The formalisation of clinical guidelines is a long and demanding task which usually involves both clinical and IT staff. Because of the features of guideline representation languages, a clear understanding of the final guideline model may prove complicated for clinicians. In this context, an assessment of the understandability of the guideline model becomes crucial. In the field of Business Process Modelling (BPM) there is research on structural metrics and their connection with the quality of process models, concretely with understandability and modifiability. In this paper we adapt the structural metrics that have been proposed in the field of BPM in terms of the features of a specific guideline representation language, which is PROforma. Additionally, we present some experiments consisting in the application of these adapted metrics to the assessment of guideline models described in PROforma. Although it has not been possible to draw meaningful conclusions on the overall quality of the models, our experiments have served to shed light on important aspects to be considered, such as the hierarchical decomposition of processes.