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Ventilator Formulas and Review

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Ventilator Formulas and Review

- http://www.drugbank.ca/drugs/DB01001
- MOA of all drugs

- 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:PaO2/PAO2 norm is 90%, >35%weaning, reflects efficiency of oxygenation as a percentage, <74% shunt, V/Q mismatch or diffusion defect

- 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
- = ( [Na+]+[K+] ) − ( [Cl−]+[HCO3−] ) with potassium

- 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:(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

- 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.

- 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.
- Ex: Vt 550 – (25 x3) = 475

- 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)

- 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: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-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: Calculate I:E ratio, rate and TCT if I-time is 1.2 seconds and E-time is 3 seconds.
- 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)

- 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

- 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
- Since I/E = 1:3, the expiratory time = 1.25 • 3 = 3.75 seconds
- Note: 1.25 + 3.75 = 5 seconds (the number of seconds per breath in this case.) ( a breath equals inspiration + expiration)

- Flow = VE x (I+E)
- Example: Calculate flow given:
- VT 600
- Rate 12
- IT 1.5
- ET 3
- 600 x 12 = 7.2 L x (1.5 +3) = 32 L

- ETT size -2 x2
- 8.0 ETT
- (8-2)2 = 12 French

- Mean Airway pressure (Paw): ½ (PIP-peep) (TI/TCT) + PEEP
- Average amount of pressure throughout the TCT
- AKA: Pmean, MAP, MAWP, PAW

- VE = actual VE x actual PaCO2
desired PaCO2

- New rate = Current rate x actual PaCO2
desired PaCO2

- New Vt = Current Vt x actual PaCO2
desired PaCO2

- Example: Calculate the new set rate for a patient,
- Current set rate: 12
- PaCO2 on ABG 48
- Desired PaCO2=40
- New Rate = 12 x 48 / 40 = 14.4 (14)

- 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

- 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 = (desired PaO2)(known FIO2) / known PaO2
- Ex: Calculate the desired FIO2 for a patient with:
- PaO2 on ABG 50
- Current FIO2 30%
- Desired PaO2 60
- 60 x 0.30/ 50 = 36%

- Cardiac Output (CO) = amt of blood ejected from the heart per minute
- CO= Stroke Volume X heart ratenormal 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

- 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

Total Ve (given from vent) –(VT mech x f mech)/ f spont (total rate – mech rate set)

SPONT VOLUME: 8.1 -5.6/10=250 ml

- Measured when machine in SIMV mode, represents what the patient is actually breathing on his/her own.
- GOAL is 5-7 ml/kg, adjust PSV to achieve this
- EX: Pt on SIMV 8, total rate is 12, Set VT 500, Ve 8.0
- 8 –(4) /4 = 1000 ml

- VE total:(VT mech x f Mech) + (VT Spont x f spont)
Calculated when patient is in SIMV mode

Example:

VT mech 700 ml, f mech 8, VT spont 250 ml, f spont 10

Total Ve –(VT mech x f mech)/ f spont

SPONT VOLUME: 8.1 -5.6/10=250 ml

- VC:Vital capacity, 65-75 mL/kg, <10mL/kg indicates support, 10-15 mL/kgweaning
- Maximum inhalation followed by a maximum exhalation
- Measured by a Wright Respirometer

- RSBI:Rapid shallow breathing index,
- RR/VT, <105 weaning
- must be calculated during spont breathing, press support reduces predictive value

- 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)

- 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: 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.

- Maximum pressure reached during a positive pressure inspiration
- Keep lower than 40 cmH2O
- Increased with:
- Secretions
- Bronchospasm
- Decreasing Compliance
- Increased RAW
- Asynchrony/fighting vent