Pharmacokinetics of Magnesium Bolus Therapy in Cardiothoracic Surgery.

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.

Current ventilation practice during general anaesthesia: a prospective audit in Melbourne, Australia.

BACKGROUND: Recent evidence suggests that the use of low tidal volume ventilation with the application of positive end-expiratory pressure (PEEP) may benefit patients at risk of respiratory complications during general anaesthesia. However current Australian practice in this area is unknown. METHODS: To describe current practice of intraoperative ventilation with regard to tidal volume and application of PEEP, we performed a multicentre audit in patients undergoing general anaesthesia across eight teaching hospitals in Melbourne, Australia. RESULTS: We obtained information including demographic characteristics, type of surgery, tidal volume and the use of PEEP in a consecutive cohort of 272 patients. The median age was 56 (IQR 42-69) years; 150 (55%) were male. Most common diagnostic groups were general surgery (31%), orthopaedic surgery (20%) and neurosurgery (9.6%). Mean FiO2 was 0.6 (IQR 0.5-0.7). Median tidal volume was 500 ml (IQR 450-550). PEEP was used in 54% of patients with a median value of 5.0 cmH2O (IQR 4.0-5.0) and median tidal volume corrected for predicted body weight was 9.5 ml/kg (IQR 8.5-10.4). Median peak inspiratory pressure was 18 cmH2O (IQR 15-22). In a cohort of patients considered at risk for respiratory complications, the median tidal volume was still 9.8 ml/kg (IQR 8.6-10.7) and PEEP was applied in 66% of patients with a median value of 5 cmH20 (IQR 4-5). On multivariate analyses positive predictors of tidal volume size included male sex (p < 0.01), height (p = 0.04) and weight (p < 0.001). Positive predictors of the use of PEEP included surgery in a tertiary hospital (OR = 3.11; 95% CI: 1.05 to 9.23) and expected prolonged duration of surgery (OR = 2.47; 95% CI: 1.04 to 5.84). CONCLUSION: In mechanically ventilated patients under general anaesthesia, tidal volume was high and PEEP was applied to the majority of patients, but at modest levels. The findings of our study suggest that the control groups of previous randomized controlled trials do not closely reflect the practice of mechanical ventilation in Australia.

Prevalence of low flow anesthesia and its cost comparison with other anesthesia techniques in an Indian tertiary care center.

BACKGROUND: The usage of high fresh gas flows with expensive volatile agents is known to cause substantial economic losses. Practicing low flow anesthesia can minimize these costs. The average costs of different anesthetic techniques, used for maintenance of general anesthesia and the frequency of use of low flows were evaluated. METHODS: We noted the consumption of anesthetic gases and volatile agents during routine surgeries in our institute for two weeks (10 consecutive full working days). According to the anesthetic agent and fresh gas flow used, all general anesthetics were divided into four groups i.e. low flow (gas flows < or = 1 L/min) and high flow anesthesia (gas flows > 1 L/min) with isoflurane, halothane anesthesia (gas flows > 1 L/min) and propofol intravenous anesthesia (gas flows > 1 L/min). We calculated the individual cost of maintenance of anesthesia on per hour basis for each group based on average flows and anesthetic concentrations used. RESULTS: Low flow anesthesia is sparingly used in our institution (4.1%). Halothane with fresh gas flow > 1 L/min is the most commonly used anesthetic technique (45.9%). Propofol anesthesia was nearly equal to low flow isoflurane anesthesia in terms of costs. Using higher fresh gas flows with isoflurane increases this cost by three times. CONCLUSIONS: Low flow anesthesia is still sparingly used. Low flow isoflurane anesthesia is equivalent in terms of costs to propofol anesthesia though more expensive than conventional anesthesia with halothane.