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.

Intranasal administration of vasoactive agents

Stabilization in cryonics requires immediate administration of vasoactive medications to maintain blood pressure, thereby assisting and enabling adequate perfusion during cardiopulmonary support. Traditionally, vasopressors such as epinephrine have been administered intravenously, requiring skilled technicians to establish an IV line as quickly as possible. Unfortunately, even the best technicians often encounter difficulties in obtaining an IV access site, thus delaying critical intervention.

Alternatively, the intranasal (IN) route is a rapidly obtainable and feasible route of administration in an emergency situation. A growing number of studies have indicated that the nasal mucosa is a suitable substrate for quick absorption of vasoactive agents into the systemic circulation. IN epinephrine has been shown to reach peak plasma concentrations in only 15 seconds, with peak systolic pressure at 200% of control value after 60 seconds (Yamada, 2004). Similarly, a comparison of IN vs. IV administration of epinephrine in a canine model of cardiac arrest and CPR demonstrated improved coronary perfusion pressure in both groups, with similar rates of successful resuscitation (Bleske, 1992).

Several factors affect successful nasal absorption of vasopressors and other drugs, including molecular weight, pH, and lipophilicity. However, absorption can be greatly improved with the use of permeation enhancers and careful modulation of dose. Bleske et al. (1996) also investigated the effect of various doses of phentolamine and epinephrine in combination on the nasal aborption of ephinephrine during CPR, and found that 0.25 mg/kg/nostril significantly enhanced absorption of ephinephrine in a canine model. Whether administration of such permeation enhancers is necessary for intranasal administration of vasopressors in cryonics remains unknown. A more detailed review of intranasal administration of therapeutic agents and its feasibility for cryonics stabilization will appear in the upcoming issue (3rd quarter 2007) of Cryonics.

Load distributing band CPS

The Autopulse presents an alternative to the (high impulse) active compression-decompression devices that cryonics organizations currently employ to provide cardiopulmonary support (CPS) during stabilization. The Autopulse uses batteries instead of compressed oxygen and is easy to set up and operate. Disadvantages include its cost, limited patient size range, and the modifications that cryonics organizations must make to protect it from water during operation in a portable ice bath. But perhaps the most serious concern that has been expressed about the Autopulse (or any other mechanical CPR devices without active decompression) is that it may be less effective for patients with flail chest during extended CPS times.

The most fundamental question, however, is whether load distributing band CPR (LDB-CPR) is superior to high impulse active compression-decompression CPR (HI-ACD CPR). Some studies report impressive performance of the Autopulse in animal models but a recent randomized trial found worse neurological outcomes versus manual CPR. An editoral discussing these apparently contradictory results contains an interesting observation:

“Standard, manual CPR may be better than is generally recognized. Preclinical investigations of the LDB-CPR device typically compared it with the only commercially available mechanical CPR device, a gas drive piston-cylinder. This device compresses the sternum 1.5 to 2 inches, presumably causing cardiac compression, the other mechanism of blood flow during CPR. In preclinical comparisons, the pistoncylinder device appears to have been adjusted to produce 20% anterior-posterior sternal displacement, similar to that produced by LDB-CPR and likely to be less than an inch in 20-kg swine. This 20% value may represent the optimal performance characteristics for LDB-CPR in small swine but may have reduced the performance of the older pistoncylinder device. A comparison of LDB-CPR to “good” manual CPR in a laboratory model could prove to be revealing.”

To our knowledge, realistic head-to-head comparisons between good mechanical HI-ACD CPR and LDS-CPR, including the use of an inspiratory impedance threshold device, such as the ResQPOD, have not been published so far.