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PubMed1989

Radio frequency (13.56 MHz) energy enhances recovery from mild hypothermia.

The rate of warming after hypothermia depends on the method of rewarming. This study compared the effectiveness of radio frequency (RF) energy against hot (41 degrees C) water immersion (HW) and an insulated cocoon (IC) for rewarming hypothermic men. Six men fasted overnight and were rewarmed for 1 h after attaining a 0.5 degree C reduction in rectal temperature (Tre). Tre and esophageal (Tes) temperature were recorded every 5 min with nonmetallic thermal probes. The base-line value for Tre and Tes just before rewarming was subtracted from each 5 min Tre and Tes during rewarming to give delta Tre and delta Tes. The 12 delta Tes values were averaged for each individual and were compared using analysis of variance. The average delta Tes for RF (1.15 +/- 0.22 degrees C/h) was faster (P less than 0.001) than either IC (0.37 +/- 0.16 degrees C/h) or HW (0.18 +/- 0.09 degree C/h). The present study shows the superiority of RF energy for rewarming mildly hypothermic men.

Adult↗
PubMed1986

Determination of safe interval of circulatory arrest from the cerebral metabolic aspect.

To determine the safe interval of hypothermic total circulatory arrest, the cerebral metabolic state was evaluated in 30 dogs. Surface cooling was achieved by deep ether anesthesia and the animals were assigned to three equal groups. Group I: 30 min circulatory arrest and surface rewarming. Group II: 60 min circulatory arrest and surface rewarming. Group III: 60 min circulatory arrest and perfusion rewarming. Brain tissue gas tension was monitored and cerebral O2 consumption was calculated. Cerebral O2 consumption reduced with cooling, parallel to the decrease in cerebral blood flow. Rapid increase in Po2, elevation of PCO2, and decrease of pH in the brain tissue were observed during circulatory arrest. Brain tissue PO2 increased significantly after circulatory arrest in Group I, but it remained low during rewarming in Groups II and III. Cerebral O2 consumption was at reduced levels in all groups during rewarming and it remained at 29% of the precooling control level in Group II at the end of rewarming, whereas it recovered to 71% and 57% of the precooling levels in Groups I and III, respectively. It was estimated that cerebral metabolism may be recovered after 30 min circulatory arrest, despite a transient reduction in cerebral O2 consumption. On the other hand, after 60 min circulatory arrest, the recovery of cerebral metabolism was delayed in Group II and organic failure might have occurred in this group. However, even after circulatory arrest for 60 min, cerebral metabolism was recovered in Group III. The safe period of circulatory arrest is considered to be prolonged by use of extra-corporeal circulation.

Animals↗
PubMed1989

Computer-based thermal imaging of human gingiva: preliminary investigation.

Computer-based thermal imaging techniques were used to compare the rewarming rates of normal and inflamed human gingiva following cooling. A Modified Gingival Index (MGI) score, gingival crevicular fluid (GCF) measurement, and clinical photograph were obtained from a maxillary or mandibular anterior facial region in 20 subjects, aged 25 to 44. Baseline thermograms of the marginal/papillary (M/P) and adjacent attached gingiva (AG) were recorded following an acclimatization period with the patient seated and chin resting on a positioning apparatus. The tissue was then cooled with a gentle stream of air and thermograms were recorded at 10 second intervals for 3 minutes as the tissue rewarmed. Rewarming slopes were calculated based on these thermograms. Correlation coefficients were calculated for MGI with GCF, as well as for both MGI and GCF with AG and M/P rewarming slopes. Mean baseline (SE) temperatures (degrees F) for AG were 75.9 (0.9), 78.0 (0.6), and 80.3 (1.3) for normal, mildly inflamed, and moderately-severely inflamed gingivae, respectively; corresponding mean (SE) temperature differences (degrees F) between AG and M/P were 0.5 (0.2), 0.9 (0.2), and 1.4 (0.3). Intragroup mean M/P temperatures were less than mean AG temperatures, with the mean regional differences increasing with greater severity of inflammation. Rewarming rates also increased as inflammation became more severe, with the rewarming slopes of both the AG and M/P having statistically significant correlations with the MGI and the GCF. The correlation between MGI and GCF was also statistically significant. This study suggests that computer-based thermal imaging techniques can detect both static and dynamic temperature differences between normal gingiva and gingivae with increasing severities of inflammation.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1975

Cerebral effects of profound hypothermia (18 degrees C) and circulatory arrest.

Clinical use of profound hypothermia and total circulatory arrest has been accompanied by occasional postoperative neurological abnormalities. In a series of infant baboons, surface cooling to 32 degrees C (brain) followed by perfusion cooling by cardiopulmonary bypass with a membrane oxygenator and heat exchanger to 18 degrees C was carried out, after which the circulation was stopped for 30 minutes. The animal was rewarmed to 35 degrees C. Marked alterations in the regional cerebral circulation were observed during perfusion cooling and rewarming. Regional cerebral ischemia was negatively correlated with jugular outflow (total cerebral blood flow) during rewarming, while regional hyperemia showed positive correlation both following perfusion cooling and after rewarming. A higher degree of ischemia in brain ischemic samples was found during rewarming than during cooling. These alterations in regional cerebral perfusion were associated with lactacidosis and hyperglycemia after rewarming, and may be considered potentially responsible for posthypothermic cerebral dysfunction.

Animals↗
PubMed1983

Transient hemodynamic dysfunction after myocardial revascularization. Temperature dependence.

We studied hemodynamics and the effects of right atrial pacing (110 beats/min) following complete myocardial revascularization and hypothermic multidose potassium crystalloid cardioplegia in 12 patients with a normal preoperative left ventricular ejection fraction (LVEF). Measurements were made immediately preoperatively, postoperatively at specified temperatures during the rewarming period (90 degrees F, 94 degrees F, and 98 degrees F), and at 24 hours. No patient had a perioperative myocardial infarction. At 90 degrees F, hemodynamics were characterized by significant decreases in cardiac index, stroke volume index, and left ventricular stroke work index (LVSW) and an increase in systemic vascular resistance index (SVRI) compared to preoperative values (p less than 0.05). Right atrial pacing significantly increased cardiac index preoperatively and 24 hours postoperatively, but not during the rewarming period. Over the entire rewarming period (90 degrees F to 98 degrees F), each of the following variables correlated with temperature: cardiac index (r = 0.71 in sinus rhythm and r = 0.66 with right atrial pacing); stroke volume index (r = 0.33 and 0.66); SVRI (r = -0.80 and -0.64); LVSW (r = 0.37 and 0.73); and heart rate in sinus rhythm (r = 0.51). During the rewarming period, there was an inverse relationship between cardiac index and SVRI (r = -0.87). In conclusion, after myocardial revascularization: (1) transient hemodynamic dysfunction occurs during the rewarming period (90 degrees F to 98 degrees F); (2) this dysfunction is temperature-dependent; and (3) right atrial pacing at 110 beats/min does not improve hemodynamic function during the rewarming period. Temperature must be considered in the evaluation of left ventricular and hemodynamic function following myocardial revascularization.

Adult↗
PubMed1980

Initial treatment of profound accidental hypothermia.

The performance of inhalation, heating pads, plumbed garment, inhalation + heating pads, inhalation + plumbed garment, and body-to-body heat exchange rewarming were compared to trunk immersion and spontaneous rewarming under laboratory conditions with mildly cooled volunteers. The experiment included 72 rewarmings. Trunk immersion exhibited the smallest afterdrop, shortest recovery period, and most rapid rewarming. Of the therapies suitable for use in the field, small afterdrops were seen with inhalation, inhalation + plumbed garment, inhalation + heating pads, and spontaneous rewarming. The largest afterdrops were seen with the heating pads and plumbed garment. Body-to-body heat exchange was seen to produce somewhat larger afterdrops than spontaneous rewarming. It is concluded that heating pads and plumbed garment should not be used in treatment of profound hypothermia. It is further concluded that, because of the depression in respiratory minute volume accompanying profound hypothermia, the heating pads and plumbed garment in combination with inhalation therapy should not be used. This leaves inhalation therapy alone as the recommended treatment for profound hypothermia in the field.

Accidents↗
PubMed1996

Detection of cerebral desaturation during cardiopulmonary bypass by cerebral oximetry.

BACKGROUND: As oxygen consumption increases acutely during rewarming period in extracorporeal cardiopulmonary bypass (CPB), it is crucial to maintain cerebral oxygen delivery at this critical moment. The aim of this study was to evaluate the effectiveness of a cerebral oximeter (INVOS 3100, Somanetics Corporation, Troy, MI, USA) in monitoring cerebral oxygen balance during rewarming in CPB by correlating optical spectroscopic determinations with jugular bulb oxyhemoglobin saturation measurements. METHODS: On trial were 15 patients who underwent cardiac surgery under moderate hypothermic nonpulsatile CPB. Infrared spectroscopic soma sensor was placed over the patient's left frontal area to continuously monitor the regional cerebral oxygen saturation (SrO2) and the ipsilateral internal jugular vein was cannulated for measurement of venous oxyhemoglobin saturation (SjO2) at the jugular bulb. The changes of SjO2, SrO2, mean arterial pressure (MAP) and nasopharyngeal temperature (NPT) were recorded 5 min before CPB, instantaneously at initiation of CPB and at the point where every increment of 1 degree C in NPT was reached during CPB rewarming. The data obtained were statistically analyzed for any correlation between two methods using repeated measurements of ANOVA and linear regression. RESULTS: There was no correlation between SjO2 and SrO2, between MAP change and SjO2 or SrO2 at the start of CPB, but a significant reduction of SjO2 during rewarming period was observed which was inversely correlative with NPT changes (r(2) = 0.64, p < 0.05). However, SrO2 did not show a similarity. CONCLUSIONS: Jugular bulb desaturation which correlated inversely with rewarming speed as disclosed in this study indicated that the temperature change during rewarming was crucial for cerebral oxygen balance. Being less sensitive and still more unspecific, the INVOS 3100 cerebral oximeter is incapable and not recommended for monitoring cerebral oxygen saturation in CPB.

Aged↗
PubMed1997

Cerebral hypoxia during cardiopulmonary bypass: a magnetic resonance imaging study.

BACKGROUND: Neurocognitive deficits after open heart operations have been correlated to jugular venous oxygen desaturation on rewarming from hypothermic cardiopulmonary bypass (CPB). Using a porcine model, we looked for evidence of cerebral hypoxia by magnetic resonance imaging during CPB. Brain oxygenation was assessed by T2*-weighted imaging, based on the blood oxygenation level-dependent effect (decreased T2*-weighted signal intensity with increased tissue concentrations of deoxyhemoglobin). METHODS: Pigs were placed on normothermic CPB, then cooled to 28 degrees C for 2 hours of hypothermic CPB, then rewarmed to baseline temperature. T2*-weighted, imaging was undertaken before CPB, during normothermic CPB, at 30-minute intervals during hypothermic CPB, after rewarming, and then 15 minutes after death. Imaging was with a Bruker 7.0 Tesla, 40-cm bore magnetic resonance scanner with actively shielded gradient coils. Regions of interest from the magnetic resonance images were analyzed to identify parenchymal hypoxia and correlated with jugular venous oxygen saturation. Post-hoc fuzzy clustering analysis was used to examine spatially distributed regions of interest whose pixels followed similar time courses. Attention was paid to pixels showing decreased T2* signal intensity over time. RESULTS: T2* signal intensity decreased with rewarming and in five of seven experiments correlated with the decrease in jugular venous oxygen saturation. T2* imaging with fuzzy clustering analysis revealed two diffusely distributed pixel groups during CPB. One large group of pixels (50% +/- 13% of total pixel count) showed increased T2* signal intensity (well-oxygenated tissue) during hypothermia, with decreased intensity on rewarming. Changes in a second group of pixels (34% +/- 8% of total pixel count) showed a progressive decrease in T2* signal intensity, independent of temperature, suggestive of increased brain hypoxia during CPB. CONCLUSIONS: Decreased T2* signal intensity in a diffuse spatial distribution indicates that a large proportion of cerebral parenchyma is hypoxic (evidenced by an increased proportion of tissue deoxyhemoglobin) during CPB in this porcine model. Neuronal damage secondary to parenchymal hypoxia may explain the postoperative neuropsychological dysfunction after cardiac operations.

Animals↗
PubMed2000

Resistive heating is more effective than metallic-foil insulation in an experimental model of accidental hypothermia: A randomized controlled trial.

STUDY OBJECTIVE: We study a resistive-heating blanket in a volunteer model of severe accidental hypothermia to evaluate differences in rates of rewarming, core temperature afterdrop, and body heat content and distribution during active and passive rewarming. METHODS: Eight volunteers participated in a crossover design on 2 days. The volunteers were anesthetized and cooled to 33 degrees C (91.4 degrees F); anesthesia was subsequently discontinued, and shivering was prevented with meperidine. On one randomly assigned day, the volunteers were rewarmed passively with reflective foil (passive insulation), whereas on the other they were covered with a carbon fiber-resistive heating blanket set to 42 degrees C (107.6 degrees F; active rewarming). Trunk and head temperature and heat content were calculated from core (tympanic membrane) temperature. Peripheral (arm and leg) tissue temperature and heat content were estimated by using fourth-order regressions and integration over volume from 30 tissue and skin temperatures. RESULTS: Core heat content increased 73+/-14 kcal (mean+/-SD) during 3 hours of active warming, but only 31+/-24 kcal with passive insulation, a difference of 41+/-20 kcal (95% confidence interval [CI] 27 to 55 kcal; P <. 001). Peripheral tissue heat content increased linearly by 111+/-16 kcal during active warming but only by 38+/-31 kcal during passive warming, a difference of 74+/-34 kcal (95% CI 50 to 97; P <.001). Consequently, total body heat increased 183+/-22 kcal during active warming but only 68+/-54 kcal with passive insulation, a difference of 115+/-42 kcal (95% CI 86 to 144 kcal; P <.001). Core temperature increased from 32.9 degrees C+/-0.2 degrees C to 35.2 degrees C+/-0. 4 degrees C during 3 hours of active warming, a difference of 2.3 degrees C+/-0.4 degrees C. In contrast, core temperature with foil insulation only increased from 32.9 degrees C+/-0.2 degrees C to 33. 8 degrees C+/-0.5 degrees C, a difference of only 0.8 degrees C+/-0. 4 degrees C. The difference in the core temperature increase between the two treatments was thus 1.5 degrees C+/-0.4 degrees C (95% CI 1. 2 degrees C to 1.7 degrees C; P <.001 between treatments). Active warming was not associated with an afterdrop, whereas the afterdrop was 0.2 degrees C+/-0.2 degrees C and lasted a median of 45 minutes (interquartile range, 41 to 64 minutes) with passive insulation. CONCLUSION: Resistive heating more than doubles the rewarming rate compared with that produced by reflective metal foil and does so without producing an afterdrop. It is therefore likely to be useful in the prehospital setting.

Adult↗
PubMed1998

Effects of angiotensin converting enzyme inhibition on systemic vascular resistance and vasoconstrictor requirements during hypothermic cardiopulmonary bypass.

OBJECTIVE: We proposed that angiotensin converting enzyme (ACE) inhibitor therapy would alter systemic vascular resistance (SVR) during rewarming and increase the requirement for vasoactive drugs in the immediate post-bypass period. METHODS: Sixty-five sequential adult patients undergoing cardiac surgical procedures requiring hypothermic (28 degrees C) cardiopulmonary bypass (CPB) were recruited. Sixty-two fitted the inclusion criteria of which 21 were receiving ACE inhibitors prior to surgery. SVR was calculated at 1 min intervals during the rewarming phase of hypothermic CPB. The use of vasoactive drugs during and immediately after termination of CPB was recorded. The doctor administering these drugs was unaware of the nature of the study. RESULTS: Mean SVR in the ACE group was 978 dyne/s per cm5 and in the control group was 1194 dyne/s per cm5 (P = 0.006). Mean arterial pressure was 48.8 mmHg in the ACE group and 56.3 mmHg in the control group (P = 0.004). There was a significant difference in vasoactive drug requirements between the groups (P < 0.01). There was no statistically significant difference in age, weight, body mass index, body surface area, theatre temperature, core temperature at which rewarming started, rate and time of rewarming, haematocrit on bypass or preoperative left ventricular function. CONCLUSION: Preoperative ACE inhibitor therapy decreases SVR during the rewarming phase of CPB and increases post-bypass vasoactive drug requirements.

Aged↗
PubMed2000

Hypothermic cardiac arrest: an 11 year review of ED management and outcome.

The purpose of this study was to examine the emergency department (ED) management of hypothermic cardiac arrest and its outcome. The medical records of all patients with hypothermic cardiac arrest treated in the ED from January 1, 1988 to January 31, 1999 were retrospectively reviewed. Data collected included initial body temperature, serum potassium, methods of rewarming, return of perfusing rhythm, and morbidity and mortality. Data were analyzed by descriptive methods. Eleven patients were treated in the ED resuscitation room for hypothermic cardiac arrest. Six patients were found in cardiac arrest in the field, one patient arrested during transport, and four patients arrested after ED arrival. The average initial temperature was 79.1 degrees F (range 69.0 degrees F to 86.7 degrees F). Seven patients received an ED thoracotomy with internal cardiac massage and warm mediastinal irrigation. Four patients had airway management in the ED and then direct transport to the operating room for cardiac bypass rewarming. Three of the seven patients who received an ED thoracotomy subsequently went to intraoperative cardiac bypass rewarming. Five of the seven (71.4%) patients who received an ED thoracotomy survived, versus none of the four patients (0%) who went directly to intraoperative cardiac bypass. A direct comparison of immediate ED thoracotomy versus intraoperative cardiac bypass without ED thoracotomy is cautiously made as this was an unmatched and nonrandomized study. Three of the surviving patients underwent intraoperative cardiac bypass rewarming after receiving an ED thoracotomy. In two of these patients a perfusing rhythm had been established after thoracotomy in the ED and before transport to the operating room for cardiac bypass. Only one of seven (14.3%) patients who arrested prehospital survived versus four of four (100%) who arrested in the ED. ED thoracotomy with internal cardiac massage and mediastinal irrigation rewarming is effective in the management of hypothermic cardiac arrest.

Adult↗
PubMed1999

Tissue heat content and distribution during and after cardiopulmonary bypass at 17 degrees C.

UNLABELLED: We measured afterdrop and peripheral tissue temperature distribution in eight patients cooled to approximately 17 degrees C during cardiopulmonary bypass and subsequently rewarmed to 36.5 degrees C. A nasopharyngeal probe evaluated trunk and head temperature and heat content. Peripheral tissue temperature (arm and leg temperature) and heat content were estimated using fourth-order regressions and integration over volume from 30 tissue and skin temperatures. Peripheral tissue temperature decreased to 19.7+/-0.9 degrees C during bypass and subsequently increased to 34.3+/-0.7 degrees C during 104+/-18 min of rewarming. The core-to-peripheral tissue temperature gradient was -5.9+/-0.9 degrees C at the end of cooling and 4.7+/-1.5 degrees C at the end of rewarming. The core-temperature afterdrop was 2.2+/-0.4 degrees C and lasted 89+/-15 min. It was associated with 1.1+/-0.7 degrees C peripheral warming. At the end of cooling, temperatures at the center of the upper and lower thigh were (respectively) 8.0+/-5.2 degrees C and 7.3+/-4.2 degrees C cooler than skin temperature. On completion of rewarming, tissue at the center of the upper and lower thigh were (respectively) 7.0+/-2.2 degrees C and 6.4+/-2.3 degrees C warmer than the skin. When estimated systemic heat loss was included in the calculation, redistribution accounted for 73% of the afterdrop, which is similar to the contribution observed previously in nonsurgical volunteers. IMPLICATIONS: Temperature afterdrop after bypass at 17 degrees C was 2.2+/-0.4 degrees C, with approximately 73% of the decrease in core temperature resulting from core-to-peripheral redistribution of body heat. Cooling and rewarming were associated with large radial tissue temperature gradients in the thigh.

Adult↗
PubMed1994

Deliberate mild intraoperative hypothermia for craniotomy.

BACKGROUND: Despite enthusiasm for the use of mild hypothermia during neurosurgical procedures, this therapy has not been evaluated systematically. This study examined the feasibility and safety of deliberate mild hypothermia and rewarming. METHODS: Thirty patients scheduled for craniotomy were assigned to either a normothermic or mildly hypothermic group. Tympanic membrane temperature was monitored at anesthetic induction, throughout the isoflurane-fentanyl-N2O-O2 anesthetic, and for 18 h postoperatively. Normothermic patients were warmed to 36.5-37.0 degrees C after an initial temperature decrease, and hypothermic patients were cooled to 35 degrees C. In the hypothermic group temperatures were allowed to drift to 34.5 degrees C before rewarming was initiated. Water blankets and convective heating devices were used to cool and rewarm. RESULTS: The minimum temperature achieved by the hypothermic group was 34.3 +/- 0.4 degrees C. Cooling occurred at a rate of 1.0 +/- 0.4 degrees C/h. Rewarming took place at a rate of 0.7 +/- 0.6 degrees C/h (range 0.1-1.8) in the hypothermic group. Hypothermia did not delay emergence from anesthesia (20 +/- 15 min) compared with normothermia (15 +/- 15 min, P = .45). Mean temperature upon intensive care unit admission was 35.8 +/- 1.0 degrees C for the hypothermic group and 37.1 +/- 0.5 degrees C for the normothermic group (P < 0.0001). The hypothermic patients had more postoperative shivering. From 8 to 18 h postoperatively the temperatures of the two groups were similar except for a slightly greater temperature in the hypothermic patients at 12 h (37.6 +/- 0.5 vs. 37.3 +/- 0.4 degrees C, P = .029). CONCLUSIONS: Although deliberate mild hypothermia is easily achieved intraoperatively, complete rewarming may be difficult to attain during craniotomy with current methods. In addition to the need for determining whether deliberate mild hypothermia confers cerebral protection in humans, the potential risks of the therapy need to be further characterized.

Aged↗
PubMed2000

Efficacy of two methods for reducing postbypass afterdrop.

BACKGROUND: Afterdrop, defined as the precipitous reduction in core temperature after cardiopulmonary bypass, results from redistribution of body heat to inadequately warmed peripheral tissues. The authors tested two methods of ameliorating afterdrop: (1) forced-air warming of peripheral tissues and (2) nitroprusside-induced vasodilation. METHODS: Patients were cooled during cardiopulmonary bypass to approximately 32 degrees C and subsequently rewarmed to a nasopharyngeal temperature near 37 degrees C and a rectal temperature near 36 degrees C. Patients in the forced-air protocol (n = 20) were assigned randomly to forced-air warming or passive insulation on the legs. Active heating started with rewarming while undergoing bypass and was continued for the remainder of surgery. Patients in the nitroprusside protocol (n = 30) were assigned randomly to either a control group or sodium nitroprusside administration. Pump flow during rewarming was maintained at 2.5 l x m(-2) x min(-1) in the control patients and at 3.0 l x m(-2) x min(-1) in those assigned to sodium nitroprusside. Sodium nitroprusside was titrated to maintain a mean arterial pressure near 60 mm Hg. In all cases, a nasopharyngeal probe evaluated core (trunk and head) temperature and heat content. Peripheral compartment (arm and leg) temperature and heat content were estimated using fourth-order regressions and integration over volume from 18 intramuscular needle thermocouples, nine skin temperatures, and "deep" hand and foot temperature. RESULTS: In patients warmed with forced air, peripheral tissue temperature was higher at the end of warming and remained higher until the end of surgery. The core temperature afterdrop was reduced from 1.2+/-0.2 degrees C to 0.5+/-0.2 degrees C by forced-air warming. The duration of afterdrop also was reduced, from 50+/-11 to 27+/-14 min. In the nitroprusside group, a rectal temperature of 36 degrees C was reached after 30+/-7 min of rewarming. This was only slightly faster than the 40+/-13 min necessary in the control group. The afterdrop was 0.8+/-0.3 degrees C with nitroprusside and lasted 34+/-10 min which was similar to the 1.1+/-0.3 degrees C afterdrop that lasted 44+/-13 min in the control group. CONCLUSIONS: Cutaneous warming reduced the core temperature afterdrop by 60%. However, heat-balance data indicate that this reduction resulted primarily because forced-air heating prevented the typical decrease in body heat content after discontinuation of bypass, rather than by reducing redistribution. Nitroprusside administration slightly increased peripheral tissue temperature and heat content at the end of rewarming. However, the core-to-peripheral temperature gradient was low in both groups. Consequently, there was little redistribution in either case.

Adult↗
PubMed2003

Hypothermia and the approximate entropy of the electroencephalogram.

BACKGROUND: The electroencephalogram is commonly used to monitor the brain during hypothermic cardiopulmonary bypass and circulatory arrest. No quantitative relationship between the electroencephalogram and temperature has been elucidated, even though the qualitative changes are well known. This study was undertaken to define a dose-response relationship for hypothermia and the approximate entropy of the electroencephalogram. METHODS: The electroencephalogram was recorded during cooling and rewarming in 14 patients undergoing hypothermic cardiopulmonary bypass and circulatory arrest. Data were digitized at 128 Hz, and approximate entropy was calculated from 8-s intervals. The dose-response relationship was derived using sigmoidal curve-fitting techniques, and statistical analysis was performed using analysis of variance techniques. RESULTS: The approximate entropy of the electroencephalogram changed in a sigmoidal fashion during cooling and rewarming. The midpoint of the curve averaged 24.7 degrees C during cooling and 28 degrees C (not significant) during rewarming. The temperature corresponding to 5% entropy (T 0.05 ) was 18.7 degrees C. The temperature corresponding to 95% entropy (T 0.95 ) was 31.3 degrees C during cooling and 38.2 degrees C during rewarming ( P < 0.02). CONCLUSIONS: Approximate entropy is a suitable analysis technique to quantify the electroencephalographic changes that occur with cooling and rewarming. It demonstrates a delay in recovery that is of the same magnitude as that seen with conventional interpretation of the analog electroencephalogram and extends these observations over a greater range of temperatures.

Adult↗
PubMedDate not supplied

Treatment of hypothermia in trauma victims: thermodynamic considerations.

The relatively high specific heat of the human body makes hypothermia very difficult to treat. Although there are many treatment methods available, most evaluations of rewarming techniques are based on clinically observed rewarming rates, and they do not take into account initial core temperature, ambient temperature, the patient's own heat production, the effects of anesthesia, paralytic agents, and other variables. A heat transfer model is proposed that simulates the flow of heat through the body of a hypothermic patient. The model uses first principles involved in heat transfer and thermodynamics to describe the effects of currently available rewarming techniques. A commercially available routine is used to solve the equations, which also include any heat exchange between the patient's body and the environment, as well as metabolic heat generation as a function of time and core temperature. This thermodynamic analysis of rewarming, based on computer modeling of heat transfer, provides a scientific basis on which to establish guidelines for appropriate selection of treatment strategies for hypothermia, and it indicates that direct blood warming or infusion of warm intravenous fluids are the most effective rewarming techniques.

Body Temperature Regulation↗
PubMed2000

Comparative analysis of alpha-stat and pH-stat strategies with a membrane oxygenator during deep hypothermic circulatory arrest in young pigs.

Using young pigs, this study compared the strategies of alpha-stat and pH-stat during deep hypothermic circulatory arrest (DHCA) for the cooling time of brains during the induction of hypothermia and rewarming time with cardiopulmonary bypass (CPB); the cerebral perfusion rate and metabolism rate, and the ratio of these 2 rates; and the extent of the cerebral edema development after circulatory arrest. Fourteen young pigs were assigned to 1 of 2 strategies of gas management. Cerebral blood flow was measured with a cerebral venous outflow technique. With CPB, core cooling was initiated and continued until the nasopharyngeal temperature fell below 20 degrees C. The flow rate was set at 2,500 ml/min. Once the temperature reached below 20 degrees C, the animals were subjected to DHCA for 40 min. During the cooling period, the acid-base balance was maintained using either alpha-stat or pH-stat strategy. After DHCA, the body was rewarmed to the normal body temperature. The animals then were sacrificed, and we measured the brain water content. The cerebral perfusion and metabolism rates were measured before the onset of CPB, before cooling, before DHCA, 15 min after rewarming, and upon the completion of rewarming. The cooling time was significantly shorter with alpha-stat than with pH-stat strategy while no significant differences were observed in the rewarming time between groups. Also, no significant differences were found in cerebral blood flow volume, metabolic rate, or flow/metabolic rate ratio between groups. In each group, the cerebral blood flow volume, metabolic rate, and flow/metabolic rate ratio showed significant differences in body temperature. Brain water content showed no significant differences between the 2 groups. In summary, this study found no significant differences between alpha-stat and pH-stat strategies, except in the cooling time. The cooling time was rather shorter with the alpha-stat than with the pH-stat strategy.

Acid-Base Equilibrium↗
PubMed1993

Right ventricular function after normothermic versus hypothermic cardiopulmonary bypass.

Normothermic systemic perfusion in patients undergoing cardiopulmonary bypass may compromise myocardial hypothermia, a mainstay for preservation of ventricular function during iatrogenic cardiac arrest. The right ventricle is the area of the heart most susceptible to rewarming. We prospectively evaluated myocardial rewarming and indexes of right ventricular function in 30 patients undergoing coronary artery bypass grafting randomized to receive moderate hypothermic (bladder temperature 25 degrees C) or normothermic perfusion and multidose cold blood cardioplegia during cardiopulmonary bypass. All patients had significant stenosis (> 70%) of the right coronary artery, and in 27 of 30 the right coronary artery was revascularized. A right ventricular ejection fraction/volumetric catheter was used to assess right ventricular function by right ventricular ejection fraction and a preload (right ventricular end-diastolic volume) normalized right ventricular stoke work index in the prebypass and postbypass periods. Findings included the following: (1) Greater rewarming of all areas of the heart occurs with normothermic bypass, with the mean temperature difference at the end of each intracardioplegic period ranging from 4.0 degrees to 6.3 degrees C warmer than with hypothermic bypass; (2) the right ventricle was not more susceptible to rewarming than the posterior left ventricle or interventricular septum in either group; (3) right ventricular function did not differ between groups at any time in the study, including the immediate postarrest period; and (4) right ventricular function was preserved and equivalent to the prebypass baseline. We conclude that the moderate myocardial rewarming that occurs with normothermic perfusion does not compromise right ventricular preservation in patients with right coronary artery disease undergoing revascularization with multidose cold blood cardioplegia to maintain electromechanical arrest.

Aged↗