Ventilator Formulas and
Review
A-a Gradient
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A-aDo2: A-a gradient,
norm 5-10 mmHg on .21, 30-60 on 100%, >
350mech support, <350 weaning.
Represents potential to Oxygenate vs. the amount
of O2 in the artery. Every 50mmHg is approx. 2
percent shunt above norm of 2-5%
Increased A-a= SHUNT
a/A ratio
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a/A ratio: PaO2/PAO2 norm is 90%, >35%weaning,
reflects efficiency of oxygenation as a percentage,
<74% shunt, V/Q mismatch or diffusion defect
Anion Gap
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Anion Gap= the difference in the measured
cations and the measured anions in serum,
plasma, or urine.
Used to assess Metabolic Acidosis or alkalosis,
normal around 8-16 mEq/L. Use MUDPILES to
determine cause of metabolic acidosis (high gap)
= ( [Na+] ) − ( [Cl−]+[HCO3−] ) without potassium
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= ( [Na+]+[K+] ) − ( [Cl−]+[HCO3−] ) with potassium
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CaO2
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CaO2: norm 20 vol%
(Hbx1.34)SaO2 + (PaO2x.003) total amount of O2
carried in 100ml of blood, combined content of O2
carried on Hb and dissolved in plasma,
(can be reduced by <Hb, anemia or <CO)
CvO2
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CvO2: (Hb x 1.34)SvO2 + (PvO2 x .003)
norm is 15 vol%, represents the value of O2 in
blood returning to the right side of the heart after
tissues have oxygenated.
C(a-v)O2 = arterial to mixed venous oxygen
content difference
Determines how well the tissues take up O2
Oxygen Consumption
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VO2: O2 consumption,
norm is 250mL/O2/L/min,
[C(a-v)O2 x QT] x 10, the amount of O2 consumed
by the body per liter of blood per minute.
Tube compliance
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Ct: Tube compliance aka compressible volume,
volume lost to pt circuit, approx 3ml/cmH20 x PIP,
deduct from VT to find actual delivered tidal
volume. VT-(PIP x 3) equals actual VT.
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Ex: Vt 550 – (25 x3) = 475
Time Constant
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Time Constant: The given % of a passively
exhaled breath of air will require a constant
amount of time to exhale
Depends on the resistance and compliance of the
lung
TC= R x C (in liters)
Time Constant
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TC: Time constant, (Raw x CS)e,
where e represents volume exhaled as a percent,
1 is 63%,
2 is 86%,
3 is 95%
5 is 100% exhaled.
TC <3 leads to air trapping.
DO2
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DO2: O2 Delivery, (CaO2 x CO) x 10, norm is
1000mL/O2/min
The ability of oxygen to tissues based on cardiac
output and Hb
I:E Calculations
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I-time = Inspiratory Time,
E-time = Expiratory time,
TCT= total cycle time (I +E)
I-time when compared to E-time will always be a 1:
something ratio.
Respiratory rate = 60 /TCT
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EXAMPLE: Calculate I:E ratio, rate and TCT if I-time is
1.2 seconds and E-time is 3 seconds.
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TCT = 1.2+ 3 = 4.2
Rate = 60/4.2 =14
I:E = TE/TI = 3/1.2 = 2.5, (I:E is 1:2.5)
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I:E Calculations
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I-time = Inspiratory Time,
E-time = Expiratory time,
TCT= total cycle time (I +E)
I-time when compared to E-time will always be a 1:
something ratio.
Respiratory rate = 60 /TCT
Example: The ventilator is set at 12 breaths per
minute with an IE ratio of 1:3. How many seconds
for inspiratory time?
Seconds per breath = 60 divided by 12 = 5 seconds
TI=5/(1+3) =5/4=1.25 seconds
I:E Calculations
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Example: The ventilator is set at 12 breaths per
minute with an IE ratio of 1:3. How many seconds
for inspiratory time?
Seconds per breath = 60 divided by 12 = 5 seconds
TI=5/(1+3) =5/4=1.25 seconds
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Since I/E = 1:3, the expiratory time = 1.25 • 3 = 3.75
seconds
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Note: 1.25 + 3.75 = 5 seconds (the number of
seconds per breath in this case.) ( a breath equals
inspiration + expiration)
Flow
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Flow = VE x (I+E)
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Example: Calculate flow given:
VT 600
Rate 12
IT 1.5
ET 3
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600 x 12 = 7.2 L x (1.5 +3) = 31.5 (32 L)
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Suction Catheter to ET tube size
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No more than ½ the internal diameter of ETT or else improper
entrainment.
French divided by 3.14 = size in mm. normal adult sizes 12 ‑ 14 fr
O.D. of catheter should not exceed 1/2 I.D.of airway
(1) to determine size of suction catheter for given ET tube: ETT size
/ 2 then x 3.14
(2) always round down – i.e. 7/2 x 3.14 = 10.99, use a 10 French
OR
Size of tube -2 then x 2.
Example: 8 ETT – 2 = 6 x 2 = 12F
PaW
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Mean Airway pressure (Paw): ½ (PIP-peep) (TI/
TCT) + PEEP
Average amount of pressure throughout the TCT
PIP 25
PEEP 5
IT 1
TCT 4
Calculation of VE for desired PaCO2
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VE = actual VE x actual PaCO2
desired PaCO2
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New rate = Current rate x actual PaCO2
desired PaCO2
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New Vt = Current Vt x actual PaCO2
desired PaCO2
Calculation of VE for desired PaCO2
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Example: Calculate the new set rate for a patient,
Current set rate: 12
PaCO2 on ABG 48
Desired PaCO2=40
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New Rate = 12 x 48 / 40 = 14.4 (14)
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Calculation of VE for desired PaCO2
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Example: calculate the new VT for a patient with:
Current VT = 500
PaCO2 on ABG 55
PaCO2 desired 40
New Vt = 500 x 55 = 687.5 (688)
40
Calculation of VE for desired PaCO2
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Example: Calculate new VE for a patient with:
Current VE = 12
PaCO2 on ABG 60
Desired PaCO2 40
VE = 12 x 60 = 18
40
Desired FIO2
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Desired FIO2 = (desired PaO2)(known FIO2) /
known PaO2
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Ex: Calculate the desired FIO2 for a patient with:
PaO2 on ABG 50
Current FIO2 30%
Desired PaO2 60
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60 x 0.30/ 50 = 36%
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Cardiac Output
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Cardiac Output (CO) = amt of blood ejected from
the heart per minute
CO= Stroke Volume X heart rate normal 4-8 L/
min
QT = cardiac output
Cardiac Index (CI)= amount of blood pumps per
minute based on body weight CI= CO/BSA,
normal 2.4-4 L/min
Shunt Equation
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QS/QT: Pulmonary Shunt equation
(CcO2-CaO2)/(CcO2-CvO2)
Norm 2-3%,
>20% vent indication,
<20% weaning,
>30% is life threatening.
Measures % of QT not exposed to ventilation, shunts
caused by atelectasis, edema, pneumonia,
pneumothorax, obstructions
CcO2: Content of pulm capillary blood oxygen at
100% FIO2, (Hbx1.34)1 + (PAO2x.003) used in shunt
equation
Spontaneous VT
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VTspont: VEtot-(VTset x RRset)/(RRtot-RRset)
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Measured when machine in SIMV mode, represents
what the patient is actually breathing on his/her own.
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Example:
Ve 10
VT set 500
RR set 10
RR total 20
10 – (500 x 10) / (20 -10) = 499
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Spontaneous Ve
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VEspont: VEtot-(VTset x RRset), norm 5-6 L/min,
Calculated when patient is in SIMV mode
Example:
Ve total= 12
VT Set = 500
RR Set= 16
12 – (500 x 16) = 7,988 ml or 7.9 L
Vital Capacity
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VC: Vital capacity, 65-75 mL/kg, <10mL/kg
indicates support, 10-15 mL/kgweaning
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Maximum inhalation followed by a maximum
exhalation
Measured by a Wright Respirometer
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RSBI
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RSBI: Rapid shallow breathing index,
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RR/VT (in liters), <100 weaning
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must be calculated during spont breathing, press
support reduces predictive value
MIP
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MIP/NIF: Max Inspiratory Press, norm -80 - -100, >
-20 support indicated, <-20 weaning (remember
that negative numbers are larger as they become
less, -25 < -20)
Hemodynamics
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PAP: pulmonary artery pressure, norm 25/10
(20-35/5-15), >35/15 is inconsistent with weaning,
pulm hypertension, left vent fail, fluid overload
PCWP: pulmonary artery wedge pressure, norm
5-10 mmHg, >18 is inconsistent with weaning, left
vent failure, fluid overload
CVP: central venous pressure, norm 2-6 mmHg,
2-6 weaning
Plateau Pressure
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Plateau pressure: The amount of pressure held in
the lung during a brief inspiratory pause. This is
used to calculate static compliance. The higher
this number the worse the patients compliance as
it represents distending pressure. Typically less
than PIP, but more than MAP.
Peak Inspiratory Pressure
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Maximum pressure reached during a positive
pressure inspiration
Keep lower than 40 cmH2O
Increased with:
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Secretions
Bronchospasm
Decreasing Compliance
Increased RAW
Asynchrony/fighting vent