The Effect of Prolonged Inspiratory Time on Pulmonary Mechanics in Obese Patients

Overview

The area of aesthesia-induced atelectasis is much larger in the obese compared with the non-obese, but there may also be more airway closure and impaired matching of ventilation and lung blood flow. When an anesthetized patient is turned to the prone position, dynamic compliance (Cdyn) decreases and peak airway pressure increases unless the abdomen hangs freely to prevent the abdominal viscera from compromising the diaphragm movement. Although the Wilson frame is designed to allow the abdomen to hang, it partially compresses the anterior abdominal wall and therefore does not allow the abdomen to hang completely, especially in obese patients. This in turn increases peak airway pressure and decreases Cdyn, oxygenation. This study aimed to investigate the effects of a prolonged I:E ratio (i.e., 1:1) compared with the conventional I:E ratio of 1:2 on respiratory mechanics and hemodynamics during spine surgery in the prone position in obese patients. We hypothesized that, compared with an I:E ratio of 1:2, a ratio of 1:1 improve oxygenation without hemodynamic instability .

After written informed consent was obtained from all patients, 50 adult patients were enrolled in the study. The patients met the following inclusion criteria: (1) body mass index (BMI, weight in kilograms divided by the square of height in metres) \> 25 kg/m2; (2) American Society of Anesthesiology (ASA) physical status classification grade I or II (BMI by itself was not used as the basis for the ASA classification); (3) aged 20 - 65 years; and (4) scheduled for elective spine surgery in prone position. Exclusion Criteria: * Patients who have severe pulmonary disease: history of chronic obstructive pulmonary disease (COPD), asthma, or pneumothorax. Patients with haemodynamic instability, hypovolaemia, bronchopleural fistula, The enrolled patients were randomly allocated according to a predetermined allocation sequence to receive an I:E ratio of either 1:1 (group 1:1) or 1:2 (group 1:2). The allocation sequence with no blocking was generated in an Internet website Standard monitoring techniques, including electrocardiography, pulse oximetry, and noninvasive arterial blood pressure measurement, are applied upon arrival at the operating room. Anesthesia was induced with intravenous propofol 1.5 mg\_kg-1 and rocuronium 0.8 mg\_kg-1 was administered intravenously. After tracheal intubation, volume-controlled ventilation was initiated with an I:E ratio of 1:2 or 1:1, no positive end-expiratory pressure, and a tidal volume of 10 mL per ideal body weight (kg). A respiratory rate was adjusted in order to end-tidal carbon dioxide (EtCO2) of 33 - 36 mmHg during surgery. Anesthesia was maintained with an end-tidal concentration of 2-2.5 vol% sevoflurane in 40% oxygen/air. The bispectral index score was monitored continuously in order to maintain an adequate anesthetic depth and was targeted at a range of 40-60 during surgery. Radial artery cannulation was conducted for monitoring continuous arterial blood pressure and blood sampling. Respiratory, hemodynamic, and arterial blood gas data were assessed and recorded at three time points: ten minutes after tracheal intubation in the supine position (T1), 30 min after prone positioning (T2), 90 min after prone positioning(T3). Respiratory data consisted of peak airway pressure, plateau airway pressure, mean airway pressure, static compliance, EtCO2, respiratory rate, and minute volume. Arterial pH, arterial oxygen tension (PaO2), arterial carbon dioxide tension (PaCO2), and lactate level were obtained from arterial blood gas analysis.