(From the Department of Pediatrics, School of Medicine,
University of Pennsylvania
and the Children's Hospital of Philadelphia)
Am. J. Obst. and Gynec. 35:1062-1065, 1938
The apparatus described herein contains certain important features not included in other incubators. The fact that it remains closed at all times permits the fulfillment of the following prerequisites hitherto considered not practicable. (1) An accurate maintenance and adjustment of temperature and humidity permits the selection necessary in such air-conditioning for infants of different weights. (2) The infant may remain without covering or clothes which condition allows great freedom of muscular activity especially for the important function of respiration. This lack of clothes also facilitates handling and prevents the usual alterations in temperature dependent upon covering and uncovering or upon changing the diaper. (3) In addition special gas mixtures, either for emergency or prolonged use, may be provided. (4) Also, a practically complete isolation is maintained against pathogenic microorganisms, whether air-borne or otherwise. (5) It permits the nurse to work in an atmosphere which is comfortable for her. In the ordinary air-conditioned room the optimum humidity and temperature for the infant are too great for the nurse's tolerance.
This incubator is made in two compartments, one above the other. The upper contains the baby, the lower houses the working unit. The upper compartment is an air-tight, heat-insulated chamber. The air for the infant is drawn from out of doors. This air is filtered and humidified before it enters the incubator and it is heated inside the chamber.
The thermostat is calibrated and is adjustable up to 99° F. There is a temperature fluctuation of less than one degree from the selected point.
The humidistat is similarly made and is adjustable up to 100 per cent relative humidity. The humidity will remain within a 5 per cent variation.
The rate of air flow is uniform at all times since it is driven by a one speed motor. The outside air is drawn in at the rate of 0.4 cu. ft. per min. It passes over the baby at a rate of 30 ft. per min. Under ordinary conditions the air is recirculated about six times, the incoming air being approximately one sixth of the total air stream. The ratio of recirculated air to outside air is adjustable for use of special gas mixtures. This is accomplished by a valve on the fresh-air pipe. Gas mixtures are admitted through a pet cock on the negative pressure side of the blower. Air samples for analysis are obtained through a pet cock on the positive pressure side of the blower. The waste air is discharged at the same rate the fresh air is admitted whether this be outside air or gas from a tank. These constant conditions are maintained by keeping the chamber closed at all times.
View is obtained into the electrically illuminated interior through the full length window. This is made of two thicknesses of noninflammable celluloid.
The baby is handled through balloon cloth sleeves, elastic about the wrist. These are kept rolled when not in use. Bottles, syringes, diapers, etc. are admitted through an air-lock. This is a box-like compartment having two doors, one opening into the chamber and one to the outside. Both doors are air-tight. When a bottle is to be given the outside one is opened and the bottle is placed inside the box. The nurse's hands, which are her only contact with the baby, are scrubbed and then introduced through the sleeves to the chamber. The inside door of the box can then be opened and the bottle taken out. One door is always locked so that there is always an air-tight partition between the chamber and the outside air.
A scale is built into the chamber and in addition to its ordinary weighing function is used as an alternate bed while the basket-bed, in which the infant usually lies, is being changed.
An opening in the top of the chamber is closed by a rubber stopper penetrated by a glass tube. Emergency carbon dioxide and oxygen can be delivered here to a tube connected to a mask or funnel outside. A stethoscope can be used with this connection and a burette can be attached with a connecting tube to a needle or rectal tip for the administration of large amounts of fluid.
The incubator is silent since the motor is mounted on rubber and there are only rubber connections between the chamber and moving parts (motor and blower).
From the nurse's standpoint, it is easier to manipulate than other incubators. The mechanism is automatic. The baby is naked, or nearly so, since the conditions of his environment are constant and handling is proportionately simplified. The only care the incubator requires is the replenishment of water to the humidifier reservoir and a monthly oiling of the motor. The reservoir is an ordinary mason jar inverted in a standard. It stands behind a slot in the door of the lower compartment so that it acts as a gauge.
Although the most reliable instruments possible are used, there is always some danger of failure of function in anything mechanical. To guard against any such failures safety devices are provided. The first is an indicator of air-flow. Should the motor fail this would no longer register. There is approximately a four-hour margin of safety in the air supply. The second is a control thermostat which would shut off the heater and the humidifier if the temperature should reach 99° F. If this temperature is reached a bell rings which stops ringing only when the temperature has fallen below this level. If the current should fail, the lights, which are on continuously, would go off.
A simple cooling unit and a dehumidifier can be installed if desired, since the incubator is an air-tight, insulated chamber.
Clinical experience with this incubator covers 25 premature babies. Because of the conditions under which the incubator was used this number has been sufficient to evince several striking results. Ten babies were in an incubator while it was kept in a 16-bed ward of other babies where cross-infections were constantly recurring. No attempt at isolation was made. Gowns and masks were not used by the incubator attendants. The only precautionary measure taken was that of washing the hands with soap and water and rinsing them in a bichloride of mercury solution before starting them through the sleeves to the baby. In this room environment no premature baby in the incubator has had a respiratory infection. Only one of the infants developed diarrhea. This was mild and transient. One baby developed a staphylococcus abscess at the site of repeated hypodermoclysis injections.
It is our feeling that this result was obtained largely by limitation of air-borne organisms as only out-of-doors air is admitted to the incubator. No ward air enters the chamber. However, it should be noted that contact and droplet infection are eliminated as well.
Another result of importance is the absence of cyanotic attacks. In some of the babies between 1 pound and 15 ounces, and 3 pounds, cyanotic attacks occurred suddenly and frequently until the oxygen content of the incubator was raised and its humidity was increased. Oxygen, introduced through the petcock for that purpose at 4 liters per minute maintains consistently the concentration of oxygen at 46 per cent. This practice is used for the very small infants. The percentage of oxygen is decreased gradually as the infant grows larger and stronger until it is discontinued completely when the baby is approximately 3 1/2 pounds. After this the oxygen tank is left attached to the incubator. At any time should the emergency arise the chamber can be flooded with oxygen instantly by turning the tank valve. In some cases a tube is left attaching the oxygen tank to the glass tube at the top of the chamber. The other end of this glass tube is connected by rubber tubing to a funnel to be put over the infant's face should cyanosis recur.
The percentage relative humidity required by infants under 4 1/2 pounds is high as shown by Blackfan and Yaglou . In this incubator infants of such weight range were kept in 70 per cent relative humidity. When the percentage was dropped below 65 per cent, cyanotic attacks became more frequent in weak infants. This would seem to be due to increased heat loss in the lower humidity range with the resultant increase in their energy expenditures. In none of the infants in the present group have cyanotic attacks occurred when humidity was kept in this high range and when the oxygen was maintained in this high configuration.
Since the temperature and the humidity are so closely controlled the infants are left uncovered and often completely naked. The temperature fluctuation as recorded by rectal thermometer is within a one degree diurnal variation. The activity of the infants under such conditions of freedom from clothes is vigorous. Even at 2 pounds they kick their feet over their heads while lying on their backs and otherwise move freely in less excursive motions. The respiratory function is unhampered by the weight of blankets.
315 Rex Avenue.
Fig. 1. -- Exterior of the incubator; showing the rolled sleeves, water gauge, lighted interior, outside air-lock door, and stopper in the top.
Fig. 2. -- Upper compartment, interior: 1, scale; 2, removable gavage bracket; 3, thermometer on the basket-bed; 4, inside air-lock door; 5, heater; 6, adjustable humidistat and thermostat, with the control thermostat between them; 7, removable tray; 8, stopper; 9, outside air-lock door.
 Am. J. Dis. Child. 46:1175, 1933.