Assessing the Onset of Regional Anaesthesia: The Role of Thermographic Imaging

The assessment of a conduction block following regional anaesthesia involves the clinical examination of motor and sensory neural pathways. Motor assessment includes the subjective evaluation of power, while sensory function is assessed using subjective perceptions of touch, cold and pain. There are considerable subjectivities and variabilities in the assessment of regional anaesthesia. Regional anaesthesia results in a blockade of not only somatosensory and motor nerve fibres but also sympathetic fibres. This results in vasodilation and an increase in blood flow, which leads to an increase in skin temperature. Multiple studies have demonstrated a high correlation between conduction block success and skin temperature changes at 10 min, detected using infrared thermography with a higher sensitivity and specificity and positive and negative predictive values up to 100%. Infrared thermography (IRT) is a non-invasive imaging tool which measures surface temperature. The role of IRT in assessing conduction blocks has been evaluated. We reviewed the literature to characterise the role of IRT in determining the onset of a conduction block following regional anaesthesia. This narrative review article synthesises the current evidence on the application of IRT in the evaluation of conduction block onset. In conclusion, IRT is a reliable tool to assess early block success as compared to routine assessment methods (touch, cold and pain perception). However, the limited studies and effects of environmental factors highlight the need for standardised protocols and multicentre studies to integrate into routine clinical practice. With further validation and integration into clinical practice, it has the potential to improve both patient safety and the reliability of block assessment.