Capnography

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Capnography

Capnography is the continuous, non-invasive measurement and graphical display of carbon dioxide (CO2) concentration in exhaled gas, breath by breath. The numeric value at the end of exhalation is called the end-tidal CO2 (ETCO2 or PetCO2). The graphical waveform is the capnogram.

1. Physiological Basis

CO2 is produced by cellular metabolism, transported via venous blood to the lungs, diffuses rapidly across the alveolar membrane, and is expelled during exhalation. Because CO2 diffuses so readily, alveolar PCO2 (PACO2) essentially equals arterial PCO2 (PaCO2). End-tidal CO2 measured from exhaled gas is therefore a close surrogate for PaCO2.

Normal PaCO2 - PetCO2 gradient: < 5 mm Hg

This small gradient exists because end-tidal gas is slightly diluted by CO2-free gas from non-perfused alveoli (alveolar dead space). The gradient widens in conditions of:

Increased alveolar dead space (pulmonary embolism, low cardiac output, pulmonary hypertension)
Pulmonary overdistension
Obstructive airway disease (air trapping, incomplete exhalation)

Normal PetCO2: 35-45 mm Hg (mirrors normal PaCO2 of 35-45 mm Hg)

Morgan and Mikhail's Clinical Anesthesiology, 7e
Miller's Anesthesia, 10e

2. Technology: How It Works

Measurement Methods

Method	Description	Advantage	Disadvantage
Mainstream (non-aspirating)	Infrared sensor placed directly in-line with the airway circuit	No sampling delay; accurate	Heavier sensor on airway; requires intubation or tight mask
Sidestream (aspirating)	Small pump aspirates a continuous gas sample from the circuit or cannula	Can be used in non-intubated patients via nasal cannula	Slight time delay; risk of water condensation/blockage
Colorimetric	pH-sensitive dye changes color (purple→yellow) with CO2	Cheap, no power needed; rapid ETT confirmation	Qualitative only; false positives from carbonated beverages or gastric acid

Display Types

Time-based capnography: CO2 concentration vs. time (most common)
Volumetric capnography: CO2 concentration vs. exhaled volume - enables direct dead space calculations and can guide PEEP titration and assess bronchodilator response

3. The Normal Capnogram Waveform

Normal capnogram and abnormal patterns - Tintinalli's Emergency Medicine

Capnogram A: Normal. B: Apnea (serially decreasing amplitudes). C: Hypoventilation (rising plateau). D: Rebreathing/air trapping (elevated baseline).

The normal capnogram has 5 phases:

Phase	Label	Description
Inspiratory baseline	W	CO2 = 0 (fresh gas inhaled)
Expiratory upstroke	V	Dead space gas clearing; rapid CO2 rise
Expiratory plateau (alveolar plateau)	X	Alveolar gas; CO2 levels out
End-tidal point	Y	Peak CO2 at end of exhalation = PetCO2
Inspiratory downstroke	Z	Rapid fall back to zero as fresh gas enters

A rising upslope or "shark fin" shape to the exhalation phase indicates obstructive airway disease (asthma, COPD) - incomplete, uneven emptying of alveoli.

Tintinalli's Emergency Medicine, p. 122

4. Abnormal Capnogram Patterns

Pattern	Appearance	Clinical Cause
Apnea	Flat line at zero	Apnea, esophageal intubation, circuit disconnection
Hypoventilation	Rising plateau and rising PetCO2	Opioid effect, CNS depression, sedation
Hyperventilation	Falling PetCO2	Anxiety, pain, mechanical over-ventilation
Elevated baseline	Baseline does not return to zero	CO2 rebreathing, exhausted soda lime, stuck inspiratory valve
Shark fin / prolonged upstroke	No plateau; slowly sloping expiration	Bronchospasm, COPD, kinked ETT
Sudden drop to zero	Waveform disappears	ETT dislodgement, circuit disconnect, cardiac arrest
Sudden decrease (not zero)	PetCO2 falls abruptly	Pulmonary embolism (sudden dead space increase), cardiac arrest
Gradual decline	Slow decrease over time	Hyperventilation, hypothermia, decreasing cardiac output
Sudden rise	Acute CO2 increase	Tourniquet release, sodium bicarbonate administration, malignant hyperthermia

Alveolar dead space fraction (AVDSf):

AVDSf = (PaCO2 - PetCO2) / PaCO2

Elevated AVDSf correlates with severity of acute hypoxemic respiratory failure and mortality in pediatric patients. - Miller's Anesthesia, 10e

5. Clinical Applications

5a. Endotracheal Tube (ETT) Confirmation

Continuous waveform capnography is the gold standard for confirming endotracheal - not esophageal - intubation. A sustained, consistent waveform across multiple breaths confirms tracheal placement.

Colorimetric detectors are adequate but can give false positives after carbonated beverages or gastric acid contamination.
Waveform capnography is superior to colorimetric devices and is the preferred method.
Tintinalli's Emergency Medicine; Barash's Clinical Anesthesia, 9e

5b. Procedural Sedation and Analgesia (PSA)

Capnography detects hypoventilation and apnea before pulse oximetry, especially when supplemental oxygen is being given (which delays oxyhemoglobin desaturation).

Key evidence:

Respiratory depression was 17.6x more likely to be detected with capnography vs. standard monitoring alone (meta-analysis by Waugh et al.)
100% of apneic episodes >20 seconds during MAC were missed by providers but caught by capnography (Soto et al.)
Pulse oximetry detected only ~50% of apnea/disordered breathing episodes detected by capnography during upper endoscopy - hypoxemia followed apnea by an average of 45.6 seconds

ACEP gives capnography a Level B recommendation as an adjunct during PSA in the ED. - Roberts & Hedges' Clinical Procedures in Emergency Medicine; Barash's Clinical Anesthesia, 9e

5c. Cardiac Arrest / CPR Monitoring

Capnography during cardiac arrest showing CPR quality monitoring

Panel A: Normal single-breath waveform. Panel B: PETCO2 during CPR - note the declining trend over time reflecting CPR fatigue.

During cardiac arrest, PetCO2 correlates with cardiac output under low-flow states:

At arrest onset: PetCO2 falls abruptly (no pulmonary perfusion)
During effective CPR: PetCO2 rises as compressions generate forward flow; correlates with coronary perfusion pressure and compression efficacy
At ROSC: Sudden, sustained rise in PetCO2 to physiologic levels (often the earliest sign of ROSC)
For termination of resuscitation: A persistently low PetCO2 after 20 minutes of ACLS may support decision to terminate efforts

The 2015 (and current) resuscitation guidelines recommend continuous waveform capnography for all intubated patients during CPR. - Roberts & Hedges' Clinical Procedures in Emergency Medicine

5d. Monitoring Ventilation Adequacy

Continuous non-invasive estimation of PaCO2 during mechanical ventilation
Detects inadvertent hyperventilation or hypoventilation
In patients receiving supplemental oxygen, SpO2 may remain normal despite significant hypoventilation - capnography reveals this early

5e. Pulmonary Embolism

Acute PE causes sudden dead space increase → abrupt fall in PetCO2 (widened a-ET gradient)
Not diagnostic alone, but clinically useful as an alert signal

5f. Malignant Hyperthermia (MH)

Early and sensitive indicator - rapidly rising PetCO2 during volatile anesthetic exposure, even before fever or rigidity
Capnography is specifically recommended as part of MH monitoring protocol

5g. Detecting Rebreathing / Equipment Failure

Elevated baseline (CO2 does not return to zero) indicates rebreathing from:
- Exhausted CO2 absorber (soda lime)
- Stuck or malfunctioning inspiratory valve
- Insufficient fresh gas flow in circle system

5h. Non-Intubated Patients

Sidestream capnography adapted for:

Nasal cannulas with integrated sampling port
Modified face masks with IV catheter sampling lines
Useful during moderate/deep sedation, sleep studies

6. Limitations

PetCO2 underestimates PaCO2 - the gradient widens in obstructive lung disease, pulmonary embolism, low cardiac output, and high dead space states
Studies of accuracy vs. ABG are mixed in ED populations - particularly unreliable in undifferentiated dyspnea, COPD exacerbations, and critically ill patients
Sidestream sampling is susceptible to water vapor condensation and secretion obstruction
Less accurate at high respiratory rates (time-based capnography) due to incomplete alveolar gas sampling
Colorimetric detectors are qualitative only and have false-positive/negative risks
Tintinalli's Emergency Medicine, p. 122

7. Summary Table: Key Uses

Application	What to Look For
ETT confirmation	Sustained waveform x6 breaths
Sedation monitoring	Rising PetCO2 or flat line (apnea)
CPR quality	PetCO2 >10-20 mmHg = adequate compressions; rise = ROSC
Malignant hyperthermia	Unexplained rising PetCO2
Bronchospasm	Shark fin waveform
Rebreathing	Elevated baseline
Pulmonary embolism	Sudden drop in PetCO2
Termination of resuscitation	PetCO2 persistently <10 mmHg after 20 min ACLS

Sources:

Tintinalli's Emergency Medicine: A Comprehensive Study Guide (p. 122)
Roberts and Hedges' Clinical Procedures in Emergency Medicine (Chapters 17, 33)
Barash, Cullen, and Stoelting's Clinical Anesthesia, 9e
Morgan and Mikhail's Clinical Anesthesiology, 7e
Miller's Anesthesia, 10e

Vaporizers

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Vaporizers

A vaporizer is a device on the anesthesia machine that converts liquid volatile anesthetic into vapor and delivers it at a precise, clinically useful concentration to the patient's breathing circuit. Because the saturated vapor pressure (SVP) of volatile agents far exceeds clinically safe concentrations, the key engineering challenge is accurate dilution and temperature compensation.

1. Basic Physics

Saturated Vapor Pressure (SVP)

Each volatile agent exerts a characteristic SVP at a given temperature - the maximum partial pressure of vapor that can exist above the liquid. SVP is independent of barometric pressure, but rises with temperature.

Agent	SVP at 20°C (mm Hg)	Boiling Point (°C)	MAC (%)
Halothane	244	50.2	0.75
Isoflurane	238	48.5	1.15
Enflurane	172	56.5	1.68
Sevoflurane	160	58.5	1.85
Desflurane	669	22.8	6.0

Desflurane's SVP of 669 mm Hg and boiling point of 22.8°C make it nearly unique - it would boil at normal OR temperatures, making a standard variable bypass vaporizer impossible to use for this agent.

Why Dilution Is Needed

Even sevoflurane at 20°C has SVP of ~160 mm Hg - that is 21% concentration at sea level. Clinical sevoflurane is used at 1-3%, so the saturated vapor from the vaporizing chamber must be heavily diluted.

2. Classification of Vaporizers

By Circuit Position

Type	Description	Use
Out-of-circuit	Located outside the breathing circuit; precise output added via fresh gas line	Virtually all modern clinical vaporizers
In-circuit	Located within the breathing circuit; patient's own breathing draws vapor through	Draw-over systems; resource-limited settings, ICU sedation

By Operating Principle

Variable bypass (plenum) vaporizers
Dual-circuit (heated/pressurized) vaporizers - for desflurane
Cassette vaporizers (e.g., Aladin)
Injection vaporizers

3. Variable Bypass Vaporizer (The Standard)

This is the type used for halothane, isoflurane, enflurane, and sevoflurane. Also called a plenum-type vaporizer.

Components

Fresh gas inlet
Concentration control dial - sets the splitting ratio
Bypass chamber - carries the majority of fresh gas that bypasses the liquid agent
Vaporizing chamber - contains liquid anesthetic + wicks/baffles
Temperature-compensating device (bimetallic strip or expansion element)
Fresh gas outlet
Agent-specific keyed filling assembly

Operating Principle

Variable bypass vaporizer with expansion element for temperature compensation - Barash's Clinical Anesthesia

Key components: liquid agent in vaporizing chamber (bottom), expansion element for temperature compensation (center), pressure compensation coil (left), concentration control dial (right), bypass chamber (top).

Fresh gas from the flowmeters enters the vaporizer inlet and is split:

Most of the gas goes through the bypass chamber - it never contacts liquid anesthetic
A small fraction is directed into the vaporizing chamber, where it flows over wicks and baffles, picks up anesthetic vapor (approaching saturation), and exits as a concentrated vapor-laden stream
The two streams recombine at the vaporizer outlet, yielding the dialed concentration

Splitting ratio = the ratio of bypass flow to vaporizing chamber flow; it is agent-specific and dial-setting-specific, set by the concentration control dial.

Example splitting ratios at 20°C for 1% output:

Isoflurane: ~45:1 (bypass:vaporizing chamber)
Sevoflurane: ~13:1
Halothane: ~4.5:1

The wicks increase surface area for vaporization. The vaporizing chamber becomes partially (not fully) saturated due to constant fresh gas inflow; full saturation is assumed conceptually for calculation.

Temperature Compensation

As vaporization occurs, the liquid agent cools (evaporative heat loss), reducing SVP and potentially decreasing output. Temperature-compensating mechanisms counteract this:

Bimetallic strip (GE Tec series): Two metals with different thermal expansion coefficients are bonded together. As temperature falls, the strip bends and diverts more gas through the vaporizing chamber (increasing output). As temperature rises, it bends the other way, diverting more gas to the bypass.
Expansion element / bellows (Dräger Vapor 2000): A wax-filled or gas-filled element expands with heat, mechanically adjusting the bypass valve.

The vaporizer body is made from materials with high specific heat and high thermal conductivity to minimize temperature fluctuations.

4. Factors Affecting Vaporizer Output

Fresh Gas Flow (FGF) Rate

Output is less than dial setting at both extremes:

Very low flows (<250 mL/min): Insufficient turbulence in vaporizing chamber; heavy vapor molecules are not adequately swept upward
Very high flows (>15 L/min): Incomplete mixing; failure to saturate carrier gas; altered resistance characteristics

Output is most accurate at intermediate flow rates (0.5-10 L/min). - Barash's Clinical Anesthesia, 9e

Temperature

Contemporary temperature-compensated vaporizers maintain nearly linear output across a wide temperature range. Without compensation, output would fall as the liquid cools during prolonged use.

Intermittent Back Pressure ("Pumping Effect")

Positive-pressure ventilation or oxygen flush valve use creates retrograde pressure waves that can push gas backward into the vaporizing chamber, causing higher-than-expected output. This is:

More pronounced at: low FGF, low dial settings, low liquid levels, rapid respiratory rates, high peak pressures
Minimized in: modern variable bypass vaporizers (outlet check valve design)

Carrier Gas Composition

Nitrous oxide (N2O) in the carrier gas can initially cause lower output vs. oxygen alone, because N2O is more soluble in the liquid anesthetic, temporarily "absorbing" into it
Helium-based carriers affect viscosity and flow resistance, potentially altering output

Altitude / Barometric Pressure

Variable bypass vaporizers: As altitude increases and barometric pressure falls, SVP (which is pressure-independent) stays constant, so the volume percent (v/v%) of anesthetic in the vaporizing chamber rises. However, because the partial pressure of anesthetic determines anesthetic depth (not volume percent), the clinical effect is minimal - no dial adjustment is needed for variable bypass vaporizers at altitude.
Mathematically: the increased volume percent is offset by the lower barometric pressure, and partial pressure output is nearly unchanged.
Barash's Clinical Anesthesia, 9e; Miller's Anesthesia, 10e

5. Desflurane Vaporizer (Tec 6 / D-Vapor)

Because desflurane cannot be used in a standard variable bypass vaporizer (SVP 669 mm Hg, boiling point 22.8°C), a purpose-built electrically heated, pressurized vaporizer was developed.

Why standard vaporizers fail for desflurane:

SVP so high that dilution would require impossibly large bypass flows (~12 L/min to achieve 6%)
Rapid evaporation would cause extreme cooling - uncompensatable with standard temperature mechanisms
Boiling point near room temperature - agent would boil in the vaporizing chamber, making output uncontrollable

Tec 6 / Dual-Circuit Design

The Tec 6 is more accurately described as a dual-gas blender than a vaporizer:

The desflurane sump is electrically heated to 39°C, generating vapor at 1300 mm Hg pressure
A shut-off valve is either fully closed (dial OFF) or fully open (dial ON)
A pressure-regulating valve downregulates desflurane vapor pressure to match fresh gas circuit pressure
The operator controls output concentration via the concentration control valve (R2) - a variable restrictor
Fresh gas and desflurane vapor run in two independent parallel circuits that combine at the outlet

Altitude effect on Tec 6: Unlike variable bypass vaporizers, the Tec 6 maintains a constant volume percent (not partial pressure) output regardless of ambient pressure. At altitude, partial pressure of desflurane falls proportionally:

Required dial setting = Normal dial setting × (760 mm Hg / ambient pressure)

So at 10,000 ft (~500 mm Hg), the dial must be increased by ~52% to maintain equivalent anesthetic depth. - Barash's Clinical Anesthesia, 9e

6. Cassette Vaporizer (GE Aladin)

The Aladin system (used on GE Aisys/Avance Carestations) is a single electronically controlled vaporizer that handles all five agents (halothane, isoflurane, enflurane, sevoflurane, desflurane) using interchangeable cassettes.

Key features:

Agent-specific, color-coded and magnetically coded cassettes (workstation auto-identifies the agent)
Functionally similar to variable bypass: bypass chamber + vaporizing chamber with fixed restrictor
Heart of the system: electronically controlled flow valve at the vaporizing chamber outlet, regulated by a CPU
CPU receives input from: concentration dial, pressure sensor, temperature sensor, bypass flow sensor, vaporizing chamber flow sensor, and flowmeter gas composition data
A one-way check valve at the vaporizing chamber inlet prevents retrograde flow - essential when delivering high-pressure desflurane vapor
Barash's Clinical Anesthesia, 9e

7. Injection Vaporizers

Some newer anesthesia platforms (e.g., Dräger Zeus, GE Aisys CS2 with certain configurations) use an injection-based system: liquid anesthetic is injected directly into the gas stream in metered doses, controlled electronically. This eliminates the traditional vaporizing chamber entirely.

8. Safety Features

Feature	Purpose
Agent-specific keyed fillers	Prevent filling with wrong agent (color-coded, shape-coded)
Filler port at maximum safe level	Prevents overfilling
Interlock system	Prevents two vaporizers operating simultaneously
Removable mounts	Allow rapid exchange; also enable removal if MH is suspected
Outlet check valve (some machines)	Prevents backflow into vaporizer during IPPV
Tipping indicator	Liquid agent in the bypass channel after tipping causes overdose

Misfilling Hazard

If a vaporizer (calibrated for isoflurane or sevoflurane) is accidentally filled with desflurane, a severe overdose results because desflurane's SVP (669 mm Hg) is far higher than the other agents. Agent-specific fillers reduce but do not eliminate this risk. Breathing circuit gas analysis is the last line of defense. - Miller's Anesthesia, 10e

9. Wash-In and Circuit Equilibration

Anesthetic delivery from the vaporizer (in L/min of gaseous drug) is approximated by:

V delivered = F_del × FGF × time

Where F_del = fractional concentration set on dial, FGF = fresh gas flow.

Circuit equilibration follows a single exponential function:

P_circ(t) = P_circ(0) + (P_del - P_circ(0)) × (1 - e^(-FGF/V_circ × t))

Time constant (τ) = V_circ / FGF
Doubling FGF halves wash-in time; doubling circuit volume doubles it
To wash anesthetic out, delivered partial pressure must be less than circuit partial pressure
Miller's Anesthesia, 10e

10. Quick Reference Summary

Feature	Variable Bypass	Tec 6 (Desflurane)	Aladin Cassette
Agent(s)	Halothane, iso, enf, sevo	Desflurane only	All 5 agents
Heating	Passive (thermal mass)	Active (39°C electric)	Passive
Temperature comp.	Bimetallic strip / expansion element	Heated sump	Electronic
Altitude behavior	Partial pressure stable; no dial change needed	Vol% stable; dial adjustment needed	Agent-dependent
Key safety risk	Misfilling, pumping effect	Requires power; heater failure alarm	Electronic failure

Sources:

Miller's Anesthesia, 10e (Chapters 18, 20)
Barash, Cullen, and Stoelting's Clinical Anesthesia, 9e (Chapter 25)

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