8.1 Upper Extremity Arterial Anatomy & Examination Protocol

8.1 Upper Extremity Arterial Anatomy & Examination Protocol

Non-invasive evaluation of the upper extremity arterial vasculature requires a rigorous understanding of regional anatomy, vascular branching patterns, and dynamic hemodynamic responses. Although peripheral arterial disease (PAD) occurs far less frequently in the upper extremities than in the lower extremities—accounting for roughly 5% to 10% of all arterial occlusive presentations—accurate duplex ultrasound interrogation is critical for diagnosing proximal inflow stenosis, upper extremity ischemia, trauma, occupational overuse syndromes, and pre-operative vascular mapping.

Upper Extremity Arterial Vasculature: Anatomic Pathway

The arterial supply to the upper extremities originates centrally within the superior mediastinum and courses through distinct muscular and osseous landmarks before arborizing into the microvascular palmar and digital beds.

1. Central Aortic Inflow & Subclavian Artery

• Asymmetric Central Origins:
  • Right Subclavian Artery: Arises from the bifurcation of the brachiocephalic (innominate) artery posterior to the right sternoclavicular joint. The brachiocephalic artery is the first and largest branch of the aortic arch, dividing into the right common carotid artery and the right subclavian artery.
  • Left Subclavian Artery: Arises directly from the aortic arch as its third major branch, originating distal to the left common carotid artery. Because of its direct mediastinal origin, the proximal left subclavian artery is deeper, more tortuous, and substantially more susceptible to atherosclerotic plaque accumulation than the right subclavian artery.
• Subclavian Course & Major Branches: The subclavian artery arches laterally over the apex of the lung and pleura, passing behind the anterior scalene muscle. It gives off five primary arterial branches before exiting the neck:
  1. Vertebral Artery: Arises from the superior aspect of the first segment of the subclavian artery, ascending through the transverse foramina of cervical vertebrae C6 through C1 to form the basilar artery.
  2. Internal Thoracic (Internal Mammary) Artery (ITA/IMA): Arises from the inferior wall opposite the vertebral artery, coursing downward behind the costal cartilages to supply the anterior chest wall and breast. It is frequently harvested as a pedicled or free conduit for coronary artery bypass grafting (CABG).
  3. Thyrocervical Trunk: Gives off the inferior thyroid, suprascapular, and transverse cervical arteries.
  4. Costocervical Trunk: Supplies the upper intercostal spaces and deep neck.
  5. Dorsal Scapular Artery: Supplies the levator scapulae and rhomboid musculature.

2. Axillary Artery

• Anatomic Boundaries: The subclavian artery becomes the axillary artery as it crosses the lateral border of the first rib. It terminates at the inferior border of the teres major muscle, where it transitions into the brachial artery.
• Surgical Segments: The axillary artery is divided into three distinct anatomical segments relative to the pectoralis minor muscle:
  • First Segment: Proximal to pectoralis minor; gives rise to the superior thoracic artery.
  • Second Segment: Posterior to pectoralis minor; gives rise to the thoracoacromial artery and lateral thoracic artery.
  • Third Segment: Distal to pectoralis minor; gives rise to the subscapular artery (largest branch), anterior humeral circumflex artery, and posterior humeral circumflex artery.

3. Brachial Artery

• Anatomic Course: Beginning at the inferior margin of the teres major muscle, the brachial artery descends through the upper arm along the medial bicipital groove, lying medial to the biceps brachii muscle and anterior to the triceps brachii and brachialis muscles.
• Neurovascular Relationship: In the proximal arm, the median nerve courses lateral to the brachial artery, crosses anteriorly across the artery in the mid-arm, and runs medial to the artery in the antecubital fossa.
• Deep Brachial Artery (Profunda Brachii): Arises from the posteromedial aspect of the proximal brachial artery and courses posteriorly with the radial nerve into the spiral groove of the humerus. The profunda brachii serves as a crucial collateral bridge (via radial and middle collateral branches) to reconstitute the radial and recurrent interosseous arteries if the distal brachial artery becomes occluded.
• Bifurcation: In the antecubital fossa, approximately $1-2\text{ cm}$ distal to the elbow joint crease at the level of the radial neck, the brachial artery bifurcates into the radial artery and the ulnar artery.

4. Forearm Runoff, Palmar Arches & Digital Vasculature

The radial and ulnar arteries descend along the lateral and medial aspects of the forearm, respectively, to supply the hand:

• Radial Artery:

  • Courses along the anterolateral aspect of the forearm beneath the brachioradialis muscle.
  • At the wrist, it lies superficially between the flexor carpi radialis tendon and the radial styloid process, serving as the standard site for radial pulse palpation and arterial blood gas (ABG) sampling.
  • The radial artery passes posteriorly around the radial styloid into the anatomical snuffbox (bounded by the tendons of abductor pollicis longus, extensor pollicis brevis, and extensor pollicis longus), traverses between the heads of the first dorsal interosseous muscle, and enters the palm to form the primary contributor to the Deep Palmar Arch (DPA).
  • Prior to entering the snuffbox, it gives off the superficial palmar branch, which courses anteriorly to communicate with the superficial arch.

• Ulnar Artery:

  • Typically larger in caliber than the radial artery at its origin.
  • Courses deep beneath the pronator teres and flexor muscles along the medial forearm.
  • Gives off the common interosseous artery, which promptly divides into the anterior interosseous artery (descending along the anterior aspect of the interosseous membrane) and the posterior interosseous artery.
  • At the wrist, the ulnar artery lies lateral to the flexor carpi ulnaris tendon and enters the hand superficial to the flexor retinaculum, passing through the canal of Guyon (alongside the ulnar nerve) adjacent to the hook of the hamate bone.
  • In the palm, the ulnar artery forms the primary contributor to the Superficial Palmar Arch (SPA).

• The Palmar Arches & Digital Vasculature:

  • Superficial Palmar Arch (SPA): Located immediately deep to the palmar aponeurosis and superficial to the flexor tendons. Formed predominantly by the termination of the ulnar artery, anastomosing with the superficial palmar branch of the radial artery. The SPA provides origin to three common palmar digital arteries, which bifurcate into proper palmar digital arteries coursing along the adjacent borders of the second through fifth digits.
  • Deep Palmar Arch (DPA): Lies approximately $1\text{ cm}$ proximal to the superficial arch, nestled deep beneath the flexor tendons directly against the bases of the metacarpal bones. Formed predominantly by the terminal radial artery, anastomosing with the deep palmar branch of the ulnar artery. The DPA gives off three palmar metacarpal arteries that travel distally to join the common digital arteries, as well as the princeps pollicis artery (dominant arterial supply to the thumb) and the radialis indicis artery (to the lateral side of the index finger).
  • Digital Arteries: Each finger receives bilateral paired digital vessels—the radial and ulnar proper digital arteries—which traverse along the anterolateral and anteromedial margins of the phalanges, supplying extensive capillary anastomotic networks to the fingertip pulp and nail beds.

Anatomical Variants of the Upper Extremity Arterial Tree

Upper extremity arterial anatomy exhibits significant embryological variation. Recognizing these variants during duplex mapping is essential to prevent diagnostic errors and catastrophic surgical complications:

Duplex Examination Protocol & Transducer Selection

A systematic upper extremity arterial duplex examination requires meticulous technical optimization and patient positioning:

1. Patient Preparation & Positioning

• The patient is positioned supine with the head of the bed slightly elevated for comfort.
• The arm undergoing examination is positioned in slight abduction and external rotation ("pledge position" or resting comfortably on an arm-board extension), allowing unhindered access from the supraclavicular fossa to the palm.
• Room Temperature Control: The ultrasound examination room must be maintained at a warm, comfortable temperature ($21^\circ\text{C} - 24^\circ\text{C}$ or $70^\circ\text{F} - 75^\circ\text{F}$). Exposure to a cold room or cold ultrasound gel induces potent cutaneous sympathetic vasoconstriction, extinguishing palmar and digital signals and creating artifactual high-resistance or absent waveforms.

2. Transducer Selection

• Subclavian & Innominate Inflow: A $7.0 - 12.0\text{ MHz}$ linear array transducer is standard for the supraclavicular and infraclavicular subclavian segments. In muscular, obese, or deep-chested patients, a $3.5 - 5.0\text{ MHz}$ curved array or phased array transducer is required to interrogate the origin of the left subclavian from the aortic arch or the right innominate bifurcation from the suprasternal notch window.
• Axillary, Brachial & Forearm Arteries: A $7.0 - 12.0\text{ MHz}$ linear array transducer provides an optimal balance between acoustic penetration and spatial resolution along the arm and forearm.
• Palmar Arches & Digital Arteries: A high-frequency $15.0 - 18.0\text{ MHz}$ linear probe or a dedicated compact "hockey-stick" transducer can be useful. The small footprint and elevated acoustic frequency resolve superficial palmar vessels and microscopic digital arteries measuring $1-2\text{ mm}$ in diameter.

3. Duplex Scanning Protocol: Step-by-Step

1. Subclavian Artery: Interrogated from supraclavicular and infraclavicular approaches. Sample the proximal, mid (behind anterior scalene), and distal segments.
2. Axillary Artery: Followed through the axilla beneath the pectoralis major and minor muscles.
3. Brachial Artery: Imaged in transverse and longitudinal planes through the upper, mid, and distal arm to the antecubital fossa, documenting the profunda brachii origin.
4. Bifurcation & Forearm: Identify the radial and ulnar bifurcations. Trace the radial artery along the lateral forearm to the wrist and anatomical snuffbox. Trace the ulnar artery along the medial forearm to the wrist and Guyon's canal, evaluating the common interosseous branch.
5. Palmar Arches & Digits: Follow the ulnar artery across Guyon's canal into the superficial arch; follow the radial artery through the first intermetatarsal space into the deep arch. Sample the common digital and proper digital arteries to each symptomatic finger.
6. Doppler Technical Optimization:
   • Align the color box and steer angle with the vessel course.
   • Maintain a Doppler insonation angle $\le 60^\circ$, oriented parallel to the acoustic vessel wall.
   • Utilize a small sample volume ($1.5 - 2.0\text{ mm}$) positioned in the centerstream of the vessel.
   • Adjust velocity scale (PRF) and wall filters to display low-flow digital signals without acoustic aliasing or baseline clipping.

Hemodynamics & Spectral Doppler Waveform Dynamics

Interpreting upper extremity spectral Doppler signals requires recognizing normal baseline resistance and understanding how physiological vasodilation alters waveform morphology.

1. Resting Waveform Characteristics

In a resting, thermo-neutral state, the upper extremity musculature and cutaneous vascular beds exhibit high peripheral resistance. Consequently, normal upper extremity arteries display a classic high-resistance, multiphasic (triphasic or biphasic) spectral Doppler profile:

• Rapid Systolic Acceleration: A sharp, nearly vertical systolic upstroke reflecting vigorous left ventricular ejection and compliant conduit elastic recoil.
• Narrow Spectral Band: A clean acoustic spectral window under the systolic peak, confirming laminar flow where red blood cells travel at uniform velocity.
• Early Diastolic Reverse Flow: Transient reverse flow below the baseline resulting from reflection of the pressure pulse against the high-resistance peripheral arteriolar beds of the resting skeletal muscles and cutaneous plexus.
• Late Diastolic Forward Flow: Low-velocity forward flow in late diastole caused by elastic recoil of the central arteries (the Windkessel effect).

Normal Peak Systolic Velocities (PSV)

Resting Triphasic High-Resistance Waveform vs Post-Vasodilation Low-Resistance Waveform:

Resting (High-Resistance):            Vasodilated (Post-Warming / Exercise):

       /\                                    /\  
      /  \                                  /  \ 
     /    \                                /    \______
    /      \                              /            \_____
───/────────\──/\───────────────      ───/───────────────────\───────
             \/ (Reverse Flow)           (Continuous Diastolic Forward Flow)

2. Dynamic Vasodilatory Conversion

Unlike the cerebral circulation (which is obligately low-resistance), the vascular resistance of the upper extremity is dynamic and under sympathetic neurovascular and local metabolic autoregulatory control:

• Physiologic Stimuli: Muscular exercise (e.g., rapid repetitive hand clenching for 30-60 seconds), localized cutaneous warming ($40^\circ\text{C}$ warm water bath or heating pad), or reactive hyperemia immediately induces profound arteriolar vasodilation in the hand and digits.
• Waveform Transition: Following vasodilation, peripheral resistance plummets. The resting triphasic waveform instantly converts into a low-resistance, broad-peaked biphasic or monophasic waveform characterized by:
  1. Loss of the early diastolic reverse flow component.
  2. Continuous, positive forward diastolic flow throughout the entire cardiac cycle, with end-diastolic velocities well above the zero baseline.
  3. Increased mean volume flow and broadened spectral envelope.