Autopeep for md Anaesthesia exam
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Auto-PEEP (Intrinsic PEEP) — MD Anaesthesia Exam Notes
Definition
Auto-PEEP (also called intrinsic PEEP, iPEEP) is positive alveolar pressure present at end-exhalation above any externally applied PEEP, caused by incomplete lung emptying before the next breath begins.
"Intrinsic PEEP is any additional end-expiratory pressure beyond what is applied to the patient." — Barash Clinical Anaesthesia, 9e
Pathophysiology
Auto-PEEP arises when exhalation is not completed before the next inspiratory cycle. The key determinant is the expiratory time constant (τ):
τ = Resistance (R) × Compliance (C)
Three time constants are required for ~95% lung emptying. If the next breath interrupts exhalation before this, residual volume and pressure accumulate — dynamic hyperinflation.
Two mechanisms:
| Mechanism | Example |
|---|---|
| Increased expiratory time constant (high R or high C) | Asthma (↑R), COPD (↑C) |
| Expiratory flow limitation (airway collapse) | Obesity, mucous plugging |
Numerical example (Barash): If R = 10 cmH₂O/L/s and C = 0.05 L/cmH₂O → τ = 0.5 s → need ~1.5 s for full exhalation.
Causes / Risk Factors
| Category | Factors |
|---|---|
| Increased resistance | Asthma, bronchospasm, COPD, secretions, kinked or small ETT |
| Increased compliance | COPD emphysema |
| Ventilator settings | ↓Expiratory time (high RR, long I:E ratio), ↑Tidal volume |
| Active expiration | Persistent inspiratory muscle activity during exhalation |
| External resistance | HME filters, narrow tubing |
Detection / Measurement
1. Flow–Time Waveform (most practical)
The flow trace does not return to zero before the next breath — a non-zero end-expiratory flow indicates ongoing alveolar emptying (= auto-PEEP present).
2. Dynamic Auto-PEEP
Tracheal pressure at the moment when the flow trace crosses zero during exhalation (expiratory port open). Underestimates true auto-PEEP in heterogeneous lungs (some alveoli are isolated by airway closure).
3. Static Auto-PEEP (Gold Standard)
Expiratory hold / occlusion manoeuvre: Occlude the expiratory port at end-exhalation for ≤4 seconds and observe the pressure rise to a plateau. Auto-PEEP = plateau pressure − applied PEEP.
Requires passive patient (no spontaneous effort). Active breathing invalidates this measurement. Oesophageal balloon manometry can assess auto-PEEP in spontaneously breathing patients.
4. Other signs
- Unexpected ↑ peak airway pressure and plateau pressure
- Apparent ↓ respiratory system compliance
- Haemodynamic compromise (hypotension, ↓ cardiac output)
- Ventilator dyssynchrony / "missed" trigger attempts
Haemodynamic Consequences
Auto-PEEP → ↑ intrathoracic pressure → ↓ venous return → ↓ preload → ↓ cardiac output and hypotension. This is especially dangerous at induction in asthmatics/COPD patients.
"With severe intrinsic PEEP, increased intra-thoracic pressure reduces venous return and lowers cardiac output." — Murray & Nadel's Respiratory Medicine
Management
A. Address the Ventilator Settings (first-line)
| Strategy | Rationale |
|---|---|
| ↓ Respiratory rate | ↑ Expiratory time |
| ↑ Inspiratory flow rate | Shortens inspiration → more time for expiration |
| ↓ I:E ratio (e.g., 1:3 or 1:4) | Lengthens expiratory phase |
| ↓ Tidal volume | Less volume to exhale |
| Permissive hypercapnia | Accept ↑ PaCO₂ to allow adequate expiratory time |
B. Treat the Underlying Cause
- Bronchodilators (β₂-agonists, anticholinergics) for bronchospasm
- Suction / physiotherapy to clear secretions
- Treat bronchospasm: in mechanically ventilated asthmatics → ketamine (bronchodilation + sedation) is preferred
C. Applied (Extrinsic) PEEP to Counter-Balance iPEEP
In spontaneously breathing patients on assisted modes (e.g., PSV), patients must generate effort equal to auto-PEEP before the ventilator triggers — causing increased work of breathing and missed triggers.
Applying extrinsic PEEP up to ~80% of intrinsic PEEP reduces triggering effort without increasing end-expiratory lung volume (EELV), because EELV only rises when extrinsic PEEP exceeds intrinsic PEEP. — Miller's Anaesthesia 10e
⚠️ In passive/controlled ventilation, adding extrinsic PEEP does NOT reduce auto-PEEP and will worsen hyperinflation — avoid this approach.
D. Emergency — Acute Haemodynamic Collapse
Disconnect the patient from the ventilator and allow passive exhalation ("the ventilator disconnect test"). This immediately relieves hyperinflation and restores venous return.
Auto-PEEP in Specific Anaesthesia Scenarios
| Scenario | Key Point |
|---|---|
| Status asthmaticus on MV | Reduce RR, accept hypercapnia; ketamine for sedation; extrinsic PEEP below iPEEP |
| COPD | High compliance + dynamic airway collapse = major risk; use expiratory hold to quantify |
| Induction of anaesthesia | May cause sudden hypotension at initiation of IPPV in obstructed patients — disconnect if BP collapses |
| One-lung ventilation | Isolated lung with dependent position and reduced expiratory time — high risk |
| Dynamic parameters (SVV/PPV) | Auto-PEEP >15 cmH₂O invalidates SVV/PPV as predictors of fluid responsiveness |
Exam Summary Box
| Definition | End-expiratory alveolar pressure > applied PEEP |
| Mechanism | Incomplete exhalation due to ↑τ or flow limitation |
| At-risk | Asthma, COPD, high RR, ↑VT, ↓I:E |
| Detect | Non-zero end-expiratory flow; expiratory hold |
| Consequences | ↑Paw, ↓compliance, ↓CO, hypotension, barotrauma |
| Fix (ventilator) | ↓RR, ↑flow rate, ↓VT, ↑I:E ratio |
| Fix (spontaneous) | Extrinsic PEEP < iPEEP to ease triggering |
| Emergency | Disconnect ventilator |
Sources: Barash Clinical Anaesthesia 9e; Miller's Anaesthesia 10e; Murray & Nadel's Respiratory Medicine
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