Method Of Controlling Tissue Hypothermia

Abstract

A pedicle flap is covered with a water-filled, latex rubber bag having a surface in contact with and substantially the same size, shape and outline as the pedicle flap. The cooling face of a thermoelectric device is placed in contact with the exposed surface of the bag. A thermostatic control system including a temperature sensitive probe which is placed between the pedicle flap and the bag maintains the predetermined temperature in the range of 15.degree.-20.degree.C. Heat generated by the thermoelectric device is carried off by a coolant which flows through a coolant system including a heat exchanger and a pump in series with the thermoelectric device.

Description

BACKGROUND OF THE INVENTION

This invention relates to methods for inducing hypothermia in pedicle flaps and other types of tissue transplants.

The effect of moderate cold on living tissues and the healing of wounds has been studied and chronicled for centuries. C. L. Kiehn and J. D. Desprez, in an article entitled "Effects of Local Hypothermia on Pedicle Flap Tissue," Journal of Plastic and Reconstructive Surgery, Vol. 4, 1960, described studies involving the cooling of experimental animal pedicle flaps to a temperature of 15.degree.-20.degree. C. Two different systems for achieving pedicle flap cooling are disclosed in the article. One of the systems relies upon the circulation of a coolant through a copper disc which is maintained in contact with the pedicle being cooled. A thermistor is utilized as a temperature sensitive probe in the vicinity of the copper disc but not in contact with the pedicle flap to operate a circuit-breaker which interrupts the power to a refrigerator unit associated with a coolant reservoir. The other system involves the use of a dry ice reservoir which is placed in thermal communication with the pedicle flap to be cooled by a cool air column. In order to control the temperature, a thermistor which is placed a considerable distance from the pedicle flap is again utilized to actuate a circuit-breaker associated with a heating element in the cold air column. The article fails to disclose the relative sizes, shapes and outlines of the pedicle flap and the copper disc on the one hand or the pedicle flap and the size of the cold air column on the other hand. Although beneficial effects of the hypothermia created by the systems in this article are beyond question, the systems themselves are too cumbersome, impractical and inexact for clinical use.

Thermoelectric devices were suggested for use in inducing local tissue hypothermia in an article entitled "A Thermoelectric Device for Inducing Local Tissue Hypothermia and Hyperthermia," Nature, Vol. 203, pp. 613-614, Aug. 8, 1964, D. M. Makow and H. C. Grice. The Makow and Grice system comprises a thermally conductive tissue holder which is placed in thermal communication with a thermoelectric device. However, the temperature is maintained by placing a thermister in thermal communication with the tissue holder but spaced from the tissue being cooled. The system is not adapted to cool transplanted tissue in situ.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a practical clinical method for inducing transplanted tissue hypothermia in situ.

It is also an object of this invention to provide a method which is capable of precisely maintaining the optimum temperature at the tissue.

It is a further specific object of this invention to maximize the benefits to be derived from tissue hypothermia.

In accordance with these and other objects of the invention, a thermal transmission medium having one surface of substantially the same size and shape as the transplanted tissue to be cooled is placed in substantial contact with the entirety of the transplanted tissue without any substantial contact with the surrounding tissue. The cooling face of a thermoelectric device is then placed in contact with an exposed surface of the thermal transmission medium to cool the transplanted tissue. A temperature sensing device such as a thermistor is placed between the transplanted tissue and the thermal transmission medium and a signal representing the sensed temperature is applied to the input to the thermostatic control which maintains the temperature of the transplanted tissue within the selected 15.degree.-20.degree. C. temperature range.

In accordance with one important aspect of the invention, the thermal transmission medium may comprise a flexible, liquid-filled bag. The bag itself may be formed so as to have one surface of the proper size, shape and outline so as to conform with the transplanted tissue to be cooled. In this connection, the bag may be formed from sheets of latex rubber which are adhesively sealed together over a substantial portion of the periphery thereof, filled with a liquid such as water, and then sealed shut.

In accordance with another object of the invention, the heat generated by the thermoelectric device is carried off by a coolant circulating system. The circulating system comprises a heat exchanger, a pump and a pressure sensitive switch which is adapted to interrupt the supply of power to the thermoelectric device when a pressure drop in the collant circulating system is encountered, as when the pump fails or the coolant otherwise fails to circulate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an embodiment of the invention being utilized to cool a pedicle flap in a laboratory animal;

FIG. 2 is an enlarged perspective view of the pedicle flap being cooled in FIG. 1 and the cooling mechanism in contact therewith;

FIGS. 3 (a--f) are simplified perspective views illustrating various steps in the formation of a water-filled bag which serves as the thermal transmission medium between the pedicle flap of FIG. 2 and a thermoelectric cooling device;

FIG. 4 is a schematic diagram of the electrical circuitry in a preferred embodiment of the invention; and

FIG. 5 is a schematic diagram of the hydraulic system in a preferred embodiment of the invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

In accordance with this invention and as depicted in FIG. 1, a thermoelectric cooling device 10 is being utilized to cool the pedicle flap of a laboratory rabbit 12 through a thermal transmission medium comprising a water-filled, latex rubber bag 14. Since the pedicle flap must be cooled over an extended period of time, the rabbit 12 is held in a substantially stationary position by a restraining box 16. Electrical lead lines 18 from temperature sensing and control devices, e.g., thermisters, and electrical-hydraulic umbilical lines 20 from the thermoelectric device 10 extend through an opening 21 in the restraining box 16. As shown in FIG. 2, the thermisters 2282 a--c are connected to the electrical leads 18.

In accordance with one very important aspect of the invention shown in FIG. 2, the water-filled rubber bag 14 has a cooling surface 23 in contact with the pedicle flap 24 which is of substantially the same size, shape and outline as the pedicle flap 24. By providing such a cooling surface 23 on the rubber bag 14, a cooling face 26 of the thermoelectric device 12 which is in contact with exposed surface 27 of the bag 14 is able to cool the pedicle flap 24 alone without any substantial cooling of the surrounding tissue. This has been found to be critical if high flap viability is to be obtained.

In accordance with another important aspect of the invention, the temperature of the pedicle flap 24 is precisely controlled to maximize the viability of the pedicle flap. In this connection, the thermister 22 a is located between and in contact with the flap 24 as well as the bag 14.

It has been found that the temperature of the pedicle flap 24 may be maintained with a good deal of precision at a preset temperature level in the range of 15.degree.-20.degree. C. Temperature at the distal end of the flap stayed, for the most part, within 1.degree.-2.degree. C. of the pre-set temperature. Temperatures taken at the sides of the flap (anteriorly and posteriorly) were at times warmer than the pre-set temperature but all peripheral parts of the flap were always at least 5.degree.-15.degree. C. (generally 10.degree. C.) cooler than the surrounding normal skin temperatures. Temperatures of the surrounding tissue, as monitored by the thermistor 22c, remained within 5.degree. of normal skin temperature. The precision in temperature control is attributed to the configuration of the bag 14 as well as the precise temperature control afforded by the thermoelectric device. In addition, it is believed that the thermal inertia provided by the liquid medium in the water-filled bag which is separate and apart from any circulating coolant is of great assistance in maintaining the proper pedicle flap temperature.

In accordance with another important aspect of the invention, the heat generated at the hot side 30 of the thermoelectric device 10 is carried away from the pedicle flap 24 by a coolant such as water which is circulated to and from the hot side 30 of the thermoelectric device 10 by means of the umbilical lines 20. This removal of heat from the thermoelectric 26 which is in close proximity to the patient permits the necessary cool temperatures at the pedicle flap 24 to be maintained as well as eliminating any discomfort to the patient due to the heating at the hot side 30. The nature of the coolant circulating system including the umbilical lines 20 will be described in somewhat further detail with reference to FIG. 5.

Referring now to FIGS. 3a--f, a preferred method of forming a water-filled bag having a cooling surface of the appropriate size, shape and outline will now be described. As shown in FIG. 3a, a sheet 40 of latex rubber or other water impervious material is being cut to form two circular segments 42. Note that the segments 42 which may be first traced on to the sheet 40 using a template are of the same shape and slightly larger size than the pedicle flap to be cooled where the dotted circular lines correspond to the size of the pedicle flap. A scalpel 44 or other suitable cutting means may be utilized to cut the segments 42 from the sheet 40.

In FIG. 3b, a rubber or dermatome cement is being applied from a tube 46 to a substantial portion of the peripheries 48 of the segments 42. Note however that a portion 50 of the periphery of each of the segments 42 does not receive the cement. In FIG. 3c, the peripheries 48 of the segments 42 are being secured to one another by sandwiching the segments 42 between a surface 52 and a weight 54. After a suitable length of time to assure a good bond along the peripheries 48 of the segments 42, water may be applied through a hose 56 or other suitable means, to an opening formed at peripheral portions 50 of the segments 42 as shown in FIG. 3d. Once the segments 42 are filled with water, the peripheral portions may be sealed shut utilizing cement from the tube 46 as shown in FIG. 3e and then brought into bonding relation by a clamp 58 shown in FIG. 3f to incapsulate the water in the bag 14.

In the electrical schematic diagram of FIG. 4, the thermoelectric device 10 is supplied by a power pack 60 (This is a simple, open chasis, center tap transformer-choke-12 automotive-type diode package with approximately 15 percent ripple at design output) which is supplied by a 110 volt AC force 62 in series with a variable transformer 64. A power switch 66 controls the application of the 110 volts to the power pack 60.

A thermostatic control 68 such as the Yellow Springs Instrument Temperature Controller, Model 73 or similar controlling unit is connected in series with the thermoelectric device 10 in response to the temperature being sensed by the thermister probe 22a.

In order to circulate the coolant through the thermoelectric device 10, a pump 70 is also connected across the 110 volt source 62. A pressure switch 72, which is responsive to the pressure of the coolant being pumped by the pump 70 is connected to relay 74 having a coil 76. When the pressure sensed by the switch 72 is at or above a predetermined leve, a movable contact bar 78 is in contact with contact points 80 to provide power from the power pack 60 to the temperature controller 68 and the thermoelectric device 10. However, when the pressure sensed by the switch 72 drops below a predetermined level, the contact between the bar 78 and the point 80 is broken and contact between a coolant bar 82 and contact points 84 is established so as to apply power from the source 62 to an indicating light 86 or other suitable alarm means while interrupting power to the thermoelectric device 10. When the light 86 is on, it may indicate a pump failure which would result in an increase in the temperature at the cooling face 26 of the thermoelectric device 10 and a resulting, potentially harmful rise in the temperature of the pedicle flap 24.

A fan 88 is also connected across the source 62. The fan circulates the air across the vanes of the heat exchanger. It also assures proper cooling of the electrical components in the system. An on/off light 89 is also provided across the source 62 so as to indicate when the power switch 66 is closed. Fuses 90 are provided in series with the power switch 66, the pump 70, the fan 88 and the power pack 60. A volt meter 91 is provided across the output terminals of the power pack 60 to permit the monitoring of the input voltage to the thermoelectric device 10.

As mentioned previously, the coolant is circulated through the thermoelectric device 10 to carry away the heat from the hot face thereof. As shown in FIG. 5, the coolant is circulated by the pump 70 through the pressure switch 72 and into and out of the thermoelectric device 10. Coolant then flows to a heat exchanger 92 and into a coolant reservoir 94. The coolant may comprise water or other suitable liquid coolants.

Various thermoelectric devices may be utilized in this system. As utilized herein, the term "thermoelectric device" embraces any and all devices which utilize the Peltier Effect involving the generation of or obstruction of heat (depending upon the direction of current) at a rate Q, at the junction between two different semiconductors when a current I flows through the semiconductors. The rate Q equals II .times. I where II is the Peltier coefficient which is a function of semiconductor conductivities.

One particularly satisfactory thermoelectric device is the model no. CP 1.4-71-06 manufactured by the Melcor Corporation, 990 Spruce Street, Trenton, N.J. This device has a cooling face of 1.17 square inches which is capable of 102 BTU's per hour with a maximum current of 6 amps. The device includes a coolant circulating path associated with the warm face of the device. This particular size thermoelectric device is particularly suitable for use with a pedicle flap of 3+ square inches. In general, it is desirable and preferred to utilize a thermoelectric device having a cooling face with area which is equal to at least 10 percent of the pedicle flap area and preferably 33 percent or more. This of course means that the thermoelectric device cooling face is equal to at least 10 percent and preferably 33 percent or more of the cooling surface of the rubber, water-filled bag in contact with the pedicle flap.

Experiments conducted on pedicle flaps which were not cooled and those which were cooled with the previously described system of this invention indicate that the latter have greatly improved chances of survival as compared with the former. Without cooling, slough of the uncooled flaps was evident by the end of the first day and well-established by the end of the third day. In contrast, the flaps which were cooled with the system of this invention were for the most part healthy afer 7 days. Without cooling, the estimated survival of viability of the flaps was 20 percent as compared with 643 percent for the flaps cooled in accordance with this invention. The survival or viability of the flaps cooled with the invention would have been higher except for the loss of one flap. If it were not for the loss of this one flap, final estimates of viability would have increased 75 percent in the cooled flaps. It has also been found that the viability of the flaps is dramatically reduced if the surrounding tissue is cooled or the temperature of the tissue is allowed to fluctuate as by spacing the control thermister from the tissue to be cooled.

Although the system has been described in conjunction with the cooling of pedicle flaps, it is understood that it is equally applicable to the cooling of grafts and other tissue transplants in situ also. Even though a particular embodiment of the invention has been shown and described in detail, it will be understood that various modifications will occur to those of ordinary skill in the art which fall within the true spirit and scope of the invention as set forth in the appended claims.

Claims

What is claimed is:

1. A method of cooling a pedicle flap or graft in situ comprising:

forming a thermal transmission medium having one surface of substantially the same size, shape and outline as the pedicle flap or graft to be cooled;

placing said one surface of said thermal transmission medium in substantial contact with the entirety of the pedicle flap or graft;

holding the cooling face of a thermoelectric device in contact with an exposed surface of the thermal transmission medium;

maintaining a temperature sensing device in position between the pedicle flap or graft and said one surface of said thermal transmission medium; and

thermostatically controlling the temperature of the cooling face of the thermoelectric device between the pedicle flap or graft and said thermal transmission medium so as to maintain the temperature of a substantial portion of the pedicle flap or graft at 15.degree.-20.degree. C.

2. The method of claim 1 wherein the temperature of the pedicle flap or graft is maintained within 2.degree. C. of a preset temperature in the range of 15.degree.-20.degree. C. over a substantial portion thereof.

3. The method of claim 1 wherein the area of the thermoelectric device cooling face is equal to at least 10 percent of the pedicle flap or graft to be cooled.

4. The method of claim 1 including the step of circulating coolant to and from said thermoelectric device so as to carry away heat generated thereby.