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Pediatric Anesthesia for Adenotonsillectomy: Viva Overview

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1. INDICATIONS FOR ADENOTONSILLECTOMY

• Recurrent/chronic tonsillitis (most common) - typically defined as >7 episodes/year or >5/year for 2 consecutive years
• Peritonsillar abscess
• Tonsillar/adenoidal hyperplasia with airway obstruction
• Obstructive Sleep Apnea Syndrome (OSAS) - most important anesthetic concern
• Failure to thrive, speech disorders, dental abnormalities from chronic obstruction
• Cardiac valvular disease with recurrent streptococcal bacteremia
• Malignant disease (lymphoma - important caveat for dexamethasone use)

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2. PREOPERATIVE EVALUATION - KEY VIVA POINTS

• Size of tonsils and adenoids
• Degree of nasopharyngeal obstruction
• Mouth opening, neck mobility

• Snoring, witnessed apneas, restless sleep, daytime somnolence, nocturnal enuresis, behavioral changes
• The "gold standard" is polysomnography (PSG)
• Apnea-Hypopnea Index (AHI): severe OSA = AHI >10 AND oxygen saturation nadir <80%
• Barash: "Despite only mild-to-moderate tonsillar enlargement, these patients have upper airway obstruction while awake and apnea during sleep - do not underestimate severity based on tonsillar size alone"

• Cor pulmonale - right ventricular hypertrophy, cardiomegaly
• Pulmonary hypertension, systemic hypertension
• ECG for RVH; CXR for cardiomegaly
• Each apneic episode spikes pulmonary artery pressure further

• Active fever or productive cough = grounds for postponement
• Recent URTI within 2-4 weeks increases risk of laryngospasm, bronchospasm

• History of bleeding, family bleeding history
• Platelet count, clotting screen if indicated

• Obese children: especially high risk for OSA, opioid sensitivity, respiratory arrest post-op (Miller 10e, Morgan 7e, Barash 9e)

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3. PEDIATRIC AIRWAY - ANATOMICAL DIFFERENCES (VIVA FAVORITE)

• (Age/4) + 4 for uncuffed; (Age/4) + 3.5 for cuffed
• Confirm with air leak at 15-20 cmH2O (uncuffed) or cuff pressure <20 cmH2O

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4. ANESTHETIC INDUCTION

• Sevoflurane is the agent of choice - greater therapeutic index than halothane
• MAC of sevoflurane is highest in infants (2.5%) vs adults (2.0%)
• Inhalational induction followed by IV cannulation
• Glycopyrrolate 10 mcg/kg IV after IV access - reduces secretions, attenuates bradycardia

• Propofol 3-4 mg/kg + fentanyl 1-2 mcg/kg
• Ketamine useful if haemodynamically compromised

• MAC is highest in infants (not neonates), decreasing with age
• Exception: sevoflurane MAC shows no difference between neonates and infants
• Physiological reason: higher alveolar ventilation/FRC ratio, greater cardiac output to vessel-rich group = faster wash-in of volatile agents (Morgan & Mikhail 7e)

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5. AIRWAY DEVICE

1. Oral RAE (Ring-Adair-Elwyn) preformed ETT - most commonly used; taped in the midline to mandible; keeps tube out of surgical field; risk of kinking with mouth gag
2. Wire-reinforced (armored) ETT - resistant to kinking
3. Flexible LMA - used in some centers; kinking/displacement with Boyle-Davis gag is a concern; requires experienced team; advantage of avoiding laryngoscopy

• Tube taped in midline of mandible (not corner of mouth)
• After gag placement: always recheck tube position - kinking or displacement is not uncommon
• ETT cuff inflation pressure must be checked after position change (neck extension for surgery)
• Throat pack placement by surgeon - mandatory documentation, must be removed before extubation (Miller 10e, p.9812; Barash 9e)

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6. MAINTENANCE OF ANESTHESIA

• Either volatile (sevoflurane/isoflurane) or TIVA (propofol infusion) - both acceptable
• TIVA preferred by some due to reduced PONV, no theatre pollution, cleaner emergence
• Opioids: Fentanyl or morphine (use cautiously in OSA - reduce dose by up to 50%)
• Dexamethasone 0.15-0.5 mg/kg IV: reduces PONV, reduces postoperative swelling, improves oral intake
  • CONTRAINDICATED if lymphoma is suspected - may cause lethal hyperkalemia from tumor lysis
• NSAIDs (e.g., ketorolac, ibuprofen): good evidence for analgesia without increased bleeding risk, though some surgeons avoid
• Paracetamol/acetaminophen: inadequate alone for tonsillectomy pain
• Ondansetron for PONV prophylaxis
• Local anaesthetic infiltration to tonsillar bed (with or without epinephrine) by surgeon - caution: injection into tonsillar vessels beneath tonsillar bed can cause seizures or cerebral infarction (Miller 10e; Barash 9e, p.10647)

• Theoretical concern about platelet dysfunction and post-tonsillectomy haemorrhage
• Evidence: bleeding rate NOT significantly increased with NSAIDs; most guidelines support their use
• Individual surgeon/institutional preference governs

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7. OSA AND OPIOID SENSITIVITY - CRITICAL VIVA POINT

• Chronic intermittent hypoxemia upregulates central opioid receptors
• Altered CO2 response curves - blunted hypercapnic drive
• Risk of post-op respiratory arrest even with standard doses
• Reduce opioid dose by up to 50% in OSA patients (Barash 9e, Key Point 3)
• Maximize non-opioid multimodal analgesia

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8. EXTUBATION - DEEP vs. AWAKE

• Patient regains protective airway reflexes
• Reduces risk of airway obstruction post-extubation
• Risk: coughing/bucking on tube may trigger bleeding from surgical site

• Smoother emergence, less coughing/straining
• Reduces bleeding risk from Valsalva-like maneuvers
• Risk: airway obstruction - must be managed in high-dependency environment
• Only appropriate if very experienced team and environment capable of rapid detection/management of obstruction

• Lignocaine (lidocaine) 1.5 mg/kg IV or topical down ETT (cuff deflated)
• Light remifentanil infusion during emergence
• Suction oropharynx thoroughly + remove throat pack before extubation
• Pass orogastric tube to decompress stomach (swallowed blood/secretions) (Miller 10e, p.9812)

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9. COMPLICATIONS

A. Post-Tonsillectomy Haemorrhage (PTH) - MOST FEARED COMPLICATION

1. Volume resuscitate first - IV access, fluids, blood if needed; hypovolemia reduces induction drug requirement - consider etomidate
2. Treat as full stomach - significant swallowed blood in stomach
3. Rapid sequence induction (RSI) with cricoid pressure - mandatory
4. Prepare for difficult airway - bleeding + swelling + anxious child = previously easy laryngoscopy can quickly become impossible
   • Have multiple laryngoscope sizes/blades available
   • Videolaryngoscope on standby
   • Have smaller ETT sizes ready
   • Vigorous suction needed during laryngoscopy
5. Surgeon must be present and scrubbed before induction

B. Laryngospasm

• Most common airway complication in pediatric anesthesia
• Risk factors: light anesthesia, secretions/blood, recent URTI
• Treatment: CPAP, jaw thrust, deepen anesthesia (propofol 0.5-1 mg/kg), suxamethonium 1-2 mg/kg IV (or 4 mg/kg IM) if complete

C. Negative Pressure Pulmonary Edema (NPPE)

• Occurs after sudden relief of chronic upper airway obstruction
• Mechanism: high negative intrapleural pressures (-30 cmH2O in obstruction vs. -10 cmH2O normally) increase venous return and disrupt pulmonary microvasculature
• Frothy pink fluid in ETT, SpO2 drop, bilateral interstitial infiltrates on CXR
• Prevention: apply moderate CPAP during induction to allow circulatory adaptation
• Treatment: supportive - oxygen, CPAP/PEEP, diuretics; usually resolves within 24 hours (Barash 9e, p.4094-4095)

D. Post-operative Nausea & Vomiting (PONV)

• Very common - multimodal prophylaxis essential
• Dexamethasone + ondansetron + TIVA where possible

E. Airway Obstruction Post-extubation

• Especially with residual tonsillar tissue, edema, or in OSA children
• Lateral "tonsil position" (lateral decubitus) for recovery

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10. OSA - POSTOPERATIVE DISPOSITION

• Age <3 years
• Severe OSA (AHI >10, SpO2 nadir <80%)
• Cor pulmonale / cardiac abnormality
• Failure to thrive / poor oral intake
• Craniofacial abnormality
• Neuromuscular disorder
• Obesity (BMI >95th percentile)
• Sickle cell disease
• Post-op SpO2 <80% in recovery
• Bleeding concerns
• Lives far from hospital / no reliable carer

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11. PHARMACOLOGY REMINDERS (VIVA SPECIFICS)

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12. SURGICAL SETUP - VIVA POINTS

• Position: supine, head ring, neck extended (shoulder roll)
• Mouth gag: Boyle-Davis gag - risk of ETT/LMA displacement, kinking; ALWAYS recheck after placement
• Throat pack: surgeon places; anaesthetist must document and ensure removal before extubation ("count" system)
• Shared airway: requires constant communication between surgeon and anaesthetist

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SUMMARY ALGORITHM FOR THE CASE

Pre-op: Screen for OSA severity → Echo/ECG if cor pulmonale → Check coags → Postpone if active URTI/fever
Induction: Inhaled sevoflurane → IV access → glycopyrrolate → RSI if full stomach/PTH
Airway: Oral RAE ETT (midline taped) or flexible LMA
Maintenance: Volatile or TIVA + reduced-dose opioids (OSA) + dexamethasone + paracetamol + NSAID
Intraop: Check ETT after gag placement; monitor EtCO2, SpO2, temp
End of case: Remove throat pack + suction oropharynx + OGT to empty stomach
Extubation: Awake preferred; deep only if skilled team; lateral recovery position
Post-op: Multimodal analgesia; PONV prophylaxis; admit if OSA criteria met
Complication readiness: PTH → RSI protocol; Laryngospasm → sux protocol; NPPE → CPAP/PEEP

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• Miller's Anesthesia 10e, Ch. 27 & 29 - Tonsillectomy and Adenoidectomy
• Morgan & Mikhail's Clinical Anesthesiology 7e, Ch. 54 - Pediatric Anesthesia
• Barash/Stoelting's Clinical Anesthesia 9e, Ch. 48 - Anesthesia for Pediatric ENT Surgery

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Pediatric Adenotonsillectomy: Complete Practical Anesthesia Reference

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STEP 1 - WEIGHT ESTIMATION (If actual weight unknown)

Most adenotonsillectomies are done in 3-10 year olds (peak 4-8 years). Use three worked examples: 4 yr/16 kg, 6 yr/22 kg, 10 yr/32 kg.

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STEP 2 - HEIGHT & BSA (Quick Reference)

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STEP 3 - BREATHING CIRCUITS

< 20 kg → Mapleson F (Jackson-Rees modification of Ayres T-piece)

• Why: Low resistance, no valves, suitable for small tidal volumes, allows IPPV and spontaneous breathing
• Fresh Gas Flow (FGF) for spontaneous breathing: 2-3 × Minute Volume (or ~200-300 mL/kg/min)
• FGF for IPPV: 1000 mL + 100 mL/kg/min
• Minute volume in child = 100-150 mL/kg/min (TV 8-10 mL/kg × RR 20-30)

• Tidal Volume = 8 mL/kg = 128 mL
• Respiratory rate = 22/min
• Minute volume = ~2.8 L/min
• FGF (spontaneous) = 3 × 2.8 = ~8-9 L/min
• FGF (IPPV) = 1000 + (100×16) = 2600 mL/min = 2.6 L/min minimum

> 20 kg → Circle System (Paediatric/Adult Circle Absorber)

• CO2 absorber (soda lime) used; lower FGF acceptable (1-2 L/min)
• Can use adult circle with smaller tidal volumes

Summary Rule:

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STEP 4 - AIRWAY EQUIPMENT (Age/Weight Based)

Endotracheal Tube Size

For adenotonsillectomy: Oral RAE tube preferred (preformed, keeps tube out of surgical field). Order size of RAE = same formula. Always have 0.5 mm smaller available.

• Cuffed ETT is now standard and safe in children >1 year (Microcuff low-pressure cuff)
• Confirms cuff pressure <20 cmH2O
• Advantage for adenotonsillectomy: protects against blood/secretion aspiration from surgical field

ETT Depth (Oral)

• (Age/2) + 12 cm at lips
• Confirm with bilateral auscultation + CXR if uncertain

Laryngoscope Blade

LMA Size (if used)

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STEP 5 - IV CANNULA SIZE

Rule of thumb for viva: "In children <20 kg, we use a 22G or 24G cannula; in children >20 kg, we upsize to 20G. For adenotonsillectomy, a single IV access is usually sufficient, but 2 IV lines are prudent in OSA children or anticipated difficult cases."

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STEP 6 - ESTIMATED BLOOD VOLUME (EBV) & MABL

EBV by Age Group

MABL Formula

MABL = EBV × (Starting Hct - Target Hct) / Starting Hct

• Target Hct for healthy children = 20-25% (i.e., Hb ~7-8 g/dL)
• Use 25% as target for OSA children (higher O2 carrying capacity needed)

Worked Examples:

• EBV = 70 × 16 = 1120 mL
• MABL = 1120 × (36-25)/36 = 1120 × 0.306 = ~342 mL

• EBV = 70 × 22 = 1540 mL
• MABL = 1540 × (38-25)/38 = 1540 × 0.342 = ~527 mL

• EBV = 70 × 32 = 2240 mL
• MABL = 2240 × (38-25)/38 = 2240 × 0.342 = ~766 mL

For adenotonsillectomy: blood loss is usually 50-100 mL, well within MABL. MABL becomes critical in post-tonsillectomy haemorrhage (PTH) where significant concealed loss (swallowed blood) has already occurred before the child reaches theatre.

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STEP 7 - FLUID MANAGEMENT (4-2-1 Rule)

Maintenance Fluids (Holliday-Segar / 4-2-1 Rule)

• 16 kg child: (4×10) + (2×6) = 40 + 12 = 52 mL/hr
• 22 kg child: (4×10) + (2×10) + (1×2) = 40 + 20 + 2 = 62 mL/hr
• 32 kg child: (4×10) + (2×10) + (1×12) = 40 + 20 + 12 = 72 mL/hr

Fluid deficit (from NPO period)

Deficit = Maintenance rate × hours of fasting Give 50% in first hour, then 25%/25% over next 2 hours

Fluid choice:

• Isotonic balanced crystalloid (Hartmann's / Lactated Ringer's) = standard
• Normal saline AVOIDED as first choice (hyperchloremic acidosis risk)
• Dextrose-containing fluids only in neonates/infants at risk of hypoglycemia

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STEP 8 - DRUG DOSES (Complete Viva Chart)

A. INDUCTION AGENTS

B. MUSCLE RELAXANTS

Note: Adenotonsillectomy does not routinely require muscle relaxants - most cases done with spontaneous breathing or IPPV using volatile ± remifentanil.

C. ANALGESICS

D. ANTIEMETICS

E. ANTICHOLINERGICS / ANTISIALAGOGUES

F. EMERGENCY DRUGS

G. MAINTENANCE INFUSIONS

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STEP 9 - COMPLETE WORKED EXAMPLE (Viva Model Answer)

"6-year-old girl, 22 kg, 116 cm, presenting for adenotonsillectomy for OSA"

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STEP 10 - MONITORING SETUP

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QUICK-RECALL CARD (Exam Summary)

WEIGHT:  (Age×2)+8 for <5 yr; (Age×3)+7 for 6-12 yr
CIRCUIT: <20 kg → Jackson-Rees (Mapleson F); >20 kg → Circle
ETT:     Uncuffed = (Age/4)+4; Cuffed = (Age/4)+3.5
DEPTH:   (Age/2)+12 cm at lips
LMA:     <10 kg=1.5; 10-20 kg=2; 20-30 kg=2.5; 30-50 kg=3
CANNULA: <10 kg=24G; 10-20 kg=22G; >20 kg=20G
EBV:     Neonate=90 mL/kg; Infant=80 mL/kg; Child=70-75 mL/kg
MABL:    EBV × (start Hct - target Hct) / start Hct
FLUIDS:  4 mL/kg for 1st 10 kg + 2 for 2nd 10 kg + 1 beyond
FENTANYL: 1-2 mcg/kg (HALVE in OSA)
SUXAMETHONIUM: 1.5-2 mg/kg IV; 4 mg/kg IM (laryngospasm)
DEXAMETHASONE: 0.15-0.5 mg/kg (AVOID if lymphoma suspected)
ATROPINE: 0.02 mg/kg (min 0.1 mg)

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Pulse Oximetry & Capnography: Complete Viva Reference

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PART A: PULSE OXIMETRY

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1. DEFINITION & PRINCIPLE

• The value measured is called SpO2 (from pulse oximetry) to distinguish from SaO2 (directly measured arterial blood gas)
• Accuracy: SpO2 is within ±2-3% of SaO2 over the range of 70-100% (Miller's Anesthesia 10e, p.5456)

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2. PHYSICS & HOW IT WORKS

Two wavelengths used:

AC/DC Separation - the key innovation:

• DC component (non-pulsatile, "direct current"): Venous blood, capillary blood, tissue, bone, pigment - all constant
• AC component (pulsatile, "alternating current"): Arterial blood only - changes with each heartbeat

R = (AC660/DC660) / (AC940/DC940)

Physical construction:

• Two LEDs (660 nm + 940 nm) that flash sequentially
• One photodetector on opposite side (transmission) or same side (reflectance)
• When both LEDs off: ambient light subtracted from signal automatically

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3. FIVE SPECIES OF HAEMOGLOBIN (Critical Viva Point)

Functional SaO2 = O2Hb / (O2Hb + deO2Hb) × 100

Pulse co-oximetry (e.g., Masimo Rad-7) uses 8 wavelengths and can measure COHb and MetHb directly.

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4. THE OXYHEMOGLOBIN DISSOCIATION CURVE (OHC)

SpO2 (%)
100 |----___
 98 |       \
 90 |        \   ← STEEP PART: small ΔPO2 = big ΔSpO2
 75 |         \
 50 |          |  P50 = 27 mmHg
    |__________|___
              PaO2 (mmHg)

1. Flat upper portion (PaO2 >70 mmHg): SpO2 cannot detect hyperoxia - dangerous in premature neonates (retinopathy of prematurity risk)
2. Steep lower portion: Once SpO2 starts falling rapidly, PaO2 is already critically low - pulse oximetry gives late warning
3. SpO2 of 90% corresponds to PaO2 ~60 mmHg (onset of respiratory failure)
4. SpO2 of 75% corresponds to PaO2 ~40 mmHg (mixed venous level)

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5. PHOTOPLETHYSMOGRAPHY - BONUS VIVA POINT

• Shows arterial blood volume changes with each heartbeat
• Respiratory variation in PPG amplitude (ΔPOP / PVI) predicts fluid responsiveness in mechanically ventilated patients
• PVI (Pleth Variability Index) - commercially available on Masimo devices
• More reliable in mechanically ventilated than spontaneously breathing patients (Miller's Anesthesia 10e, p.5461)

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6. PROBE SITES

In vasopressor-dependent patients: use ear or forehead probe - less vasoconstriction at these sites compared to fingers (Miller's Anesthesia 10e, p.5469-5470)

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7. LIMITATIONS & SOURCES OF ERROR

A. Causes of FALSELY HIGH SpO2 (reads higher than true SaO2)

B. Causes of FALSELY LOW SpO2 (reads lower than true SaO2)

C. Causes of INACCURATE or NO READING

KEY VIVA POINT - Racial bias: Studies (including Sjoding et al., NEJM 2020) showed pulse oximeters overestimate SpO2 in patients with dark skin by ~3.5%, potentially masking hypoxemia. This is an active area of regulatory review by FDA.

D. Calibration limit

• Calibration curves only validated down to SpO2 70% - accuracy below 70% is unknown

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8. CLINICAL APPLICATIONS

• Intraoperative monitoring (mandatory standard ASA/AAGBI)
• Sedation monitoring (procedural sedation, endoscopy, ICU)
• PACU / ward monitoring
• Transport monitoring
• Newborn screening for critical congenital heart disease (detects pre-ductal vs post-ductal difference)
• Pulse oximetry during COVID-19: enabled remote monitoring
• Fluid responsiveness assessment (via PPG waveform variation)

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PART B: CAPNOGRAPHY

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1. DEFINITIONS

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2. PHYSICAL PRINCIPLE

• CO2 absorbs infrared radiation at 4.28 μm wavelength
• The amount of IR absorbed is proportional to CO2 concentration (Beer-Lambert Law)
• A light source emits IR; the detector measures transmission; CO2 concentration calculated from absorption

Two system types:

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3. THE CAPNOGRAM WAVEFORM - FOUR PHASES

CO2
(mmHg)
45  |        __________D
    |       /          \
    |      /C           \
    |     /              \
  0 |____/A   B           \E___
    |←expiration→|←inspiration→|
      I  II  III              IV

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4. PaCO2 - EtCO2 GRADIENT

• Normal gradient = 2-5 mmHg (PaCO2 slightly higher than EtCO2)
• EtCO2 is always ≤ PaCO2 in healthy lungs
• Why? Because EtCO2 represents a mixture of gas from different alveoli including those with V/Q mismatch and dead space

The gradient INCREASES when dead space increases:

• Pulmonary embolism (major cause - reduced perfusion, high V/Q)
• Pulmonary hypotension
• Overdistended alveoli (high PEEP)
• Low cardiac output
• Haemorrhage / hypovolaemia

AVDSf = (PaCO2 - EtCO2) / PaCO2

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5. CAUSES OF CHANGES IN EtCO2 - COMPLETE TABLE

A. INCREASED EtCO2

B. DECREASED EtCO2

C. ABSENT EtCO2

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6. ABNORMAL WAVEFORM PATTERNS - VIVA FAVOURITES

Pattern 1: Slanted / shark-fin upstroke (Phase II-III slope increased)

     /\
    /  \
   /    \
  /      \___

• Obstructive airway disease (asthma, COPD, bronchospasm)
• Different lung units empty at different rates → prolonged, sloping plateau
• Increased alpha angle

Pattern 2: Elevated baseline (Phase I not returning to zero)

     _____
    /     \
___/       \_____  ← does not return to 0

• Rebreathing of exhaled CO2
• Exhausted soda lime
• Stuck/incompetent inspiratory valve
• Insufficient fresh gas flow (esp. Mapleson circuits)

Pattern 3: Sudden drop to zero

___|||||||||_____  (suddenly flat at 0)

• Oesophageal intubation (most critical diagnosis)
• Extubation / disconnection
• Complete circuit obstruction
• Cardiac arrest

Pattern 4: Exponential decrease toward zero

     ↓↓↓
   ...becoming smaller

• Pulmonary embolism (sudden) or decreasing cardiac output
• Cardiac arrest (gradual decline)

Pattern 5: Suddenly elevated then decreasing

    ↑↑
   ...then slowly falling

• Malignant hyperthermia onset
• CO2 insufflation starting (laparoscopy)
• Tourniquet release
• NaHCO3 bolus

Pattern 6: Cardiogenic oscillations (notches on plateau)

___/^\__/^\_____

• Heart motion transmitted to lungs during cardiac surgery
• Normal variant with slow RR and strong cardiac output

Pattern 7: Cleft/notch during Phase III

• Patient taking spontaneous breath during IPPV (curare cleft)
• Indicates neuromuscular block wearing off or patient fighting ventilator

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7. CLINICAL USES OF CAPNOGRAPHY

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8. EtCO2 IN SPECIAL SITUATIONS

Cardiac Arrest

• Before ROSC: EtCO2 <10 mmHg = inadequate CPR / very poor prognosis
• Sudden rise to >40 mmHg during CPR = ROSC (most reliable non-invasive sign)
• If EtCO2 persistently <10 mmHg after 20 min of good CPR = consider stopping resuscitation (prognostic marker)

Paediatric patients

• Same principles apply
• EtCO2-PaCO2 gradient may be slightly larger due to V/Q immaturity in neonates
• Sidestream capnography preferred in small children (lightweight)
• Accuracy reduces with very high respiratory rates and small tidal volumes (gas mixing in sidestream tubing)
• Time-based capnography less accurate at very high RRs (>40/min) - gaps between breaths too short to develop clear plateau

Laparoscopy

• CO2 absorbs from peritoneum → EtCO2 rises by 5-10 mmHg
• Increase minute ventilation by 10-15% at start
• Subcutaneous emphysema → very high EtCO2 even with hyperventilation
• Steep Trendelenburg → ↑ airway pressure + atelectasis → ↑ EtCO2

Malignant Hyperthermia

• EtCO2 rise is the earliest and most sensitive indicator - precedes temperature rise
• Often >80-100 mmHg despite maximal ventilation
• Treat with dantrolene + cooling + hyperventilation

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9. VOLUMETRIC CAPNOGRAPHY

• Provides direct Bohr dead space measurement
• Helpful for PEEP optimisation (titrate PEEP to minimise dead space without overdistension)
• Can demonstrate response to bronchodilator therapy
• Available on modern ICU ventilators (Miller's Anesthesia 10e, p.11317)

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10. TRANSCUTANEOUS CO2 (TcCO2)

• Probe heats skin → dilates capillaries → CO2 diffuses through skin → measured by IR sensor
• Continuous non-invasive PaCO2 estimate
• Useful in high-frequency ventilation where capnography inadequate
• Requires calibration against blood gas; drift over time
• Less accurate in: low cardiac output, thick skin/obesity, cyanotic heart disease (Miller's Anesthesia 10e, p.11317)

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11. COMPARISON TABLE: PULSE OXIMETRY vs CAPNOGRAPHY

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QUICK-RECALL VIVA CARD

PULSE OXIMETRY
- Wavelengths: 660 nm (red) + 940 nm (infrared)
- AC (pulsatile) = arterial; DC (steady) = everything else
- COHb: falsely HIGH SpO2 (660 nm same as O2Hb)
- MetHb: SpO2 drifts to 85% regardless
- Methylene blue: falsely LOW SpO2
- Vasoconstriction → no signal
- OHC flat top: cannot detect hyperoxia
- SpO2 90% ≈ PaO2 60 mmHg

CAPNOGRAPHY
- Phases: I=dead space, II=upstroke, III=plateau, IV=downstroke
- Normal EtCO2: 35-45 mmHg; gradient PaCO2-EtCO2 = 2-5 mmHg
- Gradient INCREASES with: PE, ↓ cardiac output, overdistension
- Sudden EtCO2 to ZERO: oesophageal intubation / disconnection / cardiac arrest
- Sudden EtCO2 RISE: MH / hypoventilation / laparoscopy / NaHCO3
- Sudden EtCO2 DROP: PE / haemorrhage / ↓ cardiac output
- CPR: EtCO2 >10 = adequate; sudden rise = ROSC
- Shark-fin: obstructive disease
- Elevated baseline: rebreathing (exhausted soda lime / bad valve)

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Pulse Oximetry & Capnography: Relevant Images with Explanations

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PULSE OXIMETRY IMAGES

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1. Oxyhemoglobin Dissociation Curve (OHC)

• Flat upper portion (PaO2 >70 mmHg): SpO2 cannot distinguish between normal and hyperoxic states - dangerous in neonates (retinopathy risk)
• Steep lower portion: Once SpO2 starts falling, PaO2 is already critically low - pulse oximetry gives late warning of deterioration
• SpO2 90% = PaO2 ~60 mmHg (onset of respiratory failure)
• Left shift (Alkalosis, Hypocarbia, Hypothermia, COHb, Fetal Hb): higher affinity, less O2 release to tissues
• Right shift (Acidosis, Hypercarbia, Hyperthermia, ↑2,3-DPG): lower affinity, better O2 delivery to tissues

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2. Haemoglobin Absorption Spectra - The Physics of Pulse Oximetry

• Panel A (left): All 5 Hb species shown across visible spectrum (500-700 nm)
• Panel B (right): The two dashed vertical lines = the two wavelengths used by pulse oximeters: 660 nm (red) and 940 nm (infrared)
• At 660 nm: Reduced (deoxy) Hb absorbs MUCH more than OxyHb - so well-oxygenated blood transmits more red light
• At 940 nm: OxyHb absorbs more than reduced Hb - infrared discriminates in the opposite direction
• Critical viva point: At 660 nm, COHb (carboxyhaemoglobin) has nearly the same extinction coefficient as O2Hb (red line ≈ blue line at 660 nm) - this explains why CO poisoning produces falsely high SpO2
• MetHb (orange) has high absorption at BOTH wavelengths - this drives the R ratio toward 1.0, which on the calibration curve corresponds to ~85% SpO2, regardless of true saturation

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3. AC/DC Pulse Principle - How Arterial Blood is Isolated

• The DC (non-pulsatile) component = background absorption from tissue, venous blood, capillary blood - constant, ignored
• The AC (pulsatile) component at the top = absorption that changes with each heartbeat = arterial blood only
• The pulse oximeter calculates the ratio R = (AC660/DC660) / (AC940/DC940) - this ratio isolates arterial saturation from all background tissue
• This is why poor peripheral perfusion (shock, hypothermia, vasoconstriction) causes loss of signal - the AC component disappears when there is no pulsatile flow

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4. Calibration Curve: R-ratio to SpO2

• The R ratio (x-axis) is mathematically derived from AC and DC light absorption at both wavelengths
• The SpO2 (y-axis) is read off this empirically-derived calibration curve built into each oximeter
• Dashed lines at R=1.0, SpO2=85%: When R=1 (equal absorption at both wavelengths, as happens with MetHb), SpO2 reads 85% - this is the classic MetHb trap
• Calibration was performed only in volunteers down to SpO2 70% - accuracy below 70% is unknown and unreliable
• Each manufacturer has their own proprietary calibration curve

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5. Photoplethysmogram (PPG Waveform)

• The pulse oximeter trace is also a photoplethysmograph - it shows arterial blood volume changes beat to beat
• Left section (60s scale): tight, small amplitude = fast heart rate
• Middle/right (3s scale): larger waves with respiratory variation in amplitude (ΔPOP)
• Point B: ventilation stopped - respiratory variation disappears
• Clinical use: Respiratory variation in PPG amplitude predicts fluid responsiveness in mechanically ventilated patients (PVI - Pleth Variability Index)
• Large respiratory swings in PPG = patient likely fluid-responsive

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CAPNOGRAPHY IMAGES

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6. Normal Capnogram - Phases Labeled (Simple View)

• Phase 1 (blue, baseline): Inspiration - dead space gas, CO2 = 0
• Phase 2 (pink, upstroke): Expiratory upstroke - mixing of dead space + alveolar gas
• Phase 3 (red, plateau): Alveolar plateau - represents alveolar gas CO2
• EtCO2 (green circle): The peak value at end of exhalation = measured end-tidal CO2
• Phase 4 (yellow, downstroke): Inspiratory downstroke - fresh gas washes out CO2

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7. Normal Capnogram - Detailed Anatomy

• Transitional part (Phase II): Mixing of dead space and alveolar gas - the upstroke represents washout of anatomical dead space
• Alveolar plateau (Phase III): Average alveolar CO2 concentration - should be nearly flat (slight upslope is normal)
• Alpha angle: Transition from Phase II to III - represents the quality of alveolar emptying. Widened (>110°) in obstructive disease (bronchospasm, COPD) because different lung units empty at different times
• End-tidal CO2: Measured at the peak of Phase III = maximal alveolar CO2 concentration
• CO2 must return to zero on inspiration - if it does not, there is rebreathing (exhausted soda lime or faulty inspiratory valve)

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8. Normal Capnogram with Phase Labels (Classic Textbook Style)

• Phase I: Respiratory baseline at 0 mmHg - dead space gas, no CO2
• Phase II: Expiratory upstroke - steep rise as alveolar gas begins to exit
• Phase III: Alveolar plateau - EtCO2 measured at the highest point
• Phase 0: Inspiratory downstroke - rapid return to zero as fresh gas enters
• Normal EtCO2 = 35-45 mmHg; Normal PaCO2-EtCO2 gradient = 2-5 mmHg

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ABNORMAL WAVEFORM PATTERNS SUMMARY TABLE

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IMAGE SOURCES

• Miller's Anesthesia 10e (Figs 37.2, 37.3, 37.4, 37.5, 37.6) - all textbook images from the medical library
• Normal waveform diagram from Don't Forget the Bubbles (Owen Hibberd, 2022)
• Capnogram anatomy from Deranged Physiology
• Phase-labeled capnogram from medical education resource via capnography.com