Effects of spinal cord temperature on the generation and transmission of temperature signals in the goat

From Nanowarming to Thermoregulation: New Multiscale Applications of Bioheat Transfer

This review explores bioheat transfer applications at multiple scales from nanoparticle (NP) heating to whole-body thermoregulation. For instance, iron oxide nanoparticles are being used for nanowarming, which uniformly and quickly rewarms 50-80-mL (≤5-cm-diameter) vitrified systems by coupling with radio-frequency (RF) fields where standard convective warming fails. A modification of this approach can also be used to successfully rewarm cryopreserved fish embryos (∼0.8 mm diameter) by heating previously injected gold nanoparticles with millisecond pulsed laser irradiation where standard convective warming fails. Finally, laser-induced heating of gold nanoparticles can improve the sensitivity of lateral flow assays (LFAs) so that they are competitive with laboratory tests such as the enzyme-linked immunosorbent assay. This approach addresses the main weakness of LFAs, which are otherwise the cheapest, easiest, and fastest to use point-of-care diagnostic tests in the world. Body core temperature manipulation has now become possible through selective thermal stimulation (STS) approaches. For instance, simple and safe heating of selected areas of the skin surface can open arteriovenous anastomosis flow in glabrous skin when it is not already established, thereby creating a convenient and effective pathway to induce heat flow between the body core and environment. This has led to new applications of STS to increase or decrease core temperatures in humans and animals to assist in surgery (perioperative warming), to aid ischemic stress recovery (cooling), and even to enhance the quality of sleep. Together, these multiscale applications of nanoparticle heating and thermoregulation point to dramatic opportunities for translation and impact in these prophylactic, preservative, diagnostic, and therapeutic applications of bioheat transfer.

From Nanowarming to Thermoregulation: New Multiscale Applications of Bioheat Transfer

This review explores bioheat transfer applications at multiple scales from nanoparticle (NP) heating to whole-body thermoregulation. For instance, iron oxide nanoparticles are being used for nanowarming, which uniformly and quickly rewarms 50-80-mL (≤5-cm-diameter) vitrified systems by coupling with radio-frequency (RF) fields where standard convective warming fails. A modification of this approach can also be used to successfully rewarm cryopreserved fish embryos (∼0.8 mm diameter) by heating previously injected gold nanoparticles with millisecond pulsed laser irradiation where standard convective warming fails. Finally, laser-induced heating of gold nanoparticles can improve the sensitivity of lateral flow assays (LFAs) so that they are competitive with laboratory tests such as the enzyme-linked immunosorbent assay. This approach addresses the main weakness of LFAs, which are otherwise the cheapest, easiest, and fastest to use point-of-care diagnostic tests in the world. Body core temperature manipulation has now become possible through selective thermal stimulation (STS) approaches. For instance, simple and safe heating of selected areas of the skin surface can open arteriovenous anastomosis flow in glabrous skin when it is not already established, thereby creating a convenient and effective pathway to induce heat flow between the body core and environment. This has led to new applications of STS to increase or decrease core temperatures in humans and animals to assist in surgery (perioperative warming), to aid ischemic stress recovery (cooling), and even to enhance the quality of sleep. Together, these multiscale applications of nanoparticle heating and thermoregulation point to dramatic opportunities for translation and impact in these prophylactic, preservative, diagnostic, and therapeutic applications of bioheat transfer.

Heat Transfer in Health and Healing

Our bodies depend on an exquisitely sensitive and refined temperature control system to maintain a state of health and homeostasis. The exceptionally broad range of physical activities that humans engage in and the diverse array of environmental conditions we face require remarkable strategies and mechanisms for regulating internal and external heat transfer processes. On the occasions for which the body suffers trauma, therapeutic temperature modulation is often the approach of choice for reversing injury and inflammation and launching a cascade of healing. The focus of human thermoregulation is maintenance of the body core temperature within a tight range of values, even as internal rates of energy generation may vary over an order of magnitude, environmental convection, and radiation heat loads may undergo large changes in the absence of any significant personal control, surface insulation may be added or removed, all occurring while the body's internal thermostat follows a diurnal circadian cycle that may be altered by illness and anesthetic agents. An advanced level of understanding of the complex physiological function and control of the human body may be combined with skill in heat transfer analysis and design to develop life-saving and injury-healing medical devices. This paper will describe some of the challenges and conquests the author has experienced related to the practice of heat transfer for maintenance of health and enhancement of healing processes.

Controlled brain hypothermia by extracorporeal carotid blood cooling at normothermic trunk temperatures in pigs

Cerebral hypothermia improves outcomes after brain injury. A technique is presented for isolated brain cooling in pigs by cooling the natural blood supply of the brain. Under general anesthesia both common carotid arteries were exteriorized. One proximal carotid artery was connected to both distal carotid arteries and a heat exchanger in this line controlled brain temperature. The second proximal carotid artery was connected to an external jugular vein and a heat exchanger in this arteriovenous shunt was used to clamp trunk temperature. Thalamic brain temperatures of anesthetized juvenile pigs (N = 8) were clamped at 38, 25, and 30 degrees C while trunk core temperature was clamped at 38 degrees C. Approximately 7 min were needed to decrease brain temperature from 38 to 25 degrees C, reducing brain electric activity by 76% and increasing the temperature differences between different brain sites. Mean arterial blood pressure, heart rate, and cardiac output showed no significant change. Re-establishment of normothermic brain temperature led to a virtually complete recovery of brain electric activity. The technique is suitable for investigations of ischemic and traumatic injuries.

Human sudomotor responses to heating and cooling upper-body skin surfaces: cutaneous thermal sensitivity

The influence of local skin temperature (Tskl) on the control of local and whole-body sweating was evaluated in eight healthy males. A water-perfusion garment (37 degrees C) and a climatic chamber (36.45 +/- 0.78 degrees C; [+/- SD]; relative humidity 60.3 +/- 1.6%) were used to raise and clamp skin and core temperatures. Warm and cool stimuli were applied to four upper-body skin regions (face, arm, forearm, hand) using perfusion patches (249.0 +/- 0.2 cm2). Heating elevated, while cooling suppressed sweat rate (msw) locally, and at other skin surfaces. However, the tendency for Tskl manipulations to induce localized sweat responses was no more powerful than it was at stimulating sweating in non-treated regions (P > 0.05). Accordingly, neither thermal stimulus produced significantly greater local sudomotor influences than were elicited contralaterally (P > 0.05). No statistical support was found for the notion of inter-regional differences in upper-body cutaneous thermal sensitivity for sudomotor control, and, regardless of the stimulation site, whole-body sudomotor responses to localized thermal treatments were equivalent (P > 0.05).

Selective brain cooling reduces respiratory water loss during heat stress

Terrestrial mammals developed several mechanisms to reduce water loss to counteract water shortage. One avenue of water loss is the evaporative heat loss by sweating and panting, which increases with body temperature. Sweating and panting are activated by temperature signals of the body, whereby the brain is the most important site in generating temperature signals. Goats, like other artiodactyls, can cool their brains selectively below the temperature of the trunk core. The aim of the present study is to determine whether an inhibition of the selective brain cooling (SBC) mechanism will increase substantially the respiratory evaporative water loss during heat stress due to the higher brain temperature. The inhibition of SBC was performed by increasing brain temperature experimentally at the same rate as trunk temperature by means of extracorporeal heat exchangers. These experiments without SBC resulted in higher respiratory evaporative water loss compared to experiments with normal SBC. Eighteen experiments in two conscious goats had shown that at a trunk temperature of 40 degrees C the respiratory water loss was reduced on average by 29 g/hr (0.7 l/day) due to the effect of SBC. This amount of water corresponds to about one third of the general water requirements. In conclusion, SBC substantially reduces the water loss in goats during heat stress and consequently improves survival chances during water shortage.

Effects of selective brain cooling on mechanisms of respiratory heat loss

Experiments (n = 36) in three conscious goats were performed at 35 degrees C air temperature and low (LH) or high (HH) humidity. Prior to the experiments the animals received carotid loops and an arteriovenous shunt, which made it possible to increase the temperature of the blood flowing to head and trunk (series A), or to increase the temperature of the trunk at constant carotid blood and hypothalamic temperature (Thyp), respectively (series B). Owing to the smaller cooling power of the inspired air in HH, the slope of respiratory evaporative heat loss versus aorta blood temperature (Taor) was reduced in series A and B. In series A the slopes of respiratory minute volume (VE) and respiratory frequency (RF) versus Taor were larger in HH than in LH. The effects were caused by a reduction of selective brain cooling in HH, which resulted in higher levels of Thyp. This is concluded from the results of series B, in which Thyp was equal in LH and HH, and the slopes of VE and RF over Taor showed no differences. Thus, selective brain cooling contributes to counteract the deterioration of the gain of the respiratory heat loss mechanism, which occurs during exposure to humid air.

Threshold and slope of selective brain cooling

Experiments (n = 50) in three conscious goats were performed in a thermoneutral environment to determine the threshold (i.e. the point at which the brain temperature is equal to the carotid blood temperature) and slope (i.e. the difference between brain and carotid blood temperatures as a function of carotid blood temperature) of selective brain cooling (SBC) and analyse the thermal inputs affecting them. Prior to the experiments the animals received carotid loops and an arteriovenous shunt to manipulate head and trunk temperatures independently of each other. The mean SBC threshold was 38.75 degrees C T(carotis) and independent of T(trunk). When body core temperature was increased from a hypo- to a moderately hyperthermic level, the SBC threshold was passed before metabolic rate had reached its minimum and before cutaneous vasodilation occurred. The mean SBC slope was 0.78 and rose with increasing Ttrunk. The degree of SBC was principally independent of respiratory heat loss: high levels of heat loss were found without SBC, and large degrees of SBC were observed at low levels of heat loss. The effect of SBC in and around normothermia is to smooth the onset of shivering or panting and to establish a range of internal temperature within which metabolic rate and respiratory heat loss are simultaneously at low levels.