Page 13 - JSOM Spring 2023

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calorimeter measured heat loss by radiation, convection, and Böhler’s book and indicates a plausibly greater severity of
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evaporation. Blood flow rates as low as 0.15mL/100mL of cold injury with tourniquet use.
hand tissue were recorded after cold exposure of the body for
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several hours. The cold eventually reduced flow 100-fold, Cold-Induced Vasodilation is a
an effect similar in size to arterial tourniquet use at warm Periodic Increase in Blood Flow to Hands and Feet
temperatures. 72–74 Cold-induced vasodilation has been studied 58,66,77 and re-
viewed 64,78–80 but is still not fully understood. During cold
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Büttner measured temperature changes in the hand exposed exposure, initial peripheral vasoconstriction is often followed
to -26°C (-14.8°F) with and without a tourniquet to occlude by a vasomotor response of spontaneous fluctuations of va-
blood flow, and results indicated that blood flow to the hand sodilation in acral tissues (e.g., fingers, toes). 66,81 Despite ex-
became practically zero when it was suddenly cold-exposed. posure of a distal limb to extreme cold, experiments have
The temperature of the back of the hand with circulating demonstrated an ability to maintain a comfortable limb tem-
blood decreased practically as fast as did that of the hand perature as long as the rest of the body is maintained at a suf-
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without circulating blood (because of tourniquet use). Local ficiently warm temperature. Investigators have found that,
convective heat flow in the blood had stopped when the cold if cold-induced vasodilation occurred, local freezing did not
had set in. The skin surface temperature had dropped in the take place because cold-induced vasodilation tempered the
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same manner as does that of a corpse starting at the same ini- effects of vasoconstriction. Other investigators 66,78 deduced
tial temperature. 14 that cold-induced vasodilation evidently acted to maintain a
balance between core body heat preservation and local tissue
Blood flow was a major source of limb heat, but when blood perfusion. However, cold-induced vasodilation becomes essen-
flow was decreased by vasoconstriction or tourniquet use, that tially absent when the core becomes hypothermic.
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blood’s heat flow was interrupted. The reduced blood flow
to the limbs in response to cold exposure served to limit heat Havenith et al demonstrated that, after cold-induced vaso-
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loss from the limbs, yet reduced blood flow simultaneously dilation onset increased hand temperature, cold-induced va-
contributed to limb cooling. 12,61 Periodic cold-induced vasodi- sodilation flow decreased, and then the hand cooled again.
lation is thought to be a way that the body attempts to rewarm In another study, cold-induced vasodilation fluctuations
its limbs; cold-induced vasodilation is addressed below. produced blood flows that ranged from 7-fold to 10-fold
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(maximum/minimum). During decreases of experimental
Using a tourniquet effect, investigators experimentally com- cold-induced vasodilation blood flow, the graphed maximal
pared the heat loss with free or arrested limb circulation to steepness of the downward slopes in skin temperature were
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calculate the heat losses from blood flow. For example, in- equal to those observed when blood flow was stopped in the
vestigators compared results of the fingertip to the whole hand limb by an inflatable cuff having a tourniquet effect on the
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of a person and showed that the maximum fingertip heat loss forearm. In a cold man, where cold-induced vasodilation
rate surpassed that of the hand by more than 10-fold. These blood flow has fallen and only the nutritive (capillary) flow
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heat loss findings quantified thermal effects of tourniquet use supplied the tissues, this flow was so small because of vasocon-
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and showed that acral fingertips lose heat faster than hands, striction that it was unable to convey any heat to the hand. A
thus informing why acral tissues are at increased risk of cold restriction of blood flow to the hand induced by a tourniquet
injury. In other analyses that included tourniquet effects, an in- made cold-induced vasodilation disappear. 14,69
vestigator identified a thermal intuition error concerning local
limb flow, namely the hand, that “[t]he heat delivered by the Altogether, evidence indicates that a tourniquet effect can me-
circulating blood is proportional not to the rate of blood flow chanically block local cold-induced vasodilation and its sub-
but to the product of the rate times temperature change as it sequent rewarming of hands or feet. If a tourniquet impairs
circulates through the hand.” Thermal intuitions, a topic of cold-induced vasodilation, then the onset of a local cold injury
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study, apply to limb cooling and thus may affect caregiver such as frostbite might plausibly be hastened. Cold-induced
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control of limb wound bleeding. vasodilation occurs only in acral tissue, whereas a tourniquet
can affect larger segments of a limb. Thus, tourniquet use may
Investigators studying contact cooling of the skin when in con- worsen frostbite by affecting a greater tissue volume than just
tact with cold objects found a difference in the shape of the that of the hand or foot.
contact cooling curve between occluded (zero) blood flow and
unoccluded (close-to-maximal) flow. Finger blood flow oc- Tourniquet Use and Frostbite: Experiments
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clusion consistently showed Newtonian cooling, whereas the In a 1947 experiment, no benefit was found in rabbits for
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unoccluded skin had a non-Newtonian cooling behavior in 13 tourniquet use to treat frostbite. In animal experiments, an
of 18 exposures, presumably the result of cold-induced vaso- association between tourniquet use and frostbite development
dilation rewarming the skin. The lack of heat input because of has been studied. 81,85–87 Investigators looking for vascular reac-
blood flow occlusion further cooled the finger skin. 12 tions associated with frostbite studied venous tourniquets ap-
plied before and after cold exposure and found that, in animals
Altogether, vasoconstriction and tourniquets act alike in that without cold exposure, venous tourniquets slightly decreased
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both can essentially stop blood flow. However, the volume of the local temperature of the limb. The clinical risk of frost-
tissue differs because the tourniquet routinely affects more of bite has been repeatedly reported for impaired circulation of
the limb. If the tourniquet affects a larger volume of the limb the limbs by a tourniquet effect—both arterial and venous—
compared with an initial frostbite lesion, a tourniquet may but this study measured limb temperature reduction and so
plausibly increase frostbite volume. Irrespective of frostbite estimated the quantity of risk. A few experiments in cold con-
or tourniquet occurring first, a risk of larger frostbite volume ditions suggested that tourniquet effects or vascular clamping
appears plausible. This deduction about volume aligns with hastened the onset and worsened the severity of frostbite. 81,87,88

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