Ventilators Flashcards
Direction of Bellows
Based on how fill on expiration
Ascending: rise on exhalation, aka standing
Descending: drop on exhalation, aka hanging
Disadvantages of descending bellows
–Draw air out of patient’s airway during expiration DT weight on bottom of bellows
–Increases work of breathing to raise bellows during inhalation (SpV)
–Can also generate sub atmospheric pressure during early part of expiration if expiratory flow not impeded, low FGFs
Advantages of Ascending Bellows
Less WOB
Does provide 2-4cm H2O PEEP
Leaks - Descending Bellows
with leaks – air dilution as bellow expands, will not reach bottom
Leaks - Ascending Bellows
detect leaks in BC by failure to rise completely
Summary of Pneumatic Driving Mechanism
compressed gas (~45-50psi) flows into bellows canister –> compression of accordion bellows –> delivery of breath
Summary Of Piston
electronically driven piston compresses bellows
LA Ventilators: Drager, Narkovet E, Surgivet
Dual circuit
Volume limited
Time cycled (fluid in drager, electronics in Narkovet, Surgivet)
Pneumatically driven
Descending Bellows
LA Ventilators: Mallard
Dual circuit
Volume limited
Time cycled
Pneumatically driven
ASCENDING Bellows
LA Ventilators: Tafonius
Single Circuit
Volume Limited
Time, volume cycled (electronic)
Piston
LA Ventilators: JD Medical/Bird
Dual Circuit
Pressure Limited
Time Cycled
Pneumatically Driven
Descending bellows
SA Ventilators: Mallard, Hallowell, Omega, Surgivet
Dual
Volume Limited
–Omega 7000 has preset minute volume
–7800 preset VY
Time cycled (all electronic)
Pneumatically driven
Ascending bellows
SA Ventilators: Ohio, Drager
Dual
Volume Limited
Time cycled - both fluid circuits
Pneumatically driven
DESCENDING bellows
SA Ventilators: Vetronics
Single Circuit
Volume limited
Time, pressure, volume cycled
Piston
SA Ventilators: Engler
Pneumatic, pressure limited ventilator
No bellows, FM, RBB: acts like NRB (no CO2)
Delivers inhalant only during inspiration via VDG through vaporizer, inflates lungs via positive pneumatic pressure
RR, PIP, FR controlled by device
Barotrauma
injury resulting from high airway pressure
Compliance
ratio of change in vol to change in pressure
o Measure of distensibility, usually expressed in mL/cm H2O
Continuous positive airway pressure (CPAP)
airway pressure maintained above ambient, usually in reference to SpV
Exhaust valve
valve in vent with bellows that opens to allow driving gas to exit bellows housing
Expiratory Flow Time
time between beginning, end of expiratory flow
Expiratory pause time
time from end of expiration flow to start of inspiratory flow
Fresh Gas Compensation
means to prevent FGF from affecting VT by measuring actual VT, using information to determine delivered VT
Fresh gas decoupling
means to prevent FGF from affecting VT by isolating FGF so doesn’t enter breathing system during inspiration
Inspiratory Flow Time
period btw beginning, end inspiratory flow
Inspiratory Pause Time
portion of inspiratory phase time during which lungs held inflated at fixed pressure or volume – ie time during which inspiratory phase has zero flow
Inspiratory Phase Time
time btw start of insp flow, beginning of expiratory flow
o Sum of inspiratory flow, insp pause times
IE Ratio
ratio of inspiratory phase time to expiratory phase
Inspiratory Flow Rate
rate at which gas flows to patient expressed as vol per unit time
Inverse ratio ventilation
ventilation in which inspiratory phase time longer than expiratory phase time
Possible for negative CV effects
Minute Volume
sum of all VT within one minute
Peak Pressure
maximum pressure during inspiratory phase time
Plateau Pressure
resting pressure during inspiratory pause, airway pressure usually falls when inspiratory pause – lower pressure = plateau pressure
PEEP
airway pressure above ambient at end of exhalation, CMV
Resistance
ratio of change in driving pressure to change in FR, commonly expressed in cm H2O per second
Sigh
deliberate increase in VT for one or more breaths
Equivalent of ARM
Solenoid
component that controls pneumatic flow by means of electronic signal
Spill Valve
valve in ax vent that allows excess gases in BS to be sent to SS after bellows or piston has become fully filled during exhalation
VT
vol of gas entering or leaving patient during insp, exp phase time
Volutrauma
injury DT overexpansion of lungs
Work of Breathing
energy expended by patient +/- ventilator to move gas in/out of lungs
o Expressed as ratio of work to vol moved, joules per liter
o Includes work needed to overcome elastic, flow-resistance forces of both resp system, apparatus
Purpose Ventilator
provide patient ventilation control; replaces reservoir bag and APL valve with bellows/piston chamber and spill valve
Basic MOA Dual Circuit, Pneumatically Driven Ventilator
Bellows housed in pressure chamber: inside of bellows connected to BS
Bellows acts as interface btw BS, VDG
* Separates breathing system gas from driving gas
* Pressure of anesthesia provider’s hand replaced by driving gas pressure that compresses bellows
Pneumatically Driven: Inspiration
driving gas delivered into space between bellows, housing
Causes bellows to be compressed gas flows into breathing system
At same time, spill valve (which vents excess gases to scavenging system) and exhaust valve (which vents driving gas) closed
Pneumatically Driven: Expiration
bellows re-expand as breathing system gases and fresh gas flow into it
Driving gas vented to atmosphere through exhaust valve
After bellows fully expanded, excess gas from BS vented to SS through spill valve
Piston Driven MOA
Electronically driven piston, eliminates need for drive gas circuit
Reservoir bag not isolated from BS during exhalation phase of automatic ventilation
* RB acts to modulate pressure increases in system
Piston: Inspiration
piston forces gases into breathing system
* Bag isolated from the breathing system
* Bag collects FGF entering BS
* Some ventilators: bag can be seen to expand, contract with respiration even though piston actually ventilating patient (eg Fabius)
Major Control Variable for Ventilators
Volume limited: Deliver set volume to patient regardless of airway pressure, stops when preset volume reached
Pressure limited: PIP determines ventilator’s action, regardless of VT
–variable based on compliance/resistance, insp flow rate, leaks, insp time, location of pressure sensor
Power Source
What is required to operate mechanical ventilator: compressed gas, electricity, both
Most modern ventilators: some electrical components regardless of what actually produces PPV
Circuit Drive Mechanism
Compressed gas (pneumatically driven) or electronically driven mechanical device (piston)
Compressed gas ventilators: dual circuit – two gas circuits; one for ventilator, one for breathing circuit
Single circuit ventilators: do not use gas to power ventilation
Can ventilators be set as dual or single circuits?
Yes - Bird Mark, Penlon Nuffield
Can be combined with bellows canister system (bag in a box) to create dual circuit ventilator
Lack of physical barrier potentially allows mixing of gas from circuit used to power ventilator with gas from patient circuit
* FGF ensures that mixing of driving gas, circuit gas doesn’t cause rebreathing
Cycling Mechanism
Most = timing
Electronic microprocessor or fluid logic (older ventilations)
Electronic Microprocessors for Cycling Mechanism
open, close valves allowing gas to drive ventilator or activate movement of piston
Fluid Logic Units
Use compressed gas normally at constant pressure to activate timing valves for inspiration and expiration
* Amt of gas needed to activate timing valves, hence inspiratory/expiratory times, can be altered by increased or decreased flow of gas into timing valve
* Valves open and closed by changes in gas pressure within timing valves
* Pressure used to produce predictable fluid timers
Related to Coanda effect
Which ventilator(s) are pressure cycled?
Bird, Engler
Timing of inspiration function of duration of time required to attain specific pressure within patient circuit during PPV
Cycling duration modified by gas FRs driving ventilator, changes in patient lung compliance
Inspiration continues until specific threshold achieved regardless of time required to achieve pressure
Components of Single Circuit Ventilators
Cylinder
Piston
Linear actuator
Rolling diaphragms
Positive/negative pressure relief valve
Single Circuit Piston Ventilators: Advantages
Use electronically controlled pistons to compress gas in BC - eliminates need for second circuit (driving gas)
More precise delivery of VT, not influenced by presence of compressible driving gas
More efficient in terms of gas: additional gas not required to drive ventilator
MOA Single Circuit Ventilators
2 rolling diaphragms that seal piston to prevent mixing of ambient and patient circuit gas
* Lower: seals breathing gas below piston in BS
* Upper seals upper side of lower diaphragm from ambient air to create space between 2 diaphragms
* Vacuum applied to space, holds two diaphragms tightly against piston, cylinder walls
Piston moves downward, space below lower diaphragm decreases, forcing gas into patients lungs
At end of inspiration patient exhales piston rises
Piston Driven Ventilators: Expiration
piston moves in response to measured airway pressure, measured at Y piece
* When airway pressure increases by .5 cmH2O, piston moved up enough to bring airway pressure back to 0
* Correction made Q5ms (200 times per second), ensures no resistance to exhalation
Bird Mark, Penlon
Key about single circuit: patient gas and ventilator drive gas move within same continuous circuit
No physical barrier between ventilator drive gas, patient gas
Can turn into dual circuit by combining with bellows in a canister
Circuits used with a Bird Mark or Penlon
Non rebreathing FR/NRB systems, prevent contamination of patient circuit with ventilator drive gas
Best suited for smaller patients
Possible to ventilate up to ~1000mL: efficiency of gas (oxygen, inhalant) consumption required markedly reduced
Penlon Nuffield
long corrugated tube that acts as a ‘reservoir,’ replaces rebreathing bag
Reservoir tube responsible for both exhaled, VDG
Inspiration: Patient valve closes, forcing ventilator driving gas into reservoir tube - compressing circuit gas back toward patient
Expiratory pause: patient valve opens, allowing ventilator driving gas to exit system via spill valve as pressure within circuit drops back to 0
–Followed closely by expired gas via high FGF
Penlon Nuffield - Key Points
–Must have adequate FGF to prevent rebreathing
–VT: volume of drive gas delivered by ventilator + volume of FGF entering circuit during inspiration
Dual Circuit
Physical separation btw two circuits of gas via compliant bellows
Primary: continuous with patient circuit = bellows, spill valve
Second: driving gas used to compress bellows
Dual Circuit: Inspiration
Drive gas allowed into bellows housing for a specific period of time, delivered at specified rate –> housing exhaust valve closed –> compression of bellows, closing of spill valve –> gas enters patient’s lungs
Dual Circuit: Expiration
Expiration: driving gas discontinued, housing pressure/patient circuit pressure drop –> gas in bellows housing allowed to escape from exhaust valve –> passive patient exhalation into bellows
Spill valve reopens: FGF from patient circuit to escape - prevents pressure from building up within patient circuit
Ascending/Standing Bellows
o Up during expiration
o Spill valves normally pose slight resistance to opening: slight PEEP (2-4cm H2O) in system to counteract tendency of bellows to collapse DT their weight, elastic nature
–Can be mitigated by application of slight vacuum to interior of bellows housing
Advantage: bellows will fail to expand fully, progressively collapse if leak in BS
Descending/Hanging Bellows
o Down during expiration
o Descent during expiration = passive, facilitated by weight placed in dependent portion of the bellows
o Can cause slight negative pressure in bellows/BS: if leak or disconnect, weight of bellows will cause normal expansion –Extraneous gases drawn into BS via leak, negative pressure relief valve
Increased WOB
Detecting a Leak with Hanging Bellows
gas in BS diluted by non-anesthetic gases, but all or some of inspiration lost through leak
Difficult to visually assess leaks vs ascending
Essentially bellows that fall very slowly or do not fall completely before next inspiration occurs
Bellows Housing
Clear plastic, direct observation of bellows movement
Exhaust Valve
Communication between inside of bellows housing, external atmosphere on pneumatically driven ventilators
Inspiration exhaust valve closed: DG builds pressure within housing, bellows compress driving air into patient
Exhalation: exhaust valve opens, DG stops, pressure decreases, bellows reexpand
Spill Valve
Replaces function of APL valve (closed during CMV)
Directs excess FG from BS into SS during exp pause
Insp: valve closed to prevent escape of gas into SS, allow PP to develop within circuit
Standing bellows: spill valve has normal opening pressure of 2-4cm H2O to offset downward force created by weight of bellows, allows complete filling during exhalation
Normally controlled pneumatically in ventilators using gas to compress bellows, open/closed electronically with piston
VDG
DG supplied to ventilator normally under intermediate pressure, 35-55 PSI
DG = typically 100% oxygen
Minimizes potential for decreased O2 concentration should leak develop between 2 ventilator circuits
Control of VDG
Electronic
Inspiratory time: Duration of time DG allowed in bellows housing
Inspiratory Flow Rate: rate at which DG flows into bellows housing
Expiratory Time: pause between inhalations
Manipulation of above 3 variables leads to control RR, VT, IE ratio
How FGF Affects VT
Increasing FGF, prolonging inspiratory time = larger VT
Significant in very small patients: FGF 1L/min +17mL of gas to VT during inspiration
How Compliance, Compression Volumes Affect VT
Increases in compliance of breathing system, decrease patient Vt - more of delivered gas volume expended expanding breathing components
- Gas volume compressible when subjected to increasing pressure
How Leaks Affect VT
Gas escapes through leaks = reduction of delivered VT
Side stream airway gas monitors aspirate small volume of gas from BS (50-250mL/min, 0.4-0.8mL/s)
Alarms for Ventilators
No standard alarm configurations for veterinary anesthesia ventilators
Low driving gas pressure alarm: when DG pressure < 35psi
* Decrease in DG pressure below a certain level - possible decrease in delivered VT
VT On Most Pneumatically Driven Ventilators
Tidal volume on most pneumatically driven ventilators limited by:
1. FR of gas entering bellows housing
2. duration of time gas is allowed to enter bellows
VCV
preset VT
If inspiratory flow too low to provide set VT, bellows/piston will not complete excursion
If flow set at faster rate than needed to provide VT = inspiratory pause
Excessively high PIP if inspiratory FR too high
Inspiratory phase may be terminated before VT delivered if peak airway pressure reaches set pressure limit
VCV - pressure waveform
Steadily increases during ventilation
VCV - volume waveform
Identical each breath
VCV - flow waveform
Square top appearance bc gas flow constant while waiting to achieve volume
Delivery of VT with CVC
For given VT, pressure in BS determined by resistance, compliance of BS, patient
Adding PEEP decreases TV delivered
* Effect greater with small TV
PCV
Operator sets inspiratory pressure at level above PEEP, vent quickly increases pressure to set level at start of inspiration –> maintains pressure until exhalation begins
PCV: flow waveform
o Inspiratory flow gas highest at beginning of inspiration, then decreases
Increased resistance may change shape of flow vs time waveform = flatter, more square-shaped pattern
* DT VT delivery shifting to latter part of inspiration
Spikey
PCV: volume waveform
TV not set/constant - fluctuates with changes in resistance/compliance, with patient-ventilator asynchrony
If resistance increases or compliance decreases, VT will decrease
PCV: volume waveform
will change with each pressure bc based on thoracic compliance
Synchronized Intermittent Mandatory Ventilation
Synchronizes ventilator-delivered breaths with patient’s spontaneous breaths
If patient inspiratory activity detected, ventilator synchronizes its mandatory breaths so set resp frequency achieved
Trigger time: ventilator checks whether airway pressure has dropped minimum amount below pressure measured at end of expiratory phase
Drop not sensed –> vent delivers a breath
Pressure Support
Augments patient’s spontaneously breathing by applying positive pressure to airway IRT patient initiated breaths
Pressure or flow initiated
Provider must set trigger sensitivity, inspiratory pressure
Triggering sensitivity set so that will respond to inspiratory effort without auto cycling in response to artefactual changes in airway pressures
Engler - features
o Non rebreathing circuit (unidirectional circuit)
o No bellows housing, no piston
o No canister for chemical CO2 absorbent
o Not intended for connection to other BS
Also no FM, RBB, no one way valves
Engler - Requires
- precision vaporizer
- breathing hose
- O2 supply 50psi
- Scavenging system that functions as ax delivery system
Engler - MOA
Ventilation drive gas moves through vaporizer, only delivers gas (and anesthetic) to patient during inspiration
Intermittent bursts of oxygen at high flow rates 2-60L/min - inflates lungs via positive pneumatic pressure
Oxygen 50 PSI for normal mode, 5 PSI for laboratory/low flow mode
Exhaled CO2 washed out via scavenge
Engler - limitations
Concern about accuracy of vaporizer output from intermittent nature of carrier gas delivery to vaporizer - VDG moves through vaporizer
Gas FR required to produce VT required for larger patients may fall outside recommended flow rates for most vaporizers
Engler - limitations
Concern about accuracy of vaporizer output from intermittent nature of carrier gas delivery to vaporizer - VDG moves through vaporizer
Gas FR required to produce VT required for larger patients may fall outside recommended flow rates for most vaporizers
JD Medical’s LAV-2000, 3000 series LA ventilators
Combined with Bird: Dual-circuit, pressure-limited, time cycled, pneumatically controlled/driven with descending bellows configuration
When operating, Bird ventilator supplies gas to pressurize space btw bellows, bellows housing (canister) = force bellows in upward motion delivering gases from bellows, through interface hose, to BS
Bird Ventilator - cycling mechanism
o Manometer, hand timer (push-pull mechanism) used to initiate/end respiration
pressure cycled ventilator unless push-pull manual cycling rod pulled out time cycled
Manual Resuscitators
o Compressible, self-expanding bag, bag-refill valve, NRB valve
o +/- attachment for oxygen
Demand Valves
Deliver oxygen when patient begins to inspire creating slight negative pressure that activates gas delivery
Manual delivery via compression of activation button
Disconnected from ETT after inspiration, decrease resistance to exhalation
Demand Valves Flow Rates
High inspiratory flow rate desirable in large animals
Low inspiratory flows: excessively long inspiratory time in patients requiring large VT
Maximum flow rate of approximately 160L/min at 50PSI, low flow 40L/min
Hudson demand valve: 200L/min if oxygen supply pressure 50 PSI, >275L/min if 80PSI
