CRNI Exam

The chapters move from the outside in. Chapter 1.1 covers the skin and the vein wall, the two structures every peripheral catheter crosses. Chapter 1.2 maps the upper-extremity veins, the nerves and arteries that sit beside them, and the central venous pathway to the heart. Chapter 1.3 explains hemostasis, thrombosis and the immune system, which underpin catheter occlusion, thrombosis, transfusion compatibility and infection. Chapter 1.4 reviews renal and hormonal regulation of fluids and the basics of acid-base balance. Every later section of this guide builds on these foundations, so treat this one as the base layer for the rest of your study.

This chapter describes the layers of the skin and the vein wall, how each layer responds to cannulation and infused solutions, and how aging changes the picture. These details explain many site complications and many "best action" choices on the exam.

The skin is the body's first line of defense and the first barrier every peripheral or percutaneous catheter breaks. It has three layers that matter for infusion practice.

Epidermis: the outermost, avascular layer of stratified squamous epithelium. Its surface layer of dead keratinized cells resists microbial entry and water loss. Thickness varies by body site; it is thin on the inner arm and dorsal hand and thick on the palms.

Dermis: the layer beneath the epidermis, containing blood and lymph capillaries, nerve endings, hair follicles, sweat and sebaceous glands, and collagen and elastic fibers. Pain from venipuncture is felt here because sensory receptors are concentrated in the dermis. Skin flora also live in hair follicles and glands, which is why antisepsis must reach beyond the surface and why it must dry fully.

Subcutaneous tissue (hypodermis): loose connective tissue and fat beneath the dermis. Superficial veins run in this layer. It is also the space used for subcutaneous infusions (hypodermoclysis) and subcutaneous immunoglobulin.

The skin hosts resident flora (for example coagulase-negative staphylococci) and transient flora picked up from the environment and from hands. Resident organisms cannot be fully eliminated, which is the reason antisepsis reduces rather than sterilizes skin and why extraluminal migration of skin organisms along the catheter is a major route of early catheter infection.

Older adults have a thinner epidermis, a flattened dermal-epidermal junction, less collagen and elastin, less subcutaneous fat, slower healing and reduced sensation. The practical effects are many.

• Medical adhesive-related skin injury (skin tears, stripping, tension blisters) is more likely when tape or dressings are removed. Use a skin barrier solution where appropriate, remove adhesives low and slow, parallel to the skin, and use adhesive remover per policy.
• Veins are more fragile and roll more easily because supporting tissue is lax. Stabilize the vein with traction below the site, use a shallow angle, and consider omitting or loosening the tourniquet (or using a blood pressure cuff at low pressure) to avoid blowing the vein.
• Reduced sensation means a patient may not feel early infiltration. Assess the site visually and by palpation more often rather than relying on patient report.
• Lower total body water and reduced thirst increase the risk of dehydration, while reduced cardiac and renal reserve increases the risk of fluid overload.

Clinical Scenario: An 84-year-old woman on a medical unit needs a peripheral catheter for 3 days of IV antibiotics. Her forearm veins are visible but mobile, and her skin is thin with old bruising. The nurse uses a loosely applied tourniquet, anchors the vein with gentle traction below the site, enters at a low angle with a 22-gauge catheter, and secures it with a dressing and an engineered securement device placed over a skin barrier film. On removal days later, the nurse uses adhesive remover and peels the dressing back slowly. The approach protects fragile skin and vein while still achieving reliable access.

Veins and arteries share three layers, but they differ in thickness and function. Knowing which layer is injured explains the type of complication that follows.

Tunica intima: the innermost layer, a single smooth layer of endothelial cells on a basement membrane. The endothelium normally prevents clotting. Any damage (catheter rubbing, rough insertion, irritating solutions, multiple attempts) exposes collagen and tissue factor, which triggers platelet adhesion and aggregation, inflammation (phlebitis) and thrombus formation. Valves are folds of the intima.

Tunica media: the middle layer of smooth muscle, elastic fibers and nerve fibers. In veins it is thinner than in arteries. Its nerve fibers respond to cold, pain, anxiety and irritating solutions by causing venous spasm, which can make a vein seem to disappear, slow an infusion, or cause cramping pain along the vein. Warmth and slower infusion rates relieve spasm.

Tunica adventitia: the outer layer of connective tissue that supports and protects the vessel. A catheter passing through the vein wall into surrounding tissue goes through the adventitia, resulting in infiltration or a hematoma.

Memory Aid: I-M-A, "Inside, Muscle, Around." The intima is inside and touches the blood, the media is the muscle that spasms, and the adventitia wraps around the vessel.

Most veins of the extremities contain one-way valves that direct blood toward the heart and prevent backflow. Valves are often found at branch points and appear as small bumps along a filled vein. Inserting a catheter directly into a valve can damage it and makes advancement difficult. If resistance is felt during advancement, the catheter may be against a valve; floating the catheter in by flushing gently while advancing, or choosing a site above or below the bifurcation, is preferred to forcing it. The central veins (superior vena cava, brachiocephalic veins) lack valves, which is one reason air can travel freely to the heart during central catheter disconnection.

Arterial puncture or cannulation during an attempted venous access is a serious error. Arteries have a thick muscular media, carry oxygenated blood under high pressure (except the pulmonary arteries), and lie deeper than most superficial veins.

If bright red, pulsatile blood appears or the patient reports severe sharp pain, remove the needle or catheter immediately and apply firm direct pressure for at least 5 minutes (longer if the patient takes anticoagulants), then reassess distal pulses, color and sensation and notify the provider. Never infuse into a suspected artery. An aberrant artery (for example a superficial ulnar or radial artery in the forearm) occurs in some people, so always palpate for a pulse before cannulating.

Exam Tip: When a stem describes bright red blood filling the flashback chamber rapidly with pulsation, the best action is to remove the device and apply pressure, not to flush it, secure it or call for help first. Assessment of the distal extremity follows.

• The dermis holds pain receptors and skin flora; antisepsis must dry completely to reach organisms in follicles and glands.
• Older adults need gentle tourniquet use, low insertion angles, skin barrier films and slow adhesive removal.
• Intimal damage triggers platelet aggregation, phlebitis and thrombosis; the media causes spasm; the adventitia is breached in infiltration.
• Valves are found at bifurcations and slow advancement; central veins have no valves.
• Bright red, pulsatile blood or sharp severe pain signals arterial puncture: remove, apply firm pressure, assess distal circulation.