Mech Vent Equations

Question 1

Cycle time =

Answer 1

(60 sec/min)/RR
AND
Ti + Te

Question 2

Ti =

Answer 2

(cycle time)/(I + E)

Question 3

Tidal Volume (VT) =

Answer 3

Flow x Ti

*flow in L/sec

Question 4

Minute Ventilation (VE) =

Answer 4

RR x VT

Question 5

Flow (l/min) =

Answer 5

(I + E ratio) x MV

Question 6

Compliance =

Answer 6

VT/(Pplat - PEEP)

Question 7

what are the units for compliance?

Answer 7

L/cmH2O

Question 8

Resistance =

Answer 8

(Ppeak - Pplat)/flow

*flow in L/sec

Question 9

what are the units for resistance?

Answer 9

cmH2O/Lsec

Question 10

DO2 =

Answer 10

CO x CaO2

Question 11

CaO2 =

Answer 11

([Hb] x 1.34 ml O2/g Hb x SaO2) + (PaO2 x 0.003)

Question 12

CvO2 =

Answer 12

([Hb] x 1.34 ml O2/g Hb x SvO2) + (PvO2 x0.003)

Question 13

VT goal, RR goal, Ve goal =

Answer 13

(PaCO2 now - PaCO2 goal)/PaCO2 goal
**is VT safe? –> < 8 ml/kg?
** is RR safe? –> < 35 bpm
Also NOTE: Pplat less than or equal to 30 cmH2O and driving pressure less than or equal to 15 cmH2O

Question 14

FiO2 goal =

Answer 14

(FiO2 now/PaO2 now) = x/PaO2 goal

Question 15

Time Constant (TC) =

Answer 15

Resistance (cmH2OL/sec) x Compliance (cmH2O/L)

Question 16

What are the set parameters in volume control?

Answer 16

VT and flow

Ti is determined by the flow and VT settings

Question 17

If you increase VT in VCAC, how will Ti change?

Answer 17

In VCAC, Ti is influenced by VT and flow.
VT = Ti x flow –> Ti = VT/flow
So if you increase VT, Ti will increase.

Question 18

How will increasing RR affect Te?

Answer 18

Increased RR will decrease the amount of time for Te

Question 19

In VCAC, how will a change in compliance change VT?

Answer 19

VT will stay the same because in VCAC, VT is set

Question 20

in VCAC, how will an increased VT change Pplat and Ppeak?

Answer 20

Pplat will increase, and thus Ppeak will increase because Ppeak is built on top of Pplat

Question 21

How will decreased compliance change Pplat and Ppeak in VCAC?

Answer 21

C = VT/(Pplat-PEEP)

So if C decreases and VT is set, then Pplat will increase and Ppeak will increase (since Ppeak is built on top of Pplat)

Question 22

How will Pplat change with increased resistance in VCAC?

Answer 22

stays the same

Question 23

In VCAC. the patient’s resistance increases, what will happen to flow?

Answer 23

Flow is a set parameter in VCAC, thus it will no change.

Question 24

How will the I:E ratio change with increased RR?

Answer 24

the I:E ratio will decrease

Question 25

In PCAC, the patient’s resistance increased, what affect will this have on VT?

Answer 25

If they are still meeting 3TCs (i.e. Ti > 3 TCs) then the pressure still has time to equalize so the delivered volume doesn’t change.
If Ti < 3 TCs (i.e. they are not meeting 3TCs) due to the increased R then VT would decrease (because increased R would increase 1 TC, and thus increase 3 TCs)

Question 26

In PCAC, how would increased resistance affect flow?

Answer 26

remember R = (Ppeak - Pplat)/Flow
With increased R, flow would decrease because a smaller diameter is more resistance to flow. Its more difficult to deliver the volume because the resistance has decreased the flow.

Question 27

In PCAC, the patient’s resistance decreased, what affect will this have on VT?

Answer 27

If they are still meeting 3TCs (i.e. Ti > 3TC) then the pressure still has time to equalize and so the volume will not change.
if they are not meeting 3TCs (i.e. Ti < 3TCs) due to the decreased resistance then VT would increase (because there is less resistance for VT to flow)

Question 28

In PCAC, the patient’s compliance increased, what affect will this have on VT?

Answer 28

Increased compliance means it will take more time for pressure to equalize because more volume is needed. .
Thus volume will increase, whether they are meeting 3 TCs or not.
*VT = C/P, and P is constant!

Question 29

In PCAC, the patient’s compliance decreased, what affect will this have on VT?

Answer 29

Decreased compliance means it will take less time for pressure to equalize because less volume is needed.
Thus, volume will decrease whether they are meeting 3 TCs or not.
*VT = C/P, and P is constant!

Question 30

In PCAC, how could a change in compliance or resistance affect Ti?

Answer 30

Ti is set in PCAC, so there would be no change to Ti.

Question 31

T/F: square waveform has a short Ti than decelerating waveform

Answer 31

True!

Question 32

T/F: decelerating waveform has a lower peak pressure than square waveform

Answer 32

True!

Question 33

T/F: there is better distribution of ventilation with square waveform

Answer 33

False! There is better distribution of ventilation with decelerating waveform - as long as you increase the flow to maintain the same Ti.
This is because 1/2 way through the breath a larger % of the volume will be delivered compared to square.

Question 34

T/F: the I:E ratio decreases when you switch to decelerating

Answer 34

False, it increases!