Automatic control of anesthetic delivery and ventilation during surgery.

Use of a closed rebreathing circuit for anesthesia delivery offers several advantages: conservation of anesthetic agent, natural heating and humidification of inspired gases, less pollution, and improved monitoring. However, the technique requires careful control of the fresh gas and anesthetic delivery. An anesthesia delivery system has been developed which automatically controls fresh gas delivery, anesthetic delivery, and ventilation in order to regulate circuit volume, oxygen concentration, end-tidal anesthetic concentration, and end-tidal PCO2. This system makes available the advantages of closed-circuit anesthesia without encumbering the anesthesiologist with its more demanding control tasks. The system has undergone clinical testing in adult surgical patients. Maintenance of circuit volume, oxygen concentration, end-tidal anesthetic concentration, and end-tidal PCO2 was achieved by the system in all patients but the first. The anesthetic delivery controller required tuning on the first patient. No further tuning was required for any of the other patients or controllers. During abdominal surgery, the end-tidal bellows position measurement and end-tidal anesthetic concentration measurement increased in variability. The cause of the variability and its elimination are discussed in the article.

Closed-circuit and high-flow systems: examining alternatives.

The nonrebreathing system has been with us since Morton demonstrated the administration of diethyl ether in 1846. Its current popularity is evidenced by the extensive use of the Bain system. The greatest advantage, its history of patient safety, is related to the circuit's simplicity and the knowledge that the delivered concentration equals the inhaled concentration. Most disadvantages of the nonrebreathing system are related to the required high delivery rates: operating room and environmental pollution, necessity of scavenging gases, cost of agents, energy loss through no-return operating room ventilation, inhalation of dry gases, and the inability of the anesthesiologist to quantitate patient uptake of oxygen and inhaled anesthetics. Partial rebreathing systems reduce the disadvantages related to high delivery flow rates but, owing to the required rebreathing, do not permit the anesthesiologist to know the inhaled anesthetic concentration. A carbon dioxide absorber is necessary. It is still impossible to quantitate uptake by the patient, and it is difficult to conclude that any real net advantage results from the use of partial rebreathing systems. When modern-day technology provides the practitioner with an appropriate anesthesia machine, it is likely that closed-circuit anesthesia will become the method of choice for anesthesia delivery. Although the economic, ecologic, and physiologic advantages of this system are important, its greatest asset is the ability to monitor important respiratory and cardiovascular variables in patients noninvasively. Important information provided to the anesthesiologist by the patient during closed-circuit anesthesia is lost through the pop-off valve when high-flow systems are used. During closed-circuit anesthesia the gas machine itself becomes a monitor. Practicing anesthesiologists will embrace closed-circuit anesthesia practice when-and if-they are convinced that it provides an opportunity for better and more efficient patient care than other systems.

Nitroprusside prevents adverse hemodynamic effects of vasopressin.

The influence of vasopressin and sodium nitroprusside on liver circulation was investigated in 18 dogs. Cardiac output was determined by the thermal dilution technique using a Swan-Ganz catheter. Hepatic artery and portal vein flows were measured with electromagnetic flowmeters. The infusion of vasopressin (0.01 unit/kg/min) caused a 13% increase in mean arterial pressure, a 38% decrease in cardiac output, a 57% decrease in portal blood flow, and a 35% decrease in portal pressure. Hepatic artery blood flow initially decreased, then increased, and eventually exceeded the baseline value by 25%. The addition of sodium nitroprusside infusion (10 microgram/kg/min) returned the mean arterial pressure to baseline value and increased cardiac output to 83% of baseline value. Portal blood flow remained unchanged, even though an additional decrease in portal pressure of 11% and a further increase in hepatic artery blood flow of 45% were observed. Nitroprusside minimizes the undesirable effects of vasopressin and augments the desirable ones in normal dogs. The combination of these drugs may be more beneficial to patients with esophageal and gastrointestinal bleeding than vasopressin alone.

Operating room scheduling by computer.

The surgical operating room schedule has been produced automatically for more than 2 years in our large teaching hospital. In order to apply computer technology to the complex surgical scheduling problem a special programming approach was devised. We discuss this approach under the headings of Expand, Sort, Order and Assign. Consistent, reliable schedules, unaffected by weekends and holidays, are produced by a clerk trained to use a computer terminal. Our program is adapttable to other institutions once the scheduling parameters and operating priorities are delineated.

Models for pulsed polarographic electrodes.

A simple model has been developed for analyzing the pulsed polarographic electrode. For short pulse durations the charge transfer kinetics must be incorporated into the boundary condition at the electrode surface. The predicted velocity dependence for long pulse durations is in good agreement with experimental data.