Shivering is a normal physiological response of an individual's sensed temperature and the thermostatic-like response of their threshold zone. Incoming signals of "cold" from the periphery provides the input information to the central control mechanism (hypothalamus) and initiates thermoregulatory responses. When hypothermia develops either accidentally or intentionally induced, the body will immediately try to counteract this disturbance to decrease heat loss by vasoconstriction and piloerection (gooseflesh) followed by shivering, a thermoregulatory mechanism.Febrile shivering is the shaking chill experienced during fever. The observed increase in skeletal muscle activity results in increased heat production until the body temperature reaches the new thermostatic set point.
Temperature reduction therapies have been proven to provide substantial protection against ischemic brain injury and also to slow and prevent brain injury. Induced moderate hypothermia which purposely lowers the body temperature below normal has seen to improve neurological outcome in survivors of cardiac arrest. Traumatic brain injury guidelines recommend mild to moderate hypothermia or normothermia for neuroprotection. But shivering, one of the common side effects seen with therapuetic cooling remains a serious limitation to this therapuetic modality and must be controlled in order to avoid serious physiologic consequences.
A growing body of evidence shows that vigorous shivering can increase metabolic heat production upto 600% above basal level, even in febrile patients. Shivering is not only uncomfortable, it also increases intracranial pressure and has undesirable effect in patients with primary neurological and post hypoxic brain injury. Shivering can double or even triple the oxygen consumption causing hypoxemia, myocardial ischemia in high risk patients because of increased myocardial demand. This has a particularly negative impact on post cardiac arrest patients whose heart has just been resuscitated. Therefore, avoidance of shivering is strongly recommended during hypothermia induction, normothermia or rewarming periods.
Shivering is most likely to occur when the core temperature is 34-36 deg celcius.The ideal goal in shivering management is prevention. Protection of cold sensitive cutaneous receptors from direct cold contact and avoiding skin exposure and contact with cold surfaces and use of warm packs should be the first step to minimize the risk of shivering. In current clinical practice, several sedatives, anaesthetics and opiate drugs and neuromuscular blocking agents are utilized to suppress shivering activities. (in neuro ICU) Many of these agents can compromise airway defense and respiration and they are used for intubated and mechanically ventilated patients only.
Neuroscience nurses often encounter a multitude of challenges managing fevers in their patient population. The efficiency of the cooling modality is critically important since the therapuetic window to implement neuroprotection is very narrow, and " time is brain" The neuro ICU nurse's bedside practice focusses on ease of initiating cooling therapy, the speed of fever reduction, and maintaining tight temperature control. Traditional cooling blankets and even the newer skin surface cooling methods have limited impact on core cooling and also induces shivering which is seen to be detrimental. For patients who need cooling measures specially with neuronal injuries intravascular cooling technology has been shown to be more effective and superior in reduction of visible and subclinical shivering compared to several methods of skin surface cooling and use of antipyretics .It is effective in transferring or removing heat directly within the core thermal compartment via a central venous catheter. This also means less usage of sedatives, neuromuscular blocking agents, opiates etc to prevent shivering thereby promoting better ventilation.
Therefore while considering methods to induce hypothermia and fever control to optimize neurological outcomes, shivering should be anticipated as a normal thermoregulatory response and as far as possible it must be prevented and controlled.