Crush Injuries and Compartment Syndrome

Prehospital Pathophysiology is a new monthly column that provides an opportunity for EMS providers of all levels to either refresh their knowledge related to the etiology of a certain disease or expand their knowledge base regarding common and not-so-common disease processes. This column is for both basic- and advanced-level prehospital care providers. The authors hope that through this column, EMS providers will gain a more thorough understanding of disease processes. If you would like to see a specific topic addressed in this column, send your request via e-mail to emseditor@aol.com.

Pathophysiology

Crush injury (CI) and compartment syndrome (CS) are different processes with very similar pathophysiology and are frequently discussed synonymously.

A crush injury results from prolonged continuous pressure on large muscles, like those of the legs or arms, which results in muscle disintegration. Compartment syndrome is defined as any condition in which a structure like a nerve or tendon has been constricted within a space. Compartment syndrome is most associated with deep tissue injury that results in a restriction of outward swelling caused by a collection of blood in the injured tissue due to inflexible muscle fasciae. This results in increased pressure within the space and swelling that is concentrated inward toward the injured and uninjured tissues and structures. This leads to restricted blood flow because capillaries are compressed by the pressure. Venous pressure increases and the arterioles spasm, leading to tissue ischemia, swelling and, potentially, tissue necrosis. Think of the pericardial tamponade patient: When the pericardial sac has been filled and stretched to the point where it will stretch no more, pressure begins to shift away from the sac and toward the heart until it can no longer beat effectively. In a crush injury, this same principle of pressure applies. As pressure builds within an extremity, the skin will only stretch so far. Eventually, the pressure will be transferred from the skin to the vessels and internal structures of the extremity.

Although trauma is the most likely cause of CS/CI, they can occur in nontraumatic situations. A stroke patient who is found after lying motionless on his arm for several hours could have an unidentified compartment syndrome.

The most prominent type of muscle in the body is skeletal muscle. The skeletal muscle cell membrane, also known as the sarcolemma, is a key factor in ensuring normal cell function and maintaining a normal cell structure. The cell membrane contains pumps that move potassium and calcium to the inside of the cell and sodium to the outside. These pumps rely on energy in the form of adenosine triphosphate (ATP). Myoglobin, which is found within the skeletal muscle cell, is responsible for supplying the skeletal and cardiac muscles with oxygen. The myoglobin has a greater affinity for oxygen than hemoglobin; thus, oxygen is drawn into the skeletal muscle cells from the blood so it may be used in normal cell metabolism. The skeletal muscle cell also contains enzymes that are not harmful to the cell itself unless the calcium level in the cell rises. When the calcium level rises, the enzymes become destructive to the cell structure and cause the cell membrane to leak or rupture.

Damage to the skeletal cell membrane, both from direct injury or the loss of energy and dysfunction of the cell membrane pumps, causes calcium and sodium to rush inside the cell, and causes substances such as myoglobin, potassium, uric acid and phosphorus to leak out of the cell. These substances are leaked into the interstitial fluid (fluid around the cell) and may be eventually picked up by the capillary network and circulated in the blood, leading to more systemic complications. This process is referred to as rhabdomyolysis.