Frank J. Sasse

Automatic control of anesthesia using two feedback variables

A new controller of anON/OFF type was implemented for halothane anesthesia. A proportional-plus-integral controller with time-delay compensation proved not to be robust enough for the known clinical situation, as shown both in computer simulations and in animal trials. TheON/OFF controller proved to be less sensitive to parameter mismatches, and repeated animal trials showed a short response time and acceptable steady-state tracking. A method for switching the controlled effect of the drug was also developed, since anesthetic agents have multiple effects. Mean arterial blood pressure and a measure ofEEG frequency were chosen as controlled variables, both being depressed by halothane. A coordinator forces the system state as near the desired values of these variables as possible, given that only one drug is used.

Use of positive airway pressure without endotracheal intubation.

Continuous positive airway pressure (CPAP) and expiratory positive airway pressure (E-PAP) may be used safely without endotracheal intubation in patients with acute respiratory failure when strict selection criteria are adhered to. The therapy should be titrated to reduce intrapulmonary shunting, improve PaO2, and reduce FIO2. Other considerations include balancing oxygen consumption against cardiac output and oxygen transport. Absolute or relative indications for abandoning the technique and using endotracheal intubation with mechanical ventilatory support include unrelenting hypoxia, patient exhaustion, rising PaCO2, development of metabolic acidosis, presence of ventricular arrhythmias, and inability to protect the airway.

The Response of Anesthetic Agent Monitors to Trifluoromethane Warns of the Presence of Carbon Monoxide from Anesthetic Breakdown

Objective. Trifluoromethane and CO are produced simultaneously duringthe breakdown of isoflurane and desflurane by dry CO2absorbents. Trifluoromethane interferes with anesthetic agent monitoring, andthe interference can be used as a marker to indicate anesthetic breakdown withCO production. This study tests representative types of gas monitors todetermine their ability to provide a clinically useful warning of COproduction in circle breathing systems. Methods. Isoflurane anddesflurane were reacted with dry Baralyme® at 45 °C. Standardizedsamples of breakdown products were created from mixtures of reacted andunreacted gases to simulate the partial degrees of reaction which might resultduring clinical episodes of anesthetic breakdown using 1% or 2% isoflurane and 6% or 12% desflurane. These mixtures were measured by the monitors tested, andthe indication of the wrong agent or a mixture of agents due to the presenceof trifluoromethane was recorded and related to the CO concentration in thegas mixtures. Results. When presented with trifluoromethane fromanesthetic breakdown, monochromatic infrared monitors displayedinappropriately large amounts of isoflurane or desflurane. Agent identifyinginfrared and Raman scattering monitors varied in their sensitivity totrifluoromethane. Mass spectrometers measuring enflurane at mass to charge= 69 were most sensitive to trifluoromethane. Conclusions. Monochromaticinfrared monitors were unable to indicate anesthetic breakdown viainterference by trifluoromethane, but did indicate falsely elevated anestheticconcentrations. Agent identifying infrared and Raman monitors provided warningof desflurane breakdown via the interference of trifluoromethane by displayingthe wrong agent or mixed agents, but may not be sensitive enough to warn ofisoflurane breakdown. Some mass spectrometers provided the most sensitivewarnings to anesthetic breakdown via trifluoromethane, but additional dataprocessing by some patient monitor units reduced their overall effectiveness.

Effects of hypocapnia on canine spinal, subcortical, and cortical somatosensory-evoked potentials during isoflurane anesthesia

Although hyperventilation with hypocapnia is frequently used in the management of neurosurgical patients in whom sensory-evoked potentials may be monitored, the effects of hypocapnia on evoked potentials have not been described with precision. In the present experiment, the effects of randomized arterial carbon dioxide tensions of 20, 25, 30, and 35 mm Hg on spinal, subcortical, and cortical somatosensory-evoked potentials (SEPs) were measured in dogs anesthetized with 1.40% isoflurane. Other variables known to affect the SEP (temperature, blood pressure, and arterial oxygen tension) were stable throughout the experiment. Hypocapnia caused reductions in the latencies of the early peaks of the spinal and subcortical SEPs. These differences were small, consisting of a 2% shortening of latency at 20 mm Hg carbon dioxide tension when compared with 35 mm Hg. No changes were detected in the later subcortical and cortical latencies. SEP amplitudes were also unchanged. These results in a controlled animal study corroborate the direction and magnitude of changes due to hypocapnia observed by other investigators in surgical patients. The magnitude of the changes indicates that SEP monitoring sensitivity is not compromised by clinically useful levels of induced hypocapnia during isoflurane anesthesia. Because hypocapnia may produce small SEP changes, baseline recordings should be acquired prior to initiation of hyperventilation. It is not warranted, however, to impute a severe deterioration of the SEP to hypocapnia alone, and causes must be sought elsewhere in a patients status and management.

Effects of halothane dose and stimulus rate on canine spinal, far-field and near-field somatosensory evoked potentials ☆

Evidence that canine spinal, far-field and near-field somatosensory evoked potentials resemble those recorded in humans and other species has been presented, and the vulnerability of each component to varying depths of halothane anesthesia is reported. Lumbar spinal peak latencies are not affected by halothane dose, but the negative peak is significantly prolonged by rapid rates of stimulation. Elevated stimulus rates and halothane doses reduce lumbar spinal cord potential amplitudes. Early far-field cephalic components are refractory to halothane. Late far-field components and near-field cortical potentials are substantially altered by increments in halothane dose. Both near-field and far-field responses are more readily identified in vertex-neck than vertex-brow derivations. Early far-field somatosensory evoked potentials recorded from vertex to neck, together with lumbar spinal cord potentials, may be the preferred monitoring technique when the use of halothane anesthesia is desired. Rapid rates of stimulation may facilitate earlier recognition of cord dysfunction, but supplement rather than replace baseline recordings at slow stimulus rates.

The “D Circle”: Closed-circuit operation of the bain circuit

SummaryA method of converting a Mapleson D (Bain) circuit to closed-circuit operation is presented, utilizing a laboratory air pump and a Waters carbon dioxide absorber canister to recirculate exhaled gas. The elimination of carbon dioxide from the circuit was studied and found to be adequate. The circuit would allow the use of low fresh gas flows for the maintenance of anaesthesia without the danger of carbon dioxide rebreathing. We suggest that such a circuit could provide appropriate conditions of gas humidity and temperature for endotracheal anaesthesia. while realizing the advantage of a circulator in mask anaesthesia is possible. Furtherdesign considerations for a “D circle” breathing system for clinical use are discussed.RésuméLes auteurs présentent une méthode de conversion ďun montage Mapleson D (Bain) en circuit fermé. Pour obtenir la recirculation de ľair expiré on utilise une pompe à air de laboratoire ainsi qu’un canister Waters pour ľabsorption du gaz carbonique dont ľélimination a été étudiée et jugée adéquate. Ce circuit permet ľemploi de volumes de gaz frais peu élevés pour le maintien de ľanesthésie sans danger de rebreathing. Les données receuillies semblent indiquer que le circuit tel que modifié pourrait fournir une humidification et une température suffisantes pour ľanesthésie endotrachéale ainsi qu’une circulation avantageuse pour ľanesthésie au masque. Les auteurs font part de modifications désirables pour ľusage clinique du système “D” selon ce mode.