1. Principles and Clinical Application
Capnography
Principles and Clinical Application

2. 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. . .
Lets start out with our 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 discuss clinical intervention.
Lets start out by discussing capnography technology.

3. 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
Most capnographs utilize infrared (IR) technology to measure and display CO2. The CO2 molecules absorb IR light energy of a specific wavelength. The amount of energy absorbed is directly proportional to the CO2 concentration . Infrared technology is particularly appropriate for measuring CO2 as CO2 exhibits a strong absorption band in the infrared spectrum. In the ICU, the CO2 band is distinct enough from other gases to minimize interference.

4. 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
You will often hear the terms capnography and capnometry. Both measure and display ETCO2 and Respiratory rate. However, only the capnograph displays both the ETCO2 value and the capnogram waveform. The capnogram is an essential piece to evaluating the ETCO2 number. Let take a closer look at this.

5. Quantitative vs. Qualitative ETCO2
Capnography
Quantitative 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
The format for reported end-tidal CO2 can be classified as quantitative (an actual numeric value) or qualitative (low medium or high).. Quantitative ETCO2 monitors provide an actual numeric value, this is the the type of technology found in capnographs and capnometers. Although not absolutely necessary for some applications, for example verification of of proper ET tube variation, quantitative ETCO2 is needed in order to take advantage of most of the major benefits of CO2 measurements. Qualitative ETCO2 monitors only provide a range of values. Another form of CO2 monitoring that is sometimes used to verify ET tube placement is the colorimetric CO2 detectors The litmus paper used in such detectors as Easy Cap and incorporated into some bag-mask resuscitators is qualitative ETCO2. They change color to indicate the presence of CO2 in exhaled gas. The difference in color becomes less visible with time, and at some point usually 1 to 2 hours does not change color at all.

6. Capnography Mainstream vs. Sidestream
Capnography units on the market today utilize both Mainstream and Sidestream technology. There are many advantages to Mainstream capnography over Sidestrean . Let take a closer look.

7. 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
With mainstream capnography, the sensor is placed directly in the ventilator circuit. The exhaled gas is sampled and measured at the “Y” of the patient circuit. The infrared sensor can not be contaminated by patient secretions, as it is protected by the airway adaptor to which the sensor is attached. Measurements taken at the ET tube result in fast response times. Measuring the exhaled CO2 at the airway eliminates the need for water traps or additional tubing. CO2 sensors today are light weight, thus alleviate any concern regarding pulling or added weight on the ET tube. Add this all together and mainstream technology results in hassle free CO2 monitoring.

8. 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
Sample measured
inside monitor
Sidestream technology on the other hand locates the sensor away form the airway and the patient. The IR sensor can be contaminated by patient secretions as moisture has access to the infrared sensor. (press down key) The actual gas sample is taken at the patient airway,(press down key) but then is pulled back through the tubing and analyzed inside the monitor. This can often result in slower response time. The water traps and tubing utilized by side stream technology require troubleshooting and maintenance.
CO2 sample
Acquired here

9. Solid State vs. Chopper Wheel
Capnography
Solid 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
There is also a difference in the way IF technology is applied. The two most common approaches are Solid State and the Chopper Wheel. Solid State CO2 Sensors do not have any moving parts which translates into durability. Dropping a solid state CO2 sensor will usually not result in a broken sensor. Solid State utilizes a beam splitter to measure IR light at two different wavelengths. The IR light source is then electronically pulsed to produce the CO2 reading.
The Chopper Wheel CO2 Sensor on the other hand does have moving parts which makes it much less durable. With this technology the IR beam is directed through the gas sample, then through the gas sample and a sealed gas reference cell, and finally through the sample cell and a dark cell with no light and 0 CO2 to obtain the reading. Any piece of equipment that has moving parts is subject to wear and break downs. Pulse oximeters last forever, because they have no moving parts. A Solid Sate CO2 monitor will offer the same durability, as they also do not have moving parts.

10. What Are We Measuring? Capnography
So now that we are through discussing technology, let take a look at just what we are measuring. Where does CO2 come form. We know that there is essentially no CO2 in inspired air.

11. Respiration - The Big Picture
Capnography
Respiration - The Big Picture 1
Cellular Metabolism of food into energy - O2 consumption & CO2 Production 2
Transport of O2 & CO2 between cells
and pulmonary capillaries 3
Ventilation between alveoli
Lets start with the big picture of Respiration. Oxygen and food is converted into energy in a process called Metabolism. We have O2 consumption by the tissues and as a by product of this process CO2 is produced. (push down key) The Transport of O2 between the lungs and the tissues, and then the transfer of CO2 back to the lung is provided by the heart and circulatory system, (push down key) in the lungs ventilation between alveoli and the pulmonary capillaries occurs.

12. Capnography Depicts Respiration
Transport
CO2
Ventilation
Because all three components of respiration, (push down key) metabolism, (push down key) transport, (push down key) and ventilation are involved in the appearance of CO2 in exhaled gas, capnography gives an excellent picture of the respiratory process. Capnography gives you a breath by breath look at ventilation just like the pulse oximeter gives a beat by beat look at oxygenation. We all know that many times the oxygenation may look good, but CO2 may be rising, especially if the patient is on supplemental oxygen. Now let take a look at normal ETCO2 and PaCO2. O2
Metabolism
CO2

13. Normal Arterial & ETCO2 Values
Capnography
Normal Arterial & ETCO2 Values
ETCO2
from Capnograph
Arterial CO2 (PaCO2)
from ABG
Normal PaCO2 Values:
Normal ETCO2 Values:
Normal PaCO2 as measured by a blood gas, is 35 to 45 mmHg. The ABG is the gold standard. However there are several issues associated with the invasiveness of obtaining a sample, and cost. Additionally the ABG is a snapshot in time. Ten minutes later everything can change and as a clinician you know that early intervention is best. Trending PCO2 give the clinician an objective reason to do an ABG. The normal value for ETCO2 is 30 to 43 mmHg.
mmHg
mmHg

14. Arterial - End Tidal CO2 Gradient
Capnography
Arterial - End Tidal CO2 Gradient
In healthy lungs the normal PaCO2 to ETCO2 gradient is 2-5 mmHg
Even in healthy lungs there is a small PaCO2 to ETCO2. In a young healthy adults the gradient may be a little as 1 or 2 mmHg. Normal range is 2 to 5 mmHg. In diseased lungs, the gradient will increase according to the ventilation/perfusion abnormality.
In diseased lungs, the gradient will increase due to ventilation/perfusion mismatch

15. Ventilation- Perfusion Relationships
Relationship between ventilated alveoli and blood flow in the pulmonary capillaries
CO2 O2
Normal
Ventilation and
perfusion is matched
Shunt perfusion
Alveoli perfused
but not ventilated
Deadspace Ventilation
Alveoli ventilated but not perfused
There are three scenarios that describe ventilation perfusion relations. Normal, (press down key) Shunt Perfusion (press down key) and Deadspace Ventilation. We’ll take a closer look at each of these ventilation perfusion relationships and how they effect ETCO2 and the ETCO2 to PaCO2 gradient, Lets start with normal V/Q.

16. . . Normal V/Q ETCO2 / PaCO2 Gradient = 2 to 4 mmHg CO2 O2
The ventilation–perfusion ratio (V/Q) describes the relationship between air flow in the alveoli and blood flow in the pulmonary capillaries. If ventilation were perfectly matched to perfusion, then V/Q would be 1. However, even in the normal lung ventilation and perfusion are not equally matched. The normal lung has an over all V/Q of 0.8. When a normal V/Q exists the ETCO2/PaCO2 gradient is 2 to 5 mmHg. Often the cardiovascular systems are normal in patients who suffer head injuries, drug overdose, or cerebral vascular accidents. In these patients the ETCO2 to PACO2 gradient may be as little as 2 or3 mmHg. Now let take a look at what happens to the ETCO2/PaCO2 gradient when V/Q is not normal.
CO2 O2

17. Shunt Perfusion – Low V/Q. .
Shunt Perfusion – Low V/Q
ETCO2 / PaCO2
Gradient =
4 to 10 mmHg
Shunt Perfusion is commonly referred to as a Low V/Q ratio. As you can see here, in shunt perfusion the abnormality is lung related, as the alveoli are perfused by not ventilated. The ETCO2 / PaCO2 Gradient in shunt perfusion is usually larger than normal in the 4 to 10 mmHg. In Shunt Perfusion the circulatory system can often provide some compensation by moving the blood flow to where the ventilation is. Additionally, oxygenation is usually more affected than CO2 removal. Why? CO2 diffuses 19 times faster and thus has the capacity to maintain normal CO2 levels longer.
No exchange of O2 or CO2

18. Shunt Perfusion – Low V/Q. .
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
Disease processes that cause Shunt Perfusion are:
Mucus plugging, ET tube in right or left main stem bronchus, Atelectasis, Pneumonia, Pulmonary edema, or in general anything that causes the alveoli to collapse, or be be full of fluid or secretions. Now let move our discussion to Dead Space Ventilation.

19. Dead Space Ventilation. .
High V/Q
ETCO2 / PaCO2
Gradient is large
Ventilation is
not the problem!
Deadspace ventilation occurs under conditions in which alveoli are ventilated but not perfused. This V/Q abnormality is often referred to as a high V/Q (push down key) Dead Space is not a ventilation abnormality. (push down key) Perfusion is the problem. No exchange of O2 or CO2 can occur. In Dead Space Ventilation, the circulatory system cannot partially compensate like we discussed in Shunt Perfusion. There is simply not enough blood flow.(push down key) In this V/Q abnormally, the ETCO2 to PaCO2 gradient is large. Why? Let take a closer look.
Perfusion is the problem
No exchange of O2 or CO2occurs

20. 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?
At this point we need stop and come to clear understanding of Alveolar Dead Space. One might ask: Why is understanding Alveolar Dead Space important? Because as Alveolar Dead Space increases, the gradient between ETCO2 and PaCO2 increases. I often have discussions with clinicians during which I hear that the ETCO2 monitor is not working. When I inquire why, the answer I get is that ETCO2 does not match the blood gas. So it is important that we understand why increased Dead Space creates a larger gradient? Lets take a closer look.

21. Dead Space Ventilation
ETCO2 = 33 mmHg
PaCO2 = 53 mmHg
Alveoli that do not
take part in gas
exchange will still
have no CO2 –
Therefore they will
dilute the CO2 from the
alveoli that were
perfused 53
This is obviously a depiction of a very simplified alveolar sac, but we see depicted is a cluster of alveoli, and that only a few of them are perfused. In the alveoli that are perfused the ETCO2 would be the same as the PaCO2 as we can see here. If technology existed that allowed us to just measure the three alveoli that did come in contact with perfusion, then ETCO2 would match PaCO2. However this technology does not exist. (push down key) Here we can see that all of the alveoli that did not come in contact with perfusion contain no CO2. During exhalation all of the alveoli eventually empty into the trachea. (push down key) Alveoli that do not take part in gas exchange will still have no CO2 – Therefore they will dilute the CO2 from the of alveoli that were perfused. (push down key) When we measure the exhaled CO2 at the patients airway the end result is a widened ETCO2 to PaCO2 gradient. In fact it may be as much as 20 to 25 mmHg. Now is it the patient or the capnograph that isn’t working? Clearly it is the patient, the capnograph can only measure and display the CO2 present in expired gas.
The result is a widened ETCO2 to PaCO2 Gradient

22. 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.
A gradient is actually a good thing, Why? Without the gradient it would be impossible to detect mismatched V/Q by capnography. It is only by comparing the expired with the arterial concentration of carbon dioxide that we gain incite. The gradient lets clinicians know when ventilation to perfusion is mismatched. If ventilation to perfusion is matched, the gradient will be only 2 to 4 mmHg wide. So by trending the gradient clinicians have another piece of objective information indicating when patient status improves. Why? Because the gradient narrows. Trending the ETCO2 as well as the ETCO2 gradient will help assess the effectiveness of medical intervention. It also aids in determining what caused a sudden drop in ETCO2. If ventilation hasn’t changed, a large drop in ETCO2, along with increased gradient indicates a change in perfusion. We’ll have a case study at the end of this presentation to illustrate this clinical scenario.

23. 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
The disease processes that may cause Dead Space Ventilation are of course all related to the cardiovascular system: Pulmonary embolism, Hypovolemia, Cardiac arrest, Shock. In short anything that causes a significant drop in pulmonary blood flow.

24. Clinical Application of Capnography
Next we’ll move our discussion to the clinical application of capnography.

25. 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
The capnogram is a graphical plot of the concentration of carbon dioxide as a function of time. The shape of the capnogram offers many clinical applications: provides validation of ETCO2 value, and visual assessment of patient airway integrity. Verifies proper ET tube placement, and assessment of ventilator, and breathing circuit integrity. Viewing a numerical value for ETCO2 without its associated capnogram is analogous to viewing the heart rate value from an electrocardiogram without the waveform. The normal capnogram has four distinct phases, let start by taking a look at phase one.

26. The Normal Capnogram
As with any technology that utilizes pattern recognition, a firm understanding of the normal capnogram is required to recognize and interpret the abnormal capnogram. There are four distinct changes to the normal capnogram, we’re going to start with phase I.

27. Normal Capnogram - Phase I
CO2 mmHg 50 25
During inspiration CO2 is essentially zero and thus inspiration is displayed at the zero baseline. Phase I occurs as exhalation begins, which is shown as A to B on the capnogram. The first gas to appear at the sampling point is the last gas that was inhaled into the conducting airways. This gas has not been subject to gas exchange and thus is essentially free of carbon dioxide and remains at the zero baseline as you see in blue here. What is anatomical dead space? A B
Beginning of expiration =
anatomical deadspace with no measurable CO2

28. Anatomical Dead Space Anatomical Dead Space
Conducting Airway - No Gas Exchange
Anatomical Dead Space
Internal volume of the upper airways
Nose
Pharynx
Trachea
Bronchi
Remember, the Anatomical Dead Space is the internal volume of the upper airways were no gas exchange takes place. This includes the nose, pharynx, trachea, and bronchi.

29. Normal Capnogram - Phase II
CO2 mmHg 50 25 C
Phase II is characterized by a rapid rise in CO2 concentration as anatomical deadspace is replaced with alveolar gas, leading to Phase III. B
Mixed CO2, rapid rise in CO2 concentration

30. Normal Capnogram - Phase III
Alveolar Plateau, all exhaled gas took part in gas exchange
CO2 mmHg 50 25 C D
End Tidal
CO2 value
In phase III all of the gas passing by the CO2 sensor is alveolar gas which causes the capnograph to flatten out. This is often called the Alveolar Plateau. (press down key) The ETCO2 value displayed on the monitor is the highest value measured during exhalation and usually occurs just prior to inspiration.
Time

31. Normal Capnogram - Phase IV
Inspiration starts,
CO2 drops off rapidly
CO2 mmHg 50 25 D
Phase four is inspiration and marked by a rapid downward direction of the capnograph. This downward stroke corresponds to the fresh gas which is essentially free of carbon dioxide that passes the CO2 sensor during inspiration. The capnograph will then remain at zero baseline throughout inspiration. Now we’ve identified what the normal capnograph looks like, let take a look at some abnormal ones. E

32. Capnogram – Valuable Tool
Alveolar Plateau
established
No Alveolar Plateau
CO2 (mmHg) 50 25
In order to evaluate the ETCO2 value the clinician needs the capnogram. If we look at the capnogram on the left we cans see that the alveolar plateau is established, giving greater confidence to the displayed ETCO2 number. (push down key). The capnogram on the right does not establish an alveolar plateau, alerting the clinician that the value of ETCO2 is likely lower than the actual ETCO2 value, further widening the ETCO2 to PaCO2 gradient. The clinician should evaluate why the patient is transitioning to inspiration before the alveolar plateau is established. Clinical events to consider would include: pain, decreased PaO2, agitation, fever and the like. Interventions should be aimed at returning the capnogram to where an alveolar plateau is present.

33. Abnormal
CO2 Waveforms
Now that we’ve identified what the normal capnogram looks like, let take a look at some abnormal ones. There are many events of clinical significance that you can observe on the CO2 waveform. Lets take a closer look.

34. Endotracheal Tube in Esophagus
Capnography
Endotracheal Tube in Esophagus
On this and on the following slides you can observe the CO2 waveform and CO2 trend. What do you suppose is happening here, CO2 is initially present, diminishes with each breath, then drops to zero.( Press down arrow) Did anybody say endotracheal tube in the esophagus? A normal capnogram is the best available evidence that the ET tube is correctly positioned and that proper ventilation is occurring. When the ET tube is in the esophagus, either no CO2 is sensed or only small transient waveform are present. Often because we bag the patient with a bag-mask-resuscitator prior to intubation, there is a small amount of CO2 in the esophagus. Additionally, if the patient imbibed any carbonated beverage prior to being intubated, CO2 may be present in the stomach. After several breaths, that CO2 will be washed out. After 3 to 6 breaths, if the ET tube is in the esophagus, little or no CO2 is present.
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

35. Obstruction in Airway or Breathing Circuit
Capnography
Obstruction in Airway or Breathing Circuit
On these capnograms we see a blunting of exhalation. What clinical scenarios should you consider? (push down arrow) It could be a partially kinked or narrowed artificial airway. Sometime secretions will dry and laminate the inside of ET tube causing a resistance to flow. As you can see here, expiratory flow is effected first and then expiratory flow. The presence of a foreign body in the airway such as secretions or a tumor, or obstruction in expiratory limb of breathing circuit can also result in a similar capnogram. Severe bronchospasms can also cause the the blunting we see here.
, such as thick secretions or Bronchospasm
Possible Causes:
Partially kinked or narrowed artificial airway
Presence of foreign body in the airway
Obstruction in expiratory limb of breathing circuit
Bronchospasm

36. Muscle Relaxants (curare cleft)
Capnography
Muscle Relaxants (curare cleft)
What do you think could cause what we see here? (push button) If the patient is on paralytics and attempts to take a breath this cleft is on The diaphragm is the first muscle of the body to recover from being paralyzed. If you are gradually removing paralytics this will alert you to the fact that spontaneous effort have returned. If you want the patient to be totally paralyzed, it lets you know that it is wearing off. The depth of the cleft is inversely proportional to the degree of drug activity. Although paralytics are used sparlly in the ICU, this curare cleft can be a tool to gain insight into the depth of muscle relaxation.
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

37. Capnography Cardiac Oscillations Characteristics:
Here we see rhythmic palpations on the inspiratory phase of the capnogram. Undulations in the capnogram that are synchronous with cardiac contractions are called carcinogenic oscillations. Usually the respiratory rate provided by the capnograph will be similar to the heart rate. This effect is synchronous with changes in pulmonary blood volume. During systole, when contraction of the right heart fills the pulmonary vascular system, a small volume of gas is expelled form the lungs. During diastole, as blood drains from the pulmonary vascular system into the atrium and fills the ventricle, the heart generates a small inspiratory movement. Capnograms from patients suffering severe emphysema tend not to register cardiogenic oscillations. This is generally a benign effect.
Characteristics:
Rhythmic and synchronized to heart rate

38. Inadequate Seal Around ET Tube
Capnography
Inadequate Seal Around ET Tube
This is the type of capnogram you may observe when the endotracheal or tracheotomy cuff has leak, an uncuffed tube, or an artificial airway that is too small for the patient. Phase II has a brisk upstroke, but due to the leak the concentration of CO2 falls.
Possible Causes:
Leaky or uncuffed endotracheal or trach tube
Artificial airway that is too small for patient

39. Hypoventilation - Increase in ETCO2
Capnography
Hypoventilation - Increase in ETCO2
What could be going one here, we can see a gradual increase in CO2 both on the capnogram and on the trend. Hypoventilation will usually result in an increase ETCO2. There are may causes of increase CO2: Decrease in respiratory rate or decrease in tidal volume. An increase in metabolic rate, rapid rise in body temperature or hyperthermia. Often patients who bypass PACU and come directly to the ICU will will be cold and have decreased body temperature. ETCO2 are often in low to mid thirties. As the patient starts to warm up and wake up you see a fairly rapid increase in ETCO2.
Possible Causes:
Decrease in respiratory rate
Decrease in tidal volume
Increase in metabolic rate
Rapid rise in body temperature

40. Hyperventilation - Decrease in ETCO2
Capnography
Hyperventilation - Decrease in ETCO2
Here we can see slow drop in ETCO2. Increase in respiratory rate, increase in tidal volume, decrease in metabolic rate, for a fall in body temperature. The difference between this and a pulmonary embolus is that the drop in ETCO2 is more gradual, and the gradient is usually not large.
Possible Causes:
Increase in respiratory rate
Increase in tidal volume
Decrease in metabolic rate
Fall in body temperature

41. Capnography Rebreathing Possible Causes:
So what do you think that we have going on here? An elevated CO2 baseline indicates CO2 rebreathing. An expiatory filter that is saturated or clogged will cause resistance to exhaled flow. Neb treatments or not changing the expiratory filter often enough can cause this to occur, as well as an expiratory valve that is sticking. Inadequate inspiratory flow, or insufficient expiratory time, in short anything that causes resistance to expired flow, can result in re-breathed CO2.
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

42. 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
A 29 year old male with head injury and 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 Minute Ventilation – 12 L/Min

43. Case Study Normal capnogram, stable trend ETCO2/PaCO2 gradient 4 mmHg
The capnogram is normal and the trend is stable. The ETCO2/PaCO2 gradient is 4 mmHg.
Normal capnogram, stable trend
ETCO2/PaCO2 gradient 4 mmHg

44. Case Study
Suddenly there is a decrease in ETCO2 from 38 mmHg to 22 mmHg and it remains there remains there. The patient becomes somewhat agitated, RR increases to 24 bpm, and his minute volume increases to 12 Lpm.
Sudden decrease in ETCO2 from 38 mmHg to 20 mmHg and remains there
RR – increases to 24 bpm
Minute Volume increases to 12 Lpm

45. Case Study ABG was drawn with the following results: PaCO2 38 mmHg
An ABG was drawn with the following results: PaCO mmHg, PaO mmHg, PaCO2/ETCO2 gradient increased to 18 mmHg from a previous gradient of 4 mmHg.
ABG was drawn with the following results:
PaCO mmHg
PaO mmHg PaCO2/ETCO2 gradient 18 mmHg

46. 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
Ventilation /perfusion lung scan was performed and the results were consistent with a pulmonary embolism. A sudden drop in ETCO2 Associated with a large increase in the PaCO2/ETCO2 gradient, often is associated with a pulmonary embolism.

47. 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
Capnography affords the clinician breath by breath trending of ETCO2 and thus ventilation, similar to pulse oximetry which provides beat by beat assessment of oxygenation. We have grown to accept the O2 saturation does not match the ABG, or reveal PaO2. But we do know that when O2 sat drops, PaO2 is also dropping. For the most part the same assumptions can be made regarding the relationship between PaCO2 and ETCO2. We know that adequate oxygenation and adequate ventilation are not always synonymous, especially if the patient is on supplemental oxygen. Often the ABG can reveal adequate oxygenation but elevated CO2 values. On the ventilated patient, setting changes and weaning can proceed if ETCO2, PaO2, RR, and HR remain stable. However when one or more of these parameters show a significant change, you now have an objective reason to get the blood gas. Remember, without the ABG you cannot observe narrowing of the gradient, which is another indicator that the patients pulmonary status is improving. Changes in ventilation and perfusion are often observed by trending the gradient. Remember, when the PaCO2 and ETCO2 differ, it is not the capnograph that isn’t working, it is the patient who’s has a ventilation perfusion mismatch. If a significant change is observed in ETCO2, either ventilation, perfusion or both have changed.