Cardiopulmonary response to knee-chest position in obese patients undergoing posterior lumbar spine surgery under general anesthesia

ABSTRACT

For spine surgery, placing the anesthetized patient in the prone position increases the risk of improper ventilation. These effects may be more pronounced in obese patients because pressure on the abdominal wall may further accentuate the restrictive nature of the pulmonary disease common in this patient population. In this study, the cardio-pulmonary response to the seated prone [knee-chest] position will be investigated as influenced by the weight of the patients [obese versus normal weight patients]. The changes in respiratory mechanics, gas exchange and hemodynamics will be investigated in patients undergoing elective posterior lumbar spine surgery under general anesthesia. Patients After approval of our local ethics committee and informed consent was obtained forty patients [19 males and 21 females], ASA I or II, who were scheduled for posterior lumbar spine surgery in the knee-chest position under general anesthesia were included in this study. They were randomly allocated into two groups according to body mass index [BMI] which was calculated as weight [kg] divided by the square height [m2]. Group I [20 patients] Non obese patients with normal BMI of <25 kgm- 2. Group II[20 patients] Obese patients with BMI >30 kgm-2. Method: After anesthesia-paralysis, patients were mechanically ventilated using closed circuit anesthesia and were turned into the knee-chest position with the abdomen hanging freely. Patients were continuously monitored for ECG, HR, Sa02, ETCO2 and MAP. The investigated hemodynamics, HR and MAP, were measured prior to induction of anesthesia and 15 min, after induction of anesthesia in the supine position. They were also recorded at 30, 45 and 60 min. after knee-chest positioning. Arterial blood samples for blood gases analysis were withdrawn via a radial artery catheter for measurement of pH, PaCO2, PaO2, SaO2, HCO3 and BE at the previously mentioned periods. The investigated respiratory mechanics pleural pressure, peak airway pressure, static lung compliance and exhaled tidal volume were measured from the BICORE. They were recorded 15 min. after induction of anesthesia in the supine position, also at 30, 45 and 60 min. after knee-chest positioning of the patients. The results of this study showed that As regards hemodynamics, there was a significant decrease in the mean values of HR and MAP at all investigated periods of the study as compared with the mean baseline value found prior to induction of anesthesia in the supine position but no statistically significant changes were found in between both groups. Regarding to the investigated respiratory mechanics, both groups showed an increase in mean value of PP after assuming the knee-chest position as compared with the value found when patients were in supine position but statistically insignificant difference was found when mean value of PP during both positions in both groups compared with each other. There was a significant increase in the mean value of PAP after knee-chest positioning as compared with baseline value in both groups. Also, group II showed higher mean value for PAP than group I at all investigated time intervals. Significant changes were found in-between both groups when the mean values were compared with each other. There was a significant decrease in the mean value of Cst, L after knee-chest positioning of the patients as compared with baseline value and the mean value was found to be statistically lower in group II than in group I at all periods of the study. Both groups of patients showed a progressive decrease in mean value Of EVT after seated prone positioning as compared with baseline value and the second group demonstrated a significantly higher mean value of EVT as compared with the first one at all investigated times of the study. As regards arterial blood gases and acid base status, no statistically significant changes were found in both groups and in-between the two studied groups during all investigated periods of the study. From the present results we can conclude that knee-chest position appeared to have minimal adverse effects on the mechanics of breathing system in anesthetized paralyzed subjects in moderately obese patients as compared with non-obese ones. As in this posture, the abdomen is hanging freely and stretching the diaphragm caudally which may counteract the effects of anesthesia-paralysis