Anaesthetic consideration in premature infantDalila Hussain

Introduction

Premature babies are defined as those born before 37 weeks

Categorization of prematurity by gestational age & birth weight

Gestational age

Birth weight

36-37 week Borderline/ near term

<2500g LBW

31-36 week Moderately premature

<1500g VLBW

24-31 week Severely premature

<1000g ELBW

Physiology Airway :

- They are obligatory nose breathers due to high resistance to airflow through oral passage .

- large & lax tongue- easily fall back & obstruct airway

- Trachea length is 4cm

• Infant’ s larynx is higher in neck (C2-C3) compared to adult’s (C4-C5)- glottis @ C3 level in premature infant

• Large , floppy & ohmega (Ω) shaped epiglottis , make visualisation of glottis difficult during intubation

Funneled shape of larynx;-Narrowest part of infant’s larynx is @ cricoid ring with an approximate 14mm2 (ETT that passes easily through the glottic opening may be tight @ level cricoid ring)

Tight fitting ETT may cause

oedema

Closing volume relatively large & encroaches normal tidal volume--predispose to:

Airway close

Atelectasis

Increase O2 consumption ( 6ml/kg/min)to meet high metabolic rate

Tidal volume is relatively fixed due to anatomic structure

Unable to increase tidal volume--compesating for increase respiratory demands through raised respiratory rate -- lead to early fatigue

Minute alveolar ventilation is more dependent on increase resp. rate than on TV

Reduce FRC which is smaller than the closing capacity --- rendering them dependent on PEEP & prone rapid desaturation

Chemoreceptor responses are blunted

Extension of head does not facilitate intubation but even obstruct airway

External laryngeal pressure facilitate intubation by bringing anterior larynx into view

- Changes in head position result in ETT movement due to shortness of trachea & large tounge

- Flexion of the neck moves the tip of tube farther out of trachea

- Extension moves the tube farther into the trachea

Sniffing position

Neck flexed forward, head extend on the

neck & jaw held forward is

required for effective

ventilation

Respiratory system

- Fetal diaphragm contain only 10% Type 1 respiratory muscle ( high oxidative & resistant to fatigue)-- contributing to apnoea during physiological stress & fatigue more quickly

- Alveoli form @ 17-28 weeks gestational age

- Pulmonary capillaries form @ 28-36 weeks gestational age

- Lung maturation @ 36 week

- Lung surfactant produce at 32-34 weeks. Surfactant function to :

Lower surface tension in alveoli

Facilitate alveolar opening

Prevent alveolar collapse at end of expiration

Prone to apnoea (pauses in breathing > 20 sec/ loss effective breathing associated with bradycardia)

Central apnoea Obstructive apnoea Mixed apnoea

- Diminishedhypercapnicresponse

- Hypoxic ventilatorydepression

- Active inhibitory reflexes

- Opposition of hypopharyngealsoft tissues

- Nasal occlusion

-obstruction followed by central pauses

Risk factor for apnoea

Hypoglycaemia Hypoxia Anaemia

Hypothermia Sepsis

Post operation, apnoea are frequent in 1st 12 hour & can continue until 48-72 hour

Premature infant exposed to mechanical ventilation & > 28 days on O2 supplementation likely to develop bronchopulmonary dysplasia (BPD)

Sx of BPD :

Increase O2 requirements

Reduced lung compliance

Reversible airway obstruction

Cardiovascular system

The ductus arteriosus often remain patent ( PDA)

Other pathology eg: atrial / VSD can cause significant :

Left to right shunting

Progressively increased pulmonary flow

Congestive cardiac failure

Hypoxia is a potent pulmonary vasoconstrictor

Raised pulmonary vascular resistance (PVR)

Lead to right to left shunting, exacerbating hypoxia & acidosis

Prolonged raise PVR lead to:

Poor right ventricular fx

Impaired cardiac output

Limited oxygen delivery

Pulmonary oedema

Sudden death

Haematology

Term baby has 18-20g/dL of Hb

Premature baby : 13-15g/dL of Hb

70-80% of which is HbF

HbF has reduced ability to release oxygen

Target haematocrit 40-45%---facilitate O2 delivery

Estimation of blood volume in premature baby: 95ml/kg

If less than target, may necessitate preop blood transfusion

Renal Ability to retain Na+ @ 32 week GA

The distal tubular response to aldosterone is low until 34 week

ADH level are high

Impaired ability to concentrate urine

Drug excretion delayed due to immature renal system

Total body water : 75-85% of body weight . Inversely related to GA

Marked transepidermal permeability & large body surface area accelereate H2O loss

Evaporative H2O increase 15 fold during 1st few days of life compare with term babies.

Temperature regulation Increase surface area to body weight ratio

Decrease brown fat store- limit heat production

Non keratinised skin , extreamly susceptible to heat loss

Inability to shivering

Thermoneutral environment for unclothed preterm baby is 34 degree celcious

@ this temp, O2 demand is minimal

Hypohermia induced stress can lead to:

Hypoglycaemia

Apnoea

Metabolic acidosis

Glucose homeostasis

Have fewer glycogen stores

Underdeveloped gluconeogenesis pathway

Prone developing hypoglycaemia during starvation

Hyperglycaemia should be avoided as a hyperosmolar state can lead to IVH, osmotic diuresis & dehydration

Gastrointestinal system

Gastro-oesophageal reflux is common resulting from underdeveloped and incompetent lower oesophageal sphinter.

This lead to:

-Laryngospasm - Chronic cough

-Laryngitis -Tracheitis

-Apnoea -Otitis media

-asthma

NEC with bowel wall necrosis and perforation can lead to systemic sepsis

Drug metabolism is immature

Central nervous system

Pain receptors develop by 20 weeks GA

Pain pathways develop by 26 weeks.

A foetus of 26 weeks may demonstrate a flexion withdrawal reflex in response to pain stimulation

Descending inhibitory pathways are immature leading to greater pain sensitivity

Risk factor for IVH or later neurodevelopmental delay include :

- RDS

- Hypotension / fluctuating blood pressure,

- the use of hypertonic infusions

- aggressive volume expansion

Pre anaesthetic assessment Consult parent regarding risk of deterioration of

pulmonary function & necessitating postoperative ventilatory support

Particular points to ascertain are:

1) Gestational age at birth and the current gestational age

2) Weight

3) Periods of mechanical ventilation, CPAP and oxygen therapy and the duration

4) Apnoeas – frequency, duration, possible triggers

5) Co-morbidities, particularly cardiac

6) General health, growth and development

7) Previous operations

8) Medications

Airway assessment

If already intubated- check the ETT size & length

Blood investigations

If anticipated blood loss greater than 10% of blood volume, crossmatch should be taken

Echocardiogram must be performed before surgery

Minimise starvation times to prevent hypoglycaemia and dehydration.

Intraoperative management

The ambient temperature minimum 27 degree celciousfor reception of the baby

Effective warming device eg: warming matress, warming air blanket

Inspired gases should be heated & humidified.

Fluids , blood, blood products & irrigation fluid must be warmed.

If 360 o access to baby is required by the surgeon( eg: laser surgery for retinopathy), access can be improved by removing the head & foot ends of operating table.

GA machine

ETCO2 detector

Tapes for securing

ETT

Laryngoscope handle

& blades: 00, 0 (premature)

ETT: portexpeadiatricsize 2.5, 3.0 & 3.5mm

Connector to fit between ETT tube & ventilation bag, circuit

Check all the

equipment

ETT size & lengthBaby weight

( kg)Tube size

(mm)Oral tube

length @ lip ( cm)

Nasal tube length @ nose (cm)

Suction tube size ( Fr)

<1.0 2.5 5.5 7.0 6

1.0 2.5-3.0 6.0 7.5 6

2.0 3.0 7.0 9.0 6

3.0 3.0 8.5 10.5 6

3.5 3.0-3.5 9.0 11.0 8

4.5 3.5 9.0 11.0 8

An alternative is to assess ET tube length by the rule of 6.

Oral tube length(cm) =: 6 + wt (kg)

Nasal tube length(cm) = 6 + (1.5 x wt)

appropriate position must always be comfirmed

Monitoring:

-BP using an appropriate sized cuff

-ECG

-capnography

-temperature

- SpO2 ( 2 oximeter probes recommended)

- pt with PDA, 1 probe placed on the right hand ( pre-ductal) & the other on lower limb ( post-ductal)

Inhalational induction is often preferred

Moderate concentration of volatile can be used to ;

- Minimized increase in PVR

- Avoid decrease in systemic arterial presure

Newer shorter acting agent, desflurane may useful for recovery in a preterm infant.

Sale, in their study on premature babies under 37 weeks gestation & under 47 week undergoing inguinal herniotomy suggested that infant wake faster from GA when maintained with desflurane as compared with sevoflurane, but no difference in postoperative respiratory events was demonstrated

A range of uncuffed tubes should be available

– a neonate of <1200g may need a 2.5mm tube

- 2 kg baby has tracheal tube positioned @ gum margin @ 8cm mark

Anticipate difficult intubation if premature infant has undergone prolong ventilation-- possibility subglottic stenosis

Exposure to high O2 levels is associated with increase morbidity & mortality.

Tacycardia & HTN can be detrimental in presence of underdeveloped cerebral autoregulation.

- Careful titration of anaesthetic & narcotic agent is necessary

Multimodal analgesia should be use for pain relief

(eg: local anesthesia, paracetamol, opioid (fentanyl)

Intaoperative fluid management

Estimated maintenance fluid requirement: 100ml/kg/24hour

Maintenance fluid should be isotonic.

Premature infant often receive a glucose-containing solution to maintain normoglycaemia.

- This should continue intraoperative

Replace on going loss:-Superficial surgery: 1-2ml/kg/hour-Thoracotomy : 4-7ml/kg/hour-Abdominal surgery :5-10ml/kg/hour

Blood / blood products Required volume

Pack cell ml = desired increment in Hb(g/dL) x weight (kg) x 3

Platelet 10-20ml/kg

FFP 10-20ml/kg

cryoprecipitate 5-10ml/kg

Sign fluid depletion: Hypotension, tacycardia, increase core peripheral temperature, delayed capillary refill time, reduce heart sound

Do not introduce air bubbles into

circulation which may transverse

right to left shunts

Post operative care

Remain intubated??? Or extubate???

- Decision should consider the preoperative state of the baby as well as the type of surgery performed.

- If plan for extubate, baby should fully awake & managing adequate TV without support

Post operation, apnoea are frequent in 1st 12 hour & can continue until 48-72 hour

Monitor in high dependency unit for at least 12 hours post operatively and for a further 12 hours following any apnoeic period.

References Anaesthesia for the preterm infant – Anaesthesia UK

Kawshala Peiris, David Fell,..The Prematurely Born Infant and Anaesthesia, Oxford Journal: Volume 9, Issue 3, Pp 73-77

Sale SM, Read JA ,..Prospective Comparison of Sevoflurane and Desflurane in Premature Infant undergoing inguinal herniotomy, Br J Anaesth.2006 Jun; 96 (6): 774-8. Epub 2006 Apr 28

Guy Bayley, Special consideration in the premature and ex-premature infant, Anaesthesia and Intensive care medicine 12:3, 2010 Elsevier Ltd

Bharti Taneja, Vinish Srivastava, Physiological and anaestheticconsideration for the preterm neonate undergoing surgery ,2012; 1:14

Neonatal Handbook