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Capnography

Principles and Clinical Application

Objectives

Describe the advantages of mainstream vs. sidestream CO2 technology.

Discuss normal and abnormal V/Q relationships. Identify a normal capnogram and discuss phase

I thru IV. Discuss the ETCO2/PaCO2 gradient and its

clinical application. Interpret abnormal capnograms and their clinical

intervention.

. .

Capnography - Technology Capnographs utilize infrared (IR) technology

CO2 molecules absorb IR light energy of a specific wavelength

Amount of energy absorbed = CO2

concentration

Infrared is particularly appropriate for measuring CO2

CO2 has a strong absorption band in the infrared spectrum

• In the ICU, the CO2 band is distinct enough from other gases to minimize interference

Capnography – Technology Capnography vs. Capnometry

Capnography

Measurement & display of ETCO2 and the CO2 capnogram

Measured by a capnograph

Capnometry

Measurement & display of the ETCO2

value Measured by a

capnometer

ETCO 2

R R

ETCO 2

R R

CapnographyQuantitative vs. Qualitative ETCO2

Quantitative ETCO2

Provides actual numeric value

Found in capnographs and capnometers

Qualitative ETCO2

Only provides range of values

Termed CO2 detectors - Easy Cap

ETCO 2

R R

mmH g

ETCO 2

0-10

11-20

21-30

31-40

over 40

0-4

5-20

>20

Capnography

Mainstream

vs. SidestreamE TC O 2

R R

E TCO 2

R R

Capnography - Mainstream Sensor placed in ventilator

circuit

Measurement made at the patient’s airway

IR sensor can not be contaminated by patient secretions!

Fast response time

No water traps or tubing needed - hassle free

Sensor

E TC O 2

R R

Capnography – Sidestream

Sensor located away from the airway

IR sensor can be contaminated by patient secretions!!

Measurement made by pump inside the monitor

Slower response time

Water traps and tubing required troubleshooting and maintenance

E TC O 2

R R

Sample measuredinside monitor

CO2 sampleAcquired here

CapnographySolid State vs. Chopper Wheel

Solid State CO2 Sensors No moving parts =

durability Uses a beam splitter to

measure IR light at two wavelengths

IR light source electronically pulsed

Chopper Wheel CO2 Sensors Spinning wheel = very fragile Spins to change parameter

measured by photodetector Gas sample to be measured

(data) Sample plus sealed gas

reference cell No light at all

Capnography

What Are We Measuring?

CapnographyRespiration - The Big Picture

1Cellular Metabolism of food into energy - O2 consumption & CO2 Production

2 Transport of O2 & CO2 between cellsand pulmonary capillaries

3 Ventilation between alveoli and pulmonary capillaries

Capnography Depicts Respiration

Capnography

Ventilation

O2

Transport

CO2

CO

2

Metabolism

CO2

E TC O 2

R R

Capnography

Arterial CO2 (PaCO2)

from ABGETCO2

from Capnograph

Normal Arterial & ETCO2 Values

Normal PaCO2 Values:

35 - 45 mmHg

Normal ETCO2 Values:

30 - 43 mmHg

E TCO 2

R R

Capnography

Arterial - End Tidal CO2 Gradient

In healthy lungs the normal PaCO2 to ETCO2 gradient is 2-5 mmHg

In diseased lungs, the gradient will increase due to ventilation/perfusion mismatch

Ventilation- Perfusion Relationships

Ventilation-Perfusion RelationshipsRelationship between ventilated alveoli and blood flow

in the pulmonary capillaries

Shunt perfusionAlveoli perfused but not ventilated

Deadspace VentilationAlveoli ventilated but not

perfused

CO2 O2

NormalVentilation and

perfusion is matched

Normal V/Q

CO2 O2

ETCO2 / PaCO2

Gradient =2 to 4 mmHg

. .

Shunt Perfusion – Low V/Q

No exchange of O2 or CO2

ETCO2 / PaCO2

Gradient =4 to 10 mmHg

. .

Shunt Perfusion – Low V/Q

Disease processes that may cause Shunt Perfusion: Mucus plugging ET tube in right or left main stem bronchus Atelectasis Pneumonia Pulmonary edema

In short anything that causes the alveoli to collapse or is alveolar filling

. .

Dead Space Ventilation

High V/Q

Perfusion is the problemNo exchange of O2 or CO2occurs

ETCO2 / PaCO2

Gradient is large

Ventilation is not the problem!

. .

Dead Space

Why is understanding Alveolar Dead Space important? As Alveolar Dead Space increases, the

gradient between ETCO2 and PaCO2

increases

Why does increased Alveolar Dead Space create a gradient?

Dead Space Ventilation

0 0 0

0

0

0 0

PaCO2 = 53 mmHg

ETCO2 = 33 mmHg

53

53

53 Alveoli that do not take part in gas exchange will still have no CO2 –Therefore they will dilute the CO2 from thealveoli that wereperfused

The result is a widened ETCO2 to PaCO2 Gradient

A Gradient is a Good Thing

Why? Lets clinicians know when patient status

improves

• PaCO2/ETCO2 gradient narrows

Aids in determining what caused a drop in ETCO2

• If ventilation hasn’t changed a sudden and large drop in ETCO2 usually indicates a change in perfusion.

Dead Space Ventilation Disease processes that may cause Dead

Space Ventilation: Pulmonary embolism Hypovolemia Cardiac arrest Shock

In short anything that causes a significant drop in pulmonary blood flow

Capnography

Clinical Application of Capnography

CO (m m Hg)2

0

37

50 Real-Tim e Trend

Capnography

Clinical utility of the CO2 Waveform or capnogram

Provides validation of ETCO2 value

Visual assessment of patient airway integrity Verification of proper ET tube placement Assessment of ventilator, and breathing circuit

integrity

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The Normal Capnogram

Normal Capnogram - Phase I

50

0

25

CO2 mmHg

Beginning of expiration =anatomical deadspace with no measurable CO2

A B

Anatomical Dead Space

Anatomical Dead Space Internal volume of the

upper airways• Nose • Pharynx• Trachea• Bronchi

Anatomical DeadspaceConducting Airway - No Gas Exchange

Normal Capnogram - Phase II

50

0

25

CO2 mmHg

Mixed CO2, rapid rise in CO2 concentration

B

C

Normal Capnogram - Phase III

50

0

25

CO2 mmHg

Time

Alveolar Plateau, all exhaled gas took part in gas exchange

End Tidal CO2 value

C D

Normal Capnogram - Phase IV

50

0

25

CO2 mmHgInspiration starts,

CO2 drops off rapidly

E

D

Capnogram – Valuable Tool

CO2 (mmHg)

0

25

50

Alveolar Plateau established

No Alveolar Plateau

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Abnormal

CO2 Waveforms

Capnography

Endotracheal Tube in Esophagus

Possible Causes: Missed Intubation A normal capnogram is the best evidence that

the ET tube correctly positioned. When the ET tube is in the esophagus, little or

no CO2 is present

CO (m m Hg)2

0

37

50 Real-Tim e Trend

Capnography

Obstruction in Airway or Breathing Circuit

Possible Causes: Partially kinked or narrowed artificial airway Presence of foreign body in the airway Obstruction in expiratory limb of breathing circuit Bronchospasm

CO (m m Hg)2

0

37

50 Real-Tim e Trend

Capnography

Muscle Relaxants (curare cleft)

Possible Causes: Patient attempts to take a breath Appear when muscle relaxants begin to subside Depth of cleft is inversely proportional to degree

of drug activity

CO (m m Hg)2

0

37

50 Real-Tim e Trend

Capnography

CO (m m Hg)2

0

37

50 Real-Tim e Trend

Cardiac Oscillations

Characteristics: Rhythmic and synchronized to heart rate

Capnography

Inadequate Seal Around ET Tube

Possible Causes: Leaky or uncuffed endotracheal or trach tube Artificial airway that is too small for patient

CO (m m Hg)2

0

37

50 Real-Tim e Trend

Capnography

Hypoventilation - Increase in ETCO2

Possible Causes:Decrease in respiratory rate Decrease in tidal volume Increase in metabolic rateRapid rise in body temperature

CO (m m Hg)2

0

37

50 Real-Tim e Trend

Capnography

Hyperventilation - Decrease in ETCO2

Possible Causes: Increase in respiratory rate Increase in tidal volume Decrease in metabolic rate Fall in body temperature

CO (m m Hg)2

0

37

50 Real-Tim e Trend

Capnography

Rebreathing

Possible Causes: Expiatory filter that is saturated or clogged,

expiratory valve that is sticking Inadequate inspiratory flow, or insufficient

expiratory time Anything that causes resistance to expired flow

CO (m m Hg)2

0

37

50 Real-Tim e Trend

Case Study

A 29 year old male with head injury, and a compound fracture of his femur sustained in a motorcycle accident

2 weeks post trauma on mechanical ventilation with the following philological values:

PaCO2 – 42 mmHg PaO2 – 95 mmHg

ETCO2 – 38 mmHg Total Rate – 14 bpm

Minute Ventilation – 7 L/Min

Case Study

CO (m m Hg)2

0

37

50 Real-Tim e Trend

Normal capnogram, stable trend ETCO2/PaCO2 gradient 4 mmHg

Case Study

CO (m m Hg)2

0

37

50 Real-Tim e Trend

Sudden decrease in ETCO2 from 38 mmHg to 20 mmHg and remains there

RR – increases to 24 bpmMinute Volume increases to 12 Lpm

Case Study

CO (m m Hg)2

0

37

50 Real-Tim e Trend

ABG was drawn with the following results:PaCO2 38 mmHgPaO2 59 mmHgPaCO2/ETCO2 gradient 18 mmHg

Case Study

Ventilation /perfusion lung scan was consistent

with a pulmonary embolism A sudden drop in ETCO2

Associated with a large increase in the PaCO2/ETCO2 gradient

Often is associated with pulmonary embolism

Summary

Capnography affords the clinician breath by breath trending of ETCO2 and thus a non- invasive look at ventilation

Provides an objective reason for ABG’s Trend ETCO2/PaCO2 gradient to observe

patient improvement

Changes in ventilation and perfusion are are often observed by trending the gradient