Physical Principles of Mechanical Ventilation
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Basic Characteristics of a ventilator include:
- -a power source or input power
- -a mechanism for delivering the gas to the patient circuit. Mechanical drive mechanism.
- -a control system with user interface that allows the operator to set and adjust various parameters.
- -an internal control system that controls ventilator function
- -an output control that determines how the mechanical drive mechanism delivers the gas.
What type of power sources can you have for a ventilator?
electrical, pneumatic, or both
What source powers the drive mechanism to deliver the gas?
the power source
What do most new ventilators require?
Electricity for the control panel (the microprocessor may not require electricity for the drive source).
What type of ventilators rely entirely on electricity to function?
Electrically powered ventilators. can be either a wall outlet or battery. can operate pistons, solenoids, motors, compressors, electromagnets and computer circuits
What does electrical power allow for?
timing mechanisms, alarms, flow controls, and microprocessor controls like wave form displays, and data storage analysis.
What type of ventilators are powered exclusively by 50 PSI gas sources (compressed o2)? Example: Bird Mark 7
pneumatically powered ventilators. may also only require one or two 50 PSI sources (compressed air and o2)
What are the two types of pneumatically powered ventilators?
pneumatic and fluidic
Bird Mark 7 = pneumatic
Fluidic powered ventilators use special circuits which are the equivalent of electronic logic circuits
What ventilators cannot operate without both power sources?
vents that use a combined power source.
- pneumatic power = deliver breath
- electrical power = functions of machine
What ventilator has a built in compressors which can provide the pneumatic power if a 50 PSI air source is not available?
How is air flow into the lungs achieved by a ventilator?
on the means of creating the pressure gradient between the machine and the patient's lungs (transrespiratory)
What three types of ventilators are there?
- Negative Pressure
- Positive Pressure
- High Frequency ventilators
What determines the functioning of the ventilator and can use either electronics, pneumatics, or fluidics as the control mechanism?
The control system
What type of system do the older non-microprocessors controlled ventilators use?
open loop control systems or "unintelligent systems". The ventilator cannot change in conditions. ex) 500cc Vt set, vent only delivers 500cc V based on the control settings, even if there is a leak
What ventilator monitors both the delivered volume and the returned volume and can then determine if a leak occurred?
Closed lop system. If it is a microprocessor controlled vent, it can even determine how much to compensate so the patient receives the set 500cc's or it sounds the alarm to alert the operator of the problem
What allows the operator to set desired parameters and alarms which the internal control system of the ventilator (microprocessor) then uses to deliver the gas to the patient?
The control panel or user interface
What 4 parameters of ventilation does the control panel allow the operator to control?
What determines how the gas moves to the patient?
The drive mechanism
What type of bellows uses a blended mixture of gas flows into the bellows?
Spring loaded bellows
What drive mechanism applies an amount of pressure which forces the gas out of the bellows when the inspiratory valve opens?
The spring tension. The tighter the spring, the greater the pressure
variation= weighted bellows where a weight is applied to determine the pressure
Describe the drive mechanism, linear drive piston. Also referred to as a constant flow generator.
an electrical motor is connected to a piston, which moves in and out in a cylinder. The piston, as it moves in, creates a constant flow rate of gas.
Describe the drive mechanism, rotary drive piston.
An electric motor rotates a wheel, which moves the piston forward, creating pressure and causing gas flow. As the wheel continues to turn, it moves the piston away. This is when exhalation occurs.
forward = more pressure, as it goes away, the pressure goes away!
What drive mechanism creates a variable flow pattern known as sine wave?
the rotary piston drive mechanism.
The fastest gas flow occurs when the wheel is at the top of the circle.
What to modern ventilators use to control gas delivery?
Flow control valves
Can be opened or closed completely or can be controlled in increments. They also have a rapid response time.
ex) proportional solenoid valves
What three ways can a solenoid valve be controlled?
- by electrical timers or microprocessors
- by manual operation
- by pressure
In a proportional solenoid valve, a microprocessor controls a motor which moves a piston up and down controlling flow to the patient.
What type of flow control device uses a motor to control a lever arm on the scissor valve?
A stepper motor with scissors valve.
Moves in discrete steps to open or close the valve.
What are ventilators classified as based on whether the gas used to power the drive mechanism also goes to the patient?
either single circuit or double circuit. This refers to the internal circuit of the ventilator
What circuit ventilator causes the gas to go directly from the power source to the patient?
the single circuit ventilator
What circuit ventilator causes the gas from the power source to compress gas in another mechanism such as a bellows, which is delivered to the patient? Source gas does not go directly to the patient.
A double circuit ventilator
Which circuit is the tubing which connects the patient to the ventilator? (2 names)
external circuit or patient circuit
What elements should an external/patient circuit contain?
an inspiratory limb though which fresh gas flows to the patient.
During inspiration and exhalation, the valve closes. The exhalation valve can be inside the vent or in the patient circuit on the exhalation limb.
the patient wye ('Y') joins two limbs together at the patient airway.
Other components include pressure manometer, a humidifier, a heater and a temperature monitoring probe, a bacteria filter on the inspiratory line, and usually on the exhalation side as well, a device to measure exhaled volume, and an apnea monitor. A nebulizer may be included.
What are the 4 phases of ventilation?
- Inspiratory Phase
- Expiratory Pause
With mechanical ventilation, what are we most concerned with? When?
how the gas moves to the patient, during inspiration
What is controlled in a number of ways knows as phase variables?
Inspiration - begun by triggerig
What three ways can be triggered to inspiration?
- time triggering - the vent ends exhalation and begins inspiration at times intervals set by the operator (as the rate which determines TCT)
- Pressure triggering - triggering can be accomplished by pressure drop. The vent senses a pressure drop set by the operator (sensitivity) and begins inspiration
- Flow triggering - The ventilator senses a drop in the flow in the circuit as the patient tries to inhale triggering the ventilator to begin inspiration.
Triggering = initiates breath
Time triggering = indicates vent initiated breaths
pressure or flow triggering = indicate patient initiated breaths
***VOLUME CANNOT ACT AS A TRIGGER!
Triggering starts inspiration, Cycling ends inspiration normally.
What can cycling be controlled by?
What is time cycling?
the vent ends inspiration after a certain time period has elapsed. This cycling mechanism is not affected by changes in patient lung characteristics. The volumes and pressures achieved can vary depending on patient lung characteristics, but should remain fairly consistent breath to breath
What is flow cycling?
involves the vent cycling out of inspiration when the flow rate drops below a certain level. Most common form of this ends inspiration when the flow has dropped to 25% of the initial peak flow. The 7200 cycles out of inspiration when the flow drops to 5 l/m.
The flow drops as the pressure gradient decreases as the lungs fill. This drop in flow indicated that inspiration is about complete. This is the most common cycling mode used with pressure support ventilation.
Disadvantage = when a patient wished to end inspiration before flow has reached the cutoff point. Creates patient ventilator asynchrony and increases WOB
What ventilator support provides continuous base flow through the vent circuit between mechanical breaths to decrease the effort required of spontaneous breaths
this is a method of flow triggering used to decrease the WOB required for a patient to trigger spontaneous breaths.
What are the two most common methods of cycling?
volume cycled(controlled/limited) or pressure cycled(controlled/limited)
What is the difference between volume controlled and pressure controlled?
the difference is which parameter is chosen to cycle the vent. The other parameter will vary depending on the characteristics of the patient's lungs
the vent will end inspiration when a preset pressure level is reached (IPPB)
This cycling mechanism has the advantage of reducing the peak pressures developed in the lung limiting barotrauma.
used extensively with infant vents and more and more with adults.
disadvantage = the V delivery is variable depending on the C and R of the patient's lungs.
Delivering adequate minute vent to achieve desired blood gas levels may be difficult if lung characteristics are changing.
What is the most common mode of mechanical ventilation for adult patients?
volume cycled ventilation
What does volume cycled ventilation do?
with volume ventilation, the ventilator ends inspiration when the set tidal volume has been delivered.
The V delivered remains constant, so blood gas levels can be maintained.
pressures generated vary depending on the C and R of the patient's lungs.
disadvantage = high pressures may be required to deliver the desired volume increasing the risk of barotrauma(ex ARDS pts.)
a leak might prevent the patient from receiving adequate volume. This can be prevented by use of an exhaled volume monitor.
Exhaled tidal volume is less than vent tidal volume by the amount lost to the tubing = effects volume ventilation
What is the amount of volume lost to the tubing known as? what is it dependent on?
tubing compliance. dependent on the peak pressure develop. This V can become very significant in infants or whenever small volumes are used. A correction factor needs to be applied to the set volume to account for this loss.
*determined at the time of vent setup
Can effect blood gas values significantly.
Newer vents calculate Vtube and correct it automatically (PB7200)
What phase variable ends inspiration abnormally?
What are the limiting factors?
those which cause inspiration to end before cycling mechanism has been reached.
Can be: Pressure, Volume, flow, or time.
- are protective of the patient
- limiting values are usually safety focused.
What phase variable ends inspiration after a certain elapsed time?
ex)a pressure cycled breath in which a leak occurred, preventing the cycling pressure from being reached.
What phase variable is common on volume ventilation to prevent barotrauma to the lungs when lung characteristics change.
This is the high pressure limit
Volume delivery ends when the pressure limit is reached even thought the set tidal volume (cycling volume) has not been reached.
What can result due to a finite fixed volume available for delivery, such as a bag or a bellows with a fixed volume?
or in some cases, a maximum delivered volume can be set. In case of a leak, this would end inspiration and alert the operator
What is flow limiting?
If the flow reaches a maximum before the end of inspiration and can't exceed that flow, then the ventilator is flow limited.
The flow would not exceed that required to deliver the set volume in the given time.
What is inhalation hold?
When inspiratory pause is used, the normal cycling mechanism no longer ends inspiration. Expiration is usually down to baseline (0 or atmospheric) but can be maintained at a pressure above baseline (PEEP)
What can be added to the inspiratory time to hold air in the lung? (plateau)
Inflation hold or inspiratory pause
This is done to improve oxygenation and improve the distribution of ventilation to peripheral lung units. Also used to determine static compliance
What is NEEP?
negative end expiratory pressure used in the past to assist exhalation.
A less than atmospheric pressure is applied at the airway opening to increase the pressure gradient between the lungs and the airway opening thereby assisting exhalation.
a feature of high frequency oscillators but is not used otherwise any longer clinically due to the risk of airway collapse and air trapping in COPD patients
What is expiratory hold (pause)?
added at end exhalation to measure air trapped in the lung at the end of exhalation.
air trapped at the end of exhalation abnormally is referred to as auto PEEP
What is expiratory retard?
the ventilator version of pursed lip breathing.
An expiratory resistance is applied that the patient exhales against to prolong exhalation and prevent airway collapse.
rarely used anymore.
PEEP is used instead to prevent airway collapse and air trapping and to improve oxygenation
What is the equivalent of PEEP?
but is used in spontaneously breathing patients either on or off the ventilator
PEEP is applied to what?
What is BiPAP?
bilevel positive pressure or a combination of PEEP and pressure support on the ventilator
PEEP or CPAP are not true modes of ventilation but are adjuncts.
What are the three types of ventilator breaths?
- mandatory - started(triggered), limited, and ended (cycled) by the vent
- assisted - pt triggered our settings
- spontaneous - pt triggered and controlled
Mode which is started by the patient, but limited and cycled by the ventilator
What mode is started, controlled, and ended by the patient?
What mode is time triggered based on the ventilator rate set by the operator? And the vent controls the breath delivery from start to finish?
If only mandatory breaths are permitted, what is the mode referred to as?
The ventilator completely controls ventilation. No amount of patient effort will start a breath.
Should only be used with apneic or paralyzed patients who make no attempts to breathe.
What is assist mode?
the patient must initiate every breath. Triggering is either pressure or flow triggering. The ventilator controls and ends the breath.
Each breath is a ventilator delivered breath based on the set parameters but initiated by the patients.
Patient controls the rate.
requires an amount of patient work, so disadvantage is that the pt may not breathe enough times per minute to maintain needed minute ventilation.
What is assist control?
a combination of the assist and control modes.
vent delivers a set number of breaths set by the operator that are time triggered. The pt can also take additional breaths that are patient triggered. Each breath delivered is a mechanical breath (machine cycled to set parameters)
When the ventilator does 100% of the WOB the mode is referred to as?
control mode is an ex of full support
When the patient does a portion of the work and the ventilator does a portion of the work it is called?
Ex = assist and assist control
What type of breath is patient triggered, patient cycled, and patient limited?
the vent provides support to overcome increased airway R or provides end expiratory pressure to prevent atelectasis.
pt must initiate all breaths and do the work of breathing
control mode- time trigger, vent delivered
assist mode patient trigger,vent delivered
those don't really exist!
ASSIST CONTROL - a combo of control and assist mode. Either patient or time triggered, vent delivered
what is IMV?
intermittent mandatory ventilation
allows spontaneous patient breaths between mandatory vent breaths
vent breaths are time triggered and bent controlled breaths
originally developed as a weaning mode to allow patients to take over more of the WOB
reduces complication of mech vent by reducing the amount of time positive pressure is applied.
Disadvantage = breath stacking = larger than normal volume being delivered
what is SIMV?
synchronized intermittent mandatory ventilation
the patient is allowed to breathe spontaneously between vent times breaths, but in this mode, the vent senses a patient breath and holds the machine breath until it is synchronized with the patient effort.
breath becomes assisted breath
most common mode today
What is MMV?
mandatory minute ventilation
a mode where the operator sets a minimum minute ventilation and the machine delivers breaths as needed to supplement patient's spontaneous breaths to maintain at least that minimum level of ventilation
What is PC-IRV?
pressure control inverse ratio ventilation
a newer mode in which the patient breathes above a higher CPAP level and this level is periodically released to baseline or a lower CPAP level
TIME CONTROL MODE
I:E ratio is the inverse of normal (2:1) with inspiration always longer than exhalation. This is the equivalent of BiPAP only with an inverse I:E ratio and no patient spontaneous breaths
what is APRV?
airway pressure release
the patient breathes above a higher CPAP level and this level is periodically released to a baseline or a lower CPAP level
TIME CONTROL MODE
I:E inverse of normal
the patient breathes above the two levels of pressure support. The high level SPAP is maintained throughout inspiration then the pressure is released to the low level CPAP for exhalation. Release to low P is shorter than the inspiratory high pressure resulting in an inverse I:E
spontaneous breathe in APRV are pressure controlled, pressure triggered, pressured limited, and pressure cycled
machine breaths in APRV are pressure controlled, time triggered, pressure limited, and time cycled. Same as pressure controlled inverse ration ventilation PC-IRV. Both used for ARDS.
proportional assist ventilation
the vent delivers variable pressure and flow based on patient effort. It is still experimental
pressure regulated volume control, mode found on the Siemens Servo-1 ventilator
Considered a A/C mode but can also be used as an SIMV mode
the first breath delivered is a volume targeted breath based on the set tidal volume with an added breath hold. Plateau pressure is measured.
The vent then uses the measured plateau pressure as the max inspiratory pressure for the next breath.
the vent then adjusts each breath to attempt to achieve the desired tidal volume
the pressure change on each breath can not be more than 3cmH2O
The pressure can increase up to 5 cmH2O below the set high pressure limit
If the Vt exceeds the desired volume, the pressure is decreased on subsequent breaths
flow rate automatically adjusts to maintain the same I:E ratio based upon the set rate and minute volume
GOAL= provide consistent tidal volume at a safe pressure as long C and R change.
pressure support ventilation
used for three functions:
- 1. to reduce the WOB of spontaneously breathing patients that is due to the increased resistance of the vent circuit, demand valve, and the ET tube.
- 2. to further reduce WOB in spontaneously breathing patients by providing support above that needed in #1. It can also provide this support for spontaneous breaths between the machine breaths in SIMV, pressure support also augments tidal volume of spontaneous breaths
- 3. PSV can be used with the assist mode to provide full vent support (PSV max)
PSV are similar to what?
the IPAP level on a BiPAP machine
breaths are patient triggered, pressure limited, and flow cycled
the speed with which the vent reaches the preset pressure is known as what?
is fixed on many machines but some have variables rise times
If rise time is too fast or too slow, what is increased?
the patient's WOB
what is PEEP?
positive end expiratory pressure
pressure maintained in the airway at end exhalation
same as CPAP or EPAP ona BiPAP machine. Raises the baseline pressure for subsequent breaths.
used to increase functional residual capacity, recruit underinflated alveoli, prevent alveolar atelectasis and improve oxygenation
5 Basic Ventilatory Patterns
- Volume = Continuous Mandatory Ventilation
- Intermittent Mandatory Ventilation
- Pressure = Continuous Mandatory Ventilation
- Intermittent Mandatory Ventilation
- Continuous Spontaneous Ventilation
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