Changes of body temperature and heat in cardiac surgical patients

Monitoring bladder temperatures in the OR

Temperature monitoring via the urinary bladder has become common in the OR, often replacing monitoring at the rectal site. A systematic, integrated review and synthesis of the literature was undertaken to assess the validity of using the urinary bladder as a site for temperature measurement in the OR. During steady thermal states, bladder temperature performed well, providing temperatures similar to those of core sites. In contrast, poor performance was demonstrated during rapid thermal changes, such as during the rapid cooling and rewarming phases of cardiopulmonary bypass. At such times, a significant lag in response rate at the bladder site was noted by multiple investigators. This delayed responsiveness during thermally dynamic states, however, may provide information regarding the adequacy of rewarming during bypass at sites intermediate between the core and periphery. Limited research indicates that urinary bladder temperature may be influenced by urine flow rate, and additional research is required in this area. The cost effectiveness of this method of temperature measurement requires investigation as well.

Venovenous perfusion-induced systemic hyperthermia: hemodynamics, blood flow, and thermal gradients

BACKGROUND

Thermal events during extracorporeal venovenous perfusion-induced systemic hyperthermia (VV-PISH) were studied and related to determination of whole-body and regional thermal isoeffect doses.

METHODS

Swine (n = 6, 77+/-4.5 kg) were heated to a target temperature of 43 degrees C for 120 minutes using VV-PISH. Colored microspheres were injected during preheat, heat induction, maintenance, cool down, and after decannulation. The esophageal, tympanic, rectal, pulmonary artery, bladder, bone marrow, kidney, brain, blood, lung, and airway temperatures were recorded continuously. The thermal dose, thermal exchange, metabolic heat production, heat loss to the environment, the change in body heat, and the thermal isoeffect dose were studied at 15-minute intervals.

RESULTS

VV-PISH increased heart rate and cardiac output and caused a redistribution of blood flow favoring the thoracoabdominal organs. Greatest thermal exchange occurred during the heating phase (total 2,162+/-143 kJ), metabolic heat production contributed in all phases (274+/-9 kJ), the greatest change in body heat occurred during heating (1,310+/-309 kJ) with a total delivered thermal dose of 298+/-21 kJ, and the total whole body thermal isoeffect dose at 100+/-5 minutes.

CONCLUSIONS

VV-PISH is feasible, is capable of transferring sufficient heat, causes a redistribution of blood flow favoring the thoracoabdominal organs, and facilitates calculation of whole-body and regional thermal isoeffect doses.

Convective warming blankets improve peroperative heat preservation in congenital heart surgery

Peroperative heat preservation, following hypothermic cardiopulmonary bypass (CPB) in children, has always been a challenge to the anaesthetist. We studied the efficiency of a convective heating system on peroperative heat preservation in 50 children undergoing congenital heart surgery. Twenty-five children, rewarmed by CPB and heating mattress, were randomly selected (Group 1). Another 25 children, rewarmed by CPB, heating mattress and convective warming blankets in addition (Group 2), were selected so the two groups were comparable regarding age, weight and anaesthetic management. The central and peripheral temperatures were measured during bypass, at the end of bypass and at the end of operation. A retrospective evaluation showed that during bypass the peripheral temperature was significantly lower in Group 2 than in Group 1, with no significant difference in central temperature. At the end of bypass there was no significant difference between the two groups. At the end of operation the central and peripheral temperatures were significantly higher in Group 2. In conclusion convective warming blankets are effective in keeping or even raising the temperature following congenital heart surgery.

Afterdrop after hypothermic cardiopulmonary bypass: the value of tympanic membrane temperature monitoring

OBJECTIVES

After weaning from cardiopulmonary bypass (CPB), a decrease in nasopharyngeal temperature (NPT) occurs (afterdrop). The pathophysiology of the afterdrop remains unclear: It might be caused by either inadequate total body rewarming on CPB or to heterogenous distribution of heat during CPB, with subsequent redistribution of heat from the warmer core to the cooler shell tissues. The study objectives were (1) to determine whether post-CPB afterdrop is the result of a negative CPB thermal balance, and (2) to investigate which sites (if any) could best predict the afterdrop.

DESIGN

Prospective evaluation using within-patient comparisons during CPB cooling, CPB rewarming, and 45 minutes post-CPB.

SETTING

Adult patients gave informed consent before a cardiac surgical procedure in a university hospital.

PARTICIPANTS

Eight patients undergoing CABG or valvular replacement with hypothermic CPB (NPT near 29 degrees C) and standardized general anesthesia.

INTERVENTIONS

Each patient was studied with temperature monitors (Mon-a-therm 7000; Mallinckrodt-Medexel, Gemenos, France) attached to disposable thermocouple probes placed as follows: urinary bladder, rectum, deltoid, esophagus, nasopharynx, tympanic membrane, and four skin sites. In addition, the temperatures from the thermistors of the pulmonary artery catheter, and the arterial and venous lines of the CPB circuit were considered. Thirteen sites for monitoring temperature were studied.

MEASUREMENTS AND MAIN RESULTS

Temperatures were recorded every 5 minutes, from the beginning of CPB to the 45th minute after CPB, and thermal exchanges were calculated: change in body heat (QBH), thermal exchanges between the patient and the pump (QCPB), metabolic heat production (Qm) (equal to calculated VO2 at the pump level), and heat loss to the environment (QS) (equal to QBH-QCPB-Qm). Thermal exchanges were obtained in six patients during the plateaus of cooling and rewarming, during the whole CPB phase, and after CPB. It was found that despite a change in QBH during rewarming (1,017 +/- 88 kJ) that was slightly greater than during cooling (-1,008 +/- 104 kJ) (mean +/- SEM), a significant decrease in post-CPB "core" temperature occurred (afterdrop: -1.4 degrees C). Magnitude of the afterdrop was directly related to the magnitude of tympanic membrane cooling and was negatively correlated to the temperature difference between the warmest site (tympanic membrane) and the coolest site (cutaneous thigh temperature) observed at the end of rewarming (r = -0.667; p < 0.05).

CONCLUSIONS

It is suggested that besides post-CPB heat loss, redistribution of heat may be involved in the mechanism of the afterdrop and that measurements of tympanic membrane and cutaneous thigh temperatures are the best monitors of adequacy of rewarming during CPB.

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.

A comparison of patient rewarming devices after cardiac surgery

Three regimens for rewarming patients after cardiac surgery involving hypothermic cardiopulmonary bypass were studied in 30 patients. The control group (n = 10) received no active rewarming, the oesophageal group (n = 10) was warmed centrally using an oesophageal heat exchanger and the radiant group (n = 10) was warmed peripherally with an overhead radiant heater. There were no statistically significant differences between the groups apart from the higher mean skin temperatures in the peripheral group.

Postoperative ventilatory and circulatory effects of extended rewarming during cardiopulmonary bypass

Postoperative effects of extended rewarming (ECR) after hypothermic cardiopulmonary bypass (CPB) were studied. All (n = 28) patients were rewarmed to a nasopharyngeal temperature exceeding 38 degrees C before terminating CPB. In 12 patients (control group) the rectal temperature (Tre) was 33.8 +/- 1.7 degrees C (mean +/- sd) at termination of CPB. In sixteen patients (ECR group) rewarming during CPB was continued to a Tre of 36.8 +/- 0.5 degrees C. Postoperative body temperatures, heat content, shivering, oxygen uptake, CO2 production and haemodynamic variables were measured. ECR reduced the heat gain required to complete core rewarming to 665 +/- 260 kJ, compared with 1037 +/- 374 kJ in the control group (p less than 0.01). The incidence of shivering was reduced (p less than 0.05) as well as shivering intensity and duration. In seven non-shivering ECR group patients this coincided with significantly reduced metabolic and ventilatory demands but these improvements were not valid for the group as a whole. The required ventilation temporarily during postoperative rewarming in both groups increased to 250 per cent of the basal need. Extending CPB rewarming (to at least 36 degrees C Tre) was inefficient when used as the sole measure to reduce the untoward effects of residual hypothermia during recovery after cardiac surgery with hypothermic CPB.

Postoperative ventilatory and circulatory effects of heating after aortocoronary bypass surgery. Extended rewarming during cardiopulmonary bypass and postoperative radiant heat supply

Twenty-four patients with stable angina pectoris were studied after aortocoronary bypass surgery with hypothermic cardiopulmonary bypass (CPB). Twelve patients (radiant heat supply group) were rewarmed during CPB to a nasopharyngeal temperature of at least 38 degrees C and a mean rectal temperature of 34.4 degrees C. Postoperatively they received radiant heat supply from a thermal ceiling. In addition, a heating water mattress was used during the end of the operation and heated, humidified inspired gases were administered intra- and postoperatively. The other 12 patients (combination heat supply group) had the rewarming during CPB extended until the rectal temperature exceeded 36 degrees C, but otherwise received the same treatment as the radiant heat supply group. The combination of extended rewarming during CPB and postoperative radiant heat supply significantly reduced oxygen uptake, carbon dioxide production and the required ventilation volumes during early recovery as compared with the values in the radiant heat supply group. The reduced metabolic demands were accompanied by lower cardiac index and oxygen delivery, which, however, were sufficient for adequate tissue perfusion as judged by the similarity in oxygen extraction and arterial base excess values in the two groups. The metabolic demands and ventilatory requirements were reduced to a level at which safe early extubation is possible.

Postoperative ventilatory and circulatory effects of heating after aortocoronary bypass surgery. Postoperative external heat supply

The effects of postoperative external heat supply on shivering, oxygen uptake, carbon dioxide production, ventilatory requirements and haemodynamic variables were studied postoperatively after aortocoronary bypass surgery in 24 men with stable angina pectoris. After hypothermic cardiopulmonary bypass (CPB) at 25 degrees C, the patients were rewarmed to a nasopharyngeal temperature of at least 38 degrees C, resulting in a rectal temperature of about 34 degrees C before termination of CPB. Twelve patients, forming the control group, were given no other external heat supply. In another group (n = 12), the "radiant heat supply group", additional external heat was provided postoperatively, the main source of which was a thermal ceiling supplemented with heated, humidified respiratory gases. In this latter group the postoperative rewarming was accomplished earlier and was converted into a mainly passive process. Shivering, oxygen uptake, CO2 production and ventilation volumes were significantly reduced compared with the control group. Cardiac index and stroke index were higher and systemic oxygen extraction was lower in the radiant heat supply group. Postoperative hypertension and vasoconstriction were greatly decreased, suggesting that residual hypothermia is an important cause of the postoperative vasoconstriction.