Respiratory Mechanics · Pressure & Flow Simulator

Pressure and flow waveforms across three modes

Curves integrated from the equation of motion: (Paw − PEEP) + Pmus = R · Flow + V/C + PEEPi. Toggle COPD mode on the CPAP panel to see how auto-PEEP creates an inspiratory threshold load and how applied PEEP counteracts it.

Spontaneous breathing

P_aw = 0 · sinusoidal flow pattern · alveolar pressure drops below atmospheric on inspiration

Pressure (cmH₂O)
Flow (L/min)
Controls
14 /min
8 cmH₂O
Tidal volume
— mL
Pressure
Airway (P_aw)
Alveolar (P_alv)
Pleural (P_pl)
Flow
Insp. positive · exp. negative
Flow follows the pressure gradient. Muscle effort drops alveolar pressure below atmospheric, gas flows in. Passive expiration follows.

CPAP

P_aw clamped at PEEP · same flow physics as spontaneous, just shifted up · WOB unchanged in normal lungs

Pressure (cmH₂O)
Flow (L/min)
Controls
COPD parameters
8 cmH₂O
8 cmH₂O
16 /min
8 cmH₂O
Tidal volume
— mL
Pressure
Airway (P_aw)
Alveolar (P_alv)
Flow
Insp. positive · exp. negative
Same flow physics as spontaneous breathing because the patient is doing all the work. Higher applied PEEP doesn't increase flow because both airway and alveolus rise together. Toggle COPD mode to see why this changes when auto-PEEP is present.

Pressure support + PEEP

Decelerating inspiratory flow · machine delivers the breath · expiratory peak flow reflects elastic recoil ÷ R

Pressure (cmH₂O)
Flow (L/min)
Controls
5 cmH₂O
12 cmH₂O
14 /min
0.2 s
3 cmH₂O
Tidal volume
— mL
Pressure
Airway (P_aw)
Alveolar (P_alv)
Flow
Decelerating insp. · passive exp.
Classic decelerating inspiratory flow: high peak at the start, falling exponentially as alveolar pressure approaches PEEP+PS. Expiratory flow is passive, peaks at start of expiration, decays with τ = R·C.
The math behind the curves
(P_aw − PEEP) + P_mus = R · Flow + V/C + PEEPi
Flow = [(P_aw − PEEP) + P_mus − V/C − PEEPi] / R
P_alv = P_aw − R · Flow
Threshold load = max(0, PEEPi − applied PEEP)
τ = R × C = 5 × 0.06 = 0.3 s
Spontaneous
Flow: sinusoidal, follows P_mus
Peak insp flow: ~30-50 L/min
Symmetric insp/exp shape
Returns to zero before next breath
CPAP (normal)
Flow: same as spontaneous
Higher PEEP → no change in V_T
WOB unchanged from baseline
Used for: alveolar recruitment, CHF
CPAP (COPD)
Auto-PEEP creates threshold load
Patient wastes effort dropping
alveolar to atmospheric first
Applied PEEP ≈ 75% of PEEPi fixes it
Auto-PEEP (PEEPi) is end-expiratory alveolar pressure above atmospheric, caused by incomplete exhalation in obstructive disease. It acts as an inspiratory threshold load: the patient must drop alveolar pressure by PEEPi before any flow can begin. Setting applied (extrinsic) PEEP to ~75-85% of measured auto-PEEP equilibrates the airway with the trapped alveolar gas, reducing the work of triggering. Going higher than auto-PEEP causes additional hyperinflation. Single-compartment linear model; severe COPD with dynamic flow limitation is approximated, not fully simulated.
Created by Gilad Tafla