R. Stuart Mackay

FAST, AUTOMATIC, ELECTRONIC TONOMETERS BASED ON AN EXACT THEORY

We have been concerned about the need for a fast, automatic, direct-reading tonometer, which is accurate, repeatable and gentle. An instrument with any of these characteristics is desirable but one with them all would be extremely useful. Such an instrument would be of great value not only to the ophthalmologist in his diagnosis and treatment of glaucoma but also to the optometrist who would be able to determine intraocular pressure in his patients without using the topical anesthetics present corneal tonometers demand. Previous tonometers, mechanical or electronic, measure either the indentation of the cornea produced by a certain size and shape plunger of a given weight, or they measure the force required to flatten a given area of the cornea. W e will not discuss these classical instruments, details of which are available in the textbooks (1). Measurement with all classical tonometers requires experience, they are somewhat tedious in their use, and are based upon questionable assumptions. Some of the tonometers to be discussed are subject to some of these same restrictions while others are quite free of them and involve basically different principles.

Automatic Tonometer with Exact Theory: Various Biological Applications

Methods for externally measuring pressure within body cavities exist. In tonometer form they are fast and gentle not requiring anesthetics for the cornea of the human eye. Readings are accurate and independent of tissue tension, corneal stiffness, astigmatic curvatures, and surface tension. There are also separate indications of corneal rigidity and relaxation, and tonographic fluid expression. Other applications include monitoring blood pressure, uterine contractions, and infant intracranial pressure.

A theoretical and experimental study of the mechanical behavior of the cornea with application to the measurement of intraocular pressure

A theoretical and experimental study was made of the mechanical behavior of the cornea. The theoretical analysis included an analytical solution for the symmetrical constraint of a thin, shallow, spherical shell by a rigid indenter. The experimental study investigated the rheology of the cornea with particular emphasis on its compliance with the requirements of the Boltzmann Superposition Principle. Representative results of tests on twenty enucleated hog eyes and two human eyes have been reported. The corneas of the human and hog eyes behaved as linear viscoelastic solids; the human eyes differed from the hog eyes in having a long term creep component. Several eyes were tested at the site of procurement, six to seven minutes after the animals death, and it was established that creep is not an artifact due to aging or enucleation. The analytical and experimental results were combined to study some instruments used to detect the level of pressure in the eye. The theoretical analysis predicted that a type of elastic instability occurs during the process of flattening a small portion of the cornea; this is discussed with reference to the Goldmann and Mackay-Marg tonometers. The role of corneal creep was considered with reference to the response of the Schiotz indentation tonometer during the time dependent process known as tonography.

DEEP BODY TEMPERATURE OF UNTETHERED DOLPHIN RECORDED BY INGESTED RADIO TRANSMITTER.

Information concerning body temperature was recorded by radio transmission from within a dolphin, Tursiops truncatus, swimming freely in a pool. The temperature varied according to the activity of the animal and appeared to oscillate when the dolphin was returned to the pool after a brief period outside it. The transmitter was passed approximately 17 hours after it had been swallowed. A new type of antenna was used with the circuit. The transmitter can be adapted for telemetering heart beat, movement, aquatic animals.

THE APPLICATION OF PHYSICAL TRANSDUCERS TO INTRACAVITY PRESSURE MEASUREMENT, WITH SPECIAL REFERENCE TO TONOMETRY.

Intracavity pressure measurement by endoradiosonde is highly developed, but is generally impossible if no orifice is accessible, e.g., for intraocular or infant intracranial pressures. Use of various physical transducers and different configurations for these are discussed. Tonometers using an insensitive flat annulus coplanar with a central transducer are described (Mackay-Marg tonometer). Several groups have accepted it as a standard of accuracy after several years of testing; in the past year, it has gone into routine use for detecting ocular hypertension, with detected glaucomas reaching the hundreds. Being fast and gentle, it can be used in mass screening without anesthesia. Sterilized tip covers prevent transfer of infection; it can be used in any position. It appears that average eye pressure may be slightly higher than thought.Indication of other variables including corneal stiffness and tonographic ease of outflow is also given. Theory is presented along with operational subtleties of performance. Brief comparison is made with indentation, aplanation, vibration, and air puff tonometry. It can be considered an absolute instrument and the principle used for other biological variables such as abdominal or blood pressure.SommaireLe procédé de mesure de pression intracavitaire par endoradiosondes est couramment employé, mais reste généralement impraticable en l’absence d’un orifice accessible, comme dans le cas de mesure de pression intra-oculaire ou intra-crânienne infantile. On examine ici la possibilité d’utiliser différents types de transducteurs physiques et de montages variés. Un tonomètre avec anneau plat et insensible, co-planaire avec un transducteur central, est décrit (Tonomètre Mackay-Marg). Certains, après plusieurs années d’expérimentation, le considèrent comme un prototype de précision. Au cours de la dernière année, l’utilisation de cet appareil est devenue un procédé de routine pour la détection de l’hypertension oculaire. Des centaines de cas de glaucome ont été révélés avec son aide. Le fonctionnement de cet appareil étant rapide et doux, il peut être employé sans anesthésie pour des examens massifs. Un fourreau de la surface de contact, stérilisable, empêche le transfert d’infection. Il peut être utilisé dans n’importe quelle position. Il apparaît que la tension oculaire moyenne pourrait être légèrement supérieure à ce que l’on pensait.Des indications sont données concernant d’autres variables, y inclus la raidaur de cornée. Un exposé théorique est développé parallèlement avec une description des subtilités opérationnelles. Une brève comparaison est tracée entre ce procédé et l’indentation, l’aplanissement, la vibration et la tonométrie des bouffées d’air. Le principe de cet instrument peut être considéré comme principe général applicable à la mesure d’autres variables biologiques, tels que les pressions abdominales et sanguine.ZusammenfassungDie Druckmessung im Inneren von Körperhöhlen mit Hilfe von Endoradiosonden ist weit verbreitet; sie ist jedoch dann unmöglich, wenn keine Öffnung zugänglich ist, d.h. bei intraokularen oder itrakraniellen Druckmessungen am Kind. Die Verwendung von physikalischen Meßwandlern und deren Bauweise werden erörtert. Es werden Tonometer beschrieben, bei denen ein unempfindlicher flacher Ring verwendet wird, der mit einem zentralen Wandler in einer Ebene liegt (Mackay-Marg Tonometer). Verschiedene Gruppen haben ihn nach mehrjährigem Testen als Maßstab für Ge nauigkeit übernommen. Im vergangenen Jahr wurde er bei der Detektion von okularer Hypertension regelmäßig verwendet; die hierbei festgestellten Glaukome gingen in die Hunderte. Da das Verfahren schnell und schmerzlos ist, kann es auch bei Massenuntersuchungen ohne Anästhesie angewandt werden Sterilisierte Überzüge für die Spitze verhindern Infektionen; das Gerät kann in jeder Lage verwendet werden. Es scheint, daß der durchschnittliche Augendruck etwas höher liegt als bisher angenommen wurde. Es werden weitere Angaben über andere Variabeln gemacht, darunter die Starre der Hornhaut. Es werden sowohl die Theorie wie auch Detailangaben über das Funktionieren des Geräts dargestellt. Es wird ein kurzer Vergleich mit der Einbuchtungs-(Indentations), der Einebnungs- der Vibrations- und der Luftdruck-Tonometrie gemacht. Das Gerät ist vielseitig verwendbar; es ist auch für die Messung der anderen biologischen Variabeln wie Bauchhöhlendruck oder Blutdruck geeignet.РезюмеНесмотря на значительное развитие измерения давления в полостях тела посредством зндорадиозондов, это все же невозможно, если нет подходящих отверстий, в случае, например, давления внутри глаза или внутри черепа ребенка. Обсуждается применение для этой цели различных модификаций физических датчиков. Описаны тонометры с плоским кольцом, имеющим в центре датчик (тонометр Маккея-Марг). После ряда лет проверки несколько гурпп исследователей приняли их как образец точности; в прошлом году он был применен для массовой диагностики повышенного глазного давления, причем глаукома была распознана в сотнях случаев. Будучи быстрым и щадящим, метод может быть применен при массовых обследованиях без местного обезболивания. Стерилизуемая покрышка на кончике тонометра предотвращает перенос инфекции; его можно применять при любом положении. Среднее глазное давление, по-видимому, может быть намного больше, чем обычно считают.Приведены данные о других параметрах, включая жесткость роговицы. Представлена теория, а также подрбности, касающиеся применения метода. Сделаны краткие сопоставления с другими методами тонометрии. Прибор можно рассматривать, как идеальный, а принцип—применить для других биологических параметров, таких как давление в брюшной полости или давление крови.

Trough height, pressure and flattening in tonometry

Abstract Our automatic, gentle, electronic tonometer which is based on an exact theory yields a characteristic tonogram. We have shown in a previous paper that the initial crest occurs when the transducer surface is just covered by the flattened cornea and that the trough occurs when it is more than covered. Using rabbit eyes, the relationship was investigated between the degree of flattening and the intraocular pressure by electronic manometry and cannulation. Then the change in manometric pressure was found at the incidence of the initial trough of the Mackay-Marg tonogram. It is demonstrated that a 1 1 2 - mm diameter circular transducer surface area requires a 3-mm diameter flattening of the cornea for the generation of the initial trough. The artificial rise in pressure caused by this applanation is 0.4 mm Hg.

Electronics in Clinical Research

Engineering training can profitably be applied to medical problems, both for better understanding of biological mechanisms and for development of more effective clinical methods. Also from clinical investigations, one can often derive concepts applicable to nonbiological systems. Examples of developments from clinically-oriented research are cited to show some of the possibilities and problems in this field.

Switching in Bistable Circuits

Switching or triggering in certain nonlinear circuits having two stable states is studied in graphic detail. The minimum and maximum pulse requirements are discussed and some of the general ideas of bistability considered. A convenient technique for studying the response of circuits, apparently in slow motion, is described.

Doppler Sonar System

A system of acoustic echo ranging for the measurement of bearing and range to an underwater object which is in motion toward or away from the detection system located at a selected position. A signal design which results in a very high ability to select only the moving objects and reject any signals which return from the surface or bottom of an ocean, river, or lake.

An Electronic Cardiac Defibrillator and Pacemaker

There is described an instrument which has been devised to stop ventricular fibrillation and to restore rhythmical beating of the heart in standstill. The instrument provides 60‐cycle sinusoidal pulses continuously variable from 0 to 3 amperes in intensity and from 0.01 to 1.0 second in duration. A single pulse may be delivered for defibrillation, or regularly repeating pulses, from 20 to 120 per minute, may be delivered for instituting rhythmical beating of the arrested heart. The time intervals produced are quite independent of line voltage and tube changes. Suitable provision is made for the protection of the operator from accidental short circuit to ground. Clinical use of the instrument has been restricted to defibrillation, where it has proven successful.