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Wednesday, January 26, 2011

Neonates should be kept warm

We had 6 day old neonate present to the emergency room in respiratory distress. When I arrived on the scene I saw a baby lying limp on the ER bed. The first thing I said was, "We need to warm that baby up."

One of the nurses proceeded to get a warm blanket, while another went up to OB to get an incubator. Within moments of being warmed the baby's respiratory rate improved, and muscle tone was much improved.

The moral of this story is one of the first things to think of when you have a neonate is to make sure it is warm. Ideally you will want a core temperature between 36.5 and 37.5 degrees celcius.

You may not think of this, but you have heat and cold receptors in your skin deep in the tissue, which send signals to the hypothalamus in the brain to release norepinepherine, which start a cascade of events.

According to S.T.A.B.L.E Program, "In response to cold stress, a series of reactions are activated for the purpose of decreasing heat loss and increasing heat production These include constriction of blood vessels in the arms and legs, increased muscle flexion activity, and metabolism of brown fat. To mount these responses, the metabolic rate must increase which, in turn, increases utilization of both oxygen and glucose."

Vasoconstriction: This prevents blood from reaching the skin to keep vital organs warm. If prolonged, oxygen delivery to the skin is prevented.

Increased muscle activity and flexion: Infants do not shiver. Instead they cry and move around to keep warm. This also reduces surface area for heat loss. If a child is flaccid, heat loss is increased due to increased surface area.

Brown fat metabolism: 6-8% of infants body weight consists of brown fat, which can be burned to create heat. Brown fat exists around vital organs such as the kidneys, adrenal glands, mediastinum, subscapular and axillary regions and the nape of the neck. When signaled to burn, brown fat creates more energy than any other tissue in the body to produce heat. This is actually called "non-shivering thermogenesis."

If cold stress gets bad enough, this can lead to pulmonary vasoconstriction, which can lead to a right to left shunt of blood through the foraman ovale and the ductus arteriosis, which can actually lead to persistent pulmonary hypertenstion and hypoxemia. The child will ultimately decrease it's respiratory rate and become flacid.

Mechanism of heat loss:

1. Conductive: This is heat loss that involves the transfer of heat between two solid surfaces, such as a metal surface the baby is set on, or cold stethoscopes, x-ray plates, blankets, your hands. These things should all be pre-warmed.

2. Convection: This is heat loss that occurs as a result of air currents, such as air conditioners, windows, drafts from air vents, or lab coats. This is why it's important to make sure the air is warm before baby is born, and limit anything that might cause a cold breeze, such as lab coats. It's also important to warm and humidify oxygen to an infant.

3. Evaporative: Occurs when moisture on the skin surface or respiratory tract mucosa is converted into vapor. The process of evaporation is always accompanies by a cooling effect. This is why it's important to dry infants with a warm towel and immediately remove wet linens.

4. Radient Heat Loss: This is heat loss by transfer of heat between solid surfaces that are not in contact with each other. The infant's skin temp is usually warmer than surrounding surfaces, so the direction of heat transfer will be from the exposed parts of the infant's body to the adjacent solid surfaces. The cooler those surfaces, the greater the heat loss. Examples include cold windows.

5. Radient Heat Gain: Just for the sake of it, a child can also pick up heat from distant objects, such as the sun poking through the window, or an incubator that's set too high.

Preterm infants actually become hypothermic faster than term babies due to larger surface area, thinner and immature skin, decreased amounts of brown fat, poor muscle tone, and poor ability to vasoconstrict. Premature infants should be placed in an incubator and wrapped in polyethylene (plastic) from neck to feet to reduce evaporative and convective heat loss.

Temperature of neonates, particularly premature neonates, should be monitored closely.

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