Recovery following outpatient anesthesia: Use of enflurane versus propofol

Study Objective: To compare the intraoperative conditions and postoperative recovery of patients following the use of either propofol-nitrous oxide (N₂O) or enfurane-N₂O for maintenance of outpatient anesthesia.

Design: Randomized, single-blind study.

Setting: University hospital outpatient surgery center.

Patients: 61 ASA physical status I and II, healthy female outpatients undergoing laparoscopic surgery.

Interventions: Patients were randomly assigned to one of three anesthetic regimes. Group 1 (control) received thiopental sodium 4 mg/kg intravenously (IV), followed by 0.5% to 1.5% enfurane and 67% N₂0 in oxygen (O₂). Group 2 received propofol 2 mg/kg IV, followed by 0.5% to 1.5% enfurane and 67% N₂O in O₂. Group 3 received propofol 2 mg/kg IV, followed by propofol 50 to 160 μg/kg/min IV and 67% N₂O in O₂. All patients received succinylcholine 1 mglkg IV to facilitate tracheal intubation and atracurium 10 to 20 mg IV to provide adequate relaxation during the maintenance period.

Measurements and Main Results: Recovery from anesthesia was assessed by a research nurse who was unaware of the anesthetic technique used. The mean ± SD time to eye opening was significantly longer in the thiopental-enflurane-N₂O group (Group 1) than in the propofol-propofol-N₂0 group (Group 3) (6.1 ± 2.5 minutes vs. 3.5 ± 2.8 minutes, respectively). In addition, the mean time to respond to verbal commands was significantly shorter in the propofol induction groups compared with the thiopental induction group. However, the use of enfurane versus propofol for maintenance of anesthesia did not significantly prolong the time from arrival in the recovery room to sitting, tolerating oral fluids, walking, or being judged “fit for discharge.” There were no differences among the three groups with respect to postoperative pain or analgesic requirements. Finally, patients who received enfurane for maintenance of anesthesia had a significantly higher frequency of nausea and vomiting than the propofol maintenance group.

Conclusion: Induction of anesthesia with propofol is associated with a more rapid emergence from anesthesia than induction with thiopental. Maintenance of anesthesia with enfurane did not prolong recovery compared with maintenance with propofol, but enfurane was associated with increased frequency of postoperative nausea and vomiting.     

Nasal continuous positive airway pressure: An alternative method for respiratory assistance

Study Objective: To examine the efficacy of a nasal continuous positive airway pressure (CPAP) system for respiratory support in patients who have respiratory insufficiency but are able to maintain spontaneous breathing without hypercapnia, respiratory acidosis, or deteriorated mental status.

Design: Prospective study.

Setting: Medical and surgical patients admitted to the intensive care unit (ICU) at the Hillel Yaffe Medical Center.

Patients: Nineteen patients with acute respiratory insufficiency and intact mental status who were able to maintain spontaneous breathing without hypercapnia or respiratory acidosis. Additional entry criteria were as follows: arterial oxygen tension (PaO₂) < 65 mmHg on inspired oxygen tension (FIO₂) ≥0.45, PaO₂/FIO₂ <150, respiratory rate >35 breaths/minute, and inability to tolerate mask CPAP.

Interventions: Nasal CPAP (10 cmH₂O) was applied to patients through two nasopharyngeal airways with an internal diameter (ID) of 8 mm each, inserted in both nostrils. During CPAP application, the patients were requested to breathe through their nose with their mouth closed. Even if they breathed through their open mouth, however, CPAP was maintained despite an observed pressure decrease of 4 cmH₂O.

Measurements and Main Results: All patients showed a constant improvement in arterial blood gases, PaO₂/FIO₂, and respiratory signs during nasal CPAP of 10 cmH₂O. PaO₂ increased from 52 ± 5.3 mmHg to 131 ± 20 mmHg with CPAP administration (p < 0.05), while arterial carbon dioxide tension (PaC02) increased from 32 ± 2 mmHg to 36 ± 2 mmHg (p < 0.05) and respiratory rate decreased from 39 ± 2.3 breaths/minute to 31 ± 1.6 breaths/minute (p < 0.05).

Conclusions: Nasal CPAP (10 cmH₂O) is a reliable alternative to support arterial oxygenation in patients with respiratory failure who are alert and vigorous enough to avoid hypercapnia and respiratory acidosis while breathing spontaneously. In addition, since the patients are able to speak and thus are capable of expressing their feelings, the anxiety observed during respiratory support can be reduced.     

Duration and recovery profile of cisatracurium after succinylcholine during propofol or isoflurane anesthesia

Study Objective: To determine the duration and recovery profile of maintenance doses of cisatracurium besylate following succinylcholine, and during propofol or isoflurane anesthesia.

Design: Randomized, open-label study.

Setting: Operating suite of a university-affiliated medical center.

Patients: Forty ASA physical status I and II adult patients having elective surgery with general anesthesia lasting longer than 90 minutes.

Interventions: Following a standardized induction sequence, a baseline electromyogram (EMG) was obtained. An intubating dose of intravenous (IV) succinylcholine 1.0 mg/kg was administered. Ventilation was maintained with a face mask until the first twitch (T₁) of the evoked train-of-four (TOF) reached 10% of control when tracheal intubation was performed. Spontaneous recovery from neuromuscular blockade was allowed to occur until the first twitch returned to 25% of control. Patients then were randomized to receive cisatracurium as follows. Group 1: 0.025 mg/kg [0.5 × 95% effective dose (ED₉₅)]; Group 2: 0.05 mg/kg (ED₉₅); Group 3: 0.05 mg/kg (ED₉₅); and Group 4: 0.1 mg/kg (2×ED₉₅). Anesthesia for Groups 1 and 2 were maintained with isoflurane 1% to 2%, 66% nitrous oxide (N₂O) in oxygen (O₂), and in Groups 3 and 4, anesthesia was maintained with propofol 80 to 160 μg/kg/min, 66% N₂O in O₂. The TOF-evoked EMG was recorded at 10-second intervals. The time for the evoked EMG to spontaneously return to 25%, 50%, and 75% of the original baseline was recorded.

Measurements and Main Results: There were 10 patients in each of the four groups. The duration of action of cisatracurium 0.05 mg/kg (ED₉₅) after an intubating dose of succinylcholine is 24.5 ± 10 minutes and 21.3 ± 9 minutes during anesthesia maintained with isoflurane and propofol, respectively. Doubling the dose of cisatracurium resulted in approximately twice the duration of action (40.2 ± 7 min) during propofol anesthesia. Following a dose of cisatracurium 0.025 mg/kg (0.5×ED₉₅), the T₁ of the EMG-evoked response did not decrease below 25% in 7 of 10 patients.

Conclusion: Following succinylcholine, the duration of action of a single dose of cisatracurium 0.05 mg/kg is 20 to 25 minutes during anesthesia maintained with propofol or isoflurane. The duration and recovery profile of cisatracurium is dose dependent during propofol and isoflurane anesthetics. Cisatracurium 0.025 mg/kg is an inadequate maintenance dose following recovery from succinylcholine and it fails to provide adequate surgical relaxation.     

Heated breathing tubes affect humidity output of circle absorber systems

Study Objective: To verify whether the airway climate in circle systems can be improved with heated breathing tubes.

Design: Randomized, controlled, prospective clinical study.

Setting: Operating theater of the Department of Maxillofacial Surgery.

Patients: 26 adult patients undergoing prolonged anesthesia.

Interventions: A total of 26 prolonged anesthetics were conducted in adult patients using a minimal fresh gas flow rate (0.6 L/min) and silicon breathing tubes (16 mm internal diameter) containing a heated coil. Group 1 (n = 10 patients) was the control group; breathing tubes were unheated. In Group 2 (n = 10 patients), breathing tubes were heated to 30°C. In Group 3 (n = 6 patients), breathing tubes were heated to 36°C.

Measurements and Main Results: Humidity and temperature were measured at the Y-piece. Inspiratory temperature in Group 2 was significantly higher than in Group 1. In Group 3, both inspiratory temperature and absolute humidity were significantly higher than in Group 1. After 5 minutes of ventilation, water content and temperature of inspiratory gases were significantly higher in Group 3 than in Group 1.

Conclusion: Low flow systems need at least 120 minutes to achieve a satisfactory airway climate. Heated breathing tubes effectively reduce this delay.     

Predictable nitrous oxide uptake enables simple oxygen uptake monitoring during low flow anaesthesia

The uptake rate of oxygen and nitrous oxide were studied during low flow anaesthesia with enflurane or isoflurane in nitrous oxide with either spontaneous or controlled ventilation. The excess gas flow and composition were analysed. The nitrous oxide uptake rate was in agreement with Severinghaus'formula _N20 1000.t^-0.5. The composition of excess gas was predictable and the following formula for oxygen uptake could be derived: O₂=fgO₂ -0.45 (fgN²O -(kg: 70.1000.t^-0.5)) where oxygen uptake rate (O₂, ml.min^-1) equals oxygen fresh gas flow (fgO₂) minus 0.45 times the difference between the fresh gas flow of nitrous oxide (fgN₂O), ml.min^-1 and estimated uptake of nitrous oxide. The equation assumes constant inspired gas concentrations of 30% oxygen and 65–70% nitrous oxide. The oxygen uptake rates calculated from this formula were in good agreement with measured uptake rates. Thus, continuous monitoring of oxygen uptake rates is possible by using only reliable flowmeters and analysis of inspried oxygen concentration.     

Total Arteriovenous CO2 Removal: Simplifying Extracorporeal Support for Respiratory Failure

Background. To reduce the complexity, complications, and cost of conventional extracorporeal membrane oxygenation, we have developed a technique of simplified arteriovenous extracorporeal CO₂ removal (AVCO₂R) with a low-resistance membrane gas exchanger for total CO₂ removal to provide lung rest in the setting of severe respiratory failure.

Methods. We initially used AVCO₂R in healthy animals to quantify the gas exchange capabilities of the system and establish ventilator management protocols for the subsequent studies of AVCO₂R in a large animal model of respiratory failure secondary to a severe smoke inhalation injury.

Results. In healthy sheep the maximum spontaneous arteriovenous flow ranged from 1,350 to 1,500 mL/min, whereas CO₂ removal plateaued at a blood flow of approximately 1,000 mL/min in which 112 ± 3 mL/min CO₂ was removed, allowing an 84% reduction in the minute ventilation of from 6.9 ± 0.8 L/min to 1.1 ± 0.4 L/min (p < 0.01) without triggering hypercapnia. A subsequent reduction in extracorporeal flow at a reduced minute volume led to the development of hypercapnia only if it decreased to less than 500 mL/min. We also applied AVCO₂R in mechanically ventilated sheep with a severe smoke inhalation injury and removed 95% (111 ± 4 mL/min) of the total CO₂ production. This allowed the minute ventilation to be reduced by 95% and the peak inspiratory pressures by 52% (both p < 0.05) over 6 hours and produced no adverse hemodynamic effects. The partial pressure of arterial oxygen was maintained above 100 mm Hg at a maximally reduced minute volume. The mean AVCO₂R flow was 1,213 ± 29 mL/min, averaging 27% ± 1% of the cardiac output.

Conclusions. We conclude that AVCO₂R in a simple arteriovenous shunt is a less complicated technique than extracorporeal membrane oxygenation and is capable of total CO₂ removal that allows a significant reduction in the minute ventilation and peak airway pressure during severe respiratory failure.     

The induction, maintenance, and recovery characteristics of spinal versus general anesthesia in elderly patients

Study Objective: To compare the induction and recovery profiles of three combinations of general anesthesia when used as an alternative to spinal anesthesia for elderly patients.

Design: Randomized, prospective, open-label study.

Setting: Large referral hospital.

Patients: 100 [ASA physical status I, II, and III] patients over 60 years of age undergoing brief transurethral surgery.

Interventions: In Groups Propofol-Propofol (P-P), Propofol-Isoflurane (P-I), and Propofol-Desflurane (P-D), anesthesia was induced with fentanyl (1 to 2 μg/kg IV) and propofol (1.0 to 2.0 mg/kg IV) and maintained with 70% nitrous oxide in oxygen and either a propofol infusion (75 to 150 μg/kg/min) or isoflurane (end-tidal 0.7% to 1.2%) or desflurane (end-tidal 1% to 4%), respectively. After induction, a laryngeal mask airway was placed and spontaneous ventilation was maintained. In Group Spinal (S), 1.5 ml 4% lidocaine (60 mg), in an equal volume of 10% dextrose, was administered intrathecally.

Measurements and Main Results: Induction and recovery characteristics were compared. Induction with propofol was technically easier and significantly (medp < 0.0001) faster (4.6 ± 1.7 min, 4.7 ± 2.2 min, and 3.8 ± 1.4 min for Groups P-P, P-I, and P-D, respectively) than induction of spinal anesthesia (9.3 ± 3.4 min). During the induction period, mean arterial blood pressure and heart rate were significantly higher in Group S. Emergence, extubation, and orientation times were similar among the general anesthesia treatment groups. In Group S, patient-generated pain scores were lower (p < 0.05) and recovery room admission longer (p < 0.001). Time to return to baseline digit symbol substitution test (DSST) scores was marginally improved in Groups P-P and P-D when compared to Group P-I. Postoperative nausea, sleepiness, anxiety, and coordination were unaffected by the treatment modality.

Conclusion: General anesthesia with propofol and desflurane facilitates shorter induction and recovery times without adversely affecting patient comfort. Therefore, this technique may be preferable to spinal anesthesia for elderly patients undergoing short transurethral surgical procedures.     

Epidural block for obstetrics: Comparison of bolus injection of local anesthetic with gravity flow technique

Study Objective: To test the hypothesis that slow administration of local anesthetic into the epidural space by gravity flow reduces the incidence of signs and symptoms of unintended injection.

Design: Prospective, randomized study.

Setting: Teaching hospital.

Patients: 600 ASA physical status I and II parturients scheduled for labor and delivery or elective cesarean section.

Interventions: After identification of the epidural space with pulsations of an air-fluid column, parturients for vaginal delivery (n = 380) were randomized to receive a test dose of 3 ml 3% 2-chloroprocaine with epinephrine 20 μg, two doses of 7 ml bupivacaine 0.03 % with sufentanil 1 μg/ml and epinephrine 2 μg/ml by either gravity flow (Group 1) given over 30 seconds or by bolus injection (Group 2) given over 5 seconds through the epidural needle; parturients for Cesarean delivery (n = 220) were randomized to receive a test dose and two doses of 6 ml lidocaine 2 % with sufentanil 1 μg/ml and epinephrine 2 μg/ml by either gravity flow or by bolus injection through the epidural needle. Changes in maternal heart rate (HR) and blood pressure, signs of intravascular injection, and adverse effects of epidural bupivacaine-sufentanil were recorded after each dose.

Measurements and Main Results: Gravity flow administration (Group 1) was associated with a smaller increase in mean maternal HR (p < 0.001), less hypotension (p < 0.01), sedation (p < 0.01), nausea (p = 0.01), and segmental spread (p < 0.0001) than were corresponding doses given by traditional bolus injection (Group 1) for vaginal or Cesarean deliveries. The incidence of systemic toxicity was zero of 300 (0%) with gravity flow and 4 of 300 (1.3%) by bolus injection, p = 0.12, Fisher's exact test. No patient in either group had an accidental intrathecal injection.

Conclusion: Gravity flow administration of local anesthetic-opioid solution during epidural block for obstetrics was associated with fewer signs of systemic drug absorption and cardiovascular perturbations than was the traditional bolus injection. This study supports the current opinion that slow administration of local anesthetic during epidural black contributes to fewer adverse events.     

The quotient end-tidal/inspired concentration of sevoflurane in a low-flow system

STUDY OBJECTIVE: To investigate the effect of two different fresh gas flows on inspired and end-tidal sevoflurane concentration for a given vaporizer setting in a low-flow anesthesia system. DESIGN: Prospective clinical study. SETTING: Department of Anesthesiology of a university teaching hospital. PATIENTS: 56 ASA physical status I and II patients without systemic diseases, having elective surgery with an expected anesthesia time of at least 120 minutes. INTERVENTIONS: Patients were randomly assigned to receive either 1.0 or 2.0 L/min fresh gas flow with the vaporizer setting fixed at 2% sevoflurane. The inspired (In), end-tidal (Et), and Et/In ratio sevoflurane concentrations were estimated. MEASUREMENTS AND MAIN RESULTS: After 120 minutes of sevoflurane anesthesia the inspired and end-tidal sevoflurane concentration were 1.45 +/- 0.10% versus 1.28 +/- 0.12% (p < 0.001) in the 1.0 L/min group and 1.64 +/- 0.08% versus 1.46 +/- 0.11% (p < 0.001) in the 2.0 L/min group. The ratio end-tidal and inspired concentrations/vaporizer setting was 0.64 +/- 0.06 and 0.73 +/- 0.05 in the 1.0 L/min group versus 0.73 +/- 0.05 and 0.82 +/- 0.04 in the 2.0 L/min group. For the ratio inspired and end-tidal/vaporizer setting there were significant difference between the groups (p < 0.001). The estimated ratio end-tidal/inspired was 0.88 +/- 0.04 in the 1.0 L/min group versus 0.89 +/- 0.04 in the 2.0 L/min group (ns). CONCLUSION: After 120 minutes of sevoflurane anesthesia at a vaporizer setting of 2% there is a significant difference between fresh gas flow of 1.0 and 2.0 L/min for inspired and end-tidal concentrations, but not for the ratio end-tidal/inspired.     

Oxygen and Carbon Dioxide Concentrations in Anaesthetic Circuits Ventilated with an AGA UV 705 in an Oxygen Consuming Model Lung

An oxygen consuming and carbon dioxide producing model lung was used for evaluation of anaesthetic circle systems. Two different circles, the AGA metal circle and the AGA Monosorbcircle ventilated hy an AGA UV-705 were tested with reference to inspired oxygen concentrations and end-tidal CO₂ levels. Cartmn dioxide is dependent on alveolar ventilation and CO₂ production and thus was stable throughout the experiments. The oxygen concentration of inspired gas decreased progressively with decreasing fresh gas flow. When using the fresh gas inlet on the ventilator outlet block, the inspired oxygen concentration fell 8% as compared to using the fresh gas inlet in the circle. Fresh gas flows under 3 1/min should only be used with oxygen monitoring in the circle. The fresh gas inlet must be directly into the circle, and not in the ventilator outlet block, to avoid hypoxia.     

Hemodynamic effects of inspired carbon dioxide after the Norwood procedure

Background. Mortality in the early postoperative period after the Norwood procedure remains substantial. Inspired carbon dioxide (CO₂) has been suggested to improve hemodynamic status in this setting. Inspired CO₂ can be delivered by one of two strategies, ie, with or without an accompanying increase in minute ventilation. The hemodynamic effects of these two strategies have not previously been studied in a controlled fashion.

Methods. Seventeen infants (median age, 9 days; range, 4 to 49 days) undergoing Norwood procedures were prospectively enrolled in this crossover study. Patients were studied while sedated, paralyzed, and mechanically ventilated 1 day to 6 days after operation. The inspired oxygen fraction was kept constant (mean value, 0.24 ± 0.01). Measurements were made at five time points: 1 = baseline; 2 = inspired CO₂ with increased ventilation; 3 = baseline; 4 = inspired CO₂ alone; and 5 = baseline. Mixed venous oxygen saturation was monitored using indwelling lines in the superior vena cava.

Results. Inspired CO₂ with increased ventilation produced a rise in mean airway pressure with no change in arterial CO₂ tension or pH. This strategy had no effect on hemodynamic status or oxygen delivery. Inspired CO₂ alone produced a rise in arterial CO₂ tension and a fall in arterial pH (respiratory acidosis). This strategy resulted in significant improvement in both variables of systemic oxygen delivery: mixed venous oxygen saturation increased from 48% ± 2% to 56% ± 2% (p < 0.05), and arteriovenous oxygen saturation difference decreased from 3% ± 2% to 26% ± 2% (p < 0.05).

Conclusions. Inspired CO₂ after the Norwood procedure can improve oxygen delivery. This improvement occurs only if minute ventilation is kept constant. There is no improvement if minute ventilation is increased. Clinical use of inspired CO₂ may be limited by the accompanying fall in pH. Differentiation of cerebral from total-body effects of inspired CO₂ will require further study.     

Desflurane potentiates atracurium in humans: A comparative study with isoflurane

Study Objective: (1) To evaluate the neuromuscular effects of desflurane and its interactions with atracurium and (2) to compare desflurane and isoflurane in these effects.

Design: Sequential entry of informed and consenting patients randomly assigned to receive desfurane (n = 25) or isofurane (n = 25).

Setting: Operating suite of a county-university medical center.

Patients: Fifty adults, ASA physical status I, undergoing elective orthopedic surgery.

Interventions: Following establishment of steady desfurane or isofurane anesthesia, at 1.25 minimum alveolar concentration (MAC) exhaled for 15 minutes, a randomly predetermined dose of atracurium (0.05, 0.1, or 0.15 mg/kg) was injected intravenously (IV). At the end of surgery, neostigmine 0.04 mglkg IV was given to reverse the residual block. The neuromuscular effects of desfurane or isofurane alone, and the dose-response relationship, time course, and reversibility of the neuromuscular effects of atracurium with either anesthetic, were examined in detail and compared using electromyographic quantification of the response of the first dorsal interosseous muscle to train-of-four (TOF) stimulation of the ulnar nerve.

Measurements and Main Results: TOF fade and depression of the first response (T₁) of the TOF were measured in response to desfurane or isofurane, atracurium, and neostigmine. Desf urane caused more TOF fade than isofurane prior to atracurium administration. The TOF ratios were 0.91 ± 0.02 and 0.98 ± 0.01, respectively (p < 0.05). For other measured neuromuscular parameters, atracurium-induced depression tended to be greater in the presence of desfurane than in the presence of isofurane, but none of the measured differences reached the statistical significance level of p < 0.05. The ED₅₀, ED₉₅, and 25–75% recovery index of atracurium were 0.038 mg/kg (95% confidence level; range 0.030 to 0.047 mg/kg), 0.11 mg/kg (0.095 to 0.14 mg/kg), and 31 ± 4 minutes (means ± SEM) with desfurane anesthesia, versus 0.043 mg/kg (0.035 to 0.052 mg/kg), 0.13 mg/kg (0.11 to 0.16 mg/kg), and 23 ± 4 minutes with isofurane anesthesia (p = 0.1-0.2). Continuation of either anesthetic at 1.25 MAC prevented complete recovery of neuromuscular functions spontaneously or following neostigmine 0.04 mg/kg.

Conclusion: In ASA physical status I adults, 9% desfurane has neuromuscular effects equal to or slightly in excess of those of 1.6% isofurane.     

Duration of carbon dioxide absorption by soda lime at low rates of fresh gas flow

STUDY OBJECTIVE: To determine the impact of a low fresh gas flow rate on the duration of carbon dioxide (CO2) absorption by soda lime. DESIGN: Nonclinical, experimental. SETTING: Experimental laboratory. METHODS: In vitro test with Sodasorb and a semiclosed breathing circle ventilating a test lung with a CO2 inflow of 250 ml per minute. Fresh gas flow rates of 0.25, 0.5, 1, 2, and 4 L/min were studied. MEASUREMENTS AND MAIN RESULTS: CO2 was measured at the breathing circuit test lung interface with a mainstream capnometer. Duration of CO2 absorption was determined as the time for the inspired CO2 tension (PICO2) to increase from 0 mm to 7 mm of mercury. The times of this interval were recorded four times for each fresh gas flow rate and compared by analysis of variance; p less than 0.05 was considered significant. Time to soda lime failure was significantly longer at 2 L/min than at 1 L/min fresh gas flow and at 1 L/min than at 0.25 L/min fresh gas flow. CONCLUSION: Because soda lime color indicators are unreliable, when a semiclosed breathing circle is used at a low rate of fresh gas flow without CO2 monitoring, the CO2 absorbent must be replaced more frequently.     

Low flow anesthesia at a fresh gas flow of 10 ml.kg-1.min-1 for hours using time-cycled ventilator]

Low flow anesthesia (LFA) at a fresh gas flow (FGF) level of 10 ml.kg-1.min-1 with oxygen flow set at 0.5 ml.kg-1.min-1: 0.5 ml.kg-1.min-1 nitrous oxide and 3% isoflurane was performed using time-cycled ventilator on 10 patients of ASA class I or II, with age of 55 +/- 13 (mean +/- SD) years and body weight of 55 +/- 10 kg for 5 h. Excessive anesthetic gases from the anesthesia gas monitor were led to an expiratory breathing tube. After rapid induction and tracheal intubation, denitrogenation was performed for about 5 min using a 100% oxygen flow of 6 l.min-1 before LFA. The inspired/expired oxygen concentration decreased gradually from 96 +/- 2%/90 +/- 2% at beginning of LFA to 42 +/- 3%/37 +/- 4% at 5 h. The operation was started after 29 +/- 10 min of beginning of LFA. The nitrous oxide concentration reached 37 +/- 4%/35 +/- 4% at the beginning of operation and further increased to 55 +/- 3%/53 +/- 3% at 5 h. The isoflurane concentration reached 1.0 +/- 0.1%/0.8 +/- 0.1% at the beginning of operation and further increased to 1.2 +/- 0.1%/1.0 +/- 0.1% at 5 h. The anesthetic potency was 1.2 +/- 0.1 MAC/1.0 +/- 0.2 MAC at the beginning of operation. The isoflurane vaporizer setting was changed only once in two cases from 3% to 2% exceeding 1.5% in inspired concentration. There was no need to change the flow of oxygen and nitrous oxide for 5 hrs. No SpO2 lower than 95% was observed during this study. This method is a clinically safe, easily applicable anesthesia method and used the smallest FGF reported in LFA without occurrence of low FIO2.     

Correlations Between Brain Tissue Oxygen Tension, Carbon Dioxide Tension, pH, and Cerebral Blood Flow-A Better Way of Monitoring The Severely Injured Brain?

Background

The ideal method for monitoring the acutely injured brain would measure substrate delivery and brain function continuously, quantitatively, and sensitively. We have tested the hypothesis that brain pO₂, pCO₂, and pH, which can now be measured continuously using a single sensor, are valid indicators of regional cerebral blood flow (CBF) and oxidative metabolism, by measuring its product, brain pCO₂.

Methods

Twenty-five patients (Glasgow Coma Score ≤ 8) were studied. A Clark electrode, combined with a fiber optic system (Paratrend 7, Biomedical Sensors, Malvern, PA) was used to measure intraparenchymal brain pO₂, pCO₂, and pH. Data were averaged over a 1-h period before and after CBF studies. Regional CBF was measured around the probe, using stable xenon computed tomography. Regression analyses and Spearman Rank tests were used for data analysis.

Results

Regional CBF and mean brain pO₂ were strongly correlated (r = 0.74, p = 0.0001). CBF values < 18 mL/100 g/min were all accompanied by brain pO₂ ≤ 26 mm Hg. The four patients with a brain pO₂ < 18 mm Hg died. Brain pCO₂ and pH, however, were not correlated with CBF (r = 0.36, p = 0.24 and r = 0.30, p = 0.43, respectively).

Conclusions

Until recently, substrate supply to the severely injured brain could only be intermittently estimated by measuring CBF. The excellent intra-regional correlation between CBF and brain pO₂, suggests that this method does allow continuous monitoring of true substrate delivery, and offers the prospect that measures to increase O₂ delivery (e.g., increasing CBF, CPP, perfluorocarbons etc.) can be reliably tested by brain pO₂ monitoring.     

Evaluation of a technique of nitrogen insufflation to decrease plastic endotracheal tube cuff flammability during carbon dioxide laser surgery

Study Objective: To determine whether nitrogen insufflation reduces the laser-induced combustibility of polyvinyl chloride (PVC) endotracheal tube cuffs.

Setting: Research laboratory of a metropolitan, university-affiliated medical center.

Design and Interventions: A plastic catheter was fastened along the shafts of 10 PVC endotracheal tubes with self-adhesive copper foil tape down to a level just above the cuff. The modified endotracheal tubes were inserted into graduated cylinders and flushed with oxygen. The cuffs were then inflated with air, and a carbon dioxide (CO₂) laser was aimed at them. Five of the modified endotracheal tubes had 10 L/min of nitrogen insuffated via the plastic tube.

Measurements and Main Results: The laser ignited only 1 of the cuffs insufflated with nitrogen; however, all 5 of the modified endotracheal tubes that were not insufflated burned (p < 0.05).

Conclusion: Nitrogen insufflation decreases CO₂ laser-induced PVC endotracheal tube cuff combustibility. However, clinical applications of this technique should be undertaken with caution, as the administration of a hypoxic mixture may be possible.     

Responses of respiratory drive and breathing pattern to inspiratory loading during nitrous oxide and isoflurane sedation

Background: Increased inspiratory resistance in combination with mild gas narcosis is common during recovery after a general anesthesia, but there are only few previous studies on inspiratory loading during subanesthetic gas narcosis.
Methods: Responses of respiratory drive (central inspiratory activity P_0.1) and ventilatory pattern to an inspiratory threshold load of –6 cm H₂O were studied in 16 healthy subjects during mild subanesthetic gas narcosis. One group (n=9) was exposed to 13, 26 and 39% nitrous oxide (N₂O) and air control (Group N). Another group (n=7) was exposed to 0.1, 0.2 and 0.3% isoflurane and air control (Group I). Measurements were done after 1 min adaptation to the load.
Results: Nitrous oxide and isoflurane had no effect on respiratory drive and V_T either during unloaded breathing or during inspiratory threshold loading. Across all gas concentrations (including 0% control), inspiratory threshold loading resulted in significant P_0.1 increases, amounting to 62% in group N and 38% in group I. At the same time V_T decreased by 11 and 12%, respectively. A significantly increased end-expired CO₂ and decreased minute volume compared to air control was found during isoflurane inhalation but could be ascribed to normalization of the hyperventilation in the control situation.
Conclusions: It is concluded that the steady-state ventilatory responses to loading, consisting of increased P_0.1 and decreased V_T, are maintained during inhalation of subanesthetic doses of N₂O (0.13–0.38 MAC) and isoflurane (0.09–0.26 MAC).     

Superiority of magnesium cardioplegia in neonatal myocardial protection

Background. We have shown that magnesium can offset the detrimental effects of normocalcemic cardioplegia in hypoxic neonatal hearts. It is not known, however, whether magnesium offers any additional benefit when used in conjunction with hypocalcemic cardioplegia.

Methods. Twenty neonatal piglets underwent 60 minutes of ventilator hypoxia (FiO₂ 8% to 10%) followed by 20 minutes of normothermic ischemia on cardiopulmonary bypass (hypoxic-ischemic stress). They then underwent 70 minutes of multidose blood cardioplegic arrest. Five (Group 1), received a hypocalcemic (Ca⁺² 0.2 to 0.4 mM/L) cardiologic solution without magnesium, whereas in 10, magnesium was added at either a low dose (5 to 6 mEq/L, Group 2) or high dose (10 to 12 mEq/L, Group 3). In the last 5 (Group 4), magnesium (10 to 12 mEq/L) was added to a normocalcemic cardioplegic solution. Function was assessed using pressure volume loops and expressed as percentage of control.

Results. Compared to hypocalcemia cardioplegic solution without magnesium (Group 1), both high- and low-dose magnesium enrichment (Groups 2 and 3) improved myocardial protection resulting in complete return of systolic (40% vs 101% vs 102%) (p < 0.001 vs Groups 2 and 3) and global myocardial function (39% vs 102% vs 101%) (p < 0.001 vs Groups 2 and 3), and reduced diastolic stiffness (267% vs 158% vs 154%) (p < 0.001 vs Groups 2 and 3). Conversely, even high-dose magnesium supplementation could not offset the detrimental effects of normocalcemic cardioplegia resulting in depressed systolic (End Systolic Elastance [EES] 41% ± 1%) (p < 0.001 vs Groups 2 and 3) and global myocardial function (40% ± 1%) (p < 0.001 vs Groups 2 and 3), and a marked rise in diastolic stiffness (258% ± 5%) (p < 0.001 vs Groups 2 and 3). Hypocalcemic magnesium cardioplegia has now been used successfully in 247 adult and pediatric patients.

Conclusions. Magnesium enrichment of hypocalcemic cardioplegic solutions improves myocardial protection resulting in complete functional preservation. However, magnesium cannot prevent the detrimental effects of normocalcemic cardioplegia when the heart is severely stressed. This study, therefore, strongly supports using both a hypocalcemic cardioplegic solution and magnesium supplementation as their benefits are additive.     

Accumulation of methane, acetone, and nitrogen in the inspired gas during closed-circuit anesthesia

During closed-circuit anesthesia, the patient's inspired gas may become progressively contaminated by nonanesthetic gases. We studied the concentrations of methane, acetone, and nitrogen as nonanesthetic gas contaminants in the circuit gas of 16 cases during closed-circuit anesthesia. After a "short" period of denitrogenation (6-8 min), average nitrogen concentration in the closed circuit increased from 6.4 to 16.2%, methane from 4.3 to 22.4 ppm, and acetone from 0.3 to 2.2 ppm. After "long" denitrogenation (33 min), average nitrogen concentration in the closed circuit increased from 1.0 to 5.1%, methane from 3.7 to 17.9 ppm, and acetone from 1.3 to 5.9 ppm. It is concluded that gases stored in tissues or produced within the body can appear in the patient's expired gas during closed-circuit anesthesia. Intermittent flushing of the circuit with high flow gases is suggested to remove these contaminants.     

Silicone-Coated Polypropylene Hollow-Fiber Oxygenator: Experimental Evaluation and Preliminary Clinical Use

Background. A membrane oxygenator consisting of a microporous polypropylene hollow fiber with a 0.2-μm ultrathin silicone layer (cyclosiloxane) was developed. Animal experimental and preliminary clinical studies evaluated its reliability in bypass procedures.

Methods. Five 24-hour venoarterial bypass periods were conducted on dogs using the oxygenator (group A). In 5 controls, bypass periods were conducted using the same oxygenator without silicone coating (group B). As a preliminary clinical study, 14 patients underwent cardiopulmonary bypass with the silicone-coated oxygenator.

Results. Eight to 16 hours (mean, 12.2 hours) after initiation of bypass, plasma leakage occurred in all group B animals, but none in group A. The O₂ and CO₂ transfer rates after 24 hours in group A were significantly higher than at termination of bypass in group B (p < 0.005 and p < 0.03, respectively). Scanning electron microscopy of silicone-coated fibers after 24 hours of bypass revealed no damage to the silicone coating of the polypropylene hollow fibers. In the clinical study, the oxygenator showed good gas transfer, acceptable pressure loss, low hemolysis, and good durability.

Conclusions. This oxygenator is more durable and offers greater gas transfer capabilities than the previous generation of oxygenators.