Extremely low birth weight babies pose special challenge to anaesthesiologist as it is a very special subset of patients with unique pathophysiology and presentation. Thorough knowledge and understanding of their pathophysiology is very important for successful outcome.
The three categories of Low birth weight babies can be divided as
- Low Birth Weight ( LBW ): Less than 2500 Gms
- Very Low Birth Weight ( VLBW ) : Less than 1500 Gms
- Extremely Low Birth Weight (ELBW ) : Less than 1000 Gms
According to National Family Health Survey -4, the incidence of LBW is 15.8% whereas the incidence of VLBW is 1.2%. Exact data about the incidence of ELBW is not available. ELBW are frequently born around 27 weeks of Gestation or even earlier. Higher gestational age and birth weight, female gender, better maternal education, and white race have been recognized as significant predictors of decreased morbidity in ELBW infants. The mortality rate has significantly reduced for this group with improved technology and better understanding of pathophysiology.
It is very important to understand the pathophysiology of ELBW babies before proceeding for any intervention.
Specific problems of ELBW:
- Thermoregulation: Prone to heat loss due to higher body surface area and thin skin.
- Respiratory Distress Syndrome: Pulmonary surfactant is produced at 24-26 weeks; alveolar development begins at 32 weeks and is complete by 18 months of age. ELBWs require pulmonary surfactant and CPAP or IPPV after delivery for survival and are susceptible to bronchopulmonary dysplasia. Surfactant deficiency causes decreased pulmonary compliance, alveolar hypotension, and an imbalance between pulmonary ventilation and perfusion. Clinically they present with tachypnoea, chest retractions, nasal flaring, cyanosis and grunting, this condition usually progresses to hypoventilation, hypoxemia and respiratory acidosis.
The INSURE technique ( Intubate-Surfactant-Extubate) technique is now gradually being replaced by LISA ( Less Invasive Surfactant Administration) which involves administrating surfactant via an intratracheal catheter while baby is breathing spontaneously on CPAP or NIPPV.
- Patent Ductus Arteriosus: Up to 80% of ELBW infants have a clinically significant patent ductus arteriosus (PDA). As a consequence of the left-to-right systemic to pulmonary shunting, they present with systolic murmur, hypotension, bounding pulses, decreased urine output, pulmonary hyperemia and edema, as well as and reduced mesenteric and cerebral perfusion. Contrary to term newborn who exhibit spontaneous ductus closure in 90% at 48 hours, it occurs in only 30 to 35% of ELBWs during the early neonatal period.
- Hemodynamic fluctuations: Hypovolemic shock should be managed by giving non-cross-matched O-Rh-negative blood or by administering an isotonic crystalloid solution; the proposed dose is 10–20 mL/kg  .Usually, mean arterial blood pressure corresponds to the gestational age .
- Central nervous system problems: Intraventricular hemorrhage (IVH) is an extravasation of blood in the brain that originates from the subependymal germinal matrix and advances into the ventricular system, most frequently occurring in the first 3 days of life . Periventricular leukomalacia (PVL) is damage to the periventricular white matter developed as a result of perinatal adverse insults such as hypoxia, hypo or hyper-perfusion, hypocarbia and chorioamnionitis combined with the defective cerebral vascular autoregulation in preterm infants. The estimated incidence of PVL is 4–15% in ELBW babies.
- Renal problems: Preterm infants exhibit increased sensitivity to impaired renal function. This is due to enhanced kidney maturation, fewer functional nephrons and higher renal filtration rate . Fluid status monitoring is a paramount. It involves daily monitoring of electrolytes, body weight, diuresis, blood pressure and insensible water loss
- Electrolyte imbalance; The ELBW infant is made up of 85% to 90% water, which is predominantly distributed in the extracellular space. During the first few postnatal days, a weight loss of 10–20% is observed which is attributable to diuresis and can be intensified by causes such as radiant warmers or phototherapy. These developments in addition to the compromised renal function constitute a setting for frequent electrolyte abnormalities such as hypo/hypernatremia and hyperkalemia 
- Impaired glucose homeostasis: Early hypoglycemia is a frequent occurrence in ELBW infants because of limited liver glycogen stores and immature endocrine mechanisms of blood glucose control.
- Anemia of prematurity: Frequently require transfusion.
- Bronchopulmonary dysplasia: BPD traditionally defined as a need for supplemental oxygen or ventilator support at 36 weeks’ post menstrual age (PMA) occurs with an incidence of around 30% in ELBW infants .
ELBWs are most commonly posted for surgical procedures for Necrotising Enterocolitis (NEC). Per se NEC gives some time to anaesthesiologist n neonatologist to stabilise the baby. But if NEC is associated with perforation of bowel, it mandates emergency exploration. Rarely, ELBWs can be posted for intracranial bleed. Procedures like ICD insertion are better performed under LA in this subset of patients.
Age is an important risk factor in anaesthesia. The risks of anaesthesia are greater in neonates and infants, even in expert hands. Anaesthesiologist should ensure full range of facilities for the perioperative care of ELBW beforehand.
- The newborn lung is small in relation to body size, tidal volumes small in absolute terms (7 ml/kg), the respiratory rate is high (40-50 breaths per minute) and there is little respiratory reserve.
- I strongly recommend hand ventilation with JR circuit to avoid barotrauma as it is difficult to achieve accurate VT even with high end workstations in ELBW babies.
- Postoperatively they are put on Pressure Control Ventilation for obvious reasons.
- The deadspace and resistance of the breathing circuit should be kept to a minimum in infants who are breathing spontaneously, to minimise the work of breathing in postoperative period.
- The tongue is relatively large, the occiput prominent so the head tips forward and the airway is easily obstructed. Airway patency is best maintained by chin lift, avoiding compression of the floor of the mouth, possibly with the use of an oropharyngeal airway.
- A proper size RBS mask or round Silicon mask are best for mask ventilation in ELBW.
- The epiglottis is long and straight and tends to flop back over the laryngeal inlet which is high and anterior; intubation is best achieved with a straight blade laryngoscope.
- The larynx is conical in shape, the narrowest portion at the level of the cricoid cartilage. Uncuffed tracheal tubes of size 2, 2.5 and 3 should be kept ready. Compatible stylets are also available to negotiate the problem of ant larynx.
- The trachea is short and endobronchial intubation is not uncommon. The position of the tracheal tube should always be checked by auscultation.
- The airways are narrow and are easily blocked by oedema or secretions. So always keep a high degree of suspicion whenever airway pressure goes up.
- Lung compliance is high and the ribs soft and elastic; chest wall compliance is higher compared with adults. The distending pressures on the lung are low and the newborn infant is prone to lung collapse, especially under general anaesthesia.
- The diaphragm is the predominant respiratory muscle in neonates but is more easily fatigable than in adults.
- A nasogastric tube should be passed to relieve gastric distension.
- Fixation of endotracheal tube is crucial. Dynaplast can damage the skin while removal. So it’s better to use Micropore, Transpore, G plast, leucoplast or similar less damaging yet better anchoring material for ET tube, NG tube or IV line fixation.
- Neonates have a high metabolic requirement for oxygen (6-8 ml/kg/min vs 4-6 ml/kg/min in adults).
- Tissue oxygen delivery is achieved by a relatively high cardiac output (300ml/kg/min vs 60-80 ml/kg/min in adults), high heart rate (120-180 beats per min) and respiratory rate (30-40 breaths per min); neonates do not tolerate bradycardia or interruption in ventilation and become hypoxic very readily. Hypoxia may lead to profound bradycardia.
- Preoxygenation with 100% oxygen for 3 to 5 minutes is very important to buy some time in case of difficult intubation.
- Equivalent haemoglobin concentrations for the same tissue oxygen delivery are 8g/dl, 6.5g/dl and 12g/dl for an adult, infant and neonate respectively. An infant tolerates anaemia fairly well, a neonate does not.
- If a child does require transfusion, they should be transfused, if possible, from one donor unit. A useful formula for transfusion is:
4ml/kg packed cells raises the Hb by 1g/dl
8ml/kg whole blood raises the Hb by 1 g/dl
Control of ventilation
- Anaesthetic agents depress ventilation in a dose dependent manner.
- Term neonates are probably not at risk of postoperative apnoea after routine minor surgery (avoiding opiates) from 1 month of age (i.e. 44 weeks PCA)
- Premature neonates are at low risk of postoperative apnoeas after 60 weeks post conception.
- Regional anaesthesia, without sedation (e.g. spinal anaesthesia for hernia repair) may reduce the risk of postoperative apnoeas.
- Cardiovascular function
- The cardiac muscle is immature at birth.
- Heart rate is an important determinant of cardiac output.
- Neonates increase cardiac output with careful volume loading (fluid bolus 5-10ml/kg), but they do not tolerate fluid overload.
- The myocardium is dependent on extracellular calcium for contraction. Hypocalcaemia may occur after large volume blood or blood product transfusion or in a child who is septic.
- Neonates are more sensitive to the negative inotropic effects of anaesthetic agents than older children; the effect is more marked with halothane than isoflurane or sevoflurane. Avoid deep anaesthesia, especially ventilation with high concentrations of volatile agents
- Atropine may counteract the reduction in cardiac output seen with volatile agents and will protect against vagally mediated reflexes, especially those associated with laryngoscopy and intubation. It is useful as premedication, although no longer routinely used, but should always be drawn up ready.
Patent arterial duct (PDA) is seen in 50% of extreme premature infants. Shunting from the aorta to the pulmonary artery results in respiratory distress syndrome and is a risk factor for intraventricular haemorrhage and necrotising enterocolitis.
Hepatic function and drug handling.
- The liver in the newborn contains 20% of the hepatocytes found in adults and continues to grow until early adulthood.
- In general, drug effects are prolonged in neonates and drugs should be titrated to effect, given by bolus rather than infusion.
- Plasma protein binding is reduced in neonates (low levels of α1-acid glycoprotein) and drugs that are plasma protein bound (such as local anaesthetics) may demonstrate increased toxicity in infants.
- Neonates have reduced hepatic stores of glycogen. Coupled with a high metabolic rate, this makes them susceptible to hypoglycaemia. Neonates should not be starved excessively (breast milk feed 4 hours preoperatively, clear fluids 2 hours preoperatively). Blood sugar should be measured during surgery. Glucose should be added to the intraoperative fluids, for instance, add 25ml of 50% dextrose to 500ml of Ringers Lactate to give a solution of 2.5% dextrose in Ringers Lactate.
- Nephrogenesis is completed at 36 weeks gestation Further increase in renal mass is due to the growth of tubules.
- The glomerular filtration rate at term is low and reaches adult indexed values only at 2 years of age. Creatinine at birth reflects the mother’s creatinine and falls to reflect renal function of the baby by 1 week of age.
- In the postoperative (catabolic) infant, renal insufficiency may become apparent and the neonate does not handle fluid or sodium overload.
Fluid and electrolyte balance
- The expanded extracellular fluid compartment results in an increased volume of distribution of commonly used drugs and increased dose requirements, despite increased sensitivity (muscle relaxants, intravenous induction agents).
- Fluids should be restricted until the postnatal weight loss has occurred. Liberal fluid regimens in the first few days of life have been shown to be associated with worse outcomes in premature infants (increased patent ductus arteriosus, necrotising enterocolitis and death). Fluid requirements increase incrementally from day 1 of life (60ml/kg/day) to 150ml/kg/day at 1 week of life (up to 180ml/kg/day in a premature neonate with high evaporative losses)
- 1 ml insulin syringes should be used for drug dilution. Total volume of drugs after dilution along with flushing volume should be taken in consideration for calculating final intake output of the baby. This amount may be significant in ELBW.
- Temperature control
- Thermoregulation in the neonate is limited and babies become cold easily.
- Heat production is limited and there is a greater potential for heat loss (high body surface area to body weight ratio, increased thermal conductance, increased evaporative heat loss through the skin).
- The newborn infant is able to increase heat production through brown fat metabolism (non shivering thermogenesis which is inhibited by volatile agents), however this is at the expense of increased oxygen consumption.
- Hypothermia is associated with hypoxia, impaired wound healing, prolonged coagulation with reduced platelet function, reduced drug metabolism, cerebral depression, myocardial depression, acidosis, decreased immunity, patient discomfort.
- The preterm baby is particularly vulnerable as the immature skin is thin and allows major heat (and evaporative fluid) losses.
- Shifting from NICU to operation theatre is crucial and must take place in wheeled radiant warmer cradle.
- OT should be prewarmed and warming mattress, warming blankets, radiant warmers and fluid warmers should be used.
- Neonates, including premature neonates, show well developed responses to painful stimuli. The foetus shows a stress response (and behavioural changes) to painful stimulation from 18-20 weeks gestation, which can be attenuated by the administration of fentanyl.
- Pain in neonates should be treated using multimodal analgesia, but opiates should be used judiciously, for both pharmacokinetic (reduced drug metabolism) and pharmacodynamic reasons (increased opiate sensitivity).
- Regional anaesthesia is preferred modality of pain management.
- Caudal epidural analgesia works very effectively in ELBWs undergoing Laparotomy for NEC.
- As the free form of LA is more, possibility of LAST should be kept in mind.
- As the loose skin of ELBWs over sacral hiatus slips over the bone, it is advisable to ask one assistant to stretch and stabilise skin over sacral area allowing anaesthesiologist to palpate hiatus with one hand and perform needle puncture with the other hand.
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By Dr. Gunjan Badwaik