Effect of diaphragmatic contraction on intramuscular pressure and vascular impedance

The purpose of the present study was to determine whether the diaphragm, like limb muscles, generates high levels of intramuscular pressure during contraction. Studies were performed on in situ diaphragm strips in 15 dogs. An isometric force transducer was used to measure strip tension, and intramuscular pressure was assessed using both wick and balloon catheter techniques. Tension and intramuscular pressures were recorded during a series of graded contractions elicited by electrically stimulating strips via intramuscular electrodes. Similar techniques were used to record the tension and intramuscular pressures developed during graded contractions of a limb muscle, the vastus medialis. Intramuscular pressure was linearly related to tension in both the diaphragm and vastus medialis. However, for a given level of tension development, vastus medialis pressures were two- to threefold greater than those developed within the diaphragm. For example, at a tension of 2 kg/cm² vastus medialis and diaphragm intramuscular pressures were 105 ± 11 and 25 ± 4 mmHg, respectively, when assessed with wick catheters, and were 159 ± 11 and 48 ± 5 mmHg, respectively, when assessed by the balloon catheter technique. These differences may reflect the fact that thick rounded muscles, like the vastus, may be capable of generating higher intramuscular pressures than the thin sheetlike diaphragm. To determine whether intradiaphragmatic pressure swings were large enough to explain the vascular compression produced by contraction, we measured phrenic arterial impedance during contraction in nine animals. In these animals the inferior phrenic artery was cannulated and pump perfused at a constant flow rate. With contraction, phrenic arterial pressure rose, i.e., phrenic arterial impedance increased. At a given level of muscle tension development, changes in phrenic arterial pressure during contraction equaled changes in balloon catheter intramuscular pressure. These data suggest that even though diaphragmatic intramuscular pressure swings are smaller than those generated in thick limb muscles during contraction, the magnitude of these swings may, nevertheless, be sufficient to account for the mechanical hindrance to phrenic arterial flow resulting from isometric diaphragmatic contraction.