Ventilator Modes & Waveforms
Pressure / flow / volume patterns for each mode, clinical use, and PB980 labels.
Reading Ventilator Waveforms
Pressure–Time (Paw)
Shows airway pressure over time. Ppeak reflects total resistive + elastic load. Pplat (end-inspiratory pause) reflects only elastic load. Pplat ≤ 30 cmH₂O targets alveolar protection in ARDS.
Flow–Time
Above zero = inspiratory flow; below zero = expiratory. Square wave = volume control. Decelerating = pressure control or PSV. Auto-PEEP appears when expiratory flow has NOT returned to zero before the next breath.
Volume–Time
Rises during inspiration, returns to zero (baseline) during expiration. Mismatch between set and exhaled VT = circuit/cuff leak. In PC mode, changing volume with stable pressure signals a compliance or resistance change.
Volume Control (VC / VC+)
- Trigger
- Time (mandatory) or patient effort (assisted)
- Limit
- Volume — constant flow until set VT delivered
- Cycle
- Volume — breath ends when set VT is delivered
Default mode for most ICU patients. ARDS lung-protection: 6 mL/kg IBW, Pplat ≤ 30 cmH₂O.
PB980 label: Volume Control (VC) / VC+
Ppeak ≠ Pplat. The difference reflects airway resistance. Always monitor Pplat for alveolar overdistension.
Pressure Control (PC)
- Trigger
- Time (mandatory) or patient effort
- Limit
- Pressure — constant pressure throughout inspiration
- Cycle
- Time — Ti set by clinician
When precise airway pressure control is needed. High resistance states, neonatal/pediatric care.
PB980 label: Pressure Control (PC)
VT varies with changes in compliance and resistance — monitor exhaled VT closely with any lung mechanics change.
Pressure Support (PSV)
- Trigger
- Patient effort (flow or pressure trigger)
- Limit
- Pressure — support level above PEEP
- Cycle
- Flow — breath ends when flow decays to 25% of peak (default)
Weaning trials, NIV (BiPAP), augmenting spontaneous breathing. Patient must have intact drive.
PB980 label: Pressure Support (PS) — within CPAP/PS mode
PS too high → patient becomes passive (no drive). Too low → increased WOB. Target RR 12–25, VT 6–8 mL/kg.
PRVC / VC+ (Pressure-Regulated Volume Control)
- Trigger
- Time or patient effort
- Limit
- Pressure — auto-adjusted breath-to-breath to reach target VT
- Cycle
- Time or flow depending on sub-mode
Combines VC volume guarantee with PC decelerating flow. Used in ARDS and post-op patients.
PB980 label: VC+ / Volume Support (VS) — see note
On PB980: VC+ = controlled mode, VS = spontaneous mode. Other vents call this PRVC, AutoFlow, APV, or Adaptive Pressure Control.
APRV / BiLevel (Airway Pressure Release Ventilation)
- Trigger
- Time (release) — patient breathes spontaneously at PHigh
- Limit
- Pressure — PHigh and PLow set independently
- Cycle
- Time — TLow ends the release
Refractory ARDS, improves oxygenation by maintaining high mean airway pressure. Allows spontaneous breathing.
PB980 label: BiLevel / APRV
TLow set to 0.2–0.8 s to prevent de-recruitment. Titrate so expiratory flow returns to 50–75% of peak at end-TLow.
SIMV (Synchronized Intermittent Mandatory Ventilation)
- Trigger
- Time (mandatory) or patient effort (spontaneous)
- Limit
- Volume or pressure (mandatory) + pressure support (spontaneous)
- Cycle
- Volume or time (mandatory) / flow (spontaneous)
Largely replaced by PSV/SBT protocols. Evidence does NOT support SIMV over PSV for weaning (Brochard 1994).
PB980 label: SIMV+PS (mandatory breaths + PS for spontaneous breaths)
SIMV does not accelerate weaning vs. PSV trials and SBTs. Avoid for primary weaning strategy when possible.
CPAP (Continuous Positive Airway Pressure)
- Trigger
- Patient effort only
- Limit
- PEEP baseline only — no breath-by-breath pressure support
- Cycle
- Patient — entirely patient-driven breathing pattern
SBT before extubation, non-invasive OSA therapy, post-extubation support.
PB980 label: CPAP/PS (at 0 PS = pure CPAP)
A 30-minute CPAP or T-piece SBT predicts successful extubation better than gradual SIMV weaning (Yang & Tobin 1991).
Puritan Bennett 980 — Mode Quick Reference
| PB980 Mode | Generic Name | Volume Guaranteed | Notes |
|---|---|---|---|
| VC | Volume Control | Yes (set VT) | Square flow, rising pressure waveform |
| 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 | Yes (target VT) | Spontaneous mode; PS auto-adjusts to maintain VT |
| 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 Vent. | No | Amplifies patient effort in proportion; needs intact drive |
| SIMV + VC/PC/VC+ | SIMV | Mandatory only | Spontaneous breaths need separate PS setting |
Source: Medtronic/Covidien PB980 Operator Manual. Verify against your unit's installed software version.
Vapotherm HVNI — High-Flow Nasal Cannula
Vapotherm is not a mechanical ventilator — it 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.
- Flow
- 1–40 L/min adult · 1–8 L/min peds/neo
- FiO₂
- 0.21–1.0 (precisely blended)
- Temperature
- 37 °C body-temperature humidification
- Cannula size
- Must cover ≤ 50% of nare diameter
- Hypoxemic respiratory failure (P/F 100–300)
- Post-extubation support
- Awake, cooperative AHRF patients (FLORALI trial)
- High-risk extubation
- Neonatal/pediatric respiratory distress
SpO₂ not improving or RR >30 after 1–2 h at ≥40 L/min and FiO₂ ≥0.60 → escalate to NIV or intubation. ROX index (SpO₂/FiO₂ ÷ RR) <4.88 at 12 hours predicts HFNC failure.
Other HFNC systems: Fisher & Paykel Optiflow (Airvo 2), Teleflex HiFlO, Breas Nippy 4+, TNI SoftFlow. Same physiology; flow rates and humidification differ.
Auto-PEEP (Intrinsic PEEP / Air Trapping)
Auto-PEEP occurs when the next breath triggers before exhalation is complete — air accumulates progressively. Common in COPD, asthma, and high RR/VT settings.
1Waveform Signs
Expiratory flow has NOT returned to zero baseline before the next breath triggers on the flow-time scalar.
2Bedside Detection
Occlude the expiratory port at end-expiration (expiratory hold) — the ventilator displays the trapped pressure.
3Management
↓ RR, ↓ VT, ↑ expiratory time (I:E ≥ 1:3), treat bronchospasm. Consider applied PEEP ≤ 80% of measured auto-PEEP.
For board exam preparation. Always consult manufacturer operator manuals and current AARC/ATS/ERS guidelines for patient care.