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Formulas and parameters of mechanical ventilation

Prof Marcelo Alcantara Holanda

-Associate Prof. of Medicine, Federal University of Ceará, Brazil
-Idealizer and founder of the Xlung Platform

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Ideal or predicted body weight (IBW or PBW), kg

Gender: Male
Equations: 50 + 0,91 * (Height - 152,4 cm).
Comments/recommendations: VT: 6 to 8ml of IBW in general.

Gender: Female
Equations: 45, 5 + 0,91 * (Height - 152,4 cm).
Comments/recommendations: VC < 6 ml/kg - ARDS.

Respiratory mechanics and related variables

Variable: Peak airway pressure
Equations: None
- Maximum airway pressure
- Keep below 35-45 cmH2O>

Variable: Plateau pressure
Equations: None
- Alveolar pressure at end inspiration during a pause of 2 s
- Keep below 15 cmH2O or lowest possible value

Variable: Driving pressure
Equations: None
- Alveolar pressure at end inspiration – PEEP
- Keep below 28-30 cmH2O or lowest possible value

Variable: Auto-PEEP or intrinsic PEEP
Equations: None
- Alveolar pressure at end expiration measured woth a pause of 2-3s
- Normal: zero, or equal to PEEP
- Try to keep < 10 cmH2O or the lowest possible value

Variable: Airway resistance (Raw)
Equations: (Peak pressure – Plateau pressure, cmH2O)) / Flow (l/s)
- Needs square wave flow type
- Normal 4-10 cmH2O/l.s
- Try to keep < 20 cmH2O/l.s in obstructive airway diseases

Variable: Static compliance(Cst)
Equations: Tidal volume (ml) / (Plateau pressure – PEEP, cmH2O)
- Normal 50 a 80 ml/cmH2O
- Higher in emphysema
- Lower in in restrictive diseases:
- Cardiogenic pulmonar edema
- Abdominal distension
- Pneumothorax

Variable: Time constant
Equations: Raw cmH2O.L.s x Cst L/cmH2O
Comments/recommendations: Note that Cst needs to be in L and not mL/cmH2O

Variable: I:E ratio
Equations: One to Exp. time / Insp. time
Example: Insp. Time of 0.5s; Exp time of 2.5s, than I:E ratio will be, 1 to (2.5/0.5) or 1:5

Gas exchange

Variable: PaO2 pressure
Equations: None
Comments/recommendations: Keep between 65 to 85mmHg with the loest possible FIO2

Variable: PaCO2
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

Variable: pH arterial
Equations: None
- Keep between 7.34-7.44
- Permissive hypercapnia (PaCO2 > 50mmHg) allow acidemia with pH > 7.20

Variable: SaO2 or SpO2
Equations: None
- Keep between 92-97%
- Make sure the SpO2 sensor provides consistent waveforms

Variable: FIO2 setting
Equations: FIO2 target = PaO2 target x (FIO2 current / PaO2 current)
Comments/recommendations: Set the lowest possible FIO2

Variable: PaO2/FIO2 ratio
Equations: PaO2 (mmHg) /FIO2 (absolute value, not in %)
- Normal > 400 a 500, sea level
- It may estimate the amount of pulmonary shunt:
- 200-300, 10-20% of shunt
- 100-199, 20-40% of shunt
- < 100, > 40% of shunt

Variable: Respiratory rate (RR) setting
Equations: RR target = RR current x PaCO2 current / PaCO2 target
Comments/recommendations: Choose a PaCO2 target according to the patient’s condition and the pH

Weaning predictors

Variable: Clinical and physiological parameters for SBT interruption
Equations: None
- Agitation, sweating, altered level of consciousness
- RR > 35 bpm
- SpO2 < 90%
- Heart Rate > 140 bpm
- Systolic blood pressure > 180 mmHg or < 90 mmHg

Variable: Maximum inspiratory pressure (MIP)
Equations: None
Comments/recommendations: MIP < |-25 to -30cmH2O| are associated with extubation failure due to respiratory muscle’s weakness

Variable: P 0.1
Equations: None
- Airway pressure measured in 100ms after initiation of breathing effort Correlates directly to the respiratory drive
- Normal < 4cmH2O, higher values may be associated to weaning failure

Variable: Rapid shallow breathing (RR/VT) ratio
Equations: RR bpm / Mean VT (L)
- Usually measured with the patient disconnected from the ventilator
- Values > 105 bpm/L are associated with a higher risk of failure during the spontaneous breathing test (SBT)

Variable: Integrative Weaning Index (IWI)
Equations: Cst ml/cmH2O × SaO2 / (RR/TV) ratio
- Cst needs to be measured in volume control ventilation with inspiratory hold for 0.5 to 1.0 second with no inspiratory efforts
- IWI>25mL/cmH2O.breaths/min/L is associated with weaning success