X-Message-Number: 7539 Date: 20 Jan 97 23:09:57 EST From: Mike Darwin <> Subject: Cryopatient Temperature Monitoring The following is a BioPreservation Technical Briefing discussing the rationale for multi-site core temperature monitoring in human cryopreservation patients. This brief also discusses the procedures for application of specific (and in some cases proprietary) temperature monitoring equipment. Temperature Monitoring in the Human Cryopreservation Patient: Theoretical Considerations and Practical Techniques by Michael Darwin Introduction The first two pillars upon which cryopatient transport rests are cardiopulmonary support and pharmacological reduction or elimination of ischemia-reperfusion injury. The third pillar of transport is induction of hypothermia. Arguably, in many patients, cooling will be the single most important injury-mediating modality that can be brought to bear. In many patients, application of cardiopulmonary support will not be effective or in some cases even possible owing to the physical condition of the patient, or the logistics of the case. In such instances, rapid induction of hypothermia will be the only way possible to slow the pace of, and/or reduce the extent of, ischemic injury. It is of critical importance to monitor and record the patient's core temperatures because this information: *Constitutes a useful record of that patient's care by providing feedback which may allow for real-time intervention to improve care. *Will likely aide future attempts at recovery by contributing to the documentation of the patient's overall condition. *Provides a medicolegal record of a critical intervention (induction of hypothermia) which is central to providing good care to the patient. *Allows for evaluation of the efficacy of the techniques applied on a given patient and contributes to the creation of a statistically meaningful database with which to evaluate the efficacy of given interventions. *Facilitates evaluation of the effectiveness of cardiopulmonary support and the duration of post arrest cerebral perfusion. *Allows for improved differential diagnosis in the interpretation of physical signs which may be present during the course of the patient's care such as rigor, secondary flaccidity, clotting, etc. Calibration of Temperature Probes Thermocouple temperature probes (the kind used exclusively in cryopreservation operations) consist of two different kinds of metals which are welded together at a junction. This bimetallic junction generates a minute electrical current which varies with the temperature of the junction. Thus, by measuring the current at the weld-point or junction, it is possible to measure the temperature of the junction. This is the principle on which thermocouple thermometry operates. The kind of thermocouple used in human cryopreservation operations is the copper-constantan thermocouple (Cu++/C) which has a useful range of temperature measurement from well above body temperature, to a little below liquid nitrogen temperature, with theoretical accuracy to a fraction of a degree Celsius. Like most other measuring equipment, thermocouple probes must be calibrated prior to use, or, if pre-calibrated, their accuracy must be determined before they are used. For transport procedures a one-point calibration or accuracy check on the probes using a slurry of crushed ice and water is sufficient. To carry out a one-point calibration with maximum accuracy it is best to use an insulated container (even a foam coffee cup will suffice) which is filled with crushed ice and sufficient water to convert the ice into a slush. The probe tip should then be held in the center of this bath of ice-slush until the temperature stabilizes at 0 C +/- 0.5 C. If a probe fails to pass this test, another probe which does read appropriately should be substituted. Alternatively, if the thermocouple meter has provisions for doing so, the meter should be re-calibrated by resetting the measured temperature to 0 C (see individual meter instructions for calibration procedures). If re-calibration or probe replacement is not possible, and the probe must be used, it is essential to note the degree of offset (i.e., inaccuracy) from 0 C in the patient temperature descent log, and to tag the probe immediately and indelibly so that a corrected cooling curve can be constructed using the probe after the conclusion of transport and removal of the probe from the patient. An incorrectly reading probe should not be left in the patient during subsequent cryoprotective perfusion or subzero cooling operations. Note: Specific instructions for calibration and use of the particular model of thermocouple meter now in use by BPI and its client cryonics organizations is provided in specific detail below. When using other equipment it is important to follow the manufacturer's instructions carefully. For this reason it is recommended that a copy of all instructional materials/manuals provided by the manufacturer be kept with the equipment to be used in the field: if possible by attaching the manual or a reduced-sized photocopy of it to the associated piece of equipment. Rationale for Multi-Site Core Temperature Monitoring The protocol specified here calls for monitoring the patient's core temperature at multiple sites including both tympanic membranes, nasopharyngeal (using a the left or right nare for probe access), deep-pharynx or esophagus, and, if possible and appropriate, the rectum. While this may seem burdensome and of questionable value at a time when almost all available resources will likely be focused on carrying out cooling and providing cardiopulmonary support, there are very good reasons for this approach. Figure 8-1: Esophageal and tympanic temperature cooling curves for a dog subjected to five hours of asanguineous extracorporeal perfusion with subsequent rewarming. This figure shows very tight (to with 0.2 degrees C)correlation of typmanic and esophageal temperature. In patients experiencing pre-arrest shock and hypotension even distribution of blood flow is disrupted. Cardiac arrest and any accompanying ischemic interval only serve to exacerbate this problem. In the case of the rectum, retained or impacted fecal material may encase the temperature probe upon insertion and render its readings inaccurate. Further, shock-induced compromise of blood flow to the bowel and rectal vascular plexus may selectively slow cooling at this site yielding data which is not representative of the rest of the body and in particular the brain, and thus is unreliable. The obverse of this may also be true in that the rate of cooling observed rectally during femoral-femoral cardiopulmonary bypass and extracorporeal cooling is often far more rapid than that observed in the brain, or when measured esophageally or pharyngeally. Another very significant problem with single-site monitoring of core temperature is that if a probe is malpositioned, becomes inaccurate, or develops an intermittent failure mode, there are no other probes to act as back-ups or to allow cross-checking of data. Further, new cooling modalities such as liquid ventilation, gastric, colonic, and peritoneal iced-fluid lavage may render esophageal and rectal temperature measurements meaningless. Finally, the use of bilateral tympanic temperature monitoring is of critical importance for several reasons which deserve both discussion and illustration with actual case data. In a hemodynamically intact animal or human, tympanic and esophageal temperatures correlate very well as is shown in Figure 8-1. Esophageal temperature can reasonably be taken to be indicative of intrathoracic and large-vessel blood temperatures (the esophagus is contained within the mediastinum in close proximity to the aorta and the superior and inferior vena cava). The brain core temperature and the brain cortical (surface) temperature (the latter only in the case of a patient with significant cerebral perfusion) will be most accurately reflected by the tympanic temperature since the blood supply for both the brain and tympanic membranes, and the associated thermal-dilutional effects will be more representative than those of the hemorrhoidal venous plexus or the esophagus. Clinical and experimental data indicate a closer correlation of tympanic temperature to brain and central venous temperature than is seen for rectal temperature 1,2,3. Tympanic temperature may vary significantly from that cerebral cortical temperature in the clinical setting4, and may vary more so in cryopatients, however, it still likely to be far more accurate in representing overall brain temperature than is either esophageal or rectal thermometry. (We have demonstrated that this is the case in-house in canine models of both surface and extracorporeally supported cooling in ischemia). Figure 8-2: Cooling curves for patient C-2150 demonstrating failure and resumption of cerebral perfusion as illustrated by the plateau in tympanic temperatures observed during the latter part of HI-ACD-CPR and continuing until the start of extracorporeal perfusion. It is a well established fact in the literature that cerebral blood flow is not maintained in conventional CPR for more than 10 to 15 minutes 5,6. Blood flow to visceral organs is maintained for considerably longer periods of time. Failure of cerebral perfusion can be documented by frequent sampling of tympanic, esophageal, and/or hepatic temperatures and plotting of these temperatures on a common graph. Failure of cerebral perfusion is demonstrated by the development of a discontinuity marked by failure of the tympanic temperatures to track the other core temperatures during non-extracorporeal supported cooling, and a resumption of comparable rates of cooling once adequate mean arterial pressure (MAP) and flow are restored by extracorporeal means. The caveat being that such restoration of MAP and flow occur before supervening processes render re-establishment of cerebral perfusion impossible (clotting, cerebral edema, etc.). This phenomenon is shown quite well in the temperature data collected from patient C-2150 shown in Figure 8-2. End of Part I BioPreservation, Inc 10743 Civic Center Drive Rancho Cucamonga, CA 91730 (909)97-3883 Rate This Message: http://www.cryonet.org/cgi-bin/rate.cgi?msg=7539