The effects of anthropometric properties on endtidal rise time of desflurane in low flow anesthesia

Objective: In low-flow anesthesia, the current practice involves maintaining a fresh gas flow of 4 L/min at the beginning for 10 minutes. However, our observations indicate that adequate desflurane intake can be achieved rapidly with a fresh gas flow of 1 L/min shortly after the initiation, but the duration varies depending on the patient's somatotype (ectomorph, mesomorph, endomorph). Considering the close relationship between somatotype and lung capacity, as well as changes in the supine position, further investigation is warranted. The primary outcome of the study is the time it takes for the desflurane concentration in the expiratory limb of the anesthesia machine to reach the target value based on somatotype (ectomorph, mesomorph, endomorph). The secondary outcome involves developing a model to predict the time to reach the target concentration based on somatotype and determining its accuracy. Additionally, the study aims to identify the relationship between the duration and gender, age, and other anthropometric features (height, body weight, body mass index). Materials and Methods: Following ethical approval (2022/11, January 20, 2022), patients aged 18-75 undergoing elective surgery under general anesthesia were included in the study. Patients with conditions affecting cardiac output and functional residual capacity, such as COPD, emphysema, a history of lung surgery, kyphoscoliosis, increased intraabdominal pressure, advanced heart failure (ejection fraction <%35, METS<4, advanced valve disease), were excluded. After standard intravenous anesthesia induction and intubation, ventilation parameters were set to maintain a constant EtCO2 level using the Primus (Draeger) anesthesia machine. The fresh gas flow was set to 1 L/min, and the desflurane vaporizer was set to 18%. Desflurane concentrations in the inspiratory and expiratory limbs were recorded at twenty-second intervals, along with hemodynamic and respiratory parameters. The follow-up was terminated when FiDes≥8% and FeDes≥6% were reached. Data analysis was performed using the R statistical program (version 4.2.2, Vienna R Foundation, Austria). Normal distribution of the data was assessed using the Kolmogorov-Smirnov test. The time to reach the inspiratory limb's desflurane concentration ≥8% and the expiratory limb's desflurane concentration ≥6% was taken as a numerical value in seconds, and body type was grouped as a 3-level ordinal categorical variable. To predict the time to reach the target desflurane concentration in the expiratory limb, a linear regression model including all patient characteristics was planned. With a type 1 error of 0.05, type 2 error of 0.05, power of 0.1, and 10 predictive parameters, the sample size was calculated as 254 using GPower 3.1.9.7. Results: A total of 307 patients were enrolled in the study. The data of 271 patients were analyzed.The time required to meet the conditions was 287±37 seconds in ectomorphic patients, 276±32 seconds in mesomorphic patients, and 266±30 seconds in endomorphic patients. The duration was 20 seconds shorter in females. A negative correlation was found between the time to reach the targeted desflurane concentration in the expiratory limb and age, body weight, and body mass index, while a positive correlation was found with height (Pearson correlation: -0.337, -0.123, -0.268, 0.251 p = 0.0001). The model including somatotype showed higher predictive power (p = 0.01). Conclusion: Somatotype was identified as an independent and effective parameter influencing the time to reach the target concentration in the expiratory limb for desflurane. Considering that the number of patients met the calculated sample size and both dependent and independent parameters were identified, we believe that further research in this regard is unnecessary. Key Words: Low Flow Anesthesia, Inhaled Anesthesia Induction, Somatotype, Desflurane