Week 9 Ventilators and Airway Monitors Flashcards
Describe possible issues with piston ventilators.
- refill even if a circuit disconnection occurs
- if a circuit leak is present, piston may entrain RA through the leak → diluting oxygen and anesthetic agent
- associated risks
- hypoxemia
- awareness
- Alarm should alert the operator
- associated risks
- positive pressure relief valve prevents excessively high breathing circuit pressure (60 to 80 cm H2O)
Distinguish ICU vents and anesthesia vents.
- ICU ventilators are more powerful allowing for greater inspiratory pressures and tidal volumes
- CO2 absorber - anesthesia vents
- ICU ventilators support more modes of ventilation
- Changing!!
- Gas supplied by the ICU ventilator directly ventilates the patient
-
Anesthesia driving gas never reaches the patient
- 100% O2 in old machines
- Air/100% O2 in newer models
Describe Pressure Support Ventilation as a mode of ventilation.
- aid in normal breathing with a predetermined level of positive airway pressure
- pt spontaneously breathing
- PSV senses patient inspiratory effort (volume or flow) and delivers pressure support
- results in larger TV than the patient would produce on their own
- PSV is useful to support MV and control arterial CO2 for spontaneously breathing patients during maintenance and emergence
How do you calculate the I : E ratio?
- Time spent in inspiration
-
TI = TV / Flow rate
- TV = IBW * 5-7 ml/kg = ____ mL
- Flow rate = 4 - 6 X minute ventilation
- MV = TV X RR
- X 60 seconds
- TE = (60 seconds / RR) - TI
What is the formula for oxygen delivery?
Oxygen delivery = CO x O2 content
Distinguish between ascending and descending bellows.
Bellows are classified by direction of bellows movement during _________.
EXPIRATION
How are we going to make sure that our ventilator settings are okay and that we are ventilating adequately?
PEtCO2
Discuss volume as a parameter used to describe ventilation.
- measure of the tidal volume delivered by the ventilator to the patient
- volume of gas patient breaths
- expressed in mls
- expressed in Ls for minute volume
Describe the disadvantage of the descending bellows type ventilator.
These bellows will continue regular upward and downward movement despite patient disconnection: the drive gas pushes the bellows upward during the inspiratory phase, and during the expiratory phase the bellows “fills” with entrained room air instead of the patient’s exhaled gas, because of the weighted bellows. The pressure monitor and the volume monitor may be fooled even if disconnection is complete.
**An essential safety feature of any anesthesia workstation that uses a descending bellows is an integrated CO2 apnea alarm that cannot be disabled while the ventilator is in use.**
Each time you increase FiO2 by _____ you increase PaO2 by _______.
10%; ~50 mmHg
- PaO2 FiO2
- 100 21%
- 150 30%
- 200 40%
- 250 50%
Discuss time as a paramter used to describe ventilation.
- divided into insp and exp periods
- expressed in seconds OR by relation of insp time to exp time expressed as I:E ratio
- used to define the number of respiratory cycles within a given time period
If ventilation is in adequate and we want to increase minute ventilation, in general, will we first increase RR or TV?
(think of Dr. E’s pearl in anesthesia, practical purposes)
- Tidal volume first
- recruit alveoli, not being ventilated as well as when patient is awake with negative pressure ventialtion
- however, must consider pressures as well
Describe features of a piston ventilator.
- use computer controlled stepper motor vs drive gas
- analagous to pushing plunger of syringe
- single circuit
-
less gas used
- great for remote locations
- more accurate TV delievery tied to piston movement
- **However, feedback mechanisms that help maintain stable tidal volume delivery are becoming increasingly more common**
- circuit compliance compensation
- use of inspired tidal volume measurement
Discuss flow rate as a parameter used to describe ventilation.
-
rate at which the gas volume is delivered to the patient
- from the patient connection of the breathing system to the patient
- refers to the volume change/time
- expressed in L/sec or L/min
FR = 4 - 6 X Minute Volume
Describe the mechanism of the Maquest FLOW-i Anesthesia System with volume reflector.
- newer vents, not commonly in use
- uses a “volume reflector”
- functional and “in circuit” during all modes of ventilation
- gases are mixing
- exhaled gases
- FGF
- breathing bag not in play during mech ventilation
- PEEP/APL valve closed during inspiration, open during expiration for scavenging
- during expiration, lots of CO2 and less oxygen in the volume reflector
- during inspiration, gas from volume reflector THEN goes through CO2 absorber and mixes with FG to go to patient
- at the end of exhalation, the volume reflector is filled at its proximal end (near the patient) with exhaled gas and is filled distally with a mixture of exhaled gases and reflector gas
- during inspiration, reflector gas module pushes the exhaled gas back out of the volume reflector, much like a piston, through the CO2 absorber, to the patient. Fresh gas combines with the volume reflector outflow to maintain the desired oxygen and anesthetic concentration
- The fresh gas modules and the reflector gas module work together in a coordinated manner to control gas flow and pressure in the breathing circuit so that operator determined ventilation parameters are maintained
Describe the flow of gas through the piston ventilator during expiration.
- FIRST STEP OF EXHALATION
- patient exhales into breathing bag
- fresh gas continues to flow in retrograde fashion
- SECOND STEP OF EXHALATION
- ventilator returns to its starting position, drawing in gas stored within the breathing bag and fresh gas from the gas supply system
- once the piston reaches the bottom of its stroke, fresh gas flow reverses course and flows in retrograde fashin toward the breathign bag and the absorber
- excess gas vents through the exhaust valve to the scavenger
Many vents can operate in VC or PC modes. Distinguish between the two.
-
Volume controlled
- terminates inspiration when a preselected TV is delivered
- most adult vents are V cycled but have a second limin in inspiratory pressure to guad against barotrauma
- a percentage of TV is always lost to the _compliance of the system (not patient)_
- usually about 4 - 5 cm H2O.
-
Pressure controlled
- cycle into expiratory phase when airway pressure reaches a predetermined level.
- TV and inspiratory time vary
- used in situations where pressures can be high
- useful in neonates/premies
**All vents are under electric control**
Explain the flow of gas through a piston ventilator during inspiration.
- The PEEP/maximum pressure (Pmax) valve is held closed
- The pressure in the breathing circuit that is generated by the ventilator closes the fresh gas decoupling valve
- This directs gas flow tward the breathign bag during inspiration so it does not interfere with TV accuracy
- Excess fresh gas flows past the open APL bypass valve, though the exhaust valve, and to the scavenger
- **Note how the breathing bag is integral to cirucit function during mechanical ventilation**
Describe CPAP as a mode of ventilation.
- CONTINUOUS POSITIVE AIRWAY PRESSURE
- positive pressure is maintained during both inspiration and expiration
- can be provided with mask
- CAUTION: if pressures > 15 cm H2O, can cause regurgitation and aspiration
Describe the mechanism of expiration in the bellow ventilator.
- the drive gas exits the bellows chamber
- pressure within bellows and the pilot drop to zero causing the ventilator relief valve (pop off valve) to open
- Exhaled pt gas fills the bellows before any scavenging occurs because the valve ball produces a 2 - 3 cm H2O back pressure
- scavenging occurs ONLY when the bellows are filled completely
- the relief/pop off valve is ONLY open during expiration, and any scavenging occurs at this point
Describe the bellows ventilator and the mechanism during inspiration.
- pneumatically driven
- bellows separate the driving gas from the pt gas circuit
- double circuit
- like practitioner squeezing the reservoir bag
- bellows serves as the reservoir for pt breathing gases
- driving force is the pressurized gas that flows into the bellows housing
- during inspiration phase, the driving gas enters the chamber and increases pressure
- The above increase in pressure casuses 2 things to occur
- the ventilator relief valve closes (pop off valve)
- so no gas can escape into the scavenger
- the bellows are then compressed and the gases in the bellows are delivered to the patient
- (analagous to you squeezing the bag)
- the ventilator relief valve closes (pop off valve)
Describe high frequency ventilation as a mode of ventilation.
- low tidal volumes, less than dead space
- high rate (60-300 bpm)
-
Typical settings
- 100-200 bpm
- IT 33%
- Drive pressure: 15-30 psi
- GOAL
- maintain pulmonary gas exchange at lower mean airway pressures
- Used in ESWL
