Sometime in the 1780s the French scientist Jacques Charles’s noted that  at constant pressure, the volume of a given mass of an ideal gas increases or decreases by the same factor as its temperature on the absolute temperature scale. Or, put more simply, the gas expands as the temperature increases. This is known as Charles’ law which can be written as:

V \propto T\,

where V is the volume of the gas; and T is the absolute temperature. The law can also be usefully expressed as follows:

\frac{V_1}{T_1} = \frac{V_2}{T_2} \qquad \mathrm{or} \qquad \frac {V_2}{V_1} = \frac{T_2}{T_1} \qquad \mathrm{or} \qquad V_1 T_2 = V_2 T_1.

The equation shows that, as absolute temperature increases, the volume of the gas also increases in proportion.

Of course, the converse of this is also true; if you cool a gas it will contract in volume. This bit of physics has surprisingly practical and immediate implications in ultraprofound hypothermia research and, of course, in the transport of cryonics patients.

One of the really nettlesome problems in dog total body washout (TBW) ultraprofound hypothermia research and in cryonics transports is that as the subject cools, the volume of gas in the endotracheal (ET ) tube balloon cuff decreases. It is, in practice, not possible to dynamically monitor and adjust this, so you are left with choice of seriously over-pressurizing the ET tube balloon, or risking aspiration of gastric contents and/or PIB water into the lungs. Even if the tube is over-pressurized to compensate for cooling-induced volume loss of the gas, there is still the substantial risk of air leaking from the balloon and aspiration occurring.   This has been  a very serious problem in our dog work in the past, and it is also a serious problem in medicine. While not a concern in cryonics, over-pressurizing the balloon cuff in clinical situations results in injury to the trachea and can even cause tracheal necrosis. And, of course, balloon pressure should vary dynamically with airway pressure: a pressure that is sufficient to maintain a seal at a low airway pressure will allow gas to leak around the balloon (thus escaping from the lungs) at a high airway pressure. Finally, someone has come up with a brilliant solution to this problem; a device that uses the airway pressure in the ventilation circuit to continually and dynamically adjust the  cuff pressure:

PressureEasy® Cuff Pressure Monitor

javascript:void(window.open('/upload/products/mainImages/pressure-easy.jpg','prodimage' ,config='height=490,width=298,left=10,top=10,scrollbars=no;return false;')) The PressureEasy® Cuff Pressure Controller is designed to continuously monitor tracheal cuff pressure. Its indicator window signals cuff pressure is maintained between 20-30cm/H2O. In addition, the airway pressure auto-feedback feature boosts cuff pressure to ensure proper sealing when high pressures are used during ventilation.

The only device of its kind, the PressureEasy® Cuff Pressure Controller offers several other advantages over traditional methods of cuff pressure control. As a single-patient use device, this cuff pressure controller reduces potential for infection and eliminates sterilization issues with quarantined or isolated patients.

The PressureEasy® Cuff Pressure Controller does away with managing and inventorying of manometers, issues of availability, calibrating, and replacement of reusable manometers.

This device ensures that even over a wide range of temperatures and pressures the seal on the ET cuff balloon, or for that matter, the balloon on any other kind of airway protection device, such as the esophageal gastric tube airway (EGTA), Combitube  or laryngeal mask airway (LMA) remains patent and at the optimum pressure regardless of variations in patient temperature or airway pressure.

I’d also like to note that there is now also a much better alternative to the EasyCap for end tidal CO2 (EtCO2) detection, to monitor the efficacy of CPS: the Stat CO2. I really like this device; it works for 24 hours, it tolerates high humidity environments such as a humidified ventilator circuit, it has a large, easy to read indicator, and it has the truly fantastic feature of allowing you to position it in the breathing circuit  on the heart-lung resuscitator (HLR) or bag-valve resuscitator up to several days before you use it. This is possible because the device has an activation a tab that is removed to activate the device. So, it can be  left in position before use and will remain ready to go until it is needed. The EasyCap rapidly deteriorates as soon as it is removed from its retort packaging, and it has a very short working life (45 minutes in practice) and is completely intolerant of high moisture conditions.

14. October 2007 · Comments Off on End tidal carbon dioxide monitoring in cryonics · Categories: Cryonics · Tags: , , ,

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.