Side Stream

Main Stream 

Infrared Analysis (most common)

Mass Spectrometry (less common)

Chemical Indicators (Most recent)

Light at 4.3 micrometers is strongly absorbed by CO2

Beer-Lambert law is used to determine CO2 concentration 

The Inspiratory Baseline

The Expiratory Upstroke

The Expiratory Plateau 

Inspiratory Downstroke 

During inspiration, fresh gas rushes by the sampling site

CO2 concentration is at 0

Mapelson Circuits always show some phase 1 CO2

Latter part of phase 1 represents exhaled dead space 

Shortly after inspiration ends the lings recoil and gas exits through the trachea causing an upstorke in capnograph

The upstroke should be steep, by may become slated if airflow is obstructed 

Phase 2 also known as transition phase 

As exhalation continues the capnogram plateaus

This occurs as lung units with a lower V/Q ratio empty into the sampling site, thus the slight incline in the plateau

Any spontaneous ventilation can cause "bumps" in the plateau

May be seen with cardiac oscillations 

Fresh gas washes away CO2 with rapid downstroke

Downstroke can be slowed by an incompetent inspiratory valve 

PaCO2-PACO2

PaCO2-true PeCO2

True PeCO2max

Reverse Gradient (PeCO2>PaCO2)

Difference between arterial and alveolar CO2

Changes in V/Q ratio increase the difference between PaCO2 and PACO2

Shunt

Dead Space

Decreased V/Q results in a very small change

Results from mixed alveolar gas not reaching the upper airway

Sample must not only reach the sampling device, but must also be there long enough to be sampled

In children the sampling size may be too large for the tidal volume or fresh gas flow

Uncuffed tube may compound problem 

Problems with the capnograph itself may result in inaccurate CO2 measurement

System leaks will reduce measured PeCO2

Reverse Gradient

(PeCO2> PaCO2)

120L of CO2 (~2L/kg)

If this tissue reserve is depleted the patient may hypoventilate in the post operative period 

Number of studies support the premise that capnography can detect problems most likely to injure the patient

Under anesthesia we attempt to normalize CO2

Causes a flattening of the expiratory upstroke

Most commonly caused by bronchospasm

But can also be: muscus plug, kinked ETT

Caused by changes in intrathoracic volume as a result of the cardiac cycle

May indicate a very high cardiac output, but if often normal especially in young healthy athletes

A sharp downstroke during the expiratory plateau indicated ventilatory attempts by the patient

aka Curare Clefts

Imply loss of neuromuscular blockade as well as increased PaCO2 

An elevated baseline indicates rebreathing

Causes: failed CO2 absrober, malfunctioning expiratory valve, inadequate fresh glas flow in a non-rebreathing circuit