Modes of Ventilation - 26 Flashcards
Describe VCV
TV selected
Constant flow rate until volume delivered
What effect does airflow at the end of inspiration during VCV have?
Peak Paw > Palveoli
The difference Paw - Palv = airway resistance
Advantages of VCV
- high airways resistance
- lung compliance dec
the ventilator still delivers desired volume
Disadvantages of VCV
- Higher airway pressures BUT they DO NOT increase risk of ventilator induced lung injury
Only an increase in alveolar pressure increases risk and this is the same in both VCV and PCV
- Constant inspiratory flow rate = short and uneven alveolar filling & inadequate with high flow demands –> decelerating pattern improves patient comfort
Describe PCV
Select inflation pressure + decelerating insp flow rate –> desired pressure quickly
Advantages of PCV
- Control peak alveolar pressure = reduce risk of injury
2. Better pt comfort - high initial flow rates, longer insp duration
At what pressure is ventilator-induced lung injury negligible?
Peak alveolar <30cmH2o
Disadvantages of PCV
- Dec alveolar volume when there is inc airway resistance or dec lung compliance = ARF
Describe PRVC
Constant TV but limits end-insp pressures
Measures lung compliance and selects lowest airway pressure needed to deliver desired TV
Describe Assist-Control Ventilation
Pt assisted or triggered BUT if not possible, vent triggered breaths @ pre-selected rate
Describes the vent triggers for ACV
- Patient triggered - pt generates -2 to -3 cm H2o –> opens pressure sens valve BUT this is double quiet breathing P = vent not always triggered
- Flow rate - little/no pressure change + flow 1-10L/min to trigger vent BUT system leaks –> auto-triggering
- Time-triggered - no neg pressure from pt = breath given by vent at preselected rate
Describe the goal of I:E
Usually 1:2
Allow enough exp time to prevent auto-PEEP
How do you increase I:E ratio if exhalation is too short?
- Inc insp flow rate
- Red TV
- Dec insp time (PCV)
Rapid breathing NOT d/t discomfort or anxiety - how do you dec resp rate? (pt unable to exhale completely)
- Sedation & insp flow adjustments = NOT successful
2. Use IMV
Describe IMV
Allows spontaneous breathing b/w vent breaths
Breathing circuit is parallel w/ vent w/ unidirectional valve that opens spontaneous breathing circuit when NO vent breath delivered.
SIMV is synchronized and can be volume or pressure-controlled
Adverse effects of IMV
- Inc work of breathing
- Dec CO - increases LV afterload
Both are d/t the spontaneous breathing period
What accounts for the inc work of breathing during spontaneous breathing during IMV?
Resistance in the vent circuit –> use pressure support at 10cmH2o to reduce work of breathing
Describe PSV
Pressure augmented spontaneous breathing - like PCV but allows patient to terminate inflation = pt control over insp time and TV
Lung inflation terminates when insp flow rate falls to 25% of peak flow rate = pt determines insp time and vol
How can PSV be used to wean from the vent?
Using 5-10cmH2o to overcome resistance to flow from the vent tubing and airways = reducing work of breathing w/o augmenting TV
Describe the pressure and collapse of airways during expiration
Progressive narrowing of distal airways during expiration –> collapse at end exp
Transpulmonary P where airways begin to collapse = closing pressure = 3cmH2o
Closing pressure HIGHER in small airway obstruction (COPD) & red lung compliance (ARDS)
What are the adverse outcomes of airspace collapse at end-expiration?
- Impaired gas exchange from atelectasis
2. Atelectrauma from repetitive opening and closing of distal airspaced
Describe the purpose of PEEP
To prevent airway pressure from < closing pressure at end expiration
How does the vent create PEEP?
Pressure relief valve in exp limb –> exhalation proceeds until pressure falls to preselected level –> then mainteined
5-7cmH2o
How does PEEP affect inflation pressures?
Upward
Higher Palv pressure and higher mean Pairway
- NOT related to PEEP level
- Change in Palv determines influence of PEEP on alv ventilation (art O2) and risk for volutrauma
Describe the effects of different levels of PEEP
- 5-10 cmH2o
- 20-30 cmH2o
- prevents collapse
2. re-open distal airways persistently collapsed = alveolar recruitment = inc area for gas exchange
What determines if PEEP will promote alveolar recruitment OR overdistension?
How do you determine which will occur?
Volume of recruitable lung (atelectasis that can be aerated, 2-25%)
- PEEP –> Lung compliance INC = recruitment (see ch. 25)
- PEEP –> PaO2/FiO2 INC = recruitment
How does PEEP dec CO?
- Dec VR
- Dec vent compliance
- Inc RV afterload
- External constraint of ventricles
More prominent in hypovlemia
How does PEEP affect O2 delivery?
Can inc alveolar recruitment and SaO2 BUT systemic O2 delivery may not improve d/t DEC CO
At what point does one achieve optimal benefit from PEEP?
Greatest inc in systemic O2 delivery - measure at inc levels of PEEP
