Mechanics of Ventilation

Lung volume and Ventilation gradient

1) downward pressure caused by weight of upper lung 2) lung volume is pushed UPWARD toward apex so that alveoli at apex are filled with FRC air, while alveoli at base are deflated 3) when you inhale VT, a larger proportion of each VT fills alveoli at the BASE

End Inspiratory Equilibrium

1) When we inspire, PA becomes negative: means gas pressure in lungs is LESS than atmospheric air since volume of lungs has increased 2) Air flows in 3) PA equilibrates with PB -- at this point, inspiration ENDS. 4) At this point, FRC has INCREASED by a volume equal to VT (FRC + VT) 5) because lungs are progressively stretched by the greater air volume, the resulting increased elastic recoil causes PL to increase (5 --> 8) to a value equal to the combined forces of Pcw + Pmuscle (-8)

INSPIRATION

1) diaphragm contracts 2) Pmuscle + Pcw > PL so outward forces > inward forces 3) chest wall EXPANDS 4) lungs are pulled OUTWARD 5) intrapleural pressure, Ppl, becomes MORE negative 6) PA becomes NEGATIVE 7) airflow is IN 8) lung volume INCREASES 9) End-inspiratory equilibrium

EXPIRATION

1) diaphragm relaxes 2) Pmuscle + Pcw < PL (outward forces < inward force) 3) chest wall RELAXES IN 4) lungs compress INWARD 5) Intrapleural pressure, Ppl, is LESS negative 6) PA is POSITIVE 7) Airflow is OUT 8) Lung volume DECREASES 9) End-expiratory equilibrium

Does the pulmonary system use positive or negative pressure ventilation?

NEGATIVE pressure

Pulmonary Pressures @ FRC

(all pressures are relative to PB so set PB (1026) = 0 cm H2O) *the 3 compartmental pressure values, PA, Ppl, PB can all be measured directly *the 3 transmural pressure values (pressure BETWEEN compartments) must be calculated indirectly from compartmental values *convention: OUTSIDE compartment values are substracted from INSIDE compartment values to obtain transmural pressures ***(INSIDE pressure minus OUTSIDE pressure) *ex: to calculate PL, the compartmental pressure OUTSIDE the lungs: PA - Ppl PA = 0 Ppl = -5 so PL = +5 **when lungs are at FRC, they exert an iNWARD force of 5 cm H2O to compress air in lungs But chest wall (Pcw = -5) and lungs (PL = +5) are OPPOSITE but equal at FRC so NET pressure is 0 (PA = 0, PB = 0 ) and no airflow gradient is generated (PRS = PA - PB =0) also respiratory muscles are relaxed (Pmuscle = 0) and exert no force

Inspiratory Respiratory Muscle Actions

= diaphragm --contracts --> expands the thorax and lungs OUTWARD --> increases thoracic volume --> creates a NEGATIVE pressure airflow gradient --> air flows INTO lungs

When does a breathing cycle begin?

At FRC At end of the previous passive expiration when lung volume is at FRC

at FRC, what is total gas pressure in atmosphere and lungs?

Both are the same: 760 mmHg or 1026 cm H2O

The relationship between airflow, air pressure, and air volume is based on

Boyle's law: P1 x V1 = P2 x V2, where P1 = P2 at equilibrium *represents 2 interconnected gas compartments (i.e. atmosphere and lungs) containing initial gas volumes (V1 & V2) that exert an initial gas pressure equal to P1 and P2 Boyles law says that at equilibrium, the product of P1 x V1 must equal P2 x V2 such that P1 = P2 (i..e no gas gradient exists). thus, no airflow occurs in or out of the lungs if V2 suddenly becomes larger (i.e. inspiration), P2 (lungs) would DECREASE relative to P1 (atmosphere). According to Boyle's Law, a proportional volume of gas, V1 (atmosphere), must flow DOWN this pressure gradient from compartment 1 (atmosphere) until both pressures requilibrate such that P1 x V1 = P2 x V2

Movement of air between atmosphere and lungs occurs by a mechanism of

Bulk airflow : caused by alternating contraction and relaxation of the respiratory muscles

During expiration, muscle relaxation causes what?

COMPRESSES chest wall around the lungs -> lung pressure becomes GREATER than atmospheric --> gases move down the gradient --> airflow OUT OF LUNGS

What pulmonary forces are passive

Chest wall + Lungs no work

Surface Tension Forces between lungs and chest wall

Chest wall and lungs move TOGETHER during the breathing cycle despite the fact that passive elastic forces of the lungs and chest wall exert opposite forces (lungs inward, chest wall outward) A thin layer of pleural fluid holds the lung and chest wall surfaces together by creating a SURFACE TENSION FORCE --> adheres the lungs and chest wall in close apposition --> prevents separation but allows the surfaces to slide freely across one another

Lung Compliance

Compliance = volume change / pressure change = measure of lungs distensibility-- i.e. how easily lungs are inflated --used to evaluate elastic recoil properties of lungs, which can be altered in diseases, which then impairs breathing *thus changes in lung compliance can alter the relative work of breathing* normal compliance of lungs = 200 ml/cm H2O

Inspiratory Pressure Gradient

Diaphragm contracts and generates OUTWARD force: Pmuscle: 0 --> -2 Pcw : -5 --> -4 Pcw + Pmuscle = -2 + -4 = -6 PL = +5 PA = -1 PRS = -1 - 0 = -1 Now outward forces (Pcw + Pmuscle) > inward force (PL) so the NET movement is air INTO lungs = inspiration

Why are expiratory muscles recruited during COPD/Emphysema?

Due to loss of much of the intrinsic elasticity of the lungs that normally allows for passive expiration

Intrapleural pressure (Ppl) between lungs and chest wall

Due to the opposing elastic forces of the lungs and chest wall, a pressure vacuum is created that would naturally act to PULL air INTO the intrapleural space from the atmosphere. (However, this doesn't occur due to the closed thoracic compartment) Instead, a NEGATIVE pressure develops within the intrapleural space (1021 cm H2O, or -5 mmHg) The Ppl value represents the MAGNITUDE of the passive elastic forces of the lungs The existence of this negative pressure compartment is important for understanding the clinical outcome of conditions that result in collapsed lungs (i.e. pneumothorax) after the closed thoracic compartment is breached: air will enter the intrapleural space

During inspiration, muscle contraction causes what?

EXPANDS the chest wall --> lung gas pressure DECREASES compared to atmospheric gas pressure --> gases move down the gradient --> airflow INTO LUNGS

What is lung volume during expiration?

FRC (bc VT is exhaled out)

What is lung volume during inspiration?

FRC + VT

Why is FRC called resting lung volume

FRC = resting lung volume bc NO airflow occurs into or out of the lungs at this volume It is the point in the breathing cycle when the mechanical forces are all at equilibrium

Inspiratory Pressure Gradient

Inspiratory muscle contraction: Pmuscle = -2 -->generates active OUTWARD force on chest wall **COMBINED forces Pcw + Pmuscle (-6 cm H2O) INCREASES total outward force relative to inward PL (+5 cm H2O) **VT flows INTO lungs (note that Ppl becomes MORE negative and outward passive force of Pcw declines during inspiration

inward elastic forces are caused by _____ while outward elastic forces are caused by _____

Inwards = lungs Outward = chest wall

How much pressure difference is required to complete a normal VT inspiratory and expiratory cycle?

Only a small amount: 1-2 cm H2O --this is important because it allows the work/energy required to breathe to remain LOW

Pulmonary Mechanical Pressures

PB = atmospheric pressure Ppl = intrapleural pressure within pleural space -- indicates that passive lung force (inward) opposes passive chest wall force + muscle force (outward) --> mainly negative PA = alveolar pressure : represents net pressure surrounding gas within alveoli and airwats that results from the NET forces of PL vs Pcw & Pmuscle PL = (PA -Ppl) = passive elastic recoil force of lungs --PL forces pull lungs and chest wall INWARD Pcw = (Ppl - PB) = passive elastic recoil force of chest wall --Pcw pulls chest wall and lungs OUTWARD PRS = (PA - PB) = net pressure difference between air in lungs and atmospheric air --relative changes in PRS initiate airflow and determines airflow DIRECTION Pmuscle = active forces of respiratory mucles --inspiratory Pmuscle = diaphragm = expands chest wall & lungs OUTWARD --expiratory Pmuscle = abdom. muscle = compresses chest wall INWARD

Pcw, declines (becomes LESS negative) during inspiration why?

Pcw becomes LESS negative because it is moving towards 0 Remember at FRC, the Pcw is 5. At FRC, chest wall is pulled slightly inward. Chest wall wants to pull outward with a force of 5 cm H20 to counterbalance that inward force. So it makes sense that if you inhale and chest wall moved from FRC OUTWARD, it moves more towards 0. So it becomes less negative. Chest wall is moving toward a more relaxed position ♣ The chest wall normally helps us inhale. ♣ The benefit of having chest wall compress inward and then having it pull outward during inhalation is that the diaphragm has to work less.

Ppl becomes MORE negative, and outward passive force of chest wall -- why?

Ppl becomes more negative because there is now a larger pressure pulling outward (muscle + chest wall) relative to pressure pulling inward (lungs) Pleural pressure is the pressure of the fluid in the thin space between the lung pleura and the chest wall pleura. As noted earlier, this pressure is normally a slight suction, which means a slightly negative pressure. The normal pleural pressure at the beginning of inspiration is about −5 centimeters of water, which is the amount of suction required to hold the lungs open to their resting level. During normal inspiration, expansion of the chest cage pulls outward on the lungs with greater force and creates more negative pressure, to an average of about −7.5 cen- timeters of water.

Intrapleural pressure gradient across lungs

Ppl is more negative near the apex relative to the base due to the more highly inflated alveoli near the lung apex ♣ Since more inflated alveoli at top at rest, its as if we took a really deep breath at apex even though we are at FRC --> much higher intrapleural pressure at apex: lungs are compressing on air much more strongly than at the base, which are less inflated i.e. at the apex, the lungs have much stronger recoil inward force so the intrapleural pressure must be larger to keep the lungs expanded *The intrapleural pressure Ppl gradient flattens during inspiration*

Pulmonary pressures at rest

Reference point = atmosphere = 1026 cm H2O = 0 cm H2O ACTIVE FORCES Pmuscles = 0 OUTWARD FORCES Pcw (chest wall) = -5 INWARD FORCES PL (lung) = + 5 PA (pressure around alveoli) = 0 PB = 0 PRS = PA - PB = 0

What pulmonary mechanical forces are active

Respiratory mucles active = work = requires ATP

Active mechanical forces: respiratory muscles

Respiratory muscles exert ACTIVE forces --inspiration is an ACTIVE process that involves contraction of DIAPHRAGM which initiates inspiration by EXPANDING the thorax The net effect of inspiratory muscle contraction is INCREASED THORACIC VOLUME which creates a NEGATIVE pressure in the lungs relative to atmosphere --Expiration is normally PASSIVE and results from RELAXATION of inspiratory muscles --Expiration can become active during exercise or voluntary hyperventilation : abdominal muscles and accessory muscles are responsible for active expiration, and function during coughing, vomiting, defecation

What is Functional Residual Capacity?

The volume remaining in lungs at the end of normal expiration == RESTING VOLUME of lungs, becuase it is the point in the breathing cycle when the mechanical forces are all at equilibrium == starting point in the resting breathing cycle when inspiration is initiated

Pneumothorax

When a wound penetrates the intrapleural space, the negative intrapleural pressure (1021 cm H2O) causes atmospheric air to rush into the pleural cavity for pressure equilibration This entry of air into the intrapleural space decreases surface tension between lungs and chest wall That causes the lungs to COLLAPSE inward --> decreaases lung volume to less than Residual Volume because FRC is lost Lungs collapse because without the Positive OUTWARD pressure of the chest wall, the inward recoil of the lungs causes lung deflation

Expiratory Respiratory Muscle Actions

abdominal muscles, accessory muscles --contracts --> compresses the thorax/lungs INWARD --> decreases thoracic volume --> creates POSITIVE pressure airflow gradient --> air flows OUT of lungs *but expiration is mostly passive, and expiratory muscles remain largely inactive during restful breathing since the PASSIVE ELASTIC INWARD force of the lungs is sufficient to expire the necessary air volume *expiratory muscles are mainly recruited during conditions that require vigorous active expiration such as exercise or pathological conditions (COPD/emphysema)

Pleural Effusion Disorders

accumulation of fluids within the pleural space (pneumothorax = AIR in the pleural space) Both pleural effusion + pneumothorax result in the loss of surface tension between chest wall and lungs --> lungs collapse --> atelectasis

airway closure

air remains in lung, even when the surrounding pressure is 0. This is called AIRWAY CLOSURE caused by trapped alveolar air within lower lung regions at very low volumes -- air becomes trapped because small airways (near resp. bronchioles) tend to close FIRST, trapping gas within more distal alveoli --normally, airway closure only occurs at very low lung volumes (but can occur in normal breathing range in the elderly) IS A NORMAL FEATURE OF LUNG COMPLIANCE CURVE

At FRC, gas pressure in lungs is equal to

atmospheric pressure (1026 cm H2O) such that NO AIRFLOW occurs

In what direction does the mediastinum shift in a pleural effusion or pneumothorax?

contralateral mediastinal shift

compliance diagram

diagram relating lung volume changes to changes in pleural pressure, which, in turn, alters transpulmonary pressure. Note that the relation is different for inspiration and expiration The characteristics of the compliance diagram are determined by the elastic forces of the lungs. These forces can be divided into two parts: (1) elastic forces of the lung tissue and (2) elastic forces caused by surface tension of the fluid that lines the inside walls of the alveoli and other lung air spaces The elastic forces of the lung tissue: In deflated lungs, the elastic fibers are in an elastically contracted and kinked state; then, when the lungs expand, the fibers become stretched and unkinked, thereby elongating and exerting even MORE elastic FORCE.

Expiratory pressure gradient

during expiration, inspiratory muscles relax: -7 --> -5 Pcw goes from -1 --> -2 this removes the dominant active outward force on the chest wall without the muscle force, the combined forces of Pcw + Pmuscle (-7) are LESS than inward PL force (+8). The now dominant force is INWARD due to lungs inward recoil force --> pulls thorax INWARD --> volume decreases --> pressure around alveoli increases (PA = 1) --> net pressure gradient is OUT of lungs VT flows down its gradient OUT of lungs

contraction and relaxation of respiratory muscles cause airflow by creating

gas pressure gradients between the total gas volume in lungs and total gas volume of atmosphere

Increased compliance

indicates a greater than normal volume change in response to a given pressure change i..e lungs are EASIER to inflate == LESS WORK needed to inflate lungs

End-inspiratory equilibrium

inspiration ends when PA equilibrates with PB PA = 0 PB = 0 PRS = 0 Pmuscle

Compliance and Lung Volume --- why is it sigmoid shaped

is SIGMOID SHAPED --means that lung compliance is not the same at all lung volumes similar to inflation of a balloon: 1) initially, an uninflated balloon is hart to start, requiring much effort (force) to inflate (volume) = *lung compliance is LOW at LOW volumes* due to high elastic recoil forces 2) Once started, the balloon becomes easily distensible, inflating more easily = *lung compliance increases greatly at volumes within normal resting breathing range* 3) However, as the ballon becomes maximally inflated, an elastic limit is reached, such that further inflation is difficult

what does mechanics of ventilation mean?

it refers to the basic process of bulk air exchange between the lungs and atmosphere

Lung elastic properties

like a balloon --when filled with air, lung elastic forces are directed INWARD --> compresses and forces air OUT of lungs

Chest Wall elastic properties

like a spring --when compressed, chest wall elastic forces PULL OUTWARD to expand the thorax

Hysteresis and Airway Closure

lung volume at any given pressure is higher during deflation (expiration) than during inflation (inspiration) = HYSTERESIS o When we inhale, and we exhale that same volume it follows a different path back to FRC o Think its related to molecules secreted by lungs called surfactant, which regulates lung elastic recoil properties o Surfactant has beneficial effect: decrease work of breathing that would normally be in lungs IS A NORMAL FEATURE OF LUNG COMPLIANCE CURVE

Expiration at rest occurs by

muscle RELAXATION--> causes airflow OUT of lungs --> return down to resting volume: FRC

Prs

o is the difference between air in lungs and atmospheric pressure ♣ represents the gradient ♣ tells you the direction of movement - is person inhaling or exhaling ♣ if lung force is greater than atmospheric force = exhaling, PRS would be positive ♣ if lung force is less than atmospheric, then the overall force is to pull air in = PRS is negative = inhalation ♣ PRS is the outcome of al the forces • And how it affects the overall pressure relationship

Passive Elastic Forces = Lungs and Chest Wall

refer to the intrinsic elastic nature of the lungs and chest wall

Inspiration is initiated by

respiratory muscle CONTRACTION --> causes airflow INTO the lungs --> INCREASES LUNG VOLUME : FRC + VT

Atelectasis

result of pneumothorax --> Lungs collapse because without the Positive OUTWARD pressure of the chest wall, the inward recoil of the lungs causes lung deflation --> incomplete inflation of lung or portion of lung detaches from chest wall

Decreased Compliance

smaller than normal volume change occurs in response to a given pressure change i.e. lungs are MORE DIFFICULT to inflate ==MORE WORK needed to inflate lungs

Ventilation mechanism involves the coordination of

the pulmonary system (lungs, chest wall, respiratory muscles) that genertes the mechanical movements associated with breathing