BACKGROUND: Increases in skin temperature may be used as an early predictor of the success of interscalene brachial plexus block (IBPB), but we lack detailed information on the thermographic response.

OBJECTIVE: To investigate and characterise the thermographic response after IBPBs.

DESIGN: Prospective observational study.

SETTING: University hospital and private hospital.

PATIENTS: Twenty-nine male and 17 female patients scheduled for ambulatory shoulder surgery. Exclusion criteria were age less than 18 years, body weight more than 120 kg and any coagulation abnormality.

INTERVENTIONS: Infrared thermographic imaging of the hand before and at 1 min intervals for 30 min after an ultrasound-guided IBPB with 20 ml ropivacaine 7.5 mg ml. Cooling of both hands was performed to standardise measurements.

MAIN OUTCOME MEASURES: Thermographic changes in skin temperature on the dorsum of the hand.

RESULTS: Forty-four blocks were successful and two were failures. Four thermographic patterns were observed after successful blocks: the increase in skin temperature was restricted to the thumb (n = 5); increase in skin temperature of the thumb and the second digit (n = 11); increase in skin temperature of the thumb, the second and fifth digits (n = 4); and an increase in skin temperature in all parts of the hand (n = 24). All successful blocks demonstrated a significant (P < 0.0001) increase in median (range) of distal skin temperature of the thumb of 6.6°C (0.7 to 17.2) by 30 min, which was already significant (P < 0.0001) by 5 min. By contrast, skin temperature decreased significantly (P < 0.0001) in the hand after failed blocks and in the contra-lateral non-blocked hand by -1.5°C (-6.2 to 4.2).

CONCLUSION: Successful IBPB resulted in four thermographic patterns. Skin temperature always increased on the thumb within 30 min and this increase achieved statistical significance at 5 min after the block.