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Fact sheet: Neonatal Biology – An Overview Part 1
Journal of Neonatal Nursing (2011) 17, 8e10
www.elsevier.com/jneo
Fact sheet: Neonatal Biology e An Overview Part 1* Julia Petty Available online 16 December 2010
The birth process imposes a significant and unique physical challenge to the neonate due to the necessity for adaptation to extra-uterine life and accomplishment of key life tasks necessary for survival. Such life tasks comprise independent breathing, the transition from fetal to neonatal circulation and the metabolic adaptation of thermoregulatory and glucose homeostasis which have been featured in previous Fact sheets (Petty, 2010a and b). For health professionals caring for neonates, it is important to understand the unique biological features of this special group in order to accurately assess normality and any potential deviation from the ‘norm’ which may require referral and further management (MacGregor, 2008; Baston and Durward, 2010; Tappero and Honeyfield, 2010). This Fact sheet provides an overview of key specific anatomical and physiological features of the term neonate as compared to the older child and/or adult. Any further differences in the preterm neonate will also be highlighted. In this issue, the respiratory, cardiovascular, haematological and immune systems of the neonate will be discussed.
*
NB: All images are taken from http://commons.wikimedia. org/ where permission is granted for use. E-mail address: [email protected]. 1355-1841/$ - see front matter doi:10.1016/j.jnn.2010.11.009
Respiratory system
Overall, the neonate’s respiratory system is smaller, shorter and less efficient in terms of the reserve and ability to cope with compromise. More specific features are as follows. An increased basal metabolic rate means that oxygen consumption is significantly higher than that of an adult. A neonate will become tachypnoeic in response to compromise (e.g. hypoxia) rather than increasing vital capacity of the lungs as an adult would do. They have a relatively high respiratory rate and short inspiratory time and are obligatory nose breathers up to 3 months of age. Structurally, the epiglottis is floppy and situated at a higher position than a child of over 8 years of age. The narrowest part of the neonate airway is the subglottic region. The airway is more of a funnel shape whereas over 8 years of age it becomes more cylindrical. The Diaphragm is the main respiratory muscle in neonates and they have shorter necks, smaller mouths, narrow nares and larger tongues compared to children or adults. Therefore, there is less airway protection and more resistance. The thorax is softer with a greater proportion of cartilage which means there is lower elastic recoil during breathing (Kanneh and Davies, 2000a and b).
Fact sheet neonatal biology e An overview part 1
9
In the preterm neonate, there is an underdeveloped respiratory centre leading to a predisposition to apnoea of prematurity. There is also immature pulmonary function due to less alveoli growth meaning the surface area for gaseous exchange is reduced and the lung functional residual capacity is lower. This is further exacerbated by potential surfactant deficiency particularly at very early gestations meaning the alveoli are more fragile with a high surface tension. Therefore the lungs are less compliant and so more easily damaged by the shearing forces of mechanical ventilation.
with high haemoglobin levels in the newborn period. These fall during the first 8e10 weeks of life to reach adult values at approximately 6 months of age. In the newborn, the majority of haemoglobin is fetal (HbF) e this has an increased affinity to oxygen according to the oxygen dissociation curve with less being given up to the tissues (left shift). This HbF is rapidly broken down by haemolysis leading to a high risk of developing physiological jaundice. Red blood cells have a shorter life span in neonatal life (60e70 days approximately whereas it is up to 120 days in adults). This further increases the chance of physiological jaundice. There is no vitamin K production until the neonate is fully fed e this, along with liver immaturity can lead to prolonged clotting times and is the rationale for administering vitamin K at birth. In the preterm neonate, the total blood volume is further reduced, the red blood cells have an even shorter life span (approximately 30e40 days) and there is a more rapid decline in HB. Clotting times are also more prolonged.
Cardiovascular and haematological systems
Immune system
As with the respiratory system, the cardiovascular system is less efficient in terms of the reserves to deal with compromise and illness. The specific features are as follows; The neonatal heart is less contractile than the adult heart with lower myocardial reserve (Kanneh and Davies, 2000a and b). Cardiac output is dependent on rate rather than stroke volume. There is also a relatively high pulmonary vascular resistance in the first few weeks of life until this lowers to reach adult values after the first month of life. Heart rate variability is wide between resting and active values. Being born preterm means the reduction in cardiac contractility is even greater and there is less sensitivity to the constricting effects of oxygen at birth to close the ductus arteriosus in the heart; therefore this may remain open. A neonate’s total blood volume is low (85 ml/ kg). There is a higher percentage of haematocrit
Neonates have reduced or immature specific and non-specific immune systems. For example. Non-specific immunity is via the action of B and T lymphocytes which form immunoglobulins against certain diseases or antigens. Immunoglobulins such as IgM and IgG start to be produced in utero in mid trimester but the full quantity required for complete immuno-protection is not reached until at least 1 year of age. IgG can cross the placenta but then the levels will fall in the first year of life rendering the neonate prone to infection. Protection against certain diseases and general defences take at least the first year to build up as immunoglobulins are produced and specific antibodies are formed against certain antigens. Immunisations are essential to further assist with the buildup of defenses against specific conditions. Prematurity means that neonates will fail to receive the transfer of IgG across the placenta during the last trimester and will be further immune-compromised. Non- specific immunity refers to the action of granulocytes which engulf bacteria and the release of Opsonins, chemical agents necessary for phagocytosis. Neonates have a less effective phagocytic
10
J. Petty
action as well as impaired opsonic activity with relatively reduced complement levels. In a subsequent issue, other systems will be outlined; namely the digestive, renal, hepatic, thermoregulatory and metabolic systems. To gain further information on neonatal anatomy and physiology, it is recommended that the reader undertakes further enquiry in this area to add depth and further details to the summary provided.
Further reading Baston, H., Durward, H., 2010. Examination of the Newborn e A Practical Guide, second ed. Routledge, London.
Kanneh, A., Davies, F., 2000a. Physical characteristics and physiological features of the full term neonate: theory practice integration e part 1. J. Neonatal. Nurs. 6 (1), 4e8. Kanneh, A., Davies, F., 2000b. Physical features of the full-term neonate: theory practice integration e part 2. J. Neonatal. Nurs. 6 (2), 49e54. MacGregor, J., 2008. Introduction to the Anatomy and Physiology of Children: a Guide for Students of Nursing, Child Care and Health. Routledge, London. Petty, J., 2010a. Normal postnatal adaptation to extra-uterine life part A e circulatory and respiratory changes at birth. J. Neonatal. Nurs. 16 (4). Petty, J., 2010b. Normal postnatal adaptation to extra-uterine life part B e thermoregulation and glucose homeostasis. J. Neonatal. Nurs. 16 (5). Tappero, E.P., Honeyfield, M.E., 2010. Physical Assessment of the Newborn e A Comprehensive Approach to the Art of Physical Examination, fourth ed. NICU-INK, CA.
Available online at www.sciencedirect.com
www.elsevier.com/jneo
Fact sheet: Neonatal Biology e An Overview Part 1* Julia Petty Available online 16 December 2010
The birth process imposes a significant and unique physical challenge to the neonate due to the necessity for adaptation to extra-uterine life and accomplishment of key life tasks necessary for survival. Such life tasks comprise independent breathing, the transition from fetal to neonatal circulation and the metabolic adaptation of thermoregulatory and glucose homeostasis which have been featured in previous Fact sheets (Petty, 2010a and b). For health professionals caring for neonates, it is important to understand the unique biological features of this special group in order to accurately assess normality and any potential deviation from the ‘norm’ which may require referral and further management (MacGregor, 2008; Baston and Durward, 2010; Tappero and Honeyfield, 2010). This Fact sheet provides an overview of key specific anatomical and physiological features of the term neonate as compared to the older child and/or adult. Any further differences in the preterm neonate will also be highlighted. In this issue, the respiratory, cardiovascular, haematological and immune systems of the neonate will be discussed.
*
NB: All images are taken from http://commons.wikimedia. org/ where permission is granted for use. E-mail address: [email protected]. 1355-1841/$ - see front matter doi:10.1016/j.jnn.2010.11.009
Respiratory system
Overall, the neonate’s respiratory system is smaller, shorter and less efficient in terms of the reserve and ability to cope with compromise. More specific features are as follows. An increased basal metabolic rate means that oxygen consumption is significantly higher than that of an adult. A neonate will become tachypnoeic in response to compromise (e.g. hypoxia) rather than increasing vital capacity of the lungs as an adult would do. They have a relatively high respiratory rate and short inspiratory time and are obligatory nose breathers up to 3 months of age. Structurally, the epiglottis is floppy and situated at a higher position than a child of over 8 years of age. The narrowest part of the neonate airway is the subglottic region. The airway is more of a funnel shape whereas over 8 years of age it becomes more cylindrical. The Diaphragm is the main respiratory muscle in neonates and they have shorter necks, smaller mouths, narrow nares and larger tongues compared to children or adults. Therefore, there is less airway protection and more resistance. The thorax is softer with a greater proportion of cartilage which means there is lower elastic recoil during breathing (Kanneh and Davies, 2000a and b).
Fact sheet neonatal biology e An overview part 1
9
In the preterm neonate, there is an underdeveloped respiratory centre leading to a predisposition to apnoea of prematurity. There is also immature pulmonary function due to less alveoli growth meaning the surface area for gaseous exchange is reduced and the lung functional residual capacity is lower. This is further exacerbated by potential surfactant deficiency particularly at very early gestations meaning the alveoli are more fragile with a high surface tension. Therefore the lungs are less compliant and so more easily damaged by the shearing forces of mechanical ventilation.
with high haemoglobin levels in the newborn period. These fall during the first 8e10 weeks of life to reach adult values at approximately 6 months of age. In the newborn, the majority of haemoglobin is fetal (HbF) e this has an increased affinity to oxygen according to the oxygen dissociation curve with less being given up to the tissues (left shift). This HbF is rapidly broken down by haemolysis leading to a high risk of developing physiological jaundice. Red blood cells have a shorter life span in neonatal life (60e70 days approximately whereas it is up to 120 days in adults). This further increases the chance of physiological jaundice. There is no vitamin K production until the neonate is fully fed e this, along with liver immaturity can lead to prolonged clotting times and is the rationale for administering vitamin K at birth. In the preterm neonate, the total blood volume is further reduced, the red blood cells have an even shorter life span (approximately 30e40 days) and there is a more rapid decline in HB. Clotting times are also more prolonged.
Cardiovascular and haematological systems
Immune system
As with the respiratory system, the cardiovascular system is less efficient in terms of the reserves to deal with compromise and illness. The specific features are as follows; The neonatal heart is less contractile than the adult heart with lower myocardial reserve (Kanneh and Davies, 2000a and b). Cardiac output is dependent on rate rather than stroke volume. There is also a relatively high pulmonary vascular resistance in the first few weeks of life until this lowers to reach adult values after the first month of life. Heart rate variability is wide between resting and active values. Being born preterm means the reduction in cardiac contractility is even greater and there is less sensitivity to the constricting effects of oxygen at birth to close the ductus arteriosus in the heart; therefore this may remain open. A neonate’s total blood volume is low (85 ml/ kg). There is a higher percentage of haematocrit
Neonates have reduced or immature specific and non-specific immune systems. For example. Non-specific immunity is via the action of B and T lymphocytes which form immunoglobulins against certain diseases or antigens. Immunoglobulins such as IgM and IgG start to be produced in utero in mid trimester but the full quantity required for complete immuno-protection is not reached until at least 1 year of age. IgG can cross the placenta but then the levels will fall in the first year of life rendering the neonate prone to infection. Protection against certain diseases and general defences take at least the first year to build up as immunoglobulins are produced and specific antibodies are formed against certain antigens. Immunisations are essential to further assist with the buildup of defenses against specific conditions. Prematurity means that neonates will fail to receive the transfer of IgG across the placenta during the last trimester and will be further immune-compromised. Non- specific immunity refers to the action of granulocytes which engulf bacteria and the release of Opsonins, chemical agents necessary for phagocytosis. Neonates have a less effective phagocytic
10
J. Petty
action as well as impaired opsonic activity with relatively reduced complement levels. In a subsequent issue, other systems will be outlined; namely the digestive, renal, hepatic, thermoregulatory and metabolic systems. To gain further information on neonatal anatomy and physiology, it is recommended that the reader undertakes further enquiry in this area to add depth and further details to the summary provided.
Further reading Baston, H., Durward, H., 2010. Examination of the Newborn e A Practical Guide, second ed. Routledge, London.
Kanneh, A., Davies, F., 2000a. Physical characteristics and physiological features of the full term neonate: theory practice integration e part 1. J. Neonatal. Nurs. 6 (1), 4e8. Kanneh, A., Davies, F., 2000b. Physical features of the full-term neonate: theory practice integration e part 2. J. Neonatal. Nurs. 6 (2), 49e54. MacGregor, J., 2008. Introduction to the Anatomy and Physiology of Children: a Guide for Students of Nursing, Child Care and Health. Routledge, London. Petty, J., 2010a. Normal postnatal adaptation to extra-uterine life part A e circulatory and respiratory changes at birth. J. Neonatal. Nurs. 16 (4). Petty, J., 2010b. Normal postnatal adaptation to extra-uterine life part B e thermoregulation and glucose homeostasis. J. Neonatal. Nurs. 16 (5). Tappero, E.P., Honeyfield, M.E., 2010. Physical Assessment of the Newborn e A Comprehensive Approach to the Art of Physical Examination, fourth ed. NICU-INK, CA.
Available online at www.sciencedirect.com