Ventilator Management

Question 1

Explain how high PEEP or positive pressure ventilation can diminish CO

Answer 1

Both hi PEEP and hi PPV increase intrathoracic pressure. This space presses on the mediastinum and diminishes venous return, causing a decreased CO. A decreased CO leads to decreased BP (MAP = SVR x CO).

Question 2

Describe a situation in which you would use an Ayre’s T-Piece or Jackson-Rees

Answer 2

Patient needs to be transported away from access to ventilator, is breathing spontaneously but not yet ready to be extubated.
Or with small animals under 7kg.

Question 3

Describe an Ayre’s T-Piece and Jackson-Rees circuit

Answer 3

Ayre’s T-Piece (Mapleson E) is tubing with a hose on the side to be attached to fresh gas flow. The apparatus is attached to an ETT. Exhaled gas is cleared out of the tubing by the FGF.
Jackson-Rees (Mapleson F) is an Ayre’s with a bag on the end so that breathing can be better observed. There is a hole on the end of the bag for breaths to be exhaled.

Question 4

Fresh gas flow in Mapleson needs to be how much higher than minute ventilation?

Answer 4

1.5-2.5x higher

Question 5

Normal values for the pressure limit (Pmax, or the highest PIP allowed) for volume control mode

Answer 5

Operator usually sets Pmax to between 30-50cmH2O

Question 6

Peak pressure - plateau pressure =

Answer 6

Resistive pressure

Question 7

Why is there resistive pressure during ventilation? How does this relate to peak and plateau pressures?

Answer 7

Resistance occurs as air moves through increasingly small pathways (circuit->ETT->bronchi etc). Once this pressure is overcome, the peak inspiratory pressure drops to the plateau pressure.

Question 8

What is difference between Pressure Controlled And Pressure Support Ventilation?

Answer 8

Pressure Control - delivers breaths at a rate and Peak inspiratory pressure (PIP) to be determined by the operator. In regards to spontaneous breathing, it gives only a small amount of additional pressure to a breath to help overcome the airway resistance of the ETT.

Pressure Support - you can set the amount of additional support given to each spontaneous breath. The support is given when a patient reaches a certain “trigger” inspiratory pressure (which also is set by the operator). Ventilator does not deliver breaths otherwise.

Question 9

Does the operator have any control over pressure while in volume control mode?

Answer 9

Yes. They can set the maximum amount of pressure to be delivered (Pmax). If this airway pressure is reached before the set tidal volume is delivered, it will stop short of delivering the set volume on that breath.

Question 10

Does the operator have any control over volume while in pressure control mode?

Answer 10

No

Question 11

Explain how “breath stacking” occurs. When is it seen?

Answer 11

Patient doesn’t have enough time to exhale entire breath and some of the tidal volume is left behind.
Occurs with high minute ventilation, airway obstruction and expiratory resistance.

Question 12

If FGF is 10L/min and I:E ratio is 1:2, how much additional volume will be delivered, on top of the set tidal volume? What if this is in Pressure support mode?

Answer 12

An additional 3.33L/min will be delivered. (I:E ratio 1:2, meaning inspiratory portion is 1/3. 1/3 x 10L/min = 3.33L/min).

This additional FGF will not effect the delivered volume in Preessure Control mode, however, the set Pressure will be reached faster. No I:E ratio in pressure control anyway (I think).

Question 13

Basic difference between PEEP and CPAP?

Answer 13

CPAP delivered throughout entire breathing cycle.

PEEP delivered only at end of expiration.

Question 14

Typical CPAP range?

Answer 14

5-10cmH2O