Metabolic effects - Heat increases metibolic rate which increases oxygen uptake and accelerates healing, though it can also become destructive. Oxygen availability is increased due to a shift in the oxygen hemoglobin dissociation curve. Cell activity and metabolic rate increase 2-3 times for each 10˚C (50˚F) temperature increase. Burns occur when tissues become too heated, around 45-50˚C (113-122˚F). 

 

Neuromuscular effects - Nerve conduction velosity is increased by approximately 2 m/sec per degree Celsius with heat. Spasms are reduced by the decreased nerve firing rate of type II muscle spindle efferents and gamma efferents which reduces the firing rate of alpha motor neurons. This also increases pain thresholds. 

 

Altered tissue extensibility - Heat applied to a tissue before stretching allows for a greater increase in length to be maintained after a stretching force is applied. The improved extensibility requires less force to achieve the increase in length and the risk of tearing is reduced. To most effectively achieve improved tissue extensibility, tissue must be heated to 40-45˚C (104-113˚F) for 5-10 minutes. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The following are methods of heat transfer:

 

Conduction: Heat transfer occuring by direct contact when there is a tempearture gradient between the two contacting surfaces (i.e. direct contact)

Convection: Direct contact of a circulating medium and another material (i.e. air, whirlpool)

Radiation: Direct transfer of energy from one material to another without an intervening medium (i.e. fire, infrared lamp)

Conversion: Nonthermal energy becomes thermal energy (i.e. therapeutic ultrasound, diathermy)

Evaporation: Absorption of energy away from the body, creating a cooling effect (i.e. vapocoolant spray, sweat)

 

Consider precautions and contraindications before introducing a patient to thermotherapy

 

 

 

 

 

 

 

 

 

 

 

 

 

Superficial heating is used to heat to a depth of 1-3 cm and is most effective in decreaseing general soreness and preparing soft tissues for stretching and joint mobs. Methods of superficial heating include:

 

• Hot packs (conduction)

  • Towels should always be used as a coupling medium due to the high temperature gradient to prevent burns. 6-8 layers of towels should be used, however a general rule of thumb is that the hot pack cover counts as two towels. 

  • On the initial use, hydrophilic silicate hot packs take approximately 2 hours to heat through in hydrocollator heated to 160˚F. After the initial use, they should only take approximately 30 minutes to reheat. 

  • The larger area the hot pack covers, the more quickly the heat will transfer. Increased tissue thickness will decrease the rate of tissue transfer. This should be taken into consideration when selecting the size of the hot pack.

  • The patient can lie supine on top of the heat pack. The patient's body weight will squeeze water from the heat pack and accelerate heat transfer. This can be good for some patients, but if it gets too hot for others, a prone position may be more comfortable. 

  • Advise the patient to call or ring if discomfort occurs and check on the patient every few minutes.

  • Treatment time should be approximately 20 minutes.

  • Skin is thinner in elderly patients and in children. Check their skin before and after, and comfort level during treatment to ensure that no burns have occurred. 

 

• Paraffin wax (conduction)

  • This treatment is good for use on the distal extremities

  • Paraffin wax is mixed with mineral oil in a vat that is heated to 118-130˚F. 

  • Dip the extremity in the wax 6-10 times, then wrap the extremity in a plastic bag and wrap that with towels to provide insulation. 

  • After these initial layers have formed a glove for the extremity, the patient can dip-reimmerse in the vat of wax and leave it there for the duration of the treatment

  • Treatment duration is 20 minutes

  • Use caution and be vigiliant of burns and edema with this treatment. 

  • Do not use paraffin was with open wounds, skin infections, or acute inflammatory joint pathologies

 

• Infrared (radiation)

  • Infrared is a superficial dry heat increasing skin temperature to by 5-10˚C to a depth of 1 cm

  • This can be a good intervention if the area that needs to be heated is tender to touch as the infra red lamp does not come into contact with the patient. The lamp should be 50-75 cm away from the patient and positioned perpendicular to the surface being treated

  • Intensity and distance vary directly. The lamp should be kept at a distance where the patient will feel a comfortable warmth

  • Treatment time is 10-30 minutes

 

• Fluidotherapy (convection)

  • A machine with a large fan circulates warm air through fine cellulos particles

  • Faster circulation creates more friction and therefore more heat

  • Temperatures can rise to 102-118˚F

  • Skin should be checked before plaing hands inside fluidotherapy

  • The patient can perform hand exercises while using fluidotherapy

 

Deep heating is used to heat to a depth of >3 cm and is accomplished with: 

• Ultrasound (conversion)

• Diathermy (conversion)