology : the essentials -- 9th edFactors Affecting Resistance
The two main patterns that characterize the flow of gas through the respiratory tract
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are laminar flow and turbulent flow . A third pattern, tracheobronchial flow, is a
combination of laminar and turbulent flow.
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Laminar flow requires less driving pressure than turbulent flow.Poiseuil e's equation describes laminar flow through a smooth, unbranched tube of
fixed dimensions (i.e., length and radius). This equation says that for gas flow to
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remain constant, the pressure is inversely proportional to the fourth power of the
airway 's radius.
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That is, by reducing the radius of a tube by half requires a 16-fold pressure increaseto maintain a constant flow (24 = 16)!
Clinical y this means that to maintain ventilation in the presence of narrowing
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airways, large increases in driving pressure may be needed, resulting in markedincreases in the work of breathing.
Airflow through large airways tends to be turbulent, whereas flow in smaller airways tends to
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be laminar. Laminar flow is described by Poiseuille's lawPoiseuil e's law
where P is the driving pressure, r radius, n viscosity, and l length.
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Chief Site of Airway Resistance? Toward the periphery of the lung :The airways become more numerous but much
narrower.
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? Based on Poiseuil e's equation with its (radius)4 term, the major part of the resistance
should lie in the very narrow airways.
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? Most of the pressure drop occurs in the airways up to the seventh generation.? The major site of resistance is the medium-sized bronchi and that the very smal
bronchioles contribute relatively little resistance.
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? Less than 20% of resistance can be attributed to airways <2 mm in diameter. The
reason for this apparent paradox is the prodigious number of smal airways.
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? The peripheral airways constitute a "silent zone," a considerable smal airway diseasecan precede detection of an abnormality of airway resistance .
? Patients who have increased airway resistance often breathe at high lung volumes; this
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helps to reduce their airway resistance.
Distribution of Resistance
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? 80% of the resistance to gas flow occurs in the nose, mouth, and large airways, where flow ismainly turbulent.
? 20% of the total resistance to flow is attributable to airways smaller than 2 mm in diameter,
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where flow is mainly laminar.
? Branching of the tracheobronchial tree increases the cross-sectional area with each airway
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generation. As gas moves from the mouth to the alveoli, the combined cross-sectional area ofthe airways increases exponentially (> 30-fold).
? The velocity of gas flow and resistance in a branching system arranged in parallel is inversely
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related to the cross-sectional area of the airways (bernoul i principle)
? The resistance to flow in small airways is very low. The driving pressure across these airways is
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less than 1% of the total driving pressure for the system.Bernoul i Principle
? In a steady flow, the sum of all forms of energy in a fluid is the same at all points along the
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path of flow. The sum of kinetic energy, potential energy, and internal energy remainsconstant.
? The Bernoulli principle states that an increase in the velocity of the fluid results in a decrease
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in the sum of its static pressure, potential energy, and internal energy.
? Fluid is flowing through a tube at a point with a certain velocity (va) and a lateral pressure
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(Pa). According to the law of continuity, as the fluid moves into the narrow or constrictedportion of the tube, its velocity must increase (vb > va). According to the Bernoulli theorem,
the higher velocity at point b should result in a lower lateral pressure at that point (Pb < Pa).
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? As a fluid flows through the constriction, its velocity increases and its lateral pressure
decreases.
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Cross-sectional area of the airways plotted against airway generation.The first 15 or airway generations represent a conducting zone : The anatomic dead space.
The gas-exchange surface increases markedly at the level of the terminal bronchiole
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? During inspiration, the stretch of surrounding lung tissue and widening transpulmonary pressure gradientincrease the diameter of the airways. The increase in airway diameter with increasing lung volume
decreases airway resistance.
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? As lung volume decreases toward RV, airway diameters decrease and airway resistance dramatically
increases; wheezing is most often heard during exhalation.
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Change in airway resistance (Raw) related to lung volume.Resistance to airflow is highly dependent on lung volume.
Mechanics of exhalation
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? Airway caliber is determined by anatomic (i.e., physical) support provided to
the airways and pressure differences across their walls.
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? The larger airways depend mainly on cartilaginous support.? The smaller airways depend on support provided by surrounding lung
parenchyma.
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? The transpulmonary pressure gradient help stabilize the smaller airways mainly.? Airway pressure varies minimally and is usually close to zero (atmospheric
pressure).
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? During a forced exhalation, contraction of expiratory muscles can increasepleural pressure above atmospheric pressure.
? During forced exhalation
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PA= pleural pressure + elastic recoil pressure of the lung.? During exhalation, the pressure along the airway decreases as gas flows
from the alveoli toward the mouth.
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? At some point along the airway, the pressure inside the airway equals the
pressure outside in the pleural space. This point is referred to as the
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equal pressure point (EPP).? Downstream from this point, pleural pressure exceeds the airway
pressure. The resulting increase in transmural pressure gradient causes
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airway compression and can lead to col apse.
? Then the airflow becomes effort independent with airway caliber and
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elastic recoil pressure determining flow.? In airways of healthy persons, airway collapse occurs only with forced
exhalation and at low lung volumes.
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Dynamic compression of airways during expiration
? Airway resistance is also affected by dynamic compression, which is the compression of airways during forced
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expiration.? An expiratory flow?volume curve: the subject performs a forced vital capacity maneuver, inspiring to total lung
capacity and then exhaling as forcibly as possible to residual volume.
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The peak of this curve represents the peak expiratory flow rate (PEFR).
The downward slope (expiratory phase) of the flow volume curve is effort-independent; during this phase of
the curve, flow is limited by dynamic compression of the airways.
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Netters
? Tissue support opposes the col apsing force created by negative transmural pressure
gradients. In pulmonary emphysema, the elastic tissue supporting the smal airways is
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damaged.
? Destruction of elastic tissue, such as occurs in emphysema, has multiple outcomes. It
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increases the compliance of the lung (i.e., elastic recoil decreases).? The combination of decreased elastic recoil and loss of support for the smal airways
al ows the airways to col apse during exhalation. Airway col apse causes air trapping
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and increase in the resting volume of the lung.
? Expiratory flow is reduced by airway col apse during exhalation (cal ed flow limitation)
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and can occur during tidal breathing when emphysematous changes in the lung aresevere.
? By exhaling through "pursed lips," a patient with emphysema changes the
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pressure at the airway opening.
? The gentle back pressure created counters the tendency for smal airways to
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col apse by moving the EPP toward larger airwaysEffects of Autonomic Nerves on Airway Resistance
Activation of parasympathetic nerves innervating smooth muscle of
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conducting airways causes bronchoconstriction and promotes glandular
secretions in the lungs.
Activation of sympathetic nerves innervating smooth muscle of conducting
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airways results in bronchodilation and reduced airway resistance (through
activation of 2-adrenergic receptor-linked pathways) in mammalian species,
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although human lungs have little sympathetic innervation.Release of epinephrine by the adrenal medulla during sympathetic activation
will also reduce airway resistance through activation of the pulmonary 2-
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receptor mechanism.WORK OF BREATHING
The respiratory muscles do the work for normal breathing.
This work requires energy to overcome the elastic and frictional forces opposing
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inflation.
Assessment of mechanical work involves measurement of the physical parameters of
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force and distance as they relate to moving air into and out of the lung.Assessment of metabolic work involves measurement of the O2 cost of breathing.
The work of exhaling is recovered from potential energy "stored" in the expanded lung
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and thorax during inhalation. However, forced exhalation requires additional work by
the expiratory muscles.
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The actual work of forced expiration depends on the mechanical properties of the lungsand thorax.
? The work of breathing can be calculated from the pressure?volume curve because pressure
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times volume (g/cm 2 ? cm 3 = g ? cm) has the same dimensions as work (force ? distance).
? The total elastic work required for inspiration is represented by the area ABCA. The actual
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elastic work required to increase the volume of the lungs alone is area ABDEA.? The amount of elastic work required to inflate the whole respiratory system is less than the
amount required to inflate the lungs alone because part of the work comes from elastic
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energy stored in the thorax. The elastic energy lost from the thorax (area AFGBA) is equal to
that gained by the lungs (area AEDCA).
Work of breathing
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? Elastic work; approximately 65% of the total work, moving inelastic tissues
(viscous resistance; 7% of total), and moving air through the respiratory
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passages (airway resistance; 28% of total).Elastic Work
? The elastic work of breathing is the work done to overcome the elastic recoil of
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the chest wal and the pulmonary parenchyma and the work done to overcome
the surface tension of the alveoli.
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? Restrictive diseases are those diseases in which the elastic work of breathingis increased.
Resistive Work
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? The resistive work of breathing is the work done to overcome the tissue resistance and the
airways resistance.
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? The tissue resistance may be elevated in conditions such as sarcoidosis.? Elevated airways resistance is much more common and is seen in obstructive diseases such
as asthma, bronchitis, and emphysema; upper airway obstruction; and accidental aspirations
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of foreign objects.? The resistive work of breathing can be extremely great during a forced expiration, when
dynamic compression occurs. This is especially true in patients who already have elevated
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airways resistance during normal, quiet breathing.
Work Done on the Lung
During inspiration, the intrapleural pressure follows the curve ABC, and the work done on
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the lung is given by the area 0ABCD0. Of this, the trapezoid 0AECD0 represents the work
required to overcome the elastic forces, and the hatched area ABCEA represents the work
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overcoming viscous (airway and tissue) resistance.The higher the airway resistance or the inspiratory flow rate, the more negative (rightward)
would be the intrapleural pressure excursion between A and C and the larger the area.
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Energy Required for Respiration? . During normal quiet respiration, only 3 to 5 percent of the total energy
expended by the body is required for pulmonary ventilation.
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? During heavy exercise, the amount of energy required can increase as much as
30-50 fold, especially if the person has any degree of increased airway
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resistance or decreased pulmonary compliance.Work of breathing: influence of breathing pattern
? Work of breathing increases when deep breaths are taken (increased TV means
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more work to overcome elastic forces).? Work of breathing increases when respiratory rate increases (increased minute
ventilation requires more flow resistance to overcome).
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? Patients with pulmonary fibrosis (more elastic work) breathe more shallow and
more rapidly.
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? Patients with obstructive lung disease (non elastic work, high resistive work)breathe more slowly and deeply.
Dynamic Compliance
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? The change in the volume of the lungs divided by the change in the alveolar-distending pressure
during the course of a breath.
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? At low breathing frequencies, < 15 breaths per minute and lower, dynamic compliance (DC) = staticcompliance(SC)
(during T V, smal change in alveolar surface area is not able to bring additional surfactant to
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surface)
? At higher breathing frequencies, DC>SC (more surfactant and more compliance)
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? In patients with elevated resistance to airflow in some of their smal airways, the DC/SC ratio falls asbreathing frequency is increased. This indicates that changes in dynamic compliance reflect changes in
airways resistance as wel as changes in the compliance of alveoli.
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? Sighing and yawning increase dynamic compliance by increasing tidal volume via restoring the
normal surfactant layer.
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