Vimpeli, Tommi Sammendrag Objective:
A decrease in CO2 follows the extended alveolar plateau and represents the appearance of the CO2-free gas from the machine-end of the inspiratory limb. During this latter phase of inhalation, CO2 concentration may reach zero. The capnogram thus created may be indistinguishable from the normal capnogram, at least during the initial phase of rebreathing.
The time capnograph is unable to reveal rebreathing because it is unaware of the beginning of actual inspiration. One of the methods suggested was to superimpose inspiratory and expiratory flow rates on the capnogram. A circle system was used with the inspiratory valve competent no rebreathing as well as with the valve displaced rebreathing.
The end of expiration almost coincides with the downslope of the CO2 waveform in the capnograms when there is no rebreathing figure A. However, in the presence of rebreathing, the alveolar plateau is prolonged figure B and includes a part of inspiration phase 0 in addition to the expiratory alveolar plateau phase III.
Breen and Bradley5 suggested that CO2 spirography CO2 concentrations versus inspired and expired volume during respiratory cycle should be used to detect rebreathing in situations in which rebreathing is not detected by time capnography.
By multiplying and integrating airway-measured flow and PCO2, they computed overall expired and inspired VCO2 CO2 volume during intermittent positive pressure ventilation in a circle anesthesia circuit.
They also recorded time capnograms.
The time capnogram did not show appreciable changes when the inspiratory valve was made incompetent. They found, however, a significant increase in inspired VCO2 when the inspiratory valve was compromised, suggesting rebreathing. They concluded that CO2 spirography in contrast to time capnography is required to detect inspiratory valve incompetence during mechanical ventilation.
However, the method suggested by Breen and Bradley5 is complex and may not be suitable for routine clinical use. Capnographs using side-stream sensor technology may not allow comparison between CO2 waveforms and flow rate waveforms because of the longer response time of side-stream CO2 analyzers, which occurs as a result of the 6-ft sampling tube.
It may be, however, that an algorithm can be built into the software of side stream capnographs to allow correction for a delayed CO2 response and thus facilitate the comparison between the two waveforms.
The simultaneous display of flow waveforms and time capnograms allows easy delineation of the inspiratory and the expiratory portions of time capnograms. This facilitates expeditious detection of rebreathing, even before the increase of the baseline phase 0 or a subsequent increase in end-tidal PCO2 as a result of rebreathing.
Moreover, delineation of segments also helps in the differential diagnosis of abnormal CO2 waveforms. As for example in a recent case report,9 the authors were unable to promptly diagnose rebreathing produced as a result of failure of CO2 absorption by the soda lime during closed circuit anesthesia because the resulting abnormal capnogram had two curves a second peak following the primary peak.
The differential diagnosis of such a capnogram could be a curare cleft if the second peak occurred during expirationor a signature capnogram if the second peak occurred during inspiration.
In addition, delineating various components of a time capnogram would widen the scope for future applications of time capnography.
For example, it would permit the identification of any differences between end-tidal CO2 at the end of expiratory flow and end-expiratory pause CO2 at the beginning of inspiratory flow particularly during low frequency ventilation.
Normally, both these values are similar when the phase III is flat. However, the difference between the two assumes importance in time capnograms where the slope of phase III is steep chronic obstructive pulmonary diseases.
This distinction may result in a more accurate prediction of arterial PCO2 using time capnography. Furthermore, the demarcation of the expiratory segment may allow the exploration of estimating physiological dead space and its components using a time capnogram as is currently possible in a volume capnogram single breath test-CO2 curve.
However, it is premature to predict whether the clinical benefits of including flow data into present capnographic technology would outweigh the economic cost involved, and further studies are required. Terminology and the current limitations of time capnography: J Clin Monit ; The single breath test for carbon dioxide Thesis.
The concept of dead space with special reference to the single breath test for CO2.Frog Heart Experiment There are a number of external in uences that can a ect cardiac output. Some of these are mediated by the autonomic nervous system and others are a response to changes in temperature and ionic concentrations.
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Conclusion: In AGA fetuses at term that develop intrapartum distress, there is evidence of prelabour redistribution of the cardiac output. The application of positive end expiratory pressure (PEEP) in mechanically ventilated (MV) patients with acute respiratory distress syndrome (ARDS) decreases cardiac output (CO).
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