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Cpap full details for md exams 10 marks answer

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CPAP (Continuous Positive Airway Pressure) - Full Details for MD Examination

Definition

Continuous Positive Airway Pressure (CPAP) is the application of a continuous positive-pressure threshold during both inspiration and expiration in a spontaneously breathing patient. It maintains a constant positive pressure throughout the entire respiratory cycle, unlike PEEP (Positive End-Expiratory Pressure), which applies positive pressure only during expiration in mechanically ventilated patients.
  • Morgan & Mikhail's Clinical Anesthesiology, 7e

Principle / Mechanism of Action

CPAP acts as a pneumatic splint to the upper airway and the alveoli:
  1. Prevents alveolar collapse - maintains functional residual capacity (FRC) above closing capacity
  2. Recruits collapsed alveoli - reexpansion occurs when pressure exceeds the inflection point on the pressure-volume curve
  3. Improves lung compliance - tidal ventilation occurs on the more compliant portion of the pressure-volume curve
  4. Corrects V/Q mismatch - reduces intrapulmonary shunting, improving arterial PO2
  5. Redistributes extravascular lung water - from alveolar-endothelial interstitial space to peribronchial/perihilar areas
  6. In OSA - acts as a pneumatic stent to prevent upper airway collapse throughout all phases of sleep breathing
  • Morgan & Mikhail's Clinical Anesthesiology, 7e; Scott-Brown's Otorhinolaryngology

CPAP vs. PEEP (Key Distinction)

FeatureCPAPPEEP
Breathing modeSpontaneous breathingMechanically ventilated
Pressure appliedDuring both inspiration AND expirationDuring expiration only
Gas flow requiredHigh continuous flow (60-90 L/min)Ventilator-cycled
Artificial airwayNot mandatory (tight mask used)Usually via ETT
Note: In clinical practice the two terms are often used interchangeably because patients may breathe with a combination of mechanical and spontaneous breaths. "Pure" CPAP provides sufficient continuous gas flow so that inspiratory airway pressure never falls perceptibly below the expiratory level.

Equipment / Delivery System

A CPAP system consists of:
  1. Flow generator - provides high-flow gas (fixed pressure or autotitrating machines)
  2. Air tubing connecting to the interface
  3. Mask interface - options include:
    • Full face mask
    • Nasal mask
    • Nasal pillows (sit on the nostril edge)
    • Tight-fitting mask for non-OSA ICU use
  4. Humidifier - recommended for patients with nasal congestion or dry mouth
  5. PEEP/threshold valve - allows expiratory flow only when airway pressure exceeds set level
Pressure delivered: Average 5-10 cmH2O for OSA; up to 15 cmH2O maximum via mask (above 15 cmH2O should only be given via endotracheal or tracheostomy tube, to avoid gastric distension and regurgitation)
  • Morgan & Mikhail's Clinical Anesthesiology, 7e; Scott-Brown's Otorhinolaryngology

Indications

A. Obstructive Sleep Apnea (OSA) - PRIMARY indication

  • Treatment of choice for moderate to severe OSA
  • Also indicated for Upper Airway Resistance Syndrome (UARS)/"sleepy snorers"
  • Recommended by NICE as treatment of choice for moderate-to-severe OSA

B. Intensive Care / Respiratory Failure

  • Decreased FRC causing hypoxemia (absolute or relative)
  • Impending respiratory failure - can avoid intubation
  • Acute Respiratory Distress Syndrome (ARDS)
  • Cardiogenic pulmonary edema / acute left ventricular failure
  • Post-extubation respiratory failure - reduces reintubation rates
  • Weaning from mechanical ventilation (5 cmH2O CPAP trials)

C. Neonatal

  • Respiratory Distress Syndrome (RDS) / hyaline membrane disease
  • Prevents atelectasis, minimizes lung injury, preserves surfactant function
  • Allows management without endotracheal intubation
  • Early delivery-room CPAP decreases need for mechanical ventilation

D. One-Lung Ventilation (thoracic surgery)

  • Applied to the non-ventilated lung to improve oxygenation
  • Scott-Brown's Otorhinolaryngology; Sabiston Textbook of Surgery; Morgan & Mikhail's Clinical Anesthesiology; Creasy & Resnik's Maternal-Fetal Medicine

CPAP Titration Methods

1. In-Lab Polysomnography (PSG) Titration

  • Diagnostic PSG in first half of night, CPAP titration in second half ("split-night")
  • Starting pressure ~4 cmH2O, increased until apneas and hypopneas eliminated
  • Technologist adjusts pressure via central computer with video access to patient
  • Disadvantage: true severity may not be captured in first half of night

2. AutoCPAP (Home Trial)

  • Patient sent home with an auto-titrating device
  • Trial of 7-14 days (preferred over 1-night trial)
  • Machine collects data: compliance, leaks, pressure profile
  • Fixed pressure then set at 90th or 95th centile pressure from autoCPAP data

3. Mathematical Formula

  • Predicted pressure (cmH2O) = (0.16 × BMI) + (0.13 × NC) + (0.04 × AHI) - 5.12
  • NC = neck circumference (cm), AHI = apnea-hypopnea index
  • Scott-Brown's Otorhinolaryngology

Physiological Effects

Pulmonary Effects

  • Increases FRC
  • Recruits collapsed alveoli (at pressures above the inflection point)
  • Improves lung compliance
  • Reduces V/Q mismatch
  • Decreases intrapulmonary shunting
  • Improves arterial oxygenation (PaO2)
  • Redistributes extravascular lung water to peribronchial areas

Cardiovascular Effects (adverse)

  • Reduces venous return to the heart (increased intrathoracic pressure)
  • Reduces cardiac output (especially at pressures >15 cmH2O)
  • Leftward shift of interventricular septum (raised RV volume) - reduces LV compliance
  • Reduced renal and hepatic blood flow
  • Decreased urinary output, GFR, free water clearance
  • May increase intracranial pressure (impedes cerebral venous drainage)
  • Morgan & Mikhail's Clinical Anesthesiology, 7e

Benefits of CPAP in OSA (Clinical Outcomes)

  • Eliminates apneas and hypopneas
  • Reduces daytime somnolence (dose-dependent: longer use per night = better improvement)
  • Improves neurocognitive function
  • Reduces cardiovascular risk (blood pressure, arrhythmia risk)
  • Improves quality of life
  • Average use in compliant patients: 2-6 hours per night

Side Effects / Adverse Effects

Mask/Interface Related

  • Nasal congestion and rhinosinusitis (most common)
  • Nasal bridge and facial skin irritation/pressure sores
  • Epistaxis (nosebleeds)
  • Claustrophobia (common cause of non-compliance)
  • Dry mouth / xerostomia

Pressure-Related

  • Gastric distention and regurgitation risk (if pressure >15 cmH2O via mask)
  • Aerophagia
  • Central sleep apnea emergence (complex/treatment-emergent sleep apnea)

Machine-Related

  • Noise of the machine causing sleep disturbance
  • Inconvenience of carrying equipment (travel)

Systemic (at high pressures)

  • Reduced cardiac output
  • Hypotension
  • Elevated intracranial pressure

Contraindications

  • Absent airway protective reflexes (risk of aspiration if mask used)
  • Facial trauma / burns precluding mask use
  • Recent facial/upper airway surgery
  • Uncooperative patient
  • CSF rhinorrhea / basal skull fracture
  • Severe hemodynamic instability
  • Need for emergent intubation
  • Sabiston Textbook of Surgery

Compliance and Follow-Up

  • Poor compliance is the main limitation of CPAP therapy
  • Causes of non-compliance: mask discomfort, noise, claustrophobia, nasal symptoms
  • Strategies to improve compliance:
    • Patient education programs (group video workshops)
    • Trying different mask interfaces
    • Adding humidifiers
    • Cognitive behavioural therapy (CBT)
  • Annual electrical safety check mandatory (NHS); engineer downloads compliance data
  • Significant weight gain may require pressure increase

Alternatives to CPAP

AlternativeNotes
BiPAP (Bilevel PAP)Separate IPAP (10-20 cmH2O) and EPAP (5-10 cmH2O); better for hypercarbic failure and CPAP-intolerant patients
Adaptive Servo-Ventilation (ASV)For central sleep apnea (CSA); specialist centre initiation required
Nasal EPAP devicesNovel; awaiting RCT vs CPAP
Mandibular advancement devicesFor mild-moderate OSA; 50-75% compliance
Weight lossAdjunct - reduces pressure requirements
SurgeryUvulopalatopharyngoplasty, maxillomandibular advancement - for selected cases
  • Scott-Brown's Otorhinolaryngology

Summary Box for Exam (Rapid Revision)

ParameterValue/Point
Pressure range (OSA)5-10 cmH2O
Max via mask15 cmH2O
Starting pressure (titration)~4 cmH2O
Average compliant use2-6 hours/night
Fixed pressure set at90th-95th centile of auto-CPAP data
Mechanism in OSAPneumatic splint - prevents airway collapse
Mechanism in ARDS/APEIncreases FRC, recruits alveoli, reduces shunt
Key side effectPoor compliance; nasal congestion
Contraindication (pressure)>15 cmH2O via mask only (use ETT/trach)
NICE recommendationTreatment of choice for moderate-severe OSA
Sources:
  • Morgan & Mikhail's Clinical Anesthesiology, 7e - CPAP vs PEEP, pulmonary effects, adverse effects
  • Scott-Brown's Otorhinolaryngology Head & Neck Surgery - CPAP in OSA, titration, alternatives
  • Sabiston Textbook of Surgery - Noninvasive ventilation, CPAP/BiPAP in ICU
  • Creasy & Resnik's Maternal-Fetal Medicine - Neonatal CPAP in RDS
  • Miller's Anesthesia, 10e - Postoperative CPAP use
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