Evaluation of techniques for measuring negative intramuscular pressures in humans

Negative external pressure has been used previously in experiments on the blood circulation in humans and is currently being considered as a counter-measure to the musculoskeletal deconditioning which occurs during the exposure to microgravity. Intramuscular pressure (IMP) measurement is an important tool for determining the effectiveness of the transmission of negative pressure. Therefore two IMP techniques, a fibre optic transducer-tipped catheter and a fluid-filled teflon catheter with side holes, were evaluated using laboratory and human tests for measuring negative pressures. For the laboratory tests, both catheters were placed inside a lower leg negative pressure (LLNP) chamber and pressures of 0, -20, -50, -80, -100 and back to 0 mmHg were applied. Both catheters measured pressures equally and remained stable over 1 min at all pressure levels. When the teflon catheter was infused at rates commonly used for IMP recordings at normal atmospheric pressures, the drip rate at the catheter tip increased inversely with the magnitude of negative pressure. The cables and pressure tubings of the catheters within the LLNP chamber were not affected by the negative pressures. For tests in humans, the catheters were inserted side by side in the tibialis anterior muscle in eight legs of four human volunteers. The leg was placed in the LLNP chamber and pressures of 0, -20, -50, -80 and back to 0 mmHg were applied. The teflon catheter was used without infusion. Both catheters measured IMP similarly at rest and both remained stable over 1 min at all pressure levels; also the IMP during muscle contractions and immediately following contractions was similarly recorded by both catheters. We concluded that both catheter systems are suitable for recording negative pressures over a wide range. As is the case with recordings at normal atmospheric pressures, the fibre optic transducer-tipped catheter system may be preferred when recording IMP in a negative pressure environment when complex limb movements are involved.