Pressure-Volume & Flow-Volume Loops Flashcards
Simple changes to what 3 things can create discrepancies between set and delivered TVs?
- Fresh gas flow (FGF)
- Respiratory rate (RR)
- I:E ratio
Compliance of the breathing circuit: For every 1 cm H20 pressure in the circuit ______ ml of volume is lost per breath r/t tubing distention and compressible volume.
2-8 ml
The greatest TV augmentation occurs at the ….?
- Highest FGF (10L/min)
- Highest I:E (1:1)
- lowest RR (8)
The lowest TV augmentation occurs at the …?
- Lowest FGF (0.25 L/min)
- Lowest I:E (1:3)
- Highest RR (16)
The magnitude of delivered TV augmentation is ____________?
- Independent of the volume control TV setting
* Thus, the percent increase is much larger for pediatric settings
What 2 methods increase TV accuracy?
- Fresh gas decoupling
2. Fresh gas compensation
What is fresh gas decoupling?
- Fresh gas is diverted by a decoupling valve to the manual breathing bag, and is thus not added to the delivered tidal volume.
- This helps ensure that the set and delivered tidal volumes are equal.
What is fresh gas compensation (volume/flow sensors)?
The volume and flow sensors provide feedback which allows the ventilator to adjust the delivered tidal volume so that it matches the set tidal volume in spite of changes in the total fresh gas flow.
How is MV (Minute Ventilation) calculated?
MV = RR x TV
A pause at the end of inspiration is a ventilator option to…?
- Benefit alveolar gas distribution from the compliant to the less-complaint alveoli while maintenance of lower airway pressures.
What is the usual I:E ratio?
1:2
Longer inspiratory times generally improve oxygenation by:______?
- Increases the mean airway pressure (longer period of high pressure increases mean airway pressure over entire respiratory cycle)
- Allows re-distribution of gas from more compliant alveoli to less compliant alveoli
What are the disadvantages of longer inspiratory times?
- Increases the risk of gas trapping, intrinsic PEEP, and barotrauma by reducing expiratory time
- Less well tolerated by the patient - requires deeper sedation levels
- Decreases peak pressure by decreasing inspiratory flow
What is compliance?
- change in volume/change in pressure
- measurement (ml/cmH2O) of how well the lung-thoracic system can change its total volume as changing pressure is applied
Compliance = ?
- Expiratory TV / (pressure at the end of inspiration - pressure at the end of expiration)
What is normal adult compliance?
35-100 ml/cmH2O
What is normal pediatric compliance?
> 15 ml/cmH2O
Compliance can easily be viewed via ___________?
Pressure-Volume Loops
How will decreased compliance be represented on pressure-volume loops?
- The slope of the pressure-volume loop will move toward the horizontal/x-axis (more pressure is required to fill the lungs to a given volume)
How will increased compliance be represented on pressure-volume loops?
The slope of the pressure-volume loop will move towards the vertical/y-axis (less pressure is required to fill the lungs to a given volume)
What is resistance (Raw)?
- The relationship between the pressure difference across the airway (between the mouth and the alveoli) and the rate at which gas flows through the airway
What is the most probable reason for a non-closing loop on the Flow-Volume Loop?
Leak
On a Pressure-Volume Loop, spontaneous ventilation is represented how?
- negative pressure
What do pressure-volume loops provide?
- information on dynamic trends of respiratory compliance
What do flow-volume loops provide?
- information on dynamic trends of respiratory resistance
Pressure-Volume Compliance Loop: Static?
- ICU vents can provide rapid airway occlusion at end-inspiration. During the pause, flow rapidly drops to zero and TV is briefly trapped inside the lung, at this point static airway pressure can be measured.
- Absence of flow equals alveolar pressure, which reflects the elastic retraction of the entire respiratory system
Pressure-volume compliance loop: Quasi-static?
- Obtained when inspiratory pause is shorter than 2 seconds & does not always reflect alveolar pressure
Pressure-volume compliance loop: Dynamic?
- Obtained without flow interruption
What is static compliance?
- Mirrors the elastic features of the respiratory system
- Only affected by variations in volume and/or complaince
What is dynamic compliance?
- Provides information on both elastic & resistive (flow-dependent) components of the airways and endotracheal tube
- Affected by variations in volume, compliance, airway secretions, bronchospasm, diameter of OET
Plateau pressures greater than _____ cm H2O are associated with shear lung damage.
35
What is normal lung and chest compliance?
50-80 ml/cm H2O
What are the advantages of PEEP?
- Improved PaO2
- Reduces right –> left intrapulmonary shunting (PEEP expands collapsed, but perfused alveoli)
- Increases FRC through maximal alveolar recruitment
- Prevents airway & alveolar closure at end-expiration
What are disadvantages of PEEP?
- Excessive distention of alveoli may cause compression of pulmonary vasculature
- decreased preload
- increased CVP
- increased risk for barotrauma
- redistribution of pulmonary blood flow
- promotion of pulmonary edema-impeding lymphatic drainage from lungs
What is “Best” PEEP?
- level of PEEP that produces maximal PaO2 with an FiO2 less than or equal to 0.5 with the least decrement in CO and over distention of alveoli
