Use of Monitoring Devices During Anesthesia and Operation

USE OF MONITORING DEVICES DURING ANESTHESIA AND OPERATION ROBERT D. DRIPPS, M.D.* Not long ago an airplane pilot "flew by the seat ofhis pants." Instruments being almost nonexistent, he relied on peering over the edge ofthe cockpit. As increasingly complex planes were developed, instruments grew apace until nearly 500 dials and levers could be counted on the panels ofBoeing's piston-engine Stratocruiser, many of them to "monitor " the performance ofthe plane and its crew. "Monitor" comes from the Latin verb monere, "to warn or bring to mind." As in aviation, so in anesthesia. Anesthetists administer some ofthe most potent drugs used in medicine. Daily they carry these substances closer to the point of irreversibility than many physicians do with other drugs during a lifetime. Anesthetists also have been flying by the seat of thenpants , sighting only crude landmarks along the route of increasing narcotic intensity—by observing the patient, appraising the rate and rhythm ofthe heart beat by feeling an artery, noting the rate and depth ofbreathing , and estimating arterial blood pressure by listening through a stethoscope . These are the common ways by which anesthetists judge the level of anesthesia and, more importantly, the effect of the anesthetic on the patient's vital functions. As more intricate and delicate operations were devised and as more desperately ill patients were accepted for operation, the anesthetist found it essential that he be "warned" oftrouble earlier than was possible with the relatively crude detection techniques ordinarily used. Some of the efforts to increase the safety of the anesthetized patient through objective measurements—the facet ofanesthesia we term "monitoring "—are described here. * Department ofAnesthesiology, Hospital ofthe University ofPennsylvania, and the University of Pennsylvania Schools of Medicine, Philadelphia 4, Pennsylvania. Experimental work in the author's laboratory has been aided by a grant from theJohn A. Hartford Foundation, Inc., New York. 362 Robert D. Dripps · Monitored Anesthesia Perspectives in Biology and Medicine · Spring 1959 I. Body Temperature Alterations in body temperature during anesthesia may presage important events. A rise, particularly in infants and children, appears to bea factor in the convulsions that sometimes occur during inhalational anesthesia. A decrease in body temperature, on the other hand, is associated with greater irritability of the heart, occasionally culminating in ventricular fibrillation. Lowered temperature of skeletal muscle causes stronger and longer action of the depolarizing relaxants, such as decamethonium and succinylcholine. Change in temperature during anesthesia is not uncommon. A child already febrile—with acute appendicitis, for example—is often given atropine as part ofthe preanesthetic medication to prevent some heat loss by means ofits drying action. When the child is anesthetized with a closedsystem apparatus, placed under the hot glare ofan operating-room light, and covered with surgical linen, temperatures of i05°-io6° F. may result. Conversely, if, to avoid this, the child is exposed with minimal covering in a coolroom, heat loss may beprofound and a temperature ofgo° F. may occur. A large volume ofwhole blood that has been stored at 40o F. in the blood bank, when given rapidly by vein to treat hemorrhage, may lower body temperature to 87o F. Finally, body temperature may be deliberately reduced in order to slow metabolism and allow tissues to survive in the absence of blood flow for longer periods than are possible at normal temperatures. These illustrations indicate the need for constant knowledge of body temperature under many circumstances. A clinical thermometer is oflittle use, since during operation it is difficult to get rectal recordings and the patient cannot keep the thermometer under his tongue. The ordinary mercury thermometer will not register a fall in temperature because it is constructed to prevent the mercury from falling back into the reservoir when the bulb is removed from the body for reading. A useful device is the thermistor, a small resistor possessing special electrical characteristics. As its temperature changes, the resistance ofthe thermistor, measured by a Wheatstone bridge circuit, changes inversely. The sensing element can be placed in a flexible tube, inserted into the esophagus or rectum, and connected to a meter. Other thermal sensing devices have been used, including a platinum-wire thermometer and a thermocouple. Units built 363 into the points of small-gauge needles...