Capnographs
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Capnographs
Terminology
There is frequent confusion between related terms, so clarity here is important:
- Capnometry - the measurement and quantification of inhaled/exhaled CO2 concentrations at the airway opening
- Capnography - not only the method of CO2 measurement, but also its graphic display as a function of time or volume
- Capnometer - a device that measures CO2 (may display a numeric value only)
- Capnograph - a device that records and displays CO2 concentrations, usually as a function of time
- Capnogram - the graphic waveform the capnograph generates
Miller's Anesthesia, 10e, p. 5482-5483
How Capnographs Work
The most common clinical method is nondispersive infrared (NDIR) absorption. CO2 absorbs infrared light over a narrow bandwidth centered around 4.26 μm. The capnograph passes an infrared beam through a gas sample; a photodetector measures the transmitted light - less transmission = more CO2.
Most devices use a rotating chopper disk (~60 rpm) that alternately directs the beam through:
- A sample cell containing the gas to be analyzed
- A reference cell free of CO2
- A fully blocked position (background)
This three-signal approach allows continuous, real-time CO2 concentration estimation. Solid-state devices may use a beam splitter instead of a chopper wheel.
Note: The absorption spectra of CO2 partially overlaps with water vapour and nitrous oxide, so infrared filters and compensation algorithms are used to minimize interference.
Miller's Anesthesia, 10e, p. 5484
Types: Sidestream vs. Mainstream
| Feature | Sidestream (Diverting) | Mainstream (Non-diverting) |
|---|---|---|
| Sensor location | At the monitor console (remote) | In-line at the airway |
| Mechanism | Pump aspirates gas via tubing | Sensor sits directly in the circuit |
| Gas withdrawal rate | 30-500 mL/min | None (no gas removed) |
| Common use | Most OR settings | NICU, infants |
| Key advantages | No added airway weight, can analyze multiple gases | No lag time, no aspiration artifacts |
| Key disadvantages | Lag time (~1 s), water condensation in tubing, gas lost from circuit | Heavier at airway, sensor can become contaminated, heated to prevent condensation |
For sidestream units, high aspiration rates (up to 250 mL/min) improve sensitivity and reduce lag time, but in small patients (pediatric), high aspiration can entrain fresh gas and dilute the ETCO2 reading. Low rates (<50 mL/min) can underestimate ETCO2 during rapid ventilation.
Morgan & Mikhail's Clinical Anesthesiology, 7e, p. 227; Miller's Anesthesia, 10e, p. 5485
The Normal Capnogram (Time-Based)
The capnogram is divided into phases:
| Phase | Segment | What it represents |
|---|---|---|
| 0 (Inspiration) | Descending to zero | Inspiratory limb; CO2 returns to zero (or baseline) |
| I | Flat at zero | Anatomical dead space gas - no CO2 |
| II | Rapid upstroke | Mixed dead space + alveolar gas emerging |
| III | Alveolar plateau | Pure alveolar gas; the peak value = PETCO2 |
| IV | Late upstroke (dashed) | Seen in some patients; reflects cardiogenic oscillations or terminal alveolar emptying |
The α angle (between phases II and III) and β angle (between phase III and the descending limb) are also informative - a widened α angle suggests obstructive pathology.
Normal PETCO2 = 35-45 mmHg (approximates PaCO2 with a 2-5 mmHg gradient; alveolar dead space accounts for PETCO2 being slightly lower than PaCO2).
Miller's Anesthesia, 10e, p. 5483
Abnormal Capnogram Patterns
| Pattern | What you see | Cause |
|---|---|---|
| A - Normal | Clear plateau reaching ~40 mmHg | Normal |
| B - COPD/Obstructive | No true alveolar plateau; continuously rising phase III; no distinct plateau before next inspiration | Obstructive lung disease (COPD, asthma) - uneven emptying of lung units |
| C - Spontaneous effort | A notch or dip during phase III | Patient breathing against the ventilator; spontaneous respiratory effort during phase III |
| D - CO2 rebreathing | Inspired baseline does not return to zero | Exhausted CO2 absorbent, incompetent expiratory valve, or inadequate fresh gas flow |
| E - Incompetent inspiratory valve | Exhaled gas persists into part of the inspiratory cycle; slow descent | Incompetent inspiratory valve in the breathing circuit |
Morgan & Mikhail's Clinical Anesthesiology, 7e, p. 227-228
Key Clinical Uses
1. Confirming ETT/LMA Placement
Capnography is the gold standard for confirming endotracheal tube placement. It rapidly and reliably detects esophageal intubation - the most critical use. Although there may be some CO2 in the stomach from swallowed expired air, this washes out within a few breaths. Capnography does not reliably detect mainstem bronchial intubation.
2. Ventilation Monitoring
- Standard of care during general anesthesia
- Mandated during moderate and deep procedural sedation
- Detects hypoventilation earlier than pulse oximetry, even with supplemental oxygen in use
3. Circuit Integrity
- Sudden drop to zero during expiration - circuit disconnection
- CO2 in the inspiratory limb - exhausted absorbent or valve failure
4. Cardiovascular Events
The PaCO2-ETCO2 gradient (normally 2-5 mmHg) reflects alveolar dead space. Any reduction in lung perfusion widens this gradient and decreases ETCO2:
- Pulmonary embolism
- Air embolism
- Decreased cardiac output / hypotension
- Cardiac arrest (ETCO2 <10 mmHg is a poor prognostic sign; rise during CPR suggests ROSC)
5. Metabolic States
- Malignant hyperthermia - causes a marked, rapid rise in ETCO2 (increased metabolic CO2 production)
- Hyperthyroidism, sepsis - elevated ETCO2
- Hypothermia, hypothyroidism - reduced ETCO2
Morgan & Mikhail's Clinical Anesthesiology, 7e, p. 227-228; Barash Clinical Anesthesia, 9e, p. 2111-2112
Limitations
- Not equivalent to ABG for PaCO2 assessment in disease states with increased dead space (e.g. emphysema, PE, low cardiac output) - the gradient widens unpredictably
- Cannot detect nitrogen directly (relevant for air embolism and preoxygenation monitoring) - infrared analyzers must infer N2
- Cannot detect xenon anesthesia; not useful with total IV anesthesia (no volatile agent)
- Water condensation in sidestream tubing can obstruct the sampling line and give erroneous readings
- In pediatric patients, high aspiration rates may dilute the sample and underestimate ETCO2
Barash Clinical Anesthesia, 9e, p. 2111-2112
Summary
A capnograph is an infrared-based CO2 monitor that displays a real-time waveform (the capnogram) of exhaled CO2 across the respiratory cycle. It is one of the most information-rich monitors in anesthesia and emergency medicine - providing simultaneous insight into ventilation adequacy, airway placement, circuit integrity, cardiac output, and metabolic state. The normal waveform has a flat phase I, rising phase II, and alveolar plateau (phase III) reaching PETCO2 ~35-45 mmHg; deviations from this pattern point to specific clinical problems.
ED 95 for 10 mark Question for MD anaesthesia exam
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ED95 of Neuromuscular Blocking Drugs
Model Answer - MD Anaesthesia (10 Marks)
1. Definition (2 marks)
ED95 stands for the Effective Dose that produces 95% depression of maximal muscle twitch response in 50% of patients studied under defined anesthetic conditions.
It is important to understand the precise terminology here, as it differs from the conventional pharmacological use of "ED95":
- In conventional pharmacology, ED95 means the dose effective in 95% of the population.
- In neuromuscular pharmacology, ED95 = the median dose (effective in 50% of patients) required to produce 95% depression of the single twitch response (measured at the adductor pollicis muscle using ulnar nerve stimulation).
This is derived from a dose-response (quantal) curve - a sigmoid curve plotting cumulative percentage of patients against dose. The ED95 is read off at the 95% twitch-suppression point on this curve.
Barash Clinical Anesthesia, 9e, p. 1604
2. How ED95 Expresses Potency (1 mark)
ED95 is the standard measure of potency of neuromuscular blocking drugs (NMBDs). A lower ED95 = more potent drug (smaller dose needed for effect). A higher ED95 = less potent drug.
Potency and onset are inversely related (see below).
3. ED95 Values of Clinically Used NMBDs (2 marks)
| Drug | Class | Duration | ED95 (mg/kg) | Intubating Dose (2× ED95) |
|---|---|---|---|---|
| Succinylcholine | Depolarizing | Ultra-short | 0.3 mg/kg* | 1.0-1.5 mg/kg |
| Rocuronium | Aminosteroid | Intermediate | 0.3 mg/kg | 0.6 mg/kg |
| Vecuronium | Aminosteroid | Intermediate | 0.05 mg/kg | 0.1 mg/kg |
| Cisatracurium | Benzylisoquinolinium | Intermediate | 0.05 mg/kg | 0.1-0.15 mg/kg |
| Atracurium | Benzylisoquinolinium | Intermediate | 0.23 mg/kg | 0.5 mg/kg |
| Pancuronium | Aminosteroid | Long | 0.07 mg/kg | 0.1 mg/kg |
| Mivacurium | Benzylisoquinolinium | Short | 0.08 mg/kg | 0.15-0.2 mg/kg |
*Succinylcholine ED95 by cumulative dose-response methods is actually <0.3 mg/kg; the conventional dose of 1 mg/kg = ~3× ED95.
Key fact: The four most commonly used NMBDs in current practice have an ED95 of either 0.3 mg/kg (succinylcholine, rocuronium) or 0.05 mg/kg (cisatracurium, vecuronium).
Barash Clinical Anesthesia, 9e, p. 1597; Miller's Anesthesia, 10e, p. 3229
4. Clinical Significance of ED95: Intubating Dose (1 mark)
The standard intubating dose = 2× ED95. This provides:
- Complete neuromuscular blockade in the vast majority of patients
- Acceptable onset time
- Reliable intubating conditions
If the trachea has already been intubated (e.g., with succinylcholine) and an NMBD is needed only for surgical relaxation, a dose slightly less than the ED95 is often sufficient for most surgical settings.
| Clinical Purpose | Dose Used |
|---|---|
| Surgical relaxation only | ~0.5-1 × ED95 |
| Standard tracheal intubation | 2 × ED95 |
| Rapid sequence intubation (rocuronium) | 3-4 × ED95 (0.9-1.2 mg/kg) |
Miller's Anesthesia, 10e, p. 3269
5. Inverse Potency-Onset Relationship (2 marks)
This is a high-yield concept for exam purposes.
The less potent the NMBD, the faster its onset of action (at equipotent doses).
Mechanism:
- At equipotent (2× ED95) doses, a less potent drug requires more molecules to achieve the same effect
- More molecules = higher molar concentration delivered to the neuromuscular junction
- Higher concentration gradient across the NMJ = faster diffusion and faster receptor occupancy
- Therefore, onset is faster
Rocuronium (ED95 = 0.3 mg/kg, molar ED95 = 0.54 μM/kg) has approximately:
- 13% the molar potency of vecuronium
- 9% the molar potency of cisatracurium
This explains why rocuronium at 1.2 mg/kg (4× ED95) achieves intubating conditions within 60 seconds, comparable to succinylcholine - by giving a very large number of molecules.
Mathematically: Onset time ∝ 1/Molar potency (i.e., onset time decreases as ED50 or ED95 increases)
This relationship holds for most steroidal NMBDs (exception: atracurium does not follow this rule).
Miller's Anesthesia, 10e, p. 3272; Barash Clinical Anesthesia, 9e, p. 1615
6. Factors Affecting the ED95 Value (1 mark)
The ED95 is not a fixed number - it changes under different conditions:
| Factor | Effect on ED95 |
|---|---|
| Volatile anesthetic agents (sevoflurane, isoflurane, desflurane) | Decrease ED95 by 20-50% (potentiate NMBDs) - so a lower dose achieves the same effect |
| Nitrous oxide | Minor potentiation |
| Hypothermia | Potentiates NMBDs (decreased metabolism, prolonged duration) |
| Acid-base status | Respiratory acidosis potentiates; alkalosis antagonizes |
| Hypermagnesaemia | Potentiates block |
| Aminoglycoside antibiotics | Potentiate block |
| Myasthenia gravis | Extreme sensitivity - ED95 much lower for NDNMBDs |
| Burns, denervation (>24h) | Resistance to NMBDs (increased ACh receptors) |
Clinically: during volatile anesthesia, the intubating dose may be reduced by ~40% relative to doses used under N2O/O2/opioid anesthesia.
Miller's Anesthesia, 10e, p. 3265; Barash Clinical Anesthesia, 9e, p. 1615
7. Interpatient Variability (1 mark)
ED95 represents the median population response, but individual variation is enormous and clinically important:
- In a classic study (Katz, 1967), 0.1 mg/kg curare caused zero twitch depression in 6% of patients yet complete abolition in 7% - the same dose!
- Duration of action at equipotent doses is equally unpredictable
- Age, sex, body build, and ASA status cannot predict individual response to a given dose
- This variability is why quantitative neuromuscular monitoring (TOF ratio) is essential and cannot be replaced by clinical assessment or time
Barash Clinical Anesthesia, 9e, p. 1606
Summary Table
| Concept | Key Point |
|---|---|
| Definition | Median dose causing 95% twitch depression in 50% of patients |
| Standard intubating dose | 2× ED95 |
| RSI dose (rocuronium) | 4× ED95 (1.2 mg/kg) |
| Low ED95 | High potency |
| High ED95 | Low potency, but faster onset |
| Volatile agents | Reduce ED95 by ~40% |
| Monitoring | Essential due to wide interpatient variation |
Sources: Barash, Cullen & Stoelting's Clinical Anesthesia 9e (pp. 1597-1615); Miller's Anesthesia 10e (pp. 3229-3275)
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Your PDF is ready. It includes all 7 sections with mark allocations, formatted tables, and source references:
ED95 NMBDs MD Anaesthesia
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The document contains:
- Title page header with exam context (10 marks)
- All 7 sections with mark breakdowns
- Colour-coded tables for ED95 values, intubating doses, factors affecting ED95, and the summary
- Source references at the bottom (Barash 9e + Miller's 10e)
Can you explain inter patient variability better
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