Chapter 27 The Baby at Birth

A newborn baby’s survival is dependent on adaptations in cardiopulmonary circulation and other physiological adjustments to replace placental function and maintain homeostasis. Birth is also the commencement of the early parent/baby relationship.

Adaptation to extrauterine life

Subjected to intermittent diminution of the oxygen supply during uterine contractions, compression followed by decompression of the head and chest, and extension of the limbs, hips and spine during birth, the baby emerges from the mother to encounter light, noises, cool air, gravity and tactile stimuli for the first time. Simultaneously, the baby has to make major adjustments in the respiratory and circulatory systems, as well as controlling body temperature.

Respiratory and cardiovascular changes are interdependent and concurrent.

Pulmonary adaptation

Prior to birth, the fetus makes breathing movements and the lungs will be mature enough, both biochemically and anatomically, to produce surfactant and will have adequate numbers of alveoli for gas exchange. The fetal lung is full of fluid, which is excreted by the lung itself.

During birth, this fluid leaves the alveoli, either by being squeezed up the airway and out of the mouth and nose, or by moving across the alveolar walls into the pulmonary lymphatic vessels and into the thoracic duct, or to the lung capillaries.

Stimuli to respiration include the mild hypercapnia, hypoxia and acidosis that result from normal labour, due partially to the intermittent cessation of maternal–placental perfusion with contractions. The rhythm of respiration changes from episodic shallow fetal respiration to regular deeper breathing, as a result of a combination of chemical and neural stimuli: notably, a fall in pH and PaO₂ and a rise in PaCO₂. Other stimuli include cold, light, noise, touch and pain.

Considerable negative intrathoracic pressure of up to 9.8 kPa (100 cm water) is exerted as the first breath is taken. Pressure exerted to effect inhalation diminishes with each breath taken until only 5 cm water pressure is required to inflate the lungs. This is an effect of surfactant, which lines the alveoli, lowering surface tension thus permitting residual air to remain in the alveoli between breaths. Surfactant is a complex of lipoproteins and proteins produced by the alveolar type 2 cells in the lungs; it is primarily concerned with the reduction in surface tension at the alveolar surface, thus reducing the work of breathing.

Cardiovascular adaptation

The baby’s circulatory system must make major adjustments in order to divert deoxygenated blood to the lungs for reoxygenation.

With the expansion of the lungs and lowered pulmonary vascular resistance, virtually all of the cardiac output is sent to the lungs.

Oxygenated blood returning to the heart from the lungs increases the pressure within the left atrium.

Pressure in the right atrium is lowered because blood ceases to flow through the cord.

A functional closure of the foramen ovale takes place. During the first days of life this closure is reversible and reopening may occur if pulmonary vascular resistance is high – e.g. when crying – resulting in transient cyanotic episodes in the baby.

The septa usually fuse within the first year of life to form the interatrial septum, though in some individuals perfect anatomical closure may never be achieved.

Contraction of the muscular walls of the ductus arteriosus takes place; this is thought to occur because of sensitivity of the muscle of the ductus arteriosus to increased oxygen tension and reduction in circulating prostaglandin. As a result of altered pressure gradients between the aorta and pulmonary artery, a temporary reverse left-to-right shunt through the ductus may persist for a few hours, though there is usually functional closure of the ductus within 8–10 hours of birth.

The remaining temporary structures of the fetal circulation – the umbilical vein, ductus venosus and hypogastric arteries – close functionally within a few minutes after birth and constriction of the cord. Anatomical closure by fibrous tissue occurs within 2–3 months, resulting in the formation of the ligamentum teres, ligamentum venosum and the obliterated hypogastric arteries. The proximal portions of the hypogastric arteries persist as the superior vesical arteries.

Thermal adaptation

The baby enters a much cooler atmosphere, the birthing room temperature of 21°C contrasting sharply with an intrauterine temperature of 37.7°C. Heat loss can be rapid, and takes place through the mechanisms listed in Box 27.1.

Box 27.1 Mechanisms of heat loss in the newborn baby

Evaporation

• Amniotic fluid evaporates from the skin. Each millilitre that evaporates removes 560 calories of heat. The baby’s large surface area:body mass ratio potentiates heat loss, especially from the head, which comprises 25% of body mass

Poor insulation

• The subcutaneous fat layer is thin, allowing rapid transfer of core heat to the skin and the environment

Conduction

• Conduction takes place when the baby is in contact with cold surfaces

Radiation

• Heat radiates to cold objects in the environment that are not in contact with the baby

Convection

• This is caused by currents of cool air passing over the surface of the body

The heat-regulating centre in the baby’s brain has the capacity to promote heat production in response to stimuli received from thermoreceptors. However, this is dependent on increased metabolic activity, compromising the baby’s ability to control body temperature, especially in adverse environmental conditions. The baby has a limited ability to shiver and is unable to increase muscle activity voluntarily in order to generate heat. Therefore the baby must depend on his or her ability to produce heat by metabolism.

The neonate has brown adipose tissue, which assists in the rapid mobilisation of heat resources (namely, free fatty acids and glycerol) in times of cold stress. This mechanism is called non-shivering thermogenesis. Babies derive most of their heat production from the metabolism of brown fat. The term baby has sufficient brown fat to meet minimum heat needs for 2–4 days after birth, but cold stress results in increased oxygen consumption as the baby strives to maintain sufficient heat for survival. Brown fat uses up to three times as much oxygen as other tissue, with the undesired effect of diverting oxygen and glucose from vital centres such as the brain and cardiac muscle. In addition, cold stress causes vasoconstriction, thus reducing pulmonary perfusion, and respiratory acidosis develops as the pH and PaO₂ of the blood decrease and the PaCO₂ increases, leading to respiratory distress, exhibited by tachypnoea, and grunting respirations. This, together with the reduction in pulmonary perfusion, may result in the reopening or maintenance of the right-to-left shunt across the ductus arteriosus. Anaerobic glycolysis (i.e. the metabolism of glucose in the absence of oxygen) results in the production of acid, compounding the situation by adding a metabolic acidosis. Protraction of cold stress, therefore, should be avoided. The peripheral vasoconstrictor mechanisms of the baby are unable to prevent the fall in core body temperature that occurs within the first few hours after birth. It is important, therefore, to minimise heat loss at birth.

Immediate care of the baby at birth

Prevention of heat loss

It is important to provide an ambient temperature in the range 21–25°C. The baby’s temperature can drop by as much as 3–4.5°C within the first minute. Measures to conserve heat include:

drying the baby at birth

removing wet towels

encouraging skin-to-skin contact with the mother

wrapping the baby in dry, prewarmed towels.

Clearing the airway

As the baby’s head is born, excess mucus may be wiped gently from the mouth. Care must be taken to avoid touching the nares, as such action may stimulate reflex inhalation of debris in the trachea. Although fetal pulmonary fluid is present in the mouth, most babies will achieve a clear airway unaided. Only rarely will it be necessary to clear the airway with the aid of a soft suction catheter attached to low-pressure (10 cm water) mechanical suction. It is important to aspirate the oropharynx prior to the nasopharynx so that, if the baby gasps as the nasal passages are aspirated, mucus or other material is not drawn down into the respiratory tract. Suction at the back of the pharynx can result in vagal stimulation, with laryngospasm and bradycardia.

Cutting the cord

The optimal time for umbilical cord clamping after birth remains unknown. Delaying cord clamping for at least 2 minutes in a term neonate can be beneficial. What is agreed is that a term baby at birth can be drawn up onto the mother’s abdomen but raised no higher, and a preterm baby should be kept at the level of the placenta. This is because:

if a preterm baby is held above the placenta, blood can drain from the baby to the placenta, resulting in anaemia

if the baby is held below the placenta, it can cause him or her to receive a blood transfusion.

Early clamping and cutting of the cord is advocated in preterm babies.

Identification

The time of birth and sex of the baby are noted and recorded.

Babies born in hospital must be readily identifiable from one another. In the UK each baby is issued with a National Unique Reference Number for receipt of Health and Social Care services.

Assessment of the baby’s condition

At 1 minute and 5 minutes after the birth, an assessment is made of the baby’s general condition using the Apgar score (Table 27.1).

The assessment at 1 minute is important for the further management of resuscitation.

An assessment at 5 minutes provides a record of response to resuscitation and immediate care needs.

The higher the score, the better the outcome for the baby. A mnemonic – APGAR – for the Apgar score is given in Box 27.2.

Table 27.1 The Apgar score

Box 27.2 A mnemonic for the Apgar score

• Appearance, i.e. colour

• Pulse, i.e. heart rate

• Grimace, i.e. response to stimuli

• Active, i.e. tone

• Respirations

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Related posts:

1. The Placenta
2. Prolonged Pregnancy and Disorders of Uterine Action
3. The Third Stage of Labour
4. Problems of Pregnancy

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