Arterial cannulation is one of the frequently performed procedures in patients with anticipated massive blood loss, significant hemodynamic changes or requirement of frequent blood gas sampling in perioperative period. Various arteries can be used for cannulation like radial, ulnar, femoral, dorsalis pedis and posterior tibial artery. Radial artery is most commonly chosen for arterial cannulation. However, Dorsalis pedis artery (DPA) is selected as an alternative to radial artery cannulation when there is no access or unsuccessful cannulation of radial artery. DPA has good collateral flow, presents minimal patient inconvenience and has low incidence of complication.

When compared to radial artery cannulation, DPA has a 12.7% lower success rate (1). Traditionally DPA is cannulated by palpatory method using navicular bone as landmark.

Ultrasound (US) is used as an aid for arterial cannulation to improve the success rate and lessen complications. Numerous approaches have been used for US guided arterial cannulation. Among them, Short Axis Out-Of-Plane (SA-OOP) and long-axis in-plane (LAIP) approaches are commonly used. In SA-OOP approach, an US image of part of the shaft or the tip of the cannula in cross- section, will appear as a hyperechoic dot; whereas in LAIP approach, the entire path of needle can be visualized as a hyperechoic line. The first attempt and overall success rate of DPA cannulation has been found to be similar in the palpatory and ultrasound guided LAIP approach (2). With both palpatory and LAIP approach, first attempt success rate of DPA cannulation have been poor (60% and 76% respectively) (2).

In short axis view the US image from the shaft as well as tip of the cannula will appear as hyperechoic dot. In order to overcome this problem dynamic needle tip (DNTP) approach was introduced. DNTP technique, a modification of SA-OOP approach, has shown to have higher first attempt and overall success rate and shorter cannulation time compared to palpatory method for DPA in infants and small children (3).

To the best of our knowledge, there are no studies that compared the SA-DNTP vs palpatory method for dorsalis pedis artery in adults. Therefore, we planned to conduct a study to compare the US guided SA-DNTP versus palpatory technique in dorsalis pedis artery. We hypothesise SA-DNTP technique has higher first attempt success rate for dorsalis pedis artery cannulation as compared to palpatory technique.

Primary objective:

First attempt success rate of dorsalis pedis artery cannulation.

Secondary objectives:

1. Overall success rate

2. Total number of attempts

3. Time required for successful cannulation

4. Incidence of posterior wall puncture

5. Incidence of hematoma formation

Study Design: A Prospective randomized controlled trial.

Study Setting:

This study will be conducted in the Department of Anaesthesiology, Pain Medicine and Critical Care, All India Institute of Medical Sciences, New Delhi after obtaining approval of institutional ethics committee and CTRI registration.

Study Duration:

The total study duration will be approximately 2 years.

Inclusion criteria

The study population will include adult patients fullfilling the following criteria -

ASA I/II/III

18 to 60 Years of age

Surgeries requiring arterial cannulation

Exclusion Criteria

Patient refusal

Peripheral vascular disease

BMI>30kg/cm²

Patients with coagulopathy

Local infection at site of cannulation

Pregnancy

Sample size calculation:

Anand et al reported first attempt success rate of DPA cannulation as 60% by palpatory method and 76% by USG guided LAIP approach. In infants and small children Takeshita et al reported first attempt success rate as 85% for DPA

cannulation with DNTP approach. We considered first attempt success rate of DNTP approach in adults would be at least similar or higher to that in infants. Therefore, assuming 85% success rate with DNTP method and 60% with palpatory method with a power of 80% and a significance level of 0.05, n=98 patients will be required. Considering 10% dropout rate, we will enrol n=108

patients.

Randomization:

Patients will be randomized using Sequentially Numbered Opaque Sealed Envelope (SNOSE) technique. Random number tables will be generated with the

help of computer software (ww.randomizer.org). The sealed envelopes will be sequentially labelled from 1 to 108. Each envelope will contain a folded slip which may have GROUP D (DNTP) or GROUP P (PALPATORY)

Blinding:

Due to nature of study blinding will not be possible. However, the patients and the data outcome assessor will be blinded to the group allocation.

Definitions

Successful arterial cannulation will be confirmed by presence of arterial waveform on the monitor.

Attempt - one new skin penetration by cannula is one attempt

First attempt success rate â€“ number (%) of patients successfully cannulated in first attempt

Overall success rate - number (%) of patients successfully cannulated within three attempts.

Time for cannulation - defined as time interval between skin penetration by cannula and confirmation of arterial waveform on monitor.

Hematoma will be defined by localised swelling on the puncture site.

Posterior wall puncture will be identified by appearance of localized hematoma beneath the artery in ultrasound scan.