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)

Representative time capnogram showing phases 0-IV across three breaths

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

Abnormal capnogram waveform patterns A through E

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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ls -la /home/daytona/workspace/ 2>/dev/null || mkdir -p /home/daytona/workspace/ed95-pdf

Running Command

mkdir -p /home/daytona/workspace/ed95-pdf

Writing File

~/ed95-pdf/generate_pdf.py

from reportlab.lib.pagesizes import A4
from reportlab.lib.styles import getSampleStyleSheet, ParagraphStyle
from reportlab.lib.units import cm
from reportlab.lib import colors
from reportlab.platypus import (
    SimpleDocTemplate, Paragraph, Spacer, Table, TableStyle, HRFlowable
)
from reportlab.lib.enums import TA_CENTER, TA_LEFT, TA_JUSTIFY
from reportlab.pdfbase import pdfmetrics

OUTPUT = "/home/daytona/workspace/ed95-pdf/ED95_NMBDs_MD_Anaesthesia.pdf"

doc = SimpleDocTemplate(
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    title="ED95 of Neuromuscular Blocking Drugs – MD Anaesthesia",
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# Table styling helper
def styled_table(data, col_widths=None, header_color=colors.HexColor("#1a3a5c")):
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story = []

# ── TITLE ──────────────────────────────────────────────────────────────────
story.append(Spacer(1, 0.3*cm))
story.append(Paragraph("ED95 of Neuromuscular Blocking Drugs", title_style))
story.append(Paragraph("Model Answer — MD Anaesthesia Examination (10 Marks)", subtitle_style))
story.append(HRFlowable(width="100%", thickness=2, color=colors.HexColor("#1a3a5c"), spaceAfter=10))

# ── SECTION 1 ──────────────────────────────────────────────────────────────
story.append(Paragraph("1. Definition", h2_style))
story.append(Paragraph("(2 Marks)", mark_style))
story.append(Paragraph(
    "<b>ED95</b> stands for the <b>Effective Dose that produces 95% depression of maximal "
    "muscle twitch response</b> in 50% of patients studied under defined anaesthetic conditions.",
    body_style
))
story.append(Paragraph(
    "Important distinction from conventional pharmacology:",
    body_style
))
tbl1 = styled_table(
    [
        ["Context", "Meaning of ED95"],
        ["Conventional pharmacology", "Dose effective in 95% of the population"],
        ["Neuromuscular pharmacology", "Median dose (effective in 50% of patients) producing 95% twitch depression"],
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    col_widths=[8*cm, 9*cm]
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story.append(tbl1)
story.append(Spacer(1, 0.3*cm))
story.append(Paragraph(
    "ED95 is derived from a <b>dose–response (quantal) sigmoid curve</b> plotting the cumulative "
    "percentage of patients against dose, read at the 95% twitch-suppression point. "
    "Measurement is at the <i>adductor pollicis</i> via ulnar nerve stimulation.",
    body_style
))
story.append(Paragraph("Source: Barash Clinical Anesthesia 9e, p. 1604", source_style))

# ── SECTION 2 ──────────────────────────────────────────────────────────────
story.append(Paragraph("2. ED95 as a Measure of Potency", h2_style))
story.append(Paragraph("(1 Mark)", mark_style))
story.append(Paragraph(
    "ED95 is the standard measure of <b>potency</b> of NMBDs:",
    body_style
))
story.append(Paragraph("• Lower ED95 = More potent drug (smaller dose achieves effect)", bullet_style))
story.append(Paragraph("• Higher ED95 = Less potent drug (more molecules required)", bullet_style))
story.append(Paragraph(
    "Potency and onset of action are inversely related — a key clinical concept (see Section 5).",
    body_style
))

# ── SECTION 3 ──────────────────────────────────────────────────────────────
story.append(Paragraph("3. ED95 Values of Clinically Used NMBDs", h2_style))
story.append(Paragraph("(2 Marks)", mark_style))
story.append(Paragraph(
    "<b>Key fact:</b> The four most commonly used NMBDs in current practice have an ED95 of either "
    "<b>0.3 mg/kg</b> (succinylcholine, rocuronium) or <b>0.05 mg/kg</b> (cisatracurium, vecuronium).",
    key_style
))
tbl2 = styled_table(
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        ["Rocuronium", "Aminosteroid", "Intermediate", "0.3", "0.6 mg/kg"],
        ["Vecuronium", "Aminosteroid", "Intermediate", "0.05", "0.1 mg/kg"],
        ["Cisatracurium", "Benzylisoquinolinium", "Intermediate", "0.05", "0.1–0.15 mg/kg"],
        ["Atracurium", "Benzylisoquinolinium", "Intermediate", "0.23", "0.5 mg/kg"],
        ["Pancuronium", "Aminosteroid", "Long", "0.07", "0.1 mg/kg"],
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story.append(Paragraph(
    "* Succinylcholine ED95 by cumulative dose-response is <0.3 mg/kg; clinical dose of 1 mg/kg ≈ 3× ED95.",
    source_style
))
story.append(Paragraph("Source: Barash Clinical Anesthesia 9e, p. 1597; Miller's Anesthesia 10e, p. 3229", source_style))

# ── SECTION 4 ──────────────────────────────────────────────────────────────
story.append(Paragraph("4. Clinical Significance: Intubating Dose", h2_style))
story.append(Paragraph("(1 Mark)", mark_style))
story.append(Paragraph(
    "The standard intubating dose = <b>2× ED95</b>. This provides complete NMB in the vast "
    "majority of patients with acceptable onset and reliable intubating conditions.",
    body_style
))
tbl3 = styled_table(
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        ["Surgical relaxation only", "~0.5–1 × ED95"],
        ["Standard tracheal intubation", "2 × ED95"],
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story.append(tbl3)
story.append(Paragraph("Source: Miller's Anesthesia 10e, p. 3269", source_style))

# ── SECTION 5 ──────────────────────────────────────────────────────────────
story.append(Paragraph("5. Inverse Potency–Onset Relationship", h2_style))
story.append(Paragraph("(2 Marks)", mark_style))
story.append(Paragraph(
    "<b>The less potent the NMBD, the faster its onset of action at equipotent doses.</b>",
    key_style
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story.append(Paragraph("<b>Mechanism:</b>", h3_style))
story.append(Paragraph(
    "• At 2× ED95 doses, a less potent drug requires more molecules to achieve the same effect.",
    bullet_style
))
story.append(Paragraph(
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    bullet_style
))
story.append(Paragraph(
    "• Higher concentration gradient = faster diffusion and faster receptor occupancy.",
    bullet_style
))
story.append(Paragraph(
    "• Therefore: <b>Onset time ∝ 1 / Molar potency</b>",
    bullet_style
))
story.append(Spacer(1, 0.2*cm))
story.append(Paragraph("<b>Example — Rocuronium:</b>", h3_style))
story.append(Paragraph(
    "Rocuronium has a molar ED95 of 0.54 μM/kg — approximately <b>13% that of vecuronium</b> and "
    "<b>9% that of cisatracurium</b>. Its low potency is exploited by giving a large dose "
    "(1.2 mg/kg = 4× ED95) to deliver enough molecules quickly, achieving RSI conditions in <b>60 seconds</b> "
    "comparable to succinylcholine.",
    body_style
))
story.append(Paragraph(
    "Note: Atracurium is an exception — its onset does not follow the inverse potency rule.",
    source_style
))
story.append(Paragraph("Source: Miller's Anesthesia 10e, p. 3272; Barash Clinical Anesthesia 9e, p. 1615", source_style))

# ── SECTION 6 ──────────────────────────────────────────────────────────────
story.append(Paragraph("6. Factors Affecting ED95", h2_style))
story.append(Paragraph("(1 Mark)", mark_style))
story.append(Paragraph(
    "ED95 is not a fixed value — it changes under the following conditions:",
    body_style
))
tbl4 = styled_table(
    [
        ["Factor", "Effect on ED95 / Block"],
        ["Volatile anaesthetics (sevoflurane, isoflurane, desflurane)", "Decrease ED95 by 20–50% (potentiate NMBDs)"],
        ["Hypothermia", "Potentiates block (decreased metabolism)"],
        ["Respiratory acidosis", "Potentiates block"],
        ["Respiratory alkalosis", "Antagonises block"],
        ["Hypermagnesaemia", "Potentiates block"],
        ["Aminoglycoside antibiotics", "Potentiate block"],
        ["Myasthenia gravis", "Extreme sensitivity to NDNMBDs"],
        ["Burns / Denervation (>24h)", "Resistance to NMBDs (↑ ACh receptors)"],
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    col_widths=[9*cm, 8*cm]
)
story.append(tbl4)
story.append(Paragraph(
    "During volatile anaesthesia, the intubating dose may be reduced by ~40% relative to "
    "doses used under N₂O/O₂/opioid anaesthesia.",
    body_style
))
story.append(Paragraph("Source: Miller's Anesthesia 10e, p. 3265; Barash Clinical Anesthesia 9e, p. 1615", source_style))

# ── SECTION 7 ──────────────────────────────────────────────────────────────
story.append(Paragraph("7. Interpatient Variability", h2_style))
story.append(Paragraph("(1 Mark)", mark_style))
story.append(Paragraph(
    "ED95 represents the median population response, but <b>individual variation is enormous</b>:",
    body_style
))
story.append(Paragraph(
    "• 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.",
    bullet_style
))
story.append(Paragraph(
    "• Duration of action at equipotent doses is equally unpredictable.",
    bullet_style
))
story.append(Paragraph(
    "• Age, sex, body build, and ASA status <b>cannot</b> predict individual response.",
    bullet_style
))
story.append(Paragraph(
    "• This variability is why <b>quantitative neuromuscular monitoring (TOF ratio)</b> is "
    "essential and cannot be replaced by clinical assessment or time.",
    bullet_style
))
story.append(Paragraph("Source: Barash Clinical Anesthesia 9e, p. 1606", source_style))

# ── SUMMARY TABLE ──────────────────────────────────────────────────────────
story.append(HRFlowable(width="100%", thickness=1, color=colors.HexColor("#1a3a5c"), spaceBefore=14, spaceAfter=8))
story.append(Paragraph("Summary", h2_style))
tbl5 = styled_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, slower onset"],
        ["High ED95", "Low potency, faster onset"],
        ["Volatile agents", "Reduce ED95 by ~40%"],
        ["Monitoring", "Essential due to wide interpatient variation"],
    ],
    col_widths=[8*cm, 9*cm]
)
story.append(tbl5)

# ── FOOTER / REFERENCES ────────────────────────────────────────────────────
story.append(Spacer(1, 0.5*cm))
story.append(HRFlowable(width="100%", thickness=0.5, color=colors.HexColor("#aaaaaa"), spaceAfter=4))
story.append(Paragraph(
    "<b>References:</b> Barash, Cullen &amp; Stoelting's Clinical Anesthesia, 9e (pp. 1597–1615) · "
    "Miller's Anesthesia, 10e (pp. 3229–3275)",
    source_style
))

doc.build(story)
print("PDF generated:", OUTPUT)

Running Command

python /home/daytona/workspace/ed95-pdf/generate_pdf.py

Your PDF is ready. It includes all 7 sections with mark allocations, formatted tables, and source references:

ED95 NMBDs MD Anaesthesia

PDF Document · PDF

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

Finding Sources

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