SIMV

SIMV

 

5 things you need to order as a physician. PEEP, FiO2, Respiratory Rate (RR) , Tidal Volume (Vt), and Pressure Support (PS).

 

Respiratory Rate (RR) and Tidal Volume (Vt).

-The patient will always receive this volume this many times per minute regardless of how they breath.

Pressure Support (PS)

-Any spontaneous breath above the set rate are pressure supported breaths. THe patient determines their own tidal volume. 

With the rate set at 12, the patient gets the set tidal volume 12 times per minute regardless of their spontaneous respiratory rate. 

Any time the patient breathes faster than the set RR, additional respirations will be spontaneous respirations supported with the PS.

PEEP FiO2

Utilize the lowest FiO2 possible to maintain SaO2 of 90%, as you go up on PEEP, Go up on FiO2 according to a PEEP table.

 

 

EXAMPLE 1

 

Vent Settings - Measured values -  

RR - 12 RR (tot) - 20

Vt - 500mL Vt (Mand) - 500mL

PS - 10 Vt (Spon) - 300mL

 

What is the Minute Ventilation? (Minute Ventilation = RR x Vt)

 

Since we have 2 different tidal volumes, we must calculate the amount of minute ventilation each type of Vt contributes to minute ventilation.

 

Since we have a RR set at 12, we know the patient will get 12 respirations at the set Vt of 500mL. No more, no less. These set breaths contribute 6L to our minute ventilation.

 

500mL x 12 respirations = 6L

 

Additional respirations above 12 will be spontaneous with the support of PS. We see in the right column, these tidal volumes are at 300mL.

 

First, I need to calculate how many breaths per minute (BPM) are spontaneous. If I have 20 BPM total and I know that 12 of these are vent breaths, that leaves 8 spontaneous respirations.

 

20 - 12 = 8.

 

So now I know that my spontaneous respirations are 8 at a Vt of 300. When I multiply these, it shows the spontaneous respirations add an additional 2.4L to my minute ventilation.

 

300mL x 8 respirations = 2.4L

 

Finally, I can add these 2 values together to see my total minute ventilation.

 

6L + 2.4L = 8.4L

 

So my total Minute Ventilation is 8.4L

 

EXAMPLE 2

 

Let us say in the above patient, I get the ABG below.

 

pH - 7.20

pCO2 - 65

HCO2 - 30

 

How will increasing PS increase minute ventilation? 

 

Increasing the PS should increase the size of my spontaneous tidal volumes. This will increase the portion of my minute ventilation from spontaneous breaths. 

 

(before change) PS 10, Spont Vt - 300mL, Spont RR - 8

8 x 300mL = 2.4L

 

(After change) PS 15, Spont Vt - 500mL, Spont RR 8

8 x 500mL = 4L

 

COUNTER ARGUMENT - If the patient can breathe spontaneously and maintain adequate Vt, I should change to spontaneous mode of ventilation and simplify management. 

EXAMPLE 3

 

In the same patient with the same ABG, how can I increase the minute ventilation by increasing the RR? 

 

The patient is receiving 12 respirations with a Vt of 500mL, and 8 respirations with a Vt of 300mL. For every 1 BPM that I increase the set RR, I increase the minute ventilation by 200mL up to the total RR.

 

(12 x 500mL) + (8 x 300mL) = 8.4L

 

(16 x 500mL) + (4 x 300mL) = 9.2L

 

20 x 500mL = 10L

 

Any increase in the RR above the total RR will increase the minute ventilation by my set Vt.

 

22 x 500mL = 11L

 

24 x 500mL = 12L

 

COUNTER ARGUMENT - If I am increasing my set RR to match the total RR, I should change the patient to A/C and make the vent easier to manage.