Ventilator Modes & Waveforms

Pressure / flow / volume waveform patterns for each mode, clinical use, and device-specific labels.

How to read ventilator waveforms

Pressure–Time (Paw)

Shows airway pressure over time. Ppeak reflects total resistive + elastic load; Pplat (end-inspiratory pause) reflects only elastic load (alveolar pressure). Pplat ≤ 30 cmH₂O targets alveolar protection in ARDS.

Flow–Time

Above zero = inspiratory flow; below zero = expiratory. A constant inspiratory flow (square wave) = volume control. A decelerating flow = pressure control or PSV. Auto-PEEP appears when expiratory flow has not returned to zero before the next breath triggers.

Volume–Time

Rises during inspiration, falls to zero (baseline) during expiration. A mismatch between set VT and exhaled VT indicates a circuit/cuff leak. In PC mode, a changing volume with stable pressure signals a compliance or resistance change.

Controlled

Volume Control (VC / VC+)

Pressure(cmH₂O)

Flow(L/min)

Volume(mL)

TriggerTime (mandatory) or patient effort (assisted)LimitVolume (constant flow delivered until set VT reached)CycleVolume (breath ends when set VT is delivered)

When to use

Default mode for most ICU patients; ARDS lung-protection (6 mL/kg IBW, Pplat ≤ 30).

PB980 label

Volume Control (VC) / VC+

Clinical tip

Ppeak ≠ Pplat — the difference reflects airway resistance. Monitor Pplat for alveolar overdistension.

Controlled

Pressure Control (PC)

Pressure(cmH₂O)

Flow(L/min)

Volume(mL)

TriggerTime (mandatory) or patient effortLimitPressure (constant pressure throughout inspiration)CycleTime (Ti set by clinician)

When to use

When precise airway pressure control is needed; high resistance states; neonatal/pediatric care.

PB980 label

Pressure Control (PC)

Clinical tip

Tidal volume varies with changes in compliance and resistance — monitor VT closely.

Spontaneous

Pressure Support (PSV)

Pressure(cmH₂O)

Flow(L/min)

Volume(mL)

TriggerPatient effort (flow or pressure trigger)LimitPressure (support level above PEEP)CycleFlow (breath ends when flow decays to 25% of peak by default)

When to use

Weaning trials, non-invasive ventilation (BiPAP), and augmenting spontaneous breathing.

PB980 label

Pressure Support (PS) — within CPAP/PS mode

Clinical tip

If PS level is too high → patient becomes passive (no drive). If too low → increased WOB. Target: RR 12–25, VT 6–8 mL/kg.

Controlled

PRVC / VC+ (Pressure-Regulated Volume Control)

Pressure(cmH₂O)

Flow(L/min)

Volume(mL)

TriggerTime or patient effortLimitPressure (auto-adjusted breath-to-breath to reach target VT)CycleTime or flow depending on sub-mode

When to use

Combine benefits of VC (volume guarantee) and PC (decelerating flow). Used in ARDS and post-op patients.

PB980 label

VC+ / Volume Support (VS) — see note

Clinical tip

On PB980: VC+ uses adaptive targeting in controlled mode; VS does the same in spontaneous mode. Other vents may call this PRVC, AutoFlow, APV, or Adaptive Pressure Control.

Hybrid

APRV / BiLevel (Airway Pressure Release Ventilation)

Pressure(cmH₂O)

Flow(L/min)

Volume(mL)

TriggerTime (release) — patient breathes spontaneously at PHighLimitPressure (PHigh and PLow set separately)CycleTime (TLow ends the release)

When to use

Refractory ARDS, improves oxygenation by maintaining mean airway pressure. Allows spontaneous breathing.

PB980 label

BiLevel / APRV

Clinical tip

TLow set short enough (0.2–0.8 s) to prevent alveolar de-recruitment on release. Commonly titrated so expiratory flow returns to 50–75% of peak flow at end of TLow.

Hybrid

SIMV (Synchronized Intermittent Mandatory Ventilation)

Pressure(cmH₂O)

Flow(L/min)

Volume(mL)

TriggerTime (mandatory) or patient effort (spontaneous)LimitVolume or pressure (for mandatory breaths) + pressure support (for spontaneous breaths)CycleVolume or time (mandatory) / flow (spontaneous)

When to use

Historically used for weaning but largely replaced by PSV trials and SBT protocols. Still used in some centers.

PB980 label

SIMV+PS (SIMV with Pressure Support for spontaneous breaths)

Clinical tip

Evidence does NOT support SIMV over PSV for weaning — PSV trials and daily SBTs have better outcomes (Brochard 1994, Esteban 1995).

Spontaneous

CPAP (Continuous Positive Airway Pressure)

Pressure(cmH₂O)

Flow(L/min)

Volume(mL)

TriggerPatient effort onlyLimitPEEP baseline only (no breath-by-breath support)CyclePatient (entirely patient-driven breathing pattern)

When to use

Spontaneous breathing trials (SBT) before extubation; non-invasive OSA/sleep therapy; post-extubation support.

PB980 label

CPAP/PS (at 0 PS = pure CPAP)

Clinical tip

A 30-minute CPAP or T-piece SBT predicts successful extubation better than gradual SIMV weaning (Yang & Tobin 1991).

Puritan Bennett 980 (PB980) — Mode Quick Reference

PB980 Mode	Generic Name	Volume Guaranteed	Notes
VC	Volume Control	Yes (set VT)	Square flow, rising pressure
PC	Pressure Control	No (varies)	Square pressure, decelerating flow
VC+	Adaptive Pressure / PRVC	Yes (target VT)	Auto-adjusts driving pressure breath-to-breath
VS	Volume Support (PSV + VT target)	Yes (target VT)	Spontaneous mode; PS auto-adjusts
CPAP/PS	CPAP + Pressure Support	No	Patient controls rate; PS augments each breath
APRV/BiLevel	APRV	No	PHigh / PLow / THigh / TLow settings
PAV+	Proportional Assist Ventilation	No	Amplifies patient's effort in proportion; requires intact drive
SIMV + VC/PC/VC+	SIMV	Mandatory breaths only	Spontaneous breaths require separate PS setting

Source: Medtronic/Covidien PB980 Operator Manual (reference your unit's specific software version for final settings).

High-Flow

Vapotherm Precision Flow — High-Velocity Nasal Insufflation (HVNI)

What it is

Vapotherm is not a mechanical ventilator — it is a High-Flow Nasal Cannula (HFNC) system that delivers heated, humidified, precisely blended air/O₂ through small-bore nasal prongs at high velocities. The high flow washes out nasopharyngeal dead space and creates a small CPAP-like effect (+1–3 cmH₂O), reducing work of breathing without intubation.

Key parameters

Flow: 1–40 L/min (adult); 1–8 L/min (pediatric/neonatal)
FiO₂: 0.21–1.0 (precisely blended)
Temperature: 37 °C (body-temperature humidification)
Cannula size: Must cover ≤50% of nare diameter to avoid pressure buildup

Clinical use

Hypoxemic respiratory failure (P/F 100–300)
Post-extubation support
Acute hypoxemic failure in awake, cooperative patients (FLORALI trial)
High-risk extubation (ICU patients with weaning risk)
Neonatal/pediatric respiratory distress as alternative to intubation

Watch for

If SpO₂ is not improving or RR >30 after 1–2 hours of HFNC at ≥40 L/min and FiO₂ ≥0.60, escalate to NIV or intubation. HFNC can mask deterioration — the ROX index (SpO₂/FiO₂ ÷ RR) <4.88 at 12 hours predicts HFNC failure with high sensitivity.

Other HFNC systems

Fisher & Paykel Optiflow (Airvo 2), Teleflex HiFlO, Breas Nippy 4+, TNI SoftFlow. All use the same physiologic principles as Vapotherm; flow rates and humidification systems differ.

Auto-PEEP (Intrinsic PEEP / Air Trapping)

Auto-PEEP occurs when the next breath is triggered before exhalation is complete — air accumulates progressively. Common in COPD, asthma, and high RR settings.

Waveform signs

Expiratory flow has NOT returned to zero before the next breath triggers on the flow-time scalar.

Bedside detection

Occlude the expiratory port at end-expiration (expiratory hold) — the ventilator displays the trapped pressure.

Management

↓ RR, ↓ VT, ↑ expiratory time (I:E ≥ 1:3), treat bronchospasm, consider applied PEEP ≤80% of auto-PEEP.

This reference is for board exam preparation. Always consult the manufacturer operator manual and current clinical guidelines (AARC, ATS/ERS) for patient care decisions.