What is the normal haemodynamic response to passive leg raise? A study of healthy volunteers

Passive leg raise (PLR) is used as self-fluid challenge to optimise fluid therapy by predicting preload responsiveness. However, there remains uncertainty around the normal haemodynamic response to PLR with resulting difficulties in application and interpretation in emergency care. We aim to define the haemodynamic responses to PLR in spontaneously breathing volunteers using a non-invasive cardiac output monitor, thoracic electrical bioimpedance, TEB (PLR-TEB).

We recruited healthy volunteers aged 18 or above. Subjects were monitored using TEB in a semirecumbent position, followed by PLR for 3 min. The procedure was repeated after 6 min at the starting position. Correlation between the two PLRs was assessed using Spearman's r (r_s). Agreement between the two PLRs was evaluated using Cohen Kappa with responsiveness defined as ≥10% increase in stroke volume. Parametric and non-parametric tests were used as appropriate to evaluate statistical significance of baseline variables between responders and non-responders.

We enrolled 50 volunteers, all haemodynamically stable at baseline, of whom 49 completed the study procedure. About half of our subjects were preload responsive. The ∆SV in the two PLRs was correlated (r_s=0.68, 95% CI 0.49 to 0.8) with 85% positive concordance. Good agreement was observed with Cohen Kappa of 0.67 (95% CI 0.45 to 0.88). Responders were older and had significantly lower baseline stroke volume and cardiac output.

Our results suggest that the PLR-TEB is a feasible method in spontaneously breathing volunteers with reasonable reproducibility. The age and baseline stroke volume effect suggests a more complex underlying physiology than commonly appreciated. The fact that half of the volunteers had a positive preload response, against the 10% threshold, leads to questions about how this measurement should be used in emergency care and will help shape future patient studies.