Ventilator setting in copd
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"mechanical ventilation" AND "COPD" AND settings
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Ventilator Settings in COPD
COPD patients have airflow obstruction, air trapping, and intrinsic PEEP (iPEEP). All ventilator decisions revolve around one central goal: maximize expiratory time to prevent dynamic hyperinflation.
1. Non-Invasive Ventilation (NIV) - First Choice in Acute Exacerbation
NIV (BiPAP) is the preferred initial ventilatory support in AECOP. It avoids intubation in the majority of patients.
| Parameter | Target |
|---|---|
| Mode | Bilevel PAP (IPAP/EPAP) |
| IPAP (Inspiratory) | Start 10-12 cmH2O, titrate up to reduce PCO2 |
| EPAP (Expiratory) | 4-5 cmH2O (to counteract iPEEP) |
| Backup RR | 10-14/min |
| FiO2 | Titrate to SpO2 88-92% |
| Goal | PCO2 reduction within 1-2 hours; pH improvement |
- Improvements in pH, PCO2, and level of consciousness within the first 1-2 hours are strong predictors of NIV success.
- Home NIV is indicated in chronic stable hypercapnic COPD patients with PaCO2 > 52 mmHg. After an acute episode, use only if hypercapnia (PCO2 > 48 mmHg) persists at 2-4 weeks post-discharge. Fixed pressure mode is preferred over auto-titrating modes.
2. Invasive Mechanical Ventilation - Indications
Invasive ventilation is needed when NIV fails or is contraindicated:
- Severe tachypnea > 35 breaths/min
- Respiratory arrest or apnea
- Severe/worsening acidosis + hypercapnia
- Hemodynamic instability (shock)
- Somnolence / altered mental status
- Life-threatening hypoxemia
3. Recommended Invasive Ventilator Settings
Mode
- Volume-controlled Assist-Control (VC-AC) is the most common initial choice - it gives control over tidal volume, which is critical to prevent dynamic hyperinflation.
- Pressure-control can also be used but requires monitoring tidal volumes closely since they vary with changing resistance and compliance.
Key Parameters
| Parameter | Recommended Setting | Rationale |
|---|---|---|
| Tidal Volume (VT) | 5-8 mL/kg predicted (IBW) | Avoid hyperinflation. 5-7 mL/kg (Murray & Nadel) or ≤8 mL/kg (Rosen's) |
| Respiratory Rate (RR) | 10-14/min | Low rate lengthens expiratory time; every decrease in rate has a large effect on expiratory time |
| I:E Ratio | 1:3 to 1:4 | Prolonged expiration allows air trapping to resolve |
| Inspiratory Flow Rate | 60-80 L/min (high flow) | Shorter inspiration = longer expiration |
| PEEP (External) | 5 cmH2O (initial) | Counterbalances iPEEP; safe for all COPD patients initially |
| FiO2 | Titrate to SpO2 88-92% | Avoid hyperoxygenation; prevents loss of hypoxic drive and worsens V/Q |
| Plateau Pressure | < 30 cmH2O | Monitor to exclude additional lung injury |
4. The iPEEP Problem - Central to COPD Ventilation
Intrinsic (auto) PEEP builds up because patients cannot fully exhale before the next breath. This causes:
- Air trapping and dynamic hyperinflation
- Reduced venous return - can mimic cardiogenic shock
- Patient-ventilator dyssynchrony (patient can't reach trigger threshold)
- Hypotension
How to detect iPEEP: End-expiratory hold maneuver on ventilator. Total PEEP minus set PEEP = iPEEP.
How to reduce iPEEP:
- Decrease RR - most effective
- Decrease tidal volume
- Shorten inspiratory time (increase flow rate)
- Bronchodilators + corticosteroids (reduce airway resistance)
- Set external PEEP at 50-80% of iPEEP (to reduce inspiratory threshold load and improve triggering)
If hemodynamic collapse occurs and iPEEP is suspected, temporarily disconnect from ventilator - rapid resolution of hypotension confirms iPEEP as the cause (not cardiogenic shock).
5. Permissive Hypercapnia
COPD patients often have chronic CO2 retention. Do not chase normal PaCO2. Attempting to normalize PaCO2 forces higher RR/VT - this worsens dynamic hyperinflation.
- Accept PCO2 elevation as long as pH > 7.2
- Tolerate respiratory acidosis - it will improve with bronchodilators and time
- If pH < 7.2: cautiously increase RR first (watch for iPEEP), then consider increasing VT
6. Pressure-Support Ventilation in COPD
PSV is useful for weaning but has a specific problem in COPD: because of the prolonged time constant (high resistance), flow takes longer to fall to the cycling threshold, so mechanical inflation persists into neural expiration. This causes:
- Expiratory muscle activation fighting the ventilator
- Patient-ventilator dyssynchrony ("fighting the ventilator")
Solution: Reduce the cycling-off threshold (increase the flow termination % from 25% to a higher value on the ventilator) to cycle the breath off sooner.
7. Troubleshooting Summary
| Problem | Action |
|---|---|
| High PaCO2, pH > 7.2 | Tolerate; treat with bronchodilators |
| High PaCO2, pH < 7.2, no iPEEP | Cautiously increase RR |
| High PaCO2, pH < 7.2, iPEEP present | Increase VT, keep rate low |
| High peak pressure (normal plateau) | Increased airway resistance - check tube, mucus plug; continue bronchodilators |
| High peak AND plateau pressure | Decreased compliance - consider pneumothorax, worsening |
| Hypotension on ventilator | Suspect iPEEP - disconnect briefly to confirm; reduce rate/VT |
Sources:
- Murray & Nadel's Textbook of Respiratory Medicine, 2-Vol (section: Invasive Mechanical Ventilation in COPD)
- Rosen's Emergency Medicine, 10e (section: Acute Exacerbation of COPD, Invasive Ventilation)
- Fishman's Pulmonary Diseases and Disorders (section: Pressure-Support Ventilation / Triggering)
Remifentanyl
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Remifentanil
Overview
Remifentanil is the first and only ultra-short-acting opioid available for clinical use. It is a piperidine derivative structurally related to fentanyl, but uniquely different because of its ester linkages - these make it susceptible to hydrolysis by blood and tissue non-specific esterases, giving it a rapid and predictable offset completely independent of hepatic or renal function.
Pharmacokinetics
| Property | Detail |
|---|---|
| Class | mu-opioid receptor (MOR) agonist |
| Onset | 1-1.5 min after IV bolus |
| Peak respiratory depression | ~5 min after bolus |
| t½ | 8-20 min; context-insensitive half-life ~3-4 min |
| Metabolism | Plasma and tissue non-specific esterases (de-esterification) |
| NOT a substrate for | Pseudocholinesterase - unaffected by pseudocholinesterase deficiency |
| Main metabolite | GI90291 (carboxylic acid) - only 0.001 to 0.003 times as potent as parent |
| Metabolite excretion | Renal |
| Protein binding | ~70% (mostly to alpha-1-acid glycoprotein) |
| pKa | 7.07 (weak base) |
| PK model | Three-compartment |
| Clearance | Several times greater than normal hepatic blood flow - confirms widespread extrahepatic metabolism |
| Lung metabolism | Not significantly metabolized in lungs |
| Recovery of respiratory function | 3-5 min after 3-5 hour infusion |
| Full effect offset | Within 15 min |
Key point: Because metabolism is by non-organ esterases, hepatic or renal failure does not significantly alter pharmacokinetics. This makes it especially useful in patients with liver/kidney failure (including infants with hepatic or renal failure).
Potency
- 100-200 times more potent than morphine
- Large inter-patient variability: the CP50 for no response to laryngoscopy/intubation ranges 50-fold (1.5 to 79 ng/mL)
- Gender differences exist: CP50 ~4.1 ng/mL in men vs 7.5 ng/mL in women (partly attributable to differences in surgical nociception)
Pharmacodynamics / Clinical Effects
CNS / Analgesia
- Potent analgesia with rapid onset and offset
- Reduces MAC of volatile anesthetics significantly (e.g., remifentanil at 3 ng/mL + sevoflurane: MAC reduced to ~0.36% from ~3.96%)
- Reduces propofol requirements >60% (propofol CP50 for laryngoscopy drops from 7 to 3 mcg/mL with remifentanil 2 ng/mL)
- At sedative doses (0.05-0.15 mcg/kg/min) can increase CBF in prefrontal, inferior parietal, and supplementary motor cortices
- At moderate anesthetic doses in craniotomy: ICP unchanged, CBF comparable to balanced anesthesia
Respiratory
- Causes dose-dependent respiratory depression
- No delay between plasma concentration and ventilatory effect (unlike slower opioids)
- Safe to use at low infusion rates (< 0.1-0.2 mcg/kg/min) in spontaneously breathing patients with adequate monitoring
Cardiovascular
- Generally stable; bolus doses can cause transient hypotension (drop in MAP), which can reflexively increase ICP - so bolus infusion should be used cautiously in head-injured patients
Dosing (Intraoperative)
| Indication | Dose |
|---|---|
| Balanced anesthesia infusion | 0.1-1.0 mcg/kg/min |
| Spontaneous breathing / sedation | < 0.1-0.2 mcg/kg/min |
| Return of spontaneous ventilation | ~0.1 ± 0.05 mcg/kg/min |
| TIVA with propofol | Target 3-8 ng/mL (titrate to response) |
| Optimal TIVA concentration for fastest awakening | Remifentanil ~4.8 ng/mL + propofol ~2.5 mcg/mL (wake-up ~7 min) |
Because of its very short duration, remifentanil must be given by continuous infusion - bolus alone is not useful for maintenance.
Special Considerations
1. Post-Operative Pain - Critical Concern
Remifentanil's rapid offset means no residual analgesia after infusion stops. Patients frequently experience significant post-operative pain ("fast-track" anesthesia problem):
- Post-op pain scores higher, morphine requirements increased after remifentanil-based anesthesia
- Strategies: Start morphine 30-45 min before end of surgery, OR give single fentanyl bolus 50 mcg or ketamine 0.125 mg/kg at end of surgery
2. Opioid-Induced Hyperalgesia (OIH)
High doses of remifentanil can paradoxically lower the pain threshold after discontinuation - a phenomenon known as OIH. This contributes to the post-operative pain problem. Ketamine (0.15 mg/kg + 2 mcg/kg/min infusion) can reduce intraoperative remifentanil requirements and post-op morphine consumption.
3. NOT for Intrathecal / Epidural Use
The formulation contains glycine (used to solubilize the free base). Glycine is an inhibitory neurotransmitter that causes reversible motor weakness when injected intrathecally - therefore remifentanil is contraindicated for spinal or epidural administration.
4. Hemorrhagic Shock
Altered pharmacokinetics in hemorrhagic shock - less remifentanil required to maintain target plasma concentration. However, because of rapid metabolism, changes in context-sensitive half-life are less clinically relevant.
5. Pediatrics / Organ Failure
Since elimination is independent of liver and kidney, it is useful in neonates and infants with hepatic or renal failure.
Clinical Indications
- Intraoperative analgesia/anesthesia supplement (TIVA with propofol, balanced anesthesia)
- Short painful procedures requiring intense analgesia
- Blunting of hemodynamic stress responses (laryngoscopy, intubation, surgical stimulus)
- PCA analgesia (with adequate monitoring)
- ICU sedation/analgesia
- Obstetric labor analgesia
- Diagnostic procedures
Comparison with Other Fentanyl Congeners
| Drug | Onset | Duration | Context-sensitive t½ |
|---|---|---|---|
| Fentanyl | Fast | Intermediate | Increases with infusion duration |
| Alfentanil | Fastest | Short | ~1 hour |
| Sufentanil | Fast | Long | Increases with infusion |
| Remifentanil | Fast | Ultra-short | ~3-4 min - does NOT accumulate |
Remifentanil is unique in that its context-sensitive half-life remains ~3-4 minutes regardless of how long the infusion runs - a property no other opioid shares.
Sources:
- Miller's Anesthesia, 10e (Ch. 22 - Opioids; Ch. 3 - Cerebrovascular effects)
- Barash, Cullen & Stoelting's Clinical Anesthesia, 9e (Ch. 20)
- Goodman & Gilman's Pharmacological Basis of Therapeutics (Ch. 7)
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