West Chapter 7: Mechanics of Breathing

What is the most important muscle of inspiration?

Diaphragm

What nerves supply the diaphragm?

Supplied by the phrenic nerves from cervical segments 3, 4, and 5

Action of the Diaphragm

• When it contracts, the abdominal contents are forced downward and forward and the vertical dimension of the chest cavity is increased

• Rib margins are lifted and moved out, causing an increase in the transverse diameter of the thorax

• During normal tidal breathing, the level of the diaphragm moves about 1 cm, but on forced inspiration and expiration, a total excursion of up to 10 cm may occur

Paradoxical Movement of the Diaphragm

When one side of the diaphragm is paralyzed and it moves up rather than down with inspiration because the intrathoracic pressure falls

Action of the External Intercostal Muscles

• Connect adjacent ribs and slope downward and forward

• When they contract, ribs are pulled upward and forward, causing an increase in the lateral and anteroposterior diameters of the thorax

• Paralysis of the intercostal muscles alone does not seriously affect breathing at rest because the diaphragm is so effective

What nerves supply the external intercostal muscles?

The intercostal nerves that come off the spinal cord at the same level

What are the muscles of inspiration?

Diaphragm

External intercostal muscles

Scalene muscles

Sternocleidomastoids

Action of the Scalene Muscles

Elevate the first two ribs

Action of the Sternocleidomastoids

Raise the sternum

Is expiration active or passive?

• Passive during quiet breathing

  • Due to elasticity of lung and chest wall, they return to equilibrium positions after being actively expanded during inspiration

• Becomes active during exercise and voluntary hyperventilation

What is the most important muscle of expiration?

Those of the abdominal wall

• Rectus abdominis, internal and external oblique muscles, and transversus abdominis

Action of the Muscles of the Abdominal Wall

When these contract, intra-abdominal pressure is ra ised and the diaphragm is pushed upward

Also contract forcefully during coughing, vomiting, and defecation

Action of the Internal Intercostal Muscles

• Assist active expiration by pulling the ribs downward and inward (opposite action to external intercostal muscles)

• Decrease thoracic volume

• Stiffen the intercostal spaces to prevent them from bulging outward during straining

Pressure-Volume Curve of the Lung

• When the pressure within the jar is reduced below atmospheric pressure, the lung expands

  • Change in volume can be measured with a spirometer

  • Pressure held at each level for a few seconds allowing the lung to come to rest

  • Allows the pressure-volume curve of the lung to be plotted

• Expanding pressure around the lung is developed by an increase in volume of the chest cage

• Lung volume at any given pressure during deflation is larger than is that during inflation

• The lung without any expanding pressure has some air inside of it

  • Even if the pressure around the lung is raised above atmospheric pressure, little further air is lost because small airways close, trapping gas in the alveoli

    • Airway closure occurs at higher lung volumes with increasing age and also in some types of lung disease (emphysema)

Hysteresis

The curves that the lung follows during inflation and deflation are different

Transpulmonary Pressure

Difference in pressure between the inside and the outside of the lung

• Numerically equal to the pressure around the lung when the alveolar pressure is atmospheric

Compliance Equation

Compliance = delta V/delta P

Compliance of the Lung

Slope of the pressure-volume curve, or the volume change per unit pressure change

• In the normal range (expanding pressure of about -5 to -10 cm water) the lung is very compliant

  • At high expanding pressure the lung is stiffer so compliance is smaller

    • Shown by the flatter slope of the curve

Causes of Reduced Compliance in the Lungs

• Increase of fibrous tissue in the lung (pulmonary fibrosis)

• Alveolar edema

  • Prevents the inflation of some alveoli

• Falls if the lung remains unventilated for a long period, especially if its volume is low

  • May be partly caused by atelectasis of some units, but increases in surface tension also occur

• Increased pulmonary venous pressure that causes the lung to become engorged with blood

Causes of Increased Compliance in the Lungs

Pulmonary emphysema

Normal aging lung

Change in the Compliance of the Lung Based on Size

Compliance is larger in larger lungs

Specific Complicance

Compliance per unit volume of lung

Elastic Recoil of the Lung

• The pressure surrounding the lung is less than atmospheric because of the elastic recoil of the lung

  • Elasticity of the lung due to elastic tissues

    • Elastin and collagen present in alveolar walls and around vessels and bronchi

    • Elastic behavior likely has less to do with simple elongation of these fibers than it does with their geometrical arrangement

Surface Tension

Force (in dynes) acting across an imaginary line 1 cm long in the surface of the liquid

• Arises because the attractive forces between adjacent molecules of the liquid are much stronger than are those between the liquid and gas, with the result that the liquid surface area becomes as small as possible

Pressure of a Soap Bubble

• Pressure of a soap bubble can be predicted from Laplace's law: P = 4T/r

  • P - pressure

  • T - surface tension

  • R - radius

  • When only one surface is involved in a liquid-lined spherical alveolus, the numerator is 2 rather than 4

Lungs Inflated with Saline vs Lungs Inflated with Air

Lungs inflated with saline have a much larger compliance than air-filled lungs because the saline abolished the surface tension forces but did not affect the tissue forces of the lung

Surfactant

• Some of the cells lining the alveoli secrete surfactant which profoundly lowers the surface tension of the alveolar lining fluid

• Phospholipid whose important constituent is dipalmitoyl phosphatidylcholine (DPPC)

  • DPPC is synthesized in the lung from fatty acids that are either extracted from the blood or synthesized in the lung

    • Synthesis is fast and there is rapid turnover of surfactant

• Formed relatively late in fetal life

What are the two types of alveolar epithelial cells?

Type I

Type II

Type I Alveolar Epithelial Cells

• Have the shape of a fried egg with long cytoplasmic extensions spreading out thinly over the alveolar walls

Type II Alveolar Epithelial Cells

More compact with lamellated bodies within them that are extruded into the alveoli and transform into surfactant

Effects of Surfactant on Surface Tension via a Surface Balance

• Consists of a tray containing saline on which a small amount of test material is placed and the area of the surface is then alternately expanded and compressed while the surface tension is measured

• Adding detergent reduces the surface tension, but this is independent of area

• Adding lung washings yields the curve in 7.7B

  • Surface tension changes greatly with the surface area and there is hysteresis

  • Surface tension falls to extremely low values when the area is small

How does surfactant reduce the surface tension so much?

• The molecules of DPPC are hydrophobic at one end and hydrophilic at the other and they align themselves in the surface

  • Their intermolecular repulsive forces oppose the normal attracting forces between the liquid surface molecules that are responsible for surface tension

• Reduction in surface tension is greater when the film is compressed because the molecules of DPPC are crowded closer together and repel each other more

What are the physiological advantages of surfactnat?

• Low surface tension in alveoli increases the compliance of the lung and reduces the work of expanding it

• Stability of the alveoli is promoted

  • Areas of atelectasis often form in the presence of disease

  • The pressure generated by a given surface force in a bubble is inversely proportional to its radius so if surface tensions are the same, the pressure inside a small bubble exceeds that in a large bubble

  • When lung washings are present, a small surface area is associated with a small surface tension so the tendency for small alveoli to empty into large alveoli is reduced

• Surfactant also helps keep alveoli dry

  • Surface tension forces tend to collapse alveoli and also tend to suck fluid out of the capillaries

  • By reducing surface forces, surfactant prevents the transudation of fluid

Consequences of Loss of Surfactant

• Stiff lungs (low compliance)

• Areas of atelectasis

• Alveoli filled with transudate

• These are the pathophysiologic features of neonatal respiratory distress syndrome, occurring in premature infants born without adequate quantities of surfactant

Interdependence

Support offered to lung units by those surrounding them

All the alveoli are surrounded by other alveoli and therefore support each other

• With a structure like this, any tendency for one group of units to reduce or increase its volume releative to the rest of the structure is opposed

Why do the lower regions of the lung ventilate more than the upper zones?

• Intrapleural pressure is less negative at the bottom than the top of the lung

  • This is due to the weight of the lung

  • Pressure near the base is higher than at the apex

• The volume of a portion of the lung expands as the pressure around it is decreased

• Pressure inside the lung is the same as atmospheric pressure

• The lung  is easier to inflate at low volumes than high volumes where it becomes stiffer

• Because the expanding pressure at the base of the lung is small, this region has a small resting volume, however because it is located on a steep part of the pressure-volume curve, it expands easily on inspiration

• The apex of the lung has a big resting volume, is situated on a flatter portion of the pressure-volume curve, has large expanding pressure, and undergoes small changes in volume in inspiration

What do regional differences in ventilation refer to?

The change in volume per unit resting volume

• The base of the lung has both a larger change in volume and smaller resting volume than the apex so its ventilation is greater

  • Although the base of the lung is relatively poorly expanded compared to the apex, it is better ventilated

Distribution of Ventilation at Low Lung Volumes

• Represents the situation at residual volume (RV), after full expiration

• The intrapleural pressures are less negative because the lung is not so well expanded and the elastic recoil forces are smaller

• The differences between apex and base are still present because of the weight of the lung

• The intrapleural pressure at the base now exceeds airway (atmospheric) pressure so the lung at the base is not being expanded, but rather compressed, and ventilation is impossible until the local intrapleural pressure falls below atmospheric pressure

• The apex of the lung is on a favorable part of the pressure-volume curve and ventilates well

• At low lung volumes the normal distribution of ventilation is inverted, the upper regions ventilating better than the lower zones

Airway Closure

• The compressed region of lung at the base does not have all its gas squeezed out

• Small airways, probably in the region of the respiratory bronchioles close first, trapping gas in the distal alveoli

  • This only occurs at very low lung volumes in young healthy individuals

  • In elderly, apparently healthy people, airway closure in the lowermost regions of the lung occurs at higher volumes and may be present at functional residual capacity (FRC)

    • This is because the aging lung loses some of its elastic recoil, and intrapleural pressures therefore become less negative

    • Dependent regions of the lung may be only intermittently ventilated leading to defective gas exchange

    • Similar situation occurs with emphysema

Elastic Properties of the Chest Wall as Illustrated by Pneumothorax

• Normally pressure outside the lung is subatmospheric

  • When air is introduced into the intrapleural space, raising the pressure to atmospheric, the lung collapses inward and the chest wall springs outward

  • Under equilibrium conditions, the chest wall is pulled inward while the lung is pulled outward

Pressure-Volume Curve for the Lung and Chest Wall

• Subject inspires or expires from a spirometer and then relaxes the respiratory muscle while the airway pressure is measured ("relaxation pressure")

• At FRC, the relaxation pressure of the lung plus chest wall is atmospheric

  • FRC is the equilibrium volume when the elastic recoil of the lung is balanced by the normal tendency for the chest wall to spring out

    • At volumes above this pressure is positive

    • At volumes below this, pressure is subatmospheric

• At zero pressure, the lung is at its minimal volume, which is below RV

• Chest wall alone

  • At FRC, the relaxation pressure is negative so at this volume the chest cage is tending to spring out

  • It is not until the volume is increased to about 75% of the vital capacity that the relaxation pressure is atmospheric, the chest wall has found its equilibrium position

• At every volume, the relaxation pressure of the lung plus chest wall is the sum of the pressures for the lung and the chest wall measured separately

  • The pressure (at a given volume) is inversely proportional to compliance, this implies that the total compliance of the lung and chest wall is the sum of the reciprocals of the lung and chest wall compliances measured separately

    • 1/C_T = 1/C_L + 1/C_CW

Laminar Flow

At low flow rates, the stream lines are parallel to the sides of the tube

Transitional Flow

As the flow rate is increased, unsteadiness develops, especially at branches and separation of the stream lines from the wall may occur, with the formation of local eddies

Turbulence

At high flow rates, complete disorganization of the stream lines is seen

Driving Pressure Equation with Laminar Flow

P = KV

Resistance Equation for Laminar FLow

R = 8nl/pi x r⁴

Radius is important, if the radius is halved, the resistance increases 16 fold

Doubling the length only doubles resistancfe

What characteristics of the gas affect the pressure-flow relationship under laminar flow conditions

The viscosity of the gas, but not its density, affects the pressure-flow relationship under laminar flow conditions

Velocity of the Gas with Laminar Flow

With laminar flow, the gas in the center of the tube moves twice as fast as the average velocity

Pressure Equation for Turbulent Flow

P = KV²

Pressure is not proportional to flow rate but approximately to its square

What characteristics of a gas are important with turbulent flow?

The viscosity of the gas becomes relatively unimportant, but an increase in gas density increases the pressure drop for a given flow

What determines if flow will be laminar or turbulent?

Depends to a large extent on the Reynolds number, Re

Reynolds Number Equation

• Re = 2rvd/n

  • D- density

  • V - average velocity

  • R- radius

  • N - viscosity

  • Gives the ratio of internal to viscous forces

When is turbulence probable?

In straight, smooth tubes, when the Reynolds number exceeds 2,000

Turbulence is most likely to occur when the velocity of flow is high and the tube diameter is large

Low-density gas such as helium tends to produce less turbulence

What conditions affect the ability for laminar flow to occur?

The entrance conditions of the tube are critical

• If eddy formation occurs upstream at a branch point, the disturbance is carried downstream some distance before it disappears

Air Flow Profiles in Throughout the Lung

• In a rapidly branching system such as the lung, laminar flow probably only occurs in the very small airways

• In most of the bronchial tree, flow is transitional

• True turbulence may occur in the trachea, especially on exercise when flow velocities are high

Airway Resistance

The pressure difference between the alveoli and the mouth divided by a flow rate

Airway Pressures During Inspiration

• Before inspiration begins, the intrapleural pressure is -5 cmH2O because of the elastic recoil of the lung and alveolar pressure is 0 because with no airflow, there is no pressure along the airways

• For inspiratory flow to occur, the alveolar pressure falls, establishing the driving pressure

  • Extent of the fall depends on the flow rate and resistance of the airways

• Intrapleural pressure falls during inspiration for two reasons

  • As the lung expands, its elastic recoil increases

  • The reduction in alveolar pressure causes a further fall in intrapleural pressure

    • Hatched area represents this additional fall in pressure

  • The vertical distance between lines ABC and AB^'C reflects the alveolar pressure at any instant

Equation of Airway Pressure

(Mouth-intrapleural) = (mouth-alveolar) + (alveolar-intrapleural)

Airway Pressures During Expiration

• Equation of pressure (mouth-intrapleural) = (mouth-alveolar) + (alveolar-intrapleural)

• Similar changes occur on expiration

  • Intrapleural pressure is less negative than it would be in the absence of airways resistance because alveolar pressure is positive

  • With forced expiration, intrapleural pressure goes above zero

• The alveolar pressure tracing will be identical to flow if airway resistances remains constant during the cycle

• The intrapleural pressure curve ABC will be the same shape as the volume tracing if the lung compliance remains constant

What is the major site of resistance in the airways?

The medium-sized bronchi

The very small bronchioles contribute relatively little resistance

• This is due to the large number of small airways

Effect of Lung Volume on Airway Resistance

Like the extra-alveolar blood vessels, the bronchi are supported by the radial traction of the surrounding lung tissue and their caliber is increased as the lung expands

As lung volume is reduced, airway resistance rises rapidly

At veery low lung volumes, the small airways may close completely, particularly at the bottom of the lung where the lung is less well expanded

• Patients with increased airway resistance often breathe at high lung volumes, helping to reduce airway resistance

Conductance

The reciprocal of resistance

• Linear relationship when plotted against lung volume

What narrows the airways and increases airway resistance?

Contraction of bronchial smooth muscle

• May occur reflexively through the stimulation of receptors in the trachea and large bronchi by irritants

What is the motor innervation for bronchial smooth muscle?

Vagus nerve

What controls the tone of the bronchial smooth muscle?

Autonomic nervous system

• B-adrenergic receptors

B1 Receptors

Primarily in the heart

B2 Receptors

Relax smooth muscle in the bronchi, blood vessels, and uterus

Stimulation of adrenergic receptors by, for example, epinephrine, causes bronchiodilation

Selective B2 agonists used for the treatment of asthma and COPD

What causes bronchoconstriction?

Parasympathetic activity and acetylcholine

Antimuscarinic agents are used in COPD and occasionally asthma

A fall of PCO2 in alveolar gas causes an increase in airway resistance as the result of direct action on bronchiolar smooth muscle

Injection of histamine into the pulmonary artery causes constriction of smooth muscle in the alveolar ducts

How do density and viscosity of the inspired gas affect the resistance to flow?

Resistance is increased during a deep dive because increased pressure raises gas density

Fact that changes in density rather than viscosity have such an influence on resistance is evidence that flow is not purely laminar in the medium-sized airways which is the main site of resistance

Flow-Volume Curve

• Flow rises very rapidly to a high value (peak expiratory flow) but then declines over most of expiration

• Whether we start exhaling slowly and then accelerate (B) or make a less forceful expiration ( C ), the descending portion of the flow-volume curve takes virtually the same path

  • Indicates that something powerful is limiting expiratory flow and over most of the lung volume, flow rate is independent of effort

Isovolume Pressure-Flow Curves

• At high lung volumes, the expiratory flow rate continues to increase with effort

• At mid or low volumes, the flow rate reaches a plateau and cannot be increased with further increase in the intrapleural pressure

  • Under these conditions, flow is effort-independent

  • The reason for this behavior is dynamic airway compression (compression of the airways by intrathoracic pressure)

Why are airways compressed during forced expiration?

• A - preinspiration

  • Airway pressure everywhere is zero (no flow)

  • Because intrapleural pressure is -5 cmH2o, there is a pressure of 5 cmH2O (transmural pressure) holding the airway open

• B - during inspiration

  • As inspiration starts, both intrapleural and alveolar pressure fall by 2 cmH2O and flow begins

  • Because of the pressure drop along the airway, the pressure inside is -1 cmH2O and there is a pressure of 6 cmH2O holding the airway open

• C - end inspiration

  • Flow is again zero and there is an airway transmural pressure of 8 cm water

• D - forced expiration

  • At the onset of forced expiration, both intrapleural pressure and alveolar pressure increase by 38 cmH2O

  • Because the pressure drop along the airway as flow begins, there is now a pressure of -11 cm of water which tends to close the airway

  • Airway compression occurs and the downstream pressure limiting flow becomes the intrapleural pressure so the effective driving pressure becomes alveolar minus intrapleural pressure

    • If intrapleural pressure is raised further by increased muscular effort in an attempt to expel gas, the effective driving pressure is unaltered because the difference between alveolar and intrapleural pressure is determined by lung volume

• Flow is independent of effort

• Flow is independent of the resistance of the airways downstream of the point of collapse

  • Equal pressure point - point of collapse

  • As expiration progresses, the equal pressure point moves distally, deeper into the lung because the resistance of the airways rises as lung volume falls and therefore the pressure within the airways falls more rapidly with distance from the alveoli

Why does maximal flow decrease with lung volume?

The difference between alveolar and intrapleural pressure decreases and airways become narrower

Factors Exaggerating Flow-Limiting Mechanism

• Factors exaggerating flow-limiting mechanism

  • Any increase in resistance of the peripheral airways

    • Magnifies the pressure drop along them and thus decreases the intrabronchial pressure during expiration

  • Low lung volume

    • Reduces the driving pressure (alveolar-intrapleural)

  • Reduced recoil pressure (e.g. emphysema)

    • Reduces driving pressure

• This type of flow limitation is seen only during forced expiration in healthy patients, it may occur during the expirations of normal breathing in patients with severe obstructive lung disease

Forced Expiratory Volume (FEV_1.0)

Volume exhaled in the first second

• Normally about 80% of the FVC

Forced Vital Capacity (FVC)

Total volume exhaled

What are the two general patterns able to be distinguished in respiratory disease?

Restrictive

Obstructive

Restrictive Respiratory Pattern

• E.g. pulmonary fibrosis

• The primary problem is expanding the respiratory system on inhalation

• Both FEV_1.0 and FVC are reduced

• Characteristically the FEV_1.0/FVC% is normal or increased

Obstructive Respiratory Pattern

• E.g. COPD or bronchial asthma

• Primary problem is obstruction to airflow on expiration

• The FEV_1.0 is reduced much more than is the FVC

• FEV_1.0/FVC% is low

What type of respiratory pattern is this?

Obstructive

What type of respiratory pattern is this?

Restrictive

Forced Expiratory Flow Rate (FEF_25-75%)

Average flow rate measured over the middle half of expiration

• Generally closely related to the FEV_1.0 but is occasionally reduced when FEV_1.0 is normal

Effect of Alterations in Compliance or Resistance on Ventilation of Lung Units

• A - normal distensibility and airway resistance

  • Volume change on inspiration is large and rapid so that it is complete before expiration for the whole lung begins

• B- low compliance

  • Change in volume is rapid but small

• C- large airway resistance

  • Inspiration is slow and not complete before the lung has begun to exhale

Time Constant

Time constant = compliance x resistance

• The shorter the time available for inspiration, the smaller the inspired volume

• Said to have a long time constant

Effect of Dilation of the Airways on Diffusion

• Dominant mechanism of ventilation of the lung beyond the terminal bronchioles is diffusion

  • Normally occurs so rapidly that differences in gas concentration in the acinus are abolished within a fraction of a second

  • If there is dilation of the airways in the region of the respiratory bronchioles, the distance to be covered by diffusion may be greatly increased

    • The inspired gas is not distributed uniformly within the respiratory zone because of uneven ventilation along the lung units

Tissue Resistance of the Lungs

• When the lung and the chest wall are moved, some pressure is required to overcome the viscous forces within the tissues as they slide over each other

  • This tissue resistance is only ~20% of the total (tissue + airway) resistance in young subjects, may increase in some diseases

• Total resistance sometimes called pulmonary resistance to distinguish from airway resistance

Total Resistance in the Lungs

Total resistance = tissue resistance + airway resistance

Work Equation

Work = pressure x volume

Work Done on the Lung

• Intrapleural pressure follows curve ABC during inspiration

  • Work done on the lung is given by the area 0ABC0

  • The trapezoid 0ABC0 represents the work required to overcome the elastic forces

  • The hatched area ABCEA represents the work 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

• The area AECFA is the work required to overcome airway (+tissue) resistance on expiration

  • Normally falls within the trapezoid 0AECD0

    • So this work can be accomplished by energy stored in the expanded elastic structures and released during passive expiration

  • The difference between the areas AECFA and 0AECD0 represents the work dissipated as heat

• The higher the breathing rate, the faster the flow rates and the larger the viscous work area ABCEA

• The larger the tidal volume, the larger the elastic work area 0AECD0

• Patients who have reduced compliance tend to take small rapid breaths, whereas patients with severe airway obstruction sometimes breathe slowly

  • These patterns tend to reduce the work done on the lungs

Efficiency % Equation

Efficiency % = (Work required to ventilate the lung/Total energy expended (or O2 cost)) x 100

What is % efficiency of breathing?

~5-10%

Cost of Breathing

Cost of quiet breathing is very small, less than 5% of total resting O2 consumption

• Possible to increase to 30% with voluntary hyperventilation

Mechanics of Positive Pressure Ventilation

• Spontaneously breathing patients generate a driving pressure by increasing the size of the thorax, thereby lowering airway pressure below atmospheric pressure

• With modern mechanical ventilators, driving pressure is primarily established by raising the pressure at the mouth (positive pressure ventilation)