Ventilator Weaning Knowledge Prep Guide

🫁 What mechanical ventilation actually does

A ventilator pushes air into the lungs using positive pressure. This is the opposite of how we normally breathe — we pull air in by contracting the diaphragm, creating negative pressure. This difference matters enormously for weaning.

Positive pressure ventilation: easier on the heart (reduces preload and afterload), but causes diaphragm atrophy over time, risk of lung injury, and infection. The longer a patient stays on it, the harder it gets to come off.

Weaning = returning to negative pressure breathing. Every step in the weaning process is about transferring work from the machine back to the patient, while checking they can handle it.

🔄 The weaning trajectory: controlled → spontaneous → extubation

Think of this as a journey with three stages. ARISE currently handles Stage 3. Phase 2 adds Stages 1 and 2.

The expert's job (and what your questions extract) is the decision logic at each arrow: what parameters trigger the step-down, and what makes them hold back?

⚖️ The supply-demand framework — the single most important concept

Weaning failure is fundamentally an imbalance between what the respiratory system needs to do (demand/load) and what it can do (supply/capacity). Every parameter in this guide sits on one side of this balance.

📊 The three weaning categories

International consensus classifies weaning difficulty into three groups. This tells you which part of ARISE Phase 2 matters for which patient:

🏥 The ventilator mode spectrum

Every ventilated patient is on a mode somewhere on a spectrum from full machine control to full patient control:

• VC-AC (Volume Control – Assist Control): Machine sets tidal volume and rate. Delivers every breath. Patient can trigger extra breaths but machine delivers each in full.
• PC-AC (Pressure Control – Assist Control): Machine sets pressure and rate. Tidal volume varies with the patient's lung compliance.
• SIMV (Synchronized Intermittent Mandatory Ventilation): Shared — machine gives mandatory breaths, patient breathes spontaneously between them. Historically used for weaning but falling out of favour.
• PSV (Pressure Support Ventilation): Patient controls everything. Machine just adds a pressure boost (e.g., PS 10 cmH₂O) to each patient-initiated breath.
• PAV+ (Proportional Assist Ventilation): Machine amplifies the patient's own effort by a set percentage (e.g., 30% assist).
• T-piece: No ventilator support at all — just humidified oxygen through the tube. Completely unassisted breathing.

🧠 P0.1 — airway occlusion pressure (Q12)

P0.1 is the pressure measured at the airway 100 milliseconds after the start of an inspiratory effort against a briefly occluded airway. It measures respiratory drive — how hard the brain is telling the muscles to breathe.

How it's measured: Most modern ventilators (Hamilton, Draeger, Maquet) can do this automatically. The ventilator briefly occludes the airway for 100ms at the start of a breath and measures the pressure drop. The patient doesn't notice. No special equipment — just a button press.

💪 NIF / MIP — muscle strength (Q13)

NIF (Negative Inspiratory Force) or MIP (Maximum Inspiratory Pressure) measures the strongest breath a patient can take. It directly measures inspiratory muscle strength, primarily the diaphragm.

Important nuance: NIF is effort-dependent — the patient has to try their hardest. A sedated or confused patient may give a falsely weak reading.

📡 Triggered breaths (Q15)

On controlled modes (VC-AC, PC-AC), the machine delivers breaths at a set rate. But patients can also trigger additional breaths by making an inspiratory effort the ventilator detects.

If a patient on VC-AC set at 14 breaths/min is actually breathing at 20/min, that means 6 breaths are patient-triggered. A high percentage of triggered breaths (>50–60%) suggests the patient's respiratory drive is active and they may be ready to do more work.

Most ventilators display this but clinicians may not systematically check it. Q15 asks whether the expert uses it.

📐 Compliance, resistance, and driving pressure (Q17)

These are the mechanical properties of the lungs and airways — all calculated from values already on the ventilator screen.

• Static compliance (Cstat) = Vt ÷ (Pplat – PEEP). How stretchy the lungs are. Low compliance = stiff lungs (ARDS, oedema, fibrosis).
• Airway resistance (Raw) = (PIP – Pplat) ÷ flow. How narrow the airways are. High resistance = obstruction (bronchospasm, secretions, kinked ETT).
• Driving pressure (ΔP) = Pplat – PEEP. How much pressure to inflate the lungs. A key safety metric — >15 cmH₂O is associated with lung injury.

Key insight: ARISE Tab 4 already collects PIP, Pplat, and PEEP. That means ARISE can auto-calculate all three — but currently doesn't. Q17 asks if experts value this.

🌬️ Auto-PEEP / intrinsic PEEP (Q16)

Auto-PEEP is trapped air pressure in the lungs at end-expiration that the ventilator doesn't know about. It happens when the patient doesn't have enough time to fully exhale before the next breath starts. Air stacks up.

Why it matters: Auto-PEEP acts as a hidden threshold load. Before the patient can start inhaling, they must first overcome this trapped pressure. It's like trying to open a door with pressure pushing against it from the other side.

Common in: COPD, asthma — narrowed airways slow expiratory flow.

How to measure: Press "expiratory hold" on the ventilator — closes both valves at end-expiration. The pressure that equilibrates = total PEEP. Subtract set PEEP = auto-PEEP. If set PEEP is 5 and total is 9, auto-PEEP is 4 cmH₂O.

📷 Diaphragm ultrasound (Q18)

Bedside ultrasound of the diaphragm measures two things:

• Diaphragm Thickening Fraction (DTF): How much the diaphragm thickens during inspiration. Measured at the zone of apposition (between ribs, right side). DTF = (thickness at inspiration – thickness at expiration) ÷ expiration thickness × 100%. DTF > 30–36% predicts successful weaning.
• Excursion: How far the diaphragm moves downward. Measured with M-mode at the subcostal window. Excursion > 10–14 mm during tidal breathing is adequate.

Q18 is partly a feasibility question. Does NUH MICU have the equipment, do consultants do it, do they trust it? If "we don't do it routinely," ARISE might include it as educational prompt rather than a required input.

❤️ Weaning-induced cardiac failure (Q19, Q28)

One of the most commonly missed causes of weaning failure. Here's why:

While on positive pressure ventilation, the machine pushes air in → positive intrathoracic pressure → reduces venous return (preload) and reduces LV afterload. The heart has it easier.

During an SBT, the patient switches to negative pressure breathing → suddenly increases venous return (more blood rushes back) AND increases LV afterload. In patients with heart failure or fluid overload, this triggers acute pulmonary oedema or ischaemia.

The patient looks like they're failing for respiratory reasons (tachypnoea, desaturation) but the real problem is the heart.

🚨 Reason-specific failure pathways

When a patient fails an SBT, the reason drives completely different management. This is the core of Phase 2 — you're extracting decision trees, not lists of interventions.

• Tachypnoea (RR >35) — Q21: Is the respiratory system overloaded or are muscles fatigued? Progressive PS weaning vs rest-then-retry vs IMT.
• Desaturation (SpO₂ <90%) — Q22: Oxygenation or ventilation problem? The decision tree: secretions → suction; fluid → diuresis; atelectasis → recruitment/PEEP; V/Q mismatch → positioning.
• Agitation/drowsiness — Q23: Sedation-related or delirium-related? Different paths: sedation → hold/reduce drugs; delirium → CAM-ICU, dexmedetomidine.
• Haemodynamic instability — Q28 (NEW): Preload, afterload, or pump failure? PLR, fluid challenge, echo, BNP.

🔧 Prolonged weaning: PS step-down protocol (Q30)

For patients who keep failing daily SBTs, the strategy shifts from "pass/fail gate" to "gradual conditioning." This is rehabilitation — building tolerance over time.

1. Start at a comfortable PS level (e.g., PS 15 cmH₂O)
2. Reduce PS by 2–4 cmH₂O every 4–24 hours, as tolerated
3. At each level, check: RR <30, no accessory muscles, SpO₂ >92%, no distress
4. If intolerant → go back up one step and rest 4–12 hours
5. When tolerating PS 5–8 → ready for SBT

Some units use "exercise and rest" cycling: 2 hours on low PS (exercise), then 4–6 hours on higher PS (rest). This mimics muscle training — work, then recover.

🔀 Patient-ventilator dyssynchrony (Q31)

Dyssynchrony = patient and ventilator are not in sync. A significant cause of distress and prolonged weaning. Each type has a specific fix:

• Trigger dyssynchrony (ineffective efforts): Patient tries to breathe, ventilator doesn't detect it. Fix → increase trigger sensitivity, reduce auto-PEEP.
• Flow dyssynchrony (flow starvation): Patient wants more airflow than ventilator delivers. The pressure curve looks "scooped out." Fix → increase peak flow or rise time.
• Cycle dyssynchrony: Ventilator ends breath too early or too late. Fix → adjust expiratory cycling threshold.
• Double-triggering: Patient's effort triggers two consecutive breaths, one stacked on another. Fix → increase inspiratory time or tidal volume.

🕐 Tracheostomy timing (Q24)

When daily SBTs keep failing and prolonged weaning protocols aren't progressing, the question of tracheostomy arises. Evidence suggests timing is highly context-dependent:

• No fixed number of failed SBTs — varies by patient profile
• Earlier consideration in neuromuscular patients (may need long-term ventilation)
• Later in post-surgical patients (more likely to recover)
• Usually discussed around 10–14 days of mechanical ventilation

💊 Sedation during active weaning (Q32)

Sedation strategy should change as patients move through the weaning trajectory — but many teams set a RASS target and leave it. Phase 2 needs to map sedation to ventilator mode stage:

• Controlled mode (early): RASS 0 to −2. Propofol/midazolam ± fentanyl. Keep comfortable, prevent dyssynchrony.
• Transitioning to spontaneous: RASS 0 to −1. Reduce sedation, consider dexmedetomidine. Patient needs to participate more.
• Pre-SBT: RASS 0. Low-dose fentanyl + dex per MICU protocol. Comfortable but awake.
• During SBT: RASS 0 to +1. Minimal or none. Need full respiratory effort for valid test.

🧪 Dexmedetomidine — why it's special for weaning

Dexmedetomidine (dex) is an alpha-2 agonist that provides light sedation without suppressing respiratory drive. This makes it uniquely suited for weaning:

• Provides anxiolysis and comfort without depressing breathing
• Treats hyperactive delirium (a cause of SBT failure)
• Your MICU protocol already mentions it pre-SBT
• Key question for Phase 2: does the expert use it more broadly during the whole weaning trajectory?

🥗 Nutrition and mobilisation (Q34)

These run parallel to ventilator management as part of the daily optimisation checklist:

• Protein: Respiratory muscle recovery requires adequate protein intake. Some experts target 1.2–1.5 g/kg/day.
• Caloric adequacy: Underfeeding worsens muscle wasting; overfeeding increases CO₂ production (more work of breathing).
• Early mobilisation: Sitting out of bed, even while ventilated, may improve SBT pass rates and reduce ICU delirium. Growing evidence base.

🫧 Dead space and ventilation efficiency

📈 Ventilator waveforms — what you need to know (Q26)

You don't need to be a waveform expert. You just need to know what the three waveforms are and what they reveal:

• Pressure-time curve: Shows airway pressure over time. Useful for: high peak pressures, plateau pressure, and dyssynchrony patterns (scooped shape = flow starvation).
• Flow-time curve: Shows airflow in/out over time. Useful for: air trapping (expiratory flow doesn't return to zero before next breath), trigger delays, and auto-PEEP detection.
• Volume-time curve: Shows tidal volume delivery over time. Useful for: air leaks (volume doesn't return to baseline), auto-cycling.

Q26 is a prioritisation question — you're asking which patterns should junior doctors learn first. The expert will likely say 2–3 high-yield ones (air trapping pattern is almost always included).

🛡️ The safety question (Q27)

This is your most important question. Save it for last, after the expert has seen everything in ARISE.

You're asking: Is there anything in ARISE that could lead a clinician to make a wrong decision? This could be an incorrect threshold, a missing safety check, or a recommendation that doesn't match evidence.

🔧 What Tab 5 does and when to use it

Tab 5 is for patients who are already awake and tolerating their tube but you're not sure they're ready to step down to less support or attempt an SBT. It's also for patients who just failed an SBT — it generates a specific 24-hour plan based on why they failed.

Think of it as answering two questions:

1. "Is this patient ready to do more work?" — The assessment section identifies barriers and gives a verdict.
2. "My patient just failed their SBT — what do I do for the next 24 hours?" — The SBT failure section gives a reason-specific plan.

This tab does NOT replace Tab 2 (SBT readiness) or Tab 4 (extubation readiness). It adds a layer for patients in the weaning trajectory who need more than a pass/fail decision.

1️⃣ Step 1: Select the ventilator mode — the most important first step

The mode dropdown is the master switch that controls everything else. Different modes mean different parameters matter. You must select the correct mode or the assessment won't work properly.

⚙️ Controlled mode inputs — what they are and why they matter

🫁 Spontaneous mode inputs — what they are and why they matter

2️⃣ Step 2: Fill the common readiness indicators

These are the same for all three modes. They establish baseline readiness before mode-specific assessment:

3️⃣ Step 3: Advanced panel (optional but powerful)

These require extra bedside tests. Click the chevron (▼) to expand. Use them when you have the data — each adds precision to the assessment.

• DTF % (Diaphragm Thickening Fraction): Measured by bedside ultrasound. Shows whether the diaphragm is actually contracting well. DTF >30–36% = adequate. Below 20% = significant diaphragm weakness — consider inspiratory muscle training.
• Excursion (cm): How far the diaphragm moves downward during each breath. Target >1.0 cm for tidal breathing.
• E/E' ratio: From bedside echo. Measures how stiff the left ventricle is during filling. Above 7.8 = diastolic dysfunction — high risk of weaning-induced pulmonary oedema (WiPO). The heart can't handle the extra load when you switch from positive to negative pressure breathing.
• AI % (Asynchrony Index): How often the patient and ventilator are out of sync. Count asynchronous breaths ÷ total breaths × 100. AI ≥10% → 33% extubation failure rate vs 6% when synchronised (Sousa 2020).
• Phosphate and Magnesium: Low levels impair muscle contraction — including the diaphragm. Phosphate <2.5 mg/dL or Mg <1.7 mg/dL = correct before weaning trial. Often missed on routine bloods.
• CAM-ICU positive: Tick this if the patient has active delirium. Delirium is a recognised cause of SBT failure and is the target for dexmedetomidine.

4️⃣ Step 4: Reading the results — what do barriers, warnings, optimise, and strengths mean?

After clicking Assess Readiness, results appear in four colour-coded categories. Understanding these is key to using the tool correctly.

5️⃣ Step 5: The verdict and what to do next

Based on the mode and barriers, the system gives a step-down recommendation:

• Controlled — READY: "Patient meets criteria for a supervised spontaneous mode trial (PSV/PAV)." → Step down to PSV 8–12 cmH₂O. Reassess in 2–4 hours. If tolerating, proceed to SBT.
• Hybrid — READY: "Consider stepping down to PSV/PAV when mandatory rate ≤4 and triggered breaths ≥80%." → Reduce SIMV rate in steps (12 → 8 → 4) over hours/days. Switch to full PSV when mandatory rate reaches 0–4.
• Spontaneous — READY: "Spontaneous parameters favourable — proceed to SBT if not yet done, then extubation readiness assessment (Tab 4)." → Confirm with Tab 2 (SBT readiness) and Tab 4 (extubation readiness).
• Any mode — NOT READY: Address the listed barriers. Re-assess when resolved.

6️⃣ The SBT Failure Section — what it is and when to use it

After any assessment, the SBT Failure — 24h Optimization Plan section appears at the bottom of Tab 5. Use it when your patient just failed an SBT.

SBT failure is not a single problem — it has different causes that need completely different responses. Treating tachypnoea the same way as desaturation or haemodynamic instability leads to incorrect management.

How to use it: Select all the signs you observed during the failed SBT. You can choose multiple reasons. The system generates a reason-specific 24-hour plan for each one.