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 Gradient also know PF ratio

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

a/A ratio

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

Anion Gap

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)

CaO2

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

CvO2

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.

Oxygen Consumption

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

Tube compliance

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

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.

Time Constant

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

DO2

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:E Calculations

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

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)

I:E Calculations

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

Flow

ETT size -2 x2

8.0 ETT

(8-2)2 = 12 French

Suction Catheter to ET tube size

Mean Airway pressure (Paw): ½ (PIP-peep) (TI/TCT) + PEEP

Average amount of pressure throughout the TCT

AKA: Pmean, MAP, MAWP, PAW

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

Calculation of VE for 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)

Calculation of VE for desired PaCO2

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

Example: Calculate new VE for a patient with:

Current VE = 12 PaCO2 on ABG 60 Desired PaCO2 40

VE = 12 x 60 = 18 40

Calculation of VE for desired PaCO2

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%

Desired FIO2

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

Cardiac Output

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

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

Spontaneous VT

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 10Total Ve –(VT mech x f mech)/ f spontSPONT VOLUME: 8.1 -5.6/10=250 ml

total Ve

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

Vital Capacity

RSBI: Rapid shallow breathing index,

RR/VT, <105 weaning

must be calculated during spont breathing, press support reduces predictive value

RSBI

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)

MIP

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

Hemodynamics

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.

Plateau Pressure

Maximum pressure reached during a positive pressure inspiration

Keep lower than 40 cmH2O Increased with:

◦ Secretions◦ Bronchospasm◦ Decreasing Compliance◦ Increased RAW◦ Asynchrony/fighting vent

Peak Inspiratory Pressure