Changes Of Oxygen Consumption And Metabolic Heat In Anesthetized Adult Humans With Isoflurane

IntroductionUnintended hypothermia occurs frequently during surgery and may have adverse effects on the cardiovascular system. Recent prospective randomized trials have shown that mild hypothermia is associated with numerous adverse outcomes, such as morbid cardiac events,[1] postoperative ventricular tachycardic,[1] higher plasma norepinephrine concentration[2] etc.The present study was to evaluate the extent to which core hypothermia decreases after induction of anesthesia, and changes of mean skin temperature, also to examine the changes of oxgen consumption and metabolic heat.Experiment materials and methodPatients: We studied 40 patients of neurosurgery selective operations, aging from 20 to 67 years , ASA I ~ II ,13 cases of meningioma, 16 cases of glioma, 9 cases of pituitary tumor, 2cases of aneurysm. None was obese, or had a history of thyroid disease, dysautonomia, or Raynaud's syndrome.Anesthesia Technique: The thermostat in the operating room was set to 21 C . All patients received their usual chronic medications (e.g.,antihypertensives, antianginals) before surgery. All patients were injected atropine 0.5mg and luminal 0.1g 30 minutes before operation, induced by fentanyl 5 g/kg, midazolam 40 g/kg, pipecuronium 0.1~0.16mg/kg and propofol 1.5~2.25mg/kg. After trachea intubation thepatients were ventilated mechanically with inspiration of 0.6-1.6% isoflurane and N2O/O2 1:1. Pipecuronium was injected 0.04mg/kg intermittently to maintain muscular relaxation. Resperation rate was 12/min, tidal volume was 8~10ml/kg,I:E was 1:2.MeasurementsHeart rate,blood pressure,pulse oxygen saturation and electrocardiogram was measured using Datex Ohmeta S/5 monitor. Nasopharynx temperature was measured using Mallinckrodt temperature monitor. Skin surface temperatures were measured using Datex Ohmeta S/5 monitor on the anterior chest, the lateral mid-upper arm, the lateral mid-thigh, and the lateral mid-calf. Mean skin temperature was calculated using the following formula: 0.3(chest + upper arm) + 0.2(thigh + calf).[3] The CO2 concentration at the end of expiration, the expiration volume per minute(Ve), the O2 concentration of inspiration and expiration(FiO2 and FeO2) were measured using Datex gas monitor. Oxygen consumption (ml/min)=Ve X ( FiO2 - FeO2 ) , metabolic heat (Kcal/h)=oxygen consumption (ml/min) X 60min X 4.82Kcal/l000ml. The data were recorded before induced time, immediately after induced time, 10 minute, 20 minute, 30 minute, 40 minute, 50 minute, 60 minute, 90 minute, 120 minute, 150 minute and 180 minute after induced time,Data analysisData are expressed as mean sd. Database were established by SPSS statistic software package. Data was analysised by ANOVA. A level of P<0.05 was considered statistically significant.ResultsAfter induced core temperature descended gradually, and reached the lowest at drug withdrawal. From then on the core tempetature steped up gradually, and approached a level of preopration. Oxygen consumption and heat production began to descend after induced, reached the lowest at about 3 hour after induced and stepped up after drug withdrawal. At 20 min after drug withdrawal they approached the level before induced.DiscussionThe study result indicated that in the anesthesia period the temperature plateau was caused by the contraction of peripheral vessels which leaded to reduced thermolysis. After induced the patients' oxygen consumption and heat production began to descend which indicated that increased peripheral heat is due to re-distribution of body heat.