End tidal carbon dioxide monitoring in cryonics
The best non-invasive indicator of cardiac output and oxygenation during cardiopulmonary support (CPS) is end tidal carbon dioxide (ETCO2). ETCO2 is the partial pressure of carbon dioxide (CO2) at the end of an exhaled breath. Until recently, cryonics standby kits were equipped with disposable colorimetric ETCO2 detectors. Some limitations of the disposable ETCO2 detectors are that they are not quantitative, not continuous, hard to read in the dark, and can give false readings. In 2006 this situation changed when Alcor used the CO2SMO, a sophisticated monitoring device that can give a complete respiratory profile of the patient, during a case.
Although devices like the CO2SMO represent the state of the art in respiratory monitoring, their cost, size and complexity may limit routine use of this equipment in remote cases. In August 2007 the cryonics company Suspended Animation added the Capnocheck to its standby equipment. The Capnocheck is similar in size to the older colorimetric ETCO2 detectors but gives quantitative and digital readings for ETCO2 and respiratory rates using infrared technology. ETCO2 readings are given in mmHg and the respiratory rate is given in breaths per minute. Some models come with an alarm that can be set for high and low ETCO2 readings.
ETCO2 can be used to evaluate the effectiveness of chest compressions and as a predictor of outcome during cardiopulmonary resuscitation. Studies have found that patients with restoration of spontaneous circulation (ROSC) have higher ETCO2 levels than patients that could not be resuscitated (levels <10 mmHg). Normal ETCO2 levels are between 35 and 45 mmHg. Because numeric readings of ETCO2 have rarely been obtained and analyzed in cryonics, knowledge about what ETCO2 levels to expect and not to expect are unknown. At this point in time, meticulous note taking of ETCO2 levels during CPS is essential to generate a series of data for cryonics patients.
Another important use of ETCO2 monitoring is that it can be used to validate correct placement of the endotracheal tube (or Combitube). If the endotracheal tube has been placed in the esophagus, or has become dislodged, one would expect to see negligible ETCO2 readings. Another issue that needs to be taken into account is the effect of stabilization medications on ETCO2. For example, administration of the vasopressor epinephrine will decrease ETCO2 readings although cerebral blood flow may be improved. Some cryonics technologies such as liquid ventilation appear to be incompatible with ETCO2 monitoring altogether.
ETCO2 monitoring does not give direct information on how well the brain of a cryonics patient is being perfused. New non-invasive technologies that can do this will be reviewed in the future.