Individual differences in the cardiovascular responses to tonic muscle pain: parallel increases or decreases in muscle sympathetic nerve activity, blood pressure and heart rate

We recently showed that acute muscle pain, induced by bolus intramuscular injection of hypertonic saline, causes a sustained increase in muscle sympathetic nerve activity (MSNA) and a modest increase in blood pressure and heart rate. However, it is not known whether long-lasting (tonic) pain, which more closely resembles chronic pain, causes a sustained increase in MSNA and blood pressure. We tested this hypothesis by recording MSNA in 12 healthy subjects. Tonic pain was induced for similar to 60 min by slow intramuscular infusion of hypertonic saline (7%) into the ipsilateral tibialis anterior muscle. Pain was sustained at a tolerable level (5/10 to 6/10 on a visual analog scale). Seven subjects showed progressive increases in mean MSNA amplitude during tonic pain, increasing to 154 +/- 17% (SEM) at 45 min and remaining essentially constant for the duration of the infusion. In these subjects, blood pressure and heart rate also increased. Conversely, for the other five subjects MSNA showed a progressive decline, with a peak fall of 67 +/- 11% at 40 min; blood pressure and heart rate also fell in these subjects. We conclude that tonic muscle pain has long-lasting effects on the sympathetic control of blood pressure, causing a sustained increase in some subjects yet a sustained decrease in others. This may have implications for individual differences in the cardiovascular consequences of chronic pain.