The Lack of Effect of Routine Magnesium Administration on Respiratory Function in Mechanically Ventilated Patients: Conclusion
The unknown extent of carryover of the effects of magnesium, therefore, limited the application of the crossover design in our data analysis. We therefore only analyzed our results from the period before infusion to that immediately following infusion of magnesium. Using this analysis, if we reject the hypothesis of a subgroup of patients and accept the normal distribution for Pimax and VC, the p-error in rejecting an effect of treatment can be calculated. Assuming a positive treatment effect of 30 percent and a onetailed a of 0.05, the P-error for VC and Pimax were 0.09 and 0.04, respectively. Thus, we are confident that our study did not reject the presence of a true magnesium effect and that other causes of respiratory muscle weakness played a more prominent role. canadian pharmacy generic viagra
As a group, our patients suffered from respiratory muscle weakness as demonstrated by their baseline values of inspiratory and expiratory muscle strength. Their mean values of Pimax and PEmax were similar to those described by Rochester in patients suffering from nutritional and nonnutritional myopathies. Along with the respiratory muscle weakness, those patients also demonstrated a restrictive pattern on lung function testing. Our subjects were similar in having reduced VCs along with reductions in Pimax and PEmax. We therefore reasoned that if magnesium did improve respiratory muscle strength, there should also be concomitant increases in VC. We did not detect a subgroup with changes in VC consistent with improved respiratory muscle strength.
Although we had hypothesized there would be a group of patients who showed increases in respiratory muscle strength, we did not, in fact, find marked changes in our measured indices of respiratory muscle strength. As well, there were no consistent changes in skeletal muscle strength as assessed by handgrip. It is likely, however, that our measurements of respiratory muscle strength are relatively insensitive. Marini and associates had suggested that measurements of maximal inspiratory pressure in intubated patients could be improved by adding a one-way valve to the distal end of the endotracheal tube. Measurements of Pimax and PEmax may still be insensitive even under such a strictly controlled setting. Multz and associates evaluated Pimax using a one-way valve, and they found a great variation in Pimax from day to day in clinically stable patients. Technically, the VC measurement is simpler and, we believe, in the setting of markedly reduced muscle strength, likely more reproducible than direct tests of Pimax or PEmax. Although some patients showed large increases in VC, and one patient showed a large (— 30 cm H20) improvement in inspiratory muscle strength following magnesium infusion, these likely represent the normal variation of these tests in the ICU setting.