Volume 2, No. 4 • Winter 1987

Refined Safety Equipment Exhibited at ASA

John H Eichhorn, M.D.

Products and equipment exhibited at the ASA annual meeting in October, 1987, in Atlanta continued to stress patient monitoring and the positive implications for anesthesia patient safety of earlier warning of untoward development during an anesthetic.

New developments in the major technologies of oximetry, capnography, and gas analysis centered more on refinements than new concepts. New brands, combinations of monitors, and also improved software, displays, and operating features were widespread. [for a more detailed discussion of these and others types of equipment, see Witcher CE, Safety technology shown at ASA. APSF Newsletter, 1986 (Dec.); 1:24.1

Pulse oximeters were offered for the first time by many manufacturers. An informal count showed at least 25 different brands represented in the very large exhibit hall. Several manufacturers have bundled together with capnographs and/or gas analyzers. Some of the instruments give a percent N20 by subtraction of the 02 and C02 from 100, but others will contain actual multigas analyzers, usually based on infrared absorption.

Among pulse oximeters, a wide spectrum of features are available. Small portable battery powered relatively simple models were shown while some of the larger models sported various alarms, displayed messages, and trending capabilities. Printers are available. While some models have automatic gain control to process the signal obtained from the sensor for maximum sensitivity, others do not and use the variation in the signal as an indicator of the adequacy of peripheral blood flow.

One new development was the introduction of a reflectance pulse oximeter. As opposed to a traditional transmittance model, which shines light through tissue, the reflectance oximeter bounces light off any flat area of skin that is well perfused (no underlying bone). This is claimed to allow sensing of sites less prone to confounding variables such as peripheral vasoconstriction.

Capnographs continue to be the mainstay of ventilatory monitoring. Again, packaging permutations abound but the underlying technology is familiar. Improved sampling via nasal prong-like devices is being claimed as an advance for monitoring patients who are not intubated. Some capno8raphs also can measure FiO2 by taking a quick reading during the inspiratory phase. Also, one model combines a capnograph with an airway pressure monitor so that there are two waveforms, ETC02 and pressure, simultaneously on a VDT screen.

Gas analysis with stand-alone mass spectrometers, infrared, or Raman analysis continue to be refined with regard to features, controls, and alarms although not all the displayed models are ready for customer purchase..

Radiostethoscopes to allow amplified heart and breath sounds at various distances from the patient continue to be available. A new entry in this area is a stethoscopic monitor based on infrared (IR) transmission (similar to a TV remote control). A stethoscope on the patient connects to a control box that sends omnidirectional IR signals to a receiver worn on the shirt of the anesthetist. This connects to a conventional ear piece. The IR signal is not subject to static interference from electrocautery as radio signals may be. This particular stethoscope can be adjusted to emphasize either heart or breath sounds. Further, there is a synthetic voice prompt that can give spoken messages in the ear piece regarding a change in temperature detected by the probe on the esophageal stethoscope. Voice prompts can also be triggered by activation of alarms on a pulse oximeter or noninvasive blood pressure cuff interfaced with the stethoscope control box. Reaction to this technology will be of interest as computer-generated voice prompts associated with monitor alarms are controversial.

Stand-alone ventilator pressure monitors with high and low pressure alarms and an alarm for PEEP level were shown.

Specific patient ventilation monitors for use with patients receiving intraspinal narcotics are claimed to allow earlier return to regular patient rooms.

A new noninvasive blood pressure monitor gives a continuous waveform display (with trending) by use of oscillotonometry of the arm. It maintains a low pressure (e-g., 20 mm HS) and senses changes in arterial wall elasticity.

Continuous real-time blood gas monitoring is offered, either as an adjunct to extra-corporeal cardiopulmonary by-pass or via intra-arterial technology. These devices are still being evaluated for potential utility as both safety and physiologic monitors.

Finally, several disposable components intended to protect immunocomprornised patients from potential exposures and to help contain secretions from infected patients were shown. One was a completely disposable single-use C02 absorber canister.

In all, the marked emphasis on patient safety through advances in technology continues. Further refinements as well as new concepts are expected in the coming year.

Dr. Eichhorn, Harvard Medical School and Beth Israel Hospital, Boston, is Editor of the APSF Newsletter.