Critical Clinical Pearls - Pressure Sore Prevention in Canine Surgery

Pressure sores, also known as pressure ulcers or decubitus ulcers, are a significant cause of morbidity in the canine surgical patient. The causes of pressure sores are multifactorial and require a multifaceted approach for prevention. This article delves into understanding the pathophysiology, risk factors, preoperative evaluation and prevention of pressure sores to provide for improved surgical patient outcomes.

Understanding pressure sores - 

The cause of pressure sores is multifactorial including but not limited to interface pressure, increased temperature, skin moisture, immobility, shear forces and time in the immobilized position. Summarily stated, pressure sores occur as a consequence of arterial and venous blood vessel occlusion leading to tissue ischemia and tissue injury. How does this happen?

Factors contributing to pressure sores -

• Pressure – pressure on soft tissue usually over bony prominences
  1. Intensity –
     1. The greatest factor contributing to the development of pressure injuries is the interface pressure between the skin of the patient and the supporting surface. With sustained interface pressure the arterial blood vessels of the skin and muscles are compressed with resultant occlusion of arterial perfusion and consequent reduction in the distribution of nutrients and oxygen to the underlying tissue. The critical arterial pressure at which this occurs is the skin capillary filling pressure which can range from 20 to 40 mmHg with the average being 32 mmHg. Compression of the veins and venules in the underlying skin and muscles have an even lower filling pressure, all of which can result in the development of a pressure sore within 2 to 6 hours.
  2. Duration –
     1. The longer a canine patient is in a compromised position and the longer the capillary arterial filling pressure is exceeded, the greater the risk of pressure sore development.

• Tissue tolerance – multifactorial
  1. Intrinsic Factors – factors intrinsic to the patient
     1. Obesity – excessive body weight increases pressure on bony prominences, increasing the risk of pressure sores.
     2. Nutrition – poor nutrition can impair skin integrity and delays wound healing.

• Hydration – is essential for skin integrity and elasticity.

1. Pre-existing Health Conditions – conditions for example, like diabetes and hypothyroidism can affect skin health and healing. Arthritis can create difficulties in positioning a patient in a preventative manner.
2. Low blood pressure – low intra-operative arterial blood pressure causes low arterial capillary filling pressures which predisposes to pressure sores.

• Extrinsic Factors – factors external to the patient
  1. Micro-environment – relates to the skin temperature and skin moisture between the patient’s skin and the supporting surface.
     1. Temperature –
        • Only 1 degree C increase above normal skin temperature correlates with a 2 mmHg increase in pressure contributes 8 to 14 times as much ischemia as a 1 mmHg increase in pressure contributes 14 times as much to the tissue damage score as 1 mmHg of pressure increases metabolic demand of the overlying skin by 10 percent higher skin temperature creates a greater need for oxygen and. Nutrients, and when pressure restricts blood flow, these needs cannot be met, leading to tissue damage and necrosis
     2. Moisture –
        • Increase in skin moisture –
          • Weakens the stratum corneum, which contributes to skin maceration and skin breakdown makes the skin more susceptible to shear forces between the patient’s skin and supporting surface, increases metabolic demand due to the increased cellular activity needed for repair and recovery from damage caused by moisture and friction can cause impaired blood flow, further reducing the tissue’s ability to withstand pressure and repair can cause a disruption of the skin’s pH balance from the moisture itself as well as body fluids, creating an environment conducive to maceration, skin irritation, and bacterial overgrowth.
        • Skin Shear –
          • Relates to the forces applied obliquely to the skin at the subcutaneous cellular level by inadequate support from the surgical patient positioner

1. The Supporting Surface –
   • The supporting surface is crucial in preventing pressure sores. It must be tissue-friendly and support every square inch of the dependent portion of the patient’s body. Not supporting a portion of the dependent portion creates increased pressure on the portions of the body that are contacting the supporting surface. There must be an equal distribution of pressure on all the support surfaces, and this equal distribution of pressure can only occur if the supporting surface conforms to the contours of the dependent portions of the patient’s anatomy.

Pressure sores can be evaluated and graded according to their pathological skin and tissue changes.

• Stage l – change in skin color or temperature
• Stage ll – abrasion, blister or skin crater
• Stage lll – deep crater with loss of skin, dead subcutaneous tissue extending down to the underlying fascia
• Stage lV – damage extends to the muscle, bone and tendons possibly accompanied by sinus tracts

Preoperative Evaluation –

• The preoperative evaluation of the patient consists of an assessment of the intrinsic or Patient-related risk factors for pressure sore development.
• Body Weight – is your patient overweight which would increase pressure on soft tissue requiring greater overall dependent anatomical support or is the patient underweight which may require greater padding under bony prominences?
• Nutrition – has your patient received the appropriate nutrition so that it is overall a healthy patient with good skin integrity and wound healing.
• Hydration – is your patient well hydrated so that in turn the skin is well hydrated with good integrity and elasticity? A well hydrated patient preoperatively goes a long way in preventing intraoperative hypotension and decreased capillary filling pressure which predisposes to pressure sore development.
• Pre-existing Health Conditions – in view of all the comorbidities your patient may have, are they well managed and in optimum condition for surgery? If your patient has diabetes, hypothyroidism or any of the host of other diseases, has their medical management been optimized? If the patient has arthritis how will you adapt the supporting surface to accommodate their decreased limb mobility?
• Your preoperative evaluation of the overall health of your patient will determine your intraoperative management.

1. The Supporting Surface –
1. Whatever the surgical procedure or the required position, the supporting surface or the positioner must accommodate the patient in a manner that provides optimal surgical exposure and pressure sore prevention. In light of the foregoing, what are the pressure sore prevention factors under our control? They are, pressure intensity and the previously discussed extrinsic factors.
2. Pressure Intensity –
1. Since the average arterial capillary filling pressure is 32 mmHg, every effort must be made to distribute the weight and dependent portion of the patient as equally as possible so that no one part of the patient’s anatomy experiences more than 32 mmHg of pressure. This pressure equalization can only happen if the positioner or supporting surface conforms to all the contours of the dependent portion of the patient’s anatomy thereby distributing equal pressure to all portions of the dependent anatomy.
3. Temperature –
1. The skin temperature is in large part determined by the management of pressure intensity. Prolonged elevated pressure leads to a rise in skin temperature primarily by restriction of blood flow which leads to heat build up in the affected area because less heat is being carried away. This again emphasizes the importance of utilizing a positioner that provides for pressure equalization.
4. Moisture –
1. We have seen how moisture between the skin and supporting surface leads to increased metabolic demand, decreased blood flow, change in skin pH, maceration and skin breakdown. Placing an absorbent fabric, like a towel,  between the skin and the supporting surface can help to wick away moisture from the skin and provide some degree of aeration to prevent skin breakdown.
5. Skin Shear -
1. Skin shear is an oblique force applied to the skin which can cause skin layers to separate and cause skin breakdown. This occurs primarily because of inadequate support of all the dependent portions of the patient’s dependent anatomy. A positioner that conforms to the entire dependent portion of the patient’s anatomy keeps the patient securely in place and keeps the pressure forces perpendicular to the skin.
6. The Supporting Surface or Positioner
1. The supporting surface or positioner is of critical importance in secure patient positioning and preventing pressure sores. Utilizing a positioner that addresses all the controllable factors in pressure sore prevention is a high standard. Does such a positioner even exist? Yes! The HUG-U-VAC patient positioners have been the standard of care in veterinary patient positioning for the past 20 years. The HUG-U-VAC positioners address pressure intensity, temperature, moisture and skin shear; all the controllable factors.

Skin temperature, as we have seen, increases when elevated pressure is exerted on an area of skin. Since the HUG-U-VAC positioners distribute pressure equally throughout the positioner, skin temperature is normalized as much as possible.

The skin moisture problem is addressed by placing an absorbent fabric or towel on the patient side of the positioner prior to placing the patient on the positioner. This will wick away moisture from the skin as well as provide some degree of aeration to prevent moisture buildup. 

Since the HUG-U-VAC conforms to all the nuances of the patient’s anatomy the patient is kept securely in place and will not shift intraoperatively so that the patient is not subjected to skin shear. This ensures that the lines of force holding the patient are perpendicular to the skin and not oriented obliquely. HUG-U-VAC positioning avoids skin shear with its oblique lines of force that affect the skin at the cellular level as well as at the interfacing skin layers to promote skin maceration and skin breakdown.