Sir,
Breakthrough breathing during intermittent positive pressure ventilation (IPPV) makes a cleft in the plateau phase of capnogram that is known as curare cleft.[1] We report a surrogate marker of breakthrough breathing in oxygen (O2) waveform. When there appears curare cleft in capnogram, a reciprocal change appears in O2 waveform. As it appears like a crest and coincides with curare cleft, we name it as curare crest to retain its utility as a monitoring tool for breakthrough breathing during IPPV [Figure 1].
Figure 1.
Curare cleft in capnogram and curare crest in oxygen waveform
Carbon dioxide (CO2) waveform and O2 waveform are mirror images of each other. The reason is reciprocal timing of changes in the concentration of CO2 and O2 at Y-piece of breathing circuit. During breakthrough respiration CO2 concentration decreases at the sampling site due to fresh gas entry, and a curare cleft is seen in the capnogram. At the same time, Oxygen concentration increases and a mirror image of curare cleft appears in the O2 waveform.
In oxygraphy, peak-to-baseline scale difference can be compressed to as low as 6 mmHg and looks similar to capnogram but mirror image. Hence, a crest cannot be missed in O2 waveform. It has been found that oxygraphy can detect the breakthrough respiration even earlier than capnography, making it superior to capnography for this purpose.[2] Hence, it can be extrapolated that oxygraphy can detect breakthrough breathing at the earliest although a randomized control trial is required to authenticate the same.
The standards for basic anesthesia monitoring of the American Society of Anesthesiologists state that the concentration of O2 in the patient breathing system shall be measured by an O2 analyzer. And, use of more than one device to monitor O2 is desirable. So, along with pulse oxymeter, O2 concentration monitoring is desired. Nowadays, anesthesia workstations are equipped for O2 concentration measurement and display of O2 waveform. Use of O2 monitoring helps to detect hypoxia, breathing circuit disconnections, and hypoventilation.[3]
Oxygen concentration is measured by paramagnetic O2 analyzers whereas CO2 concentration measurement uses infrared multi-gas analyzer, which does not measure O2. Hence, in case of nonfunctioning CO2 analyzer, oxygraphy can well serve to detect breakthrough breathing on itself. We recommend routine use of O2 waveform monitoring in our clinical practice to gather information beyond just O2 concentration measurement.
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Conflicts of interest
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References
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