Capnography

• History
• Terminology
• Why capnography
• Physics
• Types
• Basic physiology
• Components of capnography
• Clinical application
• Carry home

• 1943- luft –CO2 absorbs infrared light
• Ramwell – proved it beyond doubt
• 1978- holland the first country to adopt
• 1999 – ISA ‗desirable standard‘ in
anaesthesia monitoring standards

terminology
• Capnometry
• Capnometer
• Capnography
• Capnogram
• Capnograph

Oxygenation
• Measured by pulse oximetry (SpO2)
– Noninvasive measurement
– Percentage of oxygen in red blood cells
– Changes in ventilation take minutes
to be detected
– Affected by motion artifact, poor perfusion
and some dysrhythmias

• Capnography provides information about
CO2 production, pulmonary perfusion,
alveolar ventilation, respiratory patterns,
and elimination of CO2 from the
anesthesia circuit and ventilator.

Ventilation
• Measured by the end-tidal CO2
– Partial pressure (mmHg) or volume (% vol) of
CO2 in the airway at the end of exhalation
– Breath-to-breath measurement, provides
information within seconds
– Not affected by motion artifact, poor perfusion
or dysrhythmias

Oxygenation and Ventilation
• Oxygenation
– Oxygen for
metabolism
– SpO2 measures
% of O2 in RBC
– Reflects change in
oxygenation within
5 minutes
• Ventilation
– Carbon dioxide
from metabolism
– EtCO2 measures
exhaled CO2 at
point of exit
– Reflects change in
ventilation within
10 seconds

Why Capnography ?
• Capnography, an indirect monitor
helps in the differential diagnosis of hypoxia
to enable remedial measures to be taken before
hypoxia results in an irreversible brain damage
• Capnography has been shown to be effective in
the early detection of adverse respiratory events.

• Capnography and pulse oximetry together
could have helped in the prevention of
93% of avoidable anesthesia mishaps
according to ASA closed claim study.
• Capnography has also been shown to
facilitates better detection of potentially
life-threatening problems than clinical
judgment alone

Case Scenario
• 61 year old male
• C/O: ―short-of-breath‖ and ―exhausted‖
• H/O: > 45 years of smoking 2 packs a day,
3 heart attacks, high blood pressure
• Meds: ―too expensive to take every day ‖
• Exam: HR 92, RR 18, 160/100, 2+ pitting
edema, wheezing, crackles
What other information would help in
making assessment of this pt.?

Why Measure Ventilation—
Non-Intubated Patients
• Objectively assess acute
respiratory disorders
– Asthma
– COPD
• Possibly gauge response to treatment

Why Measure Ventilation—
Non-intubated Patients
• Gauge severity of hypoventilation states
– Drug intoxication
– Congestive heart failure
– Sedation and analgesia
– Stroke
– Head injury
• Assess perfusion status
• Noninvasive monitoring of patients in DKA

• History
• Terminology
• Why capnography
• Basic physiology
• Physics
• Types
• Components of capnography
• Clinical application
• Carry home

CO2 transport
• 60% as bicarbonate ion
• 10-20% binds to amino group of proteins
mostly hemoglobin
HALDANE EFFECT
• 5-10% directly dissolved in plasma

End-tidal CO2 (EtCO2)
r r Oxygen
O
2
CO2
O
2
VeinA te y
Ventilation
Perfusion
Pulmonary Blood Flow
Right
Ventricle
Left
Atrium

• Carbon dioxide can be measured
• Arterial blood gas is PaCO2
– Normal range: 35-45mmHg
• Mixed venous blood gas PeCO2
• Exhaled carbon dioxide is EtCO2

• Reflects changes in
– Ventilation - movement of air in and
out of the lungs
– Diffusion - exchange of gases between
the air-filled alveoli and the pulmonary
circulation
– Perfusion - circulation of blood

• Monitors changes in
– Ventilation - asthma, COPD, airway
edema, foreign body, stroke
– Diffusion - pulmonary edema,
alveolar damage, CO poisoning,
smoke inhalation
– Perfusion - shock, pulmonary
embolus, cardiac arrest,
severe dysrhythmias

a-A Gradient
r r Alveolus
PaCO2
VeinA te y
Ventilation
Perfusion
Arterial to Alveolar Difference for CO2
Right
Ventricle
Left
Atrium
EtCO2

• Normal a-A gradient
– 2-5mmHg difference between the EtCO2
and PaCO2 in a patient with healthy lungs
– Wider differences found
• In abnormal perfusion and ventilation
• Incomplete alveolar emptying
• Poor sampling

Negative a-A gradient
• Pregnancy
• Infants and children
• During and after bypass
• after coming of cardiac bypass
• Low frequency high tidal volume
ventilation

Raman effect
• Electromagnetic radiation and molecule
• The transfer of energy affects the vibration
energy associated with bonds between the
atoms in a molecule
• Absorption of radiation at a particular
wave length is associated with the specific
type of bond between atoms in a
molecule.

Absorption of radiation depends on
the wavelength of radiation

• Energy of radiation is proportional to the
frequency of radiation
• the transfer of energy between the
radiation and molecule results in a change
in the wavelength of radiation

Raman spectrography
• Raman Spectrography uses the principle of "Raman
Scattering" for CO2 measurement.
• The gas sample is aspirated into an analyzing
chamber, where the sample is illuminated by a high
intensity monochromatic argon laser beam.
• The light is absorbed by molecules which are then
excited to unstable vibrational or rotational energy
states (Raman scattering).
• The Raman scattering signals (Raman light) are of low
intensity and are measured at right angles to the laser
beam.
• The spectrum of Raman scattering lines can be used
to identify all types of molecules in the gas phase

Chemical method of CO2 measurement -
pH sensitive chemical indicator

Effect of atmospheric pressure
• FEtCO2=partial pressure(atmospheric
pressure-water vapour pressure)*100
• At atm pressure of 760mmHg,
FEtCO2=38(760-47)*100 =5%
at atm pressure of 500mmHg
FEtCO2=38(500-47)*100 =8%

Influence of water vapour
1. Effect of condensed water:
Water vapor may condense on the
window of the sensor cell and absorb IR
light, thereby produce falsely high C02
readings

2. Effect of water vapor.
The temperature of the sampling gases
may decrease during the passage from the
patient to the unit, resulting in a decrease in
the partial pressure of water vapor. This can
cause an apparent increase in C02
concentration of about 1.5-2%
FEtCO2=partial pressure(atmospheric
pressure-water vapour pressure)*100

Volume capnography Time capnography

Time capnography
Advantages
• Simple and convenient
• Monitor non-intubated patients
• Monitor dynamics of inspiration and
expiration
Disadvantages
• Poor estimation of V/Q status of lungs
• Physiologic space dead space

Side-stream Capnographs
advantages
Easy to connect
No problems with sterilization
Can be used in awake patients
Easy to use when patient is in
unusual positions such as in prone
position
Can be used in collaboration with
simultaneous oxygen
administration via a nasal prong
disadvantages
Delay in recording due to movement
of gases from the ET to the unit
Sampling tube obstruction
Water vapor pressure changes
affect CO2 concentrations
Pressure drop along the sampling
tube affects CO2 measurements

Sampling of CO2 from nasal cannulae

Adequacy of spontaneous respiration
Sampling of
CO2 from
oxygen mask

Mainstream
• Advantages
No sampling tube
No obstruction
No affect due to pressure drop
No affect due to changes in water
vapor pressure
No pollution
No deformity of capnograms due to
non dispersion of gases
No delay in recording
Suitable for neonates and children
• Disadvantages
weight of the sensor, (the newer
sensors are light weight minimizing
traction on the endotracheal tube)
Long electrical cord, but it is
lightweight.
Sensor windows may clog with
secretions( they can be replaced
easily as they are disposable)
Difficult to use in unusual patient
positioning such as in prone
positions.

Capnographic Waveform
• Normal waveform of one respiratory cycle
• Similar to ECG
– Height shows amount of CO2
– Length depicts time

• Waveforms on screen and printout
may differ in duration
– On-screen capnography waveform is
condensed to provide adequate information
the in 4-second view
– Printouts are in real-time
– Observe RR on device

• Capnograph detects only CO2
from ventilation
• No CO2 present during inspiration
– Baseline is normally zero
A B
C D
E
Baseline

Phase I Dead space ventillation
Beginning of exhalation
A B
IBaseline

Phase II Ascending Phase
Alveoli
CO2 present and increasing in exhaled air
II
A
B
C
Ascending Phase
Early Exhalation

Phase III Alveolar Plateau
CO2 exhalation wave
plateaus
A B
C D
III
Alveolar Plateau

Capnogram Phase III
End-Tidal
End of the the wave of exhalation contains the
highest concentration of CO2 - number seen on
monitor
A B
C D
End-tidal

Capnogram Phase IV
Descending Phase
• Inhalation begins
• Oxygen fills airway
• CO2 level quickly
drops to zero
Alveoli

Capnogram Phase IV
Descending Phase
Inspiratory downstroke returns to baseline
A B
C D
E
IV
Descending Phase
Inhalation

Inspiratory segment
• Phase 0:
Inspiration
• Beta Angle - Angle
between phase III
and descending
limb of inspiratory
segment

Expiratory segment
• Phase I - Anatomical
dead space
• Phase II - Mixture of
anatomical and
alveolar dead space
• Phase III - Alveolar
plateau
• Alfa angle - Angle
between phase II and
phase III (V/Q status of
lung

Capnography Waveform
Normal range is 35-45mm Hg (5% vol)
Normal Waveform
45
0

Capnography Waveform Patterns
0
45
Hypoventilation RR : EtCO2
45
0
Hyperventilation RR : EtCO2
45
0
Normal

Capnography-3 sources of information
• No. – PEtCO2 values
• Shapes of capnogram
• (a-ET)PCO2 differences

(a-ET)PCO2 differences
• (a-ET)PCO2 difference is a gradient of
alveolar dead space.
increase decrease
Age
Emphysema
Low cardiac output
states
Hypovolemia
Pulmonary embolism
Pregnancy and
Children

Five characteristics of capnogram
should be evaluated
The shape of a capnogram is identical in all
humans with healthy lungs.
Any deviations in shape must be investigated to
determine a physiological or a pathological cause
of the abnormality
• Frequency
• Rhythm
• Height
• Baseline
• Shape

• A terminal upswing
at the end of phase
3, known as phase
4, can occur in
pregnant subjects,
obese subjects
and low
compliance states

The slope the expiratory plateau is increased as a
normal physiological variation in pregnancy

Prolonged inspiratory descending limb
• due to dispersion
gases in the sampling
line or as well as
prolonged response
time of the analyzer.
Seen in children who
have faster
respiratory rates

Base line elevated in
• Inadequate fresh gas flow
• Accidental administration of CO2
• Rebreathing
• Insp / exp valve malfunction
• Exhausted CO2 absorber

Contamination of CO2 monitor
• sudden elevation
of base line and
top line

Expiratory valve malfunction
• Expiratory valve
malfunction can
result in prolonged
abnormal phase 2
and phase 0

Inspiratory valve malfunction
• Elevation of the
base line,
prolongation of
down stroke,
prolongation of
phase III

Bain circuit
• Inspiratory base
line and phase I
are elevated above
the zero due to
rebreathing. Note
the rebreathing
wave during
inspiration.

Hypoventilation
• Gradual elevation
of the height of the
capnogram, base
line remaining at
zero

hyperventillation
• Gradual decrease
in the height of the
capnogram, base
line remaining at
zero

Cardiogenic oscillations.
• Ripple effect,
superimposed on
the plateau and the
descending limb,
resulting from
small gas
movements
produced by
pulsations of the
aorta and heart.

Airway obstruction (eg., bronchospasm). Phase II and phase III
are prolonged and alpha angle (angle between phase II and
phase III) is increased

bronchospasm
during After relief

hypothermia
• A gradual decrease in
end tidal carbon
dioxide
hypothermia,
reduced metabolism,
hyperventilation,
leaks in the sampling
system

Kyphoscoliosis
• The CO2 waveform
has two humps.
resulted in a
compression of
the right lung

• Capnogram during
spontaneous
ventilation in
adults

• Sampling
problems such air
or oxygen dilution
during nasal or
mask sampling of
carbon dioxide in
spontaneously
breathing patients.

Detection of pulmonary air embolism
• A rapid decrease of PETCO2
in the absence of changes in
blood pressure, central
venous pressure and heart
rate indicates an air embolism
without systemic
hemodynamic consequences.
• as the size of air embolism
increases, a reduction in
cardiac output occurs which
further decreases PETCO2
measurement. A reduced
cardiac output by itself can
decrease PETCO2.

Effective circulating blood volume can
reduce the height of capnograms

Phases of the Capnogram
Phase I
Expiration
Represents
anatomical
dead space
Phase II
Expiration
Mixture of
anatomical and
alveolar dead
space
Phase III
Expiration
Plateau of
alveolar
expiration
Phase 0
Inspiration
Rapid fall
in CO2
concentration
Phase IV
Exhalation
Compromised
thoracic
compliance

Hyperventilation
Progressively lower plateau (phase II) segment
Baseline remains at zero
Decreasing CO2 levels

Hypoventilation
Steady increase in height of Phase II
Baseline remains constant

Spontaneous Ventilation
Short Alveolar plateau
Increased frequency of waveforms

Cardiogenic Oscillations
Ripples during Phase II and Phase III
Due to changes in pulmonary blood volume and
ultimately CO2 pressure as a result of cardiac
contractions

Curare Cleft
Shallow dips in phase II plateau
Can occur when patient is in a light plane of
anesthesia
Represent patient attempts to breathe independent of mechanical ventilation

Bronchospasm
Airway Obstruction
COPD Sloping of inspiratory and expiratory segments
Prolonged Phase II and Phase III

Rebreathing of Soda Lime
Contamination with CO2
Elevation of Phase II segment and
baseline
Elevation of baseline and Phase II, smaller inspiratory efforts
Progressive elevation of Phase II and baseline

Bain System
Smaller wave form represents rebreathing of CO2

Slow ventilation
Incompetent inspiratory valve
Prolongation of Phase 0

• Capnography provides
another objective data
point in making a
difficult decision

Capnography

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