GE HEALTHCARE B20, User'S Reference Manual

Page: 288 / 320

B40/B20 Patient Monitor
17-20
Dips in capnogram
The dips seen in the capnogram during expiration are related to the sidestream gas sampling,
the continuous gas flow to the Y-piece, and patient’s cardiac contractions, which cause intra-
thoracic pressure changes and therefore flow variations.
The alterations in expired CO
2
waveform are cardiogenic movements of exhaled and circuit
gas at the sidestream gas sampling site. When the respiratory gas flow drops below the gas
sampling rate, a variable mixture of CO
2
free fresh gas and exhaled CO
2
rich gas is sampled.
This causes variations in sampled CO
2
concentrations.
In the illustration below, CO
2
waveform is the one on top, and flow is the lower waveform.
1. Expiration
2. Cardiogenic oscillations
Cardiogenic oscillations appear when:
•
A continuous fresh gas flow is fed into the patient Y-piece.
•
Sidestream gas sampling is done at the Y-piece.
•
The patient is ventilated with a long expiration time or low respiration times, and when
there is a long zero flow at end-expiration for some other reason.
Oscillations can be eliminated by adding a spacer with a 5 ml dead space between the Y-piece
and the airway adapter. Increased dead space creates a buffer volume between the Y-piece
and the sampling point, preventing the inspiratory and expiratory air from mixing during gas
sampling. Misinterpretation of EtCO
2
information can be avoided through identifying
cardiogenic oscillation and understanding the reasons for it.
EtCO2 value % EtCO2 value mmHg Indicates
4.5 to 5.5% 34 to 41 mmHg normocapnia
< 4% < 30 mmHg hypocapnia
> 6% > 45 mmHg hypercapnia