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ACUTE RESPIRATORY DISTRESS SYNDROME (ARDS)

Notes for MD Anaesthesiology Examination

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1. DEFINITION & HISTORICAL BACKGROUND

• 1967: First described by Ashbaugh et al. — 12 patients with respiratory distress, bilateral opacities, post-trauma/viral infection, rapid onset within 48–72 hours. Initially called "Adult Respiratory Distress Syndrome."
• 1988: Murray Lung Injury Score introduced (hypoxemia + PEEP level + compliance + CXR). LIS >2.5 = ARDS.
• 1994: AECC Definition — distinguished ALI (P/F <300) from ARDS (P/F <200).
• 2012: Berlin Definition — current standard. Eliminated "ALI," added explicit timing criteria, stratified severity.

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2. BERLIN DEFINITION (2012)

Berlin Severity Classification

Key point: The PaO₂/FiO₂ criterion is assessed on ≥5 cmH₂O PEEP (by NIV or invasive ventilation). Interobserver variability remains ~50% (kappa 0.5), mainly due to CXR interpretation.

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3. EPIDEMIOLOGY

• Accounts for 10–15% of all ICU admissions
• Sepsis is the most common cause (~30% risk of developing ARDS)
• Mortality varies widely:
  • Trauma ARDS: 10–15%
  • Medical ICU ARDS: up to 60%
  • Most deaths are from the underlying cause (sepsis, trauma), not refractory hypoxemia
• Overall ICU mortality: ~27–45% depending on severity

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4. ETIOLOGY / RISK FACTORS

Direct (Pulmonary) Causes

• Pneumonia (most common direct cause)
• Aspiration of gastric contents
• Pulmonary contusion
• Inhalational injury / toxic gas inhalation
• Near-drowning
• Reperfusion injury after lung transplant

Indirect (Extrapulmonary) Causes

• Sepsis (most common overall cause)
• Major trauma / haemorrhagic shock
• Pancreatitis
• Multiple blood transfusions (TRALI)
• Burns
• Cardiopulmonary bypass
• Drug overdose (heroin, salicylates)
• DIC

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5. PATHOPHYSIOLOGY

Phases of ARDS

Phase 1 — Exudative Phase (Days 1–7)

• Trigger → activation of alveolar macrophages → release of TNF-α, IL-1β, IL-6, IL-8
• Neutrophil influx into alveolar space
• Injury to type I alveolar epithelial cells → loss of epithelial barrier
• Injury to pulmonary capillary endothelium → increased vascular permeability
• Protein-rich edema floods alveoli → diffuse alveolar damage (DAD)
• Inactivation of surfactant → alveolar collapse and atelectasis
• Hyaline membrane formation (histological hallmark)
• Net result: ↓ compliance, ↑ shunt, hypoxemia

Phase 2 — Proliferative Phase (Days 7–21)

• Type II pneumocyte proliferation (attempt at repair)
• Fibroblast activation → early fibrosis
• Gradual improvement in oxygenation in survivors

Phase 3 — Fibrotic Phase (>21 days)

• Collagen deposition, pulmonary fibrosis
• Pulmonary hypertension develops
• Increased dead space

Key Pathophysiological Mechanisms

• Diffuse Alveolar Damage (DAD): proteinaceous exudate, hyaline membranes, cellular debris
• High-permeability pulmonary edema: distinguishes from cardiogenic edema
• "Baby lung" concept: CT shows opacification predominantly in posterior dependent zones, leaving only a small normally ventilated volume (~300–500 mL) → ventilating a "baby-sized" lung
• Heterogeneous injury: not a diffuse uniform process; normal lung, recruitable lung, and consolidated lung coexist
• Pulmonary vascular injury: ↑ dead space fraction (predictive of mortality), pulmonary hypertension
• Na⁺/Water transport failure: hypoxia impairs apical Na⁺ channels and basolateral Na⁺/K⁺-ATPase on alveolar epithelium → impaired alveolar fluid clearance
• Angiopoietin-2 (Ang2): elevated in ARDS/sepsis, disrupts endothelial barrier integrity (antagonises Ang1/Tie2 signalling)

Ventilator-Induced Lung Injury (VILI)

• Volutrauma: overdistention of normally ventilated alveoli (most important mechanism)
• Atelectotrauma: cyclic collapse-and-reopening of unstable alveoli
• Barotrauma: pneumothorax, pneumomediastinum (less common with LPV)
• Biotrauma: release of inflammatory mediators from injured lung → systemic inflammation → MODS

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6. CLINICAL FEATURES

• Onset: Acute (within hours to 1 week of trigger)
• Severe dyspnoea, tachypnoea
• Hypoxemia refractory to supplemental O₂ (high intrapulmonary shunt)
• Bilateral crackles on auscultation
• Cyanosis, use of accessory muscles
• CXR: bilateral fluffy opacities, without cardiomegaly or septal lines
• CT chest: bilateral consolidation, predominantly posterior/dependent distribution (gravitational effect)
• Low static lung compliance: typically <40 mL/cmH₂O (normal ~100 mL/cmH₂O)
• High dead-space fraction (predictive of mortality)
• Pulmonary hypertension in progressive cases

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7. DIAGNOSIS

Key Investigations

• ABG: hypoxaemia, compute P/F ratio; initial respiratory alkalosis → later metabolic acidosis
• CXR: bilateral opacities (not explained by effusions, collapse, nodules)
• CT Chest: heterogeneous consolidation, dorsal predominance; identifies recruitability
• Echocardiography: exclude cardiogenic pulmonary oedema (EF, LA pressure)
• BNP/NT-proBNP: elevated in cardiogenic; may be normal/mildly raised in ARDS
• BAL: neutrophilia >60% supports ARDS; helps exclude infection
• Pulmonary artery catheter (rarely used now): PCWP <18 mmHg supports ARDS (old AECC criterion; Berlin replaced this with echo assessment)

Differentials to Exclude

• Cardiogenic pulmonary oedema
• Bilateral pneumonia
• Diffuse alveolar haemorrhage
• Acute interstitial pneumonia (AIP)
• Pulmonary vasculitis

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8. MECHANICAL VENTILATION IN ARDS ⭐ (Core Anaesthesia Topic)

A. Lung Protective Ventilation (LPV) — PROVEN MORTALITY BENEFIT

IBW Calculation: Males: 50 + 2.3 × (height in inches − 60) kg; Females: 45.5 + 2.3 × (height in inches − 60) kg. Use IBW, NOT actual body weight.

B. Driving Pressure

• ΔP = Pplat − PEEP = Vt/Crs (respiratory system compliance)
• Growing evidence that ΔP is an independent predictor of mortality in ARDS
• Target: <14 cmH₂O
• Useful guide for PEEP titration: if raising PEEP decreases ΔP → lung recruitment is occurring (beneficial)

C. PEEP Strategy

• PEEP prevents alveolar derecruitment (atelectotrauma), recruits collapsed alveoli, improves oxygenation and compliance
• No universally superior PEEP titration strategy:
  • ARDSNet PEEP/FiO₂ tables (low vs. high PEEP)
  • Esophageal pressure-guided PEEP
  • Stress index
  • Driving pressure-guided PEEP
• High PEEP (≥15 cmH₂O) may benefit moderate-severe ARDS (P/F <200) but risk overdistension and haemodynamic compromise in mild ARDS
• PEEP titration: set PEEP at point where driving pressure is minimised

D. Recruitment Manoeuvres (RM)

• Transient increase in airway pressure to open collapsed alveoli
• Methods: sustained inflation (35–40 cmH₂O × 40 sec), incremental PEEP ("staircase" RM)
• ART trial (2017): High PEEP + RM strategy increased 28-day mortality (harmful in unselected ARDS)
• RM may benefit patients with high recruitability (identified by CT or PEEP responsiveness)
• No routine recommendation; selective use in refractory hypoxaemia

E. Volume Control vs. Pressure Control

• Both modes acceptable if pressure/volume limits are respected
• Pressure-control may improve gas distribution but no proven mortality advantage
• High-frequency oscillation ventilation (HFOV): not recommended (OSCAR/OSCILLATE trials showed harm or no benefit)

F. Permissive Hypercapnia

• Deliberate acceptance of elevated PaCO₂ (50–100 mmHg) to maintain low Vt
• Usually pH 7.20–7.35 tolerated
• Contraindications: raised ICP, right heart failure (hypercapnia causes pulmonary vasoconstriction), severe metabolic acidosis

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9. PRONE POSITIONING ⭐

Mechanism of Benefit

• Redistributes lung perfusion from consolidated dorsal zones to now-dependent ventral zones
• Improves V/Q matching, reduces shunt
• More homogeneous alveolar stress distribution → less VILI
• Reduces dorsal atelectasis

Evidence

• PROSEVA Trial (2013, France): Prone positioning ≥16 hours/day in severe ARDS (P/F <150 mmHg) → significant reduction in 28-day mortality (16% vs. 32.8%; HR 0.39). This is a landmark trial.
• Earlier trials (6 hours/day) showed oxygenation improvement but NO mortality benefit
• Key: Duration matters — minimum 16 hours/day required for mortality benefit

Indications

• Moderate-severe ARDS: P/F <150 mmHg
• Despite optimised LPV and PEEP

How to Prone

• Requires neuromuscular blockade for tolerance and to prevent accidental extubation
• Padded pressure points (face, thorax, iliac crests, knees)
• Continue at least 16 hours before returning supine
• Reassess oxygenation response; responders: continue daily sessions

Contraindications

• Spinal instability or unstable fractures (absolute)
• Raised ICP, facial/chest trauma
• Anterior wounds or burns (relative)
• Obesity makes prone positioning technically challenging (not a contraindication)

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10. NEUROMUSCULAR BLOCKADE (NMB)

• ACURASYS trial: Early cisatracurium (48 hours) in P/F <150 → improved 90-day mortality, more ventilator-free days
• ROSE trial (2019): Challenged this finding — early NMB with light sedation strategy showed NO mortality benefit
• Current practice: NMB is useful for:
  • Tolerating prone positioning
  • Refractory ventilator dyssynchrony
  • Refractory hypoxaemia
• Drug of choice: Cisatracurium (Hoffmann elimination; safe in organ failure)

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11. EXTRACORPOREAL MEMBRANE OXYGENATION (ECMO)

• VV-ECMO (veno-venous) for ARDS — CO₂ removal and oxygenation support
• CESAR trial: ECMO referral centre showed improved 6-month survival
• EOLIA trial (2018): VV-ECMO for severe ARDS (P/F <80 or uncompensated hypercapnia) — 28-day mortality 35% vs. 46% (p=0.07; statistically non-significant, but crossover-adjusted effect favoured ECMO)
• Indications: Severe ARDS refractory to maximal conventional therapy including prone positioning and PEEP optimisation
• Criteria: P/F <80 on FiO₂ 1.0, Murray Score ≥3, or uncompensable respiratory acidosis

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12. NON-INVASIVE VENTILATION (NIV) AND HFNO IN ARDS

• NIV: Can be trialled in mild ARDS (P/F 200–300). Risk of delayed intubation if fails.
• HFNO (High Flow Nasal Oxygen): FLORALI trial showed benefit in non-hypercapnic ARF; may avoid intubation in mild ARDS
• In moderate-severe ARDS: Early intubation and invasive ventilation preferred
• Risk of "self-inflicted lung injury" (P-SILI) with high respiratory effort on NIV/HFNO in moderate-severe ARDS

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13. FLUID MANAGEMENT

• Conservative fluid strategy (PCWP <8 mmHg, CVP <4 mmHg) → shorter mechanical ventilation time
• FACTT trial: Conservative vs. liberal fluid — no mortality difference, but conservative → more ventilator-free days (2.5 days more)
• Avoid fluid overload; extravascular lung water directly worsens gas exchange in ARDS
• Vasopressors used to maintain MAP if needed, rather than excess fluids

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14. PHARMACOLOGICAL THERAPIES

Corticosteroids

• NOT recommended as routine therapy in established ARDS
• Methylprednisolone in early fibroproliferative phase (days 7–14): some trials show benefit in unresolved ARDS, but evidence weak
• Late steroids (≥14 days): potentially harmful
• Exception — COVID-19 ARDS: Dexamethasone reduces 28-day mortality (RECOVERY trial); WHO recommends systemic steroids in severe COVID-19

Inhaled Nitric Oxide (iNO)

• Selective pulmonary vasodilator → improves V/Q matching, reduces shunt
• Short-term oxygenation improvement
• NO mortality benefit in multiple RCTs
• Risk of renal dysfunction with prolonged use
• Reserved for refractory hypoxaemia or ARDS with pulmonary hypertension

Inhaled Prostacyclins (Epoprostenol, Iloprost)

• Mechanism similar to iNO; pulmonary vasodilation of ventilated lung
• Improves oxygenation and PAP
• No proven mortality benefit; theoretical concern: antiplatelet effect may worsen DAH
• Alternative to iNO when unavailable

β₂-Agonists (Salbutamol)

• Theoretically stimulate alveolar Na⁺ clearance via β₂ receptors
• BALTI-2 trial: IV salbutamol in ARDS → increased adverse events; STOPPED early
• Not recommended

Surfactant Replacement

• Successful in neonatal RDS; failed to show benefit in adult ARDS
• Adult ARDS involves both endothelium and epithelium (not purely surfactant deficiency)

Statins

• Anti-inflammatory properties; preclinical promise
• HARP-2 / SAILS trials: No mortality benefit; not recommended

Vitamins C, D, Zinc, Omega-3 Fatty Acids

• No consistent evidence of benefit in RCTs

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15. ADJUNCTS AND SUPPORTIVE CARE

Sedation & Analgesia

• Adequate sedation for ventilator synchrony; excessive sedation prolongs ventilation
• Analgesia-first (analgosedation) approach preferred; light sedation (RASS 0 to −1) unless contra-indicated
• Daily sedation interruption (SAT) + Spontaneous Breathing Trials (SBT) → reduces ventilator days

Nutrition

• Early enteral nutrition preferred; parenteral if enteral not feasible
• Avoid overfeeding (↑ CO₂ production)
• Immunonutrition (omega-3, antioxidants): no clear benefit in ARDS

DVT Prophylaxis

• Low-molecular-weight heparin (LMWH) unless contraindicated

Stress Ulcer Prophylaxis

• PPI or H₂ blockers in ventilated patients

Infection Control

• Treat underlying infection aggressively
• VAP bundle: HOB elevation 30–45°, oral decontamination, daily sedation holds, cuff pressure monitoring

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16. SUBPHENOTYPES OF ARDS

Clinical significance: Subphenotype-guided treatment may be the future of personalised ARDS management.

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17. COMPLICATIONS OF ARDS

Pulmonary

• Barotrauma: pneumothorax, pneumomediastinum, subcutaneous emphysema (especially with high pressures)
• Ventilator-associated pneumonia (VAP): most important infectious complication
• Pulmonary fibrosis (fibroproliferative phase)
• Pulmonary hypertension and right ventricular failure
• Tracheal injury (prolonged intubation/tracheostomy)

Systemic

• Multi-organ dysfunction syndrome (MODS) — via biotrauma, systemic inflammation
• Acute kidney injury
• ICU-acquired weakness (ICUAW)
• Gastrointestinal: ileus, stress ulcers, malnutrition

Long-Term

• Physical: reduced exercise capacity, muscle weakness, restrictive lung defect persists
• Psychological: PTSD (~25%), anxiety, depression
• Cognitive: cognitive impairment in ~30% at 12 months post-ICU

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18. PREDICTORS OF POOR PROGNOSIS

• Advanced age
• Non-pulmonary organ failure (renal, hepatic)
• High dead-space fraction (VD/VT)
• Persistent hypoxaemia despite prone positioning
• Hyperinflammatory subphenotype
• Underlying aetiology (sepsis > trauma in mortality)
• High APACHE II / SOFA score
• Immunocompromised state

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19. LONG-TERM OUTCOMES

• Most survivors have significant functional limitation for 12–24 months
• FEV₁/FVC may normalise, but DLCO (diffusion capacity) remains reduced
• Exercise limitation often due to neuromuscular weakness rather than pulmonary function
• Major contributors to morbidity: ICUAW, cognitive dysfunction, PTSD
• Recovery largely complete by 5 years in most survivors

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20. ANAESTHESIA-SPECIFIC CONSIDERATIONS ⭐

Intraoperative Management in ARDS Patients

• Use lung-protective ventilation intraoperatively (6 mL/kg IBW, Pplat ≤30, PEEP 5–10 cmH₂O)
• Avoid high FiO₂ for prolonged periods (absorption atelectasis)
• Low tidal volumes may reduce risk of developing postoperative ARDS in at-risk patients
• Set PEEP to minimise driving pressure

One-Lung Ventilation (OLV) and ARDS Risk

• OLV inherently creates large physiological dead space and shunt — increases risk of VILI
• Apply LPV during OLV: Vt 4–5 mL/kg IBW to ventilated lung

Sedation in ARDS (ICU)

• Propofol, midazolam, or dexmedetomidine (for light sedation)
• Dexmedetomidine: reduces duration of mechanical ventilation, does not cause respiratory depression — beneficial for weaning
• Ketamine: bronchodilator, haemodynamically stable; useful in haemodynamically compromised ARDS patients

Positioning in Anaesthesia

• Consider effects of supine positioning in ARDS-prone surgical patients
• Laparoscopic (Trendelenburg) position dramatically worsens oxygenation in ARDS

Tracheostomy

• Early tracheostomy (within 7 days): facilitates weaning, reduces sedation, reduces VAP in appropriate patients with anticipated prolonged ventilation

Management of Refractory Hypoxaemia — Stepwise Approach

1. Optimise FiO₂ (1.0 temporarily)
2. Optimise PEEP (use driving pressure guidance)
3. Recruitment manoeuvre (if high recruitability)
4. Prone positioning ≥16 h/day (if P/F <150)
5. Neuromuscular blockade (cisatracurium)
6. Inhaled vasodilators (iNO or inhaled prostacyclin)
7. VV-ECMO referral (if P/F <80, refractory)

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21. KEY TRIALS SUMMARY TABLE

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22. HIGH-YIELD EXAM POINTS

1. Berlin Definition: PaO₂/FiO₂ <300 on ≥5 cmH₂O PEEP, bilateral opacities, acute onset, not explained by cardiac failure
2. Baby Lung concept: ARDS is functionally a small lung — ventilate accordingly
3. LPV (6 mL/kg IBW, Pplat ≤30, ΔP <14) is the only intervention with proven mortality benefit in ARDS
4. Driving pressure = Pplat − PEEP; target <14 cmH₂O; strongly associated with outcome
5. Prone positioning ≥16 hours/day for P/F <150 — reduces mortality (PROSEVA)
6. PEEP: prevents atelectotrauma; no single titration strategy proven superior
7. ECMO for P/F <80 despite maximal therapy
8. No pharmacological therapy (steroids, statins, beta-agonists, iNO) has proven mortality benefit in general ARDS (exception: dexamethasone in COVID-19 ARDS)
9. Conservative fluid management → more ventilator-free days
10. Subphenotypes (hyperinflammatory vs. hypoinflammatory) predict outcomes and may guide treatment
11. Permissive hypercapnia: acceptable pH 7.20–7.35; contraindicated in ↑ICP, severe RV failure
12. Long-term outcomes: cognitive dysfunction in 30% at 12 months; functional limitation for 2+ years

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OXYGEN AUDIT IN ANAESTHESIOLOGY

Notes for MD Anaesthesiology Examination

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1. INTRODUCTION & DEFINITION

• Clinical audit: Ensuring oxygen is prescribed, administered, and monitored appropriately
• Equipment audit: Checking integrity of supply systems, cylinders, pipelines, and machines
• Consumption audit: Quantifying oxygen usage and preventing wastage
• Safety audit: Identifying failures and near-misses in oxygen delivery

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2. WHY OXYGEN AUDIT IS IMPORTANT IN ANAESTHESIA

1. Patient safety — oxygen supply failure during anaesthesia is immediately life-threatening
2. Prevention of hypoxic gas delivery — wrong gas supplied or pipeline crossover can cause catastrophic harm
3. Preventing oxygen toxicity — hyperoxia causes lung injury, absorptive atelectasis, retinopathy of prematurity, neonatal harm
4. Resource optimisation — oxygen is expensive; low-flow and closed-circuit anaesthesia reduce wastage
5. Regulatory compliance — hospitals and operating theatres are required to maintain documented evidence of safe gas supply and use
6. Anaesthetic machine checklist compliance — a core component of pre-use machine checks

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3. OXYGEN SOURCES IN ANAESTHESIA — THE SUPPLY CHAIN

A. Central Pipeline (Wall) Supply — Primary Source

• Hospital oxygen is supplied via a Bulk Liquid Oxygen (VIE — Vacuum-Insulated Evaporator) tank or oxygen concentrators (in low-resource settings)
• Delivered to theatre wall outlets via copper pipelines at 50–55 psig (3.4–3.8 bar)
• Connected to anaesthesia machine via DISS (Diameter Index Safety System) connectors (non-interchangeable, colour-coded)
• Wall pressure gauge must read ≥50 psig — audited daily

B. E-Cylinder (Backup/Reserve Source)

• Attached to anaesthesia machine via hanger yoke assembly with PISS (Pin Index Safety System)
• When full: pressure ≈ 2,000 psig (136 bar)
• Oxygen exists only as gas in cylinders (not liquid at room temperature) — obeys Boyle's Law
• Internal volume of E-cylinder: 4.8 L
• Volume available at 1 atm from a full E-cylinder:

Boyle's Law calculation:
P₁V₁ = P₂V₂
(2,014.7 psia × 4.8 L) = (14.7 psia × V₂)
V₂ = 658 L of oxygen at atmospheric pressure

Duration Estimation Formula (Barash's Clinical Anesthesia 9e):

Remaining time (hrs) = Cylinder pressure (psig) ÷ (200 × Flow rate [L/min])

⚠️ Critical caveat: A pneumatically-driven ventilator uses oxygen as its driving gas and can deplete a full E-cylinder in as little as 30 minutes. With manual/spontaneous ventilation and minimal FGF, the same cylinder may last several hours.

C. Cylinder Sizes and Capacities

D. Oxygen Concentrators (PSA — Pressure Swing Adsorption)

• Used in resource-limited settings
• Produce 90–96% oxygen from room air
• Cannot provide high-flow O₂ >5–10 L/min at full concentration
• Must be audited for output concentration and flow capacity

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4. SAFETY SYSTEMS — UNDERSTANDING FOR AUDIT

A. Pin Index Safety System (PISS)

• Prevents wrong cylinder from being attached to wrong yoke
• Each gas has a specific pin arrangement (O₂: pins 2 & 5)
• Audit check: Confirm correct pins, never force-fit cylinders; PISS failures have been reported — label verification is mandatory alongside PISS

B. Diameter Index Safety System (DISS)

• Non-interchangeable threaded connections for pipeline wall outlets
• Colour-coded: White = Oxygen (UK/international standard)
• Audit: Verify correct colour-coded connections at wall

C. Pipeline Pressure Gauge

• Must be visible on front of every anaesthesia machine
• Normal: 50–55 psig
• Audited daily; pressure <50 psig = suspect central supply problem

D. Oxygen Supply Failure Alarm (Fail-Safe Alarm)

• Audible + visual alarm triggered when O₂ pressure drops below manufacturer threshold
• ISO requirement for all anaesthesia machines
• Audit check: Alarm must be functional and tested during machine pre-use check

E. Oxygen Failure Cutoff ("Fail-Safe") Valve

• Located in gas line of every non-oxygen gas (N₂O, air)
• Shuts off N₂O/air supply when O₂ pressure falls below threshold (~20–30 psig)
• Protects against hypoxic mixture delivery
• Critical limitation: If hospital pipeline is contaminated (e.g., N₂O in O₂ pipeline), fail-safe valve remains open — only the oxygen analyser and clinical acumen protect the patient
• The term "fail-safe" is a misnomer — it does NOT guarantee a non-hypoxic mixture

F. Oxygen Flush Valve

• Delivers O₂ at 35–75 L/min directly to low-pressure circuit, bypassing flowmeters and vaporizer
• Audited as part of machine check: must be functional; should NOT be held open during ventilation (risk of barotrauma)

G. Proportioning Systems (Hypoxia Guard)

• Mechanically or electronically links O₂ and N₂O flowmeters
• Ensures minimum 25% O₂ in fresh gas flow when N₂O is used
• Examples: Link-25 (Ohmeda), S-ORC (Dräger)
• Audit: Verify proportioning system function if N₂O is in use

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5. OXYGEN ANALYSER — CORNERSTONE OF OXYGEN AUDIT

Importance

• The inspired O₂ concentration monitor is the primary patient-level safeguard against hypoxic mixture delivery
• Mandatory on all anaesthesia machines (ASA, AAGBI, ISO standards)
• The only device that detects pipeline contamination/crossover at the patient breathing system level

Types

Audit Requirements

• Calibrated to 21% (room air) and 100% O₂ before each use
• Low O₂ alarm set at minimum 18% FiO₂ (or per institutional standard)
• Position: placed on inspiratory limb of breathing circuit
• Document: calibration results, alarm limits set, any failures

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6. PRE-USE MACHINE CHECK — THE DAILY OXYGEN AUDIT

Key Oxygen-Specific Checks

E-Cylinder Management Rules (Audit Checklist)

1. Close cylinder valve during normal pipeline-supplied anaesthesia — prevents silent depletion
2. Open cylinder only when pipeline fails or not available
3. If both pipeline and cylinder connected with cylinder open — machine draws from pipeline preferentially (regulated cylinder pressure 40–45 psig < pipeline pressure 50–55 psig)
4. In suspected pipeline contamination: open E-cylinder AND disconnect wall source — failure to disconnect allows continued delivery of contaminated gas
5. If cylinder is primary source (remote site): ensure adequate volume for entire duration + extra; note additional consumption by pneumatic ventilator

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7. OXYGEN CONSUMPTION IN ANAESTHESIA — QUANTITATIVE AUDIT

Factors Determining Oxygen Consumption

Fresh Gas Flow Categories

Low-Flow Anaesthesia (LFA) — Key Oxygen Audit Concept

• FGF ≤1 L/min with circle absorber system
• Must monitor inspired O₂ concentration continuously (mandatory) — FiO₂ can fall unpredictably as N₂O dilutes O₂ at low flows
• At very low flows: inspired gas composition diverges from set FGF composition
• Minimum recommended FiO₂ during LFA: ≥0.3 (30%) to provide safety margin
• Closed-circuit anaesthesia: most economical; O₂ supplemented only to replace metabolic consumption (~250 mL/min) + circuit leak

Oxygen Conservation Strategies (Audit Targets)

1. Use lowest FiO₂ that maintains SpO₂ ≥94% (avoid routine 100% FiO₂)
2. Switch to low-flow or minimal-flow technique after initial equilibration phase
3. Use electrically powered ventilators (not pneumatic) when cylinders are the O₂ source
4. Avoid unnecessary O₂ flush valve activation
5. Target-controlled fresh gas flow: titrate FGF to end-tidal gas composition

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8. PIPELINE FAILURE & CROSSOVER — CRITICAL AUDIT SCENARIOS

Scenario 1: Oxygen Pipeline Pressure Failure

• Signs: Low O₂ pressure alarm sounds; O₂ flowmeter float falls; bellows may collapse
• Action:
  1. Open E-cylinder backup immediately
  2. Do NOT rely on fail-safe alarm alone to detect all forms of pipeline failure
  3. Call for engineering assistance
  4. Continue monitoring FiO₂ and SpO₂

Scenario 2: Pipeline Contamination / Gas Crossover (N₂O in O₂ line)

• Most catastrophic scenario — fail-safe valves remain open (as O₂ circuit is pressurised, albeit with N₂O)
• Only detector: inspired O₂ analyser falls, SpO₂ falls, patient hypoxic
• Action:
  1. Open E-cylinder valve
  2. Disconnect wall O₂ pipeline hose from machine (if not disconnected, contaminated gas continues to flow)
  3. Notify hospital engineering urgently
  4. Document as critical incident

Scenario 3: Silent E-Cylinder Depletion

• Occurs when cylinder valve left open during pipeline anaesthesia: pipeline fails → cylinder has already been silently depleted
• Audit prevention: Close cylinder valve after checking pressure; verify before each list

Scenario 4: Wrong Gas in Cylinder

• Prevented by PISS + label verification
• Tragic outcomes documented when PISS bypassed
• Audit action: Never accept unlabelled cylinders; always verify gas name on label AND colour-code

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9. OXYGEN TOXICITY — THE OTHER SIDE OF OXYGEN AUDIT

Mechanisms of Oxygen Toxicity

• Reactive Oxygen Species (ROS) generation: superoxide, hydroxyl radicals, hydrogen peroxide
• Overwhelm antioxidant defences (superoxide dismutase, catalase, glutathione peroxidase)
• Lipid peroxidation of cell membranes, DNA damage, protein oxidation

Clinical Manifestations

Audit Targets for Oxygen Titration

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10. CLINICAL OXYGEN AUDIT — PRESCRIPTION AND MONITORING

BTS (British Thoracic Society) Oxygen Audit Standards

1. Prescribed — written on drug chart with target SpO₂ range
2. Signed for by nursing staff on drug administration record
3. Monitored — SpO₂ documented; flow rate and delivery device documented
4. Titrated to target; escalated if not achieving target
5. Weaned as clinical condition improves

Oxygen Delivery Devices — Audit of FiO₂ Delivered

Audit Points

• Document: device, flow rate, FiO₂, target SpO₂ range, actual SpO₂
• Review for: unnecessary high FiO₂, failure to wean, no prescription, no SpO₂ monitoring

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11. CYLINDER MANAGEMENT AUDIT

Legal and Safety Requirements

• Cylinders must carry: full name of gas, chemical symbol, cylinder colour, batch number, test date
• Cylinders must be within hydrostatic test date (typically tested every 5–10 years)
• Full cylinders stored separately from empty ones
• Stored upright (or secured horizontally for large cylinders)
• Away from heat sources, oils, and flammable materials (O₂ is a powerful oxidiser)
• Never drop cylinders; handle with care

Cylinder Colour Codes (International/UK Standard — ISO 32)

Note: Many countries have transitioned to all-white cylinders for all medical gases (ISO 32:2019), with shoulder colour identifying gas type. Trainees should know both old and new systems.

Cylinder Audit Checklist

• Correct gas label and colour
• Within hydrostatic test date
• Valve cap/seal intact
• No visible damage or corrosion
• Pin Index Safety System intact
• Pressure gauge checked (O₂: should be ~2,000 psig when full)
• Valve opens smoothly (crack valve briefly before attachment to clear dust)
• Stored correctly (upright, chained, away from heat)

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12. STRUCTURED OXYGEN AUDIT TOOL FOR THEATRES

Domain 1 — Supply Infrastructure

• VIE tank level: documented and reviewed by pharmacy/engineering
• Pipeline pressure at manifold: 50–55 psig (verified on pressure gauge in plant room)
• Pipeline integrity: no known leaks; last pressure test date documented

Domain 2 — Machine-Level Audit (Per-Case)

• E-cylinder pressure documented before each list (machine check record)
• Pipeline pressure displayed and ≥50 psig
• O₂ analyser calibrated and alarm limits set
• Fail-safe alarm tested
• Oxygen flush valve checked
• Leak test performed and passed

Domain 3 — Intraoperative Monitoring

• SpO₂ monitored continuously
• FiO₂ measured continuously (O₂ analyser)
• Low O₂ alarm enabled (threshold ≤18%)
• FGF and FiO₂ documented at set intervals
• Any desaturations and FiO₂ changes documented

Domain 4 — Prescribing and Documentation

• O₂ prescribed with target SpO₂ range for peri-operative period
• Post-operative oxygen prescription documented
• Weaning plan documented

Domain 5 — Incident Reporting

• All O₂ supply failures reported as critical incidents
• Near-misses (e.g., empty cylinder found, pipeline pressure drop) documented
• Root cause analysis completed for significant events

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13. OXYGEN AUDIT IN RESOURCE-LIMITED SETTINGS

• Oxygen concentrators may be sole supply source — audit must include:
  • Output concentration (target >90%; audited with O₂ analyser)
  • Flow rate capacity
  • Power supply reliability; backup battery status
• Cylinder stock management: rotation system (FIFO — first in, first out)
• Documentation of cylinder usage per day/week
• Early warning system for stock running low (reorder level triggers)
• Training of staff on cylinder safety, connection, and colour codes

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14. HIGH-YIELD EXAM SUMMARY TABLE

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15. KEY EXAM POINTS

1. Oxygen is a drug — must be prescribed, documented, and titrated to a target SpO₂; not given "routinely" at high flow.
2. E-cylinder calculation: Boyle's Law; 658 L from full E-cylinder; use the duration formula.
3. Close the cylinder valve during pipeline anaesthesia to prevent silent depletion; open only on pipeline failure.
4. Fail-safe valves prevent hypoxic mixtures from low O₂ pressure — but NOT from pipeline crossover/contamination; only the O₂ analyser does.
5. Pipeline crossover management: open E-cylinder AND disconnect wall supply (merely turning off the machine is insufficient).
6. Low-flow anaesthesia requires mandatory continuous FiO₂ monitoring — inspired composition diverges from FGF composition.
7. Pneumatically driven ventilators consume O₂ as a driving gas — dramatically reduce cylinder duration.
8. Oxygen toxicity is dose-dependent: >0.5 FiO₂ for >24–48h causes pulmonary toxicity; neonates are at risk of ROP.
9. Absorptive atelectasis occurs at high FiO₂ — use lowest FiO₂ that maintains target SpO₂.
10. Cylinder audit: correct gas label + colour + PISS + test date + no damage + proper storage; never force-fit cylinders.

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