Peripheral intravenous catheter (PIVC) insertion is one of the most frequently performed invasive procedures in both medical and nursing practice. It is essential for blood sampling, intravenous (IV) fluid resuscitation, medication administration, blood product transfusion, and IV contrast-enhanced radiological imaging. In emergency department (ED) settings, where patients may deteriorate rapidly, timely and adequate IV access is of critical importance. Larger-bore cannulas allow faster fluid and contrast delivery and are less prone to occlusion; however, Larger-bore cannulas are often perceived to cause greater pain and procedural difficulty compared to smaller-bore cannulas. This perception may lead clinicians to prefer smaller cannulas, potentially compromising care quality in patients who may unexpectedly deteriorate.

Cannula insertion inherently causes discomfort, pain, and anxiety for patients, and multiple failed attempts further amplify these effects. Patient satisfaction, an important dimension of overall care quality, is directly influenced by pain experience and the number of insertion attempts. Patients requiring two or more attempts for successful cannulation are generally classified as having difficult intravenous access (DIVA). Repeated failed attempts are associated with vascular injury, increased patient distress, delays in diagnosis and treatment, and greater consumption of healthcare resources. The A-DIVA score, developed by Van Loon et al., enables risk stratification for IV access difficulty at triage, classifying patients as low, moderate, or high risk, and facilitating proactive measures such as ultrasound guidance or involvement of experienced personnel for high-risk patients.

This single-center, prospective, single-blind, randomized controlled trial aims to compare 18-gauge (18G) and 20-gauge (20G) PIVCs in terms of patient-reported pain intensity and nurse-reported procedural difficulty in adult ED patients with low-to-moderate A-DIVA risk. The investigators hypothesize that 18G cannulas cause similar pain and procedural difficulty compared to 20G cannulas in this patient population.

The study will be conducted at Marmara University Pendik Training and Research Hospital Emergency Department. Ethics approval has been granted by the Marmara University Faculty of Medicine Clinical Research Ethics Committee. Adult patients (18 years and older) requiring IV access for diagnostic or therapeutic purposes and classified as low or moderate A-DIVA risk will be eligible. Patients with cognitive impairment, altered mental status, or hemodynamic instability (systolic BP less than 90 mmHg or mean arterial pressure less than 65 mmHg) will be excluded.

Eligible consenting patients will be randomized 1:1 using a block randomization method (block size of 4) to either 18G or 20G cannula insertion. Cannula assignment will be delivered to nurses in sealed envelopes. Patients will be blinded to cannula size using blackout goggles. Cannulation will be performed by ED nurses following standard antiseptic and securement procedures, targeting the dorsum of the hand, forearm, or antecubital fossa. If the first attempt fails, at least one additional attempt with the assigned cannula will be required before allowing free choice of cannula and site.

The primary outcomes are patient-reported pain intensity and nurse-reported procedural difficulty, each assessed using a 10 cm Visual Analog Scale (VAS) immediately after the first cannulation attempt and compared between the 18G and 20G cannula groups. Secondary outcomes include the comparison of first-attempt cannulation success rate between the 18G and 20G cannula groups. This comparison will also be performed separately according to cannulation site, operator experience, A-DIVA risk category, patient age, and patient sex.

A total of 204 patients (102 per group) will be enrolled, based on 90% power, 5% type I error, a minimum clinically important difference of 13 mm on the VAS, and a 10% dropout allowance.

Inclusion Criteria:

• 18 years of age or older
• Requiring intravenous access for diagnostic or therapeutic purposes
• Classified as low or moderate risk according to the A-DIVA (Adult Difficult Intravenous Access) score
• Provision of informed consent by the patient, a relative, or a legal guardian

Exclusion Criteria:

• Inability to effectively communicate pain intensity due to cognitive impairment, altered mental status, or visual or language difficulties
• Hemodynamic instability defined as systolic blood pressure less than 90 mmHg or mean arterial pressure less than 65 mmHg