Isoflurane and sevoflurane produce a dose-dependent reduction in the shivering threshold in rabbits

Intraoperative secondary insults during extracranial surgery in children with traumatic brain injury

PURPOSE

Data on intraoperative secondary insults in pediatric traumatic brain injury (TBI) are limited.

METHODS

We examined intraoperative secondary insults during extracranial surgery in children with moderate-severe TBI and polytrauma and their association with postoperative head computed tomography (CT) scans, intracranial pressure (ICP), and therapeutic intensity level (TIL) scores 24 h after surgery. After IRB approval, we reviewed the records of children <18 years with a Glasgow Coma Scale score <13 who underwent extracranial surgery within 72 h of TBI. Definitions of secondary insults were as follows: systemic hypotension (SBP <70 + 2 × age or 90 mmHg), cerebral hypotension (cerebral perfusion pressure <40 mmHg), intracranial hypertension (ICP >20 mmHg), hypoxia (oxygen saturation <90 %), hypercarbia (end-tidal CO2 >45 mmHg), hypocarbia (end-tidal CO2 <30 mmHg without hypotension and in the absence of intracranial hypertension), hyperglycemia (blood glucose >200 mg/dL), hyperthermia (temperature >38 °C), and hypothermia (temperature <35 °C).

RESULTS

Data from 50 surgeries in 42 patients (median age 15.5 years, 25 males) revealed systemic hypotension during 78 %, hypocarbia during 46 %, and hypercarbia during 25 % surgeries. Intracranial hypertension occurred in 64 % and cerebral hypotension in 18 % surgeries with ICP monitoring (11/50). Hyperglycemia occurred during 17 % of the 29 surgeries with glucose monitoring. Cerebral hypotension and hypoxia were associated with postoperative intracranial hypertension (p = 0.02 and 0.03, respectively). We did not observe an association between intraoperative secondary insults and postoperative worsening of head CT scan or TIL score.

CONCLUSIONS

Intraoperative secondary insults were common during extracranial surgery in pediatric TBI. Intraoperative cerebral hypotension and hypoxia were associated with postoperative intracranial hypertension. Strategies to prevent secondary insults during extracranial surgery in TBI are needed.

Meperidine and skin surface warming additively reduce the shivering threshold: a volunteer study

Introduction

Mild therapeutic hypothermia has been shown to improve outcome for patients after cardiac arrest and may be beneficial for ischaemic stroke and myocardial ischaemia patients. However, in the awake patient, even a small decrease of core temperature provokes vigorous autonomic reactions–vasoconstriction and shivering–which both inhibit efficient core cooling. Meperidine and skin warming each linearly lower vasoconstriction and shivering thresholds. We tested whether a combination of skin warming and a medium dose of meperidine additively would reduce the shivering threshold to below 34°C without producing significant sedation or respiratory depression.

Methods

Eight healthy volunteers participated on four study days: (1) control, (2) skin warming (with forced air and warming mattress), (3) meperidine (target plasma level: 0.9 μg/ml), and (4) skin warming plus meperidine (target plasma level: 0.9 μg/ml). Volunteers were cooled with 4°C cold Ringer lactate infused over a central venous catheter (rate ≈ 2.4°C/hour core temperature drop). Shivering threshold was identified by an increase of oxygen consumption (+20% of baseline). Sedation was assessed with the Observer's Assessment of Alertness/Sedation scale.

Results

Control shivering threshold was 35.5°C ± 0.2°C. Skin warming reduced the shivering threshold to 34.9°C ± 0.5°C (p = 0.01). Meperidine reduced the shivering threshold to 34.2°C ± 0.3°C (p < 0.01). The combination of meperidine and skin warming reduced the shivering threshold to 33.8°C ± 0.2°C (p < 0.01). There were no synergistic or antagonistic effects of meperidine and skin warming (p = 0.59). Only very mild sedation occurred on meperidine days.

Conclusion

A combination of meperidine and skin surface warming reduced the shivering threshold to 33.8°C ± 0.2°C via an additive interaction and produced only very mild sedation and no respiratory toxicity.

Heat stress induces a biphasic thermoregulatory response in mice

Previous animal models of heat stress have been compromised by methodologies, such as restraint and anesthesia, that have confounded our understanding of the core temperature (Tc) responses elicited by heat stress. Using biotelemetry, we developed a heat stress model to examine Tcresponses in conscious, unrestrained C57BL/6J male mice. Before heat stress, mice were acclimated for >4 wk to an ambient temperature (Ta) of 25°C. Mice were exposed to Taof 39.5 ± 0.2°C, in the absence of food and water, until they reached maximum Tcof 42.4 ( n = 11), 42.7 ( n = 12), or 43.0°C ( n = 11), defined as mild, moderate, and extreme heat stress, respectively. Heat stress induced an ∼13% body weight loss that did not differ by final group Tc; however, survival rate was affected by final Tc(100% at 42.4°C, 92% at 42.7°C, and 46% at 43°C). Hypothermia (Tc< 34.5°C) developed after heat stress, with the depth and duration of hypothermia significantly enhanced in the moderate and extreme compared with the mild group. Regardless of heat stress severity, every mouse that transitioned out of hypothermia (survivors only) developed a virtually identical elevation in Tcthe next day, but not night, compared with nonheated controls. To test the effect of the recovery Ta, a group of mice ( n = 5) were acclimated for >4 wk and recovered at Taof 30°C after moderate heat stress. Recovery at 30°C resulted in 0% survival within ∼2 h after cessation of heat stress. Using biotelemetry to monitor Tcin the unrestrained mouse, we show that recovery from acute heat stress is associated with prolonged hypothermia followed by an elevation in daytime Tcthat is dependent on Ta. These thermoregulatory responses to heat stress are key biomarkers that may provide insight into heat stroke pathophysiology.

Biotelemetry transmitter implantation in rodents: impact on growth and circadian rhythms

The implantation of a biotelemetry transmitter for core body temperature (T(c)) and motor activity (MA) measurements is hypothesized to have effects on growth and circadian rhythmicity depending on animal body-to-transmitter (B:T) size ratio. This study examined the impact of transmitter implantation (TM) on body weight, food intake (FI), water intake (WI), and circadian T(c) and MA rhythms in mice (23.8 +/- 0.04 g) and rats (311.5 +/- 5.1 g) receiving no treatment (NT), anesthesia, laparotomy (LAP), and TM. The B:T size ratio was 6:1 and 84:1 for mice and rats, respectively. In mice, body weight required 14 days to recover to presurgical levels and never attained the level of the other groups. FI recovered in 3 days, whereas WI never reached presurgical levels. Rat body weight did not decrease below presurgical levels. FI and WI recovered to presurgical levels in rats by day 2 postsurgery. Anesthesia decreased mouse body weight for 1 wk, but was without effect in rats. LAP significantly decreased body weight for 5 days in mice and 1 day in rats, showing a significant effect of the surgical procedure in the absence of TM in both species. Circadian T(c) and MA rhythms were evident within the first week in both species, indicating dissociation between circadian rhythmicity and recovery of growth variables. Cosinor analysis showed a TM effect on T(c) min, T(c) max, mesor, amplitude, and period of mice, whereas only the amplitude of the rhythm was affected in rats. These data indicate that a large B:T size ratio is associated with minimization of the adverse effects of surgical implantation. We recommend that B:T size ratio, recovery of presurgical body weight, and display of a robust circadian T(c) and MA rhythm be established before collection of biotelemetry data collection under an experimental paradigm.