chapter 41. quiz 3. General Concepts of Exhaled CO2 Monitoring in Anesthesiology

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Questions and Answers

What is the primary piece of information that exhaled CO2 provides?

all of the above (correct)

Cardiac output

Ventilation

Pulmonary blood flow

What is the name of the equation used to estimate the ratio of physiologic dead space to tidal volume?

Bohr equation (correct)

CO2 equation

Physiologic equation

Ventilation equation

What is the term used to describe the measurement and quantification of inhaled or exhaled CO2 concentrations at the airway opening?

Ventilation measurement

Capnometry (correct)

CO2 analysis

Capnography

What is the device that records and displays CO2 concentrations, usually as a function of time?

Capnograph Signup and view all the answers

What is the graphic display generated by a capnograph?

Capnogram Signup and view all the answers

Why is continued monitoring of exhaled CO2 important during anesthesia?

To ensure correct placement of ETT or LMA Signup and view all the answers

How does the most commonly used method in clinical environments detect CO2 concentrations in respiratory gases?

By passing a beam of infrared light through a gas sample (nondispersive infrared absorption) Signup and view all the answers

What is the wavelength around which gaseous CO2 primarily absorbs light in the infrared spectrum?

4.26 μm Signup and view all the answers

How do infrared filters and compensation algorithms improve the accuracy of CO2 measurement using nondispersive infrared absorption?

By minimizing interference from other gaseous species' spectra Signup and view all the answers

What role does a reference cell with no detectable CO2 play in capnometers?

It serves as a comparison for the gas sample being analyzed Signup and view all the answers

What purpose does the chopper disk rotating at 60 revolutions per second serve in capnometers?

To direct the light alternatively through the sample and reference cells Signup and view all the answers

Most capnometers rely on infrared-light source that is focused on ?

chopper disk that rotates at approximately 60 revolutions per second Signup and view all the answers

What is the primary distinction between sidestream and mainstream capnometers?

The location of the CO2 sensor relative to the airway gases Signup and view all the answers

What is a potential issue with sidestream analyzers, especially in certain clinical scenarios?

Delay in response time to changes in CO2 concentration Signup and view all the answers

What happens to the gases aspirated by the pump or compressor in a sidestream analyzer?

They pass through filters and water traps before analysis Signup and view all the answers

What is a characteristic of the capnograms generated by sidestream analyzers?

They have an associated rise time Signup and view all the answers

What can be done with the gas volume withdrawn by a sidestream analyzer?

It can be returned to the circuit or routed to a scavenger Signup and view all the answers

What is the typical length of tubing used in sidestream analyzers?

6 feet Signup and view all the answers

What is the significance of the rise time in the context of capnometers?

Determining the time interval for the analyzer output to change from 10% to 70% of its final value Signup and view all the answers

What is the primary advantage of mainstream analyzers over sidestream analyzers when it comes to rise time?

Faster rise time Signup and view all the answers

How does heating the sample cell in mainstream analyzers to 40°C benefit the measurement process?

Prevents the bias of CO2 measurement due to water vapor condensation Signup and view all the answers

What is a potential drawback of mainstream analyzers despite their faster rise time?

Higher dead space creation Signup and view all the answers

What factors contribute to the rise time variations among commercially available capnometers?

Chopper wheel rotation, gas aspiration rate, and volume of aspiration tubing Signup and view all the answers

How does the proximity of the sensor to the patient's airway impact the rise time of mainstream analyzers?

Accelerates the rise time Signup and view all the answers

what is the Typical rise times for Sidestream Capnometers

10 to 400 ms Signup and view all the answers

Capnograms A represent ?

Normal capnogram during controlled mechanical ventilation Signup and view all the answers

Capnograms B represent ?

normal capnogram during spontaneous breathing Signup and view all the answers

Capnograms C represent ?

partially obstructed endotracheal tube Signup and view all the answers

Capnograms D represent ?

cardiogenic oscillations Signup and view all the answers

Capnograms E represent ?

spontaneous breathing efforts during controlled mechanical ventilation Signup and view all the answers

Capnograms F represent ?

esophageal intubation Signup and view all the answers

Capnograms G represent ?

rebreathing of CO2 Signup and view all the answers

Capnograms H represent ?

faulty inspiratory valve Signup and view all the answers

Capnograms i represent ?

single lung transplant Signup and view all the answers

Capnograms J represent ?

faulty inspiratory valve Signup and view all the answers

Capnograms K represent ?

leaking endotracheal tube cuff Signup and view all the answers

Capnograms L represent ?

leak in a sidestream sample Signup and view all the answers

What is the primary characteristic of phase I of the expiratory phase in a time capnogram?

Exhalation of gas with no detectable CO2 Signup and view all the answers

What is the primary reason for the slight upsloping of CO2 concentration versus time during phase III of the time capnogram?

Heterogeneous distribution of ventilation-perfusion throughout the lung Signup and view all the answers

What is the characteristic of well-ventilated and well-matched ventilation-perfusion regions in the lung?

Lower PCO2 and shorter time constants Signup and view all the answers

What is the significance of the plateau region (phase III) of the time capnogram?

It corresponds to the PCO2 in the alveolar compartment Signup and view all the answers

What is the primary characteristic of phase II of the time capnogram?

A sharp rise in PCO2 to a plateau Signup and view all the answers

What is the significance of the phase II of the time capnogram?

transitional gas between the airways and alveoli. Signup and view all the answers

What is the characteristic ofLess well-ventilated and poorly matched ventilation-perfusion regions in the lung?

higher CO2 levels, and they empty later Signup and view all the answers

What is the effect of respiratory pathologies on phase III of the time capnogram?

steeper upslope of phase III. Signup and view all the answers

What is the effect of interventions such as PEEP that improve ventilation homogeneity on phase III of the time capnogram?

A flatter slope of phase III Signup and view all the answers

What can cause mechanical disturbances during phase III of the time capnogram?

all of the above Signup and view all the answers

Why does phase III of the time capnogram have a slight upslope?

Due to ventilation heterogeneity Signup and view all the answers

What can influence the shape of phase III of the time capnogram?

Both ventilation heterogeneity and mechanical disturbances Signup and view all the answers

What is the term used to describe the sharp downstroke of PCO2 that occurs when fresh inspired gas washes out the remaining CO2 at the sampling site?

Phase 0 Signup and view all the answers

In capnography, what is the term for the sharp upstroke in PCO2 observed at the very end of phase III?

Phase IV Signup and view all the answers

What is the reason behind the sharp upstroke in PCO2 at the very end of phase III?

Closure of lung units with low PCO2 and allows for regions of higher CO2 to exhaled gas Signup and view all the answers

What phase of the time capnogram is believed to result from the closure of lung units with low PCO2 allowing for contributions from regions with higher CO2?

Phase IV Signup and view all the answers

Under what condition is PETCO2 measurement well correlated with PaCO2?

During a reasonably flat and undistorted phase III Signup and view all the answers

What may cause an inaccurate PETCO2 measurement according to the text?

Use of O2-enriched air during spontaneous breathing Signup and view all the answers

What is the cause of the increased PaCO2 − PETCO2 difference ?

high V/Q regions Signup and view all the answers

In what condition can PETCO2 be greater than PaCO2?

very low V ̇/Q ̇. Signup and view all the answers

What factor primarily determines the PETCO2 value under steady-state conditions?

CO2 production Signup and view all the answers

Which of the following can directly contribute to a high PaCO2 − PETCO2 difference?

Pulmonary embolism Signup and view all the answers

what is the normal difference between PaCO2 and PETCO2 ?

less than 5 mm Hg Signup and view all the answers

What is the main advantage of volume capnogram over time capnogram?

It allows for estimation of the relative contributions of anatomic and alveolar components of physiologic dead space Signup and view all the answers

What is the significance of the numeric integral of PCO2 as a function of volume?

It estimates the total mass of CO2 exhaled during a breath Signup and view all the answers

How does the volume capnogram differ from the time capnogram?

It provides more information about respiratory flows or volumes Signup and view all the answers

What is a limitation of time capnography?

It lacks information regarding respiratory flows or volumes Signup and view all the answers

What is the significance of the volume capnogram in detecting changes in dead space?

It is more sensitive than the time capnogram in detecting subtle changes in dead space Signup and view all the answers

Area X corresponds to ?

total volume of CO2 exhaled over a tidal breath Signup and view all the answers

Area Y corresponds to ?

wasted ventilation due to alveolar dead space Signup and view all the answers

Area Z corresponds to ?

wasted ventilation due to anatomic deadspace Signup and view all the answers

which areas represent the total physiologic dead space.

Z and Y Signup and view all the answers

this time capnograms represent?

catastrophic loss of ventilation Signup and view all the answers

this time capnograms represent?

leak or partial obstruction Signup and view all the answers

this time capnograms represent?

cardiac arrest Signup and view all the answers

this time capnograms represent?

hyperventilation Signup and view all the answers

this time capnograms represent?

all of the above Signup and view all the answers

this time capnograms represent?

release of a tourniquet Signup and view all the answers

this time capnograms represent?

contaminated sample cell Signup and view all the answers

this time capnograms represent?

rebreathing Signup and view all the answers

Study Notes

Exhaled CO2 Information

Exhaled CO2 indicates the effectiveness of ventilation and metabolic function.
The equation used to estimate the ratio of physiologic dead space to tidal volume is the Bohr equation.

CO2 Measurement and Analysis

Measurement and quantification of CO2 concentrations at the airway opening is known as capnometry.
A device that records and displays CO2 concentrations over time is called a capnograph.

Capnograph Insights

Capnograms provide a graphic display of CO2 concentration changes throughout the respiratory cycle.
Continuous monitoring of exhaled CO2 during anesthesia is crucial for assessing respiratory function and detecting hypoventilation.

Detection of CO2 Concentrations

Clinical environments commonly use infrared absorption to detect CO2 concentrations in respiratory gases.
Gaseous CO2 primarily absorbs light around the wavelength of 4.26 micrometers in the infrared spectrum.

Capnometer Functionality

Infrared filters and compensation algorithms enhance the accuracy of CO2 measurements by reducing interference.
A reference cell devoid of CO2 ensures the capnometer can accurately measure concentrations by comparing samples.

Capnometer Mechanics

The chopper disk, rotating at 60 revolutions per second, helps modulate the infrared beam for improved accuracy.
Most capnometers focus infrared light on a gas sample to detect CO2 levels.

Capnometer Types

Sidestream capnometers analyze gas drawn from a distance, while mainstream capnometers measure gases directly at the airway.

Sidestream Analyzers

Potential issues with sidestream analyzers include lag due to gas transport time in tubing.
Gases aspirated by the pump in a sidestream analyzer may be affected by humidity and temperature changes.
Capnograms generated by sidestream analyzers may exhibit larger waveform fluctuations.

Mainstream Analyzers Benefits

The rise time of mainstream analyzers is shorter because the sensor is closer to the airflow source.
Heating the sample cell to 40°C aids in minimizing condensation, thus improving measurement accuracy.
Though mainstream analyzers provide faster responses, they may be bulkier and less portable compared to sidestream models.

Rise Time Factors

Variations in rise time among capnometers depend on tubing length, gas flow dynamics, and sensor positioning relative to the airway.

Time Capnogram Phases

Phase I of a time capnogram is characterized by the initial expiration of dead space gas, showing no CO2.
Phase II reflects the mixing of dead space and alveolar gas, marked by a rapid rise in CO2 concentration.
Phase III indicates a plateau of CO2 concentrations from well-ventilated lung areas; the slight upslope during this phase arises from uneven ventilation.

Pathology Impacts

Respiratory pathologies can distort phase III shape due to mismatched ventilation-perfusion ratios.
Interventions like PEEP can improve phase III appearance by enhancing ventilation distribution.

Capnogram Transitions

A sharp downstroke of PCO2 indicates fresh inspired gas washing out remaining CO2 during the transition phase.
A sharp upstroke in PCO2 at the end of phase III is due to the closure of poorly ventilated lung units, allowing higher CO2 contributions during expiration.

PETCO2 and PaCO2 Relationship

PETCO2 correlates well with PaCO2 under stable conditions, while discrepancies can occur with hypoventilation or certain lung pathologies.
Factors influencing the PETCO2 value include metabolic rate and minute ventilation.

Volume vs Time Capnogram

Volume capnograms provide integrated data to assess lung function over total volume, and may highlight changes in dead space.
Time capnograms are limited in detecting volume changes and their dynamics can be affected by various physiological factors.

Areas in Volume Capnogram

Areas X, Y, and Z in the volume capnogram correspond to different physiological dead spaces and ventilation-perfusion matching states within the lung.

Additional Capnogram Representations

Each type of capnogram represents distinctive ventilation patterns and the respective gas exchange efficiency, highlighting pathophysiological changes.

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Description

Learn about the significance of CO2 monitoring in anesthesiology, including its role in confirming airway device placement, assessing breathing circuit integrity, and providing insights into ventilation and cardiac output adequacy.