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