Anesthesia and analgesia
-
Anesthesia and analgesia · Feb 2000
Clinical TrialInterference of cerebral near-infrared oximetry in patients with icterus.
Near-infrared spectrophotometry assesses cerebral oxygen saturation (ScO(2)) based on the absorption spectra of oxygenated and deoxygenated hemoglobin and the translucency of biological tissue in the near-infrared band. In patients with icterus, however, bilirubin can potentially hinder cerebral oximetry. In 48 patients undergoing orthotopic liver transplantation, we related total plasma bilirubin to ScO(2) as determined from spectrophotometry with wavelengths of 733 and 809 nm. Before surgery, ScO(2) was 59% (15%-78%) (median with range) and bilirubin was 71 (6-619) micromol/L with a negative correlation (r = -0.72; P < 0.05). The 95% prediction interval included the lowest measurable ScO(2) of 15% at a bilirubin level of 370 micromol/L. During reperfusion of the grafted liver, the ScO(2) increased by 7% (-8% to 17%) (P < 0.05), and bilirubin did not influence this increase. In one patient, the ScO(2) remained below 15% despite a decrease in bilirubin from 619 to 125 micromol/L, suggesting that tissue pigmentation deposits also absorb light. In conclusion, bilirubin dampens the spectrophotometry-determined cerebral oxygen saturation at 733 and 809 nm. A bilirubin level of 370 micromol/L, tissue pigment deposits, or both, may render determination of cerebral oxygen saturation impossible. Even at high bilirubin values, changes in cerebral perfusion may be visible. ⋯ In 48 patients undergoing liver transplantation, the interference of icterus on cerebral oximetry by near-infrared light was investigated. Bilirubin absorbed the near-infrared light and lowered the measured cerebral oxygen saturation. Even at high bilirubin values, changes in cerebral oxygenation, as seen during reperfusion of the grafted liver, may be visible.
-
Anesthesia and analgesia · Feb 2000
Clinical TrialChanges in cerebral blood volume with changes in position in awake and anesthetized subjects.
Changes in posture affect cerebral blood volume (CBV), and moderate head-up tilt is used as a therapeutic maneuver to reduce CBV and intracranial pressure. However, CBV is rarely measured in the clinical setting. Near-infrared spectroscopy allows real-time bedside monitoring of cerebral hemodynamics, and we have used this technique to measure changes in CBV with changes in posture in 10 normal subjects and 10 propofol-anesthetized patients. In the awake subjects, changes in CBV were correlated with the degree of table tilt. CBV decreased with 18 degrees head-up tilt and increased with 18 degrees head-down tilt (P < 0.0001, r = -0.924). In anesthetized patients, there were differences between head-up and head-down tilt. In the head-down position, CBV was also correlated with the degree of table tilt (P < 0.001, r = -0.782), whereas there was a clinically insignificant reduction in CBV in the head-up position. Near-infrared spectroscopy allows continuous, real time measurement of changes in CBV at the bedside. ⋯ Near-infrared spectroscopy, a bedside technique, has been used to measure changes in cerebral blood volume in normal subjects. We have used the same technique in anesthetized patients and have shown that, when a patient is placed in the head up position, the decrease in cerebral blood volume is attenuated, relative to normal subjects.
-
Inhaled nitric oxide (NO) is a selective pulmonary vasodilator used to treat intraoperative pulmonary hypertension and hypoxemia. In contrast to NO delivered by critical care ventilators, NO delivered by anesthesia machines can be complicated by rebreathing. We evaluated two methods of administering NO intraoperatively: via the nitrous oxide (N(2)O) flowmeter and via the INOvent (Datex-Ohmeda, Madison, WI). We hypothesized that both systems would deliver NO accurately when the fresh gas flow (FGF) rate was higher than the minute ventilation (VE). Each system was set to deliver NO to a lung model. Rebreathing of NO was obtained by decreasing FGF and by simulating partial NO uptake by the lung. At FGF > or = VE (6 L/min), both systems delivered an inspired NO concentration ([NO]) within approximately 10% of the [NO] set. At FGF < VE and complete NO uptake, the N(2)O flowmeter delivered a lower [NO] (70 and 40% of the [NO] set at 4 and 2 L/min, respectively) and the INOvent delivered a higher [NO] (10 and 23% higher than the [NO] set at 4 and 2 L/min, respectively). Decreasing the NO uptake increased the inspired [NO] similarly with both systems. At 4 L/min FGF, [NO] increased by 10%-20% with 60% uptake and by 18%-23% with 30% uptake. At 2 L/min, [NO] increased by 30%-33% with 60% uptake and by 60%-69% with 30% uptake. We conclude that intraoperative NO inhalation is accurate when administered either by the N(2)O flowmeter of an anesthesia machine or by the INOvent when FGF > or = VE. ⋯ Inhaled nitric oxide (NO) is a selective pulmonary vasodilator. In a lung model, we demonstrated that NO can be delivered accurately by a N(2)O flowmeter or by a commercial device. We provide guidelines for intraoperative NO delivery.
-
Anesthesia and analgesia · Feb 2000
Comparative StudyComparison of NAD 6000 and servo 900C ventilators in an infant lung model.
We compared the ability of the NAD 6000 (North American Dräger, Telford, PA) and the Servo 900C (Siemens-Elema AB, Solna, Sweden) anesthesia ventilators to maintain precise delivery of small tidal volumes (V(t)) and positive end-expiratory pressure using an infant test lung model. A variety of ventilator and lung model settings were selected to test clinical conditions simulating normal and extremely compromised lung function. Differences in ventilator output were analyzed by using an independent t-test with P <0.05 considered significant. With the ventilators set to deliver a V(t) of 30 mL, the actual delivered V(t) was significantly better for the NAD 6000 (25 +/- 2 mL) compared with the Servo 900C (18 +/- 3 mL), P <0.001. When the ventilators were set to deliver 100 mL V(t), their delivered V(t) were not significantly different, NAD 6000 (66 +/- 19 mL) and Servo 900C (60 +/- 12 mL), P = 0.09. The exhaled V(t) read by the anesthesia machines was significantly closer to the delivered V(t) for the NAD 6000 (11 +/- 9 mL) compared with the Servo 900C (37 +/- 11 mL), P < 0.001. Both ventilators maintained the end expiratory pressure delivered to the test lung within 2 cm H(2)O of the set positive end-expiratory pressure on average. As the conditions changed requiring the ventilator to develop a higher peak inflating pressure, both ventilators showed a decrease in V(t) delivered, which was proportionate to the tubing compression volume loss. ⋯ The NAD 6000 (North American Dräger, Telford, PA) and Servo 900C (Siemens-Elema AB, Solna, Sweden) are able to precisely deliver small Tidal Volumes. They both decreased in performance when tested under extreme conditions. Earlier studies of traditional anesthesia ventilators suggest that the NAD 6000 and Servo 900C are superior pediatric ventilators.
-
Anesthesia and analgesia · Feb 2000
The mechanical properties of intact and traumatized epidural catheters.
Comparative data on the mechanical properties of epidural catheters used clinically are not available. We performed a controlled laboratory investigation to assess the mechanical performance of three different intact or traumatized catheter types (Polyurethane, clear nylon, and radiopaque nylon catheters, designed for 18-gauge Tuohy needles). We studied a control (intact) and two trauma groups (needle bevel and surgical blade). Catheters were loaded to their breaking points by using a Lloyd LS500 material testing machine (Lloyd, Southampton, UK). Maximal load and extension values before breakage were measured, and modulus of elasticity and toughness values were calculated. Intact polyurethane catheters did not break within the limits of the experimental study (extension up to 3 times the original length of a specimen). The toughness values obtained from polyurethane and clear nylon catheters were significantly higher than those for the radiopaque catheters in intact specimens (P < 0.05). In the traumatized groups, polyurethane catheters had the highest toughness values (P < 0.05). Modulus of elasticity values were higher in both control and trauma groups of the radiopaque catheters when compared with the polyurethane and clear nylon catheters, which indicates a higher stiffness to elastic deformation (P < 0.05). In conclusion, polyurethane catheters are the most durable catheter type to tensile loading, either intact or traumatized. Mechanical properties can be used to predict complications related to the clinical use of these catheters. ⋯ Using a computer-assisted material testing machine, we studied the mechanical properties of three different types of epidural catheters, either intact or traumatized, in a blinded, controlled study. This information may be vital to clinicians who implant epidural catheters by helping them choose a catheter that has the lowest probability of failure.