Sample size calculation: Basic principles

Addressing a sample size is a practical issue that has to be solved during planning and designing stage of the study. The aim of any clinical research is to detect the actual difference between two groups (power) and to provide an estimate of the difference with a reasonable accuracy (precision). Hence, researchers should do a priori estimate of sample size well ahead, before conducting the study. Post hoc sample size computation is not encouraged conventionally. Adequate sample size minimizes the random error or in other words, lessens something happening by chance. Too small a sample may fail to answer the research question and can be of questionable validity or provide an imprecise answer while too large a sample may answer the question but is resource-intensive and also may be unethical. More transparency in the calculation of sample size is required so that it can be justified and replicated while reporting

Effect of magnesium infusion on thoracic epidural analgesia

Introduction: Patients of lung volume reduction surgery (LVRS) having an ASA status III or more are likely to be further downgraded by surgery to critical levels of pulmonary function. Aim: To compare the efficacy of thoracic epidural block with (0.125%) bupivacaine, fentanyl combination and (0.125%) bupivacaine, fentanyl combination with adjunctive intravenous magnesium infusion for the relief of postoperative pain in patients undergoing LVRS. Methods: Patients were operated under general anesthesia. Thirty minutes before the anticipated completion of skin closure in both groups, (Group A and Group B) 7 ml of (0.125%) bupivacaine calculated as 1.5 ml/thoracic segment space for achieving analgesia in dermatomes of T4, T5, T6, T7, and T8 segments, along with fentanyl 50 ΅g (0.5 ml), was administered through the catheter, activating the epidural block, and the time was noted. Thereafter, in patients of Group A, magnesium sulfate injection 30 mg/kg i.v. bolus was followed by infusion of magnesium sulfate at 10 mg/kg/hr and continued up to 24 hours. Group B was treated as control. Results and Analysis: A significant increase in the mean and maximum duration of analgesia in Group A in comparison with Group B (P<0.05) was observed. Total epidural dose of fentanyl and bupivacaine required in Group A was significantly lower in comparison with Group B in 24 hours. Discussion: Requirement of total doses of local anesthetics along with opioids could be minimized by magnesium infusion; therefore, the further downgradation of patients of LVRS may be prevented. Conclusion: Intravenous magnesium can prolong opioid-induced analgesia while minimizing nausea, pruritus, and somnolence

A comparison between volume-controlled ventilation and pressure-controlled ventilation in providing better oxygenation in obese patients undergoing laparoscopic cholecystectomy

Background: The maintenance of oxygenation is a commonly encountered problem in obese patients undergoing laparoscopic cholecystectomy. There is no specific guideline on the ventilation modes for this group of patients. Although several studies have been performed to determine the optimal ventilatory settings in these patients, the answer is yet to be found. The aim of this study was to evaluate the efficacy of pressure-controlled ventilation (PCV) in comparison with volume-controlled ventilation (VCV) for maintaining oxygenation during laparoscopic cholecystectomy in obese patients. Methods: One hundred and two adult patients of ASA physical status I and II, Body Mass Index of 30-40 kg/m 2 , scheduled for laparoscopic cholecystectomy were included in this prospective randomized open-label parallel group study. To start with, all patients received VCV. Fifteen minutes after creation of pneumoperitoneum, they were randomized to receive either VCV (Group V) or PCV (Group P). The ventilatory parameters were adjusted accordingly to maintain the end-tidal CO 2 between 35 and 40 mmHg. Respiratory rate, tidal volume, minute ventilation and peak airway pressure were noted. Arterial blood gas analyses were done 15 min after creation of pneumoperitoneum and at 20-min intervals thereafter till the end of the surgery. All data were analysed statistically. Results: Patients in Group P showed a statistically significant ( P < 0.05) higher level of PaO 2 and lower value of PAO 2 -PaO 2 than those in Group V. Conclusion: PCV is a more effective mode of ventilation in comparison with VCV regarding oxygenation in obese patients undergoing laparoscopic cholecystectomy

Effect of QTc interval on prediction of hypotension following subarachnoid block in patients undergoing cesarean section: A comparative study

Background: Previous studies have revealed that QTc interval is prolonged in pre-eclamptic parturients. Another study reflected the relationship between the sympathetic block and QTc interval. Subarachnoid block was safely administered in patients with severe pre-eclampsia. It has also been noticed that hypotension in response to spinal anesthesia is relatively less in pre-eclamptic patients than normal parturients. Aim: To compare the QTc values in normal and pre-eclamptic term parturients and to establish whether any correlation exists between the QTc interval and the systemic hypotension following subarachnoid block. Materials and Methods: Twenty-five pre-eclamptic patients (Group A) and 25 normotensive patients (Group B) were included in this study. QTc interval was recorded for each patient before subarachnoid block for cesarean section. Changes in arterial blood pressure and heart rate were measured in both the groups and compared. Results: Baseline QTc was significantly higher in the pre-eclamptic group (Group A: 0.47 ± 0.11) with that of control (Group B: 0.36. ± 0.02). Significant fall in blood pressure was seen only in one group with QTc between 0.38 and 0.39 in Group A. Hypotension was significantly more in normotensive mothers (Group B). However, no statistical correlation could be drawn from this study between QTc interval and hypotension, although a trend toward increasing hypotension with decreasing QTc was present. Discussion : The prolonged QTc intervals seen in pre-eclamptic patients may be due to the contributory effects of sympathetic hyperactivity, hypertension, and hypocalcemia secondary to underlying vasoconstriction. Decreased vagal control of heart in pre-eclampsia may have produced the difference in change in hemodynamic status between pre-eclamptic and normotensive parturient. Conclusion: Any consistent correlation between QTc and hypotension following subarachnoid block could not be derived from this study. To achieve a statistical significance a larger sample size may be required