Overview
Ventilator modes share a simplified passive lung represented by compliance C and resistance R. The equation is Paw = PEEP + V/C + R × flow, using liters and seconds internally. Inspiratory and expiratory flow, tidal volume, PIP and plateau pressure are derived from that model.
Compliance and resistance vary with the selected phenotype. Congestion reduces compliance; recruitment can improve it modestly, while overdistension reduces it. Anaphylaxis increases airway resistance; tension pneumothorax adds severe mechanical limitation. These are bounded synthetic coefficients, not measured lung mechanics.
Mode behavior
| Mode | Implemented behavior |
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| Volume control | Constant inspiratory flow delivers the target VT, with pressure determined by resistance and compliance. |
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| Pressure control | Constant pressure support during inspiration; delivered VT depends on driving pressure, compliance, time constant and incomplete expiration. |
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| PCV-VG | Required pressure is calculated to approach target VT, with a modeled driving-pressure ceiling of 35 cmH₂O. VT can remain below target. |
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| SIMV | Scheduled mandatory breaths and interspersed pressure-supported spontaneous breaths share one continuous residual-volume solution. |
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| Pressure support / NIPPV | Patient drive triggers supported breaths. Apnea does not acquire automatic backup ventilation in this model. |
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Compliance, resistance and expiration
Resting compliance starts near 0.06 L/cmH₂O and is bounded to 0.012–0.07. Normal resistance starts at 8 cmH₂O·s/L; the selected bronchospasm example starts higher. The time constant is τ = R × C. Exhaled volume decays exponentially; residual volume generates intrinsic PEEP.
The periodic breath solution carries residual volume into the next breath. SIMV solves the whole repeating sequence rather than resetting the lung at every inspiration. Short expiration, increased resistance and faster rate can cause gas trapping. PIP includes a resistive component; plateau pressure reflects elastic pressure plus PEEP.
Minute and alveolar ventilation
Minute ventilation averages delivered VT over the actual breath sequence. Alveolar ventilation subtracts dead space separately from each breath before averaging; tiny spontaneous breaths cannot cancel useful mandatory ventilation. Dead space uses 2.2 mL/kg predicted body weight, with an additional illustrative pulmonary-vascular component.
PaCO₂ = 0.863 × 200 / alveolar ventilation in L/min, bounded to 15–120 mmHg. The fixed 200 mL/min CO₂ production is a teaching assumption. Respiratory pressure-volume, recruitment and gas equations are not a validated ARDS or COPD simulator.
Settings and omissions
Rate is bounded to 6–35/min, target VT 200–900 mL, driving pressure 5–35 cmH₂O, pressure support 5–25 cmH₂O, inspiratory time 0.4–2 s and predicted body weight 40–100 kg. Available controls depend on mode. Ti is also limited relative to total cycle time.
The model does not resolve leaks, trigger sensitivity, patient–ventilator dyssynchrony, esophageal pressure, regional recruitment, chest-wall versus lung compliance, secretion obstruction, ventilator hardware algorithms or device-specific safety modes.
References & further reading
