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Anatomy & physiology

Respiratory System

 

At birth the alveoli are thick walled and only number 10% of the adult total. Lung growth occurs by alveolar multiplication until 6 - 8 years. The airways remain relatively narrow until then, which results in a high incidence of airway disease. Ventilation is almost entirely diaphragmatic.

 

During the first 2 years of life the geometry of the rib cage changes, with the gradual development of the "bucket handle" configuration seen in the adult. Ribs tend to be more horizontal in infants and this limits the potential for thoracic expansion. The "ventilatory pump" (rib cage, diaphragm, abdominal and accessory muscles) tends to be less efficient in young children due to instability of the chest wall as well as the tendency of the diaphragm to easily tire. This is a result of its relative paucity of type-1 muscle fibres. The combination of a high airway resistance and low compliance results in a short time constant and therefore a rapid respiratory rate. The respiratory pattern is sinusoidal with no expiratory pause seen.

 

Oxygen consumption in young children is high: approximately 7 ml/kg/min at birth (c.f. 3-4 ml/kg/min in adults). The metabolic cost of respiration is higher than in adults and may reach 15% of total oxygen consumption. Similarly the metabolic rate in infants is almost twice that of adults and consequently alveolar minute volume is greater and the FRC relatively low. As a result both inhalational induction and awakening at termination of anaesthesia is more rapid. Similarly, hypoxia also occurs much more rapidly.

 

The outward recoil of the chest wall in infants and young children is low, but the inward lung recoil is similar to that of a young adult. As the FRC is the lung volume at which the outward recoil of the chest wall exactly balances that of the inward recoil of the lungs, this leads to a reduced in FRC. However it is maintained by other mechanisms (e.g. laryngeal braking during expiration and active diaphragmatic and intercostal expiratory tone) in the awake state, although not necessarily during anaesthesia, contributing to rapid desaturation. The closing volume is greater than the FRC until 6-8 years due to the poor elastic properties of infant lungs, so airways closure occurs during normal tidal ventilation. This leads to an increase in alveolar-arterial oxygen tension difference and a normal PaO2 of 9-9.5 kPa.

 

The physiological dead space is approx. 30% of the tidal volume, as in adults, but the absolute volume is small, so that any increase caused by apparatus deadspace has a proportionally greater effect on small children. During anaesthesia, deadspace must be kept to a minimum, and the resistance of breathing apparatus should be kept low.

 

These changes in lung mechanics compared to those of adults increase the susceptibility of infants to respiratory infection and also to the adverse ventilatory effects of anaesthesia. Secretions resulting from cholinergic activity or an URTI may cause respiratory difficulty. This includes breath holding and coughing on induction of inhalational anaesthesia, as well as an increased incidence of bronchospasm and even more significantly, laryngospasm.

 

Distribution of ventilation and perfusion is different in children. In spontaneously breathing adults, ventilation and perfusion are distributed preferentially to dependent lung areas, with the diversion of ventilation to the non-dependent areas during IPPV. However in spontaneously breathing children, the situation is similar to that seen in the ventilated adult, with uppermost areas being better ventilated and lowermost better perfused. This leads to an increased V/Q mismatch as compared to adults.

 

Lung Mechanics of the Neonate Compared with the Adult

 

Variable

Neonate

Adult

Compliance (ml/cmH2O)

5

100

Resistance (cmH2O/l/s)

30

2

Time constant (s)

0.5

1.3

Respiratory rate (breaths/min)

32

15

 

 

Respiratory Variables in the Neonate

 

Tidal volume

7 ml/kg

Deadspace

Tidal vol. X 0.3 ml.

Respiratory rate

Neonate: 32 breaths/min

Age 1-13: (24 – age)¸ 2 breaths/min

Minute ventilation

220 ml/kg/min

Alveolar ventilation

140 ml/kg/min

FRC

30 ml/kg

 

 

The narrowest part of the pre-pubertal upper airway is at the cricoid ring C3/4, and thus if oedema develops there is no possibility of expansion. 1mm of oedema can lead to a 60% reduction in internal diameter. The larynx is high and anteriorly placed. It is opposite C3 in premature neonates, C3-4 in full term neonates and reaches C4 in adulthood. The trachea is short, only 4 cm at birth, and the angle of the carina is wider.

 

Neonates are obligatory nose breathers, which may be a problem if the nose blocked e.g. by a NG tube.

 

 

Cardiovascular System

 

Infants have a high cardiac output (commensurate with their high metabolic rate) of approx. 200 ml/kg/min (2-3 times that of adults.) 60% (by weight) of a baby's heart is non-contractile (30% in adults) with a thick muscular right ventricle. Thus ventricular compliance is poor. This increase in cardiac output is produced by an increase in heart rate, as they are unable to increase their stroke volume. Babies can tolerate a HR up to 200/min without evidence of heart failure.

 

Tachyarrythmias are uncommon in the absence of cardiac disease. Cardiac arrest is usually secondary to extreme bradycardia/asystole; VF is rare in normally connected hearts due to the low muscle mass found in infant hearts. However bradycardia occurs readily in presence of hypoxia and vagal stimulation (as children do have a relatively high vagal tone). Rapid treatment with O2 or atropine (20m g/kg iv.) is required.

 

BP is low at birth (approx. 80/50) secondary to a low SVR, due to the large proportion of vessel-rich tissues in children. BP increases within the 1st month to approx. 90/60 and reaches adult levels at approx. 16 years. As a guide for neonates, especially those born pre-term; a mean BP in mmHg of at least the gestational age in weeks should be achieved.

 

Variation of HR with Age

 

Age

Mean value

Normal range

Neonate

140

100-180

1 year

120

80-150

2 years

110

80-130

6 years

100

70-120

12 years

80

60-100

 

Neonates have a reactive pulmonary vasculature, and reversion to a transitional circulation may occur during the first few weeks of life, precipitated by an increase in PVR (e.g. secondary to acidosis, hypoxia or hypercapnia) and a decrease in SVR (e.g. most anaesthetics).

 

Monitoring of CVS

 

- ECG: RAD and RV dominance at birth, resembles adult ECG by about 1 year.

 

- NIBP, unless regular ABG are necessary. The complication rate of arterial cannulation is much higher in children. Slow continuous flushing should be used (and volume of fluid infused carefully measured) as intermittent flushing has been shown to cause retrograde flow from the radial artery to the carotid artery, risking cerebral emboli.

 

- CVP: Useful in the treatment of fluid imbalance or haemorrhage. Femoral venous cannulation is often technically easier than the internal jugular vein, especially in small children, is a safer route and carries a lower complication rate

 

Blood Volume. This can show a variation of up to +/- 20% at birth depending on the stage at which the umbilical cord is clamped. Blood losses >10% circulating volume should be replaced with blood, to maintain the Hct>30.

 

Average blood volume

 

At birth

90 ml/kg

Infant & young child

80 ml/kg

> 6-8 years

75 ml/kg

 

An alternative method of estimating blood volume at birth which takes into account degree of transfusion from the placenta is (50ml+Hct)/kg.

 

Haemoglobin

 

75-80% Hb is HbF at birth. The decrease in blood volume and HbF occur before HbA haemopoiesis is fully established at 6 months. Hb decreases from approximately 160g/l at birth to about 90-100 g/l at 6-10 weeks,

 

HbF has lower 2,3-DPG content and therefore a higher affinity for O2. The Oxygen Dissociation Curve (ODC) is thus shifted to the left. Metabolic acidosis, which persists from foetal life into infancy and a high CO2 resulting from a high metabolic rate help to improve oxygen delivery to the tissues by shifting the curve to the right. Respiratory alkalosis caused by hyperventilation reduces oxygen availability and should be avoided.

 

Haemorrhage must be monitored carefully and blood for transfusion should be warmed and filtered. If small volumes are required, or blood loss is rapid, replacement can be syringed in via 3-way tap. Otherwise a burette should be used.

 

 

Metabolism and Homeostasis

 

Renal Function and Fluid Balance

 

Total Body Water as Percentage of Body Weight:

 

Premature

85%

Neonate

80%

Infant

75%

Adult

65%

 

The proportion of total body water present as ECF exceeds ICF at birth. This ratio gradually reverses with age. There is a considerable reduction in total body water during the first few days of life, with concomitant changes in drug distribution. Fluid turnover is much greater in infants (15% of total body water/day) due to their relative inability to concentrate urine. Thus interruption of fluid intake in infants can rapidly lead to dehydration. This is especially true of premature infants, who have increased insensible losses due to a high surface area to volume ratio. However, overloading a neonate can result in the re-opening of a patent ductus ateriosus.

 

GFR and tubular reabsorption rate are reduced until 6-8 months; thus a decreased ability to handle excessive water loads exists. Both GFR and RBF are low in first 2 years of life. This immature renal function causes a relative inability to handle excessive sodium loads, and may lead to the accumulation and toxicity of drugs that are renally excreted.

 

Fluid Therapy

 

Temperature Regulation and Maintenance

 

Children have a high surface area to volume ratio (2.5 times greater in neonates than in adults); and their natural insulation is poor at birth. Thus they are susceptible to heat loss to the surrounding environment. This is reflected by the fact that the thermoneutral environment is 34° C for premature babies, 32° C for neonates and 28° C for adults. Note that even incubators are unable to provide a thermoneutral environment for a naked pre-term infant.

 

Infants less than 3 months old depend upon non-shivering thermogenesis. This is achieved by increasing the metabolism of brown fat; this causes an increase in oxygen consumption, which may stress the immature respiratory system and may even cause respiratory failure. The control of brown fat metabolism is compromised by general anaesthesia, so it is important to maintain body temperature by other means during surgery: viz. wrapping limbs in orthopaedic wool/padding or using a space blanket, placing the baby on a pre-heated blanket, humidifying and warming inspired gases. Care must also be taken not to over-heat neonates as they have poorly developed sweating mechanisms.

 

A decrease in core temperature may lead to respiratory depression, reduced cardiac output, prolongation of the action of drugs (esp. muscle relaxants) and increased risks of hypoventilation, regurgitation and aspiration in the post-operative period.

 

NB: Malignant hyperpyrexia is extremely rare in children < 3 years.

 

CNS

 

At birth myelination is incomplete. This results in an altered control of ventilation, with periodic breathing and apnoeas seen up to about 60 weeks post-conceptual age. This immaturity causes an increased sensitivity to the respiratory depressant effects of narcotics and volatile agents.

 

 


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©Charles Gomersall, March, 2007 unless otherwise stated. The author, editor and The Chinese University of Hong Kong take no responsibility for any adverse event resulting from the use of this webpage.
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