Pulmonary Mechanics Flashcards
Lung tissue has an innate desire to:
Collapse inward
Due to elastin fibers and surface tension
What is the key to keeping the lungs inflated?
Negative pressure in intrapleural space
Pleura that lies on the lungs
Visceral pleura
Parietal pleura
Lines the chest wall, mediastinum, and diaphragm
What is in the pleural cavity?
Negative pressure!!! “Gluing” lungs to chest wall
Serous fluid for lube
Role of diaphragm
Lengthens chest cavity and increases volume
Majority of for for passive tidal breathing
Moves 1-2 cm in tidal breath and up to 10 in forceful breath
Role of accessory muscles in breathing and name three
Increase anterior- posterior diameter
External intercostals
Sternocleidomastoid
Anterior serrati
Muscles of expiration
Tidal exhalation is passive! No active muscle use- recoil of elastic tissue
Active exhalation- abdominal recti and internal intercostals (pull rib cage down against diaphragm)
Most important gas law for ventilation
Boyles- at fixed temp volume and pressure are inversely related
Is spontaneous/negative pressure ventilation active or passive?
Passive movement of gas
Intrapleural pressure
Always negative under normal conditions
Pressure in pleural space
Synonymous with intrathoracic pressure
Normally -5 cm h2o
Intra-alveolar pressure
Always >than intra pleural pressure
Can be greater than or equal to atmospheric pressure depending on phase of ventilatory cycle
Negative or positive
Trans-pulmonary pressure
Positive
TPP= alveolar pressure-intrapleural pressure
What’s your transpulmonary pressure when:
Intra-alveolar pressure= -1
Intrapleural pressure = -7
+6
Difference in alveolar pressure between negative and positive pressure ventilation:
Alveolar pressure is ALWAYS positive in positive pressure ventilation
It can be negative or positive in negative pressure
Explain negative pressure ventilation
Pressure LOWER than atmospheric is applied to the extrathoracic space during inspiration
Air move down pressure gradient to lungs
Mouth is at atmospheric pressure (seperated from extrathoracic space)
So making the extrathoracic space more negative expands the chest, increasing the volume in the lungs=therefore decreasing the pressure in the lungs… if confused look at the pic
Define lung compliance
Change in volume over change in pressure
Ie how much will the lung expand at a given pressure/
What is the average compliance of both lungs in a normal adult?
200 ml of air per cmh2o trans-pulmonary pressure
Hysteresis
Distance between slope of inspiration and slope of expiration on a pressure-volume curve
What indicates decreased compliance on pressure volume curve?
More horizontal slope
Why does compliance change with a change in volume in the lungs?
There is a change in surface tension of the alveoli
Type of compliance with no airflow
Static
What determines lung compliance?
Elastin and collagen (static)
Explain surface tension and compliance
When water forms a surface with air, the water molecules have a particularly strong attraction for one another
As a result the surface is always attempting to contract
Role of surfactant
Combats the surface tension of water
Secreted by alveolar type II
Law of laplace
Describes surface tension, pressure, and radius
P= 2T/r
P-distending pressure
T-surface tension
What allows us to equally ventilate alveoli of different sizes?
Smaller alveoli have more concentrated surfactant that lowers overall pressure?
Dynamic compliance
Always less than static- because compliance is reduced by airflow resistance and thats the added variable to dynamic
Primary factor is airway resistance- it is compliance during air movement
Flow equation
Q=change in P/R
Flow is inversely related to resistance and directly related to change in pressure
Where does true laminar flow occur?
Smaller airways where diameter is small and linear velocity is low
What produces breath sounds on auscultation?
Turbulent flow
Important thing about poiseuilles law
R=8nl/3.14(r^4)
Calculates resistance in laminar flow
Radius is the biggest factor ^4
Measure of dynamic compliance
Peak pressure
Measure of static compliance
Plateau pressure
If peak pressure increases but plateau pressure is constant, what is the problem?
Resistance
If peak and plateau pressure increase what is the problem?
Compliance
Define peak pressure
Max pressure in proximal airway at end inspiration
Peak= (airway resistance)/(compliance)
Plateau pressure is a measure of what?
Compliance
Equation for static compliance
Vt/ (Pplat-PEEP)
Takeaway: plateau pressure
Dynamic compliance equation
Vt/(PIP-PEEP)
All about that PIP
Which division of the tracheobronchial tree
Is responsible for greatest
resistance to airflow?
Medium sized bronchi
B/c: Smaller airways technically have a greater cross sectional area
spirometry visual (IRV,TV,ERV,RV)
ERV
Expiratory reserve volume
Amount you can forcibly expire past normal tidal breathing
IRV
Inspiratory reserve volume
Amount you can forcibly inspire past normal Tidal breathing
Residual volume
Amount left in the lungs after forced exhalation
FRC
ERV+RV
Volume undergoing gas exchange between breaths, why we pre-oxygenate
Closing capacity
Sum of the closing volume plus residual volume
Closing volume
Volume where small airways close
What occurs when closing capacity exceeds FRC?
Some degree of airway collapse during normal tidal breathing
-this happens because everything we do in surgery/anesthesia decreases FRC
Shunt vs deadspace
Shunt- perfused but not ventilated
Deadspace-ventilated but not perfused
Anatomic dead space
Fixed per weight-2ml/kg
Volume of gas in conducting airways that do not participate in gas exchange
Alveolar deadspace
Alveoli that are ventilated but not perfused
Largest contributor to alveolar deadspace is a decrease in CO
Physiologic dead space
=anatomic+alveolar
Tidal volume calculation
Va+Vd
Alveolar ventilation + Deadspace volume
Minute ventilation calculation
RR X Vt
Alveolar ventilation calculation
RR X (Vt-Vd)
Vd-deadspace volume
Bohr equation
What does the Bohr equation give you?
Percent per volume of deadspace in Vt
Saftey release valve on mech vent (location)
Extra safety measure to regulate the pressure in the bellows housing
Spill valve
Synonymous with APL
In the patient circuit
Requires 2-4cmh2o to open, this creates (auto) peep
Location of exhaust vs spill valve
Exhaust- on the drive gas circuit
Spill- in patient circuit
Advantages of piston vents
More accurate tidal volumes to small patients or those with poor lung function
Will not deplete o2 in case of pipeline failure
No intrinsic peep
Fresh gas coupling
FGF is continuous and combined with the total volume delivered to the patients lung (ie a little extra)
FGF decoupling
FGF is diverted and not mixed with gas from ventilator
More accurate tidal volumes
Potential injury form piston vent
Negative pressure pulmonary edema
Trigger variable vs cycle variable
Trigger- vent cycles to inspiration
Cycle- tells the ventilator when to switch from inspiration to expiration
3 types of breaths on ventilator
Mandatory- triggered and supported by vent
Assisted- triggered by patient and supported by vent
Spontaneous- not triggered or supported by vent
Volume control variables
Independent- tidal volume and RR
Dependent- peak pressure (determined by system)
Other name for volume control and how to identify
Volume limited
-Shark fin in pressure
-delivered with a constant flow (top hat in flow)
-All breaths mandatory and timed
-Peak pressures vary
Disadvantage to volume control
-no control over pressure so you need a secure airway
Pressure control variables
Independent-pressure and RR
Dependent- tidal volume
Recognizing pressure control
Pressure-top hat
Flow- descending
Advantages/disadvantages of Pressure Control
Adv- increased airway pressure, better recruitment, protective against barotrauma
Dis- volume is variable and may cause volutrauma
Identifying Pressure SUPPORT
-negative inspiratory trigger before each breath
-set pressure for each breath=top hat
Pressure support variables
Independent- minimum RR, pressure for spontaneous breaths
Dependent- Tv for spontaneous breaths
When to use Pressure support
Good for LMAs
Used when weaning from vent
No mandatory breathes
SIMV independent/dependent
Spontaneous intermittent manual ventilation
Independent-
Tv for mandatory breaths,
&
Pressure support for spontaneous breaths
Pressure control- volume guarantee
Using pressure control to achieve a preset volume, I bet you can set a pressure limit.. so maybe like volume control but with some say over peak pressures
Little tooo smart- not tested
What are signs spontaneous ventilation is returning?
Negative deflection on pressure curve
Curare cleft
Normal Tv
Based on ideal body weight
6-8ml/kg usually btwn 400-500mL
Best way to offload CO2
NOT RR
Increase Tv to address the dead space ratio
What can high RR lead to?
Air trapping due to less time for exhalation
What variable is impacted when you change the RR?
Dependent
When is low fiO2 preferred
COPD
High risk of airway fire- anything above the xyphoid process
Bleomycin or BEP (chemotherapy agent) only give medical air- this can trigger pulmonary fibrosis and kill the patient if exposed to high O2 concentrations
When might we prefer to hyperoxygenate?
Colorectal surgery…..
Normal I:E ratio and COPD
Normal- 1:2
COPD- 1:3
Manipulate I:E to decrease peak pressure
Increase the I time can allow more time to achieve the Tv and decrease peak pressure
What is peep and what is peep not
Positive pressure that will remain in airway at the end of the respiratory cycle
IT IS NOT a recruitment maneuver
What does PEEP accomplish
-improves oxygenation via Henry’s law
- improves V/Q mismatch y keeping small airways open
-decreases work by keeping lungs in optimum place on compliance curve
-decreases sheer stress and bio trauma that occurs from cyclic opening and closing of alveoli with positive pressure
Consequences of PEEP
Reduced preload
Elevated plateau pressures
Impaired cerebral venous flow
Lung protective ventilation components
Low Tv
Recruitment maneuvers
PEEP
What does a recruitment maneuver do?
Transiently increases transpulmonary pressure with the goal of re-opening alveolar units that are not aerated
How to perform Recruitment Maneuver
Increase Tv until Pplat is 30cmH2O
Increase peep to 20cmh20
Manually inflate reservoir
Timing of recruitment maneuvers
Can perform for up to 30 seconds safely but peak benefit is up to ten seconds… So ONLY do it for 10 you dingus
When to avoid recruitment maneuvers and one common mistake
Avoid: elevated ICP, hemodynamic instability, uncuffed ett, bronchospasm, pneumothorax
REMEMBER- always increase peep otherwise it was useless
Distensibility
the ability of a tissue to stretch and expand when pressure is applied
Distensibility
the ability of a tissue to stretch and expand when pressure is applied
Describe flow graph in Pressure control
Descending
Define trigger level in mechanical ventilation
The amount of downward deflection needed to trigger an assisted breath by the ventilator
Peep improves oxygenation via which law?
Henrys
Pressure of gas above a solution…..
