Capnography

CAPNOGRAPHY

Our Objectives:

Capnography versus SpO2.Why capnography?How it works.Review lungs briefly.Waveforms

StandardProblems.

Why capnography??

Pulse oximetry is a direct representation of the oxygenation status of the patient

Capnography is an indirect monitor that aids in the differential diagnosis of hypoxia and enables remedial action to take place to prevent hypoxic brain damage.

Why capnography?

Capnography provides information about pulmonary perfusion, CO2 production, alveolar ventilation and respiratory patterns.

Capnography has been shown to be effective in the early detection of respiratory events.

Capnography...Reflects alveolar elimination of CO2

There is only a small difference in pulmonary capillary CO2 and pETCO2

Reflects CO2 production by the tissue of the body as occurs during aerobic metabolism.

Capnograpy: Fast & Accurate

Detect life threatening conditions such as:

malpositioning of the ET tube,

respiratory failure,

circulatory failure and

malfunction of breathing circuits.

How it works...Cough....cough.....physiology!!!

At the end of inspiration there should be no C02 present in the airway.....well, almost none.

C02 diffuses from the alveolar capillaries and equilibrates with alveolar air ( Capillary PaCO2 = 40mm/Hg)

Actual CO2 concentration depends of ventilation and perfusion

The lungsAlveoli with higher ventilation in relation to

perfusion will have lower C02 and vice versa.As you move proximally in the respiratory

tract C02 concentration will eventually fall to zero.

The volume of C02 free gas is referred to as respiratory dead space

As exhalation occurs initially the C02 free “dead space” gas will be detected by the sensor first.

At the end of the cycle C02 concentration will again fall to zero as C02 free gas is inhaled.

More about the lungs…The lower portions of the lungs tend to be

better ventilated and better perfused.In reality, the bases of lungs are better

perfused than ventilated. So this means????This results it the apex of the lungs having

a higher V/Q ratio than the bases.The lungs empty from the apex to the

bases in healthy lungs resulting in a slight increase in C02 concentration during exhalation.

The waveformPhase I: represents the

exhaled gas devoid of CO2 ( From the anatomical dead space)

Phase II: Consists of a rapid “S” like upswing ( due to mixing of gas from the dead space with alveolar gas)

And on it goes...Phase III: Consists of an

Alveolar plateau which represents CO2 rich gas from the alveoli. Almost always consisting of a positive slope for the following reasons

Steady secretion of CO2 into alveoli which become increasingly smaller with exhalation. This results in increasing alveolar CO2 concentration toward the end of

The late emptying of alveoli with lower V/Q ratios.

Alpha and beta angles

Alpha angle: Increases as phase III continues. Primarily linked to time constants within the lungs this indirectly reflects the patients V/Q status.

Beta angle: The nearly 90 degree angle between phase III and the descending limb. This can be used to assess the extent of re-breathing. In re-breathing there is an increase in the beta angle from 90 degrees. As re-breathing increases the horizontal baseline of phase 0 and phase 1 can increase.

Capnograph trace.Inspiratory baseline

Expiratory upstroke

Expiratory baseline

Inspirtory downstroke

End tidal CO2 ( pETCO2)

Rebreathing pattern

Waveform does not return to baseline

May be caused by low fresh gas flow in a closed circuit.

Sloping plateau (increased alpha angle)

Obstructive airway disease due to impairment of V/Q ratio.

Cardiac oscillations

Cardiac impulses transmitted to capnograph

Curare Cleft

Reversal of neuromuscular blockade.

Pt begins taking small breaths against circuit