Efficacy of two methods for reducing postbypass afterdrop

Performance of the transoesophageal echocardiography probe as an oesophageal temperature monitor in patients undergoing cardiac surgery with cardiopulmonary bypass: a prospective observational study

OBJECTIVES

Core temperature monitoring is critical during cardiopulmonary bypass (CPB). In this prospective observational study, we investigated the performance of the transoesophageal echocardiography (TOE) probe for core (oesophageal) temperature monitoring during CPB.

METHODS

Thirty adult patients, 18-70 years of either gender, undergoing cardiac surgery with CPB were enrolled. All patients received a reusable nasopharyngeal probe for monitoring core temperatures. In addition, the oesophageal temperatures were monitored with the TOE probe. The arterial outlet temperatures at the membrane oxygenator were also monitored and taken as the reference standard. Monitoring was performed every 5 min until 20 min, and then at 30 min during both the cooling and rewarming periods.

RESULTS

During cooling, the oesophageal and nasopharyngeal temperatures lagged behind the arterial outlet temperatures. However, the intra-class correlation of the oesophageal temperatures with the arterial outlet temperatures was better (range 0.58-0.74) than the correlation of the nasopharyngeal temperatures with the arterial outlet temperatures (range 0.46-0.62). During rewarming, the performance of the TOE probe was significantly superior to the nasopharyngeal probe. After 15 and 20 min of rewarming, there was a difference of ∼1°C between the oesophageal and nasopharyngeal temperatures. At 30 min of rewarming, the oesophageal and the arterial outlet temperatures were similar, while the nasopharyngeal temperatures still lagged by 0.5°C. Bias was significantly less both during cooling and warming between the oesophageal temperatures and arterial outlet temperatures.

CONCLUSIONS

Performance of the TOE probe as an oesophageal temperature probe is superior to the nasopharyngeal probe during CPB.

CLINICAL TRIAL REGISTRATION NUMBER

CTRI no 2020/10/028228; ctri.nic.in.

Heated Humidified Breathing Circuit Rewarming in Hypothermic Patients Post-Cardiopulmonary Bypass-Pilot Study

OBJECTIVES

Hypothermia on intensive care unit (ICU) admission after cardiac surgery and cardiopulmonary bypass is common. It contributes to postoperative complications including shivering, coagulopathy, increased blood loss and transfusion requirements, morbid cardiac events, metabolic acidosis, increased wound infections, and prolonged hospital length of stay. The current standard of care for rewarming ICU patients is forced air warming blankets. However, high-quality evidence on additional benefit rendered by other warming methods, such as heated humidified breathing circuits (HHBC), is lacking. Therefore, the authors conducted a pilot study to examine whether the addition of HHBC to standard forced air warming blankets in hypothermic patients (≤35°C) admitted to the ICU after cardiac surgery using cardiopulmonary bypass reduced time to normothermia.

DESIGN

Prospective study conducted at a single large academic medical center.

PARTICIPANTS

The study group was composed of 14 patients who were enrolled prospectively between April 1 and June 14, 2019. The study group was compared with a 2:1 matched retrospective control group. The matched group consisted of 28 patients from a 12-month period from July 1, 2018 June 30, 2019.

INTERVENTIONS

Study patients received warming via forced air warming blankets and HHBC and were compared with patients in a control group who received only warming blankets. Time to normothermia, time to extubation, time to normal pH, blood loss, blood transfusions, and coagulation profile laboratory values were compared between the study and control groups.

MEASUREMENTS AND MAIN RESULTS

The present study found no statistical difference in time to normothermia, for which the standard-of-care retrospective group achieved normothermia after a median (Q1-Q3) 4.8 (4.0-6.0) hours compared with 4.4 (3.5-5.5) hours in the prospective group receiving HHBC. All secondary outcomes, including time to extubation, time to normal pH, ICU blood product transfusion, chest tube output, and coagulation profile, were similar.

CONCLUSIONS

The present pilot study detected a similar time to normothermia, extubation, and normal pH when HHBC were added to standard forced air warming blankets in hypothermic patients (≤35°C) admitted to the ICU after cardiac surgery using cardiopulmonary bypass. A future larger prospective study designed to detect smaller, but clinically meaningful, reductions in the time to key clinical events for patients treated with HHBC is feasible and warranted.

Comparison of Effects of Propofol and Isosorbide Dinitrate during Rewarming on Cardiopulmonary Bypass

OBJECTIVES

Comparison of effects of propofol and isosorbide dinitrate during rewarming on cardiopulmonary bypass in patients undergoing coronary artery bypasses grafting.

METHODS

It was randomized prospective clinical trial. One hundred and twenty patient (120) undergoing CABG surgery were included in this study. Group-I (Study group, n=60): in which only propofol infusion used during rewarming and Group-II (control Group, n=60) in which isosorbide dinitrate and propofol infusion combination was used during rewarming. The data was entered and analyzed through SPSS Version 19. Independent sample T-test and chi-square test were used for data analysis. P value of ≤ 0.05 was taken as significant.

RESULTS

Mean arterial pressures during rewarming were 63.41±3.61 mmHg in propofol group versus 60.80±4.86 mmHg in control group (p-value 0.001). Core temperature on weaning from cardiopulmonary bypass was 37.11±0.49 °C in propofol group and 37.00±0.18 °C in control group. After drop in core temperature was little more in propofol group (1.02±0.36 °C) versus 0.96±0.37 °C in control group but this difference was not statistically significant (p-value 0.41). Mean Ventilation time after surgery in propofol group was 4.65±0.65 hours versus 5.03±0.81 hours in control group (p-value 0.006).

CONCLUSION

Propofol alone is capable of fulfilling the requirements of adequate rewarming during Cardiopulmonary bypass and can produce more hemodynamic stability and early post-operative recovery.

Accidental hypothermia in severe trauma

Hypothermia, along with acidosis and coagulopathy, is part of the lethal triad that worsen the prognosis of severe trauma patients. While accidental hypothermia is easy to identify by a simple measurement, it is no less pernicious if it is not detected or treated in the initial phase of patient care. It is a multifactorial process and is a factor of mortality in severe trauma cases. The consequences of hypothermia are many: it modifies myocardial contractions and may induce arrhythmias; it contributes to trauma-induced coagulopathy; from an immunological point of view, it diminishes inflammatory response and increases the chance of pneumonia in the patient; it inhibits the elimination of anaesthetic drugs and can complicate the calculation of dosing requirements; and it leads to an over-estimation of coagulation factor activities. This review will detail the pathophysiological consequences of hypothermia, as well as the most recent principle recommendations in dealing with it.

Steady-state and time-dependent thermodynamic modeling of the effect of intravenous infusion of warm and cold fluids

BACKGROUND

Hypothermia results in vital sign lability, coagulopathy, wound infections, and other sequelae. Normothermia can be restored by several modalities, including passive blanket heating, warm forced-air devices, and active fluid warming (AFW). In AFW, intravenously administered fluids are heated to 40 to 45 °C to minimize net thermal losses and to raise body temperature. Clinical studies have demonstrated the efficacy of AFW as part of a strategy encompassing several methods, but the isolated contribution of AFW to warming has not been theoretically examined in detail.

METHODS

A calorimetric model is derived to determine the functional dependence of warming on patient weight, hypothermia severity, infusion temperature, and volume infused. A second heat transfer model is derived to describe the time-dependent temperature changes of the periphery and core after warmed-fluid infusion.

RESULTS

There is an inverse linear relationship between the patient's initial temperature and the amount of warming achieved with a given volume. In contrast, as the temperature of the infusion approaches the desired final temperature, the volume required for a fixed temperature change increases nonlinearly. For weight-based boluses, the temperature change scales appropriately with patient mass. Infusion of 2 L of room-temperature crystalloid results in a decrease in body temperature of approximately one-third degree Celsius in the average normothermic adult. For the heat transfer model, previously reported rates of temperature drop and recovery after the intravenous infusion of cold fluids are qualitatively reproduced with a blood mixing time of approximately 15 minutes.

CONCLUSION

Our calculations reveal that AFW has a larger measurable beneficial effect for patients with more severe hypothermia, but true rewarming of the patient with AFW alone would require prohibitively large fluid volumes (more than 10 L of 40 °C fluid) or dangerously hot fluid (20 mL/kg of 80 °C fluid for a 1 °C increase). The major beneficial effect of AFW is the prevention of further net heat loss.

A model to predict patient temperature during cardiac surgery

A core temperature drop after cardiac surgery slows down the patient's recuperation process. In order to minimize the amount of the so-called afterdrop, more knowledge is needed about the impaired thermoregulatory system during anesthesia and the effect of different protocols on temperature distribution. Therefore, a computer model has been developed that describes heat transfer during cardiac surgery. The model consists of three parts: (1) a passive part, which gives a simplified description of the human geometry and the passive heat transfer processes, (2) an active part that takes into account the thermoregulatory system as a function of the amount of anesthesia and (3) submodels, through which it is possible to adjust the boundary conditions. The validity of the new model was tested by comparing the model results to the measurement results of three surgical procedures. A good resemblance was found between simulation results and the experiments. Next, a model application was shown. A parameter study was performed to study the effect of different temperature protocols on afterdrop. It was shown that the effectiveness of forced-air heating is larger than the benefits resulting from increased environmental temperature or usage of a circulating water mattress. Ultimately, the model could be used to develop a monitoring decision system that advises clinicians what temperature protocol will be best for the patient.

Reliability of temperatures measured at standard monitoring sites as an index of brain temperature during deep hypothermic cardiopulmonary bypass conducted for thoracic aortic reconstruction

OBJECTIVE

It is essential to estimate the brain temperature of patients during deliberate deep hypothermia. Using jugular bulb temperature as a standard for brain temperature, we evaluated the accuracy and precision of 5 standard temperature monitoring sites (ie, pulmonary artery, nasopharynx, forehead deep-tissue, urinary bladder, and fingertip skin-surface tissue) during deep hypothermic cardiopulmonary bypass conducted for thoracic aortic reconstruction.

METHODS

In 20 adult patients with thoracic aortic aneurysms, the 5 temperature monitoring sites were recorded every 1 minute during deep hypothermic (<20 degrees C) cardiopulmonary bypass. The accuracy was evaluated by the difference from jugular bulb temperature, and the precision was evaluated by its standard deviation, as well as by the correlation with jugular bulb temperature.

RESULTS

Pulmonary artery temperature and jugular bulb temperature began to change immediately after the start of cooling or rewarming, closely matching each other, and the other temperatures lagged behind these two temperatures. During either situation, the accuracy of pulmonary artery temperature measurement (0.3 degrees C-0.5 degrees C) was much superior to the other measurements, and its precision (standard deviation of the difference from jugular bulb temperature = 1.5 degrees C-1.8 degrees C; correlation coefficient = 0.94-0.95) was also best among the measurements, with its rank order being pulmonary artery > or = nasopharynx > forehead > bladder > fingertip. However, the accuracy and precision of pulmonary artery temperature measurement was significantly impaired during and for several minutes after infusion of cold cardioplegic solution.

CONCLUSIONS

Pulmonary artery temperature measurement is recommended to estimate brain temperature during deep hypothermic cardiopulmonary bypass, even if it is conducted with the sternum opened; however, caution needs to be exercised in interpreting its measurements during periods of the cardioplegic solution infusion.

Efficacy of forced-air warming systems with full body blankets

PURPOSE

Postoperative hypothermia after cardiac surgery is still a common problem often treated with forced-air warming. This study was conducted to determine the heat transfer efficacy of 11 forced-air warming systems with full body blankets on a validated copper manikin.

METHODS

The following systems were tested: 1) Bair Hugger 505; 2) Bair Hugger 750; 3) Life-Air 1000 S; 4) Snuggle Warm; 5) Thermacare; 6) Thermacare with reusable Optisan blanket; 7) WarmAir; 8) Warm-Gard; 9) Warm-Gard and reusable blanket; 10) WarmTouch; and 11) WarmTouch and reusable blanket. Heat transfer of forced-air warmers can be described as follows: Q = h x DeltaT x A. Where Q = heat flux (W), h = heat exchange coefficient (W x m-2 x degrees C-1), DeltaT = temperature gradient between blanket and manikin surface (degrees C), A = covered area (m2). Heat flux per unit area and surface temperature were measured with 16 heat flux transducers. Blanket temperature was measured using 16 thermocouples. The temperature gradient between blanket and surface (DeltaT) was varied and h was determined by linear regression analysis. Mean DeltaT was determined for surface temperatures between 32 degrees C and 38 degrees C. The covered area was estimated to be 1.21 m2.

RESULTS

For the 11 devices, heat transfers of 30.7 W to 77.3 W were observed for surface temperatures of 32 degrees C, and between -8.8 W to 29.6 W for surface temperatures of 38 degrees C.

CONCLUSION

There are clinically relevant differences between the tested forced-air warming systems with full body blankets. Several systems were unable to transfer heat to the manikin at a surface temperature of 38 degrees C.

Estimation of mean body temperature from mean skin and core temperature

BACKGROUND

Mean body temperature (MBT) is the mass-weighted average temperature of body tissues. Core temperature is easy to measure, but direct measurement of peripheral tissue temperature is painful and risky and requires complex calculations. Alternatively MBT can be estimated from core and mean skin temperatures with a formula proposed by Burton in 1935: MBT = 0.64 x TCore + 0.36 x TSkin. This formula remains widely used, but has not been validated in the perioperative period and seems unlikely to remain accurate in dynamic perioperative conditions such as cardiopulmonary bypass. Therefore, the authors tested the hypothesis that MBT, as estimated with Burton's formula, poorly estimates measured MBT at a temperature range between 18 degrees and 36.5 degrees C.

METHODS

The authors reevaluated four of their previously published studies in which core and mass-weighted mean peripheral tissue temperatures were measured in patients undergoing substantial thermal perturbations. Peripheral compartment temperatures were estimated using fourth-order regression and integration over volume from 18 intramuscular needle thermocouples, 9 skin temperatures, and "deep" hand and foot temperature. MBT was determined from mass-weighted average of core and peripheral tissue temperatures and estimated from core temperature and mean skin temperature (15 area-weighted sites) using Burton's formula.

RESULTS

Nine hundred thirteen data pairs from 44 study subjects were included in the analysis. Measured MBT ranged from 18 degrees to 36.5 degrees C. There was a remarkably good relation between measured and estimated MBT: MBTmeasured = 0.94 x MBTestimated + 2.15, r = 0.98. Differences between the estimated and measured values averaged -0.09 degrees +/- 0.42 degrees C.

CONCLUSIONS

The authors concluded that estimation of MBT from mean skin and core temperatures is generally accurate and precise.

End-point Temperature of Rewarming After Hypothermic Cardiopulmonary Bypass in Pediatric Patients

In an attempt to find an adequate end-point rewarming temperature after hypothermic cardiopulmonary bypass (CPB), 50 pediatric patients who underwent cardiac surgery were randomly assigned for the end-point rectal rewarming temperature at either 35.5 (Group 1) or 37.0 degrees C (Group 2). The patients' rectal temperature, with heart rate and blood pressure, was measured 0.5, 1.0, 4.0, 8.0, and 16.0 h after the arrival in the intensive care unit. For all patients, nonpulsatile perfusion with a roller pump and a membrane or bubble oxygenator was used for oxygenation. Age, sex, body surface area, total bypass time, and rewarming time were comparable in both groups. No afterdrop and no statistical differences in the rectal temperatures between the two groups were observed. Also, no statistical differences were observed between the two groups with respect to the heart rate and blood pressure. No shivering was noted in all patients. In conclusion, with the restoration of rectal temperature above 35.5 degrees C at the end of CPB in pediatric patients, the present study found no afterdrop.

Hypothermia – it's more than a toy

PURPOSE OF REVIEW

Perioperative hypothermia triples the incidence of adverse myocardial outcomes in high-risk patients; it significantly increases blood loss and augments allogeneic transfusion requirements. Even mild hypothermia increases the incidence of surgical wound infection following colon resection and therefore the duration of hospitalization. Hypothermia adversely affects antibody- and cell-mediated immune defenses, as well as the oxygen availability in the peripheral wound tissues. Mild perioperative hypothermia changes the kinetics and action of various anesthetic and paralyzing agents, increases thermal discomfort, and is associated with delayed postanesthetic recovery.

RECENT FINDINGS

On the other hand however, therapeutic hypothermia may be an interesting approach in various settings. Lowering core temperature to 32-34 degrees C may reduce cell injury by suppressing excitotoxins and oxygen radicals, stabilizing cell membranes, and reducing the number of abnormal electrical depolarizations. Evidence in animals indicates that even mild hypothermia provides substantial protection against cerebral ischemia and myocardial infarction. Mild hypothermia has been shown to improve outcome after cardiac arrest in humans. Randomized trials are in progress to evaluate the potential benefits of mild hypothermia during aneurysm clipping and after stroke or acute myocardial infarction.

SUMMARY

This article reviews recent publications in the field of accidental as well as therapeutic hypothermia, and tries to assess what evidence is available at the present time.

Comparison of fast versus slow rewarming following acute moderate hypothermia in rats

BACKGROUND

The aim of this study was to compare the biochemical and physiological responses of fast vs. slow rewarming from moderate hypothermia in anaesthetized rats.

METHODS

Anaesthetized rats were surface cooled to 28 degrees C, for 20 min, then rewarmed either quickly over 30 min or slowly over 120 min with monitoring of vital signs, systemic vascular resistance (SVR), cardiac output, biochemical changes and activity for 31 days.

RESULTS

At hypothermia, cardiac output decreased to 77 +/- 38 ml x min(-1) and lactate increased to 4.62 +/- 4.73 mmol x l(-)1. Fast rewarming caused an abrupt increase in cardiac output (270 +/- 24 ml x min(-1)) and a sharp drop in SVR (325.6 +/- 23.3 dyne x s(-1) x cm(-5)), compared with a smoother course with cardiac output (142 +/- 18 ml x min(-1), P < 0.01) and SVR (662.8 +/- 41.0 dyne x s(-1) x cm(-5), P < 0.01), measured during slow rewarming. Lactate failed to return to normal values (upon returning to normothermia) (2.5 +/- 0.75 mmol x l(-1)) only in the fast rewarming group. In both groups, activity in the open field was not different from control rats.

CONCLUSIONS

In rats, moderate hypothermia for 20 min does not appear to cause lasting biochemical or behavioural consequences, whether rewarming lasted over 30 or 120 min. However, there was a greater early change in cardiac output and heart rate, due to systemic vasodilatation in the fast rewarming animals. These acute changes may have consequences in patients with compromised cardiovascular reserves.

Safety and performance of a novel intravascular catheter for induction and reversal of hypothermia in a porcine model

OBJECTIVE

This study was undertaken to assess the acute safety and feasibility of rapidly inducing, maintaining, then reversing hypothermia using a novel heat transfer catheter and a closed-loop automatic feedback temperature control system to overcome limitations imposed by current clinical practices used for perioperative cooling and warming.

METHODS

Six swine (mean mass, 53.8 +/- 3.6 kg) were studied. The heat transfer catheter was placed in the inferior vena cava via the femoral vein. Hypothermia to 32 degrees C was induced, maintained for 6 hours, then reversed to 36 degrees C. The time needed to induce and reverse hypothermia was recorded via continuous temperature monitoring of the lower esophagus, cerebrum, and rectum. Electrocardiography provided continuous monitoring, and blood draws were made at baseline and at 2-hour intervals. Examination of the catheter in situ was performed after the animals were killed.

RESULTS

Cooling from 36.2 to 32.0 degrees C was rapid and uniform (mean, 7.3 +/- 0.7 degrees C/h), with animals reaching the target temperature within 60 minutes. Rewarming was also easily controlled, with animals' temperatures reaching 36 degrees C within 130 minutes. No arrhythmia was observed, and all hematological variables were within the normal range for swine. There was no evidence of hemolysis or platelet changes. Little to no thrombosis was observed.

CONCLUSION

The data presented here suggest that rapid induction and reversal of hypothermia are technically possible using a core intravenous cooling catheter; this method would provide a safe, rapid, and exquisitely reproducible way to induce hypothermia with subsequent restoration of normothermia.

Novel thermoregulation system for enhancing cardiac function and hemodynamics during coronary artery bypass graft surgery

BACKGROUND

Myocardial ischemia, arrhythmias, and coagulopathies are associated with postoperative hypothermia. This study assessed the efficacy of a novel thermoregulation system in alleviating these events during coronary artery bypass graft (CABG) surgery.

METHODS

Elective CABG surgery patients were randomized into either Allon thermoregulation (AT, n = 40) or routine thermal care (RTC, n = 20) groups in whom the maintenance of normothermia during the nonbypass phases of the operation was compared. The AT used patients' rectal temperature as reference data to monitor the maintenance of the water temperature circulating at 37 degrees C in a garment. Rectal temperature, patient hemodynamics, and cardiac-specific troponin I (cTnI) levels were assessed at the induction of anesthesia, 30 minutes into surgery, at discontinuation of bypass, end of surgery, and 2 hours postoperatively.

RESULTS

Body temperature was higher in the AT group compared to the RTC group at all five time points. Cardiac index (CI) (L/min) was higher in the AT group, 2.5 +/- 0.5, 2.6 +/- 0.5*, 3.2 +/- 0.6*, 3.3 +/- 0.5*, 3.1 +/- 0.7 at the respective time points, compared to the RTC group, 2.3 +/- 0.6, 2.1 +/- 0.2, 2.6 +/- 0.7, 2.7 +/- 0.7, 2.7 +/- 0.7 (*p < 0.05). Systemic vascular resistance (SVR) (dyne x s)/cm5) was consistently lower in the AT patients. Enzyme levels were elevated in both groups but were less so in the AT patients.

CONCLUSIONS

The AT system can efficiently maintain normothermia. The beneficial effects are expressed by reduced SVR, elevated CI, and lower levels of cTnI, which may show a possible attenuation of myocardial injury.

Effectiveness of resistive heating compared with passive warming in treating hypothermia associated with minor trauma: a randomized trial

OBJECTIVES

To determine the occurrence of hypothermia in patients with minor trauma, to test the hypotheses that resistive heating during transport is effective treatment for hypothermia and that this treatment reduces patients' thermal discomfort, pain, and fear, and to evaluate the accuracy of oral temperatures obtained at the scene of injury.

PATIENTS AND METHODS

In December 1999 and January 2000, 100 patients with minor trauma were randomly assigned to passive warming or resistive heating. All patients were covered with a carbon-fiber resistive warming blanket and a wool blanket, but the warming blanket was activated only in those assigned to resistive heating. Core (tympanic membrane) and oral temperatures, heart rate, pain, fear, and overall satisfaction of patients were compared between the 2 groups on arrival at a hospital.

RESULTS

Hypothermia was noted in 80 patients at the time of rescue. Mean initial core temperatures were 35.4 degrees C (95% confidence interval [CI], 35.2 degrees C - 35.6 degrees C) in the patients who received passive warming and 35.3 degrees C (95% CI, 35.1 degrees C - 35.5 degrees C) in those who received resistive heating. From the time of rescue until arrival at the hospital, mean core temperature decreased 0.4 degrees C/h (95% CI, 0.3 degrees C/h - 0.5 degrees C/h) with passive warming, whereas it increased 0.8 degrees C/h (95% CI, 0.7 degrees C/h - 0.9 degrees C/h) with resistive heating. Oral and tympanic membrane temperatures were similar. Mean heart rate decreased 23 beats/min in those assigned to resistive heating but remained unchanged in those assigned to passive warming. Patients in the resistive heating group felt warmer, had less pain and anxiety, and overall were more satisfied with their care.

CONCLUSIONS

Oral temperatures are sufficiently accurate for field use. Hypothermia is common even in persons with minor trauma. Resistive heating during transport augments thermal comfort, increases core temperature, reduces pain and anxiety, and improves overall patient satisfaction.

Update on cardiopulmonary bypass

Investigations into cardiopulmonary bypass continue to refine knowledge and clinical practice. Recent investigations have emphasized neurological complications, introducing the possibility of genetic predisposition as a risk factor. Appropriate flows, pressures, and hematocrit levels during cardiopulmonary bypass continue to create controversy. Whereas previous debate has centered around appropriate temperature management, recent discussions consider the possibility that mild hypothermia after cardiopulmonary bypass might be neuroprotective. Meta-analyses and prospective investigations continue to suggest the virtual equivalence of aprotinin and lysine analogues in reducing bleeding and transfusion after cardiopulmonary bypass. Several recent studies identified the mechanisms and severity of the inflammatory response to cardiopulmonary bypass, as well as possible techniques for attenuating inflammation.