K. Shibutani

Critical level of oxygen delivery after cardiopulmonary bypass.

The relationship between oxygen uptake (&OV0312;O2) and delivery (&OV0312;O2) was examined in 64 patients immediately after cardiopulmonary bypass. In 44 patients with lactate levels below 2.5 mmol/L, &OV0312;O2 decreased proportionally when &OV0312;O2 decreased below 300 ml/min ± m2. At a &OV0312;O2 over this level, &OV0312;O2 plateaued at 105 ± 13 (SD) ml/min ± m2. In a contrasting group of 22 patients with blood lactate levels above 2.5 mmol/L, &OV0312;O2 changes depended on changes in &OV0312;O2 both alone and below 300 ml/min ± m2.

Do changes in end-tidal PCO2 quantitatively reflect changes in cardiac output?

In anesthetized patients, acute decreases in cardiac output (CO) are often reflected as decreases in end-tidal CO2 tension (PETCO2), but the quantitative relationship between the changes in CO and the changes in PETCO2 is uncertain. We hypothesize that a quantitative relationship can be demonstrated if timing of the measurements in each episode of hemodynamic perturbation is standardized. In 24 patients undergoing abdominal aortic aneurysm surgery with constant ventilation, we prospectively performed 33 measurements of CO, PETCO2, and CO2 elimination (VECO2) within 10 min of hemodynamic changes. The percent decrease in PETCO2 directly correlated with the percent decrease in CO (slope = 0.33, r2 = 0.82). Also, the percent decrease in VECO2 correlated with the percent decrease in CO similarly (slope = 0.28, r2 = 0.84). The changes in PETCO2 and VECO2 following hemodynamic perturbation were parallel. This finding suggests that decreases in PETCO2 quantitatively reflect the decreases in CO2 elimination.

Accuracy of pharmacokinetic models for predicting plasma fentanyl concentrations in lean and obese surgical patients derivation of dosing weight: (pharmacokinetic mass)

Background: The currently available pharmacokinetic models for fentanyl were derived from normal weight patients and were not scaled to body weight. Their application to obese patients may cause overprediction of the plasma concentration of fentanyl. This study examined the influence of body weight on the predictive accuracy of two models (Anesthesiology 1990; 73:1091–102 and J Pharmacol Exp Ther 1987; 240:159–66). Further, we attempted to derive suggested dosing mass weights for fentanyl that improved predicted accuracy. Method: Seventy patients undergoing major elective surgery with total body weight (TBW) <85 kg and body mass index <30 (Group L) and 39 patients with TBW ≥85 kg and body mass index >30 (Group O) were studied. In Group L and Group O, the mean TBW was 69 kg, and 125 kg, respectively and the mean body mass index in Group L and Group O was 24 and 44, respectively. Fentanyl infusion was used during surgery and postoperatively for analgesia. Plasma fentanyl concentrations were measured and predicted concentrations were obtained by computer simulation; 465 pairs of measured and predicted values were obtained. Results: The influence of TBW on the performance errors of the original two models was examined with nonlinear regression analysis. Shafer error versus TBW showed a highly significant negative relationship (R squared = 0.689, P < 0.001); i.e., the Shafer model systematically overestimated fentanyl concentration as weight increased. The Scott and Stanski model showed greater variation (R squared = 0.303). We used the exponential equation for Shafer performance error versus TBW to derive suggested dosing weights (“pharmacokinetic mass”) for obese patients. The pharmacokinetic mass versus TBW curve was essentially linear below 100 kg (with slope of 0.65) and approached a plateau above 140 kg. For patients weighing 140 to 200 kg, dosing weights of 100–108 kg are projected. Total body clearance (ml/min) showed a strong linear correlation with pharmacokinetic mass (r = 0.793; P < 0.001), whereas the relationship with TBW was nonlinear. Conclusion: Actual body weight overestimates fentanyl dose requirements in obese patients. Dosing weight (pharmacokinetic mass) derived from the nonlinear relationship between prediction error and TBW proved to have a linear relationship with clearance.

The changes in brain surface, intracerebral tissue, and transconjunctival oxygen tension during hypo- and hyperventilation

To evaluate the validity of organ surface oxygen tension monitoring for assessment of cerebral perfusion, the oxygen tension in brain surface (PbsO2), intracerebral tissue (PicO2), and conjunctiva (PcjO2) were measured simultaneously during hypo- and hyperventilation in dogs, and the comparative study was done. p ]PbsO2 and PicO2 significantly increased during hypoventilation and decreased during hyperventilation. And the values of PbsO2 and PicO2 were correlated to the corresponding PacO2 values significantly (P < 0.001 in each case). On the contrary, PcjO2 did not change significantly during hypo- and hyperventilation.These findings indicate that PbsO2 as well as PicO2 could reflect the changes in cerebral perfusion caused by induced hyper- and hypocapnia but that PcjO2 could not.