Diabetic Ketoacidosis (DKA): Pathophysiology, Management, and Perioperative Anaesthetic Management

MD Anaesthesia Examination Long Answer

PART 1 - DEFINITION AND DIAGNOSTIC CRITERIA

1. Hyperglycaemia - blood glucose > 250 mg/dL (though euglycaemic DKA is recognised, especially with SGLT2 inhibitors)
2. Ketosis - urine ketones 2+ or more, or serum beta-hydroxybutyrate > 3.0 mmol/L
3. Metabolic acidosis - arterial/venous pH < 7.3, serum bicarbonate < 18 mmol/L

PART 2 - PATHOPHYSIOLOGY

Central Mechanism: Insulin Deficiency + Counter-Regulatory Hormone Excess

INSULIN DEFICIENCY + GLUCAGON EXCESS
         |
    _____|______________________________
    |                                  |
LIVER                           ADIPOSE/MUSCLE
    |                                  |
Gluconeogenesis ↑             FFA release ↑ (hormone-sensitive lipase activated)
Glycogenolysis ↑              Amino acid release ↑ (proteolysis)
Ketogenesis ↑                 (amino acids --> liver --> gluconeogenic substrate)
(acetoacetate, β-hydroxybutyrate, acetone)
    |
HYPERGLYCAEMIA + KETONAEMIA

1. Hyperglycaemia and Osmotic Diuresis

• Glucose cannot enter insulin-dependent peripheral cells
• Hepatic glucose output unchecked (gluconeogenesis + glycogenolysis)
• Once renal threshold (~180 mg/dL) is exceeded, glycosuria occurs
• Osmotic diuresis pulls water, Na+, K+, Mg2+, Ca2+, PO4 into urine
• This leads to profound dehydration - average fluid deficit 3-5 L in adults (up to 100-120 mL/kg in children)

2. Ketogenesis

• Insulin deficiency activates hormone-sensitive lipase in adipose tissue
• Massive release of free fatty acids (FFAs) into circulation
• In the liver: FFAs undergo incomplete beta-oxidation → acetoacetate and β-hydroxybutyrate (strong acids)
• Acetone (from spontaneous decarboxylation of acetoacetate) produces the classic fruity breath
• In DKA, β-hydroxybutyrate dominates; the beta-hydroxybutyrate: acetoacetate ratio rises to 3:1 or higher (note: nitroprusside-based urine ketone strips detect only acetoacetate, so ketosis may be UNDER-estimated)
• Cellular starvation state also decreases peripheral ketone utilisation, worsening ketonaemia

3. Metabolic Acidosis and Respiratory Compensation

• Accumulation of ketoacids generates a high anion gap metabolic acidosis
• Anion Gap = Na - (Cl + HCO3); normal 8-12 mEq/L; elevated in DKA
• Respiratory compensation: Kussmaul breathing (deep, sighing respirations) - the body eliminates CO2 to buffer the acidosis
• Bicarbonate is consumed as a buffer
• With large volumes of normal saline (0.9% NaCl) resuscitation, a hyperchloraemic normal anion gap acidosis may supervene

4. Electrolyte Disturbances

5. Precipitating Factors (The "I's")

• Infection (most common - UTI, pneumonia, cellulitis)
• Insulin omission or inadequate insulin (non-adherence, pump failure)
• Infarction (myocardial, cerebral, mesenteric)
• Iatrogenic/Drugs - corticosteroids, SGLT2 inhibitors (euglycaemic DKA), atypical antipsychotics (clozapine, olanzapine), thiazides, sympathomimetics
• Illness - pancreatitis, trauma, surgery, burns
• Illicit drugs - cocaine
• Initial presentation of type 1 diabetes
• Endocrinopathies - Cushing's, thyrotoxicosis, acromegaly

PART 3 - CLINICAL FEATURES

History

• Polyuria, polydipsia, polyphagia (or anorexia), weight loss
• Nausea, vomiting, abdominal pain (can mimic acute abdomen - due to ileus or splanchnic vasoconstriction from acidosis; rarely pancreatitis)
• Weakness, lethargy, confusion
• Preceding illness, missed insulin doses

Physical Examination

• Dehydration: dry mucous membranes, sunken eyes, reduced skin turgor, decreased JVP
• Tachycardia and orthostatic hypotension (hypovolaemia)
• Kussmaul breathing: deep, rapid respirations
• Fruity/acetone breath
• Altered mental status, stupor, or coma (reflects hyperosmolality)
• Low-grade fever (infection may be masked or absent; hypothermia is a serious sign)
• Abdominal tenderness

Investigations

• Bedside: Blood glucose, urine dipstick (glucose 4+, ketones 2-4+)
• Blood gas: pH, pCO2, HCO3 (venous acceptable for monitoring)
• Serum electrolytes: Na, K, Cl, HCO3, urea, creatinine
• Anion gap calculation
• Serum/capillary ketones: beta-hydroxybutyrate >3 mmol/L diagnostic
• Full blood count: WBC elevated (metabolic acidosis per se, not always infection)
• Blood cultures, urine culture (identify precipitant)
• HbA1c (baseline glycaemic control)
• ECG: hyperkalemia/hypokalemia changes, exclude MI as precipitant
• Serum amylase/lipase: often elevated from non-pancreatic sources; misleading
• Chest X-ray: exclude pneumonia as precipitant
• Phosphate, magnesium, calcium

PART 4 - MANAGEMENT OF DKA

Principles: The "FRIED" Approach

• F - Fluids (rehydration)
• R - Replace electrolytes (K, PO4, Mg)
• I - Insulin infusion
• E - Eliminate precipitating cause
• D - Dextrose when glucose approaches 250 mg/dL

A. FLUID RESUSCITATION

1. Haemodynamic shock (SBP < 90 mmHg, HR > 120): Give isotonic crystalloid (0.9% NaCl or Balanced salt solution like Plasmalyte/Ringer's lactate) as rapidly as possible - boluses of 500 mL-1 L until haemodynamically stable
2. Marked dehydration without shock: 1 L of 0.9% NaCl in first hour, then reassess
3. General adult protocol: 2-3 L 0.9% NaCl over first 2-3 hours, then switch to 0.45% NaCl (half-normal saline) at 250-500 mL/h, guided by corrected Na, urine output

B. POTASSIUM REPLACEMENT

• ECG monitoring for signs of hypo/hyperkalaemia
• Once patient is eating and IV fluids are discontinued, switch to oral K+ supplementation

C. INSULIN THERAPY

1. Bolus: 0.1 unit/kg IV regular insulin bolus (optional - some protocols omit this)
2. Infusion: Regular insulin 0.1 unit/kg/hour IV infusion
3. Target: Decrease blood glucose by 50-75 mg/dL/hour
4. If glucose falls > 100 mg/dL/hour, halve the infusion rate
5. When blood glucose reaches 200-250 mg/dL: reduce infusion to 0.05 unit/kg/h AND add dextrose to IV fluids
6. Continue insulin infusion until:
   • pH > 7.30
   • HCO3 > 18 mEq/L
   • Anion gap < 12 mEq/L
   • Serum/urine ketones clearing

• When patient is eating, pH normalised, anion gap closed
• Administer first dose of subcutaneous basal insulin 2 hours before stopping the IV infusion (overlap) to prevent rebound ketoacidosis

D. BICARBONATE THERAPY

• May worsen hypokalaemia
• Paradoxical CSF acidosis (CO2 crosses blood-brain barrier more freely than bicarbonate)
• May delay closure of anion gap
• Consider only if: pH < 6.9, life-threatening hyperkalaemia, severe haemodynamic compromise

E. PHOSPHATE REPLACEMENT

• Routine phosphate replacement is NOT recommended
• Indicated if: serum phosphate < 1.0 mEq/L, or if severe symptomatic hypophosphataemia (respiratory muscle weakness, haemolysis, rhabdomyolysis)
• Replace as potassium phosphate

F. MONITORING

G. TREATMENT OF PRECIPITATING CAUSE

• Blood and urine cultures
• Broad-spectrum antibiotics if infection suspected
• ECG, troponin, echo if MI suspected
• Hold SGLT2 inhibitors
• Correct all causative factors

PART 5 - PERIOPERATIVE ANAESTHETIC MANAGEMENT OF A PATIENT WITH DKA POSTED FOR SURGERY

A. GENERAL PRINCIPLES

B. PREOPERATIVE ASSESSMENT AND OPTIMISATION

• Determine type and duration of diabetes (T1DM vs T2DM; risk of DKA recurrence in T1DM)
• Assess degree of DKA severity (mild/moderate/severe per criteria above)
• Assess for precipitating cause and surgical urgency
• Assess volume status: skin turgor, capillary refill, HR, BP, orthostatic changes
• Airway assessment: Critical in diabetics

• Diabetic autonomic neuropathy can cause:
  • Cardiovascular instability: hypotension on induction, reduced heart rate variability
  • Gastroparesis: increased aspiration risk (delayed gastric emptying - always treat as full stomach)
  • Impaired hypoglycaemia awareness: silent hypoglycaemia under anaesthesia
• Test for postural hypotension, resting tachycardia, HR response to Valsalva

• ABG (pH, pCO2, pO2, HCO3, lactate, SaO2)
• Blood glucose (hourly)
• Serum electrolytes (Na, K, Cl, HCO3, phosphate, magnesium)
• Anion gap
• Serum ketones/beta-hydroxybutyrate
• Full blood count, CRP (infection screen)
• Renal function (urea, creatinine) - prerenal AKI common
• HbA1c
• 12-lead ECG (silent MI as precipitant; hyperkalaemia/hypokalaemia changes)
• Chest X-ray (precipitating pneumonia; pulmonary oedema from aggressive resuscitation)
• Coagulation profile
• Group and crossmatch (if major surgery)
• Echocardiogram if cardiac dysfunction suspected

• Postpone until DKA fully resolved:
  • pH > 7.30, HCO3 > 18 mEq/L, anion gap closed
  • Blood glucose < 200 mg/dL
  • Serum K+ 3.5-5.0 mEq/L
  • Adequate rehydration (urine output, normal BUN:creatinine)
• Optimise HbA1c (ideally < 8%) - if > 8%, refer to endocrinologist for optimisation (though this may not always be feasible)
• Hold SGLT2 inhibitors 3-7 days before surgery (risk of euglycaemic DKA)
• Hold metformin (risk of lactic acidosis with contrast/renal impairment)
• Schedule as first case of the day to minimise fasting stress

• Start DKA resuscitation IMMEDIATELY - surgery should proceed alongside resuscitation, not after
• Liaise with surgical team and endocrinologist/intensivist
• Minimum targets before theatre if time permits:
  • K+ corrected to > 3.3 mEq/L (mandatory before insulin and before induction)
  • Volume resuscitation initiated, HR < 120, SBP > 90 mmHg
  • Blood glucose trending down with insulin infusion
  • Acid-base correction underway
• Anaesthesia should not be indefinitely delayed for emergency (life-threatening) surgical pathology

C. INTRAOPERATIVE MANAGEMENT

• Standard ASA/ACC monitoring: ECG, SpO2, NIBP, EtCO2, temperature
• Invasive arterial line (A-line): Essential for:
  • Continuous BP monitoring (haemodynamic instability expected)
  • Frequent ABG sampling (pH, pCO2, glucose, K+, lactate)
• Central venous line (CVP): For major surgery, guiding fluid resuscitation, vasopressor infusion
• Urinary catheter: Urine output monitoring - target 0.5-1 mL/kg/h
• Temperature probe: Hypothermia worsens acidosis and insulin resistance
• Blood glucose every 30-60 minutes intraoperatively

• Rapid sequence induction (RSI) is mandatory in ALL DKA patients:
  • Gastroparesis (even without obvious symptoms) delays gastric emptying
  • Vomiting + aspiration risk is high in obtunded/acidotic patient
  • Precautionary use of sodium citrate (30 mL PO) and metoclopramide if time permits
• Consider pre-oxygenation with head-up position
• Difficult airway preparedness: Have video laryngoscope, bougie, supraglottic airway, and surgical airway kit readily available (stiff joint syndrome, limited neck mobility)
• Avoid nasogastric/nasotracheal routes if coagulopathy or basal skull concerns

• Prefer haemodynamically stable agents given cardiovascular instability:
  • Ketamine (1-2 mg/kg IV): Cardiovascular stimulant, bronchodilator; caution in tachycardia
  • Etomidate (0.3 mg/kg IV): Minimal cardiovascular depression; note adrenal suppression
  • Propofol with caution: Causes vasodilation and myocardial depression; reduce dose in hypovolaemic patients
• Suxamethonium for RSI: Use only if K+ is within acceptable range (< 5.5 mEq/L). Suxamethonium raises serum K+ by ~0.5-1.0 mEq/L - potentially catastrophic in hyperkalaemic DKA. If K+ is high or unknown, use rocuronium 1.2 mg/kg with sugammadex immediately available
• Pre-treat with atropine if autonomic neuropathy causes bradycardia

• Either volatile (sevoflurane/isoflurane) or TIVA (propofol infusion)
• Avoid nitrous oxide (N2O) - potential for gut distension in ileus; increases PONV
• Opioid choice: titrate carefully; DKA patients may have altered opioid pharmacokinetics
• Mechanical ventilation strategy: Critical
  • If patient had Kussmaul breathing pre-operatively (compensatory respiratory alkalosis), the ventilator must replicate the pre-operative respiratory effort
  • Target EtCO2 at the pre-operative pCO2 (typically low, e.g. 20-25 mmHg in severe DKA)
  • If you allow pCO2 to rise to "normal" values (35-40 mmHg) with controlled ventilation, the respiratory compensation is abolished, and pH will DROP dramatically - this can precipitate cardiovascular collapse
  • Formula guidance: If pH 7.2 and pCO2 20, do not allow pCO2 > 25 mmHg
• Tidal volumes: 6-8 mL/kg IBW; avoid volutrauma/barotrauma

• Continue the pre-operative DKA resuscitation protocol:
  • IV regular insulin infusion at 0.1 units/kg/h (continue pre-operative rate)
  • IV fluid (0.9% NaCl or Plasmalyte) as the primary resuscitation fluid
  • Separate dextrose-containing fluid (5% dextrose or 10% dextrose) infused concurrently when glucose < 250 mg/dL - titrate independently of the insulin line
  • Potassium in fluids as per protocol
• Blood glucose targets intraoperatively: 140-180 mg/dL (ADA; most societies agree this range is safe)
• Tight glycaemic control (80-110 mg/dL) is NOT recommended intraoperatively - risk of iatrogenic hypoglycaemia is too high
• Avoid glucose-containing fluids (Ringer's lactate with dextrose, 5% dextrose as primary resuscitation) until glucose is controlled
• Use colloids (albumin) cautiously in massive haemorrhage
• Avoid excessive 0.9% NaCl - hyperchloraemic acidosis will worsen underlying DKA acidosis and confuse interpretation of ABG

• Warming blankets, warmed IV fluids, warm environment
• Hypothermia worsens acidosis and reduces insulin effectiveness

D. POSTOPERATIVE MANAGEMENT

• All DKA patients undergoing surgery require ICU-level care postoperatively
• Continue DKA protocol monitoring (hourly glucose, 2-4 hourly electrolytes, ABG)
• Continue insulin infusion until DKA criteria for resolution are met

• Full reversal of neuromuscular blockade (TOF ratio > 0.9)
• pH > 7.30 (patient must be able to maintain respiratory compensation post-extubation)
• Alert, following commands, airway reflexes intact
• Haemodynamically stable
• Adequate analgesia (PONV will interrupt oral intake and insulin resumption)

• Regional/neuraxial analgesia preferred when feasible (reduces opioid need, attenuates surgical stress response, reduces hyperglycaemia)
• Epidural analgesia attenuates the neuroendocrine stress response and may reduce the degree of perioperative hyperglycaemia
• NSAIDs with caution (renal impairment is common in DKA)
• Multimodal analgesia

• Critical - vomiting will prevent oral intake, perpetuate dehydration, and delay transition to subcutaneous insulin
• Use ondansetron, dexamethasone (note: dexamethasone raises glucose, monitor closely), scopolamine patch, droperidol
• Avoid neostigmine (increases secretions, nausea)

• When: Patient eating well, pH > 7.30, anion gap closed, ketones cleared
• Method: Administer first dose of long-acting subcutaneous insulin (e.g. glargine) 2 hours before stopping IV insulin infusion to prevent rebound ketoacidosis
• Resume pre-admission antidiabetic regimen or endocrinology-guided new regimen
• Continue monitoring glucose 4-hourly

• Hypoglycaemia (most dangerous under sedation)
• Rebound ketoacidosis (if insulin stopped prematurely)
• Cerebral oedema (especially in children - rapid osmotic shifts)
• Pulmonary oedema (overly aggressive fluid resuscitation in elderly/cardiac patients)
• Hypokalaemia leading to arrhythmia
• Aspiration pneumonitis
• Acute kidney injury (from dehydration + contrast + NSAIDs)
• Thromboembolism (hypercoagulable state in DKA)

PART 6 - SPECIAL CONSIDERATIONS

SGLT2 Inhibitor-Associated Euglycaemic DKA

• Increasingly recognised perioperatively
• Blood glucose may be NORMAL or mildly elevated (< 250 mg/dL) despite full DKA
• High index of suspicion in patients on dapagliflozin, empagliflozin, canagliflozin
• Hold SGLT2 inhibitors 3-7 days before elective surgery (FDA recommendation 2020)

Insulin Pumps (CSII - Continuous Subcutaneous Insulin Infusion)

• For short operations: may continue pump at basal rate if accessible and glucose > 110 mg/dL
• For major/long surgery: convert to IV insulin infusion
• Know the basal rate settings; correctional dosing set at glucose > 180 mg/dL
• Hourly glucose monitoring mandatory

DKA in Children

• Extra caution: risk of cerebral oedema (leading cause of DKA mortality in paediatrics)
• Slower rehydration (48 hours), careful sodium correction
• Avoid rapid falls in osmolality
• Perform head CT if GCS deteriorates during treatment

DKA in Pregnancy

• Higher risk, can occur at lower glucose levels (near-euglycaemic DKA)
• Fetal mortality 10-30% if untreated
• Maternal respiratory compensation (pCO2 baseline is already ~30 mmHg in pregnancy)
• Aggressive insulin and fluid therapy; fetal monitoring; involve obstetrics

SUMMARY TABLE: DKA Perioperative Anaesthetic Checklist

REFERENCES (Textbook Sources)

• Rosen's Emergency Medicine: Concepts and Clinical Practice, p. 2542-2548
• Goldman-Cecil Medicine (International Edition), p. 2483-2486
• Barash, Cullen and Stoelting's Clinical Anesthesia, 9e, p. 1779-1783
• Current Surgical Therapy, 14e, p. 1510-1512
• Szabó GV et al. Balanced electrolyte solutions vs 0.9% saline in DKA. [Diabetes Metab Res Rev, 2024 - PMID 38925619]
• Alnuaimi A et al. Subcutaneous vs continuous insulin infusion in DKA. [BMC Endocr Disord, 2024 - PMID 39090718]
• Thammakosol K et al. Early subcutaneous basal insulin with IV insulin in DKA. [Diabetes Obes Metab, 2026 - PMID 41208563]

Low Flow Anaesthesia (LFA)

MD Anaesthesia Examination - Long Answer

PART 1 - DEFINITIONS AND CLASSIFICATION

Classification by Fresh Gas Flow (FGF) Rate

PART 2 - PREREQUISITES AND EQUIPMENT

1. Leak-free Breathing Circuit

• The circle system must be gas-tight
• Any leaks cause dilution of the circuit gas mixture and hypoxia risk
• Pre-use leak test is mandatory

2. Circle System Components (must be intact and functional)

• CO2 absorber with fresh absorbent (soda lime / calcium hydroxide lime / lithium hydroxide-based)
• Inspiratory and expiratory unidirectional valves (prevent stagnant mixing)
• Reservoir bag
• Fresh gas inlet
• APL (adjustable pressure-limiting) valve
• Y-piece connector

3. Monitoring Equipment

• Inspired O2 analyser - mandatory (hypoxia is the primary danger)
• Capnograph (EtCO2) - to detect CO2 absorber exhaustion
• Volatile agent analyser (inspired + expired concentrations)
• Pulse oximetry - continuous SpO2
• Anaesthetic gas monitor capable of distinguishing agent identity (to detect CO accumulation)
• Carboxyhemoglobin monitor (if CO risk suspected)

4. Vaporiser Calibrated for High Concentrations

• At low FGF, the inspired vapour concentration rises slowly toward the vaporiser dial setting
• Higher vaporiser dial settings are needed initially to achieve target alveolar concentration
• For induction and early maintenance, higher FGF (3-4 L/min) is used temporarily, then reduced to low flow once equilibrium is established (typically after 10-15 minutes)

PART 3 - PHYSIOLOGY OF LOW FLOW ANAESTHESIA

The Rebreathing Fraction

• Exhaled volatile anaesthetic (still containing anaesthetic agent)
• Water vapour (37°C, fully saturated)
• Heat
• Exhaled CO2 (removed by the absorber before rebreathing)
• Nitrogen (slowly washes out from tissues)

Uptake at time t = Initial uptake × (1/√t)

Oxygen Consumption

• Basal O2 consumption in an anaesthetised adult ≈ 3 mL/kg/min ≈ 200-250 mL/min in a 70-kg patient
• At LFA flows of 0.5-1 L/min, the delivered O2 (typically 50% of FGF = 250-500 mL/min) must exceed this consumption at ALL times
• The minimum safe FGF must deliver more O2 than the patient consumes

PART 4 - ADVANTAGES OF LOW FLOW ANAESTHESIA

A. ECONOMIC ADVANTAGES

• At high FGF, over 80% of delivered volatile agent is wasted into the scavenging system
• Miller's 10e quantifies: at moderately high FGF, delivered sevoflurane is 7.2× greater than absorbed drug; delivered isoflurane is 4.5× greater than absorbed drug
• At LFA, rebreathed anaesthetic recirculates and is re-utilised, dramatically reducing consumption
• Cost savings of up to 60-80% on volatile agent expenditure

• Less O2 and N2O wasted to atmosphere
• Reduced need for medical gas pipeline usage

• Lower gas volumes passing through the scavenging system
• Less wear on active scavenging systems

B. ENVIRONMENTAL ADVANTAGES

• Volatile anaesthetics are potent greenhouse gases with global warming potentials (GWP over 100 years) far exceeding CO2:
  • Desflurane: GWP = 2,540
  • Isoflurane: GWP = 510
  • Sevoflurane: GWP = 130
  • N2O: GWP = 265
  • CO2 (reference): GWP = 1
• One MAC-hour of desflurane at standard FGF is equivalent to driving ~190 car miles of CO2 emissions; sevoflurane ≈ 4 miles; isoflurane ≈ 8 miles
• Healthcare produces 5-8% of global greenhouse gas emissions; inhaled anaesthetics account for ~3% of healthcare's climate footprint and up to 50% of the climate impact of surgical care
• LFA reduces atmospheric emissions proportionally with flow reduction

• N2O destroys stratospheric ozone
• LFA minimises N2O waste

C. PHYSIOLOGICAL/PATIENT BENEFITS

• Exhaled gas is warm (body temperature, 37°C)
• At high FGF, cold dry fresh gases constantly enter the circuit, causing heat loss
• At LFA, the majority of gas recirculates and retains patient heat
• Reduces perioperative hypothermia - a major cause of complications (shivering, coagulopathy, cardiac events, surgical site infection, delayed recovery)
• The CO2 absorption reaction itself is exothermic, adding warmth to rebreathed gas

• Exhaled gas is fully saturated with water vapour (humidity ≈ 100% at 37°C)
• At LFA, this humidity is preserved in the rebreathed gas
• Benefits:
  • Maintains mucociliary function (ciliary transport fails when humidity < 50%)
  • Prevents drying of bronchial secretions and mucus plugging
  • Protects airway epithelial health and reduces postoperative respiratory complications
  • Reduces need for separate heat-moisture exchangers (HME) or heated humidifiers

• Once equilibrium is established (~15 minutes), circuit concentration closely mirrors alveolar concentration
• Small changes in vaporiser setting produce gradual, predictable changes in anaesthetic depth
• Less likelihood of sudden anaesthetic overdose compared to high-flow systems
• The buffering capacity of large-volume gas in the circuit stabilises concentration

• Lower gas waste means less ambient contamination of the OR environment
• Chronic occupational exposure to trace anaesthetic gases is reduced for OR personnel
• LFA with effective scavenging is the most complete solution to theatre pollution

D. MONITORING BENEFITS

• Gas analysers measure actual inspired and end-tidal agent concentrations
• Provides real-time pharmacokinetic data on uptake and distribution
• Enables anaesthetist to titrate to effect with direct feedback

PART 5 - DISADVANTAGES AND HAZARDS OF LOW FLOW ANAESTHESIA

A. HYPOXIA RISK (Most Critical Hazard)

• Nitrogen is slowly released from body tissues and dissolved in plasma
• At closed/very low circuit flows, N2 accumulates in the breathing circuit
• Can dilute the inspired O2 concentration below safe levels

• If FGF O2 fraction is inadequate or metabolic demand rises (e.g., MH, hyperthermia), inspired O2 can fall dangerously
• The anaesthetist must continuously calculate: FGF (L/min) × FiO2 > O2 consumption (~200-250 mL/min)

• Mandatory: calibrated O2 analyser with alarm in the inspiratory limb

B. ACCUMULATION OF TOXIC GASES

1. Low FGF / closed circuit (most important - concentrates compound A in recirculating gas)
2. Higher sevoflurane concentrations
3. Warm or desiccated absorbent
4. KOH/NaOH-containing absorbents (Baralyme > soda lime; newer KOH/NaOH-free absorbents generate negligible amounts)

• Desiccated strong-base CO2 absorbents (with KOH/NaOH) degrade volatile agents to clinically significant CO
• Carboxyhemoglobin levels up to 35% have been reported
• The typical scenario is first case Monday morning after high FGF left flowing over weekend, completely desiccating the absorbent
• CO production ranking (greatest to least): Desflurane ≥ Enflurane > Isoflurane >> Halothane = Sevoflurane
• At low FGF, CO can accumulate in recirculating gas without dilution

1. Turn off anaesthesia machine at end of day (most important step)
2. Change absorbent if found dry at morning check
3. Use KOH/NaOH-free absorbents
4. Rehydrate desiccated absorbent

• N2 is slowly released from tissues during long anaesthetics
• Accumulates in the closed/near-closed circuit
• Dilutes O2 and volatile agent concentration
• Prevention: occasionally "flush" the circuit with high FGF to wash out N2

C. SLOW CHANGES IN ANAESTHETIC DEPTH

• At very low FGF, vaporiser changes are buffered by the large volume of recirculating gas
• Advantage in stability becomes a disadvantage in responsiveness
• It may take 10-20 minutes for a significant change in vaporiser setting to produce a meaningful change in alveolar concentration
• This limits the anaesthetist's ability to rapidly deepen or lighten anaesthesia
• Critical in situations requiring rapid adjustment (surgical stimulation, awareness, haemodynamic instability)
• Partial remedy: temporarily increase FGF when rapid change is needed

D. TECHNICAL COMPLEXITY AND DEMANDS

• Gas-tight leak-free circle system
• Multiple gas analysers (O2, CO2, volatile agent, ideally CO)
• Calibrated FGF flowmeters with accuracy at low flows

• Exhausted absorbent causes CO2 rebreathing (hypercapnia)
• Colour indicator (purple/violet when exhausted) must be checked regularly
• At LFA, absorbent is consumed faster per unit volume than at high flow
• Failure of absorbent: rising EtCO2 and inspired CO2 - treat by increasing FGF and changing absorbent

• Some older variable bypass vaporisers may not deliver accurate output at very low FGF
• Calibration testing at low flows should be confirmed

• Initial denitrogenation requires high FGF for the first 5-10 minutes
• The transition from high to low flow must be timed correctly

E. NOT SUITABLE FOR SPECIFIC CLINICAL SCENARIOS

PART 6 - PRACTICAL PROTOCOL FOR LFA

Step 1 - Pre-anaesthetic Machine Check

• Check for circuit leaks (mandatory)
• Confirm CO2 absorbent is fresh (white colour in most soda lime types)
• Verify O2 analyser calibrated and alarm set
• Ensure gas analyser (agent/CO2/O2) functioning
• Turn off machine after last case; change absorbent if overnight flow suspected

Step 2 - Induction Phase (High Flow)

• Use FGF of 4-6 L/min with 50-66% O2 in N2O or O2/air
• Achieve target inspired volatile agent concentration rapidly
• Flush N2 out of circuit (denitrogenation)
• Continue high flow for 10-15 minutes until circuit equilibration

Step 3 - Reduction to Low Flow

• Once inspired and expired volatile concentrations are within 10-20% of each other, reduce FGF to 0.5-1 L/min
• Increase vaporiser dial setting to compensate for lower delivery
• Monitor inspired O2 and EtCO2 continuously
• Check inspired agent concentration to ensure awareness is not possible

Step 4 - Maintenance

• Titrate vaporiser to maintain desired expired anaesthetic concentration (typically 0.7-1.0 MAC)
• Check O2 analyser and EtCO2 every 5-10 minutes
• Anticipate slow changes with vaporiser adjustments
• Temporarily increase FGF when rapid depth changes needed

Step 5 - Recovery

• Increase FGF to high flow (6-8 L/min with 100% O2) 5-10 minutes before end of anaesthesia
• Flushes residual anaesthetic and CO2 from circuit
• Accelerates emergence

PART 7 - SUMMARY TABLE

PART 8 - CONTEMPORARY RELEVANCE (Exam Highlight)

1. Sustainability: Anaesthesiology sustainability checklists (ASA) target FGF of ~0.7-1 L/min during maintenance
2. Modern absorbents: KOH/NaOH-free formulations (Amsorb, Litholyme, Drägersorb Free) eliminate compound A and CO generation, removing the two major chemical hazards
3. Gas analyser technology: Modern multigas monitors make real-time inspired O2, CO2, and agent monitoring reliable and affordable
4. Desflurane phase-out: Given its extreme GWP (2,540), desflurane has been largely abandoned in many countries; sevoflurane at LFA is now the dominant maintenance technique

REFERENCES

• Miller's Anesthesia, 2-Volume Set, 10e - Chapter 18 (Uptake and Distribution) and Chapter 29 (Breathing Systems), pp. 1961-1963, 2221-2222, 2344-2345
• Barash, Cullen & Stoelting's Clinical Anesthesia, 9e - Chapter 18 (Volatile Anaesthetics) and Chapter 25 (Anaesthesia Machines), pp. 1444-1448, 2007
• Morgan & Mikhail's Clinical Anesthesiology, 7e - Chapter 3 (Breathing Systems), pp. 89-91