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Technical Brief

A Human Thermoregulation Simulator for Calibrating Water-Perfused Cooling Pad Systems for Therapeutic Hypothermia

[+] Author and Article Information
Priya S. E. Chacko

Department of Biomedical Engineering,
The University of Texas at Austin,
107 W Dean Keeton Street, Stop C0800,
Austin, TX 78712
e-mail: priya.chacko@utexas.edu

Ali Seifi

Department of Neurosurgery,
University of Texas Health Science Center at San Antonio,
MC7843,
7703 Floyd Curl Drive,
San Antonio, TX 78229
e-mail: Seifi@uthscsa.edu

Kenneth R. Diller

Fellow ASME
Department of Biomedical Engineering,
The University of Texas at Austin,
107 W Dean Keeton Street, Stop C0800,
Austin, TX 78712
e-mail: kdiller@mail.utexas.edu

1Corresponding author.

Manuscript received December 22, 2016; final manuscript received May 23, 2017; published online June 28, 2017. Assoc. Editor: Xiaoming He.

J. Med. Devices 11(3), 034506 (Jun 28, 2017) (10 pages) Paper No: MED-16-1381; doi: 10.1115/1.4037054 History: Received December 22, 2016; Revised May 23, 2017

The induction of a mild reduction in body core temperature has been demonstrated to provide neuroprotection for patients who have suffered a medical event resulting in ischemia to the brain or vital organs. Temperatures in the range of 32–34 °C provide the required level of protection and can be produced and maintained by diverse means for periods of days. Rewarming from hypothermia must be conducted slowly to avoid serious adverse consequences and usually is performed under control of the thermal therapeutic device based on a closed-loop feedback strategy based on the patient's core temperature. Given the sensitivity and criticality of this process, it is important that the device control system be able to interact with the human thermoregulation system, which itself is highly nonlinear. The therapeutic hypothermia device must be calibrated periodically to ensure that its performance is accurate and safe for the patient. In general, calibration processes are conducted with the hypothermia device operating on a passive thermal mass that behaves much differently than a living human. This project has developed and demonstrated an active human thermoregulation simulator (HTRS) that embodies major governing thermal functions such as central metabolism, tissue conduction, and convective transport between the core and the skin surface via the flow of blood and that replicates primary dimensions of the torso. When operated at physiological values for metabolism and cardiac output, the temperature gradients created across the body layers and the heat exchange with both an air environment and a clinical water-circulating cooling pad system match that which would occur in a living body. Approximately two-thirds of the heat flow between the core and surface is via convection rather than conduction, highlighting the importance of including the contribution of blood circulation to human thermoregulation in a device designed to calibrate the functioning of a therapeutic hypothermia system. The thermoregulation simulator functions as anticipated for a typical living patient during both body cooling and warming processes. This human thermoregulatory surrogate can be used to calibrate the thermal function of water-perfused cooling pads for a hypothermic temperature management system during both static and transient operation.

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References

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Figures

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Fig. 1

External water-perfused cooling pads applied to the body surface

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Fig. 2

Basic components of the HTRS arranged in three concentric shells to represent the body core, tissues, and skin

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Fig. 3

Components of HTRS center and middle containers with immersion heater and foam; pump not included

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Fig. 4

Tubing network installed on the inner surface of the outer container. The network consists of 12 parallel flow loops to represent the peripheral circulation.

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Fig. 5

Assembled HTRS during normothermia testing without an external therapeutic hypothermia water circulation pad installed

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Fig. 6

HTRS during induced hypothermia testing with the external water-perfused cooling pad in place

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Fig. 7

Philips InnerCool STx+ Surface Pads used for the testing

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Fig. 8

Top-down view of thermocouple locations in HTRS. (1) Ambient air, (2) shell outer surface, (3) shell inner surface, (4) outside tissue, (5) inside tissue, (6) fat, (7) outside core, and (8) inside core.

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Fig. 9

Thermocouple location for inline flow temperature measurements

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Fig. 10

Thermal gradients for operation of the HTRS with (a) and without (b) circulation of water during normothermia simulation after steady state has been reached

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Fig. 11

Thermal gradients for operation of the HTRS with (a) and without (b) circulation of water during therapeutic hypothermia

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Fig. 12

Thermal gradients during rewarming from induced hypothermia to normothermia with (a) and without (b) water flow in the HTRS

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Fig. 13

Thermal gradient for operation of the HTRS in a clinical setting during therapeutic hypothermia

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Fig. 14

Steady-state thermal gradients within the HTRS with and without water circulation during normothermia

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