Modes of Ventilation - 26

Question 1

Describe VCV

Answer 1

TV selected

Constant flow rate until volume delivered

Question 2

What effect does airflow at the end of inspiration during VCV have?

Answer 2

Peak Paw > Palveoli

The difference Paw - Palv = airway resistance

Question 3

Advantages of VCV

Answer 3

1. high airways resistance
2. lung compliance dec

the ventilator still delivers desired volume

Question 4

Disadvantages of VCV

Answer 4

1. 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

2. Constant inspiratory flow rate = short and uneven alveolar filling & inadequate with high flow demands –> decelerating pattern improves patient comfort

Question 5

Describe PCV

Answer 5

Select inflation pressure + decelerating insp flow rate –> desired pressure quickly

Question 6

Advantages of PCV

Answer 6

1. Control peak alveolar pressure = reduce risk of injury

2. Better pt comfort - high initial flow rates, longer insp duration

Question 7

At what pressure is ventilator-induced lung injury negligible?

Answer 7

Peak alveolar <30cmH2o

Question 8

Disadvantages of PCV

Answer 8

1. Dec alveolar volume when there is inc airway resistance or dec lung compliance = ARF

Question 9

Describe PRVC

Answer 9

Constant TV but limits end-insp pressures

Measures lung compliance and selects lowest airway pressure needed to deliver desired TV

Question 10

Describe Assist-Control Ventilation

Answer 10

Pt assisted or triggered BUT if not possible, vent triggered breaths @ pre-selected rate

Question 11

Describes the vent triggers for ACV

Answer 11

1. Patient triggered - pt generates -2 to -3 cm H2o –> opens pressure sens valve BUT this is double quiet breathing P = vent not always triggered
2. Flow rate - little/no pressure change + flow 1-10L/min to trigger vent BUT system leaks –> auto-triggering
3. Time-triggered - no neg pressure from pt = breath given by vent at preselected rate

Question 12

Describe the goal of I:E

Answer 12

Usually 1:2

Allow enough exp time to prevent auto-PEEP

Question 13

How do you increase I:E ratio if exhalation is too short?

Answer 13

1. Inc insp flow rate
2. Red TV
3. Dec insp time (PCV)

Question 14

Rapid breathing NOT d/t discomfort or anxiety - how do you dec resp rate? (pt unable to exhale completely)

Answer 14

1. Sedation & insp flow adjustments = NOT successful

2. Use IMV

Question 15

Describe IMV

Answer 15

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

Question 16

Adverse effects of IMV

Answer 16

1. Inc work of breathing
2. Dec CO - increases LV afterload

Both are d/t the spontaneous breathing period

Question 17

What accounts for the inc work of breathing during spontaneous breathing during IMV?

Answer 17

Resistance in the vent circuit –> use pressure support at 10cmH2o to reduce work of breathing

Question 18

Describe PSV

Answer 18

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

Question 19

How can PSV be used to wean from the vent?

Answer 19

Using 5-10cmH2o to overcome resistance to flow from the vent tubing and airways = reducing work of breathing w/o augmenting TV

Question 20

Describe the pressure and collapse of airways during expiration

Answer 20

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)

Question 21

What are the adverse outcomes of airspace collapse at end-expiration?

Answer 21

1. Impaired gas exchange from atelectasis

2. Atelectrauma from repetitive opening and closing of distal airspaced

Question 22

Describe the purpose of PEEP

Answer 22

To prevent airway pressure from < closing pressure at end expiration

Question 23

How does the vent create PEEP?

Answer 23

Pressure relief valve in exp limb –> exhalation proceeds until pressure falls to preselected level –> then mainteined

5-7cmH2o

Question 24

How does PEEP affect inflation pressures?

Answer 24

Upward

Higher Palv pressure and higher mean Pairway

1. NOT related to PEEP level
2. Change in Palv determines influence of PEEP on alv ventilation (art O2) and risk for volutrauma

Question 25

Describe the effects of different levels of PEEP

1. 5-10 cmH2o
2. 20-30 cmH2o

Answer 25

1. prevents collapse

2. re-open distal airways persistently collapsed = alveolar recruitment = inc area for gas exchange

Question 26

What determines if PEEP will promote alveolar recruitment OR overdistension?

How do you determine which will occur?

Answer 26

Volume of recruitable lung (atelectasis that can be aerated, 2-25%)

1. PEEP –> Lung compliance INC = recruitment (see ch. 25)
2. PEEP –> PaO2/FiO2 INC = recruitment

Question 27

How does PEEP dec CO?

Answer 27

1. Dec VR
2. Dec vent compliance
3. Inc RV afterload
4. External constraint of ventricles

More prominent in hypovlemia

Question 28

How does PEEP affect O2 delivery?

Answer 28

Can inc alveolar recruitment and SaO2 BUT systemic O2 delivery may not improve d/t DEC CO

Question 29

At what point does one achieve optimal benefit from PEEP?

Answer 29

Greatest inc in systemic O2 delivery - measure at inc levels of PEEP