Doxapram produces a dose-dependent reduction in the shivering threshold in rabbits

Novel ultrashort-acting benzodiazepine remimazolam lowers shivering threshold in rabbits

Shivering after surgery or during therapeutic hypothermia can lead to serious complications, such as myocardial infarction and respiratory failure. Although several anesthetics and opioids are shown to have anti-shivering effects, their sedative and respiratory side effects dampen the usefulness of these drugs for the prevention of shivering. In the present study, we explored the potential of a novel ultrashort-acting benzodiazepine, remimazolam, in the prevention of shivering using a rabbit model of hypothermia. Adult male Japanese white rabbits were anesthetized with isoflurane. The rabbits received saline (control), remimazolam (either 0.1 or 1 mg/kg/h), or remimazolam + flumazenil, a selective γ-aminobutyric acid (GABA) type A receptor antagonist (n = 6 each). Thirty minutes after discontinuation of the drugs, cooling was initiated by perfusing 10°C water via a plastic tube positioned in the colon until the animal shivered. Core body temperature and hemodynamic and physiological parameters were recorded. Remimazolam at 1 mg/kg/h significantly lowered the core temperature change during shivering (−2.50 ± 0.20°C vs. control: −1.00 ± 0.12°C, p = 0.0009). The effect of 1 mg/kg/h remimazolam on the core temperature change was abolished by flumazenil administration (−0.94 ± 0.16°C vs. control: −1.00 ± 0.12°C, p = 0.996). Most of the hemodynamic and physiological parameters did not differ significantly among groups during cooling. Remimazolam at a clinically relevant dose successfully suppressed shivering in rabbits via the GABA pathway even after its anesthetic effects likely disappeared. Remimazolam may have the potential to prevent shivering in patients undergoing surgery or therapeutic hypothermia.

Pharmacologic options for reducing the shivering response to therapeutic hypothermia

Recent literature has demonstrated significant improvements in neurologic outcomes in patients who have received induced hypothermia in the setting of out-of-hospital cardiac arrest. Through multiple metabolic mechanisms, the induction of hypothermia slows the progression and devastation of transient cerebral hypoxia. Despite these benefits, the desired reduction in core temperature is often a challenging venture as the body attempts to maintain homeostasis through the induction of thermoregulatory processes aimed at elevating body temperature. Shivering is an involuntary muscular activity that enhances heat production in an attempt to restore homeostasis. For successful induction and maintenance of induced hypothermia, shivering, as well as other thermoregulatory responses, must be overcome. Several pharmacologic options are available, either used alone or in combination, that safely and effectively prevent or treat shivering after the induction of hypothermia. We conducted a PubMed search (1966-March 2009) to identify all human investigations published in English that discussed pharmacologic mechanisms for the control of shivering. Among these options, clonidine, dexmedetomidine, and meperidine have demonstrated the greatest and most clinically relevant impact on depression of the shivering threshold. More research in this area is needed, however, and the role of the clinical pharmacist in the development and implementation of this therapy needs to be defined.

Isovolaemic haemodilution decreases the shivering threshold in rabbits

BACKGROUND AND OBJECTIVE

The inhibition of thermoregulatory control by anaesthesia is manifested by reduced vasoconstriction and shivering thresholds. As intraoperative bleeding can result in haemodynamic changes, including vasoconstriction, we investigated the effect of experimental bleeding on the shivering threshold in rabbits.

METHODS

Twenty-four rabbits were randomly assigned to one of three treatment strategies: (1) no blood removal (control), (2) 5 mL kg(-1) isovolaemic blood removal and (3) 10 mL kg(-1) isovolaemic blood removal. After tracheal intubation under isoflurane anaesthesia, anaesthesia was maintained with 50% nitrous oxide in oxygen. The removed blood volume was replaced with the same volume of warm hydroxyethyl starch colloid solution. Oesophageal temperature was measured as a core temperature at 1-min intervals. After blood removal, the animal's body was cooled at a rate of 2-3 degrees C h(-1) by perfusing water at 10 degrees C through a U-shaped thermode positioned in the colon. Hypothermic shivering was evaluated by visual inspection, and the core temperature at which shivering was triggered was identified as the thermoregulatory threshold for this response.

RESULTS

Just before the cooling, the body temperature of the animals was around 38.6 degrees C in all of the three groups. The shivering threshold in the control group was 37.2 +/- 0.2 degrees C (mean +/- SD). The shivering thresholds in the 5 mL kg(-1) (36.9 degrees +/- 0.3 degrees C) and 10 mL kg(-1) (36.5 degrees +/- 0.5 degrees C) blood removal groups were significantly lower and in proportion with the volume of blood removed than that in the control group.

CONCLUSION

Isovolaemic haemodilution decreased the shivering threshold in rabbits in proportion with the volume of blood removed.

Doxapram only slightly reduces the shivering threshold in healthy volunteers

We determined the effects of doxapram on the major autonomic thermoregulatory responses in humans. Nine healthy volunteers were studied on 2 days: control and doxapram (IV infusion to a plasma concentration of 2.4 +/- 0.8, 2.5 +/- 0.9, and 2.6 +/- 1.1 microg/mL at the sweating, vasoconstriction, and shivering thresholds, respectively). Each day, skin and core temperatures were increased to provoke sweating, then reduced to elicit peripheral vasoconstriction and shivering. We determined the sweating, vasoconstriction, and shivering thresholds with compensation for changes in skin temperature. Data were analyzed with paired t-tests and presented as mean +/- sd; P < 0.05 was considered statistically significant. Doxapram did not change the sweating (control: 37.5 degrees +/- 0.4 degrees C, doxapram: 37.3 degrees +/- 0.4 degrees C; P = 0.290) or the vasoconstriction threshold (36.8 degrees +/- 0.7 degrees C versus 36.4 degrees +/- 0.5 degrees C; P = 0.110). However, it significantly reduced the shivering threshold from 36.2 degrees +/- 0.5 degrees C to 35.7 degrees +/- 0.7 degrees C (P = 0.012). No sedation or symptoms of panic were observed on either study day. The observed reduction in the shivering threshold explains the drug's efficacy for treatment of postoperative shivering; however, a reduction of only 0.5 degrees C is unlikely to markedly facilitate induction of therapeutic hypothermia as a sole drug.