Mechanical Ventilation Simulator

User Manual

rogerstedman.com

Introduction

VentSim is a real-time mechanical ventilation and respiratory physiology simulator designed for clinical education and training. It runs entirely in your web browser with no installation required, and is optimised for both desktop and mobile devices.

VentSim models the complete physiological interaction between a mechanical ventilator and a patient’s respiratory system — including lung mechanics, gas exchange, and blood gas analysis. It is intended to help clinicians, trainees, and students develop intuition for ventilator management across a wide range of clinical scenarios.

The simulator is available in two tiers:

• VentSim Free — real-time waveforms, pressure-volume and flow-volume loops, full ventilator control, and lung mechanics adjustment.
• VentSim Pro — adds the complete gas exchange model, arterial blood gas sampling, SpO₂ and ETCO₂ monitoring with alarm states, alveolar ventilation display, and clinical scenarios.

The Interface at a Glance

The screen is divided into three main areas:

Header strip — runs across the top of the screen and displays key monitored values. The ventilation mode, elapsed time clock, and control buttons (ABG and PRO) sit alongside the VentSim logo.

Waveform panel — the central area shows three continuously scrolling real-time traces: airway pressure (P-time), flow (F-time), and volume (V-time), updating with every simulated breath.

Control panels — accessed via the LOOPS and CONTROLS tabs on either side of the screen. On wider screens these appear as fixed columns. On mobile they slide in as overlays.

Setting Up the Patient Model

The patient model defines the respiratory mechanics and (in Pro) gas exchange physiology. Access it via the CONTROLS panel → Patient tab.

Patient Category

Three categories are available: Adult, Child, and Infant. Selecting a category scales all default values appropriately — weight range, lung volumes, airway resistance, cardiac output, and metabolic rate all adjust automatically.

Category	Weight range	Default VO₂/kg	Default Hb
Adult	50–150 kg	3.5 ml/kg/min	14 g/dL
Child	10–50 kg	5.0 ml/kg/min	13 g/dL
Infant	0.5–10 kg	7.0 ml/kg/min	16 g/dL

Weight

Sets the patient’s body weight in kg. Weight drives the TLC calculation via the TLC Factor and scales anatomical dead space (2 ml/kg) and metabolic parameters.

TLC Factor and Total Lung Capacity

The TLC Factor (ml/kg) multiplied by weight gives the Total Lung Capacity (TLC). The default is 80 ml/kg, giving a TLC of 6,000 ml for a 75 kg adult. TLC sets the upper asymptote of the compliance curve. Adjusting either the TLC Factor or TLC slider updates the other automatically.

FRC — Functional Residual Capacity

FRC is the volume of gas remaining in the lungs at end-expiration with no applied pressure. It represents where the patient’s lungs sit on the compliance curve at zero airway pressure. The default is 37.5% of TLC (2,250 ml for a 75 kg adult).

Moving the FRC slider shifts the orange marker along the compliance curve without changing the curve’s shape — the inflection point and steepness remain constant. Clinically this represents changing the patient’s resting lung volume: reduced FRC in obesity, ARDS, or pregnancy; increased FRC in emphysema.

Note: Residual Volume (RV) is displayed as a derived value of 50% of FRC and is not independently adjustable.

Airway Resistance

Sets total airway resistance in cmH₂O/L/s. Increased resistance produces the characteristic sawtooth flow waveform of obstructive lung disease and reduces peak flow for a given driving pressure.

Clinical condition	Typical resistance
Normal adult (intubated)	5–8 cmH₂O/L/s
Mild obstruction	10–15 cmH₂O/L/s
Severe asthma / COPD	20–40 cmH₂O/L/s

Compliance

In linear mode, this slider directly sets static respiratory system compliance in ml/cmH₂O. A normal adult value is 60–100 ml/cmH₂O. In sigmoidal mode this control is disabled and instead displays the effective compliance at the current operating point (Cop) — the compliance the ventilator actually experiences at the current PEEP position on the sigmoid.

The Compliance Curve

Access the compliance curve via the LOOPS panel. A toggle switches between Linear and Sigmoidal compliance models.

Linear Compliance

In linear mode the pressure-volume relationship is a straight line: tidal volume = compliance × driving pressure throughout the breath. This is appropriate for basic teaching of ventilator mechanics.

Sigmoidal Compliance

The sigmoidal model reflects the true S-shaped pressure-volume curve of the respiratory system. Compliance is lowest at very low and very high lung volumes, and greatest near the inflection point. This allows simulation of recruitment, overdistension, and the effects of PEEP position on effective compliance.

Three controls set the shape and position of the sigmoid:

Compliance Point (cmH₂O)

Sets the absolute pressure at the inflection point of the sigmoid — the pressure at which compliance is greatest. Moving this slider shifts the entire curve left or right along the pressure axis. Two vertical crossbar lines on the canvas mark the inflection and the ±1.317 × Delta Range boundaries (the pressures at which the curve reaches 25% and 75% of TLC).

Delta Range (cmH₂O)

Controls the steepness of the sigmoid. A smaller Delta Range produces a steep, narrow curve with high peak compliance but a small safe tidal volume range. A larger value flattens the curve, reducing peak compliance but widening the range of safe operating pressures. The crossbar lines widen or narrow visibly as this is adjusted.

Hysteresis (%)

Sets the energy recovery fraction between inspiratory and expiratory limbs. At 100% there is no hysteresis. At 85% (the default) the expiratory limb sits below the inspiratory limb, as seen in real lung tissue. The expiratory curve is shown as a dashed line.

The PV loop drawn during breathing is overlaid on the compliance curve in real time, showing directly where in the pressure-volume relationship each breath is occurring. The inspiratory limb is shown in cyan, the expiratory limb in blue.

Gas Exchange Parameters

★ Pro feature: All parameters in this section require a VentSim Pro licence.

VD/VT — Physiological Dead Space Fraction

The ratio of physiological dead space to tidal volume (default 0.30). This includes anatomical dead space (fixed at 2 ml/kg) plus alveolar dead space from V/Q mismatch. Increasing VD/VT reduces alveolar ventilation, raising PaCO₂ and lowering ETCO₂. In ARDS and pulmonary embolism VD/VT can exceed 0.60.

Shunt Fraction

The percentage of cardiac output passing through non-ventilated lung units (true shunt, V/Q = 0). Default is 2%. Shunt causes hypoxaemia that is refractory to supplemental oxygen because shunted blood bypasses ventilated alveoli entirely. Increasing FiO₂ has diminishing returns as shunt fraction rises.

Recruitable

The percentage of lung that is collapsed but potentially recruitable with increased airway pressure. As this value increases, higher PEEP or mean airway pressure will progressively recruit these units, reducing effective shunt and improving oxygenation. This interacts with PEEP position on the PV loop — PEEP above the lower inflection point produces greater recruitment.

Haemoglobin

Haemoglobin concentration in g/dL. Affects oxygen carrying capacity and hence arterial oxygen content. Default values: Adult 14 g/dL, Child 13 g/dL, Infant 16 g/dL.

Base Excess

Metabolic component of acid-base status in mEq/L. Positive values indicate metabolic alkalosis; negative values metabolic acidosis. This shifts pH and bicarbonate in the ABG readout. Derived HCO₃⁻ is calculated using the Henderson-Hasselbalch relationship.

Cardiac Output

Cardiac output in L/min. Drives the rate of oxygen delivery and CO₂ removal. Reduced cardiac output increases oxygen extraction, lowering mixed venous PO₂ and worsening hypoxaemia for any given shunt fraction — the physiological basis of the haemodynamic-gas exchange interaction in critical illness.

Setting the Ventilator

Access ventilator controls via the CONTROLS panel → Ventilator tab.

Ventilation Mode

Mode	Description	Waveform characteristics
PC-AC	Pressure Control, Assist-Control. Fixed inspiratory pressure above PEEP delivered for the set Ti at the set rate. Tidal volume determined by compliance and resistance.	Square pressure trace, decelerating flow
VC-AC	Volume Control, Assist-Control. Fixed tidal volume at set flow rate. Airway pressure varies with compliance and resistance.	Square flow trace, rising pressure
Flow	Constant flow throughout inspiration. Primarily for teaching flow-pressure relationships.	Constant flow, linear pressure rise

Rate

Respiratory rate in breaths per minute (range 4–40/min). Combined with I:E ratio this sets Ti and Te.

I:E Ratio, Inspiratory Time, and Expiratory Time

These three parameters are linked. The I:E ratio buttons (1:1 through 1:4) set standard ratios; Ti and Te can also be adjusted directly. The default is a rate of 13/min with Ti 1.5 s and Te 3.0 s (I:E 1:2).

[ SCREENSHOT: I:E ratio buttons and Ti/Te displays ]

Inspiratory Pressure (PC-AC)

Pressure delivered above PEEP during inspiration in cmH₂O. Total peak airway pressure = PEEP + inspiratory pressure. Default: 10 cmH₂O above a PEEP of 5, giving a peak pressure of 15 cmH₂O.

Tidal Volume (VC-AC)

Target tidal volume in ml for volume-controlled ventilation. Default 500 ml.

Inspiratory Flow (VC-AC)

Constant inspiratory flow rate in L/min for volume-controlled ventilation. Default 60 L/min.

PEEP — Positive End-Expiratory Pressure

Applied positive pressure at end-expiration in cmH₂O. PEEP shifts the operating point on the compliance curve — the end-expiratory volume (EEV) marker on the PV loop moves upward along the expiratory limb as PEEP increases. PEEP also interacts with the recruitable shunt fraction in the gas exchange model.

Inspiratory Pause

A percentage of inspiratory time held at zero flow after the breath is delivered. Creates an end-inspiratory plateau, allowing visualisation of static compliance. Appears as a flat segment on the pressure waveform and a zero-flow period on the flow trace.

FiO₂

Fractional inspired oxygen (0.21–1.0). Drives alveolar PO₂ and arterial oxygenation in the gas exchange model (Pro). FiO₂ also appears as the denominator of the P/F ratio in the ABG modal.

Equipment Dead Space

Dead space added by ventilator circuit components (HME filters, catheter mounts, angle pieces) in ml. Additive to the patient’s anatomical dead space. Total anatomical dead space (patient + equipment) is displayed below as a derived value.

Monitored Values

Badge	Description
IP	Peak inspiratory pressure (cmH₂O)
Pplat	Plateau pressure at end-inspiration (cmH₂O)
PEEP	Applied PEEP (cmH₂O)
TV	Delivered tidal volume (ml)
MV	Minute volume (L/min)
V̇A	Alveolar minute ventilation (L/min) — Pro
SpO₂	Arterial oxygen saturation (%) — Pro
ETCO₂	End-tidal CO₂ (kPa) — Pro
MAP	Mean airway pressure (cmH₂O)
RR	Respiratory rate (/min)
Pk Flow	Peak inspiratory flow (L/s)

SpO₂ Alarm States

★ Pro feature: SpO₂ monitoring requires VentSim Pro.

Colour	SpO₂ range	Meaning
Green	≥94%	Normal
Amber	88–94%	Warning
Red	72–88%	Alarm
Red (flashing)	<72%	Critical

ETCO₂ Alarm States

★ Pro feature: ETCO₂ monitoring requires VentSim Pro.

Colour	ETCO₂ range	Meaning
Green	4.0–6.0 kPa	Normal
Amber	2.5–4.0 or 6.0–8.0 kPa	Warning (hypo/hyperventilation)
Red (flashing)	<2.5 or >8.0 kPa	Critical

Arterial Blood Gas Sampling

★ Pro feature: ABG sampling requires VentSim Pro.

Tap the ABG button in the header to take a snapshot arterial blood gas. Values outside the reference range are colour-coded amber or red.

Parameter	Reference range	Clinical note
pH	7.35–7.45
PaCO₂	4.5–6.0 kPa	Primary driver of ventilatory demand
PaO₂	10–13.3 kPa	On air; higher expected on supplemental O₂
HCO₃⁻	22–26 mEq/L	Derived from base excess
Base excess	−2 to +2 mEq/L	Set directly in patient panel (Pro)
SaO₂	>94%	From Severinghaus ODC
ETCO₂	4.0–6.0 kPa	PaCO₂ × (1 − VD/VT)
P/F ratio	>40 kPa normal	Berlin ARDS criteria applied

The P/F ratio label dynamically reflects ARDS severity: mild (<40 kPa), moderate (<26.7 kPa), severe (<13.3 kPa).

Note: ETCO₂ is always lower than PaCO₂ by the dead space fraction. As dead space increases — in ARDS, pulmonary embolism, or low cardiac output — the PaCO₂-to-ETCO₂ gradient widens.

The Flow-Volume Loop

The Flow-Volume (FV) loop plots flow (y-axis, L/s) against change in volume from FRC (x-axis, ml). Inspiration is shown above the zero-flow line; expiration below.

In normal ventilation the loop is smooth and elliptical. Increased airway resistance produces a characteristic scoop in the expiratory limb. The FV loop provides airway mechanics information that complements the PV loop.

Scenarios

★ Pro feature: Scenarios require VentSim Pro.

Scenarios allow you to save and reload complete ventilator and patient configurations. Access via the Scenarios button at the bottom of the Controls panel.

To save: enter a name and tap Save. To load: tap the scenario name. To delete: tap the × next to any scenario. Scenarios are stored locally in your browser.

Suggested scenarios to explore:

Scenario	Key settings
Normal adult	Default settings — baseline for comparison
ARDS	Reduced compliance, high PEEP (12–14), high FiO₂ (0.6–0.8), high shunt, high VD/VT, recruitable lung
COPD exacerbation	High resistance (25–35 cmH₂O/L/s), low rate, long Te (I:E 1:3 or 1:4)
Obesity / reduced FRC	Low FRC, reduced compliance, moderate shunt
Pulmonary embolism	High VD/VT (0.5–0.7), normal mechanics, low cardiac output

Reset and Pause

Pause — stops the simulation. Waveforms freeze. Useful when adjusting settings to observe the effect of a single change.

Reset — returns all settings to defaults (normal adult, PC-AC). Clears waveform buffers and PV loop history.

Getting VentSim Pro

VentSim Pro unlocks gas exchange modelling, ABG sampling, SpO₂ and ETCO₂ monitoring with alarm states, alveolar ventilation display, and clinical scenarios.

To upgrade:

• Tap the ‘Get Pro’ button
• Follow the Gumroad purchase workflow
• Your key will be displayed and also emailed to you
• Return to the activation modal, enter your key, and tap ‘Activate’
• Your licence is stored in the browser automatically — you will not need to re-enter it
• But keep you key safe – you may need to re-enter it if VentSim is upgraded

The PRO button changes to ✓ PRO once activated.

Physiological Model — Technical Notes

Compliance Model

Linear: V = C × P, where C is compliance (ml/cmH₂O) and P is airway pressure relative to FRC.

Sigmoidal: V(L) = Vmax / (1 + exp(−(P − Pinf) / Prange)), where P is absolute pressure, Pinf is the Compliance Point slider value, and Prange is the Delta Range. The expiratory limb applies a hysteresis factor h = hyst/100 to both parameters.

Gas Exchange Model

Alveolar gas composition is computed breath-by-breath using a stirred-tank model. Oxygen consumption (VO₂) and CO₂ production (VCO₂) are derived from weight, metabolic rate per kg, and cardiac output. Arterial blood gases are derived using the three-compartment shunt model:

CaO₂ = (1 − Qs) × Cc’O₂ + Qs × CvO₂

Where Qs is the effective shunt fraction, Cc’O₂ is end-capillary oxygen content (from alveolar PO₂), and CvO₂ is mixed venous oxygen content. SaO₂ is derived from PaO₂ using the Severinghaus oxyhaemoglobin dissociation curve. pH is calculated by the Henderson-Hasselbalch equation from PaCO₂ and HCO₃⁻.

ETCO₂ = PaCO₂ × (1 − VD/VT)

P/F ratio = PaO₂ (kPa) ÷ FiO₂

ARDS thresholds (Berlin definition, converted to kPa from mmHg by dividing by 7.5): mild <40 kPa (300 mmHg), moderate <26.7 kPa (200 mmHg), severe <13.3 kPa (100 mmHg).

VentSim is designed for educational purposes. It is not a medical device and should not be used for clinical decision-making.

© Roger Stedman — rogerstedman.com