Contradictory effects of dopamine at 32 degrees C in pigs anesthetized with ketamine

Altered pharmacological effects of adrenergic agonists during hypothermia

Rewarming from accidental hypothermia is often complicated by hypothermia-induced cardiac dysfunction, calling for immediate pharmacologic intervention. Studies show that although cardiac pharmacologic support is applied when rewarming these patients, a lack of updated treatment recommendations exist. Mainly due to lack of clinical and experimental data, neither of the international guidelines includes information about pharmacologic cardiac support at temperatures below 30 °C. However, core temperature of accidental hypothermia patients is often reduced below 30 °C. Few human studies exploring effects of adrenergic drugs during hypothermia have been published, and therefore prevailing information is collected from pre-clinical studies. The most prominent finding in these studies is an apparent depressive effect of adrenaline on cardiac function when used in doses which elevate cardiac output during normothermia. Also noradrenaline and isoprenaline largely lacked positive cardiac effects during hypothermia, while dopamine is a more promising drug for supporting cardiac function during rewarming. Data and information from these studies are in support of the prevailing notion; not to use adrenergic drugs at core temperatures below 30 °C.

Focal epidural cooling reduces the infarction volume of permanent middle cerebral artery occlusion in swine

BACKGROUND

The protective effect against ischemic stroke by systemic hypothermia is limited by the cooling rate and it has severe complications. This study was designed to evaluate the effect of SBH induced by epidural cooling on infarction volume in a swine model of PMCAO.

METHODS

Permanent middle cerebral artery occlusion was performed in 12 domestic swine assigned to groups A and B. In group A, the cranial and rectal temperatures were maintained at normal range (37 degrees C-39 degrees C) for 6 hours after PMCAO. In group B, cranial temperature was reduced to moderate (deep brain, <30 degrees C) and deep (brain surface, <20 degrees C) temperature and maintained at that level for 5 hours after 1 hour after PMCAO, by the epidural cooling method. All animals were euthanized 6 hours after MCAO; their brains were sectioned and stained with 2,3,5-triphenyltetrazolium chloride and their infarct volumes were calculated.

RESULTS

The moderate and deep brain temperature (at deep brain and brain surface) can be induced by rapid epidural cooling, whereas the rectal temperature was maintained within normal range. The infarction volume after PMCAO was significantly reduced by epidural cooling compared with controls (13.73% +/- 1.82% vs 5.62% +/- 2.57%, P < .05).

CONCLUSIONS

The present study has demonstrated, with histologic confirmation, that epidural cooling may be a useful strategy for reducing infarct volume after the onset of ischemia.

Epidural cooling for selective brain hypothermia in porcine model

BACKGROUND

Hypothermia has been shown to be neuroprotective in many animal models and several human trials of brain ischemic and trauma. However systemic hypothermia may result in fatal complications. This study was undertaken to test epidural cooling as a new method of inducing selective brain hypothermia.

METHOD

Six adult swine (mean mass, 33.8+/-3.6 kg) were studied. Anesthesia was maintained with pentobarbital sodium (25 mg kg-1, i.v.) and pancuronium bromide (0.5 mg kg-1 h-1, i.v.). Five probes were placed in the rectum, deep brain, brain surface, epidural space, and room air for temperature monitoring respectively. Epidural cooling was performed using cold-saline (4 degrees C) perfusion into the epidural space through a flexible double-lumen catheter. The dripping speed of cold saline was controlled to maintain the target temperature. The changes of the epidural space pressure, complete blood counts, basic metabolic panels, tests for fibrinolysis and coagulation status were monitored to assess hypothermia-induced changes.

FINDINGS

Following the epidural cooling perfused with cold-saline (4 degrees C) at mean dripping speed of 720 ml per hour, the local brain surface temperature could decrease rapidly to 17.3-21.6 degrees C within one minute, and deep brain temperature decreased to 27.2-29.7 degrees C within 5 minutes. The target temperatures were easily controlled by the dripping speed of cold saline (from mild to deep hypothermia). The rectal temperature was maintained at normal range within 6 perfusion hours. No arrhythmia was observed, and all hematological variables were within the normal range for swine. No increased intracranial pressure was induced by the cooling method.

CONCLUSIONS

The data demonstrate that epidural space cooling was technically feasible and useful for selective brain hypothermia, and the target temperatures are easily controlled. The induction of hypothermia was rapid and maintained for a long period of time, whereas the body temperature was maintained within the normal range and without hemodynamic instability.

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.

Effects of dopamine on porcine myocardial action potentials and contractions at 37 degrees C and 32 degrees C

BACKGROUND

Little information exists on the effects of drugs with cardiovascular action in hypothermia, and some findings have indicated paradoxic effects of dopamine in this setting. As we have not found any data on the electrophysiologic and contractile effects of dopamine on the heart in hypothermia, we decided to study this in pig myocardium, since pigs have a cardiovascular system more similar to that of humans than other animals.

METHODS

Excised muscle strips from pig ventricular septum were mounted in an organ bath. After 45 min of equilibration at 37 degrees C or 32 degrees C, resting and action potentials, time to peak contraction and contractile force were recorded during pacing with a frequency of 60/min. Dopamine at 4 microM or 8 microM was added and new recordings were made after 15 min.

RESULTS

Cooling to 32 degrees C caused a prolongation of contraction by 48% and the contractile force increased by 39%. The membrane action potential duration at 50% and 90% repolarization levels increased at 32 degrees C by 28% and 16% respectively. Dopamine significantly (P<0.05) increased the contractile force and membrane action potential duration at 50% and 90% repolarization levels both in normothermia and in hypothermia, whereas the duration of the contraction was not significantly changed.

CONCLUSION

Cooling to 32 degrees C significantly prolongs the myocardial action potential and the contraction duration. Dopamine increases the contractile force and prolongs the action potential both at 37 degrees C and at 32 degrees C.

Cardiovascular effects of induced hypothermia after lung transplantation

BACKGROUND

Induced hypothermia may be used to reduce metabolism in acute respiratory failure. Hypothermia is accompanied by an increase in pulmonary vascular resistance, as also seen in the early period after lung transplantation. It was our concern that the combination of the two would lead to an increased workload on the right ventricle.

METHODS

To test this hypothesis we induced hypothermia to 32 degrees C in two groups of pigs. In one group we performed left single-lung transplantation combined with right pulmectomy (TRANSP group); in the other group, only right pulmectomy was performed (PULMEC group).

RESULTS

During hypothermia, there was a significant increase in both groups in pulmonary vascular resistance (TRANSP group, 77%, p<0.05; PULMEC group, 54%, p<0.05) and a significant decrease in cardiac output (TRANSP group, 41%, p<0.05; PULMEC group, 34% p<0.05). Mean pulmonary artery pressure was unchanged, and the work done by the right ventricle was reduced (TRANSP group, 39%, p<0.05; PULMEC group, 31%).

CONCLUSIONS

Induced hypothermia to 32 degrees C after lung transplantation resulted in a significant decrease in the work done by the right ventricle despite a significant increase in pulmonary vascular resistance.