Formulas and parameters of mechanical ventilation

Marcelo Alcantara Holanda

- Pulmonologist, Critical Care Specialist, MD, Ph.D
- Associate Professor of Pulmonary and Critical Care Medicine, Federal University of Ceará, Fortaleza, Brazil
- Idealizer and Founder of the Xlung Platform for Teaching Mechanical Ventilation

Click here to see the data below in table form

Ideal Body Weight (IBW)

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

Gender: Female
Equations: 45.5 + 0.91 * (Height - 152.4 cm).
Comments/Recommendations:
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.
Comments/Recommendations:
- 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).
Comments/Recommendations:
- 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
Comments/Recommendations:
- Keep < 35 to 45 cmH2O.

Plateau pressure
Definition: Alveolar pressure measured at the end of inspiration by pause of 0.5s.
Equations: None
Comments/Recommendations:
- 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.
Comments/Recommendations:
- Normal value: zero.
- Keep < 10 cmH2O in obstructive airway diseases..

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

Gas Exchange

PaO2
Equations: None
Comments/Recommendations:
- Keep between 65 and 80 mmHg with the lowest FIO2 possible.

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

PaCO2
Equations: None
Comments/recommendations:
- 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.

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

PaO2/FIO2ratio
Equations: PaO2 (mmHg) /FIO2 (absolute value, not in %)
Comments/Recommendations:
- 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.
Comments/Recommendations:
- Keep between 92-96% (Check pulse oximetry plethysmographic waveform).

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

Respiratory rate (RR) setting
Equations: RR target = RR current * PaCO2 current / PaCO2 desired.
Comments/recommendations:
- 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)
Comments/Recommendations:
- 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
Comments/Recommendations:
- MIP > (less negative) than - 20 to -30cmH2O is associated with respiratory muscle weakness and extubation/weaning failure.

P0.1
Equations: None
Definition: Airway pressure measured at 100ms or 0.1s from the start of inspiration.
Comments/Recommendations:
- 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.

Others

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.
Comments/Recommendations:
- 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.
Comments/Recommendations:
- 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.
Comments/Recommendations:
- 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)
Comments/Recommendations:
- 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
Comments/Recommendations:
- 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.

References

Henderson WR, Chen L, Amato MBP, Brochard LJ. Fifty Years of Research in ARDS. Respiratory Mechanics in Acute Respiratory Distress Syndrome. Am J Respir Crit Care Med. 2017 Oct 1;196(7):822-833. Brazilian recommendations of mechanical ventilation 2013. Part I. Jornal Brasileiro de Pneumologia [online]. 2014, v. 40, n. 4, pp. 327-363.

ARDSNET; BROWER, R.G.; MATTHAY, M.A.; MORRIS, A.; SCHOENFELD, D. THOMPSON, B.T.; WHEELER, A. Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome. N Engl J Med. v. 342, n.18, p. 1301–1308, 2000.

Roca O, Messika J, Caralt B, García-de-Acilu M, Sztrymf B, Ricard JD, Masclans JR. Predicting success of high-flow nasal cannula in pneumonia patients with hypoxemic respiratory failure: The utility of the ROX index. J Crit Care. 2016 Oct;35:200-5. doi: 10.1016/j.jcrc.2016.05.022. Epub 2016 May 31. PMID: 27481760.

Bertoni M, Telias I, Urner M, Long M, Del Sorbo L, Fan E, Sinderby C, Beck J, Liu L, Qiu H, Wong J, Slutsky AS, Ferguson ND, Brochard LJ, Goligher EC. A novel non-invasive method to detect excessively high respiratory effort and dynamic transpulmonary driving pressure during mechanical ventilation. Crit Care. 2019 Nov 6;23(1):346.

Thille AW, Rodriguez P, Cabello B, Lellouche F, Brochard L. Patient-ventilator asynchrony during assisted mechanical ventilation. Intensive Care Med. 2006 Oct;32(10):1515-22.

VASCONCELOS, R. S.; SALES, R. P.; MELO, L. H. P.; MARINHO, L. S.; BASTOS, V. P.; NOGUEIRA, A. D. N.; FERREIRA, J. C.; HOLANDA, M. A. Influences of Duration of Inspiratory Effort, Respiratory Mechanics, and Ventilator Type on Asynchrony with Pressure Support and Proportional Assist Ventilation. Respir Care, n. 62, v. 5, p. 550-557, 2017.

Silva PL, Ball L, Rocco PRM, Pelosi P. Power to mechanical power to minimize ventilator-induced lung injury? Intensive Care Med Exp. 2019 Jul 25;7(Suppl 1):38.

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