image You can read the revised article in the book:

Introduction to Medical Equipment Repair

The book also contains a chapter on preventive maintenance, with recommendations on how to perform the various tests.
Click here for more information.

Infant Incubators

by Frank Weithöner

Infant incubators maintain a healthy environment for newborn babies. They are used for premature babies which are still small and weak as well as for some sick full-term babies.
Incubators are used in the primary care department of a hospital or in the neonatal intensive care unit, NICU (pronounced "Nickyoo").
Infant incubators should not be confused with laboratory incubators, which are used to keep reagents or samples warm.

The incubator consists of a transparent cabinet in which the baby is kept in a neutral environment for medical care. Neutral means, that the surrounding air has the optimal temperature and moisture and the baby only needs minimum of energy to maintain normal body temperature.

The infant incubator
  controls the temperature
  controls the humidity
  can deliver additional oxygen
  protects against infections and
  protects against noise

In order maintain the body temperature of the baby (36 - 37.2°C) the incubator must be able to create an ambient air of 34 - 38°C with a humidity of
40 - 80%.
The branch of medicine that deals with the medical care of newborn infants, especially with the ill or premature newborns, is called neonatology. The medical practitioner who specialises in this area is known as a neonatologist.
Paediatrics is the branch of medicine that deals with the medical care of all children in general, from birth up to 18 years of age. The medical practitioner in this area is known as a paediatrician.
A new born baby (or infant) is also called neonate.

In a typical infant incubator the baby lies on a mattress in a closed, transparent cabinet. The ambient air is fully controlled by the incubator. The cabinet has portholes so that the nurse has access to the baby without opening the cabinet.
Under the cabinet is the compartment with the technology and the control panel. The whole incubator is mounted on a trolley.

Radiant heater  
Sometimes the closed construction is not desired. When the baby needs special care (e.g. controlled ventilation, pulse oximetry, ECG) and nurses and doctors need fast and easy access it is better to have a open system. In this case the baby is warmed by a radiant heater, mounted over the cradle with the baby.

All infant incubators work on the same principle.
A fan blows filtered ambient air over a heating element and a water container. Through a control valve additional oxygen can be supplied to the air. The moistened, heated and enriched air now flows into the above cabinet with the baby. One part of the air escapes from the cabinet through vent holes, another part gets back into the air processing.

The cabinet is made out of transparent Plexiglas (Perspex). A hinged hood can be opened to put the baby in or take it out. Through portholes in the front the nurse has easy access to the baby without letting out the warm air by opening the hood.
After the desired temperature is set by the nurse, it is kept stable automatically by the incubator. Humidity and oxygen concentration are usually controlled manually.
Furthermore the incubator has a safety switch-off function when the temperature increases 40°C. Also an alarm is given when the fan does not turn in case the power fails.
The components which are used vary. Older incubators work with electrical controls, newer ones are electronically controlled and the newest generation is microprocessor controlled. But the result is the same - and the old ones are as reliable and precise as the newest ones.

The fan takes the filtered room air and blows it over or through the heating element and the humidifier. Without the fan the heat can not be conducted away from the heating element and the heating element and thus the incubator would overheat.

Simple incubators are equipped with washable foam filters. After washing and drying they can be reused. Modern incubators however usually have disposable bacterial filters. They can not be cleaned and have to be renewed.

A heating element made from coiled resistance wire as known from hair dryers or the tube type (flat or coiled) as seen in autoclaves are used to heat up the air. But unlike in autoclaves, the heater has much less power and thus does not get so hot. The power rating is between 100 W and 300 W.
The heater is controlled by an electronic temperature control unit via a relay or triac or simply by a thermostat.

Temperature control
Simple incubators are controlled by a thermostat which consists of a sensor and a pressure can. The sensor is a thin capillary tube which leads into the pressure can (or expansion chamber). This chamber has a moveable metal lid or diaphragm. This closed system contains a liquid or gas which expands when getting warmer. The lid moves and activates a connected electrical switch.
The thin capillary tube is often rolled up. This is correct. Do not cut it. It is not a wire. Also avoid bending the tube. It can get kinks and will then not work any more.

Electronic controlled system have a cable sensor. The output controls a relay (on-off) or a dimmer (infinitely variable control).
The electronically controlled type has the advantage that an additional temperature sensor can be used. This sensor is taped on the skin of the baby and measures the baby's body temperature.
The temperature display or thermometer should be separate from the temperature control unit. It can be a digital display or an alcohol glass thermometer. For safety reasons mercury thermometers must not be used in infant incubators. Mercury is toxic.

Humidity control
The heated air flows over the water in the water container and gets moistened. The humidity can be regulated by closing and opening a deflector plate over the container. Other incubators have a water heater which creates more humidity the warmer the water gets. The humidity should be adjustable between 40 and 90%.
The humidity is measured by a hygrometer, a digital or a traditional dial instrument.
The humidifier should be filled up only with distilled water only in order to avoid corrosive damage to the incubator.

Oxygen control
Most of the infant incubators have a hose connection for applying additional oxygen from an external cylinder, oxygen concentrator or from the central gas supply. In this case the warmed and moistened air gets also enriched with the oxygen.
Alternatively the baby gets the additional oxygen directly via a nasal cannula.

Alarms and safety features
For safety reasons all electronically controlled infant incubators monitor all functions and an alarm is generated when something does not work properly.
In case of a failure the following alarms are given:

  Overheating. When the air in the cabinet gets too hot.
  Fan failure. When the fan is not turning any more.
  Power off. When the power supply fails. For this to work an additional alarm battery is

Additionally all incubators, even the older non-electronic controlled ones have an over-heating protection. The heater is switched completely off when the temperature in the cabinet reaches 40°C.

Some newer infant incubators do not have washable air inlet filters but special bacterial filters. These filters have to be exchanged after 3 or 6 month and can not be cleaned.

Installation requirements
Infant incubators should be placed in a quiet environment with no exposure to direct sun light. Noisy oxygen generators should not be placed beside the incubator. Oxygen cylinders have to protected against falling over.

Note: In case of a power cut the incubator does not work any more. A small UPS as used for computers is no solution because the power consumption (of the heater) is too high.

The usage of an infant incubator is not difficult and the functions should be self-explanatory. Nevertheless the technician and the user have to read the user manual carefully before usage.
A missing user manual should be available from the manufacturer or from the Internet.
In addition to the instruction of the manufacturer here some general hints:

  Always preheat the incubator and wait for half an hour until temperature and humidity are
  Babies are kept in the incubator undressed apart from a nappy.
  Check and record the temperature every 5 hours and hourly in a critical care stage.
  Default air temperature in the incubator is 35°C.
  The setting for the humidity for small babies is 70 - 80% in the beginning, later 40%.
  Use distilled water only. It has to be drained and renewed every day.
  Do not place equipment on top of the canopy.

Some manufacturers suggest cleaning the incubator every day with a mild soap water solution, some don't. But at least the incubator has to be cleaned and disinfected thoroughly

  after each change of infant
  at least once a week.

Therefore all inserts have to be removed and cleaned with hot soap water added with antiseptic. Then the inserts and the cabinet have to be dried and ventilated before they can be reused.
The disinfection products which can be used or should not be used are noted in the user manual.
The humidifier reservoir has to be cleaned and the water changed every day.
The air inlet filter should be changed or washed according to the user manual or every 3 months.

Start the maintenance with a visual check. Ensure that the hood is free of cracks and the hinges move smoothly and all switches and knobs are OK. Check all probes, cables, and tubes for cracks and the port sleeves for tears. Check or replace also the alarm battery, if there is one.
Continue with a test run with a function test and a temperature check and a calibration if needed. The temperature check and the calibration procedure are described in the service manual.
When the service manual is not present, a typical temperature check can be performed as follows:

  Use a reference thermometer with an accuracy of 0.5° or better and place it in the centre
    of the mattress.
  Set the temperature to 36°C, wait at least 30 min and then check the temperature for
    6 hours. The temperatures should not differ more than 1°.

When the temperature differs more, the control unit has to be calibrated. In electronic controls there is always is a trim-pot for doing the adjustments. Mechanical thermostats usually do not have a calibration point. But here the knob or the pointer can be twisted.
The safety thermostat or over-temperature cut-off can be tested by bypassing the main thermostat. Then the temperature will rise above 40°C, an alarm should be given and the safety function has to switch off the heater. It is also possible to use a hair dryer to warm up the sensor, or to move it closer to the heating element.
Check also the humidity. The incubator should be able to create up to 80% humidity. On the other hand it should be possible to reduce the humidity down to 40%.

An important task during the maintenance is the thorough cleaning of the technology compartment under the cabinet.
A vacuum cleaner helps a lot to clean the inside of the incubator but it is not essential. A brush will also do. Plastic parts and everything that is water resistant should be washed with hot soap water added with antiseptic. Do not forget the air inlet filter.
A dusty fan can be cleaned easily with a brush, but this should not really be needed because of the inlet filter.

Special tools
A calibrated (electronic) thermometer with an accuracy of 0.1°C is needed. A clinical (fever) thermometer can not be used because it only shows the maximum temperature.
A hygrometer is also needed when the humidity has to be checked.

Typical problems
  Burned heating elements can be replaced against other types as long as the wattage is
    the same. Some heating elements are protected by a thermal fuse. Consider this when
    you diagnose a 'blown' heating element.
  Also, a blown heating element or thermal fuse has a reason. Check the fan operation.
  When replacing a defective thermostat do not only look only at the temperature range but
    also at the accuracy and the hysteresis. Hysteresis is temperature difference between
    switching off and switching on. For an incubator a hysteresis of 1°C is needed.
  Be careful with thermostats. The thin capillary tube is usually rolled up. This is correct.
    Do not cut it. It is not a wire, but a fine tube. Also avoid bending the tube. It can get kinks
    and will not work any more.
  Lubrication also helps when the fan gets noisy. Axial computer-type fans have a label in
    the centre which can be lifted up a bit for lubrication. One drop of oil is enough.
  Broken hood thermometers should never be replaced against mercury glass-
    thermometers. The glass can brake and mercury is toxic.
  The sleeves of the ports are often torn, filthy or missing, but you can find the material for
    replacement easily in every town.
  Casters and brakes are often damaged and spares are difficult to find. But often the
    incubators are not moved anyway and the casters are not needed. In this case it is a
    good idea to remove them completely to ensure the incubator is standing stable.

Spare parts
Modern incubators use disposable bacterial filters that can not be reused. Make sure you have some spare filters in stock.
Broken or disappeared thermometers can be replaced by mechanical or alcohol thermometers only. Mercury thermometer must not be used.

Life expectancy
According to WHO the life expectancy of infant incubators is 7 - 10 years.
Well maintained incubators can reach easily 15 years or more.

Do-it-Yourself infant incubator
Building an infant incubator is an interesting challenge for all craft skilled technicians and technically interested students. The control of temperature and humidity and some safety features can be done relatively easy electrically, electronically or with a microprocessor, depending on your preferences.
When surfing through the Internet you get the impression that do-it-yourself equipment made from cheap components is right for hospitals in developing countries. But this is wrong. What is needed is robust equipment built with proven and simple technology and not high-tech with cheap components which nobody can repair.
If you really want to design an incubator for low-income countries think about the following criteria:

The construction must be simple and easy to understand. A local technician should be able to repair the incubator one day. Mechanic thermostats are therefore better than electronic controls. But a simple electronic control is still better than a microprocessor, which needs a special program and thus can not be repaired.

Mechanically robust
Working conditions in developing countries are often rough. The cabinet should be as durable as possible and easy to clean.
Few and big knobs for setting the values are better than delicate push buttons.

Technically robust
The simpler the control the less sensitive it is against voltage fluctuations and power outages. The components also have to be protected against spikes and surges.

Cheap but good quality
The whole incubator should be much less expensive than a commercial one, but the components and the design should be of high quality. Save money by reducing the features but do not try to save money by using cheap low quality components.
Universal heating elements (or lamps), universal fans (from a computer maybe) and universal and simple control elements are needed.

Some interesting solutions I have collected here.

Links and sources
Here are some Wikipedia Medical Equipment articles:

     Neonatal intensive care unit (NICU)

image You can read the revised article in the book:

Introduction to Medical Equipment Repair

The book also contains a chapter on preventive maintenance, with recommendations on how to perform the various tests.
Click here for more information.