Critical haemoglobin concentration in anaesthetized dogs: comparison of two plasma substitutes.

We have explored systemic and regional tolerance to haemodilution during anaesthesia with two different synthetic colloids. Eighteen dogs undergoing mechanical ventilation during anaesthesia with ketamine were submitted to progressive normovolaemic haemodilution with either gelatin (GEL; n = 9) or hydroxyethylstarch (HES; n = 9) administered on a 1:1 ratio. Systemic oxygen delivery was calculated from measurement of thermodilution cardiac output and arterial oxygen content, while systemic oxygen consumption was determined from expired gas analysis. Mesenteric oxygen delivery and consumption were determined using ultrasonic flow measurements, and arterial and mesenteric venous oxygen contents. The critical haemoglobin concentration (i.e. the haemoglobin value below which oxygen consumption becomes oxygen delivery dependent) was mean 3.6 (SD 0.8) g dl-1 in the GEL and 3.5 (1.5) g dl-1 in the HES group. The mesenteric critical oxygen extraction ratio (O2ER) (GEL 50.1 (12.1)%; HES 48.5 (13.4)%) was significant lower than the systemic critical O2ER (GEL 66.1 (8.4)%; HES 67.7 (7.1)%). There were no significant differences between the GEL and HES groups for any of these variables, or in the amount of colloid administered. During the study, oxygen delivery decreased almost linearly with reduction in haemoglobin, indicating a lack of cardiac output response to anaemia during ketamine anaesthesia.

Administration of propofol by target-controlled infusion in patients undergoing coronary artery surgery.

OBJECTIVES: To study the predictive performance of a target-controlled infusion (TCI) system of propofol in patients undergoing coronary bypass graft (CABG) surgery, using a referenced pharmacokinetic set derived from healthy patients. Also, to determine the propofol concentrations required for clinically acceptable induction and maintenance of anesthesia when combined with midazolam as premedication and a continuous alfentanil infusion and to study the hemodynamic stability of this technique. DESIGN: Prospective noncomparative study analysis. SETTING: Operating room at a university hospital. PARTICIPANTS: Twenty-on patients with good left ventricular function undergoing coronary artery surgery. INTERVENTIONS: Patients were anesthetized using a continuous infusion of alfentanil (mean infusion rate: 1 microgram/kg/min) and propofol administered by TCI. MEASUREMENTS AND MAIN RESULTS: The predictive performance of the TCI system (212 arterial samples) was measured at specified time points before, during, and after bypass. The TCI system underestimated the measured blood propofol concentrations with a bias of +21.2% and +9.6% during the prebypass and the bypass periods, respectively. The predictive inaccuracy, expressed by the median absolute prediction error, was 23% and 18.5%, respectively. Mean target propofol concentrations required to induce and maintain anesthesia before bypass were 0.92 microgram/mL and 3.64 micrograms/mL, respectively. In the period during and after bypass, the mean target concentrations required to maintain anesthesia was 2.22 micrograms/mL. The administration of propofol by TCI was still associated with some short episodes of hemodynamic instability that were easily controlled by adjusting the target concentration in the majority of the patients. Therefore, the overall quality and ease of control of anesthesia were considered as being good or adequate. CONCLUSIONS: In this group of patients undergoing CABG surgery, the TCI system used underestimated the measured propofol concentrations. However, the predictive performance of the selected mean pharmacokinetic parameters derived from healthy patients was acceptable during the whole surgical procedure.

Detection of tissue hypoxia by arteriovenous gradient for PCO2 and pH in anesthetized dogs during progressive hemorrhage.

The present study tested the hypothesis that, during acute bleeding, the development of tissue hypoxia might be reflected by an abrupt widening in arteriovenous gradient for PCO2 (AV PCO2) and for pH (AV pH) as accurately as by an increase in blood lactate levels. Twenty-four anesthetized (isoflurane 1.4% end-tidal), paralyzed, and mechanically ventilated dogs submitted to progressive hemorrhage were studied. Oxygen uptake (VO2) was derived from expired gas analysis and oxygen delivery (DO2) was calculated by the product of the thermodilution cardiac index and the arterial O2 content. During the first part of the protocol, VO2 remained stable as the progressive reduction in DO2 was associated with a corresponding increase in O2 extraction (O2ER). Blood lactate increased slightly but not significantly. AV PCO2 and AV pH increased significantly, essentially related to venous respiratory acidosis. The critical value of DO2 below which VO2 decreased was 8.95 +/- 1.60 mL.min-1.kg-1. Below this value, there was a marked increase in blood lactate and an abrupt widening in AV PCO2 and AV pH gradients. The critical value of DO2 obtained from blood lactate, AV PCO2 and AV pH were similar to those obtained from VO2 (8.60 +/- 1.12; 8.73 +/- 1.40; 8.78 +/- 1.37, respectively; P = not significant). A significant correlation was found, during the hemorrhage protocol, between blood lactate and AV PCO2 (r = 0.84; P < 0.001) or AV pH (r = 0.78; P < 0.001). Therefore, AV PCO2 and AV pH represent simple but reliable indicators of tissue hypoxia during hemorrhagic shock.