Original Article
Pharmacokinetics of Magnesium Bolus Therapy in Cardiothoracic Surgery

https://doi.org/10.1053/j.jvca.2017.08.049Get rights and content

Objective

To investigate the pharmacokinetics of a 20 mmol magnesium bolus in regards to serum and urinary magnesium concentration, volume of distribution, and half-life.

Design

Prospective, experimental study.

Setting

A university-affiliated teaching hospital.

Participants

Twenty consecutive cardiac surgery patients treated with magnesium bolus therapy for prevention of arrhythmia.

Interventions

A 20-mmol bolus of magnesium sulfate was administered intravenously.

Measurements and Main Results

Median magnesium levels increased from 1.04 (interquartile range 0.94-1.23) mmol/L to 1.72 (1.57-2.14) mmol/L after 60 minutes of magnesium infusion (p < 0.001) but decreased to 1.27 (1.21-1.36) and 1.16 (1.11-1.21) mmol/L after 6 and 12 hours, respectively. Urinary magnesium concentration increased from 6.3 (4.2-14.5) mmol/L to 19.1 (7.4-34.5) mmol/L after 60 minutes (p < 0.001), followed by 22.7 (18.4-36.7) and 15 (8.4-19.7) mmol/L after 6 and 12 hours, respectively. Over the 12-hour observation period, the cumulative urinary magnesium excretion was 19.1 mmol (95.5% of the dose given). The median magnesium clearance was 10 (4.7-15.8) mL/min and increased to 14.9 (3.8-20.7; p = 0.934) mL/min at 60 minutes. The estimated volume of distribution was 0.31 (0.28-0.34) L/kg.

Conclusion

Magnesium bolus therapy after cardiac surgery leads to a significant but short-lived increase of magnesium serum concentration due to renal excretion and distribution, and the magnesium balance is neutral after 12 hours.

Section snippets

Study Inclusion Criteria

Inclusion criteria included (1) admission to the ICU after elective cardiac surgery and (2) a clinician’s decision to administer intravenous magnesium. Exclusion criteria included (1) pre-existing or acute renal failure and (2) a serum magnesium level ≥1.5 mmol/L at baseline.

Austin Health Human Research Ethics Committee approval was attained on June 23, 2016 (reference number: LNR/15/Austin/306). All included patients gave written consent to participate in the study.

Participants

The study included 20

Results

The study included 20 patients admitted to a tertiary ICU from August 1 to 31, 2016. The baseline characteristics of included patients are shown in Table 1. All patients included gave written consent to participate in the study. No further magnesium boluses, oral or intravenous, were administered after the study medication.

Crystalloids (median 1,500 mL, IQR 1,000-2,625) were used for intraoperative management rather than colloid infusions (median 0). Isotonic cardioplegic solution (16 mmol/L of

Key Findings

This study provides the first analysis of the effect of intravenous magnesium on serum and urinary magnesium concentrations in ICU patients after cardiothoracic surgery. The results demonstrate that 20 mmol of magnesium sulfate concomitantly increases the serum magnesium concentration and magnesium urinary excretion and that the volume of distribution of magnesium approximates 0.3 L/kg of body weight. Moreover, the magnesium serum concentration returned to near pretreatment levels within 12

References (17)

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Cited by (13)

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    In addition, the daily amount of 5 g corresponded to less than one-third of the daily amount prescribed in the authors’ infusion protocol. The authors recently have observed that a bolus of magnesium therapy after cardiac surgery has short-lived effects on blood levels due to urinary excretion.20 Thus, the lack of protective effect against AF reported by some studies31-34 may have been related to a non-sustained delivery method.

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    In order to study such targets and their effect on arrhythmias, however, it is crucial to understand the pharmacokinetics of intravenous magnesium administration in cardiac surgery patients so that such targets can be reliably achieved. Recently, intravenous bolus magnesium therapy was shown to fail to achieve a sustained increase of serum magnesium concentration due to the short half-life and complete renal excretion within 12 h [10]. The findings of this study, however, suggested that a combined approach of bolus dose followed by a continuous infusion, at an hourly dose based on renal elimination, may provide stable magnesium levels in the moderately elevated range.

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