The Lack of Effect of Routine Magnesium Administration on Respiratory Function in Mechanically Ventilated Patients: Materials and Methods
Subjects
We studied 20 patients admitted to our medical-surgical ICU. All patients requiring mechanical ventilation for more than 3 days were eligible for study. Exclusion criteria included the following: (1) hemodynamic instability’; (2) chest wall instability; (3) preexisting renal failure (creatinine >200 mmol/L); (4) atrioventricular conduction defects; and (5) inability to give informed consent. Patients were also screened to ensure that their baseline levels of serum magnesium were not outside the normal range for our institution (normal values, 0.7 to 1.05 mmol/L). my canadian pharmacy online
Study Design
The study was approved by our University Ethics Committee and we obtained informed consent from each subject. We obtained demographic data (age, sex, race), medication history, and a medical history. We specifically reviewed the medical history for known risks for magnesium deficiency. These included prior diuretic use, alcohol abuse,” recent aminoglycoside administration, and severe diabetes. Figure 1 outlines our experimental protocol. We measured serum concentrations of magnesium along with values for serum potassium, sodium, creatinine, ionized calcium, albumin, total protein, and phosphate. Prior to performing tests of respiratory function, patients were ventilated with 100 percent 02 for 5 min. We then measured values of vital capacity (VC), maximal inspiratory’ pressure at residual volume (Pimax), and maximal expiratory pressure at total lung capacity (PEmax). All measurements were obtained in a seated position and all patients had been at rest, receiving mechanical ventilation for at least 1 h preceding the tests. The VC was measured by a portable spirometer connected to the end of the endotracheal tube.
The patients were encouraged to obtain a maximum effort and the greatest value of three attempts was recorded as VC. Patients were then instructed to exhale to residual volume and make a maximal inspiratory effort against an obstructed airway. The most negative value of three efforts was recorded as Pimax. Patients were instructed to inspire to total lung capacity and make a maximal expiratory effort against an obstructed airway. The most positive of three efforts was recorded as PEmax. Values of Pimax and PEmax were measured at the bedside with a portable manometer. Finally, handgrip as an index of skeletal muscle strength was measured in the dominant hand using a dynamometer. Again the best of three efforts was recorded. Patients received visual feedback of their tests of respiratory function and handgrip to facilitate their generating a maximal effort.
Following these baseline measurements, patients were randomized to receive either MgSO (6 g in 500 ml of 5 percent dextrose in water [D5W]) or 500 ml D5W alone infused over a 16-h period. On the following day all biochemical measurements were repeated as were measurements of VC, Pimax, PEmax, and handgrip. Patients then received an infusion of the alternate solution, ie, those receiving MgS04 on day 1 received D5W alone on day 2 and those receiving D5W alone on day 1 received MgS04 on day 2. The next morning all measurements were repeated and the protocol was completed. Any evidence of cardiac conduction defects was assessed by the bedside nursing staff. As well, a study nurse assessed deep tendon reflexes and gross motor power hourly during the period of infusion.
Statistics
Statistics for nominal data were calculated by x2 with Yates’ correction factor and when the incidence of events was very low, a Fishers Exact Test. Differences between groups for continuous data were analyzed by a one-way analysis of variance (ANOVA). When the F statistic showed a significant difference, these values were compared by multiple t tests. Sidaks multiplicative inequality was used to correct the t statistic for the number of comparisons between groups. A p value of less than 0.05 was considered to show a significant difference. The change in VC, PEmax, and Pimax from before to after magnesium infusion was assessed by a Shapiro-Francia test to determine if it followed a normal distribution, £-Error was calculated using the method described by Lachin. All values are shown as mean ± standard deviation.
Figure 1. Outline of our experimental protocol. Patients were randomized to receive MgS04 on day 1 followed by D5W on day 2 or to receive D5W on day 1 followed by MgS04 on day 2. Measurements of VC (vital capacity), Pimax (maximal inspiratory pressure at residual volume), PEmax (maximal expiratory pressure at total lung capacity), and serum biochemistry were obtained before the study infusion and then on each subsequent study day.