Formulas and parameters of mechanical ventilation

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Click here to see the data below in table form

Ideal Body Weight (IBW)

Gender: Male
Equations: 50 + 0.91 * (Height - 152.4 cm).
VT: 6 to 8 ml/kg - Start of MV.

Gender: Female
Equations: 45.5 + 0.91 * (Height - 152.4 cm).
VT < 6 ml/kg - ARDS.

Respiratory Mechanics

Airway Resistance (Raw)
Equations: Peak p. – plateau p. (cmH2O)/Flow (L/s).
Requires square flow type in VCV mode for accurate calculation.
- Normal value: 4 to 10 cmH2O/L.s .
- Keep Raw < 20 cmH2O/L.s in obstructive airway diseases..

Static Compliance (Cst)
Equations: Tidal volume (ml) / (Plateau p. - PEEP) (cmH2O).
- Normal: 50 to 80 ml/cmH2O.
- High - Emphysema.
- Low - ARDS, pulmonary edema, abdominal distention, pneumothorax, atelectasis.

Peak airway pressure
Definition: Maximum airway pressure.
Equations: None
- Keep < 35 to 45 cmH2O.

Plateau pressure
Definition: Alveolar pressure measured at the end of inspiration by pause of 0.5s.
Equations: None
- Keep < 28 to 30cmH2O or as low as possible.

Auto-PEEP or intrinsic PEEP
Definition: Alveolar pressure measured at the end of expiration by pause of 3s.
- Normal value: zero.
- Keep < 10 cmH2O in obstructive airway diseases..

Driving pressure
Equations: Plateau pressure - PEEP (cmH2O).
- Keep < 15 cmH2O in ARDS and in patients at risk for VILI.

Gas Exchange

Equations: None
- Keep between 65 and 80 mmHg with the lowest FIO2 possible.

FIO2 to target PaO2
Equations: Target PaO2 (mmHg) * (Current FIO2 / Target PaO2 (mmHg)).

Equations: None
- Set according to the pH.
- Permissive hypercapnia (PaCO2 > 50mmHg) in ARDS, COPD or status asthmaticus.
- Keep between 35 to 38 mmHg in acute traumatic brain injury.

Equations: None
- Keep between 7.34-7.44
- Permissive hypercapnia (PaCO2 > 50mmHg with pH > 7,20).

Equations: PaO2 (mmHg) /FIO2 (absolute value, not in %)
- Normal > 400 a 500mmHg - nível do mar
- It may estimate the amount of pulmonary shunt:
- 200 to 300, 10 to 20% shunt;
- 100 to 199, 20 to 40% shunt;
- < 100, > 40% shunt.

SaO2 or SpO2
Equations: None.
- Keep between 92-96% (Check pulse oximetry plethysmographic waveform).

FIO2 setting
Equations: FIO2 target = PaO2 target (mmHg) * (FIO2 current / PaO2 current (mmHg)).
- Set the lowest possible FIO2.

Respiratory rate (RR) setting
Equations: RR target = RR current * PaCO2 current / PaCO2 desired.
- Choose a PaCO2 target according to the patient’s condition and the pH.

Predictors of successful weaning

Tobin Index or Rapid shallow breathing (RR/VT) ratio
Equations: RR bpm / VT Mean (L)
- Values ​​> 105 bpm/L are associated with weaning failure.
- Using the ventilometer, identify the minute volume and respiratory rate of the patient in spontaneous breathing, that is, disconnected from the ventilator.
- Apply the minute volume (VE) values ​​to the equation: VE = VT/RR to find the tidal volume.
- VT (mean) = VE/RR
- After that, apply the values ​​of VT and RR in the Tobin index equation: RR bpm / VT Mean (L).

Maximum Inspiratory Pressure (MIP)
Equations: None
- MIP > (less negative) than - 20 to -30cmH2O is associated with respiratory muscle weakness and extubation/weaning failure.

Equations: None
Definition: Airway pressure measured at 100ms or 0.1s from the start of inspiration.
- Normal: 1.5 – 3.5 cmH2O (normal ventilatory drive / normal ventilatory assistance)
- Low respiratory drive: < 1.0cmH2O (hypostimulated ventilatory drive / Overassistance / weaning failure)
- High respiratory drive: >4.0cmH2O (hyperstimulated ventilatory drive / under - ventilatory assistance / weaning failure)
- Some ventilators can measure P0.1 with the help of specific software.
- Clinical application: assessment of the level of ventilatory support and muscle effort, estimation of neural command or respiratory drive.


Time constant
Definition: time required for lung emptying.
Equation: Time constant = Rva (cmH2O/L.s) * Cst (L/cmH2O)
Ex.: Rva of 10 cmH2O/L.s * Cst of 0,06 L/cmH2O (Cst) = 0,6s.
- It takes 4 to 5 time constants for adequate or near complete exhalation.

Pmus (cmH2O)
Equation: Pmus = -3/4 * ∆ Pocc (occlusion pressure) or -0.75 * ∆ Pocc.
- Clinical application: Estimates Pmus during MV, without the need for an esophageal catheter.
- For this, it is necessary to find ∆ Pocc (occlusion pressure).
- To identify the ∆ Pocc: Perform an expiratory pause (a drop in pressure is observed) and observe the pressure variation in the airway.
- (Delta Pocc = PEEP - lowest pressure achieved)
- Normal values Pocc: 5 - 10 cmH2O.

Step by step

Step 1: Performs an expiratory pause (occlusion maneuver).

Step 2: Freezes the MV screen.

Step 3: With the MV cursor, identify the value of ∆Pocc (∆ Pocc = pressure drop + PEEP)

Step 4: Apply it to the formula (Pmus = -3/4 x ∆Pocc or -0.75 *∆Pocc)

- Pmus < 5cmH2O (Overassistance, low drive - e.g. sedation, muscle weakness)
- Pmus ≤ 10cmH2O (diaphragmatic protection)
- Pmus > 13-15 cmH2O (excessive muscle exertion)

ROX index
Equation: ROX = (SpO2/FIO2)*100/respiratory rate.
- ROX index ≥4.88 measured 2, 6 or 12 h after initiation of HFNC is associated with a lower risk of intubation.
- ROX index <3.85 = high risk of HFNC failure.
- If ROX from 3.85 to < 4.88, score can.

Asynchrony Index (%)
Equation: AI = number of asynchronous events / total RR (cycles triggered or not)
- AI > 10% considered severe.

Mechanical Power (J/min)
Definition: Energy transferred to the lungs by the mechanical ventilator.
Equation: MP = 0.098 x (VC/1000) x driving pressure x f
- MP < 12 J/min – normal;
- MP 13 - 17 J/min - Lung injury;
- MP 18 - 22 J/min - mild ARDS;
- MP 23 - 24 J/min - moderate ARDS;
- MP 25 - 27 J/min - severe ARDS;
- MP > 27 J/min - ECMO indication.


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