Mech Vent Equations Flashcards
Cycle time =
(60 sec/min)/RR
AND
Ti + Te
Ti =
(cycle time)/(I + E)
Tidal Volume (VT) =
Flow x Ti
*flow in L/sec
Minute Ventilation (VE) =
RR x VT
Flow (l/min) =
(I + E ratio) x MV
Compliance =
VT/(Pplat - PEEP)
what are the units for compliance?
L/cmH2O
Resistance =
(Ppeak - Pplat)/flow
*flow in L/sec
what are the units for resistance?
cmH2O/Lsec
DO2 =
CO x CaO2
CaO2 =
([Hb] x 1.34 ml O2/g Hb x SaO2) + (PaO2 x 0.003)
CvO2 =
([Hb] x 1.34 ml O2/g Hb x SvO2) + (PvO2 x0.003)
VT goal, RR goal, Ve goal =
(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
FiO2 goal =
(FiO2 now/PaO2 now) = x/PaO2 goal
Time Constant (TC) =
Resistance (cmH2OL/sec) x Compliance (cmH2O/L)
What are the set parameters in volume control?
VT and flow
Ti is determined by the flow and VT settings
If you increase VT in VCAC, how will Ti change?
In VCAC, Ti is influenced by VT and flow.
VT = Ti x flow –> Ti = VT/flow
So if you increase VT, Ti will increase.
How will increasing RR affect Te?
Increased RR will decrease the amount of time for Te
In VCAC, how will a change in compliance change VT?
VT will stay the same because in VCAC, VT is set
in VCAC, how will an increased VT change Pplat and Ppeak?
Pplat will increase, and thus Ppeak will increase because Ppeak is built on top of Pplat
How will decreased compliance change Pplat and Ppeak in VCAC?
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)
How will Pplat change with increased resistance in VCAC?
stays the same
In VCAC. the patient’s resistance increases, what will happen to flow?
Flow is a set parameter in VCAC, thus it will no change.
How will the I:E ratio change with increased RR?
the I:E ratio will decrease
In PCAC, the patient’s resistance increased, what affect will this have on VT?
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)
In PCAC, how would increased resistance affect flow?
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.
In PCAC, the patient’s resistance decreased, what affect will this have on VT?
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)
In PCAC, the patient’s compliance increased, what affect will this have on VT?
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!
In PCAC, the patient’s compliance decreased, what affect will this have on VT?
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!
In PCAC, how could a change in compliance or resistance affect Ti?
Ti is set in PCAC, so there would be no change to Ti.
T/F: square waveform has a short Ti than decelerating waveform
True!
T/F: decelerating waveform has a lower peak pressure than square waveform
True!
T/F: there is better distribution of ventilation with square waveform
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.
T/F: the I:E ratio decreases when you switch to decelerating
False, it increases!
