Hot vs Cold – The Debate – What does the Physiology Say?

The dilemma of using hot packs verse cold packs after an injury still seems to be a topic of great discussion.  Doctors, PA’s, nurses and even some PTs still are saying different things. For some reason, the hot verse cold debate still rages on despite the physiological advancements and understanding that exists today.  So, let’s look at it from a scientific and physiological perspective to gain some insight into when would be the best use of hot and cold packs.

To know when to use heat and when to use ice, first we must understand what stage of recovery the injury is in.  There are four main categories of injury to consider.  The categories are Acute-Inflammatory, Chronic-Inflammatory, Acute-Mechanical, and Chronic-Mechanical.  This terminology is more representative of the phases of injury recovery than the old paradigm that everyone might have heard of, the classic phases:  Acute, Sub-Acute, & Chronic.

This old classification of the phases of injury has been around for several decades and lends itself for people to think of using cold packs in the Acute Phase and hot packs in the Chronic Phase. However, with the advancements in physiology and the introduction of new working models the picture becomes clearer. Consider these new working model below.

Inflammatory Pain (Acute-Inflammatory & Chronic-Inflammatory)

When considering this model of injury, then treatment takes on an entirely different perspective.  As an aside, don’t let the word “Inflammatory” be too misleading. Inflammation in recent years has become a buzz word for free radicals systemically circulating in the blood stream perhaps due to poor dietary habits or autoimmune issues. That is not the type of inflammation we are talking about here, we are talking about the immune system’s response to harmful stimuli and the inflammatory response that is produced locally after injury to soft-tissue. Typically, there are two types of stimuli that create an immune response: 1) infectious factors such as bacteria and microorganisms and 2) non-infectious factors such as burns, frostbite, physical injury and trauma.  Let’s just consider the non-infectious factors such as physical injury or trauma.  Typically, the immune response to tissue injury is a sequela of cellular changes, proteins and immune response factors infiltrating an injured area. The injured cell surface receptors recognize a damaging stimulus which in turn activates an inflammatory pathway which subsequently releases protein makers that recruit inflammatory cells to the area such as monocytes, macrophages and lymphocytes. This also produces a release of cytokines and neutrophils. Neutrophils are key mediators to the immune response and active other pathways needed for healing such as T-cells and Mast cells among many others.  Platelet cells are also key to the healing of tissue and are initiated during the immune response pathway as well.  Of note, unregulated neutrophils have been shown to cause damage to host cells and tissue. So, it is important to regulate and control acute inflammation so that it does persist and turn into chronic inflammation.  Chronic inflammation occurs when the acute inflammatory mechanisms fail to eliminate tissue injury and persists for months after the initial onset.

Mechanical Pain (Acute-Mechanical and Chronic-Mechanical)

When considering the bottom half of the New Classification of Injury Model above let’s explore the definition of Mechanical pain and injury. A quick google review may mislead you when searching Mechanical pain.  Many sources cite Mechanical pain as being strictly related to low back pain lasting 6 weeks or less. However, the true understanding of Mechanical pain is pain that is fairly reproducible when a joint structure is placed in one position, moved in a specific direction, or with a specifically applied force or pressure.   That pain is also alleviated when that joint is moved out of that position or direction or when that specific force is removed.  Again, the pain is fairly reproducible and predictable when moved in the provocation position or when the provocation force is reapplied.  If this situation lasts for 6 to 8 weeks it would be considered acute, and if the pain lasts over 6 months it would consider chronic. Associated symptoms of mechanical pain include weakness, clicking, grinding, giving way, a limp and restricted joint mobility or flexibility.

Physiology of Capillaries

Located throughout the body and thus the musculoskeletal soft-tissues are infinitesimally small blood vessels called capillaries with a diameter of 5-10 micrometers. Essentially, these soft-tissue capillaries are the interface of the musculoskeletal system and the vascular system and are the transition zone where arteries turn into veins. They have very thin walls made of a single layer of cells, a basement membrane, and some connective tissue cells. They function to allow for the transfer of oxygen, nutrients and waste productions such as carbon dioxide by diffusing through that tiny cell membrane into the surrounding soft-tissue. These capillaries and the permeability of the them changes in response to injury. When soft-tissue is injured the capillaries first sustain a period of vasoconstriction (the narrowing and constriction of blood vessels) to prevent blood loss, then it reverses course fairly soon after the injury and vasodilates (the dilation and opening of blood vessels). This vasodilation increasing blood flow to the area in an attempt to bring more immune response cells and nutrients needed for healing.

Physiology of Thermal Applications

Capillaries respond to heat by vasodilating and they respond to cold by vasoconstricting.  During the Acute-Inflammatory and the Chronic-Inflammatory stages the use of cold packs are indicated. Soft-tissue when initially injured is in the Acute-Inflammatory stage and thus the blood capillaries are in a state of vasodilation, bringing in the immune response cells.  Application of a hot pack during this stage would only serve to increase the influx of the immune response and as we stated earlier causing an excessive unregulated neutrophils and other immune response mediators to infuse into the area and cause unnecessary collateral tissue damage.  Conversely, the application of a cold pack will cause vasoconstriction, which will limit the over infiltration of neutrophils and immune response mediators thus limiting the possibility of unnecessary collateral tissue damage.  Potentially unregulated neutrophil infiltration over a prolonged period could serve to unnecessarily transition the injury into the Chronic-Inflammatory stage. Once in the Chronic-Inflammatory stage the application of cold packs is still indicated due to the need for vasoconstriction limiting the over stimulation of the immune response.

The Acute-Mechanical Stage is probably the stage that confuses people the most. The pain most associated with this stage seems to be sharp and/or quick flashes of pain that dissipates with a change in position. If someone’s pain is considered Acute-Mechanical the use of cold packs is still indicated and the use heat packs is discouraged.  The reason for the mechanical, on and off nature, of the pain is most likely a function of a smaller/isolated swollen sprained structure getting pinched or compressed.  That pinching and/or compression causes a thickening and a small local inflammatory response. Think of it like this, something small gets pinched and that causes it to get swollen. Thus, the use of cold packs will help to decrease the vasodilation and profuse infusion of the immune response into that area.

The Chronic-Mechanical stage is the only stage where a hot pack seems to work the best. During this stage the structure has lost most of its inflammatory and acuteness properties. Examples of this type of pain are degenerative conditions of tendon sheaths like long-lasting Achilles tendon issues or true degenerative spinal conditions. Here the application of heat and thus the vasodilation of the capillaries can serve to bring in nutrients and proteins that can serve to rebuild the structure. Since the inflammatory process has subsided, then the influx of neutrophils and other immune system mediators would not be present and thus not be a factor.

The application of hot and cold packs has always been a confusing topic, mostly likely, because of the old working model of the Acute, Subacute and Chronic stages of injury.  I urge you to think in terms of this New Classification of Injury Model and recognize that in three of the four stages cold packs are indicated over hot packs.  When in doubt cold packs are the way to go!  However, if you have any questions regarding when to us a hot or cold pack, please feel free to reach out to an experienced fellowship trained and/or board certified licensed Physical Therapist.